Co Co

Mos

Diffuse Reflectance Infrared Fourier Transform DRT Densité de Remplissage Tassé (Compact Filling Density) HDS Hydrodesulfurization HDT Hydrotreatment HYD Hydrogenation LHSV Liquid Hourly Space Velocity LOI Loss On Ignition MES Mössbauer Emission Spectroscopy

CoMoS phase

Alumina

General Introduction

In 2009, the collapse in oil demand, combined with downstream capacity additions, led to substantially lower refinery throughputs. The global refining system was hence shifted into a level of effective "spare capacity" of more than 7 mb/d, which wasn't observed since the end of the 80's. Nevertheless, new projects coming on stream over the next five years are expected to reverse this trend; the overall refining spare could approach 10 mb/d by 2015, unless some capacity is closed. 1,2 On the other hand, driven mainly by increases in syncrudes, condensates and light crude oils, it is projected that the global crude slate will become marginally sweeter in 2015, compared to 2010. The trend then reverses towards a sourer slate, with the sulfur content slightly above 1.3 wt.% by 2035. In addition to the quality of crude oil used as a feedstock to the refining system, another significant factor affecting future downstream investment requirements is the quality specifications of finished products, especially in the road transport sector. An overall aim of policymakers is to produce transportation fuels with a sulfur content below 10 parts per million (ppm). The next step, which has already begun in several countries, is the extension of stricter sulfur specifications beyond on-road transportation to other products, particularly fuel oil, marine bunkers and jet fuel. 1 Substantial desulfurization capacity additions will thus be necessary to meet sulfur content specifications, as non-OECD (Organization for Economic Co-operation and Development) regions in particular move progressively towards low and ultra-low sulfur standards for domestic fuels, often following Euro III/IV/V standards. In short, the continued drive to tighter sulfur standards is expected to see desulfurization comprising the largest volume capacity additions from now to 2035. The majority of these units is projected in Asia, with an outlook of 10.5 mb/d, followed by the Middle East and Latin America, with 4.4 mb/d and 3.4 mb/d, respectively. The lowest desulfurization capacity additions are projected for North America and Europe, where almost all transport fuels are already following ultra-low sulfur standards. 1 Several solutions are possible to achieve the nowadays goals, that affect either the process (higher temperatures, increased hydrogen pressure, or flowsheeting modification) or the catalyst. The latter solution, which does not involve significant additional costs for refiners, is therefore the most studied. Heterogeneous catalysts are a key factor in refining R&D. The introduction of new catalyst formulations, as well as activation method modifications, has been responsible for performances increase. Most common hydrodesulfurization (HDS) catalysts nowadays are based on CoMoP or NiMoP formulations, consisting in sulfided metals from group VI.B (like Mo or W) promoted by Co or Ni, supported on a relatively high surface area alumina. Active species of molybdenum-based hydrotreating (HDT) catalysts consist in molybdenum disulfide nanoparticles. Supported MoS 2 structures have been exhaustively studied in order to rationalize their morphology, surface sites, promoters location, etc., and their impact on catalytic activity. The dispersion of the active phase is directly related to the specific surface of the support. Nevertheless, at higher molybdenum densities, the formation of refractary species such as CoMoO 4 or Co 3 O 4 by sintering during calcination and/or sulfidation steps has been reported for aluminasupported catalysts. 3 The introduction of novel, more effective porous supports could be an alternative for the modification of the active phase-support interactions and hence to increase catalytic performances. Different materials like silica, carbon, titania, zirconia have been used as supports. More recently, SBA-15 mesostructured silica has been in focus due to its large specific surface, its large pores, thick pore walls and high (hydro)thermal stability. On the other hand, polyoxometalates (POMs) have also been extensively used in diverse domains, including catalysis, due both to their strong acidity and oxidative properties, and because they can be easily tuned by simply changing poly-metal anions chemical composition. 4 These species have also been used as active phase precursors in HDT catalysts. It has been suggested that the active phase dispersion and promotion rate were improved due to the introduction of molecules containing both the active site (molybdenum) and its promoter within the same molecule. 5,6 For these reasons, much effort has been devoted to developing a synergistic combination between catalytic properties of POM and structural and textural properties of oxide supports, with the whole advantages related to the handling of a solid catalyst. 7,8 Porous materials with larger surface area may serve as support, admitting POM in their pores and/or at their walls. In the field of oxides, it has been widely reported that SBA-15 type silica has been extensively used as support and host, because of its larger pore size, thicker walls and higher hydrothermal stability rather than other mesostructured silicas such as MCM-41 or SBA-3. The first adopted strategy to include POM species into porous materials consists in post-synthetic procedures, involving wetness impregnation technique: the POM species, which are soluble in strongly polar media, are loaded into the porous matrix by impregnation of the solid with an aqueous or alcoholic solution of POM. However, this method leads to surface area loss at higher POM loadings as well as loss of homogeneity of the surface and POM dispersion by leaching. 9 Therefore, two major strategies have been developed to overcome these issues -(i) functionalization by grafting organic species on the pre-formed oxide surface to improve the linkages between POM and inorganic porous matrix, which limits their application to low temperature processes, and (ii) co-incorporation of POM clusters together with oxide precursors in a one-pot synthesis, by a sol-gel technique. The so called one-pot synthesis could lead to hybrid materials with improved pore sizes, no pore blocking (since no molecules were post inserted in the inner surface), providing a better dispersion and stability of the active phase. Here, we chose to consider this strategy to the concept of POM based catalysts, incorporated into a mesostructured silica matrix, with the objective of improving the HDT activities, after the sulfidation of the catalysts.

In this first chapter, the general context of the present study is first introduced, from the hydrotreatment process to the support synthesis and recent breakthroughs regarding the catalyst preparation.

I. Bibliographic Study I.1 Introduction

In the first part of this bibliographic chapter, the general context of the present study is introduced, beginning by a brief description of a refinery, focusing on hydrotreatment in particular: operational conditions, reactivity of organic sulfur compounds and main drawbacks. Then, the standard preparation of hydrotreating CoMo and NiMo catalysts is presented followed by a description of the different models proposed to describe CoMo catalysts active phase. The main disadvantages of the actual catalyst are discussed, and recent breakthroughs regarding the support, active phase precursors as well as introduction of the latters are described. The second part of this chapter is dedicated to the innovative methodology, the encapsulation of active phase precursors into the framework of the mesostructured support. Beginning with an introduction to the mesostructured materials synthesis, polyoxometalates precursors are at this point presented, and recent advances in the introduction of these molecules as HDT catalysts precursors into mesostructured supports are finely described and analyzed. Finally, the main conclusions obtained from the state of the art are summarized and the objectives of this PhD thesis presented.

I.2 Hydrotreatment Process

Petroleum is a naturally occurring fluid, consisting of a complex mixture of hydrocarbons such as paraffins, aromatics and naphtenes of various molecular weights, also containing small amounts of organic compounds with heteroatoms (like sulfur, nitrogen and oxygen) and metals (mostly nickel and vanadium). Crude oil also presents water, gas and inorganic compounds such as elemental sulfur and salts. Besides elemental composition, physical properties like density or viscosity may also be used to classify crude oils (density d 4 15 usually in the range 0.846 to 0.954 and viscosity from 10 up to 300 cSt). 10 Once at the refinery, crude oil is first desalinized and then processed in the atmospheric distillation (AD), which actually runs at 2-3 bar, where the different compounds are separated according to their boiling points. Table I.1, shows the different boiling points for the main "liquid" oil fractions issued from the AD. The aim of refining is to adjust raw fractions coming from the AD to the required specifications as well as physical properties. Figure I.1 presents a very simplified scheme of a modern refinery. 11 Hydrotreatment is a catalytic process spread throughout the refinery, by which undesired elements or compounds are eliminated from a petroleum cut, in the presence of hydrogen. There are different hydrotreatment processes, mainly characterized by feedstock and achieved product specification. Table I.2 shows the most common applications of the process.

Operation Petroleum Cut Purpose

HDT Naphta

Desulfurization of LPG a and Naphta cut and protection of reforming and isomerization catalysts.

HDS Kerosene

Softening of kerosene cut in order to eliminate mercaptans from Jet A1 fuel and reduction in aromatic content.

HDS

Diesel (from AD b , FCC c or Visbreaking)

Reducing sulfur levels from diesel fuel and improvement of cetane number and diesel index, related to aromatics saturation.

HDT

Heavy Cuts (AD residue and light VD d cuts)

Protection of conversion catalysts (HDM and HDS).

HDT

Gasolines from FCC Reduce sulfur level without losing significant olefins content.

HCK e

VD Distillates Protection of conversion catalysts (HDS, HDN and HYD). VD hydrotreated distillates

Decreasing molecular weight and increasing H/C ratio. The main undesired impurities are sulfur, nitrogen, oxygen, aromatics or metal compounds. Depending on these undesired compounds, hydrotreatment may consist in a hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), hydrodearomatization (HDA), or hydrodemetallization (HDM). Process conditions and applied catalysts are thus chosen accordingly. Here, we will focus mainly on the HDS process.

I.2.1 Operational conditions

Figure I.2 is a simplified diagram of a typical commercial hydrotreating unit. Typically, this process includes three fractions: a preheat system (feed-effluent exchangers and/or furnaces), the reaction area, constituted by one or more reactors, and a gas-liquid separation system.

In the simplest units, the reaction occurs over a unique catalytic bed of one reactor. However, the most complex units such as those for hydrotreating residues can include several reactors in series. Those reactors are eventually quenched with non-preheated recycle hydrogen, in order to prevent runaway situations due to the exothermic character of the main reactions. The products separation is accomplished by a single-stage flash drum, which can be pursued by more separation drums and/or stripping towers. Some units contain high and low pressure separation stages, the purpose being the maximization of both recycle H 2 gas purity and the removal of light ends (such as C 1 -C 4 ) from liquid products. For each fuel type, the current typical conditions of hydroprocessing are different, as presented in Table I. 3. The commercial hydrotreatment catalysts are applied according to the capability of reactor equipment, operating conditions feedstock type, sulfur contents and desired level of sulfur reduction. Deeper HDS may be achieved into a certain level by tailoring process conditions:

H 2 pressure/purity Improving H 2 purity in the recycled stream is better than increasing H 2 pressure since H 2 S pressure increases as well, inhibiting process conversion. A 30% H 2 purity increase may lead to a temperature decrease down to 8-9°C and a yield improvement up to 40% in sulfur removal, depending on the conditions. The H 2 purity may be achieved with a Pressure Swing Adsorption (PSA) column or a membrane system. 12 H 2 S partial pressureSince H 2 S is an inhibitor to HDS, 13 its build-up in high pressure reactions through continuous recycling can become significant. UOP improved process yield from 285 to 180 ppm w in outlet S content by partially removing H 2 S by chemical scrubbing, in an existing hydrotreater. 12

LHSV

(Liquid hourly space velocity) is defined as the ratio of the hourly volume of processed feed to the volume of catalyst. LHSV can be reduced by increasing the catalytic bed bulk or decreasing feedstock flow rate. The aim is to increase the contact time between reactants and catalyst, especially when there are more refractary sulfur compounds. Temperature Higher temperature leads to an increase in the HDS reactions rate, facilitating the high activation-energy reactions. Nevertheless, the catalyst life is reduced, which involves the change of the catalytic bed, and thus an increase of the stand-by costs.

Gas-Liquid contact

Ensuring an adequate vapor-liquid contact, one is able to enhance distillate hydrotreaters performance or decrease process temperature. A better gas-liquid contact may be achieved with appropriate distribution trays inside the reactors. Nevertheless, the above-mentioned improvements are not sufficient to achieve the new required fuel specifications. It is though essential to understand the chemical mechanisms and activities beyond the physical conditions, as well as a clear understanding of the types of sulfur compounds present in feedstock and the factors that may influence their reactivity.

I.2.2 Reactivity of organic sulfur compounds

The sulfur content of a petroleum cut usually increases with its average molecular weight. Consequently, lighter cuts as naphta contain light sulfur compounds such as mercaptans (R-SH), thiophene, sulfides (R 2 S) or disulfides (R-SS-R), etc. Middle cuts contain mostly alkyl-substituted benzothiophenes. Residue is the heaviest cut, thus → → which are then further hydrogenated. 16 Such mechanism is often discussed since the dihydro-intermediate has never been really identified nor has been the thiol intermediate, 17 although it is believed that di-hydro-di-benzo-thiophene can be further hydrolysed, or desulfurized, by elimination to provide biphenyl, which has already been proposed by Kasztelan. 18 DDS/HYD selectivity of HDT catalysts is directly related to their performances in the production of Ultra Low Sulfur Diesel (ULSD). In In fact, at that level, adsorption of refractary nitrogen species may inhibit the hydrogenation function. 19 Polyaromatic and refractary nitrogen compounds compete with sulfur compounds in hydrogenating catalyst sites since they adsorb more strongly to the catalyst surface, hence influencing the HYD route, whereas H 2 S compete with other sulfur compounds, inhibiting also the direct C-S hydrogenolysis DDS. The contact synergy model, [35][36][START_REF] Topsoe | In Studies in Surface Science and CatalysisCatalysts in Petroleum Refining[END_REF] states that Co x S y species are in contact with MoS 2 , retro-donating a hydrogen atom, that would create reduced centers on the neighboring of MoS 2 and consequently increasing the catalyst activity, as represented in [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] The CoMoS (or NiMoS) phase model, proposed by Topsøe et al., [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF][START_REF] Topsoe | In Studies in Surface Science and CatalysisCatalysts in Petroleum Refining[END_REF][START_REF] Topsoe | [END_REF] is in general the most accepted theory for the description of HDS active phase in Co-promoted Mo-based catalysts. CoMoS (or NiMoS) structures are small MoS 2 nano-crystallites, with Co or Ni atoms located at the edges of MoS 2 layers, in the same plane than Mo atoms. This model is represented in Figure I.9, as well as the promoter sulfur phases and cobalt that can be incorporated into the supports framework at high calcination temperatures. All HDT reactions involve adsorption-desorption of molecular hydrogen, which is known to occur in the active phase edges. Several theories attempted to explain experimental results correlating active phase structure with catalysts activity.

The geometric model developed by Kazstelan et al. 39 completed the CoMoS model defining the 2D morphology of active phase particles. This model was pioneer in the concept of the influence of active phase geometry on the nature of the catalytic sites present on the edges. According to this model, different morphologies illustrate the several possible forms of molybdenum sulfide particles, such as chain, triangle, hexagon and rhombohedra, to which correspond different kinds of active sites, as depicted in Figure I.10. 39 Experimentally, the particle sizes were modified varying the metal loading on the catalyst. The comparison between experimental results and geometry modeling resulted on a correlation between catalytic activity and the type of active site. The authors highlighted the importance of the edge sites and that the active phase morphology is usually of the type "truncated triangles", as further evidenced by STEM images. The reactivity of catalysts and their performance is therefore highly influenced by the morphology of the active phase.

The "Rim-Edge" model, a proposed by Daage and Chianelli, 40 also pointed out the importance of the active phase morphology, regarding the catalytic properties of the catalysts. According to this model, schematized in Figure I.11, the stacking of the MoS 2 slabs at the catalyst surface has a direct influence on the catalytic performance. The core layers -Edges -would have a different activity than the basal layers -Rim. According to this model, rim-layers are active in hydrogenation and hydrogenolysis reactions: the rim crystallites are supposed to hydrogenate and cleave C-S bonds, whereas the edge-layers are only able to cleave C-S bonds.

The authors determined hydrodesulfurization (HDS) and hydrogenation (HYD) activities by changing the sheet size and stacking, founding that lower stacking degrees increased HYD selectivity, as deduced from In order to a better understanding of the reaction mechanisms of the active phase of HDT catalyst, several authors have reported theoretical studies based on DFT calculations together with experimental data. These conducted theoretical studies were based on a same calculus base. 41 The studied structures are optimized for the thermodynamic equilibrium: n H 2 S (g) + (*) n S(*) + n H 2 (g) where H 2 and H 2 S are the gases of the sulfo-reductive atmosphere surrounding the catalyst during activation and/or test, (*) sulfur vacancy on the catalyst edges and S(*) the catalyst with sulfur adsorbed on the edges, n is the number of S atoms. The chemical potential of sulfur ΔμS(*) and hydrogen ΔμH(*) are used to describe the equilibrium states, which depend on the atmosphere P H2S /P H2 ratio and temperature, and may be determined from Figure I.12. PH2S/PH2 ratio. The two horizontal lines delimit the stability range of the sulfide phase. 42 E. Krebs and P. Raybaud, [43][44][45][46] have focused their work on the morphology of the active phase at the thermodynamic equilibrium as a function of Δμ S . As aforementioned, 39 the MoS 2 particle morphology is generally an hexagon. Nevertheless, recent advances in scanning tunneling microscopy (STM), [47][48][49][50] revealed that the active phase clusters presented different morphologies, from triangle to hexagon according to the sulfidation conditions, which also confirmed the positioning of promoters on the edges of the sheets as proposed by the CoMoS model. Depending on these conditions, not only the sulfur coverage at the edge of the nano-crystallite changes but also the shape of each individual MoS 2 nanoparticle does. For reductive environment such as in HDS conditions, the shape is close to a deformed hexagon as depicted in Figure I. 13. Two distinct edges are present, the (101 0) M-edge (or Mo-edge), consisting in exposed Mo ions with alternating bridge sulfurs and two vacant orbitals on Mo which is also called coordinately unsaturated sites (CUS), and the (1 010) Sedge, composed of sulfur atoms that bridge adjacent Mo atoms. The ratio between both types depends on the direction of crystal growth. Hence, three edges of the hexagon are of type-M, while the other three are of type-S. Certain conditions can achieve a triangular shape. In this case, crystal growth occurs in a privileged way to one of the two types of edges so that only M-edge or S-edge are exposed. 47,49 : sulfur; : molybdenum; : cobalt; : nickel. promotion rates in each edge, according to ΔμS. 39,43,45 . While coordinative unsaturated sites (CUS) are widely accepted as active center in the DDS mechanism in promoted catalysts, the specific nature of HYD sites is still a matter of discussion. Recently, Moses et al. 51 stated that brim sites in MoS 2 layers are involved in HYD reactions. The STM microscopy used by Topsøe and co-workers, 49 allowed to identify these active sites placed in the slabs edges. These sites have strong electronic states, giving origin to bright bands at the edges, as evidenced in The STM images depict the MoS2 and CoMoS particles both before (a/c) and after (d/f) exposure to atomic hydrogen. The arrows indicate the position of sulfur vacancies on the edges. 52 The Brim site model is consistent with many aspects as inhibitions, steric hindrance, poisoning effects, etc., which has been difficult to surpass using a vacancy model. Since the Brim sites are fully coordinated sites, H 2 S is not adsorbed. As very open sites, the adsorption of refractary sterically hindered species such as 4,6-DMDBT is allowed, which explains the lack of steric hindrance in the HYD pathway. Finally, the Brim sites and their neighboring acidic protons can strongly interact with alkaline nitrogen-containing molecules, explaining the inhibition effect of nitrogen compounds in HYD mechanism. This approach differs from the Raybaud et al., 42,46 though it highlights a difference in reactivity between the two different edges and reinforces the idea of controlling the reactivity of catalysts by controlling the morphology of the particles of active phase.

Nevertheless, Raybaud et al. 42,46 claimed that this Brim sites route, which was limited by the hydrogen dissociation step, did not explain the complete HDS pathway of sulfur compounds by itself.

The catalytic activity of non-promoted systems was relatively low, mainly due to the high S-Mo bond energy at the edges, which is easily poisoned by S-species, also preventing adsorption of reactants or H 2 dissociation. This limitation of MoS 2 systems was overcome through the addition of promoters.

Berhault et al. 53 showed that cobalt enhanced the stacking height of MoS 2 layers, and if the support weakly interacts with the active phase, cobalt also promotes lateral growth. However, this effect may be offset by HDS conditions, favoring the formation of single layers. Sun et al., 54,55 stated that only S-edges are decorated by cobalt, and only "M-edge" are decorated by nickel. The CoMoS phases and non-promoted MoS 2 slabs would tend to form stable species by S-H bonds, while NiMoS species would attain an energy minimum, since hydrogen atoms are directly bonded to nickel, Ni-H. Notwithstanding, a particular behavior for each edge depend on the dissociation of hydrogen, which is in good agreement with the idea of a different reactivity depending on the exposed edge.

Paul and Payen, 56 investigated the sulfur vacant sites creation. The authors found that this vacant sites creation is not thermodynamically favored. A strongly reductive atmosphere is needed (hydrogen atmosphere, temperatures above 623K) to liberate a H 2 S and give origin to a stable surface with a sulfur vacant. Thus, sulfur vacancies should not be numerous on the slabs edges, and their life time should be very short.

Synergy effect between Co or Ni and Mo

Many theories were proposed to explain the origin of such synergetic effect of Co or Ni over Mo, enhancing HDS reactions. Norskov et al. 57 admitting metal-sulfur (M-S) binding energy as critical variable, found that HDS rate is proportional to the number of S vacancies at the surface: the weaker the M-S bond the higher the HDS activity. Chianelli, 58,59 proposed that the activity of cobalt or nickel promoted MoS 2 phase is related to an electron donation from promoter to the principal metal (from Co or Ni to Mo or W), decreasing the M-S bond strength. Receiving those electrons from Co, Mo increases the number of electrons on the highest occupied molecular orbital (HOMO), filling anti-bonding orbitals and subsequently weakening the Mo-S bond energy. Cobalt by its turn, relief some electrons from antibonding orbitals, strengthening the Co-S bond. A sulfur atom shared between Co and Mo will present intermediate M-S bond strength, suggested as an optimum for HDS activity, according to Sabatier principle a interpretation. This theory was supported by theoretical a Sabatier Principle states that interactions between catalysts and substrates should not be too weak (the substrate fails to calculations and experimental techniques. In addition, Thomazeau et al. 60

I.3.2 Developing Strategies

In order to enhance the catalytic performance of HDT catalysts, different domains can be explored, namely the activation operational conditions, the active phase precursors, the catalysts preparation method or the characteristics of the support. 61 The use of different materials like silica, carbon, titania, zirconia or even mixed oxide supports in synthesis of molybdenum and tungsten HDS catalysts have been studied, as the use of additives like P, F, B and other chelating agents. Recently, SBA-15 has been extensively applied as HDT catalysts carrier, due to its large pores, thick pore walls and high thermal stability. 61 Vradman et al,. 62 demonstrated the increased catalytic activity of highly loaded NiW on SBA-15 for deep hydrotreatment of petroleum feed-stocks. Lizama and Klimova, 7 found that NiMo(W)/SBA-15 catalysts prepared by incipient wetness impregnation from methanol solutions of Keggin-type heteropolyacids, showed better performance in deep HDS than the counterparts prepared from ammonium salts. Substitution of Si 4+ ions in SBA-15 by Ti 4+ , Zr 4+ or Al 3+ ions has been studied in order to enhance the acidity and stability of mesoporous silicas and modify their catalytic properties. 61 Some authors reported that Al, Ti, Zr-containing SBA-15 provide a better dispersion of the Ni(Co)Mo(W) active components as compared to alumina supported catalysts. [63][64][65][66][67] Mesoporous silica has also been described as support for immobilization of tungsten oxide species in the reaction of selective oxidation of cyclopentene. 67,68 bind to the catalyst), neither too strong (the catalyst gets blocked by substrate).

I.3.2.1 Support Manipulation

Acidic Supports

The use of zeolites and amorphous silica-alumina as supports for HDS active phases have been the object of numerous studies. Attempts were made to increase the conversion of methyl-substituted DBT, either through dealkylation or isomerization, i.e. acid-catalyzed reactions. Zeolites are highly acidic and contain Brønsted acidity, which promotes these acidcatalyzed reactions, suppressing the alkyl-DBT steric hindrance. The zeolite-supported CoMo catalysts are more efficient than alumina-based catalysts, but additional reactions were observed as transalkylation and formation of methyl-dibenzothiophenes, which may lead to coke formation, in addition to the direct desulfurization and hydrogenation routes. This might be ascribed to a better dispersion of the molybdenum sulfide on the zeolite support than on alumina, or to an enhancement of the sulfide phase when it is in close vicinity to acid sites. 69 The main drawback using zeolites is the fact that zeolites pore size is too small (< 1nm) to accommodate the active phase and to ensure the access of bulky reactants. On the other hand, SiO 2 -Al 2 O 3 (ASA) has a milder Brønsted activity when compared to zeolites, but still enough to modify the electronic properties of active phase and to promote isomerization reactions. ASA pores size is not a limiting factor for active phase dispersion, neither for diffusion of reactant molecules within the pores.

Basic Supports

Research within alkali supports may contribute to the theory of support effects in hydrotreating catalysis. MgO supports are particularly interesting as HDT catalysts supports for two main reasons: the interaction of acidic MoO 3 with the basic MgO support would lead to the formation of a strongly bound monolayer of molybdenum oxidic species, which could give origin to a higher sulfided species dispersion, after catalysts activation. In addition, the basic properties of MgO supports may inhibit coke formation on stream (promoted by acidic supports). Notwithstanding, molybdenum-based sulfide catalysts supported over MgO have not been extensively studied, probably because the reported HDS activity was rather lowthe activity of the Mo/MgO catalyst in the HDS of benzothiophene, 70 and thiophene, 71 was about 26% and 47% of the activity of the Mo/Al 2 O 3 catalyst, respectively. Klicpera and Zdrazil, 72 prepared CoMo and NiMo catalysts from MoO 3 /MgO supports, with 250-300 m 2 g -1 . Introduction of active phase precursors was performed by impregnation with a methanol solution of Co and Ni nitrates, which prevented the damage of the supports structure during aqueous impregnation, giving origin to low surface Mg(OH) 2 . A strong synergistic effect between the promoted and non-promoted catalysts (circa 20) was observed in catalysis. The most active Co(Ni)Mo/MgO catalysts were 1.5-2.3 times more active than their Al 2 O 3supported counterparts for the hydrodesulfurization of thiophene. According to the authors, the preparation may still be improved. 72

Mixed oxides supports

TiO 2 -ZrO 2 supported MoS 2 catalysts were studied, and presented interesting HDS activities. 61 However, the specific surface area of these supports are relatively low (~ 100 m 2 /g) and porosity is not suitable for HDT applications, restricting Mo loadings. Many improvements in the preparation of these oxides were achieved, and supports with higher surface area and larger pore diameters were obtained. Molten salt preparation provided higher thermal stability and specific surface areas up to 250 m 2 /g, for ZrO 2 . However, this method, besides its intrinsic complexity, yields polluting gases such as NO and NO 2 , which can be decreased by addition of alkali metalcarbonates to the flux. 73 An alternative to overcome the low surface area of these supports is the use of TiO 2 -Al 2 O 3 . The combination of titania and alumina provided a higher specific surface area, however the supports often presented a heterogeneous dispersion of titania.

Ramirez et al. 74 showed that a better dispersion of MoS 2 on the supports is obtained when the TiO 2 content is increased, and activity measurements in thiophene HDS have shown that intrinsic activity, expressed on a Mo atom basis, sharply increases at higher TiO 2 loadings. In fact, the incorporation of TiO 2 to the support increases HDS activity, due to the electronic promoter character of Ti towards the MoS 2 phase.

Doping and Additives

Another route to improve HDS activity of the actual alumina-supported catalysts consists in adding some doping agents, the most common being P, F, B, Si, La, etc. The objective is to increase alumina acidic properties, improving the interactions between metals and the support, the dispersion of the active phase, the reducibility and the sulfidability of Mo and Co or Ni. Regarding the La-modified HDT catalysts, it has been suggested that La inhibited the lateral growth of MoS 2 slabs, by influencing the surface concentration of active components, causing a decrease in surface promoter concentration of sulfided catalysts. In addition, La atoms increase the thermal stability of Al 2 O 3 supports. [75][76][77] Fluor was also found to improve the hydrogenating properties of HDS catalysts, namely to vacuum distillates VR hydrotreating, interfering in the equilibrium of adsorption/desorption of coke precursors. On the other hand, Fluor was also found to increase the acid strength of the support, mainly due to the fulfillment of Lewis sites with Fluor, next to Brønsted acidic sites, which led to a weaker O-H bond of the Brønsted site. [START_REF] Gualda | Faculté des sciences fondamentales et appliqués[END_REF][START_REF] Marques | [END_REF] The addition of noble metals such as Ru, Pd or Pt to CoMo or NiMo catalysts has also been reported to improve HDS activity, by enhancing their hydrogenation function. [START_REF] Navarro | [END_REF]81 The presence of RuS 2 phases, recognized as one of the most active compounds for hydrotreating reactions by Pecoraro and Chianelli, 59 are located on the catalyst surface contributing to overall catalytic activity, are less prone to deactivation. It was suggested that the promoting effect of Ru-NiMo/Al 2 O 3 in HDS of DBT was due to the replacement of Ni atoms by Ru. 82 The main drawback of such catalysts is the poisoning effect of Ru in the catalyst sulfidation, giving origin to Ru-O-Mo species, less active in the HDS of thiophene than the individual ruthenium and molybdenum sulfides.

Beyond the above mentioned additives, the most often applied doping agent in recent years is phosphorous. P has been reported to improve the performance of HDT catalysts, especially in HDN of NiMo/Al 2 O 3 catalysts. Different attempts have been made to explain the influence of P on the activity of hydrotreating catalysts. The main conclusions are that phosphorous increases the metal dispersion, [83][84][85][86][87] generates new active sites with higher activity, 86,88,89 changes the superficial structures, 90 increases the amount of stacked MoS 2 combined with morphology change, 91 prevents the formation of nickel or cobalt aluminates, 92 induces the formation highly active CoMoS or NiMoS phases, [93][94][95] and tunes acidity throughout the association of phosphates and the catalyst support. 96 Despite the fact that HDS has a slight improvement by P addition, the effect on HDN is strongly positive. Studies with industrial feedstocks clearly demonstrated the positive effect and researches with model compounds indicated that phosphate, in particular, promotes the hydrogenation of aromatic rings. [102][103][104] In HDN, C-N bond cleavage is the limiting step. NH 3 -TPD analysis have shown that addition of phosphorous increases the number of strong acid sites and strengthened the moderately acid sites, which can improve C-N bond cleavage, enhancing HDN activity. Conversely, the hydrogenolysis and hydrogenation routes in HDS reaction depend on the nature of the compound, on the catalyst formulation (CoMo or NiMo), on the H 2 S partial pressure, on the total pressure, etc. Light compounds prefer hydrogenolysis rather to hydrogenation route, but alkyl-substituted benzothiophenes favor the latter. It has been shown that P addition to CoMo/Al 2 O 3 favors the HYD route in HDS of DBT. 105

Mesostructured materials as HDT catalyst supports

Recent materials designated as mesostructured molecular sieves have introduced a new degree of freedom in the conception of catalysts. There are different kinds of relevant materials obtained following different synthetic procedures (Figure I.17

Synthesis of Si-based mesostructured materials -SBA-15 Outlook

The development of porous materials with large specific surface areas is currently a domain of extensive research, particularly with regard to potential applications in catalysis, adsorption, sensor technology, gas storage, etc. 132 An upsurge began in 1992, with the development of M41S phase, by Mobil Oil Company. The use of supramolecular aggregates of ionic surfactants (long-chain alkyl trimethyl ammonium halides) as structure directing agents (SDA) or templates, was pioneering in the synthesis of these novel materials. These SDA, in the form of a lyotropic liquidcrystalline phase, led to the assembly of an ordered mesostructured composite during the condensation of the silica precursors. 132 The structure directing agents (SDA) are usually dissolved in acidic or basic media. The synthesis temperature is normally higher than the critical micelle temperature (CMT) of the SDA, corresponding to a critical micelle concentration (CMC). After the arrangement of the organic micelles, the silica precursor is added and begins to condensate around the micelles. Subsequently, silica continues to polymerize at higher temperature, in a hydrothermal treatment. This treatment induces the complete condensation of silica precursors and improves the mesoscopic regularity of the products. Hydrolysis and cross-linkage of inorganic species and assembly further proceed during this step. The hydrotreating temperature generally ranges from 80 °C to 150 °C. Higher temperature would result in the degradation of ordering and decomposition of surfactants, leading to microporous materials. As-synthesized materials are obtained after washing and drying. These materials generally have large particles which are easily filtrated, although centrifugation can sometimes be useful. Water is generally used in the washing step, though alcohol can also be used. 152 The resulting solids are then dried, from room temperature (RT) up to 50 °C, since higher temperatures may reduce the mesoscopic regularity to some extent. 153 Finally, the material porosity is obtained by surfactant elimination. The different removal methods influence the characteristics of mesostructured materials. The most commonly applied is calcination, with a multi-steps program. The temperature increase should be slow enough to prevent structural collapse caused by local overheating. The calcination temperature should be lower than the thermal stability limit of mesostructured materials, and higher enough toward the complete removal of surfactants. On the other hand, extraction may also be performed, consisting in a mild and efficient method, without distinct effects on frameworks. 154 Ethanol or THF are common organic extracting agent. 155 Extraction can partially preserve surface hydroxyl groups, enhancing the hydrophilic property and modifying the reactive ability of pore channels. 156 Nevertheless, the application of extraction is limited by the fact that surfactants cannot be completely removed, in addition to an absence of thermal stabilization of the silica framework, as when calcination is performed.

Mechanism of mesostructured materials formation

The formation of these solids has been extensively studied. Two different mechanisms of mesostructured materials formation process are known: cooperative liquid-crystal templating (CLCT) and true liquid-crystal templating (TLCT). 132 Adapted from ( 152 ).

On the other hand, in the TLCT mechanism (Figure I.20 (b)), if the concentration of SDA is high enough a liquid-crystalline phase is formed without requiring the presence of the inorganic precursor, acting like a template before the silica precursor addition. 158 Nevertheless, the differences between both mechanisms are not clearly defined since tensioactives mesophase may form and decompose continuously during synthesis. 159 Attempting to explain the interactions between the SDA and the inorganic precursor, Huo and co-workers, 160,161 proposed several synthetic routes, based on these species natures. In the particular case of a synthesis in acidic medium (below the isoelectric point of silanols Si-OH, pH∼2), the silanols are positively charged. To create an electrostatic interaction with a cationic surfactant, an intermediary anion X -is necessary, most commonly a halide (S + X -I + , Figure I.21 A). On the other hand, it is also possible for the attractive interactions to be mediated through hydrogen bonds. That's what happens when non-ionic SDA are used (S 0 , long chain amines, for example), together with non-charged precursors or ion pairs (S 0 (XI) 0 The use of glycol-based polymers has been studied, namely the Pluronic® group. These tensioactives are tri-blocked, composed by polyethylene glycol -polypropylene glycolpolyethylene glycol (PEG-PPG-PEG) groups. They are able to form liquid-crystals, and have been used in the synthesis of large pore mesostructured materials, more commonly in acidic media. 129,[162][163][164] The ethylene moieties interact with the cationic silica precursor (S 0 (XI) 0 interactions), initiating the mesostructure formation, as in the synthesis of SBA-15 silicas, for instance, where Pluronic® 123 (P123, 2D mesostructured materials with hexagonal symmetry are most easily produced, the classical products being MCM-41, FSM-16, SBA-15, etc. The ideal models for these structures are hexagonally close packed cylindrical pore channels, belonging to p6mm space group. Typical TEM images can show two features: hexagonal structures along the channel system and parallel stripes if viewed perpendicular to the channel directions, as in Figure I.23. MCM-41 is the simplest and most extensively investigated mesoporous silica molecular sieve. It can be synthesized in a wide range of conditions, with the most popular synthesis using CTAB as a structure directing agent, in an alkaline solution. In 1998, Zhao et al. 129 introduced the use of amphiphilic triblock copolymers to direct the organization of polymerizing silica species, resulting in the preparation of well-ordered hexagonal mesoporous SBA-15 silica structures, with uniform pore sizes up to 30 nm and silica wall thickness up to 6,4 nm. 129 SBA-15 preparation was performed at temperatures between 35-80 °C. At room temperature merely amorphous silica powder or poorly ordered products are obtained, whereas above 80 °C silica gel was produced. Tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), and tetrapropoxysilane (TPOS) are suitable sources of silica for the preparation of SBA-15. Hexagonal mesoporous SBA-15 has been formed in acidic media (pH < 1) using different acids, such as HCl, HBr, HI, HNO 3 , H 2 SO 4 , or H 3 PO 4 . At pH values between 2-6, which is above the isoelectric point of Si-OH groups (pH ∼ 2), no precipitation or formation of silica gel occurs, whereas at pH ∼ 7 only disordered or amorphous silicas are obtained. 122,152,158 Considering a triblock copolymer such as Pluronic® 123 as structure directing agent (SDA), the lower the pH value is, the more hydrophilic the block copolymer is, since PEO moieties are readily protonated in strongly acidic media. Conversely, acid catalyst (at low concentration) favors a slow condensation rate of silicate species, easily yielding highly ordered mesostructures. 165 When strong acids as HCl serves as a catalyst, the optimum pH value is below pH = 1, using 2 M solutions. 152 A typical procedure of SBA-15 reported by Zhao et al. 164 involves the following gel composition: 4 g P123 : 0.041 mol TEOS : 0.24 mol HCl : 6.67 mol H 2 O. The mixture was stirred at 35 °C for 20 hours, aged at 100 °C for 24 hours, and subsequently calcined in air at 500 °C for 6 h, yielding a SBA-15 with a mean pore size of 89 Å, a pore volume of 1.17 cm 3 /g, and a Brunauer-Emmett-Teller (BET) surface area of 850 m 2 /g. Three well distinguished regions of the adsorption branch in Figure I.24 are observed: (i) monolayer-multilayer adsorption, (ii) capillary condensation, and (iii) multilayer adsorption on the outer particle surfaces.

I.3.2.2 Active Phase

Improvement of catalyst impregnation techniques

The impregnation of the active components of a catalytic system plays an important role on the active phase composition, morphology and dispersion. Different methods have been extensively described in the literature for Mo and Co or Ni introduction over the supports surface, although incipient wetness impregnation (IWI) is the most widely used. 2 Briefly, wetness impregnation technique involves the impregnation of a solution with the same volume of the support pores, where the active metal precursor salt (e.g. ammonium heptamolybdate -AHM) is dissolved in water or in an organic solvent. Capillary action draws the solution into the pores in a maturation step, allowing a good dispersion of the precursors throughout the support; the impregnation solvent is then removed by a drying step to avoid steaming and/or sintering during the calcination step.

Improvements in the impregnation technique have been achieved for the preparation of highly active HDS catalysts by maximizing the formation of well dispersed CoMoS CoMoS and NiMoS phases. The addition of chelating compounds to the impregnation solution (such as glycol, nitriloacetic acid (NTA) and ethylene diamine tetra-acetic acid (EDTA)), enhances the formation of these active sites delaying the sulfidation of Co or Ni. In CoMo/Al 2 O 3 catalyst for instance, prepared with NTA, Co coverage was found to increase on the edges of MoS 2 slabs. The strong interaction between chelating ligands and Co or Ni ions leads to the sulfidation of these promoter metals at a higher temperature than that of molybdenum. Hence, sulfided Co atoms are allowed to form at the edges of the already formed MoS 2 slabs, giving rise to CoMoS or NiMoS structures. Chelating agents can also prevent strong interactions between Mo and/or Co and the support. 178,179 A strong interaction between support surface and molybdate species often occurs, and most of the Mo present as polymolybdate dissociate to monomolybdates through those interactions, giving origin to refractary compounds, such as CoAl 2 O 4 or AlMo 6 .

Chemical vapor deposition (CVD) of the promoter on the surface of the sulfided Metal/Support catalyst is another method that would lead to the selective formation of active CoMoS phase. 178,180-183 Maugé et al. 184,185 found that Co-carbonyl catalysts are twice as active as conventional CoMo/Al 2 O 3 catalysts, introducing Co(CO) 3 NO to the afore prepared MoS 2 /Al 2 O 3 . After a sulfidation step prior HDS reaction, the catalyst showed the selective formation of a CoMoS phase. Okamoto et al, 180 based on CVD technique, prepared supported Mo oxide precursor, initially sulfided at 400 °C towards MoS 2 slabs. These catalysts were then exposed to Co(CO) 3 NO vapor at room temperature for a given time (depending on the Co loading), followed by evacuation, to remove the physisorbed Co(CO) 3 NO molecules. The catalysts were finally sulfided a second time, at 400 °C. The Co coverage of MoS 2 edge sites was found to be particularly high in the catalysts prepared by CVD. Consequently, HDS activities were also substantially higher than those observed with conventional catalysts. The Co(CO) 3 NO adsorption over MoS

Polyoxometalates (POM) as new active phase precursors

The development of different catalytic precursors is another way towards catalysts improvement. 61 Polyoxometalates have been known since the beginning of the XIX th century, as it is generally admitted that the first compound (the ammonium salt of PMo 12 O 40

3-

) was discovered by Berzelius in 1826. 187 The first structural determination of the phosphotungstic The structure depicted in Figure I.28 represents the so-called primary structure. In the solid state, these ions form a crystalline phase with its counter-ion, the symmetry depending on the solid hydration degree and on the counter-ion nature. The crystal form is denoted as secondary structure, 190

Lacunary Keggin's structures

Keggin structures may also give rise to lacunary structures. The lacunary Keggin structure is an open structure where one or several sites previously occupied by one metal atom become vacant. (Figure I.29 a) Usually, the lacunary series present an atomic M/X ratio of 9 or 11, after the metallic atoms removal from the same or different M 3 O 13 groups. These compounds may be synthesized directly from a stoichiometric ratio of reactants under appropriate pH conditions, or arise from the decomposition of a saturated species exposed to an alkaline media. The degree of substitution may be controlled with the medium acidity, since mono-or polyvacant heteropolyanions may be obtained. The lacunary species may also be filled with other metals M', leading to the formation of (poly)substituted species (

In aqueous medium, the barium hydroxide gives origin to barium sulfate which precipitates, the promoter cation standing as POM counter-ion. In mild acidic medium, from pH ∼ 1.5, Keggin structures are not stable, and tend to create vacancies via the removal of one or more MoO 3 units. These vacancies can be filled up by other metals yielding mixed POM (Equation 6). Nevertheless, the stability of these structures is not yet fully understood.

Keggin heteropolyanion physico-chemical properties

From the point of view of their applications in catalysis, polyoxometalates are very interesting for their acidic and redox properties. 192,193 The redox properties of HPA, strongly depends on the nature of the metal atoms M, as well as that of the heteroatom, thus depending on the global charge. The more negatively charged the ion is, the less reducible it is (SiMo 12 O 40 4-for example, is less reducible than PMo 12 O 40 3-). In the reduced state, heteropolyanions are frequently denoted as heteropolyblues, as they usually turn dark blue. If the polyoxometalate is based on molybdenum, which has a greater reducibility than tungsten, for example, redox reactions as

• •
their pores and/or at their walls. Porous silica, active carbon, acidic ion-exchange resin, etc. are suitable supports, although periodic mesoporous silica materials, relatively inert towards POM, are the most often used due to their uniform and narrow pore size distributions, highly ordered structure and high thermal stability. [207][208][209] The first adopted strategy to include POM species into porous materials consists in post-synthetic procedures, involving wetness impregnation technique. The main drawbacks of this simple and easy method are the loss of organization and surface area of the supports at higher POM loadings. 4 Loss of POM dispersion due to leaching as well as the homogeneity of the surface may also occur in high polar medium. 9 To avoid such disadvantages, two major strategies have been developed -post-synthetic functionalization, by grafting organic species on the pre-formed oxide surface before introducing the POM clusters through either coordination bond or electrostatic interactions, and co-incorporation of POM clusters together with oxide precursors in a one-pot synthesis, by sol-gel methodology. The so called one-pot synthesis leads to hybrid materials with improved pore sizes, and no pore blocking (since no molecules were post inserted in the inner surface), providing a better dispersion and stability of the active phase. Such techniques will be describe in more detail in the next section, mirroring the broader trend in catalyst design towards improved proximity between metal precursors and active sites concerning HDS catalysis.

I.3.3.1 Traditional Active phase Impregnation

General silica supports are well known for their textural properties, although the interactions with active phase precursors are still matter of many recent researches.

Lefebvre, 210 prepared a series of impregnated H 3 PW 12 O 40 -SiO 2 catalysts, with several polyanion contents (ranging from 13 to 87 wt.%). The author observed two peaks -15 ppm and -14.5 ppm in the 31 P NMR spectrum, regardless the H 3 PW 12 O 40 (HPW) loading. Such a spectrum is different from that of pure phosphotungstic acid (in the hexahydrate form), which presents only one sharp peak at about -15 ppm. The authors attributed the peak at -14.5 ppm to PW 12 O 40 3-species, which had a slight interaction with the silica surface, and the other at -15 ppm to bulk H 3 PW 12 O 40 species without interaction with the silica surface, which has crystallized near/on the SiO 2 particles. The interaction between the support and the polyanion in such environment was described as (≡SiOH 2 +

)(H 2 PW 12 O 40 -), which suggests that the reaction of the polyanion with the support cannot be described as an exchange but as an acid-base reaction between the silanol groups, acting as the base, and the polyanion, acting as a Brønsted acid. Ghanbari-Siahkali et al. 211 prepared several HPW catalysts supported on pure silicious MCM-41 loaded with HPW from 9 to 33 wt.%. XRD measurements showed that the longrange order and periodicity of the materials decreased noticeably by the presence of HPW, above 29 wt.% of loading.

Based on a 31 P MAS NMR study, the authors also verified that the chemical shift of the HPW/MCM-41 (9 wt.%) sample was very close to the chemical shift of hydrated bulk HPW. The authors speculated that the effect of water ont the phosphorous in HPW is similar to that of the terminal hydroxyls of MCM-41 (see Figure I Based on the spectroscopic findings and catalytic results of the 1,3,5triisopropylbenzene conversion in the gaz-phase, the authors suggested that HPW forms complexes with the support, and that complexation is responsible for the high dispersion of HPW and the enhanced acidity of these systems (see Using the same preparation method, Verhoef et al. 214 supported HPW and HSiW heteropolyacids onto MCM-41 supports for liquid-and gas-phase esterification (110 °C). Good dispersion of the HPA on MCM-41 pore surface was also observed, providing more proton sites than pure HPA units. After one reaction cycle in the liquid-phase esterification of 1-propanol by hexanoic acid, the XRD of the spent catalysts showed new signals corresponding to those observed for pure HPAs, indicating substantial sintering of the catalyst. The existence of HPA clusters on MCM-41 external surface was confirmed by TEM. In vapor-phase esterification of acetic acid with 1-butanol, sintering was also observed, although to a lesser extent. It was thus suggested that esterification water, as well as ester product, may dissolve HPA units increasing their mobility onto the surface of the support.

T. Blasco et al. 213 supported HPW heteropolyacids onto different supports: nonstructured silica, high surface area non-structured aluminosilicates and mesostructured MCM-41. The HPW were introduced by stirring the support with an aqueous solution containing the required HPW amount at room temperature. The solids were dried at 100°C overnight, followed by an ageing step. The catalysts activity was evaluated by alkylation of 2butene with isobutene at 33°C and 2.5 MPa. A maximum in activity, selectivity to trimethylpenthane (TMP) and time-on-stream stability was found for the HPW/SiO 2 catalyst, with 40 wt.% of HPW. The lower activity of the mesoporous aluminosilicate supported HPW samples was attributed to the stronger interaction of HPW with the surface sites of this support, which in turn decreased both the number and the average acid strength of the Brønsted acid sites of the heteropolyacid. The all-silica MCM-41 support conversely should combine the good properties of silica with the high dispersions attainable owing to its high surface area. However, a partial blockage of the monodimensional channels of MCM-41 by small aggregates of HPW was observed, decreasing the accessibility of the acid sites to the reactants. The pore blockage was more significant as the amount of heteropolyacid in the catalyst increased, and according to the authors, can be minimized by increasing the mean pore diameter of the mesoporous molecular sieve from 40 to 66 Å. The authors also concluded that in order to increase the catalytic performance of HPA supported over mesostructured materials, the dispersion of active phase over the support should be controlled. Jalil et al. 215 have underlined the importance of the impregnation solvent to obtain better dispersions using heteropoly-compounds. To this end, Peden and co-workers proposed a very promising new way to obtain a high dispersion on mesostructured hosts. 216- 219 Metal cationic salts (CsCO 3 ) are first deposited on the mesoporous surface, forming a first precursor impregnated support. This material was calcined to raft the alkali metal over the support by creating a covalent bond. The heteropolyacid, dissolved in a solvent having a limited potential for dissolution of the first precursor, interacted with the primary deposited phase and formed the metal heteropolyacid salt (Cs x H 3 -xPW 12 O 40 ). This acidic phase is insoluble in polar solvent and consequently presented a high stability to leaching and limited cluster formation by mobile species. It was clearly demonstrated by TEM that the previous method led to the formation of clusters whereas a good HPA dispersion was obtained with the new strategy. 220 These materials were evaluated in acid catalyzed alkylation of 1,3,5 trimethylbenzene with cyclohexene, showing promising features.

HPW impregnated over silica supports were introduced as hydrotreatment catalysts by Shafi et al. 221 The catalysts were also prepared as described by Kozehvnikov et al. 212 and evaluated at dibenzothiophene (DBT) hydrodesulfurization, with a 30 wt.% loading in a trickle bed reactor at industrially relevant gas and liquid space velocities, and H 2 partial pressure of 30 bar. During activation procedure, the breakthrough of H 2 S at the reactor outlet was observed ( It was shown that SiO 2 -based HPW catalysts activity was comparable to that of conventional catalysts, despite some decrease in conversion at 350 °C (Figure I.34). 221 It was suggested that the active sulfide phase was formed by the reaction of HPW with H 2 and H 2 S from DMDS in the course of the presulfiding procedure, leading to the incorporation of sulfur into the polytungstophosphate and to the subsequent breakdown of the Keggin structure. H 3 PMo 12 O 40 had also previously been found to decompose on exposure to H 2 S/H 2 to yield an active catalyst for coal liquefaction. Lizama et al. 7 prepared HDS catalysts supported on SBA-15 mesoporous silica, from H 3 PMo 12 O 40 and H 3 PW 12 O 40 precursors, and compared them with NiMo(W)/SBA-15 catalysts prepared from traditional ammonium heptamolybdate (AHM) or metatungstate precursors impregnation, calcined for 2 h at 350 °C in air. Diffraction lines corresponding to reflections of MoO 3 in orthorhombic phase could be observed in the XRD patterns of the catalysts prepared from the conventional AHM precursor, which corresponds to a poor dispersion of the oxide Mo phase, originated from the calcination step. In contrast, no crystalline phases were detected when HPA were used as molybdenum source in the catalyst preparation. For the W series samples, the characteristic diffraction lines were detected only for the unpromoted catalyst prepared from HPW, attributed to some crystalline aggregates, located outside of the SBA-15 pores. After the addition of Ni to the HPW/SBA-15 catalyst, the diffraction peaks previously detected no longer appeared, showing that the promoter significantly improved the dispersion of the active phase. MoS 2 and WS 2 crystallites with 6.2 Å interplanar distances were observed on HRTEM micrographs of all sulfided catalysts, showing that HPA may form sulfide active phases with regular structures. The authors also observed that both HPMo-and HPW-based catalysts showed a better dispersion (smaller average stacking degree and slab length), which was attributed to a better dispersion effect of HPA precursors. The catalytic activity was evaluated in HDS of 4,6-DMDBT. All POMpromoted catalysts resulted to be significantly more active than their analogues prepared from conventional precursors. The authors attributed this catalytic behavior to the capability of phosphorus (present as heteroatom in the POM) to keep small units of Mo or W oxides together in close vicinity, as well as to keep a homogeneous distribution of the active phase.

More recently, Ramirez et al. 206 prepared NiMoP/SBA-15 catalysts from HPMo and nickel citrate as Mo and Ni precursors, respectively. The authors showed that using HPMo as Mo precursor led to a better dispersion comparing to conventionally prepared catalysts. This was attributed to the absence of NH 4 + cations to form agglomerates such as (NH 4 ) 4 [NiMo 6 ],

which often occurs with usual (NH 4 ) 6 Mo 7 O 24 and Ni(NO 3 ) 2 precursors, as also verified by Griboval et al. [97][98][99] In addition, the authors found that the conversion of 4,6-DMDBT dropped drastically with calcination at 500 °C prior to sulfidation of the catalysts, which the authors attributed to a destruction of the SBA-15 structure as observed in the adsorption-desorption The textural properties of the composite materials were characterized by N 2 sorption. HPW@SiO 2 maintained large specific surface area (600 m 2 g -1 ) relatively to the SiO 2 support (790 m 2 g -1 ), even after 30 wt.% HPW incorporation. Pore diameter was found to be practically constant and equal to 8.9 nm, with very sharp pore size distribution. The authors suggested that the loaded HPW possibly exists in the micropores of the SiO 2 framework.

The HPW@SiO 2 exhibited similar 31 P NMR spectrum as bulk HPW with a single resonance at 15.1 ppm (cf.Figure I.39) which corresponds to the retaining Keggin structures. Nevertheless, the authors observed a resonance at lower frequency (-14.2 ppm) for higher HPW loadings. This resonance was attributed to chemical interactions between HPW and the SiO 2 support, which have also been observed with impregnated samples. [190][191][192] Figure I.39: 31 P MAS NMR spectra of HPW cluster and HPW@SiO2 with different loadings (10-50 wt.%). 239

In 2009, Dufaud et al., 4 applying the one-pot synthesis method initially developed by Yun et al., 239 have used a binary system composed of ionic (CTAB (cetyltrimethylammonium bromide)) and non-ionic Pluronic® 123 (P123) structure directing agents. The presence of the cationic surfactant was supposed to modify the surface charge distribution at the surface of micelles, leading to stronger interactions between the template, TEOS and POM (HPW). 237 Part of the CTAB surfactant directly interacts with HPW, giving origin to the correspondent ammonium salt. Thus, the POM remains in the polar media, and consequently in the pore walls of the final hybrid. The resulting materials presented indeed stabilized intact Kegginunits using a CTAB/P123 ratio of 0.8, confirmed by a sharp peak around -15.3 ppm in [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P RMN. On the other hand, using higher ratios (CTAB/P123 = 1.6) or in the total absence of CTAB, the primary Keggin structure was partially or completely destroyed. The authors concluded that the CTAB/P123 ratio of 0.8 was near optimal. It is worth noting that the authors performed two different methods for template removal, methanol soxhlet extraction and calcination. Both methods led to quantitative removal of the structure directing agents, resulting in highly ordered materials, as confirmed by TGA, FTIR and XRD measurements.

The integrity of Keggin units was investigated by 31 P NMR spectroscopy. When extraction was applied to the as-synthesized hybrids, no resonance peak was observed between -200 and +200 ppm (cf. Elementary analysis showed a small decrease in the amount of tungsten, which was attributed to the loss of POM and/or partial filling of the mesostructured material by alcohol. XRD measurements showed long range order for all the hybrids, with no crystalline phases corresponding to the POM, suggesting that the POM structures were homogeneously dispersed throughout the silica matrix.

The authors proposed that during calcination the host micropores would contract, encaging the thermally degraded guest POM. The subsequent soxhlet extraction with methanol would lead to the restructuration of the POM entity entrapped in the host micropores, as well as to the removal of all POMs located at the mesopores surface ( Without this calcination/extraction sequence, the direct extraction of the assynthesized hybrids led to complete leaching of the active phase from the silica matrix, as already found by other authors. 4,9 Pursuing this method, POM clusters are preferentially located within silica matrix rather than at pores surface. 228,243 According to this synthetic route, depicted in Figure I.37 A, P123 and HCl solution are first mixed resulting in the protonation of the ethoxy moieties of P123 micelles, by the acidic conditions of the synthesis. The hydrophobic PO core remains water-free, encaged by the protonated shell, which in turn is compensated by Cl -ions. The silica precursor is then added to the system, surrounding the micelles in the outer layer of Cl -ions as silica oligomers and H 5 SiO 4 + ions after hydrolysis and protonation of TEOS, according to the mechanism:

PO m [(EO)×H 3 O + ] x ××× x Cl -×× × (H 5 SiO 4 ) + Silica condensation occurs until HPW addition. Dissolved in water or in organic solvents, POM clusters are then uniformly trapped in the hydrophilic layer, either through electrostatic interactions or hydrogen bonding. In the hydrothermal step, further polymerization of TEOS occurs, leading to well-structured hybrid SiO 2 materials with intact HPW units.

Introduction of POM prior to silica precursors hydrolysis

Another synthesis route was recently developed by Gagea et al. 8 This method involves addition of POM clusters to the acidified P123 solution prior to silica precursor addition, in order to attempt better dispersion of HPW. Thus, the ethoxy moieties of P123 will be first protonated by acid protons, and consequently compensated by Cl -ions. The addition of HPW would then induce a competition between PW 12 3-ions with Cl -ions, to compensate the protonated shell of EO moieties, according to the mechanism depicted in Figure I.37 B and described as follows:

PO m [(EO)×H 3 O + ] x ××× y Cl -; z PW 12 3-××× (H 5 SiO 4 ) + μ ν
observed in the previous incorporation strategies. The formation of HPW by the addition of P and W precursors did not disturb the formation of the mesoporous structure up to a content of 20.7 wt.% of HPW. Wide-angle XRD patterns also showed that the POM entities were well dispersed, as demonstrated by the absence of the HPW diffraction peaks. Primary Keggin structure was formed within the silica framework, as evidenced by FTIR, TEM and UV/Visible; however the synthesis efficiency was relatively low, as the authors intended a given loading of HPW and obtained a much lower loaded hybrid. For higher POM loadings, some species would not be incorporated into the walls of the mesoporous silica, but would be rather deposited in the mesoporous channels as small silica-HPW domains, as shown in The authors suggested that the hydrolysis of Si-O-W bonds in the immobilized SiW 11 Si 2 units led to the release of SiW 11 species in the reaction mixture, leaving the two SiOH groups previously inserted into the vacancy free and connected to the silica framework. Under the acidic synthesis conditions (pH ∼ 1), the removed SiW 11 would be likely converted to H 4 SiW 12 O 40 acid. Considering a balance between both aspects, the authors recommended 80°C as the optimal temperature for ageing.

• γ • • γ • • • • γ π γ • • • • •
To avoid such disadvantages, co-incorporation of POMs clusters together with oxide precursors in a one-pot synthesis, by sol-gel methodology, has been studied. The so called one-pot synthesis, developed by several authors, 4,8,9,148,228,[237][238][239]243 led to hybrid materials, i.e., metal clusters embedded in an inorganic framework, with improved pore sizes, with no pore blocking (since no molecules were post inserted in the inner surface), providing a better dispersion and stability of the active phase.

POM structures may be dispersed in the solution media at different stages of synthesis, according to its aimed location: at the surface of the pores and/or inside the silica walls. Introducing POM before or after a pre-hydrolysis of the silica precursor -TEOS -the interactions between all species may be described as:

Before POM addition: PO m [(EO)×H 3 O + ] x ××× x Cl -×× × (H 5 SiO 4 ) + After POM addition: Before TEOS PO m [(EO)×H 3 O + ] x ××× y Cl -; z PW 12 3-××× x(H 5 SiO 4 ) + After TEOS PO m [(EO)×H 3 O + ] x ××× y Cl -×× × x(H 5 SiO 4 ) + z ××× PW 12 3-
CTAB (cetyl-trimethyl-ammonium bromide) in addition to P123, as structure directing agents led to stronger interactions between the template, TEOS and POM. 237 Part of the CTAB surfactant directly interacts with the HPA, giving origin to the correspondant ammonium salt. 4 Calcined hybrids submitted to methanol soxhlet extraction led to Keggin HPA reconstruction. Without this calcination/extraction sequence, the direct extraction of the assynthesized hybrids led to complete leaching of the active phase from the silica matrix, as already found by other authors. 4

I.4.1 Main Objectives

The studies conducted by V. Dufaud et al. 4 led to an effective synthesis method to obtain highly dispersed metallic phases and a potential basis to prepare a novel hybrid catalyst. This strategy, developed during this PhD thesis, involves a one-pot synthetic procedure to prepare hybrid Mo@SBA-15 materials, further impregnated with active phase precursors -Ni or Co, according to a traditional incipient wetness impregnation. The novelty of this project is to use these catalysts for application in HDT processes. It is expected that having these elements incorporated inside the SBA-15 silica walls, a better dispersion of the active phase and a better promotion effect of cobalt or nickel, due to a better spatial proximity of Mo and Co/Ni atoms will be achieved, during activation of the catalysts by sulfidation under H 2 /H 2 S flux.

The main objective of this thesis is therefore to evaluate the potential of such approach to the creation of innovative and upgraded catalysts for the hydrotreatment of various feedstock such as diesel, gasoline or vegetable oils.

A second objective is to get more insight into the nature of the interactions between the active phase and the support, according to each preparation route. For this purpose, HDT catalysts were prepared according to different synthesis methods, different preparation routes, with various POMs, and were extensively characterized at each preparation step.

In this manuscript, the general context of the present study have been presented in Chapter I. Chapter II reveals the innovative approach to prepare non-promoted hydrotreating catalysts, with an introductory experimental part, followed by the characterization of the synthesized MoP-SBA-15 materials at each step of the preparation. The active phase -after catalysts activation -was also fully characterized and will be exploited in the end of this chapter. In Chapter III, promoted materials are introduced, based on the catalysts aforementioned in Chapter II, and are fully characterized in each step of the preparation method. Chapter IV addresses to the catalytic evaluation of non-promoted and promoted catalysts according to two model molecules: hydrogenation of toluene and hydrodesulfurization of thiophene. Finally, the most important conclusions and long-term perspectives are detailed in Chapter V. The experimental methodology used in catalysts characterization and catalytic evaluation is then described in Chapter VI, regarding the catalysts ex-situ activation, catalytic evaluation and characterization techniques.

II. Non-promoted POM-based catalysts II.1 Introduction

As aforementioned, new types of hybrid materials were prepared in order to understand the effect of encapsulation of the metal phase on the interactions between active phase and support, morphology of the active phase as well as their catalytic performances. In this chapter, the characterization of catalysts at each preparation step will be discussed: the textural and structural properties of the mesostructured support, as well as the molecular state (oxide or sulfide) of the active phase precursors.

II.2 Synthetic methodology

Non-promoted materials were prepared according to different synthesis strategies: two catalysts prepared according to an innovative one-pot method and a reference catalyst prepared through incipient wetness impregnation.

The catalysts addressed in this chapter are summarized and schematized in Table II.1.

II.2.1 Hybrid catalysts

The sol-gel encapsulation method, as already mentioned in Chapter I, involves the introduction of POM along with the silica precursor, during the synthesis of the oxide, as briefly described in Figure II.1. After the SDA dissolution in a HCl solution (1.9 M, 500 mL), 32 g of TEOS were added to the solution, and left to pre-hydrolyze for 45 minutes. At this stage, the SDA micelles are all surrounded by H 5 SiO 4 + that started to polymerize forming the silica matrix. Then, 6 g of 12phosphomolybdic acid, a H 3 PMo 12 O 40 , , were added dropwise to the milky solution, using a Pasteur pipette under 1200 rpm stirring. The vigorous stirring was very important to prevent POM from aggregating with SDA molecules until precipitation, which would result in the loss of homogeneity, loss of POM loading and consequently irreproducibility of the synthesis.

The solution was left under vigorous stirring for 20 hours at 40 °C. The colloidal light green solution due to partial reduction of Mo, was then inserted in an autoclave, and aged for 20 hours at 100 °C. The silica precursor further polymerized yielding a mesostructured SBA-15 type silica matrix, with incorporated POM. After the hydrothermal treatment, the resulting suspension was filtered, and the solid was washed twice with 125 mL of a 1.9 M HCl solution then with 250 mL of distilled water. Finally, the washed solid was left to dry overnight at 40 °C in the oven. Calcination was performed at 490 °C for 12 hours under air, with a temperature rate of 2.0 °C/min. These solids were denoted HPMo@SBA-15 C, the "@" representing an encapsulation at the SBA-15 walls. Some of the resulting hybrids were further extracted with methanol, using a soxhlet extraction technique, described in Appendix II, Several batch syntheses of these hybrid materials were prepared separately, in order to ensure the homogeneous character of the hybrid materials, which is not obviously obtained with a scale-up synthesis. Once synthesized, each hybrid materials batch was characterized by N 2 sorption and XRD measurements to ensure the uniformity of the combined lots, HPMo@SBA-15 CE or HPMo@SBA-15 C. The resulting material, issued from the assembly of 6 or 11 single batches, respectively, was ultimately characterized by N 2 sorption, XRD, IR and TEM.

The non-promoted catalysts and the supports are listed in Chapter V, section V.2 Glossary.

II.2.2 Reference catalysts

In order to be able to compare the specificity of our innovative approach, non-promoted reference materials were also prepared following a two-step process, involving the synthesis of the SBA-15 support prior to the introduction of the active phase precursor by incipient wetness impregnation method.

Preparation of SBA-15 type silica

The synthesis of the SBA-15 like support is based on the method developed by Zhao et al., 2 with a slight modification, consisting in employing a second SDA, as suggested by Dufaud et al. 1 The synthetic procedure is represented in As for the hybrid materials described above, triblock copolymer Pluronic 123® and CTAB, were used as organic SDA in the synthesis, in a 0.8 molar ratio, to maintain the support characteristics. Thus, 16 g of P123 and 0.8 g of CTAB were dissolved in 500 mL of a 1.9 M HCl solution, at 40 °C. When a clear solution was obtained, 32 g of the silica precursor, TEOS, were added dropwise to the solution. The suspension was left under stirring for 20 hours at 40 °C. The resulting suspension was inserted in an autoclave, and aged for 24 hours at 100 °C.

After the hydrothermal treatment, the silica suspension was filtered, and washed with 250 mL of distilled water. Finally, the washed solid was left to dry overnight at 40 °C in the oven. Calcination was performed at 490 °C for 12 hours under air flow, with a temperature rate of 2.0 °C/min. Several batches were prepared according to this protocol, since scale-up may lead to heterogeneously less organized solids. Each batch was characterized by N 2 sorption and XRD measurements, in order to verify the reproducibility of the synthesis and to assure the uniformity of the combined lot referred to as SBA-15 C. This latter resulted from the assembly of 19 single batches, and was ultimately characterized by N 2 sorption, XRD, IR and TEM.

POM dispersion by wetness impregnation

Firstly, one should determine the loss on ignition (LOI) of the SBA-15 like support. This analysis consists in heating a sample for 4 hours at high temperature. The most common temperatures are 550 and 1000 °C, depending on the sample. In our particular case, the LOI was performed at 550 °C. Under these conditions, all water contained in the sample vaporizes (as also some condensation of silanols may occur: 2 SiOH H 2 O + Si-O-Si). The weight loss allows for the quantification of the water content, and the determination of the dried weight of the support. The water pore volume of the supports is then measured experimentally, by impregnation of the solids with distilled water, thus determining the accessible pore volume. The amount of precursor to be introduced is then determined, according to the required contents. H 3 PMo 12 O 40 was the molybdenum precursor used to prepare the non-promoted reference catalysts. An example is shown in Appendix II, section II.7.2, pg 113.

The determined weight of metal precursors was then dissolved in distilled water, until a clear solution was obtained. The porous support was impregnated with the precursor solution, and left to maturate in a chamber saturated in water (95%), at 23 °C for 24 hours. This step allowed for a better penetration of the impregnation solution through the pores, resulting in a better dispersion of the metal phases. Water was eliminated after 24 hours at 120 °C in the oven. The impregnated non-promoted reference catalysts were named HPMo/SBA-15 D.

Elemental analysis determined the Si, Mo and P contents for each catalyst, and it is shown in Table II.2. Hybrid catalysts present Mo loadings in the range of 10-15 wt.% ± 0.3, while impregnated catalyst showed 8.3 wt.% Mo ± 0.3. All catalysts presented a Mo/P (at/at) ratio between 9-11 ± 1.8.

II.3 Catalyst characterization: oxide phases

The characterization of hybrid and reference catalysts will be advanced with a first exploitation of the textural and physical data concerning the materials structure -the silica matrix -since the encapsulation of POM inside the silica walls affects the framework. The metal content and dispersion was briefly analyzed based on XRD wide-angle diffractograms and XRF measurements, and more insights were obtained analyzing TEM images.

μ μ μ μ μ μ μ μ θ
shorter structures are able to move from the micropores that previously encaged them consequently leading to sintering of crystal forms in the pores and/or at the outer surface. On the other hand, the extracted hybrid HPMo@SBA-15 CE showed a better dispersion, with an almost negligible MoO 3 crystallization. Indeed, as already shown by Dufaud et al., 1 extraction leads to a partial leaching of incorporated POM from the pore surface, which may have eliminated most of the clusters formed during calcination. In order to get more insight into these materials structure and into the active phase precursors location and distribution, Transmission Electron Microscopy (TEM) was performed, at IFPEn Solaize.

As detailed in Chapter V, section V.4.10, TEM analysis allows the visualization of the different phases present in the catalyst, as well as the dispersion of the species over the catalyst surface. A high-contrast image is formed by blocking electrons deflected away from the optical axis of the microscope, to allow only unscattered electrons through (cf. Figure II.6), in this case one can see the silica walls in dark contrast and the pores in bright contrast. Nevertheless, TEM images may also be observed in a dark field contrast, where the contrast between heavier atoms becomes more important -using this technique, as in

II.3.2 Molecular and structural properties of introduced POMs

Polyoxometalates are relatively thermally stable, depending of their structure and chemical composition. However, when increasing temperature above 450 °C, the Keggin structure of these POMs is lost, giving origin to oxide species. After synthesis, the as-synthesized hybrids were calcined at 490°C for 12 hours, to evacuate the surfactants. Thus, the integrity of the incorporated POM was severely damaged, leading to the formation of amorphous and/or crystallized Mo and P species. In order to verify the POM state, the catalysts were analyzed using molecular techniques, such as 31 P NMR and Raman spectroscopy.

II.3.2.1 Characterization by 31 P NMR spectroscopy

Analyzing Figure II.9, one can notice that both HPMo@SBA-15 CE and HPMo/SBA-15 D 31 P NMR spectra presented a single sharp peak at -3.5 ppm, characteristic of the 12phosphomolybdic acid, suggesting that introduction of HPMo by encapsulation followed by a calcination-extraction sequence or by impregnation at room temperature, led to a structurally intact Keggin unit. This fact was also confirmed by XRF elementary analyses (Table II.2). When considering the calcined catalyst, HPMo@SBA-15 C, one can notice that a broad peak between -37 and -12 ppm appeared along with the peak at -3.5 ppm, indicating that the POM structure was partially lost during calcination. Nevertheless, as the hybrid solids remained in contact with air for an extended period, the water present in the atmosphere was sufficient to partially reformulate the destructed POM clusters.

These materials have Mo loadings varying from 7.2 up to 15.5 wt.%, which corresponded to a Mo/P molar ratios varying between 9 and 11 ± 1.8, as determined by FX elementary analysis (see Table II

II.4 Catalysts characterization: sulfide phase

The active phase of hydrotreating catalysts is characterized by layered sulfide slabs, which may be decorated with transition metals, such as Ni or Co atoms, on the slab edges (NiMoS or CoMoS slabs). The formation of this active phase, obtained upon sulfidation, depends not only on the activation conditions (temperature, pressure, hydrogen sulfide partial pressure, time, etc.) but also on the active phase precursors (ammonium heptamolybdate, MoO 3 , polyoxometalates, such as H 3 PMo 12 O 40 or H 3 PW 12 O 40 ), the nature of the support and the interactions between active phase precursors and the carrier.

In order to understand the different properties of the active phases formed either from "encapsulated" or impregnated based catalysts, several characterization techniques were used to determine their morphology and their composition. In this regards, the non-promoted catalysts were sulfided ex situ prior to analysis, at 350 °C for 2 hours under H 2 /H 2 S flow, using the sulfidation protocol, described in detail in Chapter V, section V.3.

II.4.1 Catalysts characterization by TEM

TEM is a widely used technique to characterize sulfided catalysts. 9 Nevertheless, TEM of conventionally supported catalysts only allows to distinguish the active phase slabs through which the electron flow crosses their length. In addition, TEM observations cannot provide the geometry of the slabs: only information on particle size and their stacking may be obtained. 10 These observations have been performed at IFPEn, Solaize.

Consequently, sulfided catalysts were also analyzed by High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) and Bright Field STEM (BF-STEM), in addition to traditional TEM, at the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), in the context of the Post-Doctoral research of Maria Girleanu. BF-and HAADF-STEM uses high angles diffracted electrons, instead of transmitted electrons in the incident beam axis. Thus, the sensitivity to the atomic number of the element that deflected the electron is enhanced, leading to a better contrast. 9 MoS 2 slabs placed normally (perpendicularly) over the support pores are thus visible.

The three non-promoted catalysts HPMo@SBA-15 CE, HPMo@SBA-15 C and HPMo/SBA-15 D where chosen to be analyzed. To simplify the reading of these results, the HPMo@SBA-15 C catalyst results are shown in Appendix II, pg. 115, since no significant differences were observed between both "encapsulated" catalysts, in terms of morphology or active phase distribution.

Firstly, regarding the HPMo@SBA-15 CE, one should recall that before sulfidation, this catalyst presented well dispersed molybdenum clusters throughout the host SBA-15, which were mainly located at the silica walls, as shown before in Otherwise stated, few MoS 2 slabs present in the mesopores did not entirely plug the pore, leaving a de-obstructed entrance for reactants to pass through, as evidenced and schematized in Figure II.14. This slab morphology could suggest some sulfur vacancy sites, arising from the atomic tension that could eventually be exerted.

Δμ Δμ

sections showed MoS 2 slabs clusters with an average length of 19 nm, varying from 9 up to 36 nm, (from the analysis of ca. 200 slabs, Appendix II, Figure II.36, pg. 115). Comparing qualitatively both hybrid-based catalysts, the HPMo@SBA-15 CE catalyst appears to present a higher amount of MoS 2 slabs.

Concerning the impregnated catalyst -HPMo/SBA-15 D, one has seen that there was a good dispersion of the molybdenum species over the catalyst surface, located mainly at the pores surface, as in

Why does sulfidation occur mainly at the edges of the mesostructured domains?

As aforementioned, sulfidation was not fully accomplished, as evidenced by XPS and discussed below. This limitation may be due to diffusional limitations of the H 2 S flow into the core of the mesostructure, leading to sulfidation of MoS 2 at the catalyst surface and periphery, simultaneously leading to sintering of sulfided species at the pores entrances.

Conclusive remarks

HPMo@SBA-15 CE hybrid catalyst presented molybdenum clusters mainly located at the silica walls, with relatively good dispersion. After sulfidation, it was shown a homogeneous dispersion of the active phase over the whole solid, since all analyzed micrographs presented the same active phase distribution: partially blocked channels, with MoS 2 slabs, and Mo clusters of ca. 1 nm at the silica walls. Some of the channels presented MoS 2 slabs clogging the surface, although leaving a de-obstructed entrance for reactants to pass through the channels. Small MoS 2 slabs were observed inside the walls and over the pores surface, suggesting an early MoS 2 slabs formation. The observation of flat MoS 2 slabs has evidenced a hexagonal morphology, which is in good agreement with what would be predictable from the sulfidation conditions, according to the models proposed by C. Arrouvel, 11 and P. Raybaud et al. 12,13 HPMo@SBA-15 C catalyst showed the same active phase distribution, although it appears to present a smaller amount of MoS 2 slabs, referring to HPMo@SBA-15 CE catalyst. However, one should note that TEM analyses show a limited sector of several chosen regions for each sample, which could not represent 100 % of the sample.

Regarding the impregnated non-sulfided catalyst, HPMo/SBA-15 D, a good dispersion of the Mo species, mainly at the pores surface was observed. Once sulfided, the metal species were mainly present at the edges of the mesostructured domains, and only few micrographs have shown highly dispersed MoS 2 slabs in the mesopores. A few mesopores were completely fulfilled with molybdenum sulfide, and cluster agglomeration was observed at the periphery of the silica grain. Nevertheless, observing more precisely the core of the mesostructured support, metallic clusters inside the mesopores and, unexpectedly, also inside the silica walls were detected. These metallic clusters were not clearly identified, as they may represent oxide or oxy-sulfide clusters or MoS 2 structures at an early formation phase.

II.4.2 Characterization by 31 P NMR spectroscopy

The ex-situ sulfided samples were analyzed by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] HPMo@SBA-15 CE and HPMo@SBA-15 C catalysts present a large band at ca. -23 ppm, followed by a second broad resonance centered at ca. -36 ppm, which can be attributed to the same oxide species which was already present, although in a lower extent, in the HPMo@SBA-15 C "oxide" catalyst, before activation. A small shoulder is also distinguished with the HPMo@SBA-15 CE catalyst at -11 ppm. These species are very difficult to identify, most probably being mixtures of species, issued from POM partial decomposition, giving origin to mixed oxides, lacunary POM, or oxy-sulfide species, including or in the neighboring of phosphorous. On the other hand, the HPMo/SBA-15 D 31 P NMR spectrum presented two evident resonances at -11 ppm and -22 ppm, and may be attributed to POM decomposed species. The peak at 0 ppm indicates the presence of phosphates. Nevertheless, it is worth noting that despite the common species appearing at -22 ppm for all three analyzed solids, the impregnated catalyst showed a different species distribution: the shoulder observed for hybrid catalysts is clearly observed at -11 ppm, whereas the peak at -36 ppm is not evidently detectable in the HPMo/SBA-15 D spectrum.

These results could point towards different decomposition kinetics, depending on the different preparation methods, most probably due to the different interactions between the metallic phase and the support, as schematized in After 2 hours of sulfidation (see Figure II.24), the peak at 2118 cm -1 disappeared, also confirming the intermediary nature of these sites, as previously observed with HPMo@SBA-15 CE. The bands attributed to the MoS 2 phase are clearly evidenced at 2108 cm -1 and 2065 cm -1 , and a small shoulder at 2098 cm -1 indicates the presence of Mo atoms of small MoS 2 structures in early formation, or corner Mo. 17 Finally, the only interaction of CO with Mo sites is evidenced at ca. 2110 cm -1 , attributed to Mo atoms at the M-edges of the MoS 2 slabs. 15,16,19,2015,16,19,2015,16,19,2014,15,18,1913,14,17,18 Figure II.26: Subtracted spectra in the CO interaction range (2300-1950 cm-1) of the HPMo@SBA-15 CE catalyst, at high CO coverage.

In conclusion, sulfided encapsulated hybrid catalysts showed a different pattern of IR-CO adsorption spectra than the impregnated HPMo/SBA-15 D catalyst. The interactions between POMs entrapped inside the silica walls or at the mesopores surface are not the same, as previously deduced from Raman spectroscopy, leading to different sulfidation kinetics, which is in good agreement with the 31 P NMR spectra of the sulfided catalysts.

Analyzing the IR spectra at low iso-CO coverage depicted in Figure II.27, one may conclude that the higher specific surface area of impregnated catalyst (∼ 700 m 2 g -1 ), regarding the encapsulated catalysts (∼ 350 m 2 g -1 ), led most probably to a very important interaction of CO with surface silanols, masking the MoS 2 slabs contribution. These latter showed similar IR-CO spectra patterns suggesting isotropic MoS 2 slabs, whether starting from intact HPMo Keggin units or from partially decomposed HPMo units, encapsulated inside the silica walls. The hybrid catalysts have shown the presence of different molybdenum sites, after two hours of sulfidation: M-edge, at 2110 cm -1 , S-edge at 2060 cm -1 , and a third molybdenum site, attributed to Mo of M-edges deficient in sulfur, at 2090 cm -1 . This last band was more important for the HPMo@SBA-15 CE catalyst (Figure II.27). The impregnated catalyst did not clearly show these bands, possibly masked by the silanol interactions response. Nevertheless, these results are in good agreement with the previous characterizations of the sulfided catalysts: [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR showed different phosphorus environments for hybrids and impregnated sulfided catalysts, which has been attributed to the different interactions between POM and silanol and/or siloxane functionalities, according to the POM location: in the walls or at the mesopores surface. TEM, STEM and BF/HAADF-STEM have clearly shown the presence of small MoS 2 slabs for hybrid catalysts at the pores and/or walls of the silica support, which was not observed for the impregnated HPMo/SBA-15 D catalyst. These small MoS 2 slabs may have a better propensity to generate deficit sites in sulfur, on one hand due to its early/incomplete formation state, and, on the other hand, possibly due to its curved morphology. These curved slabs, adopting the mesopores surface form, may generate some tension at atomic scale, allowing a sulfur atom to exit the slab, as in Figure II.14. Nevertheless, the nowadays technologies do not allow to get further insight into this interpretation. Conclusive remarks on [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and IR-CO Spectroscopies [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR results pointed out that impregnation or encapsulation preparation methods did not result in the same sulfidation kinetic. Taken together, [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR, Raman and IR-CO spectroscopies of the sulfided catalysts, one can stipulate that this difference is most probably due to the different interactions between active phase precursor clusters and the silica support, whether they are located at the mesopores surface or in the silica walls microporosity. In the latter, POMs are surrounded by silanol and/or siloxane functionalities, which can stabilize the POMs (as schematized in Figure II. 19). On the other hand, at the mesopores surface, the interactions are weaker, which may leave POMs much more susceptible to transform under sulfidation conditions. IR spectroscopy with CO as probe molecule has also shown the presence of intermediary species after 30 minutes of sulfidation for the HPMo@SBA-15 C catalyst, attributed to an oxysulfide state, which was also observed with the HPMo@SBA-15 CE catalyst during temperature rise, and which practically disappeared after two hours. The hybrid catalysts also showed the presence of Mo "corner" atoms, with intermediary features between M-edges and S-edges, which could be attributed to MoS 2 structures at early formation. 17 These sites were in accordance with what was observed by TEM measurements, in section II.4.1, where small clusters were observed for the "encapsulated" catalysts. Some sulfur vacancy sites could also be present in these catalysts, from the curved MoS 2 slabs observed in some TEM images (see Figure II.14), which may induce sulfur vacancy sites formation, probably due to atomic tension. In order to confirm this statement, it would be interesting to perform theoretical calculations, in order to determine the probabilities of this sulfur vacancies creation to occur.

II.4.4 Characterization by XPS

The sulfided non-promoted catalysts were also analyzed by X-ray Photoelectron Spectroscopy. The XPS analysis provides information on the chemical composition and relative concentrations of the different species present at the catalysts surface. Fully assignment of spectra decomposition and the different contributions have been provided in Appendix II, section II.8.5, pg. 126, and the exploitation data is summarized in Table II.6.

The results were obtained after two analyses for each catalyst. The XPS acquisitions were performed according to the protocol detailed in Chapter V, section V.4.8, and the resulting data were expressed in different terms: Molybdenum sulfur rate (MoSR) and Global sulfur rate (GSR), determined according to Equations 1 and 2.

! "

• 100

& '•() • 100 (1)
MoSR is the percentage of molybdenum that gave origin to MoS 2 structures, whereas GSR is determined based on the assumption that all molybdenum is transformed into MoS 2 , and gives us the percentage of sulfur that gave origin to MoS 2 structures. As one can see analyzing Table II.6, the sulfidation of hybrid materials seems to be more difficult to achieve, regarding the reference HPMo/SBA-15 D catalyst. The total amount of oxide and oxy-sulfide intermediary species is higher for these hybrid-based catalysts, as the amount of sulfide molybdenum is relatively low. Concerning the global sulfidation rate, also shown in Table II. 6, one can confirm that hybrid materials were not fully sulfided, in particular the HPMo@SBA-15 CE catalyst, which has shown a MoSR of 50 %, after 2 hours of sulfidation at 350 °C. These results were also confirmed by "CHNS" elemental analyses, which showed a lower sulfidation rate for both "encapsulated" catalysts.

These results arise most probably from the fact that impregnated catalysts have more accessible active phase precursors than "encapsulated" catalysts, and in weaker interactions with the silica support. In addition, before sulfidation, the impregnated catalyst already presented the active phase precursors closer to the external surface of the catalyst grain, which means that these species are more easily reached than the species inside the grain. In fact, as we have seen earlier with TEM analysis (section II.3.1, pg. 79, and II.4.1, pg. 85), the metal clusters in the core of the mesostructured domains are present mostly as oxide or oxy-sulfided, as there were no MoS 2 slabs observable in this region of the solid. Nevertheless, the species closer to the periphery of the mesostructured domains were able to sulfide, giving origin to fulfilled mesopores closer to the edges of the mesostructured domains or to large clusters deposed at the external surface.

On the other hand, from the same TEM images of the HPMo@SBA-15 CE catalyst, one could also deduce that molybdenum species could give origin to other MoS 2 structures, other than the MoS 2 slabs. Actually, TEM and HR-STEM images have shown that the walls of hybrid and even impregnated catalysts presented numerous molybdenum clusters, attributed to MoS 2 slabs in early formation, which was also detected via IR-CO analyses. However, with the nowadays technology one cannot clearly confirm the nature of these species.

Anyhow, HPMo@SBA-15 CE catalyst presented the lowest sulfidation rate, which is also in accordance with previous results: the POM is homogeneously dispersed over the mesostructured support, either in the silica walls or at mesopores surface, which most probably remains inaccessible to the H 2 S flow, or requires more severe sulfidation conditions to reach the same degree of sulfidation than for an impregnated catalyst. Actually, Lefebvre et al., 21 had already stated that regarding the HDT catalysts sulfidation, the active phase formation is much slower when POM are used as precursor, but the same sulfidation rate may be achieved regarding typical ammonium heptamolybdate (AHM) precursor.

In order to confirm this apparently slow sulfidation rate of hybrid materials, several sulfidation conditions were also tested, namely the duration and the temperature of the catalysts activation step. Thus, the HPMo@SBA-15 CE catalyst was also sulfided at 350 °C for 4 hours and at 600 °C for 2 hours, and finally, in order to compare these results to the IR-CO data presented in the last section, the HPMo@SBA-15 C catalyst was sulfided at 350 °C for only 30 minutes. These results are also listed in Table II. 6.

In fact, with longer sulfidation period or higher temperature, one may obtain higher sulfidation rates, which can be respectively up to 16% and 49% higher than conventional sulfidation conditions. At 600 °C, one can obtain a similar global sulfidation rate for hybrid catalysts to what was achieved at 350 °C, with a traditionally impregnated catalyst. These results are in good agreement with the previous statements, which indicate that encapsulation leads to stabilized species dispersed all over the support, which consequently leads to a more difficult sulfidation.

Regardless, these results should still be confronted to XAS analysis, in order to assure the consistency of this analysis.

II.4.5 Characterization by XANES/EXAFS

X-ray Absorption Spectroscopy (XAS) analysis provides information on the local structure of the metal atoms present in the sample: their nature, the number and distance of neighboring atoms absorbing X-rays. The precision in the number of neighboring atoms is relatively low, in the order of ± 0,5, although the distance of these neighbors in quite precise (± 0,5 Å). The X-ray Absorption Near-Edge Structure (XANES) spectra analysis further provides the electronic structure (oxidation degree) and 3D structure (octahedral, tetrahedral, etc…) of the atoms. Further information regarding the XANES/EXAFS techniques is detailed in Chapter V, section V.4.9.

XANES spectra analysis

The XANES spectrum of Mo provides qualitative insight into the environment of molybdenum. The position of the energy E 0 threshold and the signal form are sensitive to the elements state: metal, sulfide or oxide. To highlight the intermediate sulfide phase, a quantitative analysis of the EXAFS spectrum of the HPMo/SBA-15 D catalyst was performed, neglecting the oxide phase. For this purpose, the crystal structures of MoO x S y , 22 MoS 3 , 23 and MoS 2 , as well as the Artemis software were used, to determine the coordination numbers and distances of the different coordination spheres. Table II.8 summarizes the results issued from the adjustments of the distances (R), made with the FEFF application.

II.5 Conclusions

The one-pot synthesis used to prepared hybrid catalysts based on the encapsulation of POM clusters into a mesostructured silica framework led to well-structured materials, with a relatively high specific surface area and molybdenum loading, between 11-16 wt.%. Calcination at 490 °C of the hybrids led to the formation of MoO 3 phase and methanol extraction allowed for the reformulation of the POMs into intact Keggin structures, embedded into the microporosity of the silica framework walls. These hybrids presented a better dispersion of the metal phase mainly in the walls comparing with traditional incipient wetness impregnation, without significant structure loss, as schematized and reminded in Scheme 1, for the non-promoted catalyst before sulfidation.

Scheme 2: Sketch for the non-promoted oxide catalysts.

Hybrid catalysts showed stronger interactions with the silica host material with regards to impregnated catalysts, as evidenced by Raman and IR-CO spectroscopic analyses, which results in a slower sulfidation of molybdenum species, as deduced from XPS and EXAFS/XANES analyses. IR-CO spectroscopy pointed out intermediary Mo species, attributed to oxy-sulfided molybdenum atoms, which gave rise to a MoS 2 phase after two hours of sulfidation. Nevertheless, a band at 2099 cm -1 indicated a new active site, which can be attributed to corner Mo or to a Mo at a MoS 2 structure in early formation, with intermediary features between Mo at M-edges and Mo at S-edges, which was in good agreement with TEM observations. TEM and STEM microscopy results showed that sulfided hybrid HPMo@SBA-15 CE catalyst presented molybdenum clusters mainly located at the silica walls, with relatively good dispersion, which led to a homogeneous distribution of the molybdenum sulfide phase over the whole solid: blocked channels -yet not completely fulfilled, with MoS 2 slabs, and Mo clusters of ca. 1 nm at the silica walls were observed, as also some clogged channels with de-obstructed entrance for reactants to pass through the channels. Small MoS 2 slabs were also observed inside the walls and over the pores surface, suggesting an early MoS 2 slabs formation. Flat MoS 2 slabs observed have evidenced a hexagonal morphology, which is in good agreement with what would be predictable from the sulfidation conditions, according to the models proposed by C. Arrouvel, 11 and P. Raybaud et al. 12,13 The impregnated non-sulfided catalyst, HPMo/SBA-15 D, presented good dispersion of the Mo species, mainly at the pores surface. Once sulfided, the metal species were mainly present at the edges of the mesostructured domains, and only few micrographs have shown highly dispersed MoS 2 slabs in the mesopores. Few mesopores were completely fulfilled with molybdenum sulfide, and cluster agglomeration was observed at the periphery of the silica grain. Observing the core of the mesostructured support with HAAF-STEM showed metallic clusters inside the mesopores and, unexpectedly, also inside the silica. These metallic clusters are not clearly identified, as they may represent oxide or oxy-sulfide clusters or MoS 2 structures at an early formation phase. Scheme 2 represents a sketch of the non-promoted catalyst after activation at 350 °C for 2 hours under a 15 v/v% H 2 S/H 2 flow, deduced from the results aforementioned discussed.

II.6 Summary of catalysts characterization

In order to assemble all catalysts characterization, purpose and main results/observations are summarized in the Tables below, before and after sulfidation.

XRD

Small-angle (S-A) patterns allow for the identification of the mesostructure and order of the framework, while wide-angle (W-A) patterns give us the presence or absence of crystallites in material.

S-A: p6mm hexagonal symmetry with a high degree of hexagonal mesoscopic organization for all solids. W-A: MoO 3 crystallites observed for the HPMo@SBA-15 C hybrid (and for HPMo@SBA-15 CE in a less extent). No crystallites observed for the HPMo/SBA-15 D catalyst.

TEM

TEM analysis allows for the visualization of the different phases present in the catalyst, as well as the dispersion of the species over the catalyst surface.

Both encapsulated HPMo@SBA-15 CE and HPMo@SBA-15 C catalysts showed small Mo clusters highly dispersed throughout the solid, mainly in the host silica walls, and also at the mesopores surface. The presence of these clusters into the framework led to the distortion of the walls of the host material, in the case of the HPMo@SBA-15 CE;

P NMR

Molecular integrity of the POM precursor

Both HPMo@SBA-15 CE and HPMo/SBA-15 D catalysts presented a single sharp peak at -3.5 ppm, characteristic of intact 12-phosphomolybdic acid The calcined catalyst, HPMo@SBA-15 C, showed a broad peak between -37 and -12 ppm appearing along with the peak at -3.5 ppm, indicating that the POM structure was partially lost during calcination. HPMo@SBA-15 CE Homogeneous dispersion of the active phase for hybrid catalysts: all pore channels contained Mo, some of them blocked, although not completely fulfilled. No sintering was observed at the catalysts outer surface. Mo clusters of 1 nm are present at the silica walls and at the pores surface, as also small MoS 2 slabs, suggesting early slabs formation. The MoS 2 slabs blocking the mesopores adopt the form of the surface of the support in the basal plane, which did not entirely plug the pore, leaving a de-obstructed entrance -this morphology could led to sulfur vacancy sites, arising from the atomic tension that could eventually be exerted. MoS 2 slabs presented an anisotropic hexagonal morphology, with edges that seem to have a different number of Mo atoms HPMo/SBA-15 D Metal species are present mainly at the edges of the mesostructured domains, whereas no metal was detected in the core of the mesostructured domain with TEM analysis, at a first glance.

Raman

A few mesopores were completely fulfilled with molybdenum sulfide cluster agglomeration, mainly at the periphery of the silica grain. HRTEM allowed for the observation of metallic clusters in the silica framework (undetectable with TEM), attributed to oxide or oxy-sulfide clusters, or MoS 2 structures at an early formation point.

P NMR

Phosphorous environment in the sulfided catalysts Different 31 P environments suggested different decomposition kinetics when using encapsulation or impregnation, most probably due to the different interactions between the metallic phase and the support: Encapsulation > POMs are surrounded by silanol and siloxane functionalities that may deeply stabilize the POM, via hydrogen bonds or electrostatic with the negative charge of the POM Impregnation > the interactions between POMs and its environment are less strong, and thus POMs are much more susceptible to decompose under sulfidation conditions.

IR-CO

Qualitative analysis of the different active sites

Intermediary species after 30 minutes of sulfidation were observed for encapsulated catalysts, attributed to an oxy-sulfide state, which practically disappeared after two hours. Hybrid catalysts also showed the presence of Mo "corner" atoms, with intermediary features between M-edges and S-edges, which could be attributed to MoS 2 structures at early formation.

The MoS 2 slabs contribution of the impregnated catalyst was covered by the band assigned to the interactions of CO with surface silanols, most probably due to the higher specific surface area of impregnated catalyst (∼ 700 m 2 g -1

).

XPS

Chemical composition and relative concentrations of the different species present on the catalysts surface

Sulfidation of hybrid materials is more difficult to achieve: the total amount of oxide and oxysulfide intermediary species is higher for hybrid-based catalysts, as hybrid materials were not fully sulfided. Longer sulfidation period or higher temperature led up to 16% and 49% higher sulfidation rates than conventional sulfidation conditions. At 600 °C, one can obtain a similar global sulfidation rate for hybrid catalysts to what was achieved at 350 °C, with a traditionally impregnated catalyst.

CHNS

II.8.2 Reference catalysts preparation

The preparation of the non-promoted reference catalysts comprises the impregnation of the active phase POM precursor over the previously prepared SBA-15 like support. An example of the promoter weight determination is shown below.

II.8.4.2 Bulk Oxide H3PMo12O40

The spectrum of the bulk H 3 PMo 12 O 40 , presented in Figure II.40, showed one specie in interaction with CO, at 2163 cm -1 , after CO desorption, under primary and secondary vacuum. Since CO is a reductive gas with regards to HPMo, the latter is probably partially reduced, giving origin to this resonance.

Resume Oxide > Sulfide transition -Low CO coverage

In order to get more insight into the transition from the oxide into the sulfide state of this catalyst, three spectra were superposed, corresponding to the "oxide", "30 minutes sulfided" and "2 hours" sulfided HPMo@SBA-15 C catalyst and it is shown in Figure II.47.

Here we can see that actually, the CO-HPMo interaction bands of the oxide catalyst (2173-2208 cm -1 ) are no longer present after 30 minutes under H 2 S/H 2 flow at 350 °C. Instead, a broad band 2118 cm -1 is preponderant, which has been attributed to intermediary oxy-sulfide species. After 2 hours of sulfidation, the catalyst spectra showed different molybdenum sites in interaction with the silica support: 2108 cm -1 and 2068 cm -1 , which can be attributed to Mo on the M-edges and S-edges of the MoS 2 slabs, respectively. One can thus conclude that the intermediary species were considerably converted into the MoS 2 active phase, as this interaction vibration resonance is no longer detectable after 2 hours of sulfidation.

II.8.4.5 HPMo/SBA-15 D catalyst -sulfided

CO interactions region -high CO coverage

Figure II.48 shows the high CO coverage spectra in the CO region of the sulfided HPMo/SBA-15 D catalyst. One can observe two bands, at 2157 cm -1 and 2137 cm -1 , assigned to the interactions of CO with silanol groups, and physically adsorbed CO, respectively. In addition, a smaller band at ca. 2110 cm -1 is detectable, assigned to Mo atoms at the M-edges of the MoS 2 slabs.

II.8.5 XPS appendix

Quantitative analysis

XPS analysis was performed over the non-promoted selected catalysts: HPMo@SBA-15 CE, HPMo@SBA-15 C and HPMo/SBA-15 D. In Table II.9 one can see the quantitative analyses of these catalysts, presenting a relative error of ± 20%, considering the average between the two analyses performed for each sample.

Spectra decomposition

The obtained spectra were decomposed in order to integrate the relative quantities of each species present in the catalyst surface. This data analysis procedure is described in detail in Chapter V, section V.4.8, and is exemplified more in detail for the HPMo@SBA-15 CE catalyst. ).

II.8.6 EXAFS analysis appendix

Chapter III Promoted POM-based catalysts

III. Promoted POM-based catalysts III.1 Introduction

In the previous chapter, we have seen that innovative hybrid materials based on the encapsulation of polyoxometalates into mesostructured silica matrix could be achieved. In this chapter, one will discuss the properties of promoted hybrid catalysts, prepared by incipient wetness impregnation of the promoters precursors (Ni or Co nitrates) over the hybrid HPMo@SBA-15 materials.

In order to assess the molecular state of the introduced species and the effect of Ni or Co being close to Mo in the silica walls, these solids were also compared to reference catalysts, obtained from the impregnation of different Mo, Co and Ni precursors over the SBA-15 like support.

The main aim of this chapter is thus to get more insight into this catalysts properties, which was achieved by different techniques performed over hybrid and reference catalysts at the oxide state, and, for certain catalysts, at sulfide state.

III.2 Synthesis methodology

Promoted materials were prepared according to two different synthesis strategies: (i) the impregnation of Co or Ni promoters over hybrid catalysts, containing already the molybdenum phase embedded into the silica framework (hybrid catalysts) and (ii) the conventional impregnation technique, where Mo and promoters (Co or Ni) precursors are simultaneously introduced by wetness impregnation on the support in the appropriate molar ratio (reference catalysts).

The catalysts addressed in this chapter are summarized and schematized in Table III

III.2.1 Promoted hybrid catalysts

In order to introduce the promoter of the active phase, the above mentioned encapsulated hybrids HPMo@SBA-15 C and HPMo@SBA-15 CE (Chapter II) were further impregnated according to an incipient wetness impregnation method.

The appropriate weight of Ni(NO 3 ) 2 •6H 2 O or Co(NO 3 ) 2 •6H 2 O precursors was dissolved in distilled water, until a clear solution was obtained. The hybrids were impregnated with the precursor solution, and left to maturate in a 95% water environment, at 23 °C for 24 hours. The excess of water was then eliminated by a 24 hours drying step at 120 °C in the oven. Part of the catalysts was calcined under air at 450 °C for 2 hours, with a ramp rate of 5 °C.min -1 .

The final catalysts prepared according to this two-step synthesis -primary hybrid synthesis followed by promoter impregnation, were named according to the promoter nature and loading, as well as the final treatment, as detailed in Table III.1 and in Chapter V, section V.3 Glossary.

III.2.2.2 NiMoP or CoMoP POM impregnated catalysts

Modified polyoxometalates synthesis

To favor the proximity between the active metal (Mo) and its promoter (Pr = Co or Ni), various POMs containing both components within the same molecule with the promoter, either as counter-ion and/or included in the POM structure were synthesized, namely Co NiO 40 H. The synthesis of these species is derived from 12-phosphomolybdic acid. According to the concentration of promoter precursors and to the pH of the medium, one may achieve different degrees of substitution. Equation 1 describes the substitution of the proton of 12-phosphomolybdic acid by cobalt or nickel ions, at pH ~ 1. In aqueous medium, the barium hydroxide gives origin to barium sulfate after exchange with the promoter sulfate, and precipitates, the promoter cation standing as POM counter-ion. In weak acidic medium, pH ~ 2-3, Keggin structures are not stable and have tendency to create vacancies via the removal of one or more MoO 3 units. These vacancies can be filled up by other metals yielding mixed POM, as described by equation 2.

H PMo O • H O , , BaSO ↓ % Pr , PMo O Pr/Mo = 0.125 (1)
H PMo O • H O , BaSO ↓ % Pr PMo PrO H Pr/Mo = 0.364 (2)
Thus, 40 g of H 3 PMo 12 O 40 were dissolved in 150 mL of distilled water, at room temperature. Ba(OH) 2 was added to the clear yellow POM solution, and the suspension was stirred for 30 minutes, until dissolution was accomplished. Then, NiSO 4 or CoSO 4 were introduced to the solution under stirring, and the reaction mixture stood for 2 hours, until BaSO 4 precipitation. The suspension was filtered twice, in order to completely eliminate the sulfate. The limpid resulting solution was concentrated in a rotary evaporator. The brown (for cobalt) or green (for nickel) dark solution gave rise to POM crystals after a few days at room temperature. Synthesis details are presented in Table III CoO 40 H POM was evaluated and monitored by Raman spectroscopy in liquid state (cf. Section III.6.1.1). These species seems to be able to keep their structure between 3.0 < pH < 5.3. Outside this range, lacunary species and/or the acid form may be generated. However, having the different structure components in the same solution, namely Ni or Co ions (from the original NiMoP or CoMoP polyoxometalate) and the H 3 PMo 12 O 40 acid, these structures seem able to reformulate into their original form, by simply varying the pH, without significant loss of the Raman intensity.

NiMoP and CoMoP based reference catalysts preparation

Pr 1,5 PMo 12 O 40 or Pr 3 PMo 11 PrO 40 H, with Pr = Co or Ni were then impregnated by the classical incipient wetness impregnation method over the SBA-15 like support, and optionally calcined, as described above for the other promoted catalysts. The catalysts were further referred according to the nomenclature that follows, regarding the promoter nature, loading and the final treatment, as described in Table III.1 and in Chapter V, section V.3 Glossary.

Nomenclature

III.3 Catalysts characterization: oxide phases

The prepared catalysts were extensively characterized in order to investigate the nature of the active phase for each material. Textural properties of the promoted catalysts were analyzed by nitrogen sorptions, XRD and TEM measurements. Complementary XRF, IR, [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and TEM analyses were also performed, in an attempt to identify the species present in the catalysts after impregnation.

III.3.1 Reference impregnated catalysts III.3.1.1 Co-impregnation of HPMo and Pr(NO3)2 over SBA-15 like support

The reference catalysts, prepared by co-impregnation of 12-phosphomolybdic acid together with Ni(NO 3 ) 2 or Co(NO 3 ) 2 over SBA-15 like support, were firstly analyzed by XRD. Figure III.1 presents the XRD patterns of the dried catalysts.

X-Ray diffractograms of all prepared materials show that these catalysts preserved the solids hexagonal symmetry after impregnation, as one can distinguish three well-resolved peaks indexed as (100), ( 110) and (200) reflections, in the small-angle XRD patterns. 1 Analyzing Table III. 3, one can see that the cell parameter slightly decreases after impregnation (determined from d 100 of the (100) reflection), as well as the wall thickness, although the pore diameters remained very similar. These phenomena could arise from the incertitude of the XRD data exploitation (d 100 determination), which should be taken in consideration.

Analyzing the wide angles range in Figure III.1, one can also state that the impregnated POM and Pr(NO 3 ) 2 (Pr = Co or Ni) species were highly dispersed over the support, since no crystallites were observable in the amorphous pattern of the support. The co-impregnated dried reference catalysts presented type IV isotherms, with a hysteresis loop varying from P/P 0 = 0.6 to 0.75, typical of mesoporous materials, as also shown for the SBA-15 like support (Figure III.2, left). Nonetheless, it is worth noting that these catalysts showed narrower pore distributions, centered at ca. 5.5 nm, with respect to the SBA-15 like support, which could suggest that the active phase precursors are most likely found at the largest pores surface, of the parent silica ( From Table III.3, one can also note that the specific surface area was slightly decreased (from 918 to 683 m²g -1 ) as the porous volume (from 1.2 to 0.78 mLg -1 ), since we have more weight and less surface in the catalysts, as the active phase precursors were deposed over the pores surface.

Contrarily, the microporous surface was nearly maintained and the microporous volume increased with the promoters introduction, from 0.029 up to 0.080 mLg -1 , which could explain the low variation in the BET specific surface. In order to get more insight into the molecular nature of these species, these catalysts were also analyzed by Raman spectroscopy. The Raman spectra of Co-promoted catalysts are shown in Nevertheless, with respect to the 5%Co/HPMo/SBA-15 catalysts (C and D), one can state that there were no traces of the 12-phosphomolybdic acid. Looking at the Co 3 PMo 11 CoO 40 H structure spectrum, one may guess a similarity with the co-impregnated catalysts spectra. However, the main peak of the Co 3 PMo 11 CoO 40 H at 979 cm -1 was not clearly distinguished.

In order to investigate this observation, the pH of the impregnation solution was measured (Ni(NO 3 ) 2 or Co(NO 3 ) 2 together with H 3 PMo 12 O 40 ), and was found to be in the range of 1.1 up to 1.3, as shown in Appendix III, Table III Finally, the elementary content of these catalysts was quantified by XRF, and is listed in Appendix III, pg. 180. The Mo content varies between 6.3 and 7.8 wt.% and the Ni or Co content between 0.7 and 1.8 wt.%, which corresponds to the introduced quantity of Mo and Co or Ni by impregnation. Ni/Mo atomic ratios vary between 0.43 and 0.48 ± 0.2 for high promoter loadings and between 0.16 and 0.17 ± 0.2 for low promoter loadings, which is slightly different than the targeted values (0.36 and 0.12, respectively). The Mo/P atomic ratios vary between 9 and 10 ± 1.1, which is slightly below the targeted Mo/P = 12.

Conclusive remarks

Co-impregnation of 12-phosphomolybdic acid together with Ni(NO 3 ) 2 or Co(NO 3 ) 2 led to wellstructured materials, with non-crystallized CoMoP or NiMoP species over the relative high surface area of the support, as verified by XRD measurements and N 2 sorption. These catalysts presented a relative narrow pore size distribution centered at 5.5 nm, and an average wall thickness of 5 nm, as determined from XRD and N 2 isotherms data. [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and Raman investigations led us to believe that Ni(NO 3 ) 2 or Co(NO 3 ) 2 promoter precursors together with 12-phosphomolybdic acid led to the formation of CoMoP and NiMoP POMs of different structures (lacunary, Keggin), according to the promoter loading. The lacunary species were possibly formed due to local pH changes by NO . ions, in particular when higher promotion loadings were considered (cf. Scheme 1). However, one could not conclude whether the promoter ion was as counter-ion of the anions, inside the structure or merely in the neighborhood.

III.3.1.2 Impregnation of NiMoP and CoMoP polyoxometalates over SBA-15 like support

The reference catalysts prepared by impregnation of modified polyoxometalates containing both Mo and Ni or Co atoms were analyzed by XRD, in order to get more insight into the structure, order and active phase dispersion. The low and wide-angles XRD patterns of these catalysts are represented in Figure III.5 Analyzing the XRD patterns of these catalysts, one can see that as for the previous solids, the mesostructuration and hexagonal symmetry were preserved after impregnation. The (100) reflections in the small-angle XRD patterns are well distinguished for all dried catalysts, as well as the (110) and (200) reflections, suggesting that the long range order was also maintained. In Table III. 4, one can see that the contraction of the silica framework also occurred, as observed with the co-impregnated catalysts. Concerning the BET data, all these catalysts presented type IV isotherms, with a hysteresis loop between P/P 0 = 0.6 up to 0.75, typical of mesoporous materials (cf. Figure III.6). As also observed with the previous catalysts, the impregnated POM-based dried catalysts showed relatively narrow pore distributions, which suggest that POMs are most likely placed at the largest pores surface. The pore diameters varied between 5.3 and 5.5 nm, showing a slight decrease, with regards to the SBA-15 support. From Table III.4, one can notice that the NiMoP and CoMoP based catalysts showed a slight decrease in the specific surface area from 918 down to 580 m 2 g -1 , as the pore volume, which decreased from 1.2 down to 0.81 mL.g -1 . On the other hand, the microporous surface and volume were nearly maintained. These catalysts were finally analyzed by X-Ray Fluorescence. The results are listed in Appendix III, pg. 183. In brief, Mo content varied between 6.0 and 8.1 wt.% and the Ni or Co content between 0.7 and 2.2 wt.%. Ni/Mo atomic ratios were in the range of 0.16 up to 0.18 ± 0.2 for lower promotion and 0.43 ± 0.2 for high promoter loadings, whereas Mo/P atomic ratios ranged between 8 and 10 ± 1.8, which was nearly expected from the precursors stoichiometry. The porous volume decreased with Ni or Co impregnation, unlike the microporous surface and the microporous volume that slightly increased. This fact, together with the constant pore diameter, may suggest that it might have been pore blockage to some extent, which has been confirmed by TEM images (shown and discussed below, in Nevertheless, with promoters impregnation, the encapsulated POMs were probably able to migrate from the silica framework into the mesopores, possibly due to a partial decomposition into smaller clusters (either already present in the parent HPMo@SBA-15 CE catalysts, or decomposed upon impregnation). This decomposition may be achieved due to the pH range of the impregnation solution (shown below in Table III The integrity of POMs was investigated by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR analysis, and the MAS NMR spectra are depicted in Figure III.12. In fact, the Keggin structure of the POM after Ni or Co impregnation of hybrid materials was partially decomposed, as all impregnated hybrid catalysts showed broader NMR resonances between -3.5 and -40 ppm, with respect to the non-promoted HPMo@SBA-15 CE catalyst. The Co-promoted catalysts did not present the peak at -3.5 ppm corresponding to the H 3 PMo 12 O 40 acid, which could be also due to the paramagnetic character of the promoter. On the other hand, Ni-promoted catalysts presented the Keggin structure characteristic peak at -3.5 ppm, although broader with respect to parent catalyst, which could arise from the paramagnetic effect of nickel. A close-up of the mesopores, present in micrograph C, showed that the metallic clusters were mainly located in the silica walls (bright spots), measuring less than 1 nm. EDX analysis revealed that these particles were mainly composed of molybdenum (no Ni was detected), meaning that POM were actually inside the silica walls, and that Ni atoms did not achieve all encapsulated POM.

Notwithstanding, few images, shown in Figure III.15, micrographs D and E, evidenced some pores completely fulfilled with molybdenum particles, as stated by EDX analysis, which is in good agreement with N 2 Isotherm data. XRF elemental quantification was performed, and is listed in Table III.15, Appendix III pg. 184. Generally, these catalysts presented a Mo loading in the range of 10 up to 11 wt.%, and a Ni or Co content of 1.1 up to 3.0 wt.%. Ni/Mo atomic ratio varied between 0.45-0.48 ± 0.2 for higher promotion rate and 0.16-0.18 ± 0.2 for lower promotion rate, which is slightly above the targeted ratios (0.12 and 0.36, respectively). Mo/P atomic ratio was 10 ± 1.1 for all promoted HPMo@SBA-15 CE-based hybrid catalysts.

D E A B C μ μ μ μ μ μ μ μ
All solids presented type IV isotherms, with a hysteresis loop (P/P 0 = 0.7 up to 0.85) typical of mesosporous materials, and narrow pore size distributions, as obtained for the non-promoted parent catalyst HPMo@SBA-15 C (cf. Figure III.17). The specific surface area of these hybrid promoted catalysts was slightly decreased, together with the total porous volume. Microporous surface and volume were also decreased, contrarily to HPMo@SBA-15 CE-based catalysts, suggesting that Ni/Co are located inside the micro-and mesopores. The decrease in total porous volume with Ni or Co impregnation, together with the constant pore diameter suggests that it might have been pore blockage, by the same mechanism proposed in Figure III.11 for the previous catalysts. With promoters impregnation, the POMs still encapsulated in the pore walls would be partially decomposed and thus able to migrate from the walls into the mesopores, agglomerating at the pores surface hence decreasing the total porous volume.

The pH of the impregnation solutions used to prepare these catalysts was checked, and are listed in Table III.8. Analyzing Table III.8, one can see that the pH of the impregnation solutions were in the range of 4.7 to 5.6. This means that the Keggin structures remaining in the silica walls after calcination were most likely partially decomposed after impregnation, since the HPMo stability In general, the catalysts exhibit a combination of species in almost every case, as also previously observed with HPMo@SBA-15 CE based catalysts. Observing the 5%Co/HPMo@SBA-15 C-D catalyst, one can also see a very broad peak centered at ca. 932 cm -1 , as for the 5%Co/HPMo@SBA-15 CE catalyst, whereas 5%Ni/HPMo@SBA-15 C-D catalyst presents the same mixture of species observed with the 5%Ni/HPMo@SBA-15 CE catalyst, presenting a major peak centered at ca. 969 cm -1 , and three small shoulders at 875 cm -1 , 905 cm -1 and 935 cm -1 .

Table III.8: pH of the impregnation solution for the

These results suggest that the species present at the catalysts surface after impregnation of Pr(NO 3 ) 2 precursors (Pr = Ni ou Co) over a HPMo@SBA-15 CE or HPMo@SBA-15 C catalyst are very similar, which was also supported by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR results. In other words, the impregnation solutions, presenting a pH range between 4.7 and 5.2 for the HPMo@SBA-15 C-based catalysts, could lead to the formation of lacunary species from partially decomposed 12-phosphomolybdic acid, previously encapsulated in the silica framework, as observed for HPMo@SBA-15 CE-based catalysts.

showed MoO 3 crystallites agglomeration over the catalysts surface. These refractary species suggest the degradation of the Keggin structure into some extent, that could arise from different phenomena: (i) POM decomposition during the synthesis calcination that did not reformulate by methanol extraction, and that could have been leached and agglomerate with the impregnation solution (ii) partial decomposition upon impregnation due to high pH that could possibly create lacunary species, which, associated with hot points during the drying step, could have generated the MoO 3 species. These results were further confirmed by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and Raman spectroscopies.

In addition, STEM dark field analysis, performed on the 2%Ni/HPMo@SBA-15 CE catalyst, showed that there were metallic clusters mainly in the silica walls, measuring less than 1 nm, which were found to be mainly composed by molybdenum (no Ni was detected). This result pointed out that POMs were actually present inside the silica walls, whereas Ni did not achieve all the encapsulated Mo clusters, for this particular catalyst (cf. Scheme 2). Few micrographs also evidenced some pore blockage by molybdenum particles, which is consistent with the porous volume decrease observed by N 2 sorption analysis, and the MoO 3 crystallites agglomeration observed by XRD.

Promoted HPMo@SBA-15 CE and HPMo@SBA-15 C based hybrid catalysts can be schematized as in Scheme 2.

III.3.3 Catalyst characterization: sulfide phase

As we have seen before, the active phase of hydrotreating (HDT) catalysts is characterized by layered sulfided slabs, decorated with transition metals, such as Ni or Co atoms, on the edge of the layers (NiMoS or CoMoS phase). In order to understand the different properties of the active phase formed from promoted "encapsulated" or impregnated catalysts, TEM and XPS spectroscopy were performed to determine the morphology, the composition, the nature of species and the interactions with the support, after sulfidation.

Catalysts were sulfided ex situ for analysis purposes. Briefly, 0.5 g of catalyst was inserted in a glass cell, heated up to 350 °C under 15 v/v% H 2 S/H 2 flow, and stood for two hours. More details of sulfidation conditions can be found in Chapter V, section V.2.4.

III.3.3.1 Characterization by XPS

To get more insight into the sulfide phase characterization, some of the promoted catalysts were further analyzed by X-ray Photoelectron Spectroscopy technique (details of the technique are present in Chapter V, section V. 4.8). The XPS analysis provides information on the chemical composition and relative concentrations of the various species present on the different catalysts surface, allowing for the estimation of the sulfidation and promotion rates, according to the different preparation methods. The results of the Mo, Co or Ni and S spectra decomposition are listed in Table III.9, and the corresponding spectra are presented in Appendix III, section III.6.3, pg. 187.

First of all, one should note that promoted reference catalysts presented better molybdenum sulfidation rates (in the range of 66 -72 %) than hybrid-based catalysts (between 36 -39 %), as specified in Table III.9. These different sulfidation rates arise most probably from the sintered MoO 3 phase detected for hybrid catalysts by Raman and XRD analyses, which was also confirmed by TEM images (shown later in this section). This fact is also in accordance with the percentage of Mo 6+ present in each catalyst, determined from the XPS spectra: impregnated catalysts presented between 8 -14 % of oxide molybdenum, whereas hybrid catalysts presented 36 up to 41 %. One should still mention that the promotion ratios of the active phase slabs ((Pr/Mo) slabs ), derived from XP spectroscopy, were in good agreement with TEM EDX analysis, except for the Ni 3 Mo 11 Ni-based catalyst, which has shown a lower promotion rate in TEM.

In conclusion, the Pr/Mo ratio determined from XPS analyses were also in agreement with the XRF data, for impregnated catalysts. However, hybrid-base materials showed a lower promotion ratio in XPS than what would be expected from the XRF results (at higher loadings). This fact supports the hypothesis that Ni or Co atoms did not promote MoS 2 slabs either because of the small amount of these structures (caused by the higher amount of MoO 3 refractary species than found in impregnated catalysts), or due to accessibility issues towards Mo sites, since the promoters were effectively present at the catalyst surface, although the amount of Ni or Co that gave origin to promoted slabs was considerably low (26 -34 %).

In addition, there were also some regions with large clusters, mostly at the edges of the material (micrograph D, Figure III.21). These clusters were mostly composed by a large agglomeration of MoO 3 refractary species, confirming the presence of Mo 6+ in the sample from XPS data, although some regions also demonstrated the presence of large clusters of MoS 2 slabs, with a slight Ni promotion. This heterogeneous active phase distribution may find origin in the preparation method (incipient wetness impregnation). The support SBA-15 like material has high specific surface area, with relatively large mesopores. Thus, during impregnation, several effects could be observed, as schematized in Figure III [1] Some mesostructured domains have the pore entrances directed to the outer surface and thus to the impregnation solution, which will give origin to particles highly charged in active phase precursors, leading eventually to some pore blockage. The solution remaining in the outer surface of the catalyst grain and/or between mesostructured domains, may lead to sintering of large clusters, as depicted in micrograph D, [2] However, not all mesopores are conveniently directed, which could lead to silica domains less loaded in Mo and Ni precursors. If the solution is able to get inside the pore, during maturation it will go further inside the mesopore by a concentration gradient effect, giving origin to well dispersed MoS 2 slabs, as observed in Figure III.21 B and C; [3] On the other hand, the silica mesostructured domains in the core of the grain may remain plain as if the precursors solution does not attain those particles during the maturation period (ca. 12 hours), as observed Figure III.21 A.

Nonetheless, before sulfidation, XRD did not show any signs of crystallites. Another explanation for this distribution would be a non-homogeneous sulfidation of the catalysts. Perhaps the temperature promotes the migration of the active phase precursors at the catalyst surface, since the interactions between these species and the support are relatively weak, leading to sintering of large clusters. On the other hand, there could be some H 2 S flow diffusion limitations throughout the catalyst during sulfidation, leading to an incomplete sulfidation.

As an alternative, lower temperature rate or milder sulfidation conditions associated with longer sulfidation treatments would prevent the sintering of active phase precursors at the surface, simultaneously enhancing a better active phase formation. However, one should also take into consideration that longer sulfidation could also lead to sintering phenomena. Thus, one has to consider a tradeoff between both operational conditions (temperature and time), which should be optimized in order to achieve a maximum sulfidation rate with a minimum clusters formation. Ni and Mo (not visible in the micrographs) were found over the apparently plain silica domains, with a Ni/Mo atomic ratio of 0.25. These results suggest that the sulfidation of this catalyst was not completely achieved, as well dispersed metal particles remained even after 2 hours at 350 °C under H 2 S/H 2 flow. These results also support the hypotheses pointed out with previous catalysts. We thus believe that the incipient wetness impregnation leads to a heterogeneous dispersion of the active phase. One alternative would be to introduce the active phase precursors via a wet impregnation technique, or to apply a longer maturation period (in order to give active phase precursors more time to migrate and achieve the core of catalysts grains), together with milder sulfidation conditions. , one can see that the scenario is not much different from the impregnated catalysts. These catalysts presented a heterogeneous dispersion of the active phase over the silica matrix: metallic clusters at the edges of the SBA-15 domains as well as some pore channels completely fulfilled by the NiMoS/CoMoS slabs were found, as in micrographs A, B and C in Figure III.27, for the 2%Ni/HPMo@SBA-15 CE catalyst. Nevertheless, these catalysts did not show any silica domain without active phase, as previously observed for both reference catalyst families.

Hybrid catalysts -Impregnation of

In addition to the active NiMoS or MoS 2 slabs well resolved in Figure III.27, one could also observe some clusters sintering as in micrograph A, demonstrating the sintering of MoO 3 crystallites previously estimated from XRD, Raman and XPS analyses. In fact, one has shown that hybrid catalysts presented an oxide molybdenum content corresponding to 35-40% of all Mo present in the sample, which clearly aggregated into clusters that can attain 80 nm.

Regarding the active slabs in micrographs C and D in Figure III.27, one can observe a curved shape around the pore surface, as better evidenced in close-up D. In addition, this catalyst also showed some sections with well dispersed NiMoS slabs, as shown in micrograph D, some of them also presenting a slight bend. As aforementioned in the previous chapter for the non-promoted hybrid catalysts, this disposition of the MoS 2 may give origin to highly active sites: (i) the curved morphology may induce sulfur vacancies, (ii) the unblocked channel entrances still allow the reactant/product molecules to migrate easily through the mesopores, being constantly in contact with the clogged mesopores surface. The analysis over the widely dispersed slabs showed an average stacking degree of 2.8 ± 1.4 and an average length of 3.7 ± 1. The 5%Ni/HPMo@SBA-15 CE catalyst, represented in Figure III.29, is very similar to the latter catalyst with lower loading. One can distinguish a global homogeneous dispersion of the active phase, as there is no silica domains without active slabs, nevertheless one could see some sintering in one micrograph, presented in Figure III.29 A, also consistent with XRD, Raman and XPS results. The metal phase also formed large clusters, up to 100 nm of length, mainly inside the pores.

At the nanoscale, one can see that the active phase slabs formed large hanks, occupying the mesopore volume -as also evidenced by a decrease in total volume, nevertheless, in micrograph With respect to the Co-promoted catalysts, a similar trend was observed, however with more active phase sintering, as shown in Appendix III, section III.6.2.3, pg. 184.

• • •

Hybrid promoted catalysts

Both hybrid HPMo@SBA-15 CEand HPMo@SBA-15 C-based catalysts have shown similar properties, at the oxide state. STEM dark field analysis, performed over the 2%Ni/HPMo@SBA-15 CE catalyst, has shown that molybdenum clusters are mainly found in the silica walls. Few micrographs also evidenced some pore blockage by molybdenum particles after impregnation of the promoters, which is consistent with the porous volume decrease observed by N 2 sorption analysis, and the MoO 3 crystallites agglomeration observed by XRD. Several species were found at these catalysts: intact Keggin units, partially decomposed POM and MoO 3 (cf. Sulfided hybrid promoted catalysts presented a scenario similar to the impregnated catalysts: heterogeneous dispersion of the active phase over the silica matrix, with metallic clusters at the edges of the SBA-15 domains as well as some pore channels completely fulfilled by the NiMoS/CoMoS slabs. Nevertheless, all silica domain presented metallic clusters or MoS 2 slabs, unlike the impregnated reference catalysts, which have shown some plain silica domains.

The Ni-promoted hybrid catalysts also showed particular MoS 2 slabs morphologies, which have also been observed with the non-promoted hybrid catalysts: (i) curved morphology, which may induce sulfur vacancies, (ii) unblocked channel entrances, allowing the reactant/product molecules to migrate easily through the mesopores, being constantly in contact with the clogged mesopores surface. Some of these catalysts also evidenced the formation of large hanks, occupying the mesopore volume, as well as long and flat MoS 2 slabs clogging the mesopore surface, however leaving the pore entrance free.

Regarding the Co-promoted catalysts, one may state that the trend is very similar to what was observed for the Ni-promoted catalysts, though more active phase sintering was observed than when nickel was used as promoter. These results, suggest that encapsulating the molybdenum precursor inside the silica walls may enhance the active phase dispersion of these catalysts into some extent. There were no mesostructured domains without MoS 2 clusters, as observed with the impregnated catalysts.

Hybrid-based catalysts presented lower molybdenum sulfidation rates (between 36 and 39 %) than promoted reference catalysts (in the range of 66 up to 72 %)Erreur ! Source du renvoi introuvable.. This different sulfidation rates was attributed to the sintered MoO 3 phase detected for hybrid catalysts by Raman and XRD analyses, which was also confirmed by TEM images. This fact is also in accordance with the percentage of Mo 6+ present in each catalyst, determined from the XPS spectra: impregnated catalysts presented between 8 -14 % of oxide molybdenum, whereas hybrid catalysts presented 36 up to 41 %. These catalysts are also presented in Figure III.31.

Promotion of the MoS 2 active slabs

The XPS analysis also provided information on the promotion rates from the chemical composition of the different catalysts.

Co-Impregnated or NiMoP/CoMoP-based catalysts have shown a percentage of decorating promoters of 45 -50 %, while encapsulated catalysts presented 26 -29 % for lower loadings and 33 -34 % for higher loadings. This could be related either to the smaller number of molybdenum sulfide sites over hybrid catalysts, decreasing the probability of decorated NiMoS or CoMoS slabs formation, or to diffusional limitations during impregnation associated with the limited accessibility of Ni or Co towards the molybdenum sites, inside the silica framework.

Nevertheless, regarding the NiMoS or CoMoS slabs promotion rates, impregnated and "encapsulated" catalysts present appreciably the same Ni/Mo or Co/Mo ratios, despite the differences in molybdenum sulfidation and global promotion rates, suggesting that NiMoS or CoMoS slabs are decorated likewise, regardless the preparation method.

Next Chapter will present the catalytic evaluation of these catalysts, through the toluene hydrogenation reaction. The objective is the correlation of the active phase properties evidenced in Chapter II and Chapter III with the activity performance through this benchmark reaction.

III.6

Appendix III

III.6.1 NiMoP and CoMoP polyoxometalates characterization

The molecular NiMoP and CoMoP POM composition was analyzed by XRF. Analyzing the results in Table III.10, one can see that the Ni/P or Co/Mo atomic ratios are in accordance with what would be expected for all compounds. Mo/P ratios were acceptable, with the exception of the Co 1.5 PMo 12 O 40 polyoxometalate, which presented a higher Mo/P ratio than expected (Mo/P = 12). These solids were also analyzed by UV-Vis spectroscopy, dissolving a few mg of each POM in distilled water at different concentrations. The respective spectra are present in

III.6.1.1 NiMoP and CoMoP polyoxometalates stability experiments monitored by Raman spectroscopy

For the purpose of this work, the analysis of Ni 3 PMo 11 NiO 40 H and Co 3 PMo 11 CoO 40 H polyoxometalates stability in aqueous solutions at different pH was evaluated. A small amount of POM (20 mg) was diluted in the minimum of distilled water. The pH was then adjusted using HCl 37wt.% and/or sodium silicate, for lower or higher pH values, respectively. The analysis was performed in liquid state, using the same apparatus described above, using a LabRam HR (Horiba Jobin Yvon) spectrometer equipped with a BXFM confocal microscope, interference and Notch filters and charge-coupled device (CCD) detector cooled at 198 K by Peltier effect. The excitation was provided by the 514.53 nm line of an Ar + -Kr + ion laser (Spectra Physics) focused with a 50× long working distance objective. Its power was carefully adjusted to work with negligible laser heating effect. Therefore, the spectra were achieved with a maximum power of 1 mW at the sample. The diffused light was dispersed with a 1800 grooves mm -1 diffraction grating, leading to spectral resolution of 1 cm -1 . To measure the pH of the solution, a Eutech Instruments, pH 510 pH meter was used. The Raman at different pH for both polyoxometalates are represented in 70, one can see that as we have seen for higher pH, the POM reformulates to its original form, presenting the exactly same Raman spectrum than at pH = 4.95. Decreasing the pH again down to 0.80, the 12-phosphomolybdenic acid was re-formed, and was possibly in equilibrium with other species, not clearly identified.

By its turn, the Co 3 PMo 11 CoO 40 H solution presented a pH of 5.04. Increasing the solution pH, the POM starts to decompose up to pH = 6.33, as the Raman shift starts to enlarge and decrease in intensity from 5.39. A small peak at ca. 900 cm -1 starts to intensify, and becomes clearly visible at pH = 6.33. This peak may be attributed to lacunary species, or to the different oxide components of the initial POM. Looking in the other sense, decreasing the pH from pH = 5.04 down to pH = 1.28, one can see that the POM starts to decompose near 3.08, starting to form lacunary structures and at pH = 1. 79

N 2 Sorption measurements

Elementary contents from X-Ray Fluorescence

TEM analysis

Elementary contents from X-Ray Fluorescence

Elementary contents from X-Ray Fluorescence

TEM analysis

Regarding the sulfided Co-promoted catalysts, one may state that the trend is very similar to what was observed for the Ni-promoted hybrid-based catalysts. Nevertheless, looking at the Copromoted catalysts, it appears to be more active phase sintering than when nickel was used as promoter (cf.

Chapter IV Catalytic evaluation of HDT catalysts

IV. Catalytic evaluation of HDT catalysts IV.1 Introduction

The catalysts described in Chapters II and III were evaluated in several benchmark reactions, as a complement of the physical and chemical characterizations, with the overall objective of delineating structure-activity relationships.

In a first instance, the Hydrogenation of toluene at constant sulfur rate (HSC) was used in order to evaluate the potential of all the prepared catalysts through their intrinsic activity, since hydrogenation is known to occur in the well known mechanisms of hydrotreatment, as we have already seen in Chapter I.

Afterwards, the Hydrodesulfurization of thiophene (HDS of thiophene) was carried out merely over the non-promoted catalysts, at atmospheric pressure, in order to evaluate their intrinsic activities in HDS reaction.

IV.2 HSC -Toluene hydrogenation test IV.2.1 Experimental set-up

The catalytic performance of the prepared catalysts was first evaluated in toluene hydrogenation reaction (HYD), under hydrogen pressure and at constant sulfur rate (HSC test). This reaction provides some indications on the catalysts activity in hydrodesulfurization (HDS), since HYD occurs in the two well-known HDS mechanisms (Chapter I, Figure I.4). Pre-sulfidation and catalytic test were conducted in gas phase, using a fixed bed microreactor, loaded with 0.9 cm 3 of catalyst, consisting in powder particles sieved between 0.1 and 0.5 mm, unless otherwise stated. The DRT (Densité de Remplissage Tassé -Compact Filling Density) allows charging the reactor at isovolume. To determine the DRT, the catalyst is inserted in a graduated cylinder with a standard known volume. Then, the cylinder is submitted to 400 hits for 5 times, adding more catalyst at each interval, in order to entirely fulfill the cylinder. Once fulfilled, one determines the weight of catalyst, and thus, since the exact volume of the graduated cylinder is known, the DRT. On the other hand, HRTEM analyses over the sulfided catalysts have shown the presence of small MoS 2 slabs in the pores of the HPMo@SBA-15 CE catalyst, which were not observable in the HPMo@SBA-15 C catalyst, in addition to the metallic particles inside the pore walls and the slab heaps at the mesopores, present in both catalysts (see Chapter II and Scheme 2). Thus, one can assume that intact Keggin units of H 3 PMo 12 O 40 gave origin to more and smaller MoS 2 slabs (probably arising from an incomplete slab formation), between silica walls and mesopores surface. Comparing the evaluated catalysts in terms of the preparation methods, one can see that encapsulation followed by calcination and extraction is more performing in toluene hydrogenation than impregnation of Keggin units over the SBA-15 support. One should recall that both HPMo@SBA-15 CE and HPMo/SBA-15 D catalysts have intact Keggin structures before sulfidation, as verified by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR, Raman and IR spectroscopies (Chapter II). Nevertheless, the HPMo@SBA-15 CE catalyst presented better active phase dispersion, as evidenced by TEM: the POMs were trapped inside the silica micropores, and thus could not migrate to form large cluster in the outer surface of the catalyst, as observed when impregnation technique was used. In addition, even though HPMo@SBA-15 CE presents some pore blockage, it has shown globally uniform active phase dispersion, contrarily to the impregnated HPMo/SBA-15 D catalyst, which presented several domains with absolutely no MoS 2 slabs and few zones with MoS 2 clusters agglomeration, in addition to the Mo clusters located inside the silica framework.

Thus, with the one-pot synthesis and post active phase reformulation of the entrapped Keggin units, one has, on one hand, better dispersion and, on the other hand, the intrinsic ability of intact Keggin units to form smaller molybdenum sulfide slabs, at traditional activation conditions. One can see that the intrinsic activity of the HPMo@SBA-15 CE increased considerably, when expressed per mole of sulfided molybdenum. This catalyst has shown by IR-CO, XPS and XAFS/XANES techniques a very slow sulfidation kinetics, possibly due to stronger interactions with the silanols and/or siloxanes of the silica support, giving rise to small active MoS 2 slabs placed between mesopores and silica walls, thus representing a gain in active sites vis a vis the larger and higher stacking slabs, obtained for the other catalysts. Observing these results, one may also state that HPMo@SBA-15 C and HPMo/SBA-15 D present sensitively the same activity, since MoSR of calcined hybrid catalyst was 10% lower than that of the impregnated reference. This suggests that encapsulation followed by calcination and extraction leads to active sites intrinsically more active in toluene HYD, than wetness impregnation technique. This fact could be attributed to the active phase dispersion before the activation of the catalysts, and/or to the intact Keggin units encapsulated before sulfidation, giving origin to smaller MoS 2 slabs. Nonetheless, the morphology of the MoS 2 slabs is limited by the traditional sulfidation conditions, which appears to be a limiting factor since POM in interaction with silanols and/or siloxanes of the silica framework are relatively harder to sulfide.

a Sulfided molybdenum atoms determined by multiplying the total Mo atoms of the catalyst, obtained from XRF measurements, by the molybdenum sulfidation rate MoSR, obtained from XPS measurements.

IV.2.3 Promoted catalysts IV.2.3.1 Reference promoted catalysts

One will now discuss the catalytic performances of the promoted catalysts, presented in Chapter III. In order to evaluate the potential of these materials, two commercial catalysts were also tested: CoMoP/Al 2 O 3 and NiMoP/Al 2 O 3 , prepared by co-impregnation of ammonium heptamolybdate and Ni(NO 3 ) 2 or Co(NO 3 ) 2 , which properties are listed in Table IV.3. These catalysts were used in a trilobe extrudate shape. Comparing to the commercial NiMoP catalyst, one can conclude that impregnation over mesostructured silica led to much more effective catalysts in this benchmark reaction. This can be related to several different aspects: [1] Interactions with the support -the mesostructured SBA-15 type support is known for the weaker interactions with active phase precursors comparing with alumina, which leads to different mobility and dispersion of the active phase; [2] The specific surface area of the co-impregnated catalysts (708 m 2 g -1 ) is much higher than that of the commercial alumina-supported catalyst (200 m 2 g -1 ), which can also affect the active phase dispersion; [3] The molybdenum loading of co-impregnated catalysts (∼11-12 wt.%) over the high specific surface support gave origin to a Mo density of Mo/nm² = 0.7, which is much lower than the Mo density of commercial catalyst (∼22 wt.%), presenting Mo/nm² = 5 ( cf.

Table IV.3 and Table IV.4);

Regarding the Co-promoted reference catalysts, one can notice that the most loaded (Co/Mo = 0.4), both dried and calcined after impregnation, have better catalytic activities than the commercial CoMoP catalysts (Co/Mo = 0.3). The dried low Co loaded catalysts (Co/Mo = 0.2) showed inferior activities comparing with commercial reference catalyst, which may be attributed to the lower promotion rate; although, calcined 2%Co/HPMo/SBA-15 C catalyst presented slightly better activities, even with a lower promotion rate. Regardless, these materials are much less active than their nickel counterparts, which was expected since Co-promoted catalysts are more suitable in low-pressure processes. With respect to the calcination treatment, one can see that calcination did not improve

NiMoP and CoMoP POM-based promoted reference catalysts

Using NiMoP and CoMoP polyoxometalates as active phase precursors, we should expect better promotion rates than using a classical co-impregnation technique, since Ni or Co and Mo are located near to each other during the sulfidation step. However, as we have seen in Chapter III, co-impregnation of 12-phosphomolybdic acid together with Ni(NO 3 ) 2 or Co(NO 3 ) 2 seems to lead to the formation of CoMoP and NiMoP POMs, with similar promotion rates, even after calcination, which was highlighted by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR Raman and XPS spectroscopies (cf. Scheme 3).

IV.2.3.2 Hybrid promoted catalysts

With the innovative method of encapsulation, one should expect better performances in HYD of toluene not only because of the better active phase dispersion observed with the non-promoted catalysts, but also from an improvement of the active phase promotion due to a better confinement of Mo and Ni or Co precursors, during the catalysts activation.

However, as we have seen by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR, after impregnation of the active phase promoters, the encapsulated HPMo were not in their initial form, indicating a partial or complete destruction of the Keggin structure, giving origin to decomposed POMs and MoO 3 crystallites into some extent. Consequently, the encapsulated POMs were able to migrate from the silica walls into the mesopores, blocking some of the channels and giving origin to large sulfided clusters. In addition, well dispersed small MoS 2 slabs and clogged mesopores were also observed in TEM images, shown in Chapter III, and depicted in Scheme 4. Analyzing Figure IV.7, one can see that the encapsulated HPMo@SBA-15 CE-based catalysts are also more performing than commercial catalysts with Ni promotion, although Co-promoted catalysts showed similar activity. However, when compared to impregnated reference catalysts, these catalytic activities are significantly lower. These inferior performances could be attributed to the MoO 3 crystallites present at the catalysts surface before the sulfidation of encapsulated catalysts, which led to a lower sulfidation rate and thus minor number of active sites, comparing with the impregnated reference catalysts. In addition, considering the Ni-promoted catalysts, analyzing Table IV.6 one can see that the 2%Ni/HPMo@SBA-15 CE and 5%Ni/HPMo@SBA-15 CE catalysts have very similar molybdenum sulfidation rates, and the same global promotion rate (%NiMoS). This means that the percentage of sulfided Mo (MoS 2 ) and the percentage of Ni which forms the NiMoS phase are the same for both catalysts. As we have appreciably the same Mo loading (16.8-16.1 MoO 3 wt.%), one may say that one has the same amount of MoS 2 phase for both catalysts. However, 2%Ni/HPMo@SBA-15 CE contains 1.2 wt.% Ni while 5%Ni/HPMo@SBA-15 CE contains 3.0 wt.% Ni, suggesting that the absolute amount of NiMoS sites is higher for the most loaded catalyst. Nevertheless, analyzing Figure IV.7, one can see that both Ni-promoted catalysts present the same catalytic activity, despite the higher promotion rate of the 5%Ni/HPMo@SBA-15 CE catalyst. This statement may suggest two phenomena:

(i) The 5%Ni/HPMo@SBA-15 CE catalyst present more Ni x S y or Ni x O w S y species, a which could agglomerate and induce a loss of dispersion and/or blockage of the pore entries, and thus the access to the NiMoS active phase. TEM analysis clearly showed that both 2%Ni/HPMo@SBA-15 CE and 5%Ni/HPMo@SBA-15 CE catalysts presented pore blockage (Chapter III, section III.3.1). Nevertheless, after analyzing all TEM micrographs of both catalysts, one cannot rule out that 5%Ni/HPMo@SBA-15 CE has significantly higher amount of bulky species blocking the pore entrances, regarding the 2%Ni/HPMo@SBA-15 CE catalyst, or a worst dispersion of the metallic phase. (ii) The NiMoS slabs of 5%Ni/HPMo@SBA-15 CE catalyst contain more decorating Ni than the 2%Ni/HPMo@SBA-15 CE catalyst, which is in agreement with the Ni/Mo ratio found by EDX: Ni/Mo = 0.41 and Ni/Mo = 0.15, respectively. Hence, we could hypothesize that there is an optimal decoration rate for MoS 2 slabs, corresponding to a maximal number of active Ni-Mo sites, which is inferior to Ni/Mo < 0.4, as also suggested by Marchand et al. 1

HPMo@SBA-15 C -based catalysts

As previously observed in Chapter III, HPMo@SBA-15 C and HPMo@SBA-15 CE-based catalysts have shown similar properties, at the oxide state. After sulfidation, a heterogeneous dispersion of the active phase over the silica matrix was found, with metallic clusters at the edges of the SBA-15 domains as well as some pore channels completely fulfilled by the NiMoS/CoMoS slabs, as represented in Scheme 5. In addition, as also observed with HPMo@SBA-15 CE-based catalysts, despite the higher promotion rate for the 5%Ni/HPMo@SBA-15 C-D catalyst, the same activity was found between 5%Ni-and 2%Ni/HPMo@SBA-15 C-D catalysts, which was slightly less obvious with Co-promotion. This result could be related to the aforementioned hypothesis that in this particular case, the higher Ni/Mo ratio led to the formation of more species such as Ni x S y , a inducing a loss of dispersion and/or blockage of the pore entries, and thus the access to the active sites Nevertheless it is interesting to note that calcined catalysts (after impregnation) showed a considerably lower catalytic performance comparing with the dried materials, which can be attributed to a double sintering effect. As previously observed in Chapter III, these catalysts presented MoO 3 signature in the XRD spectra, before catalysts activation, revealing a partial decomposition of incorporated POMs into MoO 3 . TEM images have shown that the cobalt-promoted calcined 2%Co/HPMo@SBA-15 C catalysts presented pore blockage, mainly at the edges of the silica domains, which supports these phenomena. The more the catalyst is calcined, more MoO 3 crystallites will form at the surface, leading to a lower sulfidation, and thus generating fewer MoS 2 slabs. In addition, 5%Ni/HPMo@SBA-15 C showed a catalytic performance lower than that of the 2%Ni/HPMo@SBA-15 C catalyst.

IV.2.4 HSC results discussion

As we have seen, the non-promoted HPMo@SBA-15 CE catalyst showed an intrinsic activity 2-fold higher than HPMo@SBA-15 C and HPMo/SBA-15 D, when taking into consideration the sulfided molybdenum atoms present in each catalyst. This higher performance in HSC reaction test could be attributed to small active MoS 2 slabs placed between mesopores and silica walls (as observed in Chapter II), eventually resulting from the strong interactions with silanols and/or siloxanes of the silica support which prevent larger and higher stacking slabs formation, as obtained for the other two catalysts. These strong interactions prevent, however, fully sulfidation of the active phase precursors, as confirmed by XPS and EXAFS measurements, reflecting the potential of this catalyst if sulfidation could achieve 100%.

Let's now take a look into the Ni-promoted catalysts, since Ni-promoted catalysts were more suitable for toluene HYD reaction than Co-promoted ones. In order to compare the different preparation methods under the same conditions, the intrinsic velocities were expressed in molecules of converted toluene normalized per mole of nickel in NiMoS nanocrystallites (NiMoS). Analyzing Figure IV.9, one can clearly see that the low-promotion hybrid basedcatalysts have on average two-fold the catalytic performances of the high-promoted catalysts, in toluene hydrogenation reaction. These results suggest that at higher promotion rates, one may obtain highly decorated NiMoS slab edges. Nevertheless, the active sites are composed by both Ni and Mo atoms, which means that fully decorated NiMoS slabs do not enhance the catalytic activity, as the edges of the slabs become saturated in promoter atoms. With a Ni/Mo = 0.2, the catalytic performances were considerably enhanced, suggesting the existence of an optimum promotion rate for HDT catalysts, as also suggested by Ben Tayeb et al. 2 Nevertheless more formulations should be considered and tested, in order to get more insights about the active phase optimal composition. In addition, comparing the high promoted catalysts, one can see that the toluene hydrogenation intrinsic activities are similar for all catalysts, suggesting that regardless the These results may suggest two different hypotheses:

(i) Encapsulated catalysts would generate more hydrogenating sites, while impregnation generates hydrodesulfurization sites. Nevertheless, recalling Chapter I, one has seen that the edges of the MoS 2 slabs as well as the basal planes are active in HDS reactions. HYD only takes place in the basal planes. This suggests that the interactions between POM and the silanols and/or siloxanes within the silica framework led to very large MoS 2 slabs, with very low stacking degree (according to the Rim-Edge model, proposed by Daage and Chianelli 4 ), which was not observed. One has seen large MoS 2 slabs, but mostly small MoS 2 slabs at early formation, at the walls and and/or at the pores surface in Chapter II, by TEM and HRSTEM techniques. Thus, a second hypothesis could be put forward:

(ii) The accessibility of the active centers would not be the same in encapsulated and impregnated catalysts. Hydrogenation reactions are performed at high pressure (60 bar), while hydrodesulfuration of thiophene occurred at atmospheric pressure. Thus, if active sites are not easily to achieve, the pressure of the medium may improve the diffusion to the active sites. As we have seen before, impregnated catalysts presented MoS 2 hanks mainly in the external surface of silica mesostructured domain, while encapsulated catalysts presented the active phase rather inside the mesopores. Thus, it is more easily to reach a MoS 2 hank at the surface of the grain or near the pores entrances, than inside the mesopores channels, mainly at low pressure processes. However, in order to answer this last question, catalysts should be tested in the 4,6-di-methyl di-benzothiophene (

V. Glossary and Characterization techniques V.1 Introduction

As aforementioned, the main objective of this work is the evaluation of new HDT catalysts based on an innovative preparation method consisting in the encapsulation of polyoxometalates into the framework of mesostructured SBA-15 type silica. To evaluate and demonstrate the potential of these new catalytic materials, various HDT catalysts were also prepared according to conventional methodologies and all catalysts were thoroughly characterized at each step of the preparation, using a large panel of physical and spectroscopic techniques. Different b enchmark reactions (Hydrogenation, Hydrodesulfurization) were used to evaluate the catalysts performance and to be able to make meaningful comparisons between the different approaches.

In this chapter, the optimization of these catalysts preparation is described, as well as the glossary of all catalysts discussed in this manuscript. Finally, the characterization techniques and devices applied are described.

V.2 Glossary

In order to facilitate the reading and understanding of the present work, all prepared catalysts are listed below, with their nomenclature and main preparation features.

V.2.1 Non-promoted catalysts

The prepared supports and non-promoted catalysts main preparation features and nomenclature are listed in Table V.1. The preparation scheme of these catalysts is represented in Figure

V.1.
order to eliminate the remaining gases present in the reactor. The sample was then recovered from the glass bulb of the reactor which was sealed in a flame torch. The recovery technique and storage of sulfided catalyst in a sealed bulb prevents re-oxidation of the sample.

V.4 Characterization techniques V.4.1 X-ray Diffraction (XRD)

XRD is a non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of different materials. When a monochromatic X-ray beam with wavelength lambda is projected onto a crystalline material at an angle theta, diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by a complete number n of wavelengths. Plotting the angular positions and intensities of the resultant diffracted peaks of radiation produces a pattern, which is characteristic of the sample. When a mixture of different phases is present, the resultant diffractogram is formed by the addition of the individual patterns. λ θ θ temperature did not exceed 600 °C in order to prevent MoO 3 sublimation, but in the case of all silica samples up to 1000 °C some residual weight loss was observed, attributed to residual water resulting from silanol groups condensation.

V.4.4 Raman Spectroscopy

V.4.4.1 General

Raman spectroscopy probes the sample vibration states, allowing structural analysis of the catalyst adsorbed species. Raman experiments were made at IRCELyon (Institut de Recherches sur la Catalyse et l'Environnement de Lyon) using a LabRam HR (Horiba Jobin Yvon) spectrometer equipped with a BXFM confocal microscope, interference and Notch filters and charge-coupled device (CCD) detector cooled at 198 K by Peltier effect. The excitation was provided by the 514.53 nm line of an Ar + -Kr + ion laser (Spectra Physics) focused with a 50× long working distance objective. Its power was carefully adjusted to work with negligible laser heating effect. Therefore, the spectra were achieved with a maximum power of 1 mW at the sample. The diffused light was dispersed with a 1800 grooves mm -1 diffraction grating, leading to spectral resolution of 1 cm -1 .

Before spectra acquisition, all samples were submitted to a potency study, to prevent laser heating leading to structural modifications. To this purpose, several filters were introduced, and the spectra compared after normalization. Micro-Raman mappings were also achieved for the HPMo/SBA-15 D and HPMo@SBA-15 CE catalysts, to check the homogeneity of samples at the micron scale. The spectra were acquired using a VarianTM Cary 4G spectrometer equipped with a praying Mantis (HarrickTM) for solid analysis. The acquisition domain was 900-190 nm with an acquisition step of 1 nm. The liquid samples were acquired in absorbance mode and the solid samples in diffuse reflection mode (DRS).

V.4.5 UV-Visible Spectroscopy

V.4.6 Infrared Spectroscopy with CO as probe molecule

The Infrared spectroscopy with CO as adsorbed probe molecule can provide the qualitative and semi-quantitative analysis of the different sites, present in the sulfided catalysts. In this study, the IR-CO spectroscopy allowed the identification of several sulfide, oxide and oxy-sulfide intermediate species over the non-promoted catalysts: HPMo/SBA-15 D, HPMo@SBA-15 C and HPMo@SBA-15 CE.

The interaction between CO and the catalysts surface is very weak at RT. Thus, all experiments were carried out at 77 K (liquid nitrogen temperature). The spectra acquisition in the range of 4000 to 400 cm -1 was performed with a Fisher Scientific Nicolet spectrometer type Nexus, with a resolution of 4 cm -1 , and an acquisition number c.a. 128 in order to obtain a good quality signal/noise ratio. The spectra analysis was performed with the Omnic 6.0 program.

The IR cell was constituted by a double glass envelope with CaF 2 windows. The samples were sulfided ex-situ prior to the introduction in the glove-box with controlled H 2 O and O 2 contents (below 10 ppm). The powdery samples were then pressed as a self-supported pellet of c.a. 20 mg, which was then positioned on a sample-holder and transferred under inert atmosphere to the infrared cell (Figure V.5).

The sample was treated directly in the IR cell connected to a vacuum apparatus. Prior to the absorption measurements, the sample was treated at 200 °C for 10 hours, after which the IR spectra were recorded at 77 K.

The adsorption measurements were carried out with the introduction of a known amount of CO into the cell followed by the spectrum recording. This operation is repeated by gradually increasing the amount of CO introduced, until saturation of the sample.

The amounts of CO introduced into the cell were calculated using the ideal gas law according to equation 1:

• (1)
where n CO is the number of moles of introduced CO, P CO the total pressure of CO in atm, V standard the standard volume of the setting (1.59x10 -3 L), R the perfect gases constant R = 0.082 atm.K -1 .mol -1 and T the temperature in K.

Results exploitation

The results exploitation was performed according to a protocol developed at IFPEN by A. A. Quoineaud. 5 The spectral range from 4000 to 3300 cm -1 corresponds to hydroxyl groups, represented in Figure V.6 and listed in Table V.5.

ν ν ν ν γ Δ Δ ν
The sulfided analyzed samples were conditioned in a glove box under argon sweep. The samples transfer into the analysis chamber was performed without air replacement.

Results exploitation

The obtained spectra were decomposed after subtracting a Shirley-type background, applying a linear or peak shape of Gaussian (30%)-Lorentzian (70%). Three independent contributions were used to decompose the signal of the oxygen O 1s or carbon C 1s. Only one contribution was used for phosphorus P 2p.

The decomposition of sulfur S 2p signal was also performed. Three oxidation states were considered: sulfide, sulfate and oxy-sulfide. Constraints of binding energy, intensity and width at half height were previously determined at IFP Energies Nouvelles with reference sulfide catalysts, and are listed in Table V.7, Appendix VII, pg. 249. 8 For the decomposition of the signal of molybdenum Mo 3d six contributions were considered. These contributions highlighted on reference samples studied at IFP Energies Nouvelles, 8 resulted from three different oxidation states (IV, V, VI) at the 3d 3/2 and 3d 5/2 electron layers. The signal of the S 2s electrons was also observed, and was considered in the decomposition but not in the quantification. Constraints of binding energy, intensity and width at half height contributions are summarized in Table V.8, Appendix VII, pg. 249.

The decomposition of nickel Ni 2p signal was also performed. Contributions came from three different chemical forms: oxide, sulfide and 'NiMoS', i.e., decorating MoS 2 slabs, and for each chemical form the 2p 3/2 and 2p 1/2 contributions and the associated satellite. 8 For Co, the same contributions were considered: oxide, sulfide and 'CoMoS', i.e., decorating MoS 2 slabs, and for each chemical form the 2p 3/2 and 2p 1/2 contributions and the associated satellite. 8 Constraints of binding energy, intensity and width at half height contributions for these two elements are summarized in Table V.9 and Table V.10, Appendix VII, pg. 249.

The resulting spectra were decomposed using CasaXPS, version 2.0.71.

From the intensity of XPS signals obtained for each element analyzed on a given sample (Mo, Al, S, Ni, Co, P, O, C), it is possible to quantify the actual elemental concentration , according to equation 2:

∑ • 100 (2)
where S i is the sensibility factor gave by the constructor, and I stands for intensity.

From the spectra decompositions, it is possible to determine the relative concentrations of species at the catalyst surface, taking into account the relative intensities of each species. In the present study, most information was provided by the XPS sulfidation and promotion rates, determined as follows.

a) Sulfidation rate

The sulfidation rate may be expressed in several forms. The molybdenum sulfidation rate considers only the MoS 2 phase, and may be determined as in equation 3.

!"

# $% & # $% & ' # $% )* ' # $% +* • 100 (3)
The global sulfidation rate considers all the sulfided species present in the sample and may be determined as in equation 4.

,!" -. /01 2 3

b) Promotion rate

The promotion rate, corresponding to the nickel or cobalt in decoration of the MoS 2 slabs (NiMoS or CoMoS) compared to the total Ni (or Co) amount, may be determined as in equation 6.

D ! # E $% # E $% '# E &* '# E /01 2 • 100 (6)
The effective content of mixed NiMoS or CoMoS is then obtained by multiplying the promotion rate by the total promoter content:

D ! 9:86. • 9: 211 FGG (7)
The Ni/Mo ratio in the promoted NiMoS or CoMoS slabs may be determined as in equation 8: On an average of two tests per sample, the repeatability of the relative contents of different chemical forms was estimated to be ± 7% for species containing cobalt or nickel and ± 4% for molybdenum species.

V.4.9 X-Ray Adsorption Spectroscopy (XAS)

The analysis of Extend X-ray absorption Fine Structure (EXAFS) provides information on the local structure of atoms: the nature, number and distance of neighboring atoms. This analysis was performed over the non-promoted catalysts, discussed in Chapter II, in order to get more insight to these atomic properties. The analyzed energy absorption edges were the K-edge of molybdenum. The reliability of the number of shell neighbors is relatively low, presenting an error range of ± 0.5. However, the distance of the neighbors is obtained with a high accuracy (± 0.05 Å). The X-ray Absorption Near-Edge Structure (XANES) spectrum analysis provided information on the electronic structure (oxidation) and the 3D structure of atoms.

V.4.9.1 XAS Acquisition EXAFS were performed at HASYLAB laboratory using DORIS III, a storage ring operated with electron energy of 4.45 GeV and a current of 120 mA. Data were collected on the X1 station, a beam line placed after a bending magnet which is equipped with a double crystal monochromator, Si(3 1 1) for the Mo K-edge (20000 eV). Measurements were done in transmission mode using ionisation chambers filled with krypton to absorb around 30% of the X-ray beam in the first ion chamber, around 40% of the X-ray beam in the second ion chamber and the remainder of the Xray beam in the third ion chamber (used for online recording of reference spectra obtained from metallic foils of Mo). At Mo K-edge, each spectrum was recorded twice with a step of 3 eV (from 19,850 eV to 21,000 eV) and 2 s per point. The energy/angle calibration was performed using a Mo foil as references. After sieving, the particles in the 100-200 mm range were pressed and loaded into a sample holder sealed by self-adhesive tape made of Capton1. The EXAFS spectra were recorded at room temperature under atmospheric pressure. Fourier transforms (FT) of the k 3 -weighted EXAFS functions were obtained using a Hanning type window ranging between 3.10 Å -1 and 13.1 Å -1 beyond the Mo K-edge (the spectra are noisy above those limits). In this whole work, all FT are calculated and presented without a phase correction. The inverse FT was obtained in the range between 1.30 Å and 3.60 Å for molybdenum.

V.4.10 Transmission Electron Microscopy (TEM)

The TEM analysis allows visualizing the morphology of the different phases present on the catalyst, such as the MoS 2 crystallites or bulk structures. The sample is traversed by an electron beam and an image of the object being analyzed is achieved through a complex system of electromagnetic lenses. A high-contrast image is formed by blocking electrons deflected away from the optical axis of the microscope, to allow only unscattered electrons through (Bright Field technique).

TEM images of sulfided catalysts can also show the dispersion of the active phase. Only slabs parallel oriented to the electron beam are visualized by TEM -the slabs parallel to the support do not appear on the photographs, due to insufficient resolution. The determination of the morphological characteristics of MoS 2 slabs (histograms of length and stacking slabs) was μ Samples were prepared involving the powder-formed sample in a resin, cut the ensemble by ultramicrotomy in 40 nm thick blades. These blades are further deposed at a carbon membrane of the TEM grid.

V.4.11 Quantitative Determination

V.4.11.1 X-ray Fluorescence (XRF) XRF analyses were made using a Philips TM spectrometer PW 2404 providing quantitative results with 2% incertitude for cobalt, nickel, molybdenum, phosphorus and silicium elements. In this method, high energy X-ray radiation excites an inner electron of the analyzed element. The excited electron is ejected and the vacancy is quickly filled by an upper-level electron. The energy involved in this relaxation process is then evacuated as an X-photon. Each element possesses a characteristic emission wavelength and its intensity is proportional to the element concentration in the sample.

V.4.11.2 Elemental Analysis by CHNS methods

This analysis was made using a CE Instruments TM EA1110 CHNS, operating at 1000 °C. This equipment is used for the simultaneous quantification of carbon, hydrogen, nitrogen and sulfur organic or inorganic samples in solid, liquid or gaseous state. The average error is 5% for solids and gases and 10% for liquids.

The analysis is divided in 3 stages:

1. 1-3 mg of sample, which are introduced in a tin (Sn) carrier, are precisely weighted and suddenly exposed to a temperature superior to 1000 °C (flash) in the presence of oxygen diluted in helium;

2. The combustion gases (CO, CO 2 , H 2 O, NO, NO 2 , SO 2 and SO 3 ) are then transported by the vector gas onto an oxidation and reduction catalyst. The obtained gases (N 2 , CO 2 , H 2 O and SO 2 ) are afterwards separated in a chromatographic column;

3. The gases that leave the column are analyzed by a thermal conductivity detector (TCD) previously calibrated. The TCD signal is converted by software to the CHNS elemental composition. This analysis was used to measure the amount of organic molecules in the samples.

The raw data is given in weight %. The overall rate of sulfurization of the catalyst may thus be determined, based on the concentrations of metals (Ni, Co, Mo, W) derived from X-ray fluorescence.

V.5

Appendix VII V.5.1 X-ray Photoelectron Spectroscopy (XPS) Since the active phase of HDT catalysts formulation is composed by Ni/Mo/P or Co/Mo/P, the strategy we envisioned to introduce the active phase promoters consists in an incipient wetness impregnation, using the traditional Ni(NO 3 ) 2 or Co(NO 3 ) 2 salts, over the hybrid Mocontaining SBA-15 material, as schematized below.

One of the objectives of our work was to evaluate the potential of such catalysts in the hydrotreatment of various feedstocks such as diesel, gasoline or vegetable oils. For that purpose, we also synthesized reference catalysts, non-promoted in a first instance, and promoted afterwards, prepared by traditional (conventional) wetness impregnation over SBA-15 silica support using POM active phase precursors, for comparison with hybrid and also commercial HDT catalysts. These catalysts were thus evaluated through two benchmark reactions: toluene hydrogenation reaction (HSC), and thiophene hydrodesulfurization reaction (HDS).

A second objective was to get more insight into the nature of the interactions between the active phase and the support, according to each preparation route. Thus, all catalysts were extensively characterized, at each preparation step, namely Raman spectroscopy and HR-TEM which were particularly instructive.

Non-promoted POM-based catalysts

Starting with the simplest case of the non-promoted catalysts, the one-pot synthesis used to prepared hybrid catalysts based on the encapsulation of POM clusters into a mesostructured silica framework led to well-structured materials, with a relatively high specific surface area and molybdenum loading, between 11-16 wt.%. Calcination at 490 °C of the hybrids led to the formation of MoO 3 phase and methanol extraction allowed for the reformulation of the POMs into intact Keggin structures, embedded into the microporosity of the silica framework walls, as confirmed by TEM observations. At the oxide state, these hybrids presented a better dispersion of the metal phase, comparing with traditional incipient wetness impregnation, without significant structure loss. Raman spectroscopy still brought relevant information concerning the interactions between support and Mo precursors. Very briefly, hybrid HPMo@SBA-15 CE catalyst has shown broadened Mo-O d vibration modes, indicating stronger interactions with neighboring silica functionalities, siloxanes and silanols, which were not observed with the reference HPMo/SBA-15 D catalyst, as schematized in Scheme 1. The non-promoted catalysts catalytic performances were evaluated in the toluene HSC reaction, after in situ sulfidation. The innovative HPMo@SBA-15 CE catalyst was almost twice as active as the calcined counterpart, HPMo@SBA-15 C, and reference impregnated catalyst HPMo/SBA-15 D. This activity enhancement could be explained by two different features of the catalysts:

Keggin units state before sulfidation

As schematized in Scheme 1, the HPMo@SBA-15 C presented active phase dispersion similar to the HPMo@SBA-15 CE catalyst, as well as similar textural properties, only differing in the molecular state of the molybdenum precursor before the activation step of the catalyst: intact Keggin structure in the case of HPMo@SBA-15 CE, or partially decomposed POM for HPMo@SBA-15 C, both presenting MoO 3 crystallites. HRTEM analyses over the sulfided catalysts have shown the presence of small MoS 2 slabs in the pores of the HPMo@SBA-15 CE catalyst, which were less observable in the HPMo@SBA-15 C catalyst, in addition to the metallic particles inside the pore walls and the slab heaps at the mesopores, present in both catalysts. Thus, one can assume that intact Keggin units of H 3 PMo 12 O 40 gave origin to more and smaller MoS 2 slabs (probably arising from an incomplete slab formation), between silica walls and mesopores surface, as schematized in Scheme 2.

Scheme 2: Sketch of the non-promoted sulfided catalysts.

Dispersion of the Mo species before sulfidation

Both HPMo@SBA-15 CE and HPMo/SBA-15 D catalysts have intact Keggin structures before sulfidation, as confirmed by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR, Raman and IR spectroscopies (Chapter II). What could distinguish these two catalysts before sulfidation is the dispersion of the metallic clusters: HPMo@SBA-15 CE catalyst presented better dispersion of Keggin units, as evidenced by TEM: the POMs were trapped inside the silica walls, and thus could not migrate to form large cluster in the outer surface of the catalyst, as observed with the impregnated reference catalyst. Even though, after sulfidation, HPMo@SBA-15 CE presented some pore blockage, it has shown globally uniform active phase dispersion, contrarily to the impregnated HPMo/SBA-15 D catalyst, which presented several domains with absolutely no MoS 2 slabs and few zones with MoS 2 clusters agglomeration, in addition to the Mo clusters located inside the silica framework, as depicted in Scheme 2.

Thus, with the one-pot synthesis and post active phase reformulation of the entrapped Keggin units, one has, on one hand, better dispersion and, on the other hand, the intrinsic ability of intact Keggin units to form smaller molybdenum sulfide slabs, under traditional activation conditions. Nevertheless, these catalysts have shown lower sulfidation rates in XPS measurements, which represent an important limitation towards the development of highly effective HDT catalysts. Comparing the toluene conversion for each catalyst per sulfided molybdenum, as in Figure 1, one can clearly distinguish the potential of the HPMo@SBA-15 CE hybrid catalyst. Thus, continuous efforts must be made to go beyond these limitations. As already mentioned, IR-CO, XPS and XAFS/XANES techniques have pointed out very slow sulfidation kinetics and low sulfidation rate for hybrid catalysts, possibly due to stronger interactions of POMs with the silanols and/or siloxanes of the silica support. These interactions could give rise to small active MoS 2 slabs placed between mesopores and silica walls, representing a gain in active sites vis a vis the larger and higher stacking slabs, obtained for HPMo@SBA-15 C and HPMo/SBA-15 D catalysts. In addition, one should note that these latter catalysts showed appreciably the same intrinsic activity. Thus, we believe that encapsulation of active phase precursors such as 12-phosphomolybdic acid inside the silica framework leads to intrinsically more or more active HYD sites. To overcome the lower sulfidation rates of the encapsulated catalysts, one can consider harsher sulfidation conditions, such as higher temperature or longer sulfidation treatments, in order to attain the same sulfidation degree than impregnated reference catalysts, which was actually demonstrated in Chapter II: Molybdenum sulfidation rates of 82%, obtained after sulfidation at 600 °C for 2 hours under 15 v/v% H 2 S/H 2 , and 64%, obtained after sulfidation at 350 °C for 4 hours, respectively, versus 55% obtained after 2 hours at 350 °C).

Promoted hybrid POM-based catalysts

The non-promoted hybrid catalysts were afterwards impregnated with the active phase promoter precursor: Pr(NO 3 ) 2 (Pr = Ni or Co), in order to form NiMoS or CoMoS active slabs in a confined space, and evaluate the effect of the proximity between Mo and Pr, directly comparing with the commercial catalyst. Reference catalysts were also prepared by wetness impregnation, either of HPMo together with Pr(NO 3 ) 2 or of Ni-or Co-containing polyoxometalates. All solids maintained their hexagonal mesostructured organization, however with a slight decrease in the specific surface area. (cf. Scheme 3.) Encapsulating the molybdenum precursor inside the silica walls should enhance the active phase dispersion of these catalysts into some extent, since there were no mesostructured domains without MoS 2 clusters, as observed with reference impregnated catalysts. Nevertheless, as the interactions of these active clusters with the silica support are relatively weak with respect to alumina, one could observe some sintering of the active phase into large clusters inside the mesopores, in addition to the molybdenum species located inside the silica walls. The sintering of the active phase could occur during impregnation of promoters precursors and/or during sulfidation of the catalysts, as schematized in Scheme 3.

Impregnation of active phase promoters

Impregnation of Pr(NO 3 ) 2 , in pH range of 4.0-6.0 could lead to the partial or complete decomposition of the encapsulated POM, which consequently could migrate from the silica walls into the mesopores surface. The decomposed POM already present in the parent catalyst together with bulk crystals eventually formed during the drying step (most probably arising from the formation of hot points in the catalyst), led to the sintering of MoO 3 crystallites, preventing the formation of MoS 2 slabs, and thus leading to a loss on active sites.

Sulfidation of hybrid promoted catalysts

Sulfidation was carried out at 350 °C for 2 hours under 15 v/v% H 2 S/H 2 flow. At these conditions, POMs start to decompose and under the effect of temperature may migrate and form large clusters at the pores surface, also leading to a loss on MoS 2 slabs.

The encapsulated promoted catalysts were evaluated and compared with commercial NiMoP and CoMoP catalysts in the toluene hydrogenation reaction. We have seen that HPMo@SBA-15 CE-based catalysts presented enhanced catalytic performances compared to traditional alumina-based catalysts when nickel was used as active phase promoter, although Copromoted catalysts showed similar activity (cf. Figure 2). However, two major limitations should be considered and discussed in the interpretation of these results, which could be matter of future investigation for further insights in the development of enhanced hybrid HDT catalysts: Sulfidation and Promotion Rate.

Sulfidation rate of hybrid catalysts

As already observed with the non-promoted catalysts, promoted HPMo@SBA-15-based catalysts present lower sulfidation rates, when compared to the reference impregnated catalysts: 41-46 % vs 70-77 %. As shown for the non-promoted catalysts, severer sulfidation conditions (longer treatments -4 hours, or higher temperatures -600 °C) could lead to a better sulfidation rate, increasing the yield in active sites formation. However, under these conditions, the probability to form MoO 3 crystallites also increases together with the mobility of species at the catalyst surface. Therefore, there is a tradeoff between sulfidation rate and dispersion of active phase that should be evaluated.

Promotion rate of hybrid catalysts

Both Ni-promoted catalysts present the same catalytic activity, despite the higher promotion rate of the 5%Ni/HPMo@SBA-15 CE catalyst. On the other hand, 2%Ni/HPMo@SBA-15 CE and 5%Ni/HPMo@SBA-15 CE catalysts have very similar molybdenum sulfidation rates and the same global promotion rate (% NiMoS), which means that the percentage of sulfided Mo (MoS 2 ) and the percentage of Ni which forms the NiMoS phase are the same for both catalysts.

As we have the same amount of MoS 2 phase for both catalysts (same molybdenum content), and a higher Ni loading for the 5%Ni/HPMo@SBA-15 CE catalyst (suggesting more NiMoS sites), one can raise two hypothesis: (i) formation of more Ni x S y or Ni x O w S y species, which could agglomerate and induce a loss of dispersion and/or blockage of the pore entries, and thus the access to the NiMoS active phase; (ii) the Ni/Mo ratio on the NiMoS slabs of 5%Ni/HPMo@SBA-15 CE catalyst (Ni/Mo = 0.4) is too high, giving origin to a loss on active sites by excessive Nidecoration, than the 2%Ni/HPMo@SBA-15 CE catalyst (Ni/Mo = 0.2).

In order to overcome these drawbacks associated with the synthesis of promoted hybrid materials, a synthesis procedure with NiMoP or CoMoP polyoxometalates encapsulated inside the silica walls should be developed (avoiding the impregnation step), using a mild calcination that would prevent the formation of MoO 3 crystallites while evacuating the structure directing agents (SDA). Finally, a compromise between sulfidation rate and active phase dispersion should also be investigated.

New insights have been explored towards these alternative syntheses, and will be further detailed in Perspectives.

Promoted reference catalysts

The reference catalysts prepared either by co-impregnation of HPMo with Ni(NO 3 ) 2 /Co(NO 3 ) 2 or impregnation of NiMoP/CoMoP POMs were very similar in terms of textural properties, molecular composition and catalytic performance: both presented the same promotion rate, active phase dispersion, and activity in HYD of toluene. This leads us to believe that coimpregnation of 12-phosphomolybdic acid together with Ni(NO 3 ) 2 or Co(NO 3 ) 2 would lead to the formation of similar compounds (CoMoP and NiMoP POMs), which was highlighted by [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and Raman spectroscopies. Consequently, these compounds lead to a similar active phase, with the same dispersion over the support and the same morphology and nature, as observed by TEM images.

In general, one should also note that these reference catalysts -co-impregnated or POMbased, are all intrinsically more performing than the commercial NiMoP and CoMoP catalysts and, more performing than hybrid promoted catalysts. This catalytic enhancement with regards to alumina-supported catalysts, could be attributed to the intrinsic properties of mesostructured silica, i.e. specific surface area, porous volume etc., which can give origin to a better active phase dispersion.

Finally, the intrinsic velocities of the Ni-promoted catalysts, prepared according to the different preparation routes, were expressed in molecules of converted toluene normalized per nickel atom engaged into MoS 2 (NiMoS) (cf. Figure 3).

It was shown that low-promoted hybrid based-catalysts presented nearly twice the catalytic performances of high promoted hybrid and impregnated reference catalysts, which was related to a Ni/Mo promotion ratio near optimal, giving rise to intrinsically enhanced catalysts. However, the major drawback of these hybrid catalysts is the crystallization of MoO 3 phase during synthesis calcination and/or impregnation/drying step, preventing the complete sulfidation of molybdenum, consequently reducing the absolute number of promoted active sites, relatively to impregnated catalysts.

Comparing the reference catalysts, prepared either by co-impregnation or from NiMoP POM, one can see that both have intrinsically comparable activities, supporting the similarity between these solids: preparing a molecular NiMoP POM in order to obtain Ni and Mo in proximal vicinity led to materials having the same catalytic behavior than a catalyst prepared by coimpregnation of H 3 PMo 12 O 40 and Ni(NO 3 ) 2 solution. Finally, we have seen that SBA-15 -supported catalysts are intrinsically more active in toluene HYD than commercial alumina supported NiMoP catalyst, even at equal Ni/Mo promotion ratio. The minor activity of commercial catalysts highlights the interest of using mesostructured silica materials as active phase support, enhancing the active phase dispersion and/or the quality of the active sites: on one hand we could have higher number of active sites (Ni-Mo or Co-Mo), when all introduced Ni or Co (in an optimal Pr/Mo promotion ratio), give origin to an active site; on the other hand, one may have intrinsically more performing sites, associated to the curvature on the basal plane, on silica supported catalysts. Ni-promoted NiMoP commercial

9.8

HPMo@SBA-15 As-synthesized -low temperature (near zero Celsius)

Finally, HPMo@SBA-15 as-synthesized material was recovered by filtration, with no further treatment. The resulting material was then perched in distilled water under vigorous stirring, presenting a pH of 1, and the temperature of the suspension was cooled down to 1 °C. NaOH (∼10 -3 M) was then added several times until pH stabilization at 4.0. A large excess of Ni(NO 3 ) 2 (3 fold the stoichiometric amount needed to form Ni 3 PMo 11 NiO 40 H), was dissolved in distilled water and then added to the suspension. After filtration, the resulting solid was dried at 50 °C overnight. This catalyst presented a well ordered mesostructure, as evidenced by XRD measurements, suggesting that there were no textural changes with the low-temperature treatment. XRF analyses reported a Mo/P ratio of 12 and Ni/Mo ratio of 0.10, nearly corresponding to a Ni 1,5 PMo 12 O 40 structure, which was in agreement with 31 P NMR measurements.

To provide a better understanding of the adopted concept, a scheme is presented below against, representing a suggestion of what could happen during this post-synthetic treatment.

As one can see in Scheme 3, as-synthesized hybrids have the hydrophobic micelle inside the pores, which impedes the access to the POM embedded inside the walls. That is the reason why room temperature treatments did not allow for the inclusion of promotors nearby the Mo centers.

However, when cooling the temperature down to near zero Celsius, the P123 micelle would shrink, giving origin to a pass-through. Hence, the POM clusters would be in contact with both the higher pH NaOH solution, eventually leading to the formation of lacunary POM species, and the promoter ions which, at least, could substitute the protons of 12phosphomolybdic acid, to generate Ni

Regeneration of HDT hybrid catalysts

The regeneration of hybrid catalysts has not been brought to the subject yet. However, regeneration is a very important property of hybrid materials that could lower considerably the catalyst costs in a refinery. Since the active phase is supposedly trapped inside the silica walls, one could expect that a regeneration treatment would lead to a neat active phase precursor, without complete leaching of these species from the matrix.

In order to get more insights in these catalysts regeneration, the non-promoted hybrid catalysts, recovered from the thiophene HDS reaction, were firstly calcined at 490 °C for 2 hours. Once calcined, the catalysts were further extracted with methanol for 4 hours, as described for the classical hybrid synthesis. [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR Spectroscopy have shown one single peak at -4.0 ppm, attributed to the 12phosphomolybdid acid. The peaks present at the catalysts before regeneration, corresponding to sulfide and partially oxidized compounds were no longer visible, being presumably leached from the catalysts surface during the methanol extraction, and/or being at the origin of the HPMo reformulation.

Nevertheless, 31 P NMR is not enough to conclude about the regeneration efficiency on these catalysts. The integrity of POMs could be confirmed by Raman and/or IR spectroscopies. In addition, it would be interesting to determine the Mo content by XRF, in order to verify the recovery rate of these catalysts. A post-regeneration impregnation would be feasible to compensate the leached Mo atoms, during methanol extraction.

Wide-angles XRD measurements could also indicate the presence or absence of refractory species that could still be present at the surface, as well as TEM analysis would give a picture of the catalyst. Low-angles XRD and N 2 sorptions would allow the investigation about the consequences of these treatments on the support structure and porosity Finally, and a new catalytic test would lead to a final statement about the effectiveness of the catalyst regeneration treatment.

Concluding, the innovative approach of these PhD project, based on the understanding of different features of HDT catalysts issued from the encapsulation of POM in a mesostructured silica framework, allowed for the conception of extremely promising non-promoted catalysts. Although these solids still require additional investigation, in order to minimize the formation of refractary species such as MoO 3 . Promoted catalysts further highlighted this synthesis drawback, presenting inferior catalytic performances than reference impregnated catalysts. However this work raised some hints in new research fields, towards enhanced active phase dispersion, optimized promoter/molybdenum ratios, and thus catalytic performances.

aFigure I. 1 :

 1 Figure I.1: Refinery simplified Scheme. 11

Figure I. 2 :

 2 Figure I.2: Hydrotreatment process scheme.

12

 12

Figure I. 4 :

 4 Figure I.4: DDS and HYD routes in HDS of 4,6-DMDBT.

Figure I. 7 :

 7 Figure I.7: Monolayer model representation.[START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF]

  Figure I.8.

Figure I. 8 :

 8 Figure I.8: Contact synergy model representation.[START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF]

Figure I. 9 :

 9 Figure I.9: CoMoS model schematic representation. 31,37,38

Figure I. 10 :

 10 Figure I.10: Different active sites present in the MoS2 slab edges according to different morphologies.39

  Figure I.11.

Figure I. 11 :

 11 Figure I.11: Rim-Edge model scheme for non-promoted MoS2 catalysts.

Figure I. 12 :

 12 Figure I.12: Chart of ΔμS as a function of the thermodynamic conditions applied. Each line represents aPH2S/PH2 ratio. The two horizontal lines delimit the stability range of the sulfide phase.42

Figure I. 13 :

 13 Figure I.13: DFT calculated equilibrium morphologies in HDS conditions of the (a) non-promoted MoS 2 active phase, (b) CoMoS and (c) NiMoS.

Figure I. 14 :

 14 Figure I.14: Phase diagram representing the evolution of slabs morphology, edges coverage andpromotion rates in each edge, according to ΔμS.39,43,45 .

  Figure I.15.

Figure I. 15 :

 15 Figure I.15: Summary of edge structures exposed by the unpromoted and promoted MoS2 nanoclusters. The STM images depict the MoS2 and CoMoS particles both before (a/c) and after (d/f) exposure to atomic hydrogen. The arrows indicate the position of sulfur vacancies on the edges.52

  has shown by Density Functional Theory (DFT)-volcano curve predictions, based on sulfur bond energy calculation and experimental work, that NiMoWS catalysts would be more performing than conventional NiWS, NiMoS or CoMoS catalysts, as depicted on Figure I.16.

Figure I. 16 :

 16 Figure I.16: Schematic representation of the relative positions of Co(Ni)MoWS systems on the volcano curve.

  ): the M41S family of silica and aluminosilicates introduced by Mobil group, 106-119 which includes hexagonal MCM-41, cubic MCM-48 and lamellar MCM-50 phases; the hexagonal mesoporous silicas (HMS

  Mesoporous materials are obtained by subsequent removal of the surfactant by extraction or calcination. The general synthesis procedure is represented in Figure I.19.

Figure I. 19 :

 19 Figure I.19: Sequential synthesis steps of a mesostructured material -a) pre-hydrolysis of silica precursors followed by hydrothermal treatment or ageing, b) template removal -typically calcination or extraction.

  In CLCT (Figure I.20 (a)), the liquid-crystal phase forms at low concentrations of surfactant molecules, by cooperative self-assembly between SDA molecules and the inorganic species. 157

Figure I. 20 :

 20 Figure I.20: formation of mesostructured materials by SDAs -(A) CLCT mechanism (B) TLCT mechanism.Adapted from ( 152 ).

Figure I. 21 :

 21 Figure I.21: Interactions between the inorganic species and the head group of the surfactant with consideration of the possible synthetic pathway in acidic media.

  Figure I.22) is used.

Figure I. 22 :

 22 Figure I.22: Pluronic ® 123 structure.

Figure I. 23 :

 23 Figure I.23: TEM images of SBA-15. Electron beam parallel (a) and perpendicular (b) to the main axis of the pores. 93

Figure I. 25 :

 25 Figure I.25: Powder XRD pattern of calcined SBA-15. 129

  2 /Al 2 O 3 are schematically represented in Figure I.26. Co(CO) 3 NO molecules preferentially interact with MoS 2 edge sites (CUS), 184,186 giving rise to CoMoS phases in the second sulfidation step. Nevertheless, adsorbed in MoS 2 partially occupied by Co atoms, Co(CO) 3 NO molecules are transformed into separate Co sulfide clusters.

Figure I. 26 :

 26 Figure I.26: Sketch of the CVD method with Co(CO3)NO molecules, absorbed on the edges of MoS2 slabs and on partially occupied MoS2 edges.

Figure I. 28 :

 28 Figure I.28: Keggin-type heteropolyanion.

  Figure I.29 b).

Figure I. 29 :

 29 Figure I.29: (a) Lacunary Keggin and (b) substituted Keggin heteropolyanion. Substitution may occur even if the substitution atom has an oxidation number lower than the original M atom, such as Co, Ni, Fe etc. 191 For instance, the synthesis of Ni 1,5 PMo 12 O 40 and Ni 3 PMo 11 NiO 40 H are based on 12-phosphomolybdic acid. According to the concentration of Ni precursors and to the pH of the medium, one may achieve different degrees of substitution, as depicted in Equations 5 and 6. In the first case the Ni/Co ion is present as the POM counter ion, whereas in the second case Ni/Co substitutes one Mo atom inside the Keggin structure.

  .30). At 23 wt.% loadings on HPW/MCM-41 catalysts, two additional lines appeared at -12.1 and -10.7 ppm, which the authors attributed to H 3 PW 12 O 40 located on the external surface, comparing this spectra to that of the dehydrated H 3 PW 12 O 40 (Figure I.30 b and d ).

Figure I. 30 :

 30 Figure I.30: 31P MAS NMR spectra of (a) hydrated HPW; (b) dehydrated HPW; (c) HPW/MCM-41 (9wt.%); (d) HPW/MCM-41 (23wt.%). 211

Figure I. 32 :

 32 Figure I.32: Schematic binding of species A and B to siliceous surface.

  Figure I.33). In presulfiding conventional hydrodesulfurization catalysts (CoMo/Al 2 O 3 or NiMo/Al 2 O 3 ), this breakthrough indicates the active phase formation. As depicted in Figure I.33, the breakthrough is much slower for the HPW catalyst, but the same level is eventually achieved. This period of activation was ascribed to the destruction of the Keggin structure.

Figure I. 33 :

 33 Figure I.33: H2S breakthrough during presulfiding for HPW and CoMo catalysts. 221

  222

Figure I. 34 :

 34 Figure I.34: DBT hydroconversion for HPA and CoMo catalysts at H2 30 bar, GSHV 600 h -1 and LHSV 6 h -1 , at different temperatures. 221

  be introduced in the solution medium at different stages of the synthesis, depending on the targeted location: at the surface of the pores, inside the silica walls or both (Figure I.37).

Figure I. 37 :

 37 Figure I.37: POMs encapsulation into SBA-15 silica synthesis routes. 242 ( P123 micelle, Cl -chloride ion, HPA, H5SiO 4+ ion)

Figure I. 38 :

 38 Figure I.38:TEM image of mesoporous SiO2 (A) and HPW@SiO2 (B).239

  Figure I.40 (a)), suggesting that all polyoxometalates have been extracted along with the templates, which was confirmed by elementary analysis. When calcination was performed at 490 °C under air flow, a broad and intense series of resonances between -10 and -30 ppm surrounding the central peak at -15 ppm was shown (Figure I.40 (b)), suggesting that calcination led to significant decomposition of Keggin structures. However, when calcined hybrids were submitted to methanol soxhlet extraction, evidences for the Keggin unit reconstruction was found, with the appearance of a single resonance at -15 ppm corresponding to the hydrated POM molecular moiety (cf.Figure I.40 (c)).

Figure

Figure I. 40 :

 40 Figure I.40: 31 P NMR of the hybrids after (a) extraction, (b) calcination (c) calcination followed by extraction.4

Figure I. 41 :

 41 Figure I.41: Scheme for entrapped and restructured HPA at SBA-15 matrix by methanol extraction, directly after synthesis and after calcination, proposed Dufaud et al. 4

Figure I. 44 .

 44 The presence of POM in the synthesized hybrids was also confirmed by[START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR measurements. The samples presented the typical peak at -15.3 ppm, with a less intense resonance at -14.1 ppm, which was attributed to chemical interactions between HPW and the silica framework.

Figure I. 44 :

 44 Figure I.44: TEM images of (a,b) 23 wt.% HPW, (c,d) 20 wt.% HPW and (e,f) 13 wt.% HPW loaded SBA-15 15 samples. 238Keggin units incorporation by covalent linkages

Figure I. 45 :

 45 Figure I.45: Hydrolysis of SiW11Si2 and removal of SiW11. 240

( 90 )

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Figure II. 1 :

 1 Figure II.1: Synthetic sequence for encapsulated hybrid POM materials. 16 g of triblock copolymer Pluronic 123® (HO(PEO) 20 (PPO) 70 (PEO) 20 H -P123®) and atypically 0.8 g of cetyltrimethylammonium bromide ((C 16 H 33 )N(CH 3 ) 3 Br -CTAB), were used as organic structure directing agents (SDA) in the gel synthesis, in a 0.8 molar ratio. The CTAB was introduced to favor stronger interactions between the template, TEOS -the silica precursor (tetraethyl orthosilicate -Si(OEt) 4 ), and POM.

  Figure II.32, pg. 112. The extraction was performed at 90 °C for 4 hours. As stated by Dufaud et al., 1 the extraction allows the reformulation of Keggin structures placed within the silica framework, destroyed during calcination, simultaneously leaching the POMs laying at the pores surface. The resulting solids were finally dried at room temperature under air, overnight. These solids were referred to as HPMo@SBA-15 CE. a Determined from ⁄

  Figure II.2.

Figure II. 2 :

 2 Figure II.2: Synthetic sequence for SBA-15 like synthesis.

Figure II. 5 :

 5 Figure II.5: Wide-angles XRD patterns of the non-promoted catalysts. MoO3 diffractogram is also depicted for comparison.

  Figure II.7, the contrasts reverse: silica walls become bright and the pores darker. As evidenced in Figure II.6, TEM images have shown to be in good agreement with XRD and N 2 sorption measurements, since the hexagonally organized domains are clearly evidenced for SBA-15, HPMo@SBA-15 CE and HPMo/SBA-15 D catalysts.

Figure II. 6 :Figure II. 7 :

 67 Figure II.6: TEM analysis of SBA-15-like (A and B), HPMo@SBA-15 CE (C) and HPMo/SBA-15 D (D and E). Dark field micrographs of the HPMo@SBA-15 CE catalyst, shown in Figure II.7, have evidenced that the Mo sites are mostly located inside the silica framework, with some small clusters at the surface of the mesopores. In Figure II.7, one can still state that the introduction of POM into the silica mesostructure led to slightly deformed walls, fulfilling the microporosity intrinsic to the support, as also verified by a decrease in the microporosity volume and surface of the HPMo@SBA-15 CE catalyst. Finally, the impregnated HPMo/SBA-15 D catalyst showed well dispersed molybdenum sites, as evidenced in Figure II.6 D and E, where only one micrograph has shown molybdenum sintering (micrograph D), which could be attributed to the incipient wetness impregnation of POM over the high specific surface area support SBA-15. In STEM mode, this catalyst showed well dispersed molybdenum particles (as confirmed by EDX), mostly in the mesopore channels, as evidenced in Figure II.8.

Figure II. 8 :

 8 Figure II.8: STEM analysis of the HPMo/SBA-15 D catalyst (Dark Field) -longitudinal cut (left) perpendicular cut (right).

Figure II. 9 :

 9 Figure II.9: Solid state hpdec 31 P NMR of non-promoted catalysts.

  .2). HPMo@SBA-15 CE HPMo@SBA-15 C HPMo/SBA-15 D Mo = 10.6 wt.% Mo = 15.5 wt.% Mo = 8.3 wt.

  Figure II.6 to Figure II.8.Once sulfided, this catalyst showed a homogeneous dispersion of the active phase over the whole surface, since all analyzed micrographs exhibited the same active phase distribution. All pore channels contained Mo, as show micrographs A and B, in Figure II.12. Some of the pore channels are blocked, as evidence in Figure II.12, C -although not completely fulfilled, forming clusters of ca. 35 nm length, in average. Along through the pore channels, these clusters of MoS 2 slabs could attain up to 60 nm length, built from ca. 9 nm MoS 2 independent blocks, (Figure II.12, A and B). No sintering was observed at the catalyst outer surface.

Figure II. 12 :

 12 Figure II.12: TEM (A) and BF-STEM (B, C) analysis of the sulfided HPMo@SBA-15 CE catalyst. (Bright -pores; Dark -walls and/or Mo particles) Observing in the nanometer scale, one can perceive that Mo clusters of 1 nm are present at the silica walls, as in Figure II.13. Small MoS 2 slabs are also observed inside the walls and over the pores surface, suggesting early slabs formation, as in micrographs D and E. The MoS 2 slabs blocking the mesopores adopt the form of the surface of the support in the basal plane, as evidenced in micrograph F.

Figure II. 13 :

 13 Figure II.13: HAADF-STEM analysis of the HPMo@SBA-15 CE catalyst, transversally to the pores entrance.(Bright -walls and/or Mo particles; Dark -pores)

  Figure II.8, with little sintering at the catalyst surface, probably due to some clusters agglomeration during maturation. However, only one single micrograph showed sintering signs (D, in Figure II.6). Once sulfided, one can notice that the metal species are present mainly at the edges of the mesostructured domains, as depicted in Figure II.16, micrographs A, B and C. TEM images may suggest no metal phase in the core of the mesostructured domain, although a few micrographs showed highly dispersed MoS 2 slabs in the mesopores, with a stacking number of 1 to 2 sheets measuring between 2 to 3 nm (see B in Figure II.16). Only few mesopores were completely fulfilled with molybdenum sulfide, as in micrograph F. In addition, HAADF-STEM images showed some cluster agglomeration at the periphery of the silica grain, as evidenced in the micrographs D and E in Figure II.16, essentially formed by MoS 2 slabs.

Figure II. 16 :

 16 Figure II.16: TEM (A, B, F), BF-STEM (C) (Bright -pores; Dark -walls and/or Mo particles)and HAADF-STEM (D/E) analysis of the HPMo/SBA-15 D catalyst. (Bright -walls and/or Mo particles; Dark -pores) Nevertheless, observing Figure II.17, one can distinguish metallic clusters in the core of the mesostructured support, where nothing was detectable at first glance. These metallic clusters

  P NMR spectroscopy. As depicted in Figure II.18, one can see that the phosphorus local environment is considerably different between encapsulated and impregnated catalysts.

Figure II. 18 :

 18 Figure II.18: Phosphorus 31 P NMR of the sulfided (black) and oxide (grey) non-promoted catalysts.

  Figure II.19. In the silica framework microporosity, we could expect that POMs are surrounded by silanol and siloxane functionalities was further confirmed by XPS (discussed later in section II.4.4), which showed the presence of Mo 6+ and Mo 5+ species in addition to Mo 4+ .

Figure II. 23 :

 23 Figure II.23: Subtracted spectra in the CO interaction range (2300-1950 cm -1 ) of the HPMo@SBA-15 C sample after 30 minutes of sulfidation, at low CO coverage.

Figure II. 24 :

 24 Figure II.24:Subtracted spectra in the CO interaction range (2300-1950 cm -1 ) of the HPMo@SBA-15 C sample after 2 hours of sulfidation, at low CO coverage.

Figure II. 27 :

 27 Figure II.27:Subtracted spectra in the CO interaction range (2300-1950 cm -1 ) of the three nonpromoted catalysts: HPMo@SBA-15 CE, HPMo@SBA-15 C, and HPMo/SBA-15 D, at low iso-CO coverage.

  :1101659-001 SUPPO après sulfuration 350°C pulse 09 05 mbar CO Résultat de Soustraction:1101659-002 CO après SULF pulse 09 05 mbar Résultat de Soustraction:1101659-003 Suppo pulse 09 05 mbar -

  Figure II.28 presents a comparison of XANES spectra of molybdenum Mo 0 (blue spectrum), MoO 3 (green spectrum) and MoS 2 (red spectrum).

Figure II. 30 :

 30 Figure II.30: |TF| 2 of the EXAFS spectra between 0-7 Å of MoS 2 (blue) and non-promoted catalysts: HPMo/SBA-15 D (red), HPMo@SBA-15 CE (green), and HPMo@SBA-15 C (purple).

  atoms; b -distance of neighboring atoms from Mo; c -Debye-Waller factor, associated with the thermal and structural disorder in the considered coordination shell; e -quality of the fit: the lower the R-factor the better the theoretical model fits.

Figure II. 31

 31 Figure II.31 shows that the inclusion of an intermediate sulfide phase significantly improves the quality of the fit around 2.4 Å. Nevertheless it was not possible to distinguish between MoOS 2 and MOS 3 .

Figure II. 31 :

 31 Figure II.31: Comparison between |TF| 2 of the EXAFS spectrum of HPMo/SBA-15 D (blue), with theoretical fitting: "fit 1" corresponds to the "MoS2 + MoOS2" model, "fit 2" to the "MoS2 +MoS3" model and "fit 4" to MoS2.

Scheme 3 :

 3 Scheme 3: Sketch for the non-promoted sulfided catalysts.

  .%) HPMo@SBA-15 CE = 5.9, GSR = 84% S (wt.%) HPMo@SBA-15 C = 6.3, GSR = 61% S (wt.%) HPMo/SBA-15 D = 5.1, GR = 92 % EXAFS Local structure of the metal atoms present in the sample: their nature, the number and distance of neighboring atoms absorbing X-rays Hybrid catalysts: 46-48 % Mo 4+ HPMo/SBA-15 D: 60 % Mo 4+ Oxy-Sulfur species present for all catalysts The solid catalysts were extracted according to a Soxhlet setting, as schematized in Figure II.32. The solid is introduced in a paper thimble, inserted in a glass container, positioned over a solvent balloon and below a condenser, were the coolant circulates. Methanol was used as solvent, heated at 110 °C, and water as coolant, at sensibly 15 °C.The extraction begins when methanol begins to condensate at the bottom of the condenser, cumulating inside the thimble, in contact with the solid. As methanol surpasses a given level, a syphon system recirculates MeOH back to the balloon.

Figure II. 32 :

 32 Figure II.32: Soxhlet extraction schematic mechanism.

m 1 =

 1 Example (HPMo/SBA15): Target catalyst weight: 10 g; Support water content (LOI): 10 wt.%; Porous volume: 2.02 cc/g (determined experimentally); Mo target content: 10 wt.% Molybdenum precursor: H 3 PMo 12 O 40 , MW: 1825 g.mol -1 m catalyst = m dried support + m catalyst ⋅ %Mo m dried support = m catalyst (1-0.10) = 9 g Mo = m catalyst ⋅ %Mo = 1 g ⇒ n Mo = /0./The calcined HPMo@SBA-15 C material was also analyzed by BF-/HAADF-STEM. Selected micographs of this catalyst are shown below.

Figure II. 35 :

 35 Figure II.35: BF-STEM (left) and HAADF-STEM (right) images for the HPMo@SBA-15 C catalyst at different scales.

Figure II. 36 :Figure II. 39 :

 3639 Figure II.36: HAADF-STEM images of MoS2 slabs of the HPMo@SBA-15 C.

Figure II. 40 :

 40 Figure II.40:Subtracted spectra in the CO interaction range (2300-1950 cm-1) after CO desorption under primary and secondary vacuum (res = 1 cm -1 ) of the bulk HPMo.

-Figure II. 46 :

 46 Figure II.46:Subtracted spectra in the CO interaction range (2300-1950 cm-1) of the fully sulfided HPMo@SBA-15 C catalyst, at high CO coverage.

Figure II. 47 : 15 C

 4715 Figure II.47: Subtracted spectra in the CO interaction range (2300-1950 cm -1 ) of (a) the HPMo@SBA-15 C sample before sulfidation (b) after 30 minutes of sulfidation and (c) after 2 hours of sulfidation, at low CO coverage.

-Figure II. 48 :

 48 Figure II.48: Subtracted spectra in the CO interaction range (2300-1950 cm-1) of the HPMo/SBA-15 D catalyst, at high CO coverage.

Figure II. 49 :II. 8 . 5 . 2

 49852 Figure II.49: XPS spectra decomposition of Mo3d (above) and S 2p (below) of the HPMo@SBA-15 CE catalyst.

Figure II. 51 :Figure II. 52 :

 5152 Figure II.51: XPS spectra decomposition of Mo3d (above) and S 2p (below) of the HPMo@SBA-15 C catalyst.

Figure II. 53 :

 53 Figure II.53: Linear fitting of the XANES spectra of the HPMo@SBA-15 CE catalyst. Composition: 13 % MoO 3 , 39 % MoOS 2 , 48 % MoS 2 (FM: 2,6.10 -4 ).

Figure II. 54 :

 54 Figure II.54: Linear fitting of the XANES spectra of the HPMo@SBA-15 C catalyst. Composition: 13 % MoO 3 , 41 % MoOS 2 , 46 % MoS 2 (FM: 2,5.10 -4 ).

Figure II. 55 :

 55 Figure II.55: Linear fitting of the XANES spectra of the HPMo/SBA-15 D catalyst. Composition: 5 % MoO 3 , 35 % MoOS 2 , 60 % MoS 2 (FM : 5.10 -5

Table III. 1 :

 1 Preparation features and schematic representation of the referred promoted catalysts. Pr = Co or Ni. POM, partially decomposed POM, substituted POM, Co ions, CoO.

  NiMoP/CoMoP-based catalysts: Pr 1.5 PMo 12 /SBA-15-Y or Pr 3 PMo 11 Pr/SBA-15-Y, where Pr = Co or Ni and Y (C -calcined or D -dried) represents the final treatment.

Figure III. 1 :

 1 Figure III.1: XRD measurements of the reference catalysts prepared by co-impregnation, dried 24 hours at 120 °C after impregnation, in the low range (left) and wide range angles (right).

  Figure III.2, right).

Figure III. 2 :

 2 Figure III.2: N2 Isotherms (left) and pore size distributions (right) of the dried promoted reference catalysts, prepared from the co-impregnation of 12-phosphomolybdic acid together with Ni(NO3)2 or Co(NO3)2, over the SBA-15 like support.

Figure III. 3 :

 3 Figure III.3: Solid state 31 P NMR of promoted reference catalysts, prepared by co-impregnation of 12phosphomolybdic acid together with Ni(NO3)2 or Co(NO3)2, over the SBA-15 like support.

Figure III. 4 ,

 4 and Ni-promoted catalysts spectra, showing similar results, are shown in Appendix III, Figure III.42, pg. 180. The 2%Co/HPMo/SBA-15 D catalyst showed a very broad band in the 900-1000 cm -1 region, which is maintained after calcination at 450 °C for 2 hours, as the Mo-O a stretching mode band at 250 cm -1 . Comparing these spectra with the Co 1.5 PMo 12 O 40 and bulk H 3 PMo 12 O 40 spectra, one cannot rule out whether the cobalt (or nickel) ion is present as counterion of the POM anion or not, although Co 1.5 PMo 12 O 40 and 2%Co/HPMo/SBA-15 C catalyst shared the peak at attributed to the Mo-O d vibration mode, at 995 cm -1 .

Figure III. 4 :

 4 Figure III.4: Raman spectroscopy of promoted reference catalysts, prepared by co-impregnation of 12phosphomolybdic acid together with Co(NO3)2, over the SBA-15 like support.

Scheme 1 :

 1 Scheme 1: Expected (left) and presumably obtained (right) catalyst, prepared by the co-impregnation of H3PMo12O40 and 3 eq. of Co(NO3)2, from 31 P NMR and Raman spectroscopies results.

Figure III. 5 :

 5 Figure III.5: XRD measurements of the reference catalysts prepared by impregnation of NiMoP or CoMoP polyoxometalates over a SBA-15 like support, dried at 120 °C after impregnation, in the low range (left) and wide range angles (right).

  The wide angles XRD range in Figure III.6 also showed the absence of crystallites over the supports, even after calcination (cf. Appendix III, Figure III.44, pg. 181).

Figure III. 6 :

 6 Figure III.6: N2 Isotherms (left) and pore size distributions (right) of the dried promoted reference catalysts, prepared from the impregnation of NiMoP and CoMoP polyoxometalates, over the SBA-15 like support.

  31 P NMR spectroscopy was performed in order to verify the molecular state of the impregnated POMs. The 31 P NMR spectra of dried and calcined catalysts are presented in Figure III.7.

Figure III. 7 :

 7 Figure III.7: Solid state 31 P NMR of promoted reference catalysts, prepared by impregnation of NiMoP and CoMoP POM over the SBA-15 like support. One can see that the dried Co 1.5 PMo 12 /SBA-15 D and Ni 1.5 PMo 12 /SBA-15 D catalysts presented a single resonance at ca. -3.5 ppm, typical of the Keggin structure, with a broadening that revealed to be more important with cobalt promotion. After calcination, no significant changes were observed. With respect to the Co 3 PMo 11 Co/SBA-15 D and Ni 3 PMo 11 Ni/SBA-15 D catalysts, no resonance was observed in the 40 to -40 ppm range, or in a larger range, as evidenced in Appendix III (Figure III.46, pg. 182). After calcination of these materials, both Co 3 PMo 11 Co/SBA-15 -C and Ni 3 PMo 11 Ni/SBA-15 -C catalysts showed the appearance of a new peak at -3.5 ppm, corresponding to the phosphomolybdic PMo 12 Keggin structure anion

Figure III. 8 :

 8 Figure III.8: Raman spectroscopy of promoted reference catalysts, prepared by the impregnation of CoMoP polyoxometalates, over the SBA-15 like support.

  Figure III.15, pg. 152): the nonpromoted hybrid HPMo@SBA-15 CE showed a lower microporous volume comparing with the silica support, which was attributed to the HPMo placed inside the silica walls, as schematized in Figure III.11, at left, creating a certain roughness of the mesopores surface.

Figure III. 11 :

 11 Figure III.11: Scheme of POM migration with the impregnation solution, used in the promoted HPMo@SBA-15 CE based catalysts preparation.

  .6), or to local hot points during the drying step, giving origin to crystallized MoO 3 (cf. Figure III.11, right).

Figure III. 14 :

 14 Figure III.14: MET images in STEM mode of the 2%Ni/HPMo@SBA-15 CE sample, before catalyst activation. Bright -higher atomic density (silica walls, Mo particles), Dark -lower atomic density (pores)

Figure III. 15 :

 15 Figure III.15: MET images in STEM mode of the 2%Ni/HPMo@SBA-15 CE sample, before catalyst activation.

Figure III. 17 :

 17 Figure III.17: N2 Isotherms (left) and average pore size distributions (right) of the promoted hybrid catalysts, prepared from the impregnation of Ni(NO3)2 or Co(NO3)2, over the HPMo@SBA-15 C catalyst, dried after impregnation.

  HPMo@SBA-15 C based catalysts.

  range is below pH = 1.5. The state of the Keggin units in the impregnated catalysts was verified by[START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR spectroscopy, and is presented in Figure III.18.

Figure III. 18 :

 18 Figure III.18: Solid state MAS 31 P NMR of promoted catalysts, prepared from the impregnation of Ni(NO3)2 or Co(NO3)2, over the HPMo@SBA-15 C hybrid catalyst. From the analysis of Figure III.18, one can confirm that the POMs structure of the impregnated-dried catalysts are at least partially decomposed. The Co-promoted catalyst showed no peak at -3.5 ppm, characteristic of the starting HPMo acid, which could be due to the paramagnetic character of cobalt. The resonance of the intact Keggin structure is nevertheless present for the Ni-promoted catalysts spectra, although much less intense (2%Ni/HPMo@SBA-15 C-C) or totally absent (5%Ni/HPMo@SBA-15 C-D), after calcination. These catalysts were also analyzed by Raman spectroscopy. The spectra are presented in Figure III.19 for the dried catalysts, and in Appendix III, Figure III.52, for the calcined counterparts.

Scheme 2 :

 2 Scheme 2: Expected (left) and presumably obtained (right) catalysts, prepared by impregnation of Co(NO 3 ) 2 precursor over A) HPMo@SBA-15 CE and B) HPMo@SBA-15 C catalysts. Partially decomposed POM, MoO 3 .

Co 9 S 8 (

 98 GSR) = [S]/([Ni]+2[Mo]) or (GSR) = [S]/(8/9[Co]+2[Mo]); c -Global promotion rate (GPR) = %PrMoS; d -Slabs promotion (Pr/Mo) slabs = (%PrMoS*[Pr])/(%Mo 4+ *[Mo]). As these hybrid-based catalysts present much less molybdenum sulfide phase, the probability of a Ni or Co atom to achieve the decoration of the MoS 2 slabs is relatively low, when compared to impregnated catalysts. In fact, co-impregnated or NiMoP/CoMoP-based catalysts have shown a percentage of decorating promoters of 45 -50 %, while encapsulated catalysts presented 26 -29 % for lower loadings and 33 -34 % for higher loadings, representing almost 30% less of promoting Ni or Co atoms, as evidenced in Figure III.20. As a result, hybrid catalysts would present systematically less Ni or Co atoms engaged in the promotion of MoS 2 slabs, also corresponding to a loss in active sites.

Figure III. 20 :

 20 Figure III.20: Global promotion rate (GPR) and Slabs promotion rate ((Pr/Mo)slabs) of reference (dark blue) and hybrid (light blue) catalysts. Impregnated samples have molybdenum and its promoters in the same impregnation solution, laying both Ni or Co and Mo at the supports surface next to each other, whereas "encapsulated" catalysts have part of the molybdenum sites inside the walls of the silica matrix, and the promoters are afterwards impregnated over the hybrid materials. At the impregnation stage, Ni or Co atoms have to achieve the Mo located in the silica walls, in order to sit in the Mo neighborhood for the sulfidation step. During the catalyst sulfidation, only the promoter atoms that reached the Mo sites may give origin to the promoted NiMoS or CoMoS slabs.Nevertheless, regarding the NiMoS or CoMoS slabs promotion, impregnated and "encapsulated" catalysts presented appreciably the same Ni/Mo or Co/Mo ratios (0.37 -0.40 for impregnated catalysts and 0.28 -0.51 for "encapsulated" catalysts), despite the differences in molybdenum sulfidation and global promotion rates, as evidenced in Figure III.20. This fact suggests that NiMoS or CoMoS slabs are decorated likewise, regardless the preparation method.

Figure III. 22 :

 22 Figure III.22: Stacking degree and length distributions of the sulfided 2%Ni/HPMo/SBA-15 C catalyst.

Figure III. 23 :

 23 Figure III.23: TEM images of the sulfided 5%Ni/HPMo/SBA-15 C catalysts, prepared from the co-impregnation of 12-phosphomolybdic acid together with Ni(NO3)2.

  .

Figure III. 24 :

 24 Figure III.24: Schematic drawing of a catalyst grain after impregnation and maturation (the relative proportion of 1, 2 or 3 is represented as an example).

  Figure III.21;

  The reference catalysts prepared by wetness impregnation of NiMoP POMs over the SBA-15 like support presented nearly the same active phase distribution over the supports than reference catalysts obtained from co-impregnation. TEM images of the Ni-promoted calcined catalysts are shown in Figure III.25 and Figure III.26. The Ni 1.5 PMo 12 /SBA-15 C catalyst, shown in Figure III.25, presented NiMoS slabs in most of the analyzed support. Nevertheless, few mesostructured domains did not show any metallic phase, as in micrograph A. NiMoS slabs were mainly assembled in large clusters, blocking the pore entrances (micrograph B), where the slabs were slightly curved, as evidenced in close-up C. It was thus very difficult to evaluate the average stacking and length of these slabs, although, there were some regions where it was possible to visualize some well dispersed NiMoS slabs, as shown in micrograph D.

Figure III. 25 :

 25 Figure III.25: TEM images of Ni1.5PMo12/SBA-15 C, prepared from impregnation of Ni1.5PMo12O40 over SBA-15 like support. The Ni 3 PMo 11 Ni/SBA-15 C catalyst evidenced the formation of large clusters, mainly in the edges of the mesostructured silica domains, as shown in Figure III.26 A and B. The active phase was assembled in large clusters inside the pores near to the silica edges, as one can notice in micrographs C and D. This catalyst also showed some regions with well distributed slabs as the previous catalyst, with an average Ni/Mo ratio of 0.17.

Figure III. 26 :

 26 Figure III.26: TEM images of Ni3PMo11Ni/SBA-15 C, prepared from impregnation of Ni3PMo11NiO40H over the SBA-15 like support.

  Ni(NO 3 ) 2 or Co(NO 3 ) 2 precursors over the HPMo@SBA-15 CE catalyst Analyzing the "encapsulated" catalysts, presented in Figure III.27 and in Appendix III (Figure III.48 and Figure III.49)

Figure III. 27 :

 27 Figure III.27: TEM images of the 2%Ni/HPMo@SBA-15 CE, prepared from impregnation of Ni(NO3)2 over the HPMo@SBA-15 CE catalyst.

5 .

 5 The stacking and length distributions of this catalyst are shown in Figure III.28. One can see that, comparing to the previous analyzed catalyst, a larger distribution of the stacking number and length of the active phase slabs were observed with the 2%Ni/HPMo/SBA-15 C catalyst.

Figure III. 28 :

 28 Figure III.28: Stacking degree and length distributions of the 2%Ni/HPMo@SBA-15 CE catalyst.

  can clearly state that the MoS 2 slabs also may grow longitudinally, clogging the mesopore surface, leaving the pore entrance free, as also shown in micrograph D, in Figure III.29.

Figure III. 29 :

 29 Figure III.29: TEM images of the 5%Ni/HPMo@SBA-15 CE, prepared from impregnation of Ni(NO3)2 over the HPMo@SBA-15 CE catalyst.

  MoO 3

  Figure III.31).

Figure IV. 30 :

 30 Figure IV. 30: Schematic representation of the different impregnated and sulfided catalysts.

8 Figure III. 31 :

 831 Figure III.31: UV-Visible spectra of molecular NiMoP and CoMoP polyoxometalates, at lower concentration (∼10 -3 M). At higher concentrations (∼10 -1 M, Figure III.32), the absorption bands of cobalt and nickel are isolated. Only the presence of octahedral Ni or Co species can be observed, assigned to the

Figure III. 33 :

 33 Figure III.33: Raman spectra of the molecular NiMoP and CoMoP polyoxometalates.

  CoO 40 H polyoxometalates are very sensitive to the pH medium, maintaining their structure approximately between 3.00 < pH < 5.30. In addition, one may also notice that, having the different components in solution, namely Ni or Co ions (from the original NiMoP or CoMoP polyoxometalate) and H 3 PMo 12 O 40 acid, they are able to reformulate into their original form by simply varying the pH, without significant loss of the Raman intensity.

Ni 3

 3 Figure III.34 represents the solid state 31 P NMR spectra of the NiMoP and CoMoP polyoxometalates, containing Co 2+ and Ni 2+ as counter ion of the PMo 12 O 40 3-anion. Observing the Ni salt, one can see one peak at -3.5 ppm whereas the cobalt ion induced an intense broadening of the signal, due to its paramagnetic character.

Figure III. 34 :

 34 Figure III.34: Solid state 31 P NMR of the molecular synthesized Ni3/2PMo12O40 and Co3/2PMo12O40 polyoxometalates. * spinning side bands. Regarding the substituted POM, the 31 P NMR spectra of Co 3 PMo 11 CoO 40 H and Ni 3 PMo 11 NiO 40 H are presented in Figure III.35. Both compounds exhibited a very broad signal, which could be attributed to the paramagnetic character of Co or Ni outside the Keggin structure, which are placed inside (filling a lacuna) and outside (as counter-ion) of the Keggin structure. This broad signal is most likely assign to Keggin species inside a bulk grain. Nevertheless, one can still discern a peak centered at ca. 403 or 417 ppm, corresponding to a downfield shift caused by the Co or Ni atoms of Keggin species, respectively, rather placed outside the bulk grain.

Figure III. 35 :

 35 Figure III.35: Solid state 31 P NMR of the molecular synthesized Ni3PMo11NiO40H and Co3PMo11CoO40H polyoxometalates.

  Figure III.36 and Figure III.37.

Figure III. 36 :

 36 Figure III.36: Study of the Ni3PMo11NiO40H stability at different pH, monitored by Raman spectroscopy. Ni 3 PMo 11 NiO 40 H solution presented a pH of 4.95. As one increases the solution pH, the POM slowly starts to decompose up to pH = 5.56, as the Raman shift starts to enlarge and decrease in intensity. As one decreases the pH down to 3.63 (after being at 5.56), the POM partially reformulates, as the spectrum gains some intensity in the 997 cm -1 peak. Looking in the other sense, decreasing the pH from pH = 4.95 down to pH = 0.8, one can see that the POM starts to change near 2.42, and at pH = 1.75 one can already notice the presence of the band of the H 3 PMo 12 O 40 acid at ca. 998 cm -1 . If one re-rises the pH to 4.70, one can see that as we have seen for higher pH, the POM reformulates to its original form, presenting the exactly same Raman spectrum than at pH = 4.95. Decreasing the pH again down to 0.80, the 12-phosphomolybdenic acid was re-formed, and was possibly in equilibrium with other species, not clearly identified.

  one can already see the Mo-O d vibration mode of the H 3 PMo 12 O 40 acid at ca. 998 cm -1 , which remains at pH = 1.28 If one re-rises the pH to 3.90, one can see that the peak of the HPMo acid disappears, and the Co 3 PMo 11 CoO 40 H POM reforms.

Figure III. 37 :Figure III. 38 :

 3738 Figure III.37: Study of the Co3PMo11CoO40H stability at different pH, monitored by Raman spectroscopy. In conclusion, Ni 3 PMo 11 NiO 40 H and Co 3 PMo 11 CoO 40 H polyoxometalates are very sensitive to the pH of the medium, maintaining their structure approximately between 3.0 < pH < 5.3. In addition, on may also notice that having the different components in solution, namely Ni or Co oxide precursors (from the original NiMoP or CoMoP polyoxometalate) and the H 3 PMo 12 O 40 acid, they seem able to reformulate into their original form by simply varying the pH, without significant loss of the Raman intensity.

Figure III. 39 :Figure III. 40 :

 3940 Figure III.39: N2 Isotherms (left) and pore size distributions (right) of the calcined promoted reference catalysts, prepared from the co-impregnation of 12-phosphomolybdic acid together with Ni(NO3)2 or Co(NO3)2, over the SBA-15 like support.

Figure III. 41 :Figure III. 42 :

 4142 Figure III.41: Solid state 31 P NMR of the 5%Ni/HPMo/SBA-15 C from 1500 to -1500 ppm.

Figure III. 43 :III. 6 . 2 . 2 Figure III. 44 :

 4362244 Figure III.43: TEM images of the sulfided 2%Co/HPMo/SBA-15 C catalyst, prepared from the co-impregnation of 12-phosphomolybdic acid together with Ni(NO3)2. III.6.2.2 Reference catalysts : impregnation of NiMoP and CoMoP polyoxometalates over SBA-15 like support XRD measurements

Figure III. 45 :

 45 Figure III.45: N2 Isotherms (left) and pore size distributions (right) of the calcined promoted reference catalysts, prepared from the impregnation of modified POM over the SBA-15 like support, dried (left) and dried + calcined (right).

31 P NMR measurements The Co 3

 3 PMo 11 /SBA-15 D and Ni 3 PMo 11 /SBA-15 D catalysts were analyzed in a larger 31 P NMR range -between [-10000; 30000] ppm, in order to verify the presence or absence of signal. One should recall that these catalysts did not present any peak in the [-40; 40] ppm range (cf. Figure III.7), nor in a larger range (cf. Figure III.46). When examining a larger ppm region, ranging between [-10000; 20000] ppm, one can see very broad signs, probably masking the CoMo and NiMo species, expected at 417 and 403 ppm, respectively.

Figure III. 46 :Figure III. 47 :

 4647 Figure III.46: Solid state 31 P NMR of the Co3PMo11Co/SBA-15 D and Ni3PMo11Ni/SBA-15 D catalysts at wide range (* -carrier).

III. 6 . 2 . 3

 623 Promoted hybrid catalysts: impregnation of Ni and Co promoters over the encapsulated HPMo@SBA-15 CE and HPMo@SBA-15 C catalyst a) HPMo@SBA-15 CE based promoted hybrid catalysts

  Figure III.48 and Figure III.49).

Figure III. 48 :

 48 Figure III.48: TEM images of the sulfided 2%Co/HPMo@SBA-15 CE, prepared from impregnation of Ni(NO3)2 over the HPMo@SBA-15 CE catalyst. The sulfided 2%Co/HPMo@SBA-15 CE catalyst TEM is shown in Figure III.48, and 5%Co/HPMo@SBA-15 CE in Figure III.49. These catalysts displayed a local heterogeneous dispersion of the active phase, presenting mostly large molybdenum clusters inside the mesopores. Some well distributed slabs were also observed, as shown in the right side of Figure III.48, presenting curved morphologies.

Figure III. 49 :Figure III. 50 :

 4950 Figure III.49: TEM images of the 5%Co/HPMo@SBA-15 CE, prepared from impregnation of Co(NO3)2 over the HPMo@SBA-15 CE catalyst.

III. 6 . 3 Figure III. 53 :

 6353 Figure III.53: XPS spectra decomposition of Mo3d of the 5%Ni/HPMo/SBA-15 C catalyst.

Figure III. 54 :

 54 Figure III.54: XPS spectra decomposition of Ni 2p of the 5%Ni/HPMo/SBA-15 C catalyst.

Figure III. 55 :

 55 Figure III.55: XPS spectra decomposition of S 2p of the 5%Ni/HPMo/SBA-15 C catalyst.

Figure III. 56 :

 56 Figure III.56: XPS spectra decomposition of Mo 3d of the 5%Co/HPMo/SBA-15 C catalyst.

Figure III. 57 :III. 6 . 3 . 2 Figure III. 58 :

 5763258 Figure III.57: XPS spectra decomposition of S 2p of the 5%Co/HPMo/SBA-15 C catalyst.

Figure III. 59 :

 59 Figure III.59: XPS spectra decomposition of Ni 2p of the Ni3PMo11Ni/SBA-15 C catalyst.

Figure III. 60 :III. 6 . 3 . 3

 60633 Figure III.60: XPS spectra decomposition of S 2p of the Ni3PMo11Ni/SBA-15 C catalyst.III.6.3.3 Impregnation of promoters over the encapsulated HPMo@SBA-15 CE catalyst

Figure III. 62 :

 62 Figure III.62: XPS spectra decomposition of Ni 2p of the 2%Ni/HPMo@SBA-15 CE catalyst.

Figure III. 63 :

 63 Figure III.63: XPS spectra decomposition of S 2p of the 2%Ni/HPMo@SBA-15 CE catalyst.

Figure III. 64 :

 64 Figure III.64: XPS spectra decomposition of Mo 3d of the 5%Ni/HPMo@SBA-15 CE catalyst.

Figure III. 65 :

 65 Figure III.65: XPS spectra decomposition of Ni 2p of the 5%Ni/HPMo@SBA-15 CE catalyst.

Figure III. 66 :

 66 Figure III.66: XPS spectra decomposition of S 2p of the 5%Ni/HPMo@SBA-15 CE catalyst.

Figure III. 67 :

 67 Figure III.67: XPS spectra decomposition of Mo 3d of the 5%Co/HPMo@SBA-15 CE catalyst.

Figure III. 68 :

 68 Figure III.68: XPS spectra decomposition of Co 2p of the 5%Co/HPMo@SBA-15 CE catalyst.

Figure III. 69 :

 69 Figure III.69: XPS spectra decomposition of S 2p of the 5%Co/HPMo@SBA-15 CE catalyst.

Scheme 1 :

 1 Scheme 1: Sketch for the non-promoted oxide catalysts.

Scheme 2 :

 2 Scheme 2: Sketch for the non-promoted sulfide catalysts.

  Taking into consideration the sulfidation rate of molybdenum (MoSR a ) derived from XPS, for each catalyst, one can represent the intrinsic activity in converted toluene molecules, per atom of sulfided molybdenum, per hour. The purpose is to evaluate the intrinsic activity, regarding only the active sites of each catalyst. The catalytic performances of the non-promoted catalysts are presented in Figure IV.3.

Figure IV. 3 :

 3 Figure IV.3: HSC test results of the non-promoted catalysts, expressed in converted toluene molecules, per atom of sulfided molybdenum, per hour.

  The catalytic performances of these catalysts are represented in Figure IV.4 and expressed in terms of converted toluene molecules, per atom of molybdenum present in the catalyst, per hour.As one can see analyzing Figure IV.4, the co-impregnated promoted catalysts showed a better performance when cobalt and particularly nickel are promoting the active phase. As we have mentioned before (Chapter I), the MoS 2 sheets are promoted by the nickel (or cobalt) atoms according to the NiMoS (or CoMoS) model, and NiMoS slabs are known to be more effective than Co in high pressure processes, such as the HSC test carried out at 60 bar.

Figure IV. 4 :

 4 Figure IV.4: HSC test results for the promoted reference catalysts, prepared by co-impregnation of Ni(NO3)2 or Co(NO3)2 together with H3PMo12O40, over SBA-15 like support.

  -promoted catalysts hydrogenating (HYD) activity, although one can distinguish a slight enhancement for Co-promoted catalysts.

Scheme 3 :

 3 Scheme 3: Recapitulative sketch of the impregnated reference catalysts before and after sulfidation.

Figure IV. 6 :

 6 Figure IV.6: HSC test results comparison between co-impregnated and NiMoP/CoMoP POM-based reference catalysts.

Scheme 4 :

 4 Scheme 4: Recapitulative sketch of the promoted encapsulated catalysts, prepared by the impregnation of Ni(NO3)2 over the HPMo@SBA-15 CE catalyst, before and after sulfidation.These catalysts presented a Ni/Mo and Co/Mo between 0.17-0.18 up to 0.46-0.48 (at/at), although, as we have seen in Chapter III, they presented sensitively half of the global promotion rate (GPR) determined for reference catalysts. As hybrid-based catalysts presented much less molybdenum sulfide phase due to the presence of MoO 3 crystallites before sulfidation, the probability of a Ni or Co atom to achieve decorating MoS 2 slabs is relatively low, when compared to impregnated catalysts. As a result, hybrid catalysts would present systematically less Ni or Co atoms engaged in the promotion of the MoS 2 slabs, also corresponding to a loss in active sites.

Figure IV. 7 :

 7 Figure IV.7: HSC test results of the promoted HPMo@SBA-15 CE based catalysts prepared by encapsulation of H3PMo12O40 polyoxometalate followed by promoters impregnation.

a

  Calculated from XPS and XRF data: %NiS x [Ni] catalyst

Scheme 5 :

 5 Scheme 5: Recapitulative sketch of the promoted encapsulated catalysts, prepared by the impregnation of Ni(NO3)2 over the HPMo@SBA-15 C catalyst, before and after sulfidation. Thus, as expected, HPMo@SBA-15 C-based dried catalysts presented similar catalytic performances than the HPMo@SBA-15 CE-based catalysts. The dried and calcined catalysts properties are resumed in Table IV.7, and their catalytic results are shown in Figure IV.8.

Figure IV. 8 :

 8 Figure IV.8: HSC test results of the promoted HPMo@SBA-15 C-based catalysts, prepared by encapsulation of H3PMo12O40 polyoxometalate followed by promoters impregnation.

a

  Determined from XPS and XRF data: %NiS x [Ni] catalyst

Figure IV. 9 :

 9 Figure IV.9: HSC test results of the Ni-promoted catalysts expressed in terms of converted toluene molecules normalized per mol of Ni in NiMoS nanocrystallites, as quantified by XPS.

  emphasizing the potential of HPMo@SBA-15 CE vis a vis the HPMo@SBA-15 C catalyst.

Figure IV. 12 :

 12 Figure IV.12: HDS of thiophene test results of the non-promoted catalysts, expressed in terms of converted thiophene molecules, per atom of molybdenum present in the catalyst, per hour.

Figure V. 4 :

 4 Figure V.4: Sulfidation cell scheme, for catalysts activation at low vacuum.

  UV-Visible Spectroscopy was used analyze the bulk HPMo-based compounds: Pr 1.5 Mo 12 O 40 and Pr 3 PMo 11 PrO 40 H, Pr = Ni or Co.

Figure V. 5 :

 5 Figure V.5: IR-CO spectroscopy setting scheme.

Scheme 1 :

 1 Scheme 1: Sketch for the non-promoted oxide catalysts.

Figure 1 :

 1 Figure 1: HSC test results of the non-promoted catalysts, expressed in converted toluene molecules, per atom of sulfided molybdenum, per hour.

Scheme 3 :

 3 Scheme 3: Sketch of the promoted oxide and sulfided hybrid catalysts.

Figure 2 :

 2 Figure 2: HSC test results of the promoted HPMo@SBA-15 CE-based catalysts prepared by encapsulation of H3PMo12O40 polyoxometalate followed by promoters impregnation.

Figure 3 :

 3 Figure 3: HSC test results of the Ni-promoted catalysts expressed in terms of converted toluene molecules normalized per mol of Ni in NiMoS nanocrystallites, as quantified by XPS.

1 . 5

 15 PMo 12 O 40 species. Even though these results suggest a new insight into the synthesis of promoted hybrid catalysts, there is still matter of research with respect to the promotion ratio: even with 3 times the stoichiometric Ni amount, one has not achieved the Ni 3 PMo 11 NiO 40 H structure.

Scheme 4 :

 4 Scheme 4: Post-treatment of hybrid materials towards promoted NiMoP@SBA-15 materials.

Table I . 1 :

 I1 Ebullition temperature of oil fractions.

	Oil Fractions	Range of ebullition temperatures (°C)
	Gasoline	20-185
	Kerosene	185-230
	Diesel	230-370
	Distillates	370-600
	Residue	> 600

Table I . 2 :

 I2 Specific hydrotreatment applications.

Table I . 3 :

 I3 Typical hydroprocessing conditions used in industry.

	Fuel	Pressure (bar)	LHSV (h -1 )	Temperature (°C)
	Naphta (gasoline)	1-5	2-6	290-370
	Kerosene/Gas Oil (jet/diesel fuels)	3-10	0.5-3.0	315-400
	FCC feed pretreatment	7-21	0.5-2.0	370-425

  Griboval et al. 97- 99 used P-based Keggin heteropolymolybdates (H 3 PMo 12 O 40 or HPMo) as starting material in impregnation solutions over alumina. They have shown by 31 P RMN that an interaction between HPMo and Co 2+ ions exists, due to the absence of counterions such as NH 4 + or NO 3 -, which is also verified with Si-based Keggin units (H 4 SiMo 12 O 40 or HSiMo).

100

These interactions prevent Co atoms to penetrate alumina or to precipitate, remaining available to the formation of active phase CoMoS upon sulfidation. Phosphate interacts strongly with alumina, not only reducing the surface area, but also eliminating sites, with which molybdate ions would have interacted. Consequentially, Mo is found in the polyanion structure, and further sulfided to MoS 2 crystallites. Furthermore, a weaker interaction with a minor surface area leads to larger MoS 2 slabs with a higher stacking degree.

101

  and if the counter-ion is the oxonium ion (H 5 O 2 + ) then the structure is named heteropolyacid (HPA). At lower concentration, the HPA crystallizes as hydrates, containing from 29 up to 31 water molecules, and is easily tuned into a 6 water molecules hydrate, which is considerably more stable. The H n XM 12 O 40 HPA are bonded to some of these water molecules to form (H 5 O) 3 + ions, as for 12-phosphotungstic acid H 3 PW 12 O 40 •6H 2 O

	(or PW 12 O 40 3-; 3(H 5 O 2 ) + ).
	Polyoxometalates may be synthesized in a large range of compositions and structures.
	Isopolyanions denoted as M m O y n-are synthesized from oxoanions condensation (MO 4	n-, M
	for metal as Mo, W or V). The condensation takes place after a solution acidification with
	water elimination, and oxo-bridge formation between the metallic atoms, as in reaction (1):
	7 MoO	8 H → Mo O	4 H O			heptamolybdate	(1)
	If another oxoanion is present (XO 4	n-, X for a different atom from M and not
	necessarily a metal -B, Si, P, Ge), the metal atoms will associate around those heteroatoms,
	giving rise, after polycondensation, to a heteropolyanion denoted as X x M m O y n-:
	12 MoO	23 H	HPO → PMo O	12 H O	12-molybdophosphate (2)
	12 WO	23 H	HPO → PW O	12 H O	12-tungstophosphate (3)
	12 WO	22 H	SiO → SiW O		11 H O	12-tungstosilicate	(4)

Table II .

 II

1: Non-promoted prepared catalysts: nomenclature, preparation features and schematic representation. POM, Partially decomposed POM Catalysts

Preparation strategy Final treatments Catalyst representation Hybrid

HPMo@SBA-15 C Encapsulated Calcination HPMo@SBA-15 CE Encapsulated Calcination + Extraction Reference HPMo/SBA-15 D Impregnated Drying

Table II . 2 :

 II2 Elemental content of the hybrid materials.

	Catalyst	LOI wt.%	Si ± 1.2 wt.%	P ± 0.02 wt.%	Mo ± 0.3 wt.%	MoO 3 ± 0.3 wt.%	Mo/P ± 1.8 (at/at)
	HPMo@SBA15 CE 9	31	0.4	10.6	14.5	9
	HPMo@SBA15 C	4	31	0.4	15.5	22.2	11
	HPMo/SBA-15 D	10	31	0.3	8.3	11.2	9

Table II . 6 :

 II6 Relative amounts of molybdenum species in atomic %.

	Molybdenum spectra exploitation Catalysts	% Mo Ox.	6+	% Mo Ox-Sulf 5+	XPS % Mo MoSR 4+	GSR (%)	S/Mo at/at	Elemental analysis S GSR S/Mo (%) (%) at/at
	HPMo@SBA15 CE -350 °C, 2hours	25		25	50	55	1.82	5.9	84	1.7
	HPMo@SBA15 CE -350 °C, 4hours	25		21	54	64	2.03	-	-	-
	HPMo@SBA15 CE -600 °C, 2hours	16		21	63	82	1.96	-	-	-
	HPMo@SBA15 C -350 °C, 2 hours	22		17	61	77	1.94	6.3	61	1.2
	HPMo@SBA15 C -350 °C, 30 minutes	28		12	60	66	1.94	-	-	-
	HPMo/SBA15 D -350 °C, 2 hours	4		19	77	87	1.91	5.1	92	1.8

Table II . 8 :

 II8 Mo-K edge EXAFS derived structure parameters for the HPMo/SBA-15 D catalyst (with S0 2 determined from reference MoS2 at 0,95).

	Fitting Model	Shell a Coordination number	R (Å) b	σ² (x10 -3 Ų) c	ΔE 0 (eV) d	R-factor e
	Fit 1	Mo-O Mo-S Mo-Mo Mo-Mo	0,1 ± 0,2 4,8 ± 0,5 0,2 ± 0,2 2,4 ± 1,2	2,00 ± 0,02 2,40 ± 0.01 2,78 ± 0,08 3,16 ± 0,01	5 ± 1 5 ± 2	0,8	0,0027
		Mo-S	4,8 ± 0,5	2,40 ± 0.01	5 ± 1
	Fit 2	Mo-Mo Mo-Mo	0,2 ± 0,2 2,4 ± 1,2	2,78 ± 0,08 3,16 ± 0,01	5 ± 2	0,8	0,0029

  PMo 12 O 40 (HPMo) bands were broadened for encapsulated catalysts, specially the Mo-O d vibration mode, indicating stronger interactions with neighboring silica siloxane and silanol functionalities at the silica walls microporosity.The interactions of impregnated catalyst with the silica support were weaker, since the Raman spectra are very similar to that of the bulk-HPMo, and not significantly broadened.

	MET	Objective Observation of the active phase: Information on dispersion, particle size and stacking degree (TEM) Information on active phase location and morphology (HAADF-STEM and BF-STEM)	The H 3 SULFIDE STATE	Main Conclusions/Observations
		Molecular integrity of the POM
		precursor
		Interactions of POM with the
		silica framework

Table II . 9 :

 II9 XPS quantitative analyses of the non-promoted catalysts.

HPMo@SBA-15 CE HPMo@SBA-15 C HPMo/SBA-15 D Mo (wt%)

		2.95	5.95	2.9
	S (wt%)	1.1	3.05	1.7
	Si (wt%)	42.9	40.95	42.35
	Si/Mo (at/at)	14.5	6.9	14.6

  .1, and the description of each catalyst and respective nomenclature are detailed in sections III.2.1 and III.2.2.

  1,5 PMo 12 O 40 , Ni 1,5 PMo 12 O 40 , Co 3 PMo 11 CoO 40 H and Ni 3 PMo 11

Table III . 2 :

 III2 .2. NiMoP and CoMoP polyoxometalates synthesis details.

	Polyoxometalate H

3 PMo 12 O 40 Ba(OH) 2 Ni(NO 3 ) 2 or Co(NO 3 ) 2 Ni 1,5 PMo 12 O 40

  PMo 12 O 40 , Ni 1,5 PMo 12 O 40 , Co 3 PMo 11 CoO 40 H and Ni 3 PMo 11 NiO 40 H structures specifications were verified, with Ni/Mo and Co/Mo ratios between 0.1 and 0.4, as expected. In addition, the stability of Ni 3 PMo 11 NiO 40 H and Co 3 PMo 11

		40 g	10 g	8.6 g
	Co 1,5 PMo 12 O 40	40 g	10 g	9.4 g
	Ni 3 PMo 11 NiO 40 H	40 g	24 g	23 g
	Co 3 PMo 11 CoO 40 H	40 g	24 g	25 g
	The molecular polyoxometalates obtained were further characterized by XRF, UV-Vis, Raman
	and 31 P NMR spectroscopies, presented and discussed in Appendix III, section III.6.1, pg. 172. The
	Co 1,5

  .12. Hence, at a first glance, these Co 3 PMo 11 CoO 40 H structures should not form in the catalysts surface. Nonetheless, with NO . ions present in the solution, the pH could have locally increased during maturation and/or drying steps, giving origin to lacunary species. Indeed, the H 3 PMo 12 O 40 Keggin structure starts to decompose into lacunary species at pH > 1. In addition, attentively observing the study reported in Appendix III, section III.6.1.1, with respect to the stability of Co 3 PMo 11 CoO 40 H polyoxometalates with the pH variation (cf. Figure III.37), one may see that the substituted Co 3 PMo 11 CoO 40 H structure may also give origin to Hence, one could expect that with local pH increase, the H 3 PMo 12 O 40 , together with Ni(NO 3 ) 2 or Co(NO 3 ) 2 , could give origin to lacunary species, though, comparing with the spectra in Figure III.37, one cannot rule out the placement of Co atoms in these catalysts: as counter-ion of the POM structures or inside the Keggin structure fulfilling a lacuna.

	-3.5 lacunary species at pH > 5.4. These species at 5.4 < pH < 6, presented Raman spectra very similar -3.5
	to those of 5%Co/HPMo/SBA-15 catalysts (C and D).
	1748 scans	51914 scans
	55296 scans	8918 scans
	23431 scans	55296 scans
	3851 scans	1373 scans
	3055 scans	3162 scans

Table III . 4 :

 III4 Textural properties of the promoted catalysts, prepared from the impregnation of modified POM over the SBA-15 like support.

	Catalysts	Mo ± 0.2 (wt.%)	S BET (m 2 g -1 )	Mo (at/nm²)	V p (mLg a -1 )	S µ (m²g -1 )	V µ b (mLg -1 )	D BJH c (Å)	d 100 d (Å)	a 0 e (Å)	f Wall thickness (Å)
	SBA-15	-	918	-	1.2	79	0.029	58	103 119	61
	Ni 1.5 PMo 12 O 40 /SBA-15 -D	8.2	685	0.75	0.90	74	0.028	55	92	106	51
	Ni 1.5 PMo 12 O 40 /SBA-15 -C	8.6	685	0.79	0.91	69	0.026	55	93	107	52
	Ni 3 PMo 11 Ni/SBA-15 -D	7.9	622	0.80	0.81	64	0.024	53	93	107	54
	Ni 3 PMo 11 Ni/SBA-15 -C	8.6	619	0.87	0.81	61	0.023	53	90	104	51
	Co 1.5 PMo 12 /SBA-15 -D	8.2	683	0.75	0.90	70	0.027	55	93	107	52
	Co 1.5 PMo 12 /SBA-15 -C	8.5	682	0.78	0.90	64	0.024	55	92	106	51
	Co 3 PMo 11 Co/SBA-15 -D	7.2	609	0.74	0.69	105	0.039	48	90	104	56
	Co 3 PMo 11 Co/SBA-15 -C	7.6	580	0.82	0.66	89	0.032	47	88	102	55
	HPMo/SBA-15 D	8.3	720	0.72	0.98	81	0.032	55	97	112	57
	HPMo/SBA-15 C	7.2	743	0.61	1.0	81	0.030	55	92	106	51

a Total pore volume at P/P0 = 0.99, b Micropore volume determined using the t-plot method , c Pore size from desorption branch applying the BJH pore analysis, d d(100) spacing, e a 0 = 2d √3 ⁄ -hexagonal lattice parameter calculated from XRD, f Calculated by a 0 -pore size.

Table III . 9 :

 III9 XPS spectra decomposition: molybdenum, nickel and cobalt species relative amounts in atomic %, and respective sulfidation and promotion rates. Comparison with promotion rates determined from TEM of sulfided catalysts (by EDX) and XRF analyses.

	Catalysts	% Mo 6+	% Mo 5+	% Mo 4+ MoSR a	GSR b (%)	% Pr	2+	% PrS	% PrMoS GPR c	(Pr/Mo) slabs	d	(Pr/Mo) slabs EDX	Pr/Mo XRF	Pr/Mo Targete d
	HPMo/SBA15 D	4	19	77	87	-		-	-	-		-	-	-
	5%Ni/HPMo/SBA-15 C	14	14	72	77	7		48	45	0.40		0.41	0.48	0.36
	5%Co/HPMo/SBA-15 C	16	18	66	70	47		9	44	0.37			0.45	0.36
	Ni 3 PMo 11 Ni/SBA-15 D	8	24	67	74	4		46	50	0.38		0.17	0.43	0.36
	HPMo@SBA15 CE	25	25	50	55	-		-	-	-		-	-	-
	2%Ni/HPMo@SBA-15 CE	40	24	36	41	28		39	33	0.28		0.15	0.18	0.12
	5%Ni/HPMo@SBA-15 CE	36	25	39	46	9		57	34	0.51		0.41	0.45	0.36
	5%Co/HPMo@SBA-15 CE	37	26	37	46	37		38	26	0.37			0.48	0.36
	HPMo@SBA15 C	22	17	61	77	-		-	-	-		-	-	-
	2%Ni/HPMo@SBA-15 C-D 41	21	38	41	29		43	29	0.30			0.2	0.12

a -Molybdenum sulfidation rate is the percentage of molybdenum that gave origin to MoS 2 structures (MoSR) = %Mo 4+ ; b -Global sulfidation rate is determined based on the assumption that all molybdenum is transformed into MoS 2 , and all promoter in NiS or

Table III . 10 :

 III10 NiMoP and CoMoP polyoxometalates elementary contents, determined by XRF measurements.

	Solid	Ni or Co ± 0.2 (wt.%)	P ± 0.05 (wt.%)	Mo ± 0.8 (wt.%)	Mo/P ± 0.5 at/at	Ni or Co/P ± 0.7 at/at	Ni or Co/Mo ± 0.7 at/at
	Co 1,5 PMo 12 O 40	4.4	1.1	61	17	2.0	0.12
	Ni 1,5 PMo 12 O 40	4.4	1.2	51	13	1.9	0.14
	Co 3 PMo 11 CoO 40 H	11	1.4	38	9	4.0	0.47
	Ni 3 PMo 11 NiO 40 H	10	1.2	39	10	4.4	0.42

Table III . 11 :

 III11 Raman spectroscopy: vibration modes attribution.

10 Vibration mode H 3 PMo 12 O 40 Co 1.5 PMo 12 O 40 Co 3 PMo 11 CoO 40 H Ni 1.5 PMo 12 O 40 Ni 3 PMo 11 NiO 40 H

  PMo 11 NiO 40 H and Co 3 PMo 11 CoO 40 H compounds in different pH media was also analyzed by Raman spectroscopy, in liquid state. The details of this interesting study are found in Appendix III, III.6.1.1, pg. 176. One has seen that Ni 3 PMo 11 NiO 40 H and Co 3 PMo 11

	Symmetric Mo-O d	1001	995	979	996	975
	P-O a vibration	974	978	932	975	930
	Stretching in opposite phase of M-O d	908	870	877	870	877
	Symmetric Mo-O c -Mo	605	602	Very broad	602	Very broad
	Mo=O a	250	253	235	235	234
	The stability of Ni 3

Table III . 13 :

 III13 Elemental content of the promoted reference catalysts, prepared from co-impregnation of 12-phosphomolybdic acid together with Ni(NO3)2 or Co(NO3)2, over the SBA-15 like support.

	Catalysts	LOI wt.%

Si (wt.%) ± 1.2 P (wt.%) ± 0,02 Mo (wt.%) ± 0.2 Ni or Co (wt.%) ± 0.1 MoO 3 (wt.%) ± 0.2 Ni(Co)O (wt.%) ± 0.1 Ni(Co)/Mo (at/at) ± 0.2

										Mo/P
										(at/at)
										± 1.8
	HPMo/SBA-15 D	10	31	0.26	7.5	-	11.2	-	-	9
	HPMo/SBA-15 C	7	34	0.23	6.7	-	10.0	-	-	9
	2%Co/HPMo/SBA-15 -D	12	29	0.27	7.8	0.8	11.8	1.0	0.16	9
	2%Co/HPMo/SBA-15 -C	6	35	0.22	6.6	0.7	10.0	0.8	0.16	10
	5%Co/HPMo/SBA-15 -D	13	28	0.24	6.9	1.8	10.4	2.3	0.43	9
	5%Co/HPMo/SBA-15 -C	8	33	0.21	6.3	1.8	9.4	2.2	0.45	10
	2%Ni/HPMo/SBA-15 -D	11	31	0.21	6.9	0.7	10.3	0.9	0.17	10
	2%Ni/HPMo/SBA-15 -C	7	33	0.23	7.0	0.7	10.6	0.9	0.17	10
	5%Ni/HPMo/SBA-15 -D	12	30	0.21	6.4	1.8	9.5	2.2	0.45	10
	5%Ni/HPMo/SBA-15 -C	9	31	0.22	6.3	1.8	9.4	2.3	0.48	9

Table III . 14 :

 III14 Elemental content of the promoted reference catalysts, prepared from impregnation of NiMoP or CoMoP POM over the SBA-15 like support.

	a.u.					Evaluation Copy
	100	200	300	400	500	600	700	800	900	1 000	1 100
	Catalysts			LOI wt.%	Si (wt.%) ± 1.2	P (wt.%) ± 0,02	Mo (wt.%) ± 0.2	Ni or Co (wt.%) ± 0.1	MoO 3 (wt.%) ± 0.2	Ni(Co)O (wt.%) ± 0.1	Ni(Co)/Mo (at/at) ± 0.2	Mo/P (at/at) ± 1.1
	HPMo/SBA-15 D		10	31	0.26	7.5	-	11.2	-	-	9
	HPMo/SBA-15 C		7	34	0.23	6.7	-	10.0	-	-	9
	Co 1.5 PMo 12 O 40 /SBA-15-D	12	29	0.23	7.2	0.7	10.8	0.9	0.16	10
	Co 1.5 PMo 12 O 40 /SBA-15 -C	8	32	0.27	7.8	0.8	11.7	1.0	0.17	9
	Co 3 PMo 11 CoO 40 H/SBA-15-D	17	29	0.23	6.0	1.6	9.0	2.0	0.43	8
	Co 3 PMo 11 CoO 40 H/SBA-15 -C	12	29	0.25	6.7	1.8	10.0	2.2	0.43	8
	Ni 1.5 PMo 12 O 40 /SBA-15 -D	12	29	0.24	7.2	0.8	10.9	1.0	0.18	10
	Ni 1.5 PMo 12 O 40 /SBA-15 -C	8	32	0.28	8.0	0.8	11.9	1.1	0.17	9
	Ni 3 PMo 11 NiO 40 H/SBA-15-D	13	28	0.24	6.9	1.8	10.3	2.3	0.43	9
	Ni 3 PMo 11 NiO 40 H/SBA-15 -C	5	33	0.31	8.1	2.2	12.2	2.8	0.43	8

Table III . 15 :

 III15 Elemental content of promoted hybrid catalysts, prepared from prepared from the impregnation of Ni(NO3)2 or Co(NO3)2, over the HPMo@SBA-15 CE hybrid catalyst.

			Si	P	Mo	Ni or Co	MoO 3	Ni(Co)O	Ni(Co)/Mo	Mo/P
	Catalysts	LOI	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(at/at)	(at/at)
			± 1.2	± 0,02	± 0.2	± 0.1	± 0.2	± 0.1	± 0.2	± 1.1
	HPMo@SBA-15 CE	9%	31	0.36	9.7	-	14.5	-	-	-
	2%Co/HPMo@SBA-15 CE 7%	30	0.35	11	1.1	16.6	1.4	0.16	10
	5%Co/HPMo@SBA-15 CE 9%	28	0.33	10	3.0	15.3	3.8	0.48	10
	2%Ni/HPMo@SBA-15 CE	8%	29	0.35	10	1.1	15.5	1.4	0.18	10
	5%Ni/HPMo@SBA-15 CE 11%	27	0.32	9.5	2.7	14.3	3.4	0.45	10

Table III . 16 :

 III16 Elemental content of promoted hybrid catalysts, prepared from prepared from the impregnation of Ni(NO3)2 or Co(NO3)2, over the HPMo@SBA-15 C hybrid catalyst.

			Si	P	Mo	Ni or Co	MoO 3	Ni(Co)O	Ni(Co)/Mo	Mo/P
	Catalysts	LOI	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(at/at)	(at/at)
			± 1.2	± 0,02	± 0.2	± 0.1	± 0.2	± 0.1	± 0.2	± 1.1
	HPMo@SBA-15 C	4%	31	0.44	14.8	-	22.2	-	-
	2%Co/HPMo@SBA-15 C-D	7%	30	0.41	12.5	1.4	18.7	1.8	0.18
	2%Co/HPMo@SBA-15 C-Calc	4%	32	0.44	13.4	1.5	20.1	2.0	0.19
	5%Co/HPMo@SBA-15 C-D	10%	26	0.37	11.3	3.7	16.9	4.7	0.53
	5%Co/HPMo@SBA-15 C-Calc	3%	32	0.42	13.0	4.2	19.5	5.4	0.53
	2%Ni/HPMo@SBA-15 C-D	7%	30	0.42	12.2	1.5	18.4	1.9	0.20	9
	2%Ni/HPMo@SBA-15 C-Calc	3%	32	0.45	13.6	1.6	20.3	2.1	0.20
	5%Ni/HPMo@SBA-15 C-D	10%	26	0.39	10.9	3.7	16.4	4.7	0.56	9
	5%Ni/HPMo@SBA-15 C-Calc	3%	31	0.44	13.1	4.6	19.7	5.8	0.56

evaluation of HDT catalysts

  ............................................. IV.1 Introduction ............................................................................................................... Experimental set-up ..................................................................................... IV.2.2 Non-promoted hybrid catalysts ................................................................... IV.2.3 Promoted catalysts ....................................................................................... IV.2.3.1 Reference promoted catalysts .............................................................. Co-impregnated promoted reference catalysts...................................................... NiMoP and CoMoP POM-based promoted reference catalysts .............................. IV.2.3.2 Hybrid promoted catalysts .................................................................... HPMo@SBA-15 CE-based catalysts ........................................................................ HPMo@SBA-15 C -based catalysts ......................................................................... IV.2.4 HSC results discussion ..................................................................................

	Table of Contents
	Chapter IV.............................................................................................
	IV. Catalytic IV.2 HSC -Toluene hydrogenation test.........................................................................
	IV.2.1

IV.3 HDS -Thiophene hydrodesulfurization test

  ....................................................... IV.3.1 Experimental set-up ..................................................................................... IV.3.2 Non-promoted catalysts ............................................................................... IV.4 Conclusions ................................................................................................................ Bibliographic references .........................................................................................

	IV.5

Table IV . 3 :

 IV3 Properties of the CoMoP and NiMoP commercial catalysts.

	Catalyst	Pr/Mo (at/at)	MoO3 (wt.%)	rHYD (molTol.(atMoh) -1 )	rHYD a (molTolmolPrMoS -1 )	Mo/nm 2
	Co/Mo/P	0.33	22.1	0.88	8.9	4.7
	Ni/Mo/P	0.41	22.5	1.24	9.8	4.8

a Calculations based on the work of Marchand et al. 1 Co-

impregnated promoted reference catalysts

  The co-impregnated promoted catalysts, prepared by co-impregnation of Ni or Co precursors together with H 3 PMo 12 O 40 over the SBA-15 like support, were evaluated in the toluene hydrogenation reaction, in the same conditions than the non-promoted catalysts. The properties of these catalysts are recalled in TableIV.4.

Table IV . 4 :

 IV4 Physico-chemical properties of the promoted reference catalysts, prepared by coimpregnation of H3PMo12O40 together with Ni(NO3)2 or Co(NO3)2 .

	Catalysts	S BET m 2 g -1	D BJH a Å	MoO 3 ± 0.2 wt.%	Mo/nm	2 Pr/Mo (at/at)	Pr/Mo targeted	%MoSR	b %PrMoS c
	2%Ni/HPMo/SBA-15 -D	858	56	11.6		0.6	0.17	0.12	na	na
	2%Ni/HPMo/SBA-15 -C	768	57	11.4		0.6	0.17	0.12	na	na
	5%Ni/HPMo/SBA-15 -D	685	55	10.8		0.7	0.45	0.36	na	na
	5%Ni/HPMo/SBA-15 -C	708	55	10.4		0.6	0.47	0.36	72	45
	2%Co/HPMo/SBA-15 -D	683	55	13.4		0.8	0.17	0.12	na	na
	2%Co/HPMo/SBA-15 -C	722	55	10.7		0.6	0.16	0.12	na	na
	5%Co/HPMo/SBA-15 -D	665	55	12.0		0.8	0.43	0.36	na	na
	5%Co/HPMo/SBA-15 -C	834	55	10.2		0.5	0.46	0.36	66	44
	HPMo/SBA-15 D	720	55	12.5		0.7	-	-	77	-
	HPMo/SBA-15 C	743	55	10.8		0.6	-	-	na	-
	a Pore size from desorption branch applying the BJH pore analysis;	b -Molybdenum sulfidation rate is the percentage of
	molybdenum that gave origin to MoS 2 structures (MoSR) = %Mo	4+ ;	c -Global promotion rate (GPR) = %PrMoS.

Table IV .6: Physico-chemical properties of the hybrid HPMo@SBA-15 CE-based promoted catalysts. Catalysts S BET m 2 g -1 D BJH a Å MoO 3 ± 0.2 wt.% Mo/nm 2 Pr/Mo (at/at) Pr/Mo Targeted %MoSR b %PrMoS c HPMo@SBA-15 CE 362

 IV

			76	15.9	1.8	-		-	50	-
	2%Ni/HPMo@SBA-15 CE	433	75	16.8	1.6	0.18		0.12	36	33
	5%Ni/HPMo@SBA-15 CE	388	76	16.1	1.7	0.46		0.36	39	34
	2%Co/HPMo@SBA-15 CE 347	73	17.9	2.2	0.16		0.12	na	na
	5%Co/HPMo@SBA-15 CE 326	75	16.8	2.2	0.48		0.36	37	26
	a Pore size from desorption branch applying the BJH pore analysis;	b -Molybdenum sulfidation rate is the
	percentage of molybdenum that gave origin to MoS 2 structures (MoSR) = %Mo	4+ ;	c -Global promotion rate (GPR) =
	%PrMoS.

Table IV . 7 :

 IV7 Properties of the promoted HPMo@SBA-15 C-based catalysts, prepared by encapsulation of H3PMo12O40 polyoxometalate followed by promoters impregnation.

	Catalysts	S BET m 2 g -1	D BJH a Å	MoO 3 ± 0.2 wt.%	Mo/nm	2 Pr/Mo (at/at)	Pr/Mo targeted	%MoSR	b %PrMoS c
	HPMo@SBA15 C	345	77	23.3	2.8	-	-	61	-
	2%Co/HPMo@SBA-15 C-D	324	76	20.1	2.6	0.18	0.12	na	na
	2%Co/HPMo@SBA-15 C-C	333	76	20.9	2.6	0.19	0.12	na	na
	5%Co/HPMo@SBA-15 C-D	293	78	18.8	2.7	0.53	0.36	na	na
	5%Co/HPMo@SBA-15 C-C	296	78	20.3	2.7	0.53	0.36	na	na
	2%Ni/HPMo@SBA-15 C-D	326	76	19.8	2.5	0.20	0.12	38	29
	2%Ni/HPMo@SBA-15 C-C	336	76	21.0	2.6	0.20	0.12	na	na
	5%Ni/HPMo@SBA-15 C-D	302	78	18.3	2.5	0.55	0.36	na	na
	5%Ni/HPMo@SBA-15 C-C	302	76	20.3	2.8	0.57	0.36	na	na

a Pore size from desorption branch applying the BJH pore analysis; b -Molybdenum sulfidation rate is the percentage of molybdenum that gave origin to MoS 2 structures (MoSR) = %Mo 4+ ; c -Global promotion rate (GPR) = %PrMoS.

  Considering that all nickel and cobalt sulfides are present as NiS and Co 9 S 8 , respectively, one can thus determine the S/Mo ratio at the MoS 2 slabs, according to equation 5.

				4 5	6 211 '7 86 211	• 100	or			,!"	-. /01 2 3 9: 211 '7 86 211	• 100	(4)
		where -! ;<= :> 3 is all sulfur in the sulfide form.
	?	. 86	@ ;=AB;	-. /01 2 3 211 86. & 211 C -9: /01 2 3 211	or	?	. 86	@ ;=AB;	-. /01 2 3 211 86. & 211 C 4 5 -6 /01 2 3 211

Table V . 6 :

 V6 Recording protocol of XAS data of the K-edge of molybdenum.

		Before Edge	Edge	After Edge
	Mo (K) (Transmission)	[1750-19970] 10 eV step 0,5 s step	[19970-20030] 0,5 eV step 1,5 s step	[20030-30000] 1,245 eV step 1,5 s step

Table V . 7 :

 V7 XPS parameters used in the contributions of each sulfur phase.

		Binding energy (± ± ± ± 0.1 eV) Peak area Width at half height
	S 2p 3/2 sulfide	162.1 (A)	D	G
	S 2p 1/2 sulfide	A +1.2	D x 0.61	G x 1.12
	S 2p 3/2 oxy-sulfide	163.2 (B)	E	G x 1.2
	S 2p 1/2 oxy-sulfide	B +1.2	E x 0.61	G x 1.32
	S 2p 3/2 sulfate	169.4 (C)	F	H
	S 2p 1/2 sulfate	C +1.2	F x 0.61	H x 1.12

Table V . 8 :

 V8 XPS parameters used in the contributions of each molybdenum phase. A-H referenced to Table V.7.

		Binding energy (± ± ± ± 0.1 eV) Peak area Width at half height
	S 2s sulfide	226.6 (I)	M	Q
	S 2s oxy-sulfide	I + (B-A)	M x E/D	Q x 1.2
	S 2s sulfate	I + (C-A)	M x F/D	Q x H/G
	MoS 2 3d 5/2	229.4 (J)	N	R
	MoS 2 3d 3/2	J + 3.1	N x 0.7	R x 1.2
	Mo 6+ 3d 5/2	230.6 (K)	O	S
	Mo 6+ 3d 3/2	K + 3.1	O x 0.6	S x 1.1
	Mo 5+ 3d 5/2	233.2 (L)	P	T
	Mo 5+ 3d 3/2	L +3.1	P x 0.65	T x 1.1

Table V . 9 :

 V9 XPS parameters used in the contributions of each nickel chemical form.

		Binding energy (± ± ± ± 0.1 eV) Peak area Width at half height
	Ni 2+ 2p 3/2	856.0 (A)	B	C
	Ni 2+ 2p 1/2	A + 17.4	B x 0.38	C x 0.96
	Ni 2+ shup1	A + 6.1	B x 0.75	C x 1.8
	Ni 2+ shup2	A + 24.1	B x 0.69	C x 2.55
	Ni 2+ shup3	A + 10.7	B x 0.11	C x 1.39
	Ni 2+ shup4	A + 2.2	B x 0.24	C x 1.24
	Ni 2+ shup5	A + 19.1	B x 0.17	C x 1.14
	Ni Sulf 2p 3/2	853.1 (D)	E	F
	Ni Sulf 2p 1/2	D + 17.3	E x 0.5	F x 1.46
	Ni Sulf shup1	D + 1.8	E x 0.58	F x 2.04
	Ni Sulf shup2	D + 5.8	E x 0.42	F x 3.18
	Ni Sulf shup3	D + 19.3	E x 0.4	F x 2.98
	Ni Sulf shup4	D + 23.9	E x 0.5	F x 4.65
	NiMoS 2p 3/2	D + 0.9	G	H
	NiMoS 2p 1/2	D + 18.2	G x 0.52	H x 1.41
	NiMoS shup1	D + 8.1	G x 0.23	H x 2.20
	NiMoS shup2	D + 23.5	G x 0.30	H x 3.33
	NiMoS shup3	D + 14.9	G x 0.09	H x 2.23
	NiMoS shup4	D + 4.6	G x 0.05	H x 1.59

Table V .

 V 10: XPS parameters used in the contributions of each cobalt chemical form.

		Binding energy (± ± ± ± 0.1 eV) Peak area Width at half height
	Co 2+ 2p 3/2	781.5 (A)	B	C
	Co 2+ 2p 1/2	A + 15.9	B x 0.42	C x 1.02
	Co 2+ shup1	A + 2.1	B x 0.27	C x 0.92
	Co 2+ shup2	A + 5.1	B x 0.94	C x 1.97
	Co 2+ shup3	A + 9.9	B x 0.01	C x 0.49
	Co 2+ shup4	A + 17.5	B x 0.14	C x 1.08
	Co 2+ shup5	A + 21.7	B x 0.47	C x 1.62
	Co 2+ shup6	A + 25.2	B x 0.04	C x 0.99
	Co 2+ LMM	A + 4.3	B x 0.12	C x 1.46
	Co Sulf 2p 3/2	778.1 (D)	E	F
	Co Sulf 2p 1/2	D + 15.1	E x 0.53	F x 1.30
	Co Sulf shup1	D + 2.0	E x 0.6	F x 2.16
	Co Sulf shup2	D + 17.8	E x 0.21	F x 1.56
	Co Sulf shup3	D + 23.0	E x 0.3	F x 3.80
	CoMoS 2p 3/2	D + 0.50	G	H
	CoMoS 2p 1/2	D + 15.4	G x 0.5	H x 1.45
	CoMoS shup1	D + 1.8	G x 0.15	H x 1.13
	CoMoS shup2	D + 5.7	G x 0.06	H x 2.26
	CoMoS shup3	D + 17.3	G x 0.15	H x 1.66
	CoMoS shup4	D + 22.0	G x 0.18	H x 3.17

a This model was developed for unpromoted bulk catalysts, based on DBT catalytic conversion results, which limits its applicability.

P NMR measurements

rHYD (molTol(atMoSulfh)-1

Based on the work published in US

2011/0081416, and Jammaer , J. et al., Journal of Materials Chemistry 2009, 19, 8290-8293.

Chapter I Bibliographic Study

γ Nomenclature Impregnation over HPMo@SBA-15 catalysts: %Pr/HPMo@SBA-15-Y-Z, where (2 or 5) represents the estimated NiO or CoO relative amount at the final catalyst, Y (C -calcined or CEcalcined and extracted) stands for the final treatment of the hybrid material, before impregnation, and Z (C -calcined or D -dried) is the final treatment, after impregnation. In general, as in the previous Chapter, the "@" represents an encapsulation at the SBA-15 walls. Here, one has Pr/HPMo@SBA-15 since Pr is impregnated over a hybrid with HPMo encapsulated at the walls of a SBA-15 material.

III.2.2 Promoted reference catalysts

Two reference catalyst families were prepared, using different active phase precursors: (i) classical Ni(NO 3 ) 2 or Co(NO 3 ) 2 precursors were impregnated together with H 3 PMo 12 O 40 , as the traditional catalysts, and (ii) NiMoP and CoMoP polyoxometalates were impregnated in order to study the effect of the proximity between the Ni/Co atoms and the molybdenum in the active phase.

III.2.2.1 Co-impregnated catalysts, using HPMo and Ni(NO3)2 or Co(NO3)2 precursors

For promoted reference catalysts, the determined weight of metal precursorscorresponding to a promoter/Mo molar ratio of 0.12 or 0.36, was dissolved in distilled water, and the SBA-15 like support was impregnated with this solution. These ratios where used in order to compare these catalysts with material solids prepared from molecular NiMoP or CoMoP POM precursors impregnation, having similar composition (cf. section III.2.2.2). The final solids were maturated, dried and optionally calcined following the same operational method than hybridbased catalysts.

The final catalysts were named according to the promoter nature, loading and final treatment, as follows and as detailed in Table III.1 and in Chapter V, section V.3 Glossary.

Nomenclature

Co-impregnated catalysts:

%Pr/HPMo/SBA-15-Y, where (2 or 5) represents the estimated NiO or CoO relative amount at the final catalyst and Y (C -calcined or D -dried) stands for the final treatment. In general, the "/" represents an impregnation over the SBA-15 support. Here, one has Pr/HPMo/SBA-15 since Pr and HPMo are both impregnated in the same solution, over the SBA-15 support. Similar textural properties derived from XRD and N 2 sorption were observed for the materials after calcination (cf. Table III.3 and section III.6.2.1 in Appendix III, pg.178). It is still worth noting that these materials did not show any crystallites in the XRD wide range angles, suggesting the absence of crystallites or clusters formation. In fact, the SBA-15 support has a specific surface area large enough to prevent clusters agglomeration (for the impregnated loading Mo ∼ 8 wt.%). Calculated by a 0 -pore size. [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR spectroscopy was used to assess the state of the active phase precursors in the impregnated catalysts. The spectra of dried and calcined co-impregnated reference catalysts are presented in Figure III.3. The Ni-promoted catalysts present a peak at ca. -3.5 ppm, slightly broadened relatively to the H 3 PMo 12 O 40 , which can be due to the Ni 2+ counter-ion. After calcination, this peak was also observed, meaning that the Keggin structure of the HPMo acid was reformulated by simply exposition to air, with the water present in the atmosphere. Conversely, the 2%Co/HPMo/SBA-15 catalysts (dried and calcined) presented broader resonances centered at ca. -3.5 ppm, which may be due to the cobalt paramagnetic character, as already observed by other authors. 2 In addition, the 5%Co/HPMo/SBA-15 catalysts showed no resonances in the (1000; -1000 ppm) range, which could suggest the absence of Keggin structures, as it might be considered from the absence of peaks at -3.5 ppm or near 400 ppm (cf.

V.2.2 Promoted POM-based catalysts

Wetness impregnation was performed to prepare the reference catalysts and to complete the two-step synthesis of hybrid catalysts.

V.2.2.1 Promoted reference impregnated catalysts

In Figure V.2 are schematized the impregnations performed over the SBA-15 type silica supports. The co-impregnated reference catalysts nomenclature is listed in Table V.2.

Catalyst

V.3 Ex-situ catalyst sulfidation protocol

The active phase of HDT catalysts is known as MoS 2 slabs, which may be decorated with Ni or Co atoms on the slabs edges (NiMoS or CoMoS). To obtain these active phase, sulfidation of the oxide (or other) precursors is necessary. For analysis purposes, the catalysts were activated by means of an ex-situ gas-phase sulfidation. The catalysts were activated in a sulfidation frame, consisting of a glass reactor, a furnace with controlled temperature, power and gas pumping systems under low vacuum. The reactor, represented in Figure V.4, presents a glass cell equipped with a filter, a glass bulb for catalyst recovery and two gas connections. These connections are equipped with Teflon screw-caps to isolate the sulfidation cell. The temperature is controlled by a thermocouple inserted into the thermocouple shaft. Typically, 0.5 g of catalysts was placed inside the sulfidation cell. The reactor was placed inside the oven and connected to the gas lines (inlet, outlet). The gas flow was determined according to the mass of catalyst (2 l/h/g catalyst ), and to the reduction gas composition -usually H 2 S/H 2 15 v/v%. This gas flow insures at least a 5-fold excess sulfur in relation to MoS 2 stoichiometry.

Once established the gas flow, the samples were submitted to a temperature rise from RT to 350 °C at 5 ºC/min. At this stage the molybdenum and promoters species are reduced to the corresponding sulfurs, giving rise to the active slabs. After a 2 hours dwell, the temperature was decreased at 12 °C/min to 250 °C, and the H 2 S/H 2 feed was interrupted and replaced by an argon flow. The cell remained 2 hours at this temperature, to evacuate the H 2 S physically adsorbed at the catalyst surface and present in the reactor. Afterwards, the temperature decreased to 120 °C at a 20 °C/min rate to finish the process. The gas flow was then interrupted, and the system was submitted to a low vacuum in order to eliminate the gas phase. The gas inlets were then closed with two screw-caps, to isolate the cell from the exterior. A primary vacuum was then applied, in Perspective towards an enhanced HDT catalyst based on the findings of this project will now be explored in the next section, namely:

Milder synthesis calcination conditions Sulfidation Post-synthetic treatment in order to introduce Ni and Mo in proximal vicinity

Perspectives Perspectives

During the three years of this PhD project, several ideas showed up as possible solutions for the restrictions encountered in the development of our innovative materials. Part of these ideas has already been explored into some extent. However, the short delay to develop the experimental method and the delay of certain techniques extremely helpful to understand all the phenomena behind the catalyst conception, did not allow the conclusion of these recent theories. Therefore, we present, here, as perspectives the most important clues that came out from our work in order to continue raising and answering questions, or simply to develop new ideas that could be interesting not only at an academic level but also at an industrial level.

Synthesis calcination of hybrid material

The calcination of as-synthesized hybrid materials may lead to the partial or complete decomposition of encapsulated POM, as indeed observed in Chapters II and III. Thus, a milder calcination, performed at 130 °C for 72 hours, was considered and experienced over a nonpromoted hybrid, referred to as HPMo@SBA-15 C130. This hybrid catalyst presented 12 wt.% of molybdenum content. [START_REF] Tops›e | Hydrotreating Catalysis -Science and Technology[END_REF] P NMR and Raman spectroscopies confirmed the presence of the 12-phosphomolybdic acid, and, as expected, MoO 3 crystallites were not observed. XRD measurements showed a highly ordered mesostructure, without crystallized species in the wide-angles range. Toluene hydrogenation reaction has shown 20% higher performances than HPMo@SBA-15 CE catalyst, expressed in mol Tol (at Mo h) -1 , suggesting a better sulfidation, dispersion and/or accessibility of the active MoS 2 slabs, which must be confirmed by XPS, XAS and TEM analyses.

Sulfidation of hybrid material

As we have already seen and discussed, hybrid HPMo@SBA-15 based-materials are harder to sulfide, when compared to reference impregnated HPMo/SBA-15 catalysts. In order to achieve the same degree of global sulfidation, a hybrid catalyst has to be submitted to severer sulfidation conditions: higher temperature, longer sulfidation treatments, as confirmed in this work with the HPMo@SBA-15 CE catalyst. However, this limitation for hybrid catalysts should be explored meticulously in order to obtain the optimized sulfidation conditions, not only for non-promoted catalysts but also when nickel or cobalt are present and able to form NiMoS or CoMoS active slabs, which certainly requires different operating conditions.

Promotion of hybrid material

The promotion of hybrid materials has been performed by an incipient wetness impregnation of traditional Ni(NO 3 ) 2 or Co(NO 3 ) 2 precursors over the Mo-containing hybrid materials. Nevertheless, we have seen that this method is not well adapted to hybrid materials.

When faced to this situation, we have envisioned several methods to obtain the active phase precursors in close proximity to molybdenum within the silica framework.

The main interest in using POM as active phase precursors consists in the proximity of principal metal (Mo) and its promoter (Ni or Co) within the same structure. These POMs, such as Pr 1,5 PMo 12 O 40 or Pr 3 PMo 11 PrO 40 H, are stable at a pH range varying between 3 to 5.2. The one-pot synthesis applied to synthesize HPMo@SBA-15 based-catalysts was carried out in highly acidic medium (pH ≤ 1), which is not compatible with the stability range of these modified CoMoP or NiMoP polyoxometalates.

Consequently, we were forced to change the strategy, in order to obtain Ni/Co atoms next to Mo atoms inside the silica walls before catalysts sulfidation. Several directions were explored, and will be briefly described, in order to a better understanding of our choice to overcome this promotion restraint.

Synthesis One-Pot -quasi-neutral pH 1

The first adopted strategy was based on the concept of using a buffer solution of citric acid and trisodium nitrate to maintain the pH nearly stable at ca. 4.0. Sodium silicate was used as silica source, and the pH was adjusted several times with a NaOH solution, in order to keep the pH range between 3.0 and 5.2. Pr 3 PMo 11 PrO 40 H was then added dropwise. However, only HPMo@SBA-15 materials were obtained. The promoter, was found to form soluble citrate salts (such as C 12 H 10 Ni 3 O 14 ), which were filtered off with the solution. XRF measurements did not evidence the presence of Ni or Co.

Post-synthesis treatment: reacting H 3 PMo 12 O 40 towards Pr 3 PMo 11 PrO 40 H compounds

HPMo@SBA-15 CE hybrid HPMo@SBA-15 CE catalyst was immerged in a NaOH solution, with a pH near 4.0. Ni(NO 3 ) 2 precursor was then added to the suspension. The solution turned light green immediately, as the solid turned white, indicating complete leaching of all encapsulated HPMo from the silica walls, as confirmed by XRF measurements.

HPMo@SBA-15 As-synthesized -room temperature

After a regular synthesis at pH below 1.0, the resulting as-synthesized hybrid was washed and filtered. The idea was to recover this hybrid and dip it in a NaOH solution at room temperature (NH 3 was also experimented), with controlled pH in the range of 3.0 to 5.2 to generate lacunary structures. Then, promoter precursors were added, and left to interact with the hybrid material from 1 hour up to 24 hours. Nevertheless, XRF analysis revealed that no Ni was present in the solid after the treatment.