Keywords:

Preparation and characterization of the supported oxidic precursors 1B2.

General Introduction 6

Chapter 1 Synthesis of mixed H4SiMonW12-nO40 (n = 1 and 3) heteropolyacids using lacunar salts 61B2.2. 1.3 One-step synthesis of mixed H4SiMonW12-nO40 (n = 6 and 9) heteropolyacids

54B2.2.2
Characterization of H4SiMonW12-nO40 Keggin- Transmission electron microscopy (TEM) 128

General introduction

Recently, much attention has been paid to the quality of the produced fuel, especially diesel and gasoline. As the world demand for these types of fuel increases every year, environmental standards for diesel fuel have noticeably tightened, especially concerning the content of total sulfur (≤10 ppm) and polyaromatic hydrocarbons (≤11 wt.%). At the same time, the period of light oil is coming to an end and lately oil refining has been paying more and more attention to the use of heavy viscous sour oil sources, with an increase of the volume of heavy sulfur and high-sulfur oils to be treated [1]. In this regard, hydrotreating process is still one of the most important catalytic processes in oil refining.

Deep hydrotreating of petroleum fractions to produce ultra-clean fuels is possible only in the presence of highly efficient catalysts.

For more than a century, academic and industrial hydrotreating catalysts have been based on a general composition -Co(Ni)Mo(W)S/Al2O3. The development of researches on sulfide catalysis has made possible to increase the activity of catalysts tenfold and to create hundreds of brands of industrial hydrotreating catalysts. Recently, there has been a growing interest in the use of mixed metal sulfides for hydrotreating catalysts, which may include two or more structural-forming metals, such as molybdenum [2,3], tungsten [4] and niobium [5].

The development of the trimetallic NiMoWS bulk catalyst «Nebula» from Albemarle was a real breakthrough in oil refining. This catalyst exhibits activity at least three times higher than the best conventional supported hydrotreating catalyst used in industry [6]. According to the results of many studies, it was found that the specific catalytic activity of bulk catalysts is several times higher than the supported one [4,7,8]. However, bulk catalysts are much more expensive than supported ones and are used only in batch loading together with supported ones, which should also have high activity.

In addition to the modification of the active phase, research is underway on the use of new materials for support, such as mesostructured silica (MCM-48 [START_REF] Harris | Catalysis by transition metal sulfides: A theoretical and experimental study of the relation between the synergic systems and the binary transition metal sulfides[END_REF], SBA-15 (16), etc), carbon, zeolites and their alumina composite materials (Al2O3-B2O3, Al2O3-TiO2, Al2O3-SBA-15, etc). Among them, mesostructured silica have been explored as support for hydrotreating catalysts, their main advantage being their high thermal stability and well-developed texture [9,10].

But even now, most of the developed catalytic systems do not cope with modern tasks, such as the deterioration of the quality of raw materials and the tightening of environmental standards.

Therefore, the development of new highly active catalysts for hydrofining petroleum fractions is a real challenge for both science and industry.

The aim of this work is the development of (Ni)MoW catalysts prepared from mixed Keggintype SiMonW12-n heteropolyacids as precursors, with a large range of Mo/W ratio, ranging from 1/11 to 9/3. The effect of the Mo/(Mo +W) molar ratio on the activity of the prepared MoW and NiMoW bulk and supported catalysts in hydrotreating of model and real feedstocks will be explored.

The thesis consists of five chapters. Chapter I provides an overview of the literature on the role of hydrotreatment in oil refinery, main compounds and target reactions of hydrotreating. The composition and structure of supported and bulk hydrotreating catalysts are described in detail, as well as methods for their preparation, the structure of the active phase and active sites. The advantages and potential of using mixed MoW sulfided systems are considered. The use of heteropolyanions as promising oxide precursors is described.

Chapter II focuses on a new method for the synthesis of mixed H4SiMonW12-nO40 heteropolyacids (HPA) with high molybdenum content (Mo/W=6/6 and 9/3), as well as advanced physico-chemical characterization of starting precursors and corresponding prepared catalysts supported on alumina. The catalysts were tested in model hydrotreating reactions (DBT HDS and naphthalene HYD). The composition of the active phase was studied by X-ray photoelectron spectroscopy (XPS) and the crystallite structure was analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The influence of the Mo/W ratio together with the nature of the precursors (mixed HPA or mixture of monometallic HPAs) on the active phase and catalytic activity is considered.

Chapter III presents studies of the influence of the atomic Mo/W ratio on the catalytic activity of Ni-promoted MoW/Al2O3 catalysts tested in hydrodesulfurization of dibenzothiophene and naphthalene hydrogenation with the addition of nitrogen-containing compound and hydrotreating of a straight-run diesel fraction. Catalyst samples were analyzed by TEM and XPS, and the composition and structure of the active phase were studied by extended X-ray absorption fine structure (EXAFS), and HAADF. The main goal of this section is to select the optimal catalyst composition for the process of hydrotreating diesel fractions, taking into account the effect of inhibition by nitrogencontaining components.

Chapter IV is devoted to the preparation of unsupported MoW catalysts by HF acid etching of the support and the investigation of their physicochemical and catalytic properties in hydrodesulfurization of dibenzothiophene and naphthalene hydrogenation. This chapter presented the results of a study of the composition and structure of non-supported catalysts by EXAFS and ToF-SIMS.

Chapter V examined the possibility of replacing traditional alumina with a more promising support, such as mesostructured silica, in order to improve catalytic properties. The obtained samples were tested in the reactions of DBT HDS and HYD of naphthalene.

Chapter 1

Literature review Introduction

The first chapter provides an analysis of the scientific literature as well as the general context of the present study. The beginning of the section is devoted to general problems of oil refining and, in particular, the process of hydrotreating. The main aspects of the hydrotreating are considered, such as the purpose of the process, the main reactions and the role of the catalyst. The composition and structure of traditional catalysts, as well as the main directions of their modernization, such as the introduction of additives, replacement of the support and the use of new precursors of the active phase are described. The second part of this chapter deals with the development of mixed MoW catalytic systems, studies of mixed active phase including simultaneously two structure-forming metals (Mo and W) together with the synthesis of mixed MoW precursors such as heperopolyacids. Finally, the main methods of promoted and non-promoted unsupported catalysts synthesis as well as successful attempts to introduce bulk catalysts in industry, are then reported. The main conclusions obtained from the state of the art are summarized and allow to draw the objectives of the thesis devoted to the development of (Ni)MoW supported and bulk sulfide catalysts for hydrotreatment.

The development of hydrotreating processes 1.2.1 The role and purpose of hydrogenation processes in oil refining

Hydrotreating is the most used process for refining of components of crude oil over the last 60 years [1]. The constant growth in the consumption of liquid hydrocarbons and the tightening of environmental standards are the main reasons for the high rates of development of the hydrotreating process. According to the U.S. Energy Information Administration for 2018, global liquid fuels demand will increase more than 20% in 2050 [2] (Fig. 1.1), and the demand for motor fuels will reach 107 million barrels per day [3]. Chapter 1

Analysts predict an increase in consumption of diesel fuel, which means that the hydrotreating process will be in demand in the future. Straight-run distillates such as kerosene and gas oil cannot be used directly as motor fuels due to several technical and chemical limitations such as high amounts of impurities (sulfur, nitrogen, and aromatics) and low cetane numbers in the case of diesel fuels.

According to the worldwide refining survey for 2010, hydrotreatment has the largest processing capacity among all refining operations (45, 4 mil. barrels per calendar day) [4]. In the structure of the refinery, this process plays a key role in the production of all types of liquid fuel (Fig. 1.2). Usually, the hydrotreating process can be split into two major areas: (i) postprocessing of distillates and (ii) feed pretreatment for conversion processes such as isomerization, catalytic reforming, catalytic cracking. At the same time, there has been a steady trend towards the involvement of heavy secondary fractions in the hydrorefining process. Light cycle oil, thermal cracking gas oil, gasoline and light delayed coking gas oil, as well as heavy straight-run fractions are widely involved in hydrotreating of diesel fractions for the last 15-20 years [5]. This trend is due to the fact that plants are seeking to increase the production of light petroleum products. Secondary distillates are subjected to more difficult hydrotreating than that of straight-run fractions due to the presence of a large content of Chapter 1

unsaturated hydrocarbons (aromatic, olefinic, diolefinic, etc.), polynuclear aromatics (PNA), asphaltresinous substances, heavy sulfur compounds of cyclic structure [16,17]. The sulfur removal and hydrogenation of polynuclear aromatics (PNA) are of primary concern because these compounds are among the major targets of environmental regulations.

Main compounds and target reactions of hydrotreating

The content of organic sulfur and polyaromatic compounds primarily depends on the composition and type of raw materials. Many studies of the composition of organic sulfur compounds, including the content of organic sulfur compounds in straight-run gas oils, have been carried out. It was found that for the kerosene and gas oil middle distillate fractions, thiophenic compounds containing benzo-and dibenzothiophene structures dominate. Increasing the molecular weight and the number of aromatic rings in the structure reduces the reactivity in the HDS reaction (Table 1.1)

as evidenced by the decrease of the pseudo-first-rate constant, because the aromatic structures become more stable increasingly [9]. Among the numerous reactions occurring during HDS, the slowest are the hydrogenolysis reactions of DBT and its derivatives.

Table 1. 1 The reactivities of the different types of S-compounds [10] *Catalyst: Co/Mo/Al2O3 (CoO, 3.7%; MoO3, 13%; SiO2, 1.8%); LHSV: 6 h-1; reactor type: trickle bed; feed: 10 wt.% sulfur in tetralin.

The transformation of S-containing molecules with aromatic rings occurs through two parallel reactions (Fig. 1.4.): for HYD route, the primary stage is the hydrogenation of one of the aromatic rings of dibenzothiophene, and then hydrogenolysis of the C -S bond to form cyclohexylbenzene (CHB); during DDS route the hydrogenolysis of C-S-C bonds occurs, leading to the formation of biphenyl (BP) [21][22][23]. The HYD pathway is the predominant one for unpromoted catalysts in DBT HDS, while the DDS pathway is the predominant one for promoted catalysts. The greatest attention should be paid to dibenzothiophene (DBT) with alkyl substituents, which can reduce the reactivity in the HDS reaction, especially when they are adjacent to the S-atom in the dibenzothiophene core, due to steric effects. [16,24,25]. As the alkyl substituents approach the sulfur heteroatom, the reactivity of the component decreases ( Sulfur compounds with alkyl substituents at the 4-and 6-positions are the most resistant to desulfurization due to the steric hindrance around the sulfur atom. Such structure makes the σ-type interaction between the sulfur and the vacancy difficult [16]. In contrast, 2,8-and 3,6alkyldibenzothiophenes are almost as reactive as dibenzothiophenes, and some of the alkylsubstituted ones are even more reactive, an effect attributed to increased electron density on the S atom (inductive effect) [23,27,28].

As mentioned earlier, another important indicator of the quality of the resulting fuels is the content of polycyclic aromatic hydrocarbons. Moreover, the presence of aromatic hydrocarbons has a negative effect on the hydrodesulfurization reaction due to competitive adsorption [1,29]. The content of aromatic hydrocarbons in straight-run gas oil is approximately 25-30% and depends on the composition of the oil itself [21]. The involvement of secondary fractions in hydroprocessing significantly increases the content of aromatic hydrocarbons in the feed. For example, light cycle oil (LCO) from the FCC can contain up to 70% aromatic hydrocarbons [1,24,30]. Hydrogenation of PNA compounds (Fig. 1.6) [22] proceeds sequentially, from ring to ring. Chapter 1

The kinetic study showed that each subsequent ring is much more difficult to hydrogenate than the previous one [30,32]. The arrangement of the rings also affects the hydrogenation activity of aromatic hydrocarbons. The hydrogenation rate increases in the following order:

It was found that the presence of alkyl substituents on the rings of aromatics hardly affects the strength of adsorption [24]. In addition, PNAs are able to undergo polycondensation reactions, which lead to the formation of coke on the catalyst surface. Coking of the catalyst is one of the main reasons for the decrease in its activity.

Nitrogen-containing organic compounds have a strong influence on both HDS and HYD reactions. Interest in the study of the inhibitory effect of nitrogen-containing compounds increased when the need arose for the production of fuels with ultra-low sulfur content. The straight-run diesel fraction can contain up to 300 ppm of nitrogen [19], but when secondary processes are involved in gas oil hydrotreating, this content can significantly increase [25]. At the moment, the nitrogen content in fuel is not regulated by standards, but the presence of even a small amount (~30 ppm) of Ncontaining compounds in the feed significantly reduces the depth of sulfur removal [23]. Basic nitrogen compounds such as quinoline and acridine have the greatest inhibitory effect [35,36]. In many works it was noted that hydrogenation reactions are mainly inhibited by nitrogen compounds due to their stronger adsorption [36][37][START_REF] Hagenbach | Physicochemical investigations and catalytic activity measurements on crystallized molydbenum sulfide-cobalt sulfide mixed catalysts[END_REF]. In addition, the study of model compounds showed that hydrodenitrogenation of various cyclic nitrogen-containing compounds proceeds along the path of preliminary hydrogenation of the nitrogen-containing ring, and only then the cleavage of the C-N bond with the formation of an aliphatic amine, which is rapidly converted into ammonia and hydrocarbon [24,[START_REF] Grange | The role of cobalt and molybdenum sulphides in hydrodesulphurisation catalysts: A review[END_REF][START_REF] Topsøe | Characterization of the structures and active sites in sulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts by NO chemisorption[END_REF]. For this reason, it is necessary to consider not only the HYD and HDS activity, but also to take into account the effect of nitrogen-containing inhibitors in the development and research of hydrotreating catalysts.

Hydrotreating sulfide catalysts

The composition and structure of the active phase

The active phase of hydrotreating sulfide catalysts consist in small MoS2 (WS2) crystallites (Fig. 1.7) with a layered structure. Several structural models for promoted and unpromoted Mo(W)S2

were proposed and described in the literature [START_REF] Harris | Catalysis by transition metal sulfides: A theoretical and experimental study of the relation between the synergic systems and the binary transition metal sulfides[END_REF][START_REF] Eijsbouts | MoS2 structures in high-activity hydrotreating catalysts. I. Semi-quantitative method for evaluation of transmission electron microscopy results. Correlations between hydrodesulfurization and hydrodenitrogenation activities and MoS2 dispersion[END_REF][START_REF] Paul | Vacancy formation on MoS2 hydrodesulfurization catalyst: DFT study of the mechanism[END_REF][START_REF] Cristol | Theoretical study of benzothiophene hydrodesulfurization on MoS2[END_REF].

Relative reactivity

Top view Side view

Chapter 1

sulfur covalent bonds on catalytic activity. In the type I "CoMoS phase", substantial interaction with the support was observed. This interaction decreases in type II "CoMoS phase", and the nature of the metal-sulfur bonds becomes more covalent, leading to an increase in catalyst activity in the HDS reaction.

On active phase and active sites of unpromoted hydrotreating catalysts

Each Mo atom is surrounded by six sulfur atoms in trigonal-prismatic coordination inside the crystallite (Fig. 1.9), where each sulfur atom is strongly bonded to three Mo atoms [14]. It is known that active centers are located on the edges of the MoS2 (WS2) slabs and contain coordination unsaturated molybdenum atoms and sulfur atoms. In Fig. 1.9, the structure of molybdenum disulfide in the bonds form allow to present in detail the surface atoms of sulfur and molybdenum. On the metallic edge (Me edge), each Mo atom is coordinated to four sulfur atoms, and on the sulfur edge (S edge), each S atom is coordinated to two Mo atoms.

It was found that under the reaction conditions, these edges are unstable [START_REF] Nagai | Poisoning effect of nitrogen compounds on dibenzothiophene hydrodesulfurization on sulfided NiMo/Al2O3 catalysts and relation to gas-phase basicity[END_REF][START_REF] Song | New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization[END_REF][START_REF] Moses | The hydrogenation and direct desulfurization reaction pathway in thiophene hydrodesulfurization over MoS2 catalysts at realistic conditions: A density functional study[END_REF]: hydrogen can react with sulfur located on S edges, forming anion vacancies or coordination-unsaturated sites (CUS)

-molybdenum atoms, and hydrogen sulfide can dissociate after adsorption on the CUS on Me edges.

Fig. 1.9 Hexagonal MoS2 crystallite exposing Mo-and S-edges (left) [START_REF] Sun | Theoretical investigations of the structures and properties of molybdenum-based sulfide catalysts[END_REF] and Cell showing the MoS2 surface, stable upon classical reaction conditions (right) [START_REF] Cristol | Theoretical study of the MoSi (100) surface: A chemical potential analysis of sulfur and hydrogen coverage[END_REF] First, it was assumed that the centers of hydrogenation and direct desulfurization were the same [START_REF] Zeuthen | Kinetics for simultaneous HDS, HDN and hydrogenation model reactions on a Co-Mo/Al2O3 catalyst[END_REF]. Then, two different types of active centers were involved in HDS reactions. According to the "rim-edge" model [START_REF] Daage | Structure-function relations in molybdenum sulfide catalysts: The 'Rim-Edge' model[END_REF] (Fig. 1.10) developed for bulk catalysts and later adapted for supported catalysts, the centers of the HDS and HYD reactions have a similar structure, but a different location:

the HDS and HYD active centers are on the rim edges and the HDS centers are located on the edges of the MoS2 crystallites. Salmeron et al. [START_REF] Salmeron | The adsorption and binding of thiophene, butene and H2S on the basal plane of MoS2 single crystals[END_REF] found that sulfides of the basal plane do not participate in hydrogenation and hydrogenolysis reactions and are almost inactive. that results in improved catalytic activity. A further increase in electron density enhances the interaction between the products and the catalyst, which leads to a decrease in catalytic activity.

The influence of the support on the catalytic activity

The nature of the support has a great influence on the dispersion of the oxide and sulfide phases of catalysts, the sulfidation of the deposited precursor, the composition of particles on the surface, the morphology of active phase particles, catalytic properties and selectivity, deactivation, etc. There are numerous types of supports reported in literature: carbon, simple oxides (Al2O3, SiO2, TiO2, ZrO2), binary oxides (TiO2-Al2O3, TiO2-ZrO2, etc.), silica-alumina, zeolites and clays used in hydroprocessing. However, most of them did not find industrial application [START_REF] Breysse | Recent studies on the preparation, activation and design of active phases and supports of hydrotreating catalysts[END_REF], because of impairing drawbacks. For example, most mesoporous carbons have poor crushing strengths, low bulk density or low surface area [START_REF] Vissers | Carboncovered alumina as a support for sulfide catalysts[END_REF] .

It was recognized very early that TiO2 and ZrO2 could impart four to five times higher activities in Mo(W) catalysts in comparison to alumina. But at the same time these supports systematically lose their advantages over alumina for the promoted catalysts [START_REF] Trong On | Perspectives in catalytic applications of mesostructured materials[END_REF][START_REF] Bagshaw | Templating of mesoporous molecular sieves by nonionic polyethylene oxide surfactants[END_REF].

Silica-alumina is promising support for HDS as their Brønsted acidity can improve HDS conversion of refractory compounds such as 4,6-DMDBT by enhancing the hydrogenation activity Chapter 1

of the active phase or by suppressing the steric hindrance brought by the alkyl groups, through previous isomerization [START_REF] Sánchez-Minero | Kinetic study of the HDS of 4,6-DMDBT over NiMo/Al2O3-SiO2(x) catalysts[END_REF]. On the other hand, the high acid properties of these catalysts tend to accelerate the formation of coke on the surface of the catalysts leading to deactivation [START_REF] Muralidhar | Catalytic functionalities of supported sulfides. I. Effect of support and additives on the CoMo catalyst[END_REF]. Moreover, the presence of strong Brønsted sites also induces significant cracking [START_REF] Breysse | Recent studies on the preparation, activation and design of active phases and supports of hydrotreating catalysts[END_REF]. The difficulty in obtaining well dispersed active phase species of HDS catalysts supported on zeolites and the small size of the zeolite pores will impede the application of zeolites as support for hydrotreatment [START_REF] Richard | Reaction mechanism of 4,6-dimethyldibenzothiophene desulfurization over sulfided NiMoP/Al2O3-zeolite catalysts[END_REF].

The absolute leader among hydrotreating catalyst carriers is γ-Al2O3, which has a developed surface, the possibility to control the porous structure, high bulk density, thermal resistance and strength and ability to undergo regeneration [START_REF] Rana | Effect of alumina preparation on hydrodemetallization and hydrodesulfurization of Maya crude[END_REF].

Mesostructured silica as a promising support for hydrotreating catalysts

In more and more works of researchers, various meso-structured silica materials are proposed as support [START_REF] Dhar | Physico-chemical characterization and catalysis on SBA-15 supported molybdenum hydrotreating catalysts[END_REF][START_REF] Kokliukhin | CoMo Hydrotreating Catalysts Supported on Al2O3, SiO2 and SBA-15 Prepared from Single Co2Mo10-Heteropolyacid: In Search of Self-Promotion Effect[END_REF][START_REF] Wang | Hydrodesulfurization of dibenzothiophene over siliceous MCM-41-supported catalysts: I. Sulfided Co-Mo catalysts[END_REF][START_REF] Mendoza-Nieto | Support effect on the catalytic performance of trimetallic NiMoW catalysts prepared with citric acid in HDS of dibenzothiophenes[END_REF].

Trong On et al. [START_REF] Trong On | Perspectives in catalytic applications of mesostructured materials[END_REF] divided all the most well-known mesostructured silica (MSS) into three large groups obtained following different synthetic procedures. The first one is so-called M41S family of silica and aluminosilicates, prepared using ionic surfactants, such as cetyltrimethylammonium bromide (CTAB) as structuring agents. The second group (HMS and MSU) was developed by Pinnavaia and co-workers, who produced these materials using two neutral routes based on hydrogen bonding and self-assembly of non-ionic primary amines such as hexadecyl amine or polyethylene oxide (EO) surfactants and neutral oligomeric silica precursors S0I0 [START_REF] Vradman | High loading of short WS2 slabs inside SBA-15: Promotion with nickel and performance in hydrodesulfurization and hydrogenation[END_REF][START_REF] Gutiérrez | SBA-15 supports modified by Ti and Zr grafting for NiMo hydrodesulfurization catalysts[END_REF][START_REF] Saleh | Applying Nanotechnology to the Desulfurization Process in Petroleum Engineering[END_REF]. Finally, the third group of silicates (such as SBA-15 and COK-12) was developed by Stucky et al. [START_REF] Zhao | Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores[END_REF] and Jammaer et al.

[72] respectively, using di-and tri-block copolymers as organic structuring agents. These materials, with hexagonal (p6mm) structure, have long-range order and thick walls (typically between 3 and 9 nm) which make them thermally and hydrothermally more stable than previous materials. These findings were attributed to the optimal morphology of sulfide particles and the Co/Mo promotion degree due to the more suitable interaction between the active phase species and the support resulting in an increase of the stacking number and decrease of the average slab length of the CoMoS species located inside of mesoporous silicate channels as compared to the SiO2 supported Chapter 1

samples. Ordered mesoporous materials, modified or not by incorporation of different heteroatoms (Al, Ti or Zr), were shown to present attractive properties (high surface area, large pore volume and thermal stability) leading to high performance HDT catalysts [START_REF] Sigurdson | Phosphorus promoted trimetallic NiMoW/γ-Al2O3 sulfide catalysts in gas oil hydrotreating[END_REF][START_REF] An | A study on the morphology of unsupported Ni-Mo-W sulfide hydrotreating catalysts through high-resolution transmission electron microscopy[END_REF][START_REF] Huirache-Acuña | Comparison of the morphology and HDS activity of ternary Ni(Co)-Mo-W catalysts supported on Al-HMS and Al-SBA-16 substrates[END_REF][START_REF] Mendoza-Nieto | Development of new trimetallic NiMoW catalysts supported on SBA-15 for deep hydrodesulfurization[END_REF].

Supported MoWS catalysts

Recently, in the field of sulfide catalysis the attention of researchers was paid to the creation of mixed catalytic systems including two structure-forming atoms, such as molybdenum and tungsten.

However, on the way of designing such systems, many obstacles arise, such as the stronger interaction of W species with the support than that of molybdenum [START_REF] Saleh | Applying Nanotechnology to the Desulfurization Process in Petroleum Engineering[END_REF] or lower sulfidation degree of tungsten oxide than that of molybdenum oxide, due to the stronger W-O bond strength [START_REF] Van Der Vlies | Chemical principles of the sulfidation of tungsten oxides[END_REF]. But research is ongoing and first results have already been achieved. Moreover, the temperature of the sulfidation increase also contributes to the formation of slabs with a random distribution of atoms, which was explained by the sintering of particles together with the formation of slabs of greater length. Each structure was evaluated in only one reaction: the coreshell structure in the thiophene HDS reaction, the random structure -in the DBT HDS reaction, but in both cases, no pronounced synergistic effect between Mo and W was observed. More interesting data were obtained in in HDS of gas oil, according to the results of which it was found that mixed NiMo0.75W0.25 was the most active, which shows the potential of mixed systems [START_REF] Van Haandel | Structure-Activity Correlations in Hydrodesulfurization Reactions over Ni-Promoted MoxW(1-x)S2/Al2O3 Catalysts[END_REF].

Sigurdson et al. [START_REF] Sigurdson | Phosphorus promoted trimetallic NiMoW/γ-Al2O3 sulfide catalysts in gas oil hydrotreating[END_REF] reported that the catalytic activity in HDN and HDS reactions increases when phosphorus is added to NiMoW/Al2O3 as a promoter thanks to the P doping improving the dispersion and number of surface-active sites. A series of P-doped catalysts with different amount of P (0-2.5 wt.%) were prepared by impregnation of alumina with aqueous solution (pH ∼ 4) containing

Chapter 1

ammonium heptamolybdate, ammonium tungstate, nickel nitrate. and phosphoric acid. The catalysts were tested under conditions of industrial hydrotreatment of light coking gas oil in a trickle bed reactor. The phosphorous content of 1.6 wt.% is optimal and the trimetallic catalyst exhibits better hydrotreating activity than bimetallic and commercial catalysts.

Various types of studies of mixed MoW systems can be found in the literature, and most of them are related to synthesis using traditional precursors of the active phase, such as AHM, AHT, AMT and ammonium tungstate [START_REF] Cabello | Influence of cobalt content on the hydrotreatment catalytic activity of CoMo6/γ-Al2O3 heteropolyoxomolybdate-based catalyst[END_REF][START_REF] Cabello | Decamolybdodicobaltate(III) heteropolyanion: Structural, spectroscopical, thermal and hydrotreating catalytic properties[END_REF][START_REF] Lamonier | Molybdocobaltate cobalt salts: New starting materials for hydrotreating catalysts[END_REF]. Heteropolyanions are complex inorganic compounds, which composition can be described as an assembly of oxygen polyhedra (octahedra and tetrahedra) of limited extent obtained by sharing one or more oxo (or hydroxo) ligands, the polyhedra being joined at their corners, edges or faces [START_REF] Toulhoat | Prediction of optimal catalysts for a given chemical reaction[END_REF].

The stability of the HPA is influenced primarily by the composition of the ligand sphere: HPA with W is much more stable than HPA with Mo [START_REF] Pope | Heteropoly and isopoly oxometalates[END_REF].

Heteropolyanions as precursor for hydrotreating catalysts

Hetoropolyanions can be derived from the acids (heteropoly acids) and corresponding salts (Co, Ni, etc.) (heteropoly compounds). Heteropolyanions are favorable for the preparation of catalysts because of the uniform adsorption of HPA on the surface of the support [START_REF] Kozhevnikov | Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions[END_REF][START_REF] Lizama | Highly active deep HDS catalysts prepared using Mo and W heteropolyacids supported on SBA-15[END_REF], the possibility to

I II III

Chapter 1

avoid the calcination step due to the absence of NH4 + , NO3 -counterions (if metallic salts of heteropolyanions are used for example) and a higher sulfidation degree of metals, which contributes to an increase in catalytic activity.

The main advantage of Anderson HPA is the presence of the 3d-metal as heteroatom in the composition of the HPA, including the traditional promoters of Ni (Co) or Fe [START_REF] Pope | Heteropoly and isopoly oxometalates[END_REF]. The literature data [START_REF] Trong On | Perspectives in catalytic applications of mesostructured materials[END_REF][START_REF] Keggin | The structure and formula of 12-phosphotungstic acid[END_REF][START_REF] Tomina | Influence of Anderson and Keggin heteropoly compounds as precursors of oxide phases in hydrotreating catalysts on their performance[END_REF] shows that the activity of catalysts synthesized from HPA in the thiophene HDS depends on the molar ratio of promoter/molybdenum [START_REF] Trong On | Perspectives in catalytic applications of mesostructured materials[END_REF][START_REF] Keggin | The structure and formula of 12-phosphotungstic acid[END_REF] In turn, the Keggin type HPA and its derivatives (Fig. 14 I) are more stable than Anderson HPC, accessible and important for catalysis [START_REF] Spojakina | Thiophene conversion and ethanol oxidation on SiO2-supported 12-PMoV-mixed heteropoly compounds[END_REF][START_REF] Nikulshina | Molecular approach to prepare mixed MoW alumina supported hydrotreatment catalysts using H4SiMonW12-nO40 heteropolyacids[END_REF]. In the Keggin-type heteropolyanions, one non-metallic atom, such as P, Si, As, B, etc., exists in a regular tetrahedron, combined with 12 MO6

octahedra (where M is Mo VI , W VI , V V ), which are connected by shared edges to form trimetallic M3O13 groups joined together by their vertices [START_REF] Keggin | The structure and formula of 12-phosphotungstic acid[END_REF]. Depending on the synthesis conditions, various isomers (α, β, etc) can be isolated.

It has been shown that supported phosphomolybdic heteropolyacid (H3PMo12O40) and its Co and Ni salts are efficient oxide precursors for thiophene HDS [START_REF] Damyanova | Surface properties of titania-supported 12molybdophosphoric acid hydrodesulphurization catalysts[END_REF]. Moreover, Ni promoted molybdenum and tungsten HPA-based catalysts showed better performance in deep HDS of 4,6-DMDBT [START_REF] Lizama | Highly active deep HDS catalysts prepared using Mo and W heteropolyacids supported on SBA-15[END_REF] and diesel hydrotreating [START_REF] Tomina | Influence of Anderson and Keggin heteropoly compounds as precursors of oxide phases in hydrotreating catalysts on their performance[END_REF] than the counterparts prepared from traditionally used Mo(W) ammonium salts.

Blanchard et al. have shown that catalysts obtained from the PMo-HPA with Keggin structure are more active in the thiophene HDS than those prepared from AHM and H3PO4 due to a better metal dispersion and the absence of ammonium counterions [START_REF] Blanchard | New insight in the preparation of alumina supported hydrotreatment oxidic precursors: A molecular approach[END_REF]. Griboval et al. [START_REF] Griboval | Alumina supported HDS catalysts prepared by impregnation with new heteropolycompounds. Comparison with catalysts prepared by conventional Co-Mo-P coimpregnation[END_REF] showed by using of SiMo12 and PMo12HPAs that the nature of the heteroatom (Si or P) of these HPAs does not affect the thiophene HDS catalytic activity of the obtained catalysts.

Mixed MoW-heteropolyanions as interesting precursors for hydrotreating catalysts

The literature also describes the synthesis of mixed HPA with Keggin structure, containing two structure-forming metals at once. Mixed heteropolyanions, such as [PVxMo12-xO40] (3+x)-where (x= 1-3) [START_REF] Pope | Heteropoly and isopoly oxometalates[END_REF], [PVxW12-xO40] (3+x)-where (x= 1-4) [START_REF] Devers | Hydrothermal syntheses and catalytic properties of dispersed molybdenum sulfides[END_REF][START_REF] Li | Hydrothermal synthesis of MoS2 nanowires[END_REF], were described and prepared for different applications. Mixed PMo(12-x)VxO40 (3+x)-generally used in the field of mild oxidation catalysis have been prepared for hydrodemetallation catalysis [START_REF] Soogund | New Mo-V based oxidic precursor for the hydrotreatment of residues[END_REF]. Spojakina et al. [START_REF] Spojakina | Thiophene conversion and ethanol oxidation on SiO2-supported 12-PMoV-mixed heteropoly compounds[END_REF] also reported that catalyst Chapter 1 [START_REF] Pol | Micro to nano conversion: A one-step, environmentally friendly, solid state, bulk fabrication of WS2 and MoS2 nanoplates[END_REF][START_REF] Uchida | Preparation of MoS2 Catalyst for Ultra-deep Desulfurization of Diesel Oil by a Media Agitation Mill. Effect of Media Filling Ratio[END_REF]) (GS -gas phase sulfidation; LS -liquid phase sulfidation)

Unsupported (bulk) sulfide hydrotreating catalysts

Recently, the number of studies aimed at the development of unsupported sulfide hydrotreating catalysts has significantly increased. Currently, new catalysts appeared in the market, which contains very large amounts of active components and can be considered as bulk catalysts. The metal composition of the active phase in bulk catalysts is the same than that in the supported one, but at the same time, their hydrotreating volume activity may exceed two or more times that of supported catalysts [START_REF] Zhang | Study on high-performance unsupported Ni-Mo-W hydrotreating catalyst[END_REF]. The concentration of the active component can vary from 80% (if various additives and modifiers are used or if the role of the support is played by inactive sulfide) to 100% (pure bulk catalyst).

Methods for the synthesis of unsupported sulfide hydrotreating catalysts

The literature has described various methods for preparing bulk catalysts. One of the first methods for growing nano-sized MoS2 particles inside inverse micellar cages in nonaqueous solvents was proposed more than 20 years ago and described by Wilcoxon and Samara [START_REF] Wilcoxon | Strong quantum-size effects in a layered semiconductor: MoS2 nanoclusters[END_REF]. The MoS2 clusters are formed by first dissolving a molybdenum (IV) halide salt in solution with micellar cages and then combining this solution with another inverse micelle solution containing a sulfiding agent (e.g., metal sulfide or H2S). The authors managed to get MoS2 nanoparticles ranging in size from 2 to 15 nm. The size of the particles depends on the size of the initial micelles containing salt of Mo(IV).

Methods of hydrothermal [START_REF] Li | Application of HF etching in a HRTEM study of supported MoS2 catalysts[END_REF][START_REF] Varakin | Comparable investigation of unsupported MoS2 hydrodesulfurization catalysts prepared by different techniques: Advantages of support leaching method[END_REF] and solvothermal [START_REF] Richardson | Electronic properties of unsupported cobalt-promoted molybdenum sulfide[END_REF][START_REF] Licea | Unsupported trimetallic Ni(Co)-Mo-W sulphide catalysts prepared from mixed oxides: Characterisation and catalytic tests for simultaneous tetralin HDA and dibenzothiophene HDS reactions[END_REF] processes for the synthesis of bulk catalysts have also been described. Devers et al. [START_REF] Devers | Hydrothermal syntheses and catalytic properties of dispersed molybdenum sulfides[END_REF] synthesized unsupported MoS2 by Chapter 1 hydrothermal method using (NH4)2MoS4 (ATTM) as Mo source. The catalysts consisted in multilayered slabs with an average length of 5 to 50 nm and a surface area (from 50 to 100 m 2 /g) depending on the synthesis conditions (temperature and acidity of the reaction mixture). It was found that samples with a large particle length exhibit a higher activity in hydrogenation (HYD), while samples with a low particle length are more active in hydrodesulfurization (HDS). Vrinat and co-workers [START_REF] Rueda | Highly dispersed, nonsupported molybdenum sulfides[END_REF] investigated bulk molybdenum disulfide obtained by the solvothermal method using molybdenum naphthenate as precursor and n-hexadecane or 1-methylnaphthalene as hydrocarbon solution. The samples had a high surface area (more than 200 m 2 /g) and showed higher catalytic activity in thiophene HDS than the sample obtained by the hydrothermal method. A variable amount of a carbonaceous phase was detected in catalysts (6-8 wt.% of the total solids). It has been suggested that the coke phase originates from the organic-molybdenum precursor, while the organic solvent is not involved in the formation of a carbonaceous phase. The authors did not establish a direct dependence of the catalytic activity on the surface area. However, changes in the texture of coke can affect the accessibility of reagents to the MoS2 surface and, therefore, the catalytic activity can depend on the interface between coke and molybdenum sulfide [START_REF] Rueda | Highly dispersed, nonsupported molybdenum sulfides[END_REF].

Polyakov et al. [START_REF] Polyakov | Mechanochemical activation of MoS2-Surface properties and catalytic activities in hydrogenation and isomerization of alkenes and in H2/D2 exchange[END_REF] reported the method of high temperature decomposition of ATTM. The catalysts were tested in the model reactions of hydrogenation and isomerization of alkenes, as well as deuterium exchange. It was found that MoS2 microcrystalline obtained at high-temperature was completely inactive in hydrocarbon transformations (isomerization of alkenes and ethylene hydrogenation) and H2/D2 scrambling, but after high-energy ball milling was transformed into an active catalyst. The activity of MoS2 correlates with the degree of structural defects such as split nanoslab projecting from an aggregation of bent nanoslabs and stepwise truncations of the layers (Fig

1.17.).

Fig. 1.17 structural defects of MoS2 crystallites. Adapted from [START_REF] Polyakov | Mechanochemical activation of MoS2-Surface properties and catalytic activities in hydrogenation and isomerization of alkenes and in H2/D2 exchange[END_REF] Chapter 1

The direct method of producing WS2 and MoS2 nanoplates in one stage in environmentally friendly conditions was also described in [START_REF] Pol | Micro to nano conversion: A one-step, environmentally friendly, solid state, bulk fabrication of WS2 and MoS2 nanoplates[END_REF]. The process was realized in stainless steel reactor at room temperature in a nitrogen filled glovebox using elemental sulfur, tungsten and molybdenum powders (all micrometer sized). The temperature of the tube furnace was raised to 750 °C at a rate of 15°C/min and the temperature maintained at 750°C for 1-3 h. The sizes (diameter) of the WS2 and MoS2 particles were 1-5 μm and 3-10 μm respectively.

A different interesting method for the synthesis of unsupported catalysts has also been described [START_REF] Yoosuk | Effects of preparation conditions in hydrothermal synthesis of highly active unsupported NiMo sulfide catalysts for simultaneous hydrodesulfurization of dibenzothiophene and 4,6dimethyldibenzothiophene[END_REF][START_REF] Pollack | High-reflectance and single layer MoS2: two new forms[END_REF]. The catalysts were prepared from purified molybdenite MoS2 mineral in an activation mill. The grinding conditions were as follows: the raw MoS2 and the grinding media with a diameter of 1 mm in the ratio of 1/75, butanol as the milling additive, at an agitating rate of 2600 rpm for 10 h in an argon atmosphere. The ground MoS2 powder containing a high concentration of crystalline defects, dislocations or discontinuities in the random layer structure, can make it more active in the hydrodesulfurization reaction. The catalysts were evaluated in the model reaction of hydrogenolysis of dibenzothiophene, as well as hydrotreatment of heavy oil fraction. It was found that bulk MoS2 catalyzed the desulfurization of 4,6-DMDBT more than the industrial catalyst under the equivalent molybdenum weight, while as for DBT HDS it was inferior to the industrial sample [START_REF] Kouzu | Micro-crystalline molybdenum sulfide prepared by mechanical milling as an unsupported model catalyst for the hydrodesulfurization of diesel fuel[END_REF].

Li et al [START_REF] Li | Application of HF etching in a HRTEM study of supported MoS2 catalysts[END_REF] used the HF etching for MoS2 structural studies removing Al2O3 from supported MoS2 catalysts. The application of HF etching may affect the size distribution of the slabs and change the size distribution of the MoS2 phase to more stacking and longer particle size, no matter what kind of metal loading or precursors is used.

Varakin et al. [START_REF] Varakin | Comparable investigation of unsupported MoS2 hydrodesulfurization catalysts prepared by different techniques: Advantages of support leaching method[END_REF] used the same method to prepare bulk catalysts (Et-MoS2 and Et-MoS2/C) by removal of alumina using HF etching (Et) from supported MoS2/Al2O3 and MoS2/C/Al2O3. These bulk systems had greater catalytic activity in DBT HDS than the samples obtained by decomposition of ATTM (Ref-MoS2), solvothermal synthesis using a hexadecane/water mixture in the volume ratio of 1/3 at 350 °C and under 4 or 14 MPa of hydrogen (4-MoS2, 14-MoS2) and hydrothermal synthesis with surfactants Triton X114 (Trit-MoS2) and chitosan (Chit-MoS2). In bulk catalysts, as in supported catalysts, the morphology of the active phase particles plays an important role. The authors, using the 3D diagram for rate constant kHDS as a function of average length and average stacking number (Fig

1.
18), demonstrated that the maximum HDS activity is achieved with a good dispersion of the active phase (the shortest length and the largest stacking), which corresponds to the catalysts obtained by etching the support. It was also found that nitrogen content in bulk catalysts, which the samples acquired during synthesis, strongly impacted catalytic activity. The authors suggested that incomplete removal or presence of ammonium from ATTM during the activation process had led to the nitrogen incorporation into the MoS2 crystallite and, as a result, inhibited activity in DBT HDS. In the case of Et-MoS2 and Et-MoS2/C samples, it was assumed that nitrogen comes from the support after its peptization in nitric acid.

Promoted unsupported sulfide catalysts

Introduction of the promoter always represents a delicate procedure in order to form a good composite of MoS2/WS2 and Co/Ni salt to obtain high active multi-metal unsupported catalysts with high surface area [START_REF] Zhang | Study on high-performance unsupported Ni-Mo-W hydrotreating catalyst[END_REF]. For this reason, more and more researches are aimed at creating bi-and polymetallic unsupported catalysts.

More than thirty years ago, Richardson [START_REF] Richardson | Electronic properties of unsupported cobalt-promoted molybdenum sulfide[END_REF] proposed a method for the preparation of promoted unsupported sulfide catalysts. Unsupported molybdenum sulfide was obtained by precipitation of ammonium heptamolybdate (NH4)6Mo7O24, which was then air-dried and cobalt acetate solution was further added to impregnate the samples with the desired compositions. It was found that the HDS activity of the samples is directly dependent on the concentration of cobalt.

The Co(Ni)MoS2 catalyst without carrier was proposed in [START_REF] Licea | Unsupported trimetallic Ni(Co)-Mo-W sulphide catalysts prepared from mixed oxides: Characterisation and catalytic tests for simultaneous tetralin HDA and dibenzothiophene HDS reactions[END_REF] by hydrothermal synthesis using water and organic solvent. The samples showed a higher activity in hydrodesulfurization of 4,6-dibenzothiophene in comparison with the supported analogs. The catalysts had a good pore volume (VP = 0.72 m 3 /g) and a high specific surface area (SBET = 320 m 2 /g), while most bulk catalysts Chapter 1 traditionally have a low specific surface area, not exceeding 100 m 2 /g [7,[START_REF] Grange | Catalytic Hydrodesulfurization[END_REF][START_REF] Thakur | A comparison of some catalytic, structural and textural properties of unsupported MoS2 and WS2 hydrodesulfurization catalysts promoted by Group VIII metals[END_REF]. For this reason, various organic additives and surfactants are added during the synthesis of bulk catalysts [START_REF] Genuit | Solution syntheses of unsupported Co(Ni)-Mo-S hydrotreating catalysts[END_REF]. Later Yoosuk et al. [START_REF] Yoosuk | Effects of preparation conditions in hydrothermal synthesis of highly active unsupported NiMo sulfide catalysts for simultaneous hydrodesulfurization of dibenzothiophene and 4,6dimethyldibenzothiophene[END_REF] showed effects of preparation conditions in hydrothermal synthesis of highly active unsupported NiMo sulfide catalysts for simultaneous hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene. Studies were carried out in the temperature range from 300 to 375⁰C and pressures from 1.4 MPa to 3.4 MPa. The highest temperature and highest H2 pressure in the preparation led to an increase in surface area and pore volume of NiMo bulk catalysts, as well as to highest activity of the resulting catalyst in simultaneous HDS of DBT and 4,6-DMDBT.

Genuit et al [START_REF] Genuit | Solution syntheses of unsupported Co(Ni)-Mo-S hydrotreating catalysts[END_REF] described the methods for preparing Ni(Co)Mo bulk catalysts by solution reactions. In this work, different variations of the initial reagents and their influence on the catalytic activity were examined in detail. For the preparation of bulk promoted Ni(Co)Mo catalyst, ammonium thiodimolybdate (NH4)2Mo2S12 (TDM), Co(NO3)2 and acetylacetonate (Co(acac)2) and ethylene glycol (EG) as organic agent were used. Several surfactants of the same family (Triton X114, Triton X100, Tergitol NPX and Imbentin N 60) were tested as textural promoters. It was observed that impregnation of dispersed unsupported MoS2 catalysts with aqueous Co nitrate led to a poor promoting effect. The negative influence of Co nitrate was explained by the oxidative reaction of MoS2 by nitrate anions. The use of Co(acac)2 contributed to the improvement of the promoting effect.

It was also found that carbonaceous species formed through the decomposition of EG during the sulfiding treatment of the catalyst precursors disperse well over the sulfide particles, preventing MoS2 slabs from sintering. Earlier, Rueda et al. [START_REF] Rueda | Highly dispersed, nonsupported molybdenum sulfides[END_REF] showed that the use of organic additives, which are a source of carbon for catalysts, contributes to an increase in the surface area of unsupported catalysts, thereby increasing their HDS activity.

Pollack and co-workers [START_REF] Pollack | High-reflectance and single layer MoS2: two new forms[END_REF] first proposed the HF etching for structural studies of Ni-MoS2/Al2O3 supported catalysts. At that time, there were almost no ideas about the formation of a mixed NiMoS phase. Therefore, the authors suggested that only MoS2 and Ni9S8 is formed. Using the TEM analysis method, authors have established that molybdenum disulfide can form multilayer particles with an average length of 3 to 5 nm.

De la Rosa et al. [START_REF] De | Structural studies of catalytically stabilized model and industrialsupported hydrodesulfurization catalysts[END_REF] proposed the HF etching for structural studies of alumina and silica supported CoMo catalysts. HF can react and dissolve not only alumina, but also silica for eliminating the support. However, in their study, De la Rosa et al. found that the dissolution of the support by HF solution led to a reorganization of the MoS2 particles into less stacked slabs and partial destruction of the active phase with loss of cobalt [START_REF] De | Structural studies of catalytically stabilized model and industrialsupported hydrodesulfurization catalysts[END_REF].

Delvaux et al. [START_REF] Le | Solution synthesis of the unsupported Ni-W sulfide hydrotreating catalysts[END_REF][START_REF] Olivas | The catalytic activity of Ni/W bimetallic sulfide nanostructured catalysts in the hydrodesulfurization of dibenzothiophene[END_REF] conducted a large series of experiments devoted to the research of the promoting effect in unsupported CoMo catalysts obtained by the so-called "co-maceration method".

Chapter 1

Molybdenum oxide (MoO3) and cobalt oxide (Co3O4) were co-digested in an aqueous ammonium sulfide solution for 6 h at 70°C. The compounds obtained were subsequently heated under an H2S-Ar atmosphere for 4 h at 400°C before evacuation [START_REF] Delvaux | X-ray photoelectron spectroscopic study of unsupported cobalt-molybdenum sulfide catalysts[END_REF].

In the same time, several groups of researchers were studying the promoting effect in Ni-W bulk systems in the reactions of hydrogenation of unsaturated hydrocarbons and the hydrogenolysis of sulfur compounds [START_REF] Cristol | Theoretical study of benzothiophene hydrodesulfurization on MoS2[END_REF][START_REF] Wang | Ultra-deep hydrodesulfurization of diesel fuels on trimetallic NiMoW sulfide catalysts[END_REF]. The studies of the promoting effect in unsupported Ni-W and Co-Mo catalysts carried out by Delvaux et al. [START_REF] Delvaux | Influence of the preparation temperature on the physicochemical characteristics and catalytic properties of unsupported sulfided cobalt/molybdenum catalysts[END_REF] and Delmon et al. [START_REF] Thakur | A comparison of some catalytic, structural and textural properties of unsupported MoS2 and WS2 hydrodesulfurization catalysts promoted by Group VIII metals[END_REF] were systemized by Grange [START_REF] Grange | Catalytic Hydrodesulfurization[END_REF] and are summarized in At low cobalt concentrations, a layered structure is formed, the same as that of bulk MoS2. The first increase in k"HDS was attributed to the decoration of the MoS2 edges by the Co ions. Similar concepts have been proposed by Halbert et al. [START_REF] Halbert | Synthesis and activity of hydrotreating catalysts prepared via promotion by low-valent transition-metal complexes[END_REF] In their work, they suggested that the addition of cobalt has two effects. In the first case "electronic promotion," meaning that Co and Mo act together to create sites or vacancies that are more active than sites on either single component (pseudo-binary).

The second concept implies that when Co is added, there is a "structural stimulation" by the interaction of Co and Mo, which increases the phase dispersion, thereby increasing the activity.

The explanation for the second promotion effect for Co/(Co+Mo) = 0.2÷0.4 might be related to the formation of Co9S8 crystallites. Delmon and co-workers [START_REF] Hagenbach | Physicochemical investigations and catalytic activity measurements on crystallized molydbenum sulfide-cobalt sulfide mixed catalysts[END_REF] found that the unsupported catalysts, which had simultaneously MoS2 and Co9S8 phases present were able to work together by being in close contact and the hydrodesulfurization occurred at the interface between these two bulk sulfide phases, but there was no insight how these two bulk phases interacted to produce a synergistic effect.

Based on the correlation between the energy of the metal-sulfur bond calculated by DFT (density functional theory) and HDS performances evaluated in DBT HDS, Ramos et al. [START_REF] Ramos | HRTEM and molecular modeling of the MoS2-Co9S8 interface: Understanding the promotion effect in bulk HDS catalysts[END_REF] showed the model of the surface contact region between the two bulk Co9S8/MoS2 phases (Fig Mo also occurs through the intermediate sulfur atom bonded to both metals [START_REF] Ramos | HRTEM and molecular modeling of the MoS2-Co9S8 interface: Understanding the promotion effect in bulk HDS catalysts[END_REF].

The same synergistic effect was found for unsupported NiW catalysts. Le and co-workers [START_REF] Le | Solution synthesis of the unsupported Ni-W sulfide hydrotreating catalysts[END_REF] also supposed that the catalyst is represented by core shell of NiS particles covered by curved WS2 layers. But at the same time, Olivas et al. [START_REF] Olivas | The catalytic activity of Ni/W bimetallic sulfide nanostructured catalysts in the hydrodesulfurization of dibenzothiophene[END_REF] reported that a NiW catalyst containing mainly nanorods has a higher catalytic activity compared to a NiW catalyst containing nanoparticles.

Unsupported mixed (Ni)MoW catalysts

A new trimetallic NiMoW bulk catalyst was synthesized by Wang et al. [START_REF] Wang | Ultra-deep hydrodesulfurization of diesel fuels on trimetallic NiMoW sulfide catalysts[END_REF]. It was found that the NiMoW sulfide catalyst has a high HDS activity which is achieved mainly by forming a unique multilayered structure of the NiMoW sulfide, in which the Ni7S6 species with layered structure act as a support for Mo, W, or MoW sulfide particles. Furthermore, the formation of Mo-W composite species was confirmed by EXAFS. The Mo-W composite species could interact with Ni species to form Ni-Mo-W-S active species [START_REF] Wang | Ultra-deep hydrodesulfurization of diesel fuels on trimetallic NiMoW sulfide catalysts[END_REF]. Mixed Mo-W particles were formed only on the surface of The question of the cost of bulk catalysts and the method of their regeneration remains open. It is assumed that the bulk catalysts will have a high cost, since a large amount of precursor is consumed for synthesis in comparison with supported catalysts. However, as reported by manufacturers, bulk catalysts will have a short payback period, which makes them competitive.

After analyzing all the available information, we can conclude that the development of catalysts does not stand still, in particular the development of bulk catalysts. For this reason, the search, development and improvement of catalytic systems is an important task.

Chapter 1

Conclusion

Literature review has shown that mixed MoW supported and unsupported sulfides are efficient catalysts that revealed most of the time better catalytic performances than that of their monometallic Mo or W counterparts. Most of previously reported MoW mixed supported and bulk catalysts were prepared using as precursors (together or sequentially) ammonium heptamoybdate and ammonium metatungstate or ammonium thiomolybdate (NH4)2MoS4 and ammonium thiotungstate (NH4)2WS4.

The efficiency of mixed MoW supported catalysts was generally proven in HDS using alumina as the most choosen support. Particular attention was paid to the use of mesostructured silica. As noted in literature, mesostructured silica has several advantages over traditional Al2O3, such as high surface area, monomodal distribution of pore size and thermal stability. Moreover, sulfide catalysts based on mesostructured silica showed higher catalytic activity in the reactions of HDS and HYD.

Our work will be focused on MoW catalysts synthesized from mixed H4SiMonW12-nO40 heteropolyacids (HPA). Talking into account previous catalytic study showing that the increase of Mo/W ratio in mixed HPA leads to improved catalytic activity, we proposed to extend the known MoW series (with molar Mo/W ratio equal to 1/11 and 3/9) to new mixed HPA using Mo/W ratio equal to 9/3 and 6/6, which synthesis is not described in the literature. The use of MoW HPAs will allow to start from a single molecular entity to introduce together Mo and W. As suggested by literature, this initial nanoscale proximity of Mo and W could induce and facilitate after sulfidation the formation of the mixed (MoW)S2 phase which formation was claimed to explain better catalytic performances of MoW catalysts.

Taking into account the advantages and potential of using mixed MoW systems for hydrotreating of petroleum feedstock, MoW catalysts supported on mesostructured silica will be investigated. Special attention will be also paid to bulk catalysts, which have already found industrial application and have shown excellent results. The literature describes many methods for the synthesis of bulk catalysts. In the present work, new bulk mixed MoWS catalysts will be synthesized using the HF etching method. The physico-chemical properties of the catalysts obtained will be investigated by advanced characterization methods and their catalytic performances will be investigated in HDS and hydrogenation reactions under the conditions of a fixed-bed microreactor.

Introduction

The literature review allows concluding that the use of mixed MoW catalysts is a promising method for improving the hydrotreating process. Various types of mixed MoW systems can be found in the literature, which preparation mostly involves traditional precursors of the active phase, such as ammonium heptamolybdate (AHM), ammonium paratungstate (APT), ammonium metatungstate (AMT) and ammonium tungstate [1][2][3]. Heteropolycompounds, such as Keggin and Kegginderivated heteropolymolybdate and heteropolytungstate structures, can be used as starting oxidic precursor for highly active hydrotreating catalysts [4][5][6][7].

A method was previously proposed to synthesize MoW catalysts using mixed H4[SiMo1W11O40] and H4[SiMo3W9O40] Keggin-type heteropolyacids (HPA) prepared from corresponding lacunar salts, allowing to introduce both Mo and W metals in a catalyst from a single molecular precursor [8]. It was found that such catalysts are more efficient in DBT HDS and naphthalene HYD than their counterparts obtained from a mixture of monometallic H4SiMo12O40 and H4SiW12O40 HPAs with the same Mo/(Mo+W) ratio. The use of mixed HPAs as a precursor led to the formation of mixed MoWS2 sulfides, the composition and structure of which were studied in detail by HAADF and EXAFS analysis. [9,10]. Moreover, it was found that when increasing the Mo/W atomic ratios from 1/11 to 3/9, the catalytic activity also increases.

We thus intended to synthetize HPA with higher Mo/W ratio than the one previously studied, as we assumed that it may have a strong effect on the structure of the active phase and thus on the catalytic activity. Various mixed polyanions are mentioned in the literature, but most of synthesis cannot provide a high yield of the product and/or lead to the formation of a mixture of various mixed anions, which are almost impossible to separate. In this chapter, a new one-stage method was proposed for the synthesis of mixed HPAs with Mo/W ratio 6/6 and 9/3, based on monometallic H4SiMo12O40 and H4SiW12O40 HPAs without prior formation of lacunar salts. The structure of the obtained compounds was identified by single-crystalline XRD, IR and Raman spectroscopy.

Catalysts with different Mo/(Mo+W) atomic ratio were synthesized using corresponding mixed HPAs, characterized by advanced analysis methods and tested under the conditions of a micro-flow unit in model reactions of DBT HDS and naphthalene HYD. method described in reference [11]. 6 ml of solution of nitric acid (13 M) was added dropwise to 10 ml of sodium molybdate dihydrate solution (1 M), and then the dried K8[SiW11O39] salt was added in small portions with stirring. After 3 h stirring, the salt was isolated by filtration.

The mixed H4[SiMo3W9O40] HPA was obtained in almost the same way using the corresponding lacunar β-Na9[SiW9O34H] salt obtained by dissolving sodium metasilicate pentahydrate (12 g) and sodium tungstate dihydrate (150 g) in 250 mL of cold water [15]. Then, 95 ml of hydrochloric acid solution (6 M) was slowly added dropwise into the vigorously stirred solution.

If a suspension of silica is formed, the solution must be filtered. The purified solution was stirred for 1 hour and then transferred to a refrigerator (4℃) for 48 hours to crystallize the β-Na9[SiW9O34H] salt. The mixed potassium salt β-K8[SiMo3W9O39] was prepared following ref. [11]. 60 ml of hydrochloric acid (12 M) was added into 40 ml of sodium molybdate dihydrate solution (1 M) with stirring, and then β-Na9[SiW9O34H] salt (27 g) was added in small portions. The resulting solution was stirred for 1 hour. The mixed β-K8[SiMo3W9O39] salt was precipitated by the addition of potassium chloride (5 g) followed by filtration.

The corresponding mixed α-K4[SiMo1W11O40] and β-K8[SiMo3W9O39] salts were dissolved in distilled water and isolated by the "ether" extraction (described in section 2.1.1). HPAs were crystallized from an aqueous solution at a temperature of 4℃.

One-step synthesis of mixed H 4 SiMo n W 12-n O 40 (n = 6 and 9) heteropolyacids

The synthesis of polyvacant lacunar compounds of molybdenum or tungsten with a large number of vacant positions (more than three) with Si as a heteroatom did not appear possible to consider as a starting step to prepare the desired mixed compounds. The main problem lies in the isolation of pure high-lacunar W or Mo based compounds from solution for further synthesis of the corresponding mixed ones. To prepare mixed MoW heteropolyacids with high Mo/W ratio, we had to develop a new synthesis route. Several methods of preparation have been proposed in the literature.

In 1942, Brown and Frazer [16] proposed a method for the synthesis of mixed SiMonW12-nHPAs from sodium molybdate, sodium tungstate, sodium silicate and followed by the stepwise addition of hydrochloric acid. However, the work did not present any data confirming the formation of a "mixed" structure. In this study, attempts were made to reproduce this technique and analyze the compounds.

It was found that, a mixture of two HPA forms in the crystal, as was also observed when the corresponding monometallic H4SiMo12O40 and H4SiW12O40 HPAs were boiled. This section thus presents a one-step protocol for the synthesis of two new SiMonW12-n HPAs with Mo/W ratio equal to 6/6 and 9/3 using monometallic H4[SiMo12O40] and H4[SiW12O40] HPAs as starting materials:

-for Mo/W = 6/6: 4.12 g of SiMo12 and 6.21 g of SiW12 acids were dissolved in 40 ml of distilled water;

-for Mo/W = 9/3: 6.62 g of SiMo12 and 3.44 g of SiW12 acids were dissolved in 48 ml of distilled water.

Yellow solutions were obtained in both cases. The acid solutions were heated to 80℃ and kept for 1 h. After that, potassium hydroxide was added to each solution in small portions until pH reaches 4. After the addition of alkali, the solution became slightly greenish. Both solutions were boiled for another 30 min at 80℃ and then cooled down to room temperature. It was noticed that the mixed acid was sufficiently stable to be crystallized from aqueous solution by using the etherate method. The extraction and the crystallization were carried out according to the procedure described in section 2.1.1.

The elemental composition and structures of the mixed heteropolyacids were confirmed by XRD, IR and Raman spectroscopy. vibrations. The increase of the Mo content in SiMonW12-nO40 4-is reflected by a slight shift to lower wavenumbers for Mo(W)-Ot and Mo(W)-Ob-Mo(W) lines [12,17].

Characterization of H

Raman spectra of monometallic and mixed MoW HPAs (Fig. 2.2) are dominated by a main peak corresponding to symmetric νs(Mo(W)-Ot) terminal vibration [12,18,19]. For the high W content mixed HPA, SiMo1W11 and SiMo3W9 HPAs, the position of this line (1005 cm -1 ) is similar to that observed for SiW12O40 while a more important shift towards low wavenumbers is observed with the increase of Mo content, which, possibly, can be associated to a change in the bond strength between oxygen and metal. Previously, a similar effect was found in a work devoted to the synthesis of mixed potassium salt K4[β-SiMo3W9O40] [20]. An important difference is observed for vibrations νs(W-Oc-W) and νs(Mo-Oc-Mo), which correspond to peaks at 552 and 638 cm -1 . In the case of spectra for mixed HPAs, a weak peak is observed, which lies between 552 and 638 cm -1 , which may indicate the presence of different νs(Mo(W)-Oc-W(Mo)) vibrations.

Preparation and characterization of the supported oxidic precursors

The bimetallic MonW12-n catalysts were prepared by using the corresponding mixed H4[SiMonWn-12O40] HPAs and will be referred to as MonW12-n/Al2O3. Reference catalysts were also prepared using the impregnating solutions obtained by mixing SiMo12 and SiW12 in an aqueous solution with the same Mo/W ratio than the corresponding mixed HPAs and were denoted as Mon+W12-n/Al2O3. The catalysts with surface density of the metals d(Mo + W) equal to 3.9 at nm -2 were prepared by incipient wetness impregnation of alumina support [γ-Al2O3 (Norton), specific area: 240 m 2 g -1 , pore volume: 0.9 mL g -1 ]. To evidence the behavior of the HPAs after deposition on the support, Raman analysis was performed and obtained Raman spectra of the oxidic catalysts are shown in Fig. 2.6. For comparison purposes, Raman spectra of the oxidic precursors prepared from a mixture of monometallic HPA SiMo12 and SiW12 are also reported.

For precursors prepared from a mixture of HPAs, the main Raman line observed around 970 cm -1 and corresponding to Mo(W)-O terminal vibrations presents an important shoulder at 950 cm -1 , increasing with Mo content, witnessing the presence of Keggin entities with other species.

Mo12/Al2O3 presents the Raman spectrum of AlMo6O24H6 3-(AlMo6) Anderson species in agreement with results previously reported [21,22], with a main peak at 944 cm -1 . For oxidic catalysts prepared wavenumbers due to interaction with the support is observed [9]. According to the obtained spectral data, it can be assumed that the mixed MoW structure is preserved even after maturation and drying.

Characterization of supported Mo(W)/Al 2 O 3 sulfide catalysts

Before characterization, the catalysts were sulfided in a flow of H2S/H2 (10 vol. %) at atmospheric pressure and 400 °C for 2 h. For characterization of the active phase, sulfided catalysts were unloaded from the reactor for analysis in a glove box in an inert gas atmosphere (argon).

Transmission electron microscopy (TEM)

All catalysts were characterized by TEM to obtain more information about the dispersion of Moreover, the dispersion of the active phase of mixed MonW12-n catalysts decreased from 0.33 to 0.29 with an increasing Mo/W ratio from 3/9 to 9/3, while it increased from 0.29 to 0.32 for the ones based on the mixture of HPAs counterparts.

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)

The TEM method does not allow to determine the structure and arrangement of metals in the composition of the active phase, but gives only general morphological information about the sulfided phase. Contrariwise, HAADF allows visualization of the basal planes of the sulfide phase, where the Z-contrast between Mo and W atoms facilitates their discrimination. Typical HAADF images of sulfided Mo3W9/Al2O3, Mo6W6/Al2O3, Mo9W3/Al2O3 and W12/Al2O3 catalysts are presented in Fig.

2.7.

The overwhelming majority of clusters, both in the case of a monometallic W12/Al2O3 catalyst and in the case of mixed MonW12-n/Al2O3 samples, have an irregular shape, which may be associated to the interaction of the active phase with an inhomogeneous support surface. In the micrographs of the catalysts, apart from the main clusters, isolated metal atoms and groups of atoms (less than 1 nm in size) are observed, which are not observed in the TEM images.

Most attention was paid to the monolayer particles and their structure. For the monometallic W12/Al2O3 catalyst, particles with a homogeneous structure are observed in all images. For MoW catalysts, difference in contrast of the atoms in the slabs are clearly observed. The intensity in HAADF images, for a given thickness and density and depending on the collection angles, is a function of the atomic number (Z-number), being approximately proportional to Z 1.7 [25], as a result of which W atoms (Z=74) appear brighter than Mo atoms (Z= 42) in HAADF images (Fig 2 .8). This is confirmed by the intensity profiles obtained in a row of atoms, as illustrated for Mo3W9 sample in Fig. 2.8, where the ratio in intensities corresponds to the ratio of Z 1.7 . We can thus conclude that, as previously observed for Mo3W9/Al2O3 [10], the use of mixed SiMo6W6 and SiMo9W3 HPAs promotes the formation of particles with a mixed structure after gas-phase sulfidation. In catalysts prepared from a mixture of two monometallic SiMo12 and SiW12 HPAs, like for Mo3+W9 [10], a majority of monometallic particles is observed, together with very few mixed clusters. HAADF imaging, it can be concluded that mixed MoWS2 particles are observed for all mixed MoW samples but that the distribution of the atoms depends on the ratio of molybdenum and tungsten in the structure. Therefore, when the content of Mo increases from 50 (in Mo6W6/Al2O3 catalyst) to 75 at % (in Mo9W3/Al2O3), a transition occurs from an ordered core-shell structure to a disordered structure in which tungsten atoms are randomly located.

X-ray photoelectron spectroscopy (XPS)

Information about the composition of sulfide particles on the surface of the synthesized catalysts was obtained in detail by XPS. The XPS spectra of MonW12-n/Al2O3 and Mon+W12-n/Al2O3

were decomposed according to previous works [9,10] using the appropriate oxide and sulfided references as supported monometallic catalysts (Fig. Tungsten oxide sulfides at higher temperatures than molybdenum due to thermodynamics and lower reactivity [26]. For W analysis, as shown in The spectral characteristics of W5p3/2, W4f5/2, W4f7/2 are interdependent and simulated by doublets corresponding to W4f5/2, W4f7/2 core levels, using constraints in area, full width at half maximum (FWHM) and binding energies (BE). The W4f spectra contain three 4f doublets: the doublet with binding energies (BE) at 32.1 and 34.3 eV is associated to W4f7/2 and W4f5/2 of W 4+ species of the WS2 phase, the doublet with binding energies at 33.0 and 35.2 eV to W 5+ species of a WSxOy oxysulfide species, and finally the doublet with binding energies at 36.0 and 37.9 eV is correlated with W 6+ oxide species.

The results of the XPS decomposition for the metal fractions of molybdenum and tungsten species of the sulfided MonW12-n/Al2O3 and Mon+W12-n/Al2O3 are reported in Table 2.4. In all bimetallic catalysts, the sulfidation degree of molybdenum was found higher than in the monometallic Mo12 sample: from 70% in Mo12 to higher than 80% in all bimetallic samples. Replacing a quarter of the Mo atoms with W made it possible to increase the sulfidation degree by more than 20 rel. % compared to Mo12/Al2O3 catalyst. In parallel with this, tungsten sulfidation occurs, only 51% in W12/Al2O3, which is associated with a stronger interaction with the support and is consistent with the literature data [27][28][29]. Increase in the molybdenum content contributed to an increase in the degree of sulfidation of both metals in mixed MonW12-n/Al2O3 catalysts, the highest values for both Mo (91%) and W (85%) being obtained for Mo9W3/Al2O3 catalyst. It should also be noted that the sulfidation degree of Mo in mixed MonW12-n/Al2O3 catalysts was higher than in bimetallic Mon+W12-n/Al2O3 samples prepared from mixture of HPAs. Using in situ time-resolved characterizations of the sulfidation by H2S/H2 of the catalysts by Quick-XAS associated with MCR-ALS chemometric method, differences were revealed in sulfidation pathways and formation of intermediates between monometallic and mixed bimetallic Keggin HPAs based catalysts [30]. Previously, the authors suggested that the sulfidation of Mo3W9/Al2O3 catalysts begins with the formation of MoS2 nuclei, which contribute to the activation of H2S, which allows increasing the sulfidation degree of WO3 and raising the number of active sites [8] . According to XAS analysis, it was found that the proximity of metals in the H4[SiMo3W9O40] has a significant effect and changes the sulfidation kinetics of both tungsten (lowering the sulfiding temperature and increasing sulfidation degree) and molybdenum (the formation of intermediate products occurs at a higher temperature in comparison with the monometallic Mo12/Al2O3).

Influence of the Mo/(Mo+W) atomic ratio in the active phase on the catalytic properties

The catalytic activities of the sulfided MonW12-n/Al2O3 and Mon+W12-n/Al2O3 catalysts in co-HDT of DBT and naphthalene are presented in Table 2. 5 and Fig. 2.10. Increasing the molybdenum content up to Mo/(Mo+W) = 0.5, by replacing 1, 3 and 6 tungsten atoms by molybdenum ones, led to an increase in activity both in HDS and HYD, whatever the precursor

[ (W) ] (%) = (W) (W) (W) (W) × 100
С Mo(W)S = [Mo(W)S] × 𝐶𝐶(𝑀𝑀𝑀𝑀(𝑊𝑊)) 𝑇𝑇 , (2)
where 𝐶𝐶(𝑀𝑀𝑀𝑀(𝑊𝑊)) 𝑇𝑇 represents the effective concentration of Mo(W) determined by XPS (at.%).

The number of edge sites of the active phase was calculated as follows:

MoWS egde = 𝑊𝑊 ⋅ � С WS 2 𝐴𝐴𝑟𝑟 𝑊𝑊 + С MoS 2 𝐴𝐴𝑟𝑟 𝑀𝑀𝑀𝑀 � ⋅ 𝐷𝐷 ⋅ 3600 ( 3
)
where W is the weight of the catalyst (g); С WS 2 and С MoS 2 are the effective content of W and Mo in WS2 and MoS2 states, respectively (wt. %); D is the dispersion of MonW12-nS2 species; 𝐴𝐴𝑟𝑟 𝑊𝑊 and 𝐴𝐴𝑟𝑟 𝑀𝑀𝑀𝑀 are the standard atomic weight of tungsten (183.9 g/mol) and molybdenum (95.9 g/mol), respectively. equal, the key effect on the number of edge centers was exerted only by the sulfidation degree of metals. An increase in the molybdenum content led to an increase in the content of edge sites for both types of MoW catalysts due to the higher degree of sulfidation of both metals compared to monometallic ones (Table 2.4). However, the number of edge sites is similar for MoW catalysts and their Mo+W counterparts for each Mo/(Mo+W) ratio, indicating that the difference in performance between the two series is not related to a different number of active sites but could be due to a difference in the quality of the active sites. As confirmed by HAADF analysis and previously by XAS analysis on Mo3W9 catalyst, mixed MoWS2 slabs are observed in samples that have been synthesized from mixed HPAs [30], which was attributed to the close interaction of both metals in the mixed oxide precursor. The presence of this mixed structure in MoW catalysts resulted in higher activity both in HDS and HYD compared to catalysts Mo+W, where mainly monometallic slabs were observed (with an equivalent number of active sites). The structure of the mixed slabs seems to also XPS data showed that the sulfidation degree in bimetallic catalysts is higher than in monometallic ones. This effect is enhanced by the use of mixed HPAs, which is explained by the closer proximity of metal atoms at the molecular level in the precursor. The simultaneous presence of Mo and W leads to a modification of the metal sulfidation kinetics, according to previously obtained XAS results.

The results of catalytic tests showed that Mo6W6 and Mo9W3 alumina supported samples were more active in DBT HDS and naphthalene HYD reactions than their Mo+W references prepared using their monometallic counterparts. Similar quantity of edge sites was calculated for both MonW12-n and Mon+W12-n catalysts with the same Mo/Mo+W ratio, indicating that the difference in performance is more related to the presence of the mixed MoWS2 active phase in MoW catalysts. The maximum conversion of reagents in the studied reactions was achieved on Mo6W6/Al2O3 due to the formation of the optimal structure of mixed slabs rich in edge sites.

Introduction

Previous results have shown that the use of mixed SiMonW12-n HPAs makes it possible to increase the activity of MoW/Al2O3 catalysts through the formation of a mixed MoWS2 phase (chapter 2). However, the industry mainly uses promoted systems for the hydrotreating of real feedstock. Based on the literature review [1][2][3], it was shown that the promotion of bimetallic MoW catalysts by cobalt atoms is less effective than the promotion by nickel due to the higher binding energy metal-sulfur. Therefore, this chapter is devoted to the study of Ni promoted MoW catalysts supported on alumina, to investigate if the mixed phase observed on MoW catalysts is maintained in Ni promoted solids and could contribute to improved catalytic performance. The effect of Mo/W ratio between 1/11 and 9/3 on activity will also be considered.

The NiMonW12-n/Al2O3 samples were tested in model reactions (HDS, HYD and HDN), as well as in the hydrotreating of straight-run gas oil (SRGO) of West Siberian oil with boiling range 180-360°С, to evaluate the efficiency of catalysts on real feedstock. The inhibitory effect of nitrogen-containing compounds on a model mixture (DBT + naphthalene + quinolone) and real feed was also investigated. Prior analysis and catalytic evaluations, all catalysts were sulfided at hydrogen pressure by a mixture of dimethyl disulfide (DMDS) (6 wt. % of sulfur) in toluene. The catalysts were fully characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). For a more detailed study of the composition and structure of the active phase of the Ni-promoted systems, high angle annular dark field imaging (HAADF) and Quick X-ray absorption spectroscopy (XAS) were also used on selected systems.

All the obtained results were presented in an article published in the Journal of Catalysis (DOI:

10.1016/j.jcat.2021.02.019).

Synthesis of NiMo(W)/Al 2 O 3 catalysts

Catalysts with fixed surface density of metals d(Mo+W) equal to 4 at nm -2 and Ni/(Mo+W) ratio equal to 0.5 were synthesized by simultaneous impregnation of alumina [γ-Al2O3, specific area: 240 m 2 g -1 , pore volume: 0.9 mL g -1 ] via the wetness impregnation method using aqueous solution of HPAs, citric acid (CA) and nickel carbonate (CA/Ni molar ratio equal to 1:1). Four trimetallic catalysts were prepared using mixed HPAs (hereafter NiMonW12-n/Al2O3, n = 1, 3, 6 and 9).

Trimetallic reference samples with a Mo/W ratio corresponding to the composition of the mixed SiMonW12-n HPAs were synthesized using a mixture of monometallic Mo and W HPAs, and will be denoted Ni(Mon+W12-n)/Al2O3. The bimetallic catalysts based on monometallic H4[SiMo12O40] and H4[SiW12O40] HPAs were prepared for comparison and denoted as NiMo12/Al2O3 and NiW12/Al2O3, respectively. After maturation, the oxidic catalysts were dried at 60°C (4 h), 80°C (2 h) and 100°C (4 h) in air atmosphere without further calcination. The metal compositions in the solids were obtained using an EDX800HS Shimadzu X-ray fluorescence analyzer and are given in Table 3.1. For further investigation, all catalysts were sulfided with the same protocol than prior the catalytic test, at 3.0 MPa of hydrogen, 2 h -1 LHSV and a 300 NL L -1 volume ratio of hydrogen to feed by a mixture of dimethyl disulfide (DMDS) with 6 wt. % of sulfur in toluene at 240°С for 10 h and at 340°С for 8 h. For characterization of the active phase, sulfided catalysts were unloaded from the reactor for analysis in a glove box in an inert gas atmosphere (argon). The samples were transferred to glass vials with n-heptane to avoid reoxidation. The textural properties of the liquid phase sulfided samples are summarized in Table 3.1. All catalysts have close textural characteristics. The surface area varied slightly from 202 to 240 m 2 g -1 and the average pore volume was about 0.5 cm 3 g -1 . All catalysts present an average pore size around 8.4 nm.

Transmission electron microscopy (TEM)

Liquid sulfided NiMo(W)/Al2O3 catalysts were characterized by HRTEM in order to obtain information about the global dispersion of the active phase particles, without possible discrimination between Mo and W containing particles. Typical micrographs are presented in Fig. 3.1 and the average values of stacking degree and slab length of active phase particles in Table 3.2. For all catalysts, the average length and stacking varied from 3.4 to 4.1 nm and from 1.4 to 2.1, respectively.

NiMo12/Al2O3 had the slabs with the shortest length ~3.4 nm among all studied catalysts and an average stacking number of 1.6, corresponding to high dispersion of the active phase (0.34).

NiW12/Al2O3 catalyst consisted of slabs with an average length ~ 3.9 nm and stacking number ~ 2.1.

The Mo/(Mo+W) atomic ratio had almost no effect on the geometric characteristics of the active phase. By comparison with the active phase morphology of unpromoted catalysts, promotion of Mo(W)S2 phase by Ni is reported to decrease the average slab length due to the decorating action of the Ni atoms, which fixed on the edges of Mo(W)S2 during the formation of the disulfide slabs hindering their further growth [4][5][6]. However, in our case, the non-promoted were sulfided in gas phase, preventing direct comparison. in an oxidic environment, respectively [7 -9]. The decomposition of the XPS spectra revealed the relative amounts of nickel, molybdenum and tungsten species present on the surface of the sulfided catalysts, summarized in Table 3.3.

The sulfidation degree of Mo, expressed as the percentage of MoS2, varied from 60 to 74% for all catalysts and almost did not depend on the Mo/(Mo+W) atomic ratio. In mixed HPA based samples, as noted for unpromoted catalysts, Mo and W sulfidation degree appears slightly higher than in the corresponding reference samples, the difference being more significant for W (between 17 and 25%) than for Mo (below 15%). This difference in sulfidation of metals in MoW samples was explained for non-promoted solids by the difference in the sulfidation steps for Mo and W in MoW and Mo+W samples, which could affect the final sulfidation degree of both metals [10,11].

Shan et al. [12] also prepared NiMoW/Al2O3 catalysts and observed high sulfidation of metals. They claimed that it was due to the close interaction of Mo and W provided by the hydrothermal deposition method and its ability to generate highly dispersed W/Mo/Ni oxide species easier to reduce and to sulfide. The authors also noted in their work that the precursor has a direct effect on the interaction of metals.

In the trimetallic catalysts, the fraction of Ni species involved in NiMo(W)S active phase increased from 37% to 42 % in NiMo1W11 and NiMo3W9, respectively and then remains almost constant when increasing the Mo content. For Ni(Mo+W), the NiMoWS phase percentage remains between 35 and 39%. The values are always over that obtained in the NiW sample.

Except for the NiMo12 sample, the main species is always NiS (about or more 43% of the Ni relative percentage) together with 5-20% of Ni in oxidic environment. That means that more than 80 % of Ni is sulfided, which may be due to the addition of citric acid introduced in the impregnating solution for the preparation of the oxidic precursors. Citric acid chelates the Ni ions, to improve their dispersion of Ni species on the support, to decrease the interaction of the Ni ions with the support and thus to improve the sulfidation of Ni [6]. However, the excess of Ni that is not used for promotion is present as a pure NiS species that remain at the surface. At the same time, for catalysts based on mixed HPAs, the fraction of bulk nickel sulfide is slightly higher than for samples Ni(Mon+W12-n)/Al2O3, which in turn may be associated to an enhancement of the metallic properties of Mo due to the extra electrons that W provides to the unitary cell [13], which is why Ni atoms are more difficult to incorporate into the structure of the active phase. This also confirms the data that the bond between molybdenum and tungsten is retained even during sulfidation [10]. The high proportion of bulk nickel sulfide is also associated to the type of sulfidation. Eijsbouts et al. [14] noted in their work that Ni is more susceptible to segregation into Ni2S3 particles under conditions

Determination of the composition and structure of the active phase

Liquid phase sulfidation prior to catalytic tests was chosen to be close to the real conditions of hydrotreating reactions, as a consequence, the same type of activation was used before TEM and XPS analysis. To obtain information about the presence of mixed active phase in the promoted catalysts as observed in non-promoted ones, XAS and HAADF were considered. Unfortunately, at the time of the experiments, no liquid phase sulfidation could be followed by in situ XAS on the ROCK beamline at Soleil synchrotron.

We however decided to study selected NiMoW catalysts by this technique and by HAADF after gas phase sulfidation. Indeed, previous work comparing gas and liquid phase sulfidation of non-promoted MoW catalyst [15] evidenced that regardless of the activation type, mixed HPA based catalysts had high amount of mixed MoWS2 clusters, while samples based on a mixture of two monometallic HPAs had predominantly monometallic MoS2 and WS2 slabs, the only difference being the arrangement of the Mo and W atoms in the slabs. Based on this result, the main task of HAADF and EXAFS characterization in this part will be to determine how the addition of nickel to MoW systems affected the formation of the active phase, assuming that the effect will be the same whatever the sulfidation procedure.

The following paragraphs thus present the HAADF and XAS analysis of NiMo 3 W 9 /Al 2 O 3 and Ni(Mo 3 +W 9 )/Al 2 O 3 catalysts after sulfidation in a flow of 10 % H2S in H2 at atmospheric pressure with heating from room temperature to 400 °C with a heating rate of 3°C/min followed by a plateau of 2 h. catalyst is quite similar to the one observed for unpromoted (Mo3+W9)/Al2O3 sample sulfided under gas or liquid phase [15]. Hensen and co-workers [17] found that NiMo0.75W0.25/Al2O3 catalyst with a random distribution of Mo and W atoms in mixed MoWS2 slabs was formed in liquid phase sulfidation conditions and was more active in the gas oil hydrotreatment compared to NiMo/Al2O3. 3.4. At both Mo K-and W L3-edges, the first FT contribution with a maximum at 2.0 Å (not phase corrected), was attributed to the metal-sulfur first coordination. The coordination numbers of the Mo-S and W-S contributions varied from 5.4 to 5.9 and from 5.6 to 6.1, respectively, thus indicating the high sulfidation degree of metals (Table 3.4). In the case of unpromoted catalysts, the coordination

HAADF characterization of gas phase sulfided

Ni(Mo3+W9)/Al2O3

Probe position/nm Intensity/a.u.

Chapter 3

numbers of the Mo-S and W-S were lower and varied from 5.1 to 5.3 and from 4.3 to 4.8, respectively.

The contribution around 2.9 Å (not phase corrected) at both edges was assigned to metal backscatter within Mo(W)S2 structure. A noticeable decrease can be seen in the second coordination shell when comparing trimetallic samples to bimetallic NiMo and NiW references (Fig. 3.6), particularly for mixed HPA based catalyst. The same behavior was already observed for unpromoted samples [11], the second coordination shell was more modified for the catalysts prepared from Summarizing, the HAADF and XAS data showed that the promotion by Ni did not prevent the formation of mixed MoWS2 slabs during gas-phase sulfiding, and based on previous results for unpromoted systems [15], the same observation is assumed for liquid activation, despite possible differences in the arrangement of Mo and W atoms inside the plates. By HAADF it was visualized that the mixed slabs are formed with small Mo islands surrounded by W atoms under gas phase sulfidation. However, the active phase of the catalyst prepared from separate monometallic HPAs consisted mainly of monometallic sulfide particles. The changes of the Ni K-edge XANES spectra confirm the formation of NiMo(W)S phase along with nickel sulfide. Chapter 3

Catalytic tests in hydrotreating of model and real feeds

All the prepared promoted catalysts have been evaluated in the process of co-hydrotreating of DBT (1000 ppm S) and naphthalene (3 wt. %). Further addition of a nitrogen-containing component (quinoline, 500 ppm N) was considered in order to determine the inhibition effect on trimetallic NiMoW/Al2O3 catalysts. To confirm the results obtained in model reactions, the trimetallic NiMoW/Al2O3 catalysts were also investigated in the process of hydrotreating of straight-run gas oil (SRGO). West Siberian oil with boiling range 180-360°С was used as feedstock, with a density at 20°С of 0.841 kg/m 3 , sulfur and nitrogen contents of 0.815 wt. % and 156 ppm, respectively. The tests were carried out at 340°C, 4.0 MPa of hydrogen, 2 h -1 LHSV and a 700 NL L -1 volume ratio of hydrogen to feed.

NiMo(W)/Al 2 O 3 catalysts in hydrotreating of DBT and naphthalene

Results of the catalytic activities of the sulfided NiMo(W)/Al2O3 catalysts in hydrotreating of DBT and naphthalene are presented in Table 3.5. DBT HDS and naphthalene HYD conversions varied in a wide range, from 42 to 75% and from 32 to 49 %, respectively. Some catalysts were tested twice, after which the average conversion values of the reagents were calculated. The experiment data show that the accuracy of determining the rate constant is on average 3%. HPAs show a different behavior: the HDS activity noticeably increases when incorporating one and three Mo atoms in the structure: from 16 mol h -1 g -1 for NiW12 to 22 and 32 mol h -1 g -1 for NiMo1W11 and NiMo3W9, respectively, while further increase in molybdenum content resulted in only a very slight increase in performance, with 31 and 34 mol h -1 g -1 for NiMo6W6 and NiMo9W3. NiMo3W9 catalyst is 2.4 times more active than Ni(Mo3+W9), but this difference decreases when increasing the Mo content. When comparing catalysts with equal Mo/W ratio, the percentage of Ni atoms in the NiMoWS phase is slightly higher for the NiMoW catalysts than for their reference counterparts.

However, these little differences in promotion of the slabs, that are similar for all MoW and Mo+W solids whatever the Mo/W ratio, cannot explain the observed discrepancies in activity. These results thus highlight the influence of the formation of the mixed phase present in a larger extend in NiMoW catalysts than in Ni(Mo+W) catalysts. On non-promoted catalysts, HDS activity shows a maximum for Mo6W6 catalyst, a further increase in molybdenum content leading to a less organized mixed phase that appeared to be less active in HDS. On promoted catalysts, the effect of the mixed nature of the slabs on the activity of the promoted sites is more noticeable at low Mo contents, the largest difference between NiMoW and Ni(Mo+W) catalysts being observed for a ratio Mo/W of 3/9.

The HDS DBT reaction mainly proceeds along the preliminary hydrogenation route for unpromoted catalysts. Promotion by Ni atoms leads to a sharp change in selectivity and shifts the reaction to the path of direct desulfurization (DDS), which also agrees with the literature data [21], [22].

Chapter 3

The maximum activity in all model reactions was observed for the NiMo1W11/Al2O3 sample (Fig. 3.9), as evidenced by the calculated reaction rate constants. It was previously noted by Lercher and co-workers [23] that the hydrogenating activity correlated with concentration of SH-groups formed due to the dissociative adsorption of H2 and H2S. In another work [24], investigating catalytic activity of unsupported trimetallic NiMoW systems with different Mo/W atomic ratios (1:2, 1:1, and 2:1), the authors found by using method of continuous-flow H2-D2 scrambling that the concentration of SH-groups directly depends on the tungsten content in mixed NiMoW systems.

This can explain the higher HDS, HYD and HDN activities of the NiMo1W11/Al2O3. Further from 67 to 90 % (Fig. 3.10b). The catalysts with high content of Mo were the most affected by quinoline inhibition effect. Naphthalene HYD performance of catalysts prepared from a mixture of monometallic HPAs are a little more affected by quinoline than the samples prepared from the mixture of monometallic HPAs. As in the case of the HDS reaction, the inhibition degree in the naphthalene HYD depends on the atomic ratio of the structure-forming metals of the active phase.

It should be noted that the increase in the atomic ratio more than 0.25 almost does not change the degree of inhibition, and varies around 1-2%.

Hydrotreating of SRGO

The contents of stable hydrogenation products obtained after hydrotreating of SRGO over NiMo(W)/Al2O3 catalysts are shown in Table 3.7. The catalytic activity was evaluated by residual sulfur content in the hydrogenated product. All studied catalysts exhibited stable performance during a 48h period. In order to minimize errors in the determination of the rate constants, all catalysts were tested twice, after which the average conversion values of the reagents were calculated. The experiment data show that the accuracy of determining the rate constant is on average 3%. There was no acid component in the catalysts, for this reason no cracking products of model compounds were found in the hydrogenation product. The yield of hydrogenated diesel fraction was close to 96-97 %.

The highest residual sulfur and nitrogen contents were observed over NiW12/Al2O3 catalyst. The shift in the maximum of HDS activity in comparison with the model feedstock containing quinolone may be associated with a lower nitrogen content (147 ppm versus 500 ppm) in SRGO.

Replacement of only one

As noted earlier in model reactions, quinoline and its by-products can block the HYD centers, thereby opening vacant HDS centers through anchor adsorption. Since the content of quinoline in SRGO was much lower, inhibition affect was to a lesser extent. As a result, the high HYD activity of catalysts with a high molybdenum percentage was observed. In contrast to HDS (Fig. 3.11a), the HYD (Fig. 3.11b) function did not change significantly with the increase of Mo/(Mo+W) from 0.25 to 1.00 ratio in the catalysts. Mixed HPAs based samples exhibited higher HYD activity and, as in model reactions without quinoline, no strong effect on the HYD activity was observed with the increase in the fraction of molybdenum in the catalyst.

The most abrupt increase in activity was found when one tungsten atom was replaced by molybdenum, a further increase is noted for NiMo3W9 but for higher Mo content, the values do not change much. For Ni(Mon+W12-n)/Al2O3 catalysts, no clearly pronounced dependence of the HYD activity on the atomic ratio of Mo and W is observed. High-efficiency liquid chromatographic analysis of hydrogenated products showed that only bi-and tricyclic aromatic hydrocarbons are hydrogenated, while monocyclic ones underwent almost no transformation.

It should be noted that the maximum performance in the three reactions HDN, HDS and HYD refers to the NiMo3W9/Al2O3 catalyst. Catalytic tests were performed using liquid phase sulfidation in a bench scale flow reactor, for HDS of DBT and HYD of naphthalene with or without quinoline, as well as in the hydrotreating of a real feed (SRGO). In all reactions, mixed HPAs based NiMonW12-n/Al2O3 catalysts presented higher activity than their Ni(Mon+W12-n) counterparts, which was associated to the presence of mixed promoted MoWS2 slabs as evidenced by EXAFS and HAADF results. As for non-promoted solids, the proximity of Mo and W atoms in the starting material resulted in a synergetic effect between them during sulfidation through the formation of mixed slabs, which higher efficiency was maintained after promotion.

In the presence of quinoline in the model feed, NiMo1W11/Al2O3 catalyst was the most active in HDS, HYD and HDN reactions, this catalyst being the less sensitive to the inhibiting effect of quinoline. In SRGO HDT, the optimum Mo/W ratio is shifted to NiMo3W9/Al2O3 sample, this difference being attributed to the lower concentration of N compounds in the SRGO. Enhanced N-

Introduction

In order to explore possible ways of increasing the performance of MoW based catalysts, we studied in the previous chapter the effect of support. In this chapter, we will consider another option, consisting in the removal of the support to obtain mixed bulk catalysts. Recently, there has been an increased interest in the development of bulk hydrotreating catalysts, which contain more than 90% of active phase. This interest is primarily due to the development and introduction into the industry of the bulk NiMoW catalyst (NEBULA and Celestia) by ExxonMobil and Albemarle. Many methods have been proposed for the synthesis of bulk catalysts, only some of them making it possible to obtain bulk MoWS catalysts with a mixed active phase. As already mentioned in the literature review, one of the promising methods for the synthesis of bulk catalysts is the removal of the support by acid HF etching. Moreover, unsupported MoS2 catalysts synthesized via etching of alumina were found with higher catalytic properties compared to those prepared by other methods, due to high dispersion of active sulfide particles and good accessibility to active sites [25].

This chapter is devoted to the synthesis of mixed MoWS bulk catalysts using previously obtained supported MoWS/Al2O3 samples based on mixed H4SiMo1W11O40 and H4SiMo3W9O40 in order to produced systems with a previously formed mixed MoWS2 active phase. The effects of catalyst composition on catalytic performance were studied in the co-hydrotreating of DBT and naphthalene. Bulk catalysts were characterized by nitrogen physisorption, powder X-ray diffraction (XRD), extended X-ray absorption fine stricture spectroscopy (EXAFS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

All the obtained results were presented in an article published in the journal Catalysis Today (doi.org/10.1016/j.cattod.2020.07.018). In the previous sections, (Ni)MoW catalysts supported on alumina, the classical HDT support, were synthesized with different Mo/(Mo+W) atomic ratio, which had a significant effect on activity.

The advantage of using mixed SiMonW12-nHPAs has been demonstrated for these catalysts containing two structure-forming active phase metals in a same molecular precursor. At the same time, it was found that the activity of such catalysts is significantly higher than that of the samples obtained from a mixture of two corresponding monometallic HPA.

In the literature, there are many works in which various other oxides (SiO2, TiO, MgO, etc) are used as support. One of the promising supports is the mesostructured silica, which differs from traditional alumina by highly developed surface area and specific mesoporosity. Many methods for the synthesis of mesostructured silica have been proposed by many researchers. Using various polymers as a structure-forming material, as well as changing the conditions and temperature of synthesis, it is possible to obtain a support with different textural properties depending on the requirements of the process. There are very few studies related to the influence of the Mo/(Mo+W) ratio on the catalytic performance of catalysts based on mesostructured silica. This chapter deals with MoW mesostructured silica supported catalysts prepared from mixed HPA and corresponding mixture and different Mo/(Mo+W) ratio hoping to keep the beneficial effects observed on aluminabased catalysts.

SBA-15 and СOK-12 were chosen as supports. SBA-15 has established itself as a support that can be an excellent replacement for traditional alumina [1][2][3]. However, it is very difficult to synthesize this material at large scale due to the use of a sol-gel method starting from tetraethylorthosilicate, as the source of silicon. COK-12 is an analogue of SBA-15, a mesostructured silica for which a continuous synthesis method could be developed, COK-12 also presents a platelets morphology avoiding long channels present in SBA-15 [4,5] Therefore, in this chapter, a comparative analysis of catalysts based on SBA-15 and COK-12

with traditional catalysts supported on alumina was carried out. The samples were investigated in model reactions of HDS DBT and HYD naphthalene.

Comparison of the catalytic properties of supported mixed catalysts based on alumina

and mesostructured silica (SBA-15 and COK-12).

Supports and catalysts preparation

Mesostructured silica COK-12 and SBA-15 were synthesized according to the methods proposed by Jammaer et al. [6] and Stucky et al [7], respectively, using di-and tri-block copolymers as organic structuring agent.

SBA-15: Triblock copolymer Pluronic P123 (М = 5800, EO20PO70EO20, Aldrich) was used as a structure-forming agent and tetraethyl orthosilicate was used as a source of silica. A weighed portion of Pluronic Р123 (4 g) was dissolved in water (30 mL) containing 2 М HCl (120 mL) at a temperature of 35°С. The weighed portion of tetraethyl orthosilicate (8.5 g) was added dropwise under vigorous stirring to the obtained solution and the resulting mixture was held for 20 h at 35°С

under stirring. After stirring, the solution was transferred to a polypropylene bottle (1000 cm 3 ) and placed in an oven for 48 h at a temperature of 80°С. The product was cooled to room temperature, filtered, washed with deionized water, dried at 60°С (5h), 80°С (2 h) and 100°С (5 h), and finally calcined at 240°С (4h), then 540°С (6 h).

COK-12: Triblock copolymer Pluronic P123 (4.0 g) was dissolved in distilled water (107.5 mL) and a buffer mixture composed of citric acid (3.68 g) and sodium citrate (2.54 g) were added to buffer the pH of the synthesis at 5. Under stirring 10.4 g of Na2SiO3 aqueous solution (NaOH, 10 wt %;

SiO2, 27 wt %; Merck, рН = 11.0 -11.5) was added dropwise and water (30 mL) was poured through a separating funnel. pH was stabilized within 30 s after the addition and the pH was maintained around 5-5.5. The solution was stirred for 5 min and the resulting mixture was kept at room temperature for 24 h. Synthesis was accompanied by the stage of aging at 90°С (without stirring) to increase the volume and average diameter of pores [6]. The material was isolated from the solution using vacuum filtration, dried at 60°С for 8 h and calcined in an air stream at 300°С for 8 h and at 550°С for the next 8 h with a heating rate of 1°С/min.

The reference support is alumina, which was used in Chapter 2.

Mo3W9/Sup (Sup = Al2O3, SBA-15 and COK-12) catalysts and the corresponding bimetallic Mo3+W9/Sup reference samples were prepared by the single incipient wetness impregnation with the aqueous solution of the SiMo3W9 heteropolyacid and mixture of H4SiMo12O40 and H4SiW12O40 heteropolyacids, respectively. All catalysts were prepared with the similar loading of metals. After impregnation, all samples were dried at temperatures of 60 (5 h), 80 (2 h), and 100°С (5 h) without calcination. Before physicochemical studies and catalytic tests, oxide samples were sulfided in an H2S/H2 flow (10 vol% H2S), according to the conditions described in Chapter 2.

Characterization of MoW/Sup catalysts

The textural characteristics of the sulfided samples were measured by low-temperature nitrogen adsorption. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) model at Р/Р0 = 0.05-0.3. The total pore volume and pore size distribution were calculated from the desorption curve and within the framework of the Barrett-Joyner-Halenda model. The Mo and W content and textural properties of the synthesized samples and supports are summarized in Table 5.1. proposed by Gregg and Sing [8]. Type IV isotherms exhibiting hysteresis loops of type H1 corresponds to most synthetic mesostructured materials [5,9]. Alumina has a type IV isotherm with a large type H2 hysteresis loop. This type of hysteresis is characteristic of materials with nearly cylindrical channels or consisting of aggregates or agglomerates of spheroidal particles of irregular size or shape. The pore distribution for SBA-15 and COK-12 is monomodal with narrow pore size distribution in agreement with the expected mesoporosity of these supports in the case of alumina, this distribution varies over a wider range from 5 to 11.5 nm.

After impregnation and sulfidation, alumina supported catalysts textural properties remain stable with only a small decrease (less than 10%) of surface area and pore diameter. It indicates the presence of well-dispersed particles at the alumina surface. A significant decrease in pore volume is associated with the deposition of a large amount of the active component inside the pores. COK-12

and SBA-15 present a high specific surface area (720 m 2 /g and 850 m 2 /g, respectively), related to mesostructured materials [4,10]. However after metal deposition and sulfidation their specific surface area decrease by 300-400 m 2 g -1 together with their pore volume (by 0.25-0.55 mL g -1 ) due to the partial filling of pores with deposited particles, in agreement with published data [2,11]. The decrease of the total surface area and the pore volume is more important for SBA-15 supported catalysts presenting the lowest pore size diameter. For the three supports, the average diameter of pores remains stable whatever the sample indicating that a part of the porosity is not affected by metal deposition and sulfidation.

High-resolution transmission electron microscopy (HRTEM)

TEM micrographs are presented in Fig. 3.2. Regardless of the type of precursor, the average length of particles on the mesostructured supports (COK-12: 3.7 and 4.0 nm) (SBA-15: 3.9 and 4.1 nm) and deposited on Al2O3 (4.0 and 3.6 nm) are very similar, leading to very close dispersion values (D). (Table 5.2). We can notice a higher average stacking (≥2.3) on mesostructured support compared to alumina (around 2.0/2.3) which is explained by the lack of interaction between the silica supports and the active phase [1]. The distribution in length from statistical analysis shows only small differences between supports with sulfide phase predominantly represented by slabs with a length mainly between 2 to 6 nm. A stacking equal to two and three is mainly observed on mesostructured silica, while on alumina, single-layer and two-layer particles slightly predominate (Fig. 5.3). Higher stacking number can be explained by an initial weaker oxide-oxide interaction of active-phase precursor particles with the SiO2 surface than that with the Al2O3 surface, as reported for the catalysts supported on SBA-15 [2] and МСМ-41 [12]. In our case, same Mo and W loadings have been chosen for the catalyst preparation leading to metallic atomic density d(Mo+W) equal 3.9 and 1.2 at nm -2 for alumina and silica respectively, explaining that the support has almost no effect on the dispersion of the active phase, even if a slightly higher stacking is observed for silica-based catalysts.

X-ray photoelectron spectroscopy (XPS) of MoW/Sup catalysts

Information about nature and relative amount of Mo and W species on the surface of the synthesized catalysts were obtained by XPS. Fig. 5. 4 shows the decomposition of Mo3d-S2s and W4f photoelectron spectra recorded for Mo3W9/Sup and Mo3+W9/Sup catalysts. Decomposition of the spectra was performed similarly to Chapter 2 using the appropriate monometallic sulfide catalysts.

The results of the XPS decomposition for the metal fractions of molybdenum and tungsten species of the sulfided Mo3W9/Sup and Mo3+W9/Sup are reported in Table 5.3.

In bimetallic catalysts, the sulfidation degree of molybdenum was found higher than 80 rel. % for Mo and 60 rel. % of W, respectively. Similar and high degree of sulfidation of tungsten in Mo3W9 and Mo3+W9 for a same silica support are observed. Differences in the degree of sulfidation of tungsten depending on the nature of the precursorsmixed SiMonW12-n HPA or mixture of SiMo12 and SiW12 HPAs, which were previously observed in Chapter 2 for catalysts supported on alumina, are not observed for catalysts supported on mesostructured silica. The sulfidation degrees of metals in С -12 supported catalysts are the highest with values close to 100% for Mo and 80% for W, and turns out to be higher than with SBA-15. A detailed analysis of the global XPS survey reveals the presence of sodium atoms (0.4 wt. %) in the catalysts based on COK-12, which presumably remained in the structure of the support during its synthesis, since the source of silicon oxide is an aqueous solution of sodium silicate. According to the literature [14][15][16], the presence of alkali metals in small amounts could contribute to an increase in the degree of sulfidation. A high amount of alkali atoms (K(Na)/Mo > 2) could contribute to the formation of two types of molybdenum disulfide (MoS-1T and MoS-2H) [16] but in our case, the remaining amount is too low to form 1T MoS2 phase. These results show that the washing of the material after filtration did not completely remove the inhibitor atoms.

MoW/Sup catalysts in hydrotreating of mixture of DBT and naphthalene.

The catalytic properties of the synthesized catalyst samples were studied in the co-hydrotreating of DBT and naphthalene under microflow unit conditions. The degree of conversion of dibenzothiophene and naphthalene varied from 22.4 to 52.7 % (Table 5.4) with a mass of 0.6 g for alumina-based catalysts and 0.4 g for the silica ones. In order to better compare the catalysts, the reaction rate constants were calculated per gram of catalyst corresponding to the same amount of Mo and W.

The activity of the catalysts supported on COK-12 is much inferior to the SBA-15 and alumina supported samples. As noted earlier, the structure of COK-12 contains sodium atoms, which are known to have a very high inhibitory activity. The presence of alkali metals in the composition of hydrotreating catalysts primarily affects the hydrogenation function, as evidenced by the low values of selectivity. For catalysts for the hydrotreating of FCC naphtha, this approach is used to selectively reduce the hydrogenation function of the catalysts in order to maintain a high olefin content [13], [14]. In the present study, inhibition is not wanted and then completely negative.

The catalysts based on SBA-15 demonstrate the highest performances in both naphthalene hydrogenation (230.8 and 214.1 mol h -1 g -1 for Mo3W9/SBA-15 and Mo3+W9/SBA-15 respectively) and dibenzothiophene hydrodesulfurization (64.6 and 56.9 mol h -1 g -1 for Mo3W9/SBA-15 and Mo3+W9/SBA-15 respectively). These two catalysts are more efficient than the best catalyst supported on alumina prepared from the mixed HPA. It is noticeable that no significant difference is observed between the catalyst prepared from mixed HPAs and one obtained from a mixture of two monometallic HPAs for SBA-15 based catalysts, as noted in the alumina supported ones. Moreover, these alumina catalysts presented very different SHYD/DDS selectivity ratio showing that preliminary hydrogenation route compared with direct desulfurization was largely favored when using Mo3W9/Al2O3. This behavior together with best catalytic results were related to the mixed sulfide phase MoWS2 formation starting from mixed HPA, as demonstrated in Chapter 2. This hydrogenation performance was also reflected by efficiency in naphthalene hydrogenation. For SBA-15 supported catalysts, such a behavior is not observed with also quite similar values for SHYD/DDS.

(Test conditions: T=320℃, LHSV= 10 h -1 , KH=500 NL L -1 , Р = 3.0 MPa, mcat = 0.4 (0.6*) g)
These observations lead to propose that the use of SBA-15 support does not allow to form the mixed MoWS2 phase even when starting from the mixed Mo3W9 HPA and then the active phase could be constituted mainly with separate MoS2 and WS2 phases. Nevertheless, improved catalytic results are obtained for both types of precursors.

Better catalytic results with high hydrogenating activity of MoW/SBA-15 catalysts compared to that of Mo3+W9/Al2O3 could be explained by different quality of MoS2 and WS2 active sites keeping in mind that similar active phase morphology and sulfidation rates were observed for these catalysts. It could be related to a weaker interaction of sulfide particles with SBA-15 support and as a result a better mobility of SH groups responsible for the hydrogenating function. Mesostructured silica is then a promising replacement for traditional alumina support.

Influence of the Mo/W ratio on the catalytic activity of mixed SBA-15 based catalysts

Preparation and characterization of the Mo(W)/SBA-15 catalysts

The bimetallic MonW12-n/SBA-15 catalysts with surface density of metals d(Mo+W) equal to 1.2 at nm -2 were prepared by using the corresponding mixed H4[SiMonWn-12O40] HPAs, n = 0, 3, 6, 9, 12. Reference catalysts were also prepared using the impregnating solutions obtained by mixing SiMo12 and SiW12 in an aqueous solution with the same Mo/W ratio than the corresponding mixed HPAs and were denoted as Mon+W12-n/Al2O3. The catalysts supported on SBA-15 were prepared by incipient wetness impregnation of prepared support fraction (0.5-0.25 mm). The oxidic catalyst precursors after maturation were dried at 60° (4 h) 80° (2 h) and 100° (4 h) in air atmosphere without further calcination. The catalysts were activated in gas phase sulfidation. The compositions of oxidic and textural characteristics of sulfided samples are given in Table 5.5.

All the obtained samples had approximately the same textural characteristics, regardless of the type of precursor. After impregnation and sulfidation, the surface area decreased by more than 40%, while the specific pore volume also decreased, which indicates that the active components are blocking partially the pores. The average pore diameter remained almost unchanged, which indicates that the pores are available for internal diffusion of molecules to active sites.

Transmission electron microscopy (TEM)

All catalysts were characterized by TEM. Morphological information of SBA-15 based catalysts, through the distributions in stacking degree and length of the Mo(W)S2 slabs, presented together with the corresponding average values are reported in Table 5.5.

The average particle length and stacking number ranged from 3.0 to 4.4 nm and from 2.0 to 2.7, respectively. Monometallic W12/SBA-15 sample has the highest particle length and stacking number as a consequence of the lowest dispersion of the active phase, which is typical for a tungsten-based catalyst. The simultaneous presence of Mo and W leads to lower average particles length comparing to those of the monometallic samples and taking into account their relative amounts. It can be noted that the greatest dispersion (0.38 and 0.37) of the active phase is achieved at Mo/W ratio of 0.5 due to the smallest particle length, regardless of the type of precursor. Changing of Mo/(Mo+W) ratio has the same effect on the average particle length and stacking of both types of catalysts regardless of the type of precursor. A similar behavior was also reported for alumina with close morphology of MoW catalysts at a given Mo/(Mo+W) ratio. As previously observed for Mo/(Mo+W) ratio equal to 0.25, the stacking of catalysts on SBA-15 is higher than that of samples supported on alumina.

X-ray photoelectron spectroscopy (XPS)

The results of the XPS decomposition for the metal fractions of molybdenum and tungsten species of the sulfided MonW12-n/SBA-15 and Mon+W12-n/SBA-15 are reported in Table 5.6. In all bimetallic catalysts, the sulfidation degree of molybdenum was found higher than in the monometallic Mo12 sample: from 74% in Mo12 to higher than 80% in all bimetallic samples. The introduction of tungsten increases the degree of sulfidation of molybdenum. The highest sulfidation degree of molybdenum (86-88 rel.%) was obtained on catalysts with a low tungsten content; a further increase in the content led to a slight decrease sulfidation of molybdenum. In the case of tungsten, no trend can be proposed. The highest sulfidation degree of tungsten, exceeding the value for the monometallic W12/SBA-15 sample, was obtained at a Mo/(Mo+W) ratio equal to 0.25. However, the type of precursor does not have a definite effect on the W sulfidation contrary to the case of alumina-based compounds where all the catalysts prepared from mixed HPA presented better W sulfidation rate than their counterpart highlighting different sulfidation behaviors attributed to mixed MoWS2 phase formation. SBA-15 supported catalysts are more efficient than the alumina ones whatever the Mo/(Mo+W) ratio and a same trend is observed for the evolution of the rate constant with the Mo/(Mo+W) ratio.

Nevertheless, as showed in Chapter 2, the alumina-based catalysts obtained by using mixed HPAs are significantly superior to the samples prepared from a mixture of two HPAs showing a different behavior between alumina and SBA-15 catalysts. On alumina, the improved performances were attributed to the formation of a mixed MoWS2 active phase, which produces an increased hydrogenating activity letting again to think that this mixed phase does not form on SBA-15.

Indeed, the use of mesostructured silica made it possible to significantly improve the catalytic activity of MonW12-n and Mon+W12-n bimetallic catalysts, but at the same time deprives the advantages of using mixed HPAs, which may indicate that during sulfidation, a mixed active phase is not formed on SBA-15, due different support interaction by comparison with alumina. As noted earlier, on Mon+W12-n/Al2O3 catalysts, after impregnation and drying, SiMo12HPA interacts with the support transforming into AlMo6 Anderson species, due to which the sulfidation of molybdenum passes through other intermediates, and as a result, affects the heat of sulfidation of molybdenum. In the case of SBA-15, we see that, for all the studied ratios of metals, the degree of sulfidation does not depend on the type of precursor, which may indicate that the SiMo12 HPA structure is retained after impregnation and drying. Even if the use of mixed MoW HPA as precursor is not favorable compared to conventional ones, the simultaneous presence of Mo and W in the catalysts appears to lead to higher catalytic performance than those of monometallic catalysts, as it provides catalysts with a higher content of active phase due to a higher degree of sulfidation of metals.

Conclusions

In this section, mesostructured silica (SBA-15 and COK-12) supports were synthesized and used as an alternative replacement for traditional alumina. Comparative analysis of bimetallic Mo3W9/Sup and Mo3+W9/Sup systems in HDS DBT and HYD naphthalene showed that not all types of mesostructured silica supports are suitable for the preparation of catalysts. It was found that the use of sodium silicate as a source of silicon in COK-12 negatively affects the activity due to the presence of sodium atoms in the structure. SBA-15 showed significantly higher reaction rate constants both in the HDS of DBT and HYD of naphthalene whatever the type of precursors used, which is in agreement with the literature data [17][18][19].

The study of the influence of the Mo/(Mo+W) atomic ratio and the type of precursor on the catalytic activity of catalysts supported on SBA-15 showed that the type of precursor has almost no effect on catalytic activity, as evidenced by the close values of the reaction rate constants. However, it was found that the ratio Mo/(Mo+W) of metals has an effect on the amount of the active phase, and as a result on catalytic activity with better values obtained at high Mo content.

Comparative analysis of the activities of the catalysts supported on SBA-15 and Al2O3 showed that the catalysts supported on mesostructured silica for all studied metal ratios are superior to that on alumina-based samples. At the same time, the synergistic effect provided from the use of mixed

HPAs on sulfidation degree and rate constants was not evidenced on SBA-15. Almost the same characterization results for both types of catalysts also permit to rule out the possibility to have in both cases the mixed-phase on SBA-15 support. To confirm this assumption deduced from similar catalytic behavior and Mo sulfidation rate for both type of precursors, additional advanced characterization (HAADF, ToF-SIMS) of SBA-15 sulfide catalysts would be required as previously done for alumina-based catalysts.

General conclusions

Due to the constant growth in demand for liquid fuels, and in particular diesel fuel, as well as the deterioration of the quality of raw materials and the introduction of new, more stringent environmental standards, researchers are forced to develop new, highly active catalytic systems.

Analysis of the literature data showed that more and more attention of researchers is directed to the creation of polymetallic bulk and supported catalytic systems, in which two structure-forming active phase metals (Mo and W) are present at once. This trend is due to the higher activity of these systems in comparison with traditional catalysts, as well as their high resistance to the inhibitory effect of nitrogen-containing compounds, according to many studies. A particularly important factor in the preparation of such catalysts is the presence of a mixed active phase, which ensures high activity.

It was noted that the formation of a mixed MoWS active phase is facilitated by the use of mixed oxidic precursors, in the molecule of which Mo and W atoms are already in chemical interaction.

Mixed heteropolyacids (HPAs), and in particular Keggin-type SiMonW12-nHPA, are a promising precursor of the active phase. The use of HPAs with different structure and composition as a precursor provides a higher degree of sulfidation and a uniform distribution of the active phase over the support surface as compared to traditional ammonium heptamolybdate (AHM) and ammonium metatungstate (AMT) precursors. A series of works devoted to the study of SiMo3W9 HPA showed a number of advantages of this precursor, the main of which is the formation of a mixed MoW active phase on alumina supported catalysts regardless of the type of sulfidation. Comparative analysis with SiMo1W11 HPA showed that an increase in the content of molybdenum in mixed HPAs promotes an increase in HDS and HYD activity due to a higher degree of sulfidation of metals. DFT calculations reported in literature also indicate an optimal bond energy between metal and sulfur at higher molybdenum contents. The aim of this work is then the study of MoW catalysts supported on alumina with a larger range of Mo/W content, up to 9/3 starting from mixed MoW Keggin HPA. However, in the literature, there were no established protocols for the synthesis of HPA with a high content of molybdenum. In fact, SiMo1W11 HPA, like SiMo3W9 HPA, were obtained from the corresponding lacunar salts, which was not possible for HPA with higher Mo content. The primary objective of this study was thus the development of a new method for the synthesis of mixed SiMonW12-n HPA with a high molybdenum content for the development of highly active mixed catalysts for deep hydrotreating of oil fractions. In Chapter 2 of the thesis, a new synthesis protocol was developed, which made it possible to obtain mixed SiMonW12-nHPA with Mo/W ratio of 6/6 and 9/3, the composition and structure of which were confirmed by Raman and IR spectroscopy, as well as single-crystal XRD.

The new protocol makes it possible to obtain mixed HPAs in one step, without preparation of corresponding lacunar salts, which greatly facilitates the synthesis. This made it possible to supplement the previous results of the study of the effect of using mixed Keggin-type HPAs for the synthesis of unpromoted alumina supported mixed catalysts, as well as to determine the effect of the ratio of structure-forming metals of the active phase on the composition and structure of the active phase and its activity in the reactions of HDS DBT and HYD naphthalene. The composition and structure of the active MoWS2 phase in MoW/Al2O3 catalysts obtained by gas-phase sulfidation were studied by TEM, XPS, and HAADF. It has been found that the use of new SiMo6W6HPA and SiMo9W3HPA also allows the production of highly active mixed MoWS2 phase. Higher activity in the reaction of HDS DBT and HYD naphthalene was measured compared to monometallic catalysts and bimetallic references obtained from the corresponding monometallic SiMo12 and SiW12 HPAs with the corresponding Mo/W ratio. It was shown that Mo/(Mo+W) atomic ratio affects not only the activity of mixed catalysts, but also affects the structure of the active phase itself. HAADF analysis showed that an ordered core-shell structure is formed at Mo/W metal ratios equal to 3/9 and 6/6. A further increase in the proportion of molybdenum led to disordering of the structure, which had a negative effect on the activity in the DBT HDS and naphthalene HYD. However, the mixed Mo9W3/Al2O3 catalyst remained more active than the sample prepared from a mixture of two monometallic heteropolyacids with a corresponding metal ratio.

The introduction of promoters into the composition of the catalyst has a huge effect on the activity. According to literature data, the synergistic effect from the use of two structure-forming metals of the active phase (Mo and W) is preserved when Ni is used as a promoter. This effect was not observed for Co-promoted systems. The question of the formation of a mixed NiMoWS active phase for alumina supported catalysts based on mixed SiMonW12-nHPAs has not been studied in detail. For this reason, Chapter 3 examined the effect of using mixed HPAs and of the Mo/(Mo+W) ratio on the formation and structure of the active NiMoWS phase, as well as on the catalytic activity in HDS of DBT and HYD of naphthalene with or without quinoline, as well as in the hydrotreating of a real feed (SRGO) under liquid-phase sulfidation conditions. By EXAFS and HAADF it was shown that the behavior of the promoted samples towards the formation of the sulfide slabs is similar to that observed with unpromoted ones and that the presence of nickel does not hinder the formation of mixed clusters in HPA based catalysts. In contract, the active phase of Ni(Mon+W12-n)/Al2O3 sulfided under gas phase consisted mainly of monometallic slabs, as previously observed for nonpromoted catalysts. In all reactions, mixed HPAs based NiMonW12-n/Al2O3 catalysts presented higher activity than their Ni(Mon+W12-n) counterparts, which was associated to the presence of mixed promoted MoWS2 slabs as evidenced by EXAFS and HAADF results. In the presence of quinoline in the model feedstock, the NiMo1W11/Al2O3 catalyst was the most active in the HDS, HYD, and HDN reactions, due to its higher resistance to the inhibitory effect of quinoline. In SRGO HDT, the optimal Mo/W ratio is shifted to the NiMo3W9/Al2O3 sample, which is explained by the lower concentration of N compounds in SRGO. Increased resistance to nitrogen inhibition is observed at low Mo content, however, the Mo/W ratio in the catalysts should be selected depending on the content of nitrogen-containing compounds in the feed. For petroleum fractions with a high nitrogen content, NiMoW catalysts with a high tungsten content are most preferred due to their higher resistance to inhibitory effect.

Due to the increased interest in bulk catalytic systems in Chapter 3, unsupported samples were obtained by acid (HF) etching of alumina support. The samples obtained had an active phase concentration of more than 90% with high dispersion of nanoparticles by XRD and TEM results. The use of mixed SiMonW12-nHPAs as starting material contributed to an increase in the content of mixed sulfides compared to their counterparts, which was confirmed by the results of EXAFS and TOF-SIMS. Total DBT HDS and naphthalene HYD activities were correlated with the W-Mo coordination number. According to the catalytic tests, it was found that the catalyst based on the mixed SiMo3W9HPA is more active in model reactions due to the high concentration of the mixed MoWS2 active phase.

Another strategy to increase catalytic performance is to explore the effect of support.

Mesostructured silica was thus chosen instead of alumina and it was found to promote an increase in the activity of bimetallic MoW catalysts in the reactions of HDS of DBT and HYD of naphthalene due to the weaker interaction of the active phase with the support. At the same time, it was found that the use of mixed HPAs compared to separate precursors did not lead to an increase in activity, as was observed for catalysts supported on alumina. It was assumed that on supported SBA-15 catalysts, a mixed active phase was not formed on the catalyst; nevertheless, whatever the precursors, the activity of bimetallic samples was higher than that of monometallic catalysts due to the high degree of sulfidation of metals and good dispersion of the active phase.

Thus, within the framework of this Ph.D. thesis, new Keggin-type SiMonW12-n HPA with high Mo content, not previously described in the literature, were presented. The use of these compounds as precursors made it possible to fully reveal their prospects. The data obtained for Ni-promoted catalysts can be further used to create industrial catalysts for hydrotreating for each type of feedstock, varying Mo/W ratio. The combination of bulk and supported catalysts further facilitates the production of ultrapure fuels. Layered loading of catalysts, as has already been implemented for the NEBULA catalysts, will allow the catalysts to be adapted to the selected process. The use of acid etching of the support opens up the possibility of using already spent catalysts that cannot be regenerated to obtain a highly active MoW bulk catalyst with good dispersion of the active phase.

Mesostructured silica as a catalyst support also opens up additional opportunities in the development of hydrotreating catalysts. Their high hydrogenation capacity will provide a lower PAH content, which is regulated by current standards. However, this direction of development needs additional research. Increasingly, researchers are combining such materials with aluminum to provide even greater gains in activity. Perhaps this approach will reveal the additional potential of using mixed heteropolyacids.

Physicochemical methods of analysis 1.1 Raman spectroscopy

Raman spectroscopy is a molecular spectroscopy technique based on irradiation on the sample surface by monochromatic light, the main part of the light is elastically scattered with the same frequency (Rayleigh diffusion). When scattering, two more types of phonon can be observed: (i) when the molecule can go into a phonon state and release a photon with a lower energy compared to the incident photon (Stokes shift of the Raman distribution). (ii) when the molecule is in the phonon state, is excited and goes into a virtual state, then returns to the ground state and emits a photon with a higher energy than the incident light photon (anti-Stokes Raman distribution) (Fig. 1). This method of characterization is important for catalysts from the impregnation solution to the sulfided state.

The Raman spectra of the HPAs and oxidic samples were recorded at RT using an Infinity XY Horiba Jobin-Yvon Raman microprobe equipped with a photodiode array detector. The excitation laser source was the 532.16 nm line of a Nd-YAG laser. The wavenumber accuracy was 4 cm -1 .

IR-spectroscopy

IR spectroscopy is a method for the analysis of chemical compounds in which the energy of the infrared region of electromagnetic radiation is absorbed. The absorbed energy causes transitions between vibrational and rotational levels of molecules. The set of spectral bands characterizes the studied molecular structure rather completely.

In the region of near-infrared radiation (12500 -4000 cm -1 ), many bands usually appear, corresponding to the overtones of fundamental or composite vibrations. For medium IR radiation, the group frequency region (4000 -1300 cm -1 ) and the fingerprint region (1300 -650 cm -1 ) are distinguished. In the first region, group vibrations ascribed to some pairs of atoms of the molecule appear: from 4000 to 2500 cm -1 -vibrations with the participation of hydrogen atoms, from 2500 to 2000 cm -1 -vibrations of triple bonds, from 2000 to 1540 cm -1 -vibrations double bonds. In the area of fingerprints, there are deformation and skeletal vibrations of polyatomic systems [1].

IR spectra of H4[SiMonWn-12O40] HPAs (as KBr pellets) (400-4000 cm -1 ) were measured on a Shimadzu IR Prestige-21 FT-IR spectrophotometer.

Single-crystal X-ray diffraction (XRD)

Single-crystal X-ray Diffraction (XRD) is a method for studying the crystal structure of a substance to determine the symmetry of the crystal, the amount and content of symmetry elements in it. Moreover, it is used for determination of the unit cell dimensions, bond-lengths, bond-angles and details of site-ordering [2]. Single-crystal XRD is a non-destructive analytical technique based on constructive interference of monochromatic X-rays and a crystalline sample (Fig 2 ).

Fig. 2 Schematic single-crystal X-ray diffraction technique

The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray). Based on Bragg's law (Fig. 3), knowing the glancing angle and the intensity of the diffracted rays, it is possible to create a three-dimensional picture of the electron density in the crystal. correction was applied using SADABS [3]. The structure was solved using Superflip [4] and refined using Jana 2006 [5]. Specificities for each collection and refinement are given in the crystal structure dedicated sections.

Powder XRD

Powder X-ray diffraction (XRD) is a method for studying the structural characteristics of a material using X-ray diffraction (X-ray structural analysis) on a powder, the result of which is the dependence of the intensity of scattered radiation on the scattering angle.

The main difference between powder XRD and single-crystal XRD is the degree of texturing of the material. Single crystals have the maximum texture and are considered anisotropic, while in powder diffraction the samples are considered isotropic (every possible crystal orientation is represented in the same way). Powder XRD operates on the assumption that the sample is randomized, which follows that a statistically significant amount of each plane of the crystal structure will have the correct orientation for X-ray diffraction and each plane will be represented in the signal.

In practice, it is sometimes necessary to rotate the orientation of the sample to eliminate texturing effects and achieve true randomness [6].

In contrast to a single crystal, in the analysis of which the scattered radiation is collected on a flat plate detector as discrete Laue spots, in the case of a powder rotational averaging leads to smooth diffraction rings around the beam axis (Debye-Scherrer rings). The angle between the ring and the axis of the beam is called the scattering angle and is denoted as 2θ. According to Bragg's law, the reciprocal lattice vector G corresponding to each ring of the sample is determined by the equation (1) [7]:

𝐺 ⃗ 𝑞 2𝑘 sin 𝜃 sin 𝜃 (1)
where 𝐺 ⃗ is the reciprocal lattice vector, q is the length of the reciprocal lattice vector, k is the momentum transfer vector, θ is half of the scattering angle, and λ is the wavelength of the source.

Powder XRD data collection was performed on the ARLX'TRA diffractometer with Cu Kα emission (λ = 1.54056 Å) operating at 43 kV and 38 mA and identified using standard JCPDS files.

The approximate crystallite dimensions of the MoS2 slabs was calculated using the Debye-Scherrer relation [8]:

𝐷 • • (2)
where 𝐷002 is the mean size of ordered (crystalline) domains (Å) along the stacking direction; λ is the X-ray wavelength; θ is the Bragg angle; β002 (or FWHM) is the line broadening at half maximum intensity; k002 is the dimensionless shape factor. The shape factor k002 depends on the crystal shape and is close to 0.9 for WS2 [9] and to 0.76 for MoS2 [8]. The average number of layers N was calculated using the equation N = D002/6.17, where 6.17 Å corresponds to the value of the interlayer spacing in the 2H-WS2 structure.

The crystallite size along the basal direction was calculated using the Debye-Scherrer equation

(2) applied to the broadening of the diffraction peak [START_REF] Nikulshina | MoW synergetic effect supported by HAADF for alumina based catalysts prepared from mixed SiMonW12-n heteropolyacids[END_REF]. The same calculation method was previously used by de la Rosa et al. [8] for MoS2 prepared by HF acid etching of the support. The peak [START_REF] Nikulshina | MoW synergetic effect supported by HAADF for alumina based catalysts prepared from mixed SiMonW12-n heteropolyacids[END_REF] is not affected by imperfect stacking or bending/folding of layers. However, Liang et al.

[10] found that the shape factor k110 depends on the β110 angular line width, and this correlation between β110 and k110 was taken into account. According to the experimental angular line widths, the shape factor k110 was equal to 1.49.

Low-temperature adsorption of nitrogen

The method of low-temperature adsorption of nitrogen is designed to determine the specific surface area by the BET method; it is measured at the boiling point of liquid nitrogen (77 K) and a partial pressure p/p0 = 0.05 -0.35. These conditions favoring polymolecular adsorption can be regarded as intermediate between the conditions of monomolecular adsorption (according to Langmuir) and the conditions of condensation of saturated vapor.

The main quantity that allows calculating the specific surface area is the monolayer capacity (or maximum adsorption), which is equal to the number of adsorbate molecules in a completely filled monolayer on the adsorbate surface. In practice, when nitrogen adsorption is carried out, monomolecular adsorption according to the Langmuir theory is not realized, and the surface filling is not limited to a monolayer. In this case, the Brunauer, Emmett, Teller (BET) method is used to calculate the specific surface area [11]. This method is an extension of Langmuir's theory of monomolecular adsorption to the case of polymolecular adsorption.

The BET method is based on the following provisions:  all adsorption centers on the surface of a solid are energetically identical;

 only one adsorbate molecule can be adsorbed on one adsorption center;

 gas molecules can be adsorbed on the surface of a solid in the form of layers, the number of which is not limited and the filling of the layers does not necessarily occur uniformly;  there is no interaction between any adjacent adsorbed molecules;  Langmuir's theory of monomolecular adsorption is valid for the first adsorption layer;

 the second and subsequent layers are formed due to gas condensation;  gas adsorption proceeds in equilibrium.

The BET equation is

• (3)
where 𝑝 and 𝑝0 are the equilibrium and the saturation pressure of adsorbates, respectively, 𝜐𝑚 is the volume adsorbed in one complete monomolecular layer, and 𝑐 is the BET constant (𝑒 ).

The specific surface area (SSA) can be calculated by the equation:

𝑆 • • • ( 4
)
where 𝑎 is effective cross-sectional area of one adsorbate molecule (m 2 ) for nitrogen is 0.162 nm 2 , 𝑁 is Avogadro constant (6.022 × 10 23 mol -1 ), 𝑚 is mass of test powder (g).

The Barrett-Joyner-Halenda (BJH) method was originally developed for relatively wide pore adsorbents with a wide pore size distribution. However, it has been repeatedly shown that this method can be successfully applied to almost all types of porous materials. The model is based on the assumption of the cylindrical shape of the pores and the fact that the pore radius is equal to the sum of the Kelvin radius and the thickness of the film adsorbed on the pore wall (capillary condensation) [12]. The desorption branch of the isotherm is used as the initial data for calculations by the BJH method (although the use of the adsorption branch is also possible) in the pressure range p/p0 = 0.4 -0.967. In this calculation, pores with a diameter of more than 60 nm are not taken into account.

The textural characteristics of the catalysts were measured on a Quantachrome Autosorb-1 adsorption porosimeter by low-temperature nitrogen adsorption at 77 K. Before analysis, the samples were outgassed under vacuum (< 10 -1 Pa) at 300°C for 3 h. The specific surface area (SSA) was calculated using the BET method at relative partial pressures (p/p0) ranging from 0.05 to 0.3. Total pore volume (at p/p0 of 0.99) and pore size distribution were obtained using the desorption curve and the BJH model.

          t i i i t i i n n n W W W D .. 1 2 .. 1 T c e 1 3 3 6 6 , ( 5
)
where ni is the number of Mo(W) atoms along one side of the Mo(W)S2 slab, as determined by its length, and t is the total number of slabs in the TEM micrograph.

The number of slabs per stack was determined to obtain the average stacking degree ( N ):

     t i t i i i n N n N .. 1 .. 1 , ( 6
)
where ni is the number of stacks in Ni layers.

High resolution high-angle annular dark-field scanning transmission electron microscopy (HR HAADF-STEM)

One of the types of TEM is scanning transmission electron microscopy (STEM), the main feature of which is that the electron beam is focused into a thin spot (with a typical spot size of 0.05 -0.2 nm), which is then scanned on a sample in a raster illumination system built in this way that the sample is illuminated at each point using a beam parallel to the optical axis.

High-angle annular dark-field imaging (HAADF) is a STEM technique where only electrons scattered at high angles are collected to build the image [15]. This technique is highly sensitive to variations in the atomic number of atoms in the sample (Z-contrast images). Higher Z elements scatter more electrons at larger angles due to greater electrostatic interaction between the nucleus and the electron beam, which makes the HAADF detector pick up a stronger signal from atoms with high Z [16], making these atoms appear brighter in the image (Fig. 5). But high-resolution scanning transmission electron microscopes require extremely stable room conditions. To obtain atomicresolution images in STEM, vibration, temperature fluctuations, electromagnetic and acoustic waves must be avoided in the room in which the microscope is located [17]. (the intensity between Mo and W is proportional to Z 1.7 ).

HAADF-STEM analyses have been performed using a FEG TEM/STEM system (Titan Themis FEI) operated at 300 kV, equipped with a monochromator and a probe Cs corrector. For HAADF acquisition, the spot size was 9 (probe size of the order of 500 pm) with a screen current of ∼50 pA and collection angles for the HAADF detector of ∼50 and ∼200 mrad, respectively. The probe semiconvergence angle has been 21 mrad. All samples were ground under an inert atmosphere. In order to avoid contamination, the samples have been deposited in the form of dry powder, without solvents, on cupper grids with the lacey carbon film.

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a widely used method of chemical surface analysis.

This method can be applied to a wide range of applications, from identifying surface contamination to characterizing materials as process control or as a method for characterizing new materials in a research environment. More detailed analysis may require visualization of the sample or depth profiles, and the use of numerical techniques such as curve fitting, layer-by-layer modeling, or linear least squares fit to more accurately describe the material. XPS is based on measuring the energy of photoelectrons knocked out from different energy levels of atoms when a substance is irradiated with X-ray radiation. Under the action of a quantum of light, electrons are knocked out of the substance, the energy of the quantum hʋ, in accordance with the energy conservation law, is spent on the ionization energy Eb and the transfer of kinetic energy to this electron (Ekin = mν 2 /2). In the case of a metal, the Fermi level is at the upper point of the filled valence band and is separated from the vacuum level by the potential of the work function φ. If the absorption of a photon occurs at the internal level, the binding energy of the electron is Eb. The emitted electron is recorded with kinetic energy:

𝐸 ℎ𝑣 𝐸 𝜑 (7)
where Eb is the electron binding energy, hʋ is the energy of the exciting photon, Ekin is the kinetic energy of an electron recorded in the experiment, φ is the work function of the spectrometer.

In the method of X-ray photoelectron spectroscopy (XPS), an X-ray tube with one anode with a wide range of operating energies (from 0 (Fermi level) to 1250 eV and more) is used as a radiation source, and covers almost all electronic levels of chemical elements [18]. This allows to: carry out a qualitative analysis, determine the composition of the surface, establish the valence of elements and study the electronic structure of valence states close to the Fermi level [19].

The spectra were recorded on a Kratos Axis Ultra DLD spectrometer using a monochromatic Al Kα source (h = 1486.6 eV, 150 W). The samples were mounted on a holder using double-sided adhesive tape. For the non-conductive samples, the Kratos charge neutraliser system was used and The collected spectra were analysed using the CasaXPS software program (Version 2.3.16)

after applying a Shirley background subtraction. Gaussian (30%) -Lorentzian (70%) peaks were used for spectra decomposition. All XPS spectra were carefully decomposed according to previous works [20], [21].

ToF-SIMS measurements

Time of flight secondary ion mass spectrometry (Tof SIMS) is based on the measurement of charged particles emitted from a surface as a result of bombardment with high-energy ions (such as Ga + , In + , Ar + , O2 + , Cs + , SF 5+ , Aux + , Bix 2+ , and C60 + ). When a primary ion collides with the surface of a solid, its kinetic energy is scattered by the solid through cascades of collisions in the near-surface region. As a result, bonds near the impact site are broken, and fragments, which are atoms, molecular fragments and molecules, are ejected from the upper one to three atomic layers (Fig. 6). Most discarded species have no net charge. However, a small fraction of these species will be either positively or negatively charged and only these species (ions) are detected by the mass spectrometer [22].

Modern spectrometers are equipped with two ion beams at once, since difficulties often arise in the analysis of dielectric structures, as well as thin-film poorly conducting structures on dielectric substrates. The dual sputter beam completely neutralizes the charge caused by the analyzing ion beam with a low current value. To compensate for the charge caused by sputtering ion beams with a much higher current (hundreds of nA), additional time delays are introduced in the "sputtering-analysis" sequence in a very wide range from a few microseconds to several seconds.

Fig. 6

Schematic ToF-SIMS analysis [23].

In a TOF spectrometer analyzed ions with a charge q are subjected to an extraction field (Ve) of several thousand volts, imparting each ion with a fixed kinetic energy (Ek). The kinetic energy is given by the expression Ek=qVe, which is also equal to 1/2mv 2 , where m is the mass of the ion and v is the velocity of the ion [22]. Since the kinetic energy is constant for all ions, the speed of each ion is inversely proportional to its mass. The heavier the ions, the slower their speed in the extraction field. Consequently, for a given flight length l, the flight time t is shorter for lighter ions t = l/v. Thus, the mass of any ion with a unit charge can be determined from the flight time:

m = 2qVet 2 /l 2 (8)
This method of analysis is of great importance in determining the composition of the active phase of mixed MoW catalysts and makes it possible to analyze not only the composition of the supported catalyst, but is also suitable for the analysis of bulk samples. Moreover, it allows you to quantify the content of mixed MoW sulfides.

ToF-SIMS measurements were performed with a TOF.SIMS 5 spectrometer (ION-TOF GmbH Germany) equipped with a bismuth liquid metal ion gun (LMIG). The compacted samples were bombarded with pulsed Bi 3+ primary ion beam (25 keV, 0.25 pA) rastered over a 100 × 100 m 2 surface area. With 30 scans and 128x128 pixels, the total primary ion dose does not amount up to 1012 ions/cm 2 ensuring static conditions. Charge effects due to primary ion beam were compensated by means of a 20 eV pulsed electron flood gun. Cycle time was fixed at 150 µs in order to detect secondary molecular ions up to 2000 m/z. The mass resolution (m/∆m) measured on our spectra was about 4000 at m/z = 143 for MoO3 -. This good mass resolution allowed us to identify high m/z ionic fragments by their exact mass and the attribution could be confirmed most of the time by the simulated isotopic pattern.

EXAFS spectroscopy

XAFS spectroscopy consists in studying the X-ray absorption fine structure (XAFS) that appears in the X-ray absorption spectra near the absorption edges of the atoms constituting the substance. The study of the short-range fine structure in the energy range extending from the absorption edge to an energy of about 30 eV above it (XANES region) (Fig. The main advantage of EXAFS is the ability to determine the atomic structure of the environment of only one chemical element in a crystal lattice of almost any complexity. As the energy of the incoming X-ray increases, the photo-excited electron has sufficient energy to escape the atom.

The outgoing photo-electron has a wavelength, determined by its kinetic energy, and is scattered by The rate constants of the pseudo-first-order reactions of the DBT HDS and naphthalene HYD were determined using the equations presented in works [32], [33]. The rate constants of were determined using the following equations: The HDS products from DBT included biphenyl (BP) via the direct desulfurization (DDS) pathway, as well as cyclohexylbenzene (CHB) and dicyclohexyl (DCH) from the HYD pathway. The inhibiting factor for the DBT HDS and naphthalene HYD reaction in the presence of quinoline was calculated using the following equations: 𝜃 100% and 𝜃 100%

where k 0 is the rate constant (mol g -1 h -1 ) in absence of quinoline; k is the rate constant with the addition of quinoline in feedstock (mol g -1 h -1 ).

The turnover frequencies (TOF, s -1 ) normalized on edge sites of MonW12-nS2 slabs for the HDS of DBT, HYD of naphthalene allowed us to get more complete understanding of the catalytic properties of the active phase species. TOF values were calculated using the following equations: Ar are the standard atomic weights of tungsten (183.9 g/mol) and molybdenum (95.9 g/mol), respectively.

Evaluation of catalytic activities in hydrotreating of SRGO

For testing on real raw materials, a more modified setup was used (Fig. 9). Samples of NiMo(W) catalysts (10 g) with a particle size of 1-3 mm (the bulk density equal to 0.9±0.02 сm 3 g -1 )

and low-surface-area sieved carborundum (0.2-0.4 mm) were loaded into a steel reactor bed at a ratio of 1:2. This loading system with inert material was used to ensure uniform distribution of the feedstock over the catalyst bed.

The tests were carried out at 340°C, 4.0 MPa of hydrogen, 2 h -1 LHSV and a 700 NL L -1 volume ratio of hydrogen to feed. Before testing, the catalysts had been sulfided according to the procedure described above. SRGO of West Siberian oil with boiling range 180-360°С was used as feedstock with a density at 20°С of 0.841 kg/m 3 , the sulfur and nitrogen content are 0.815 wt. % and 156 ppm, respectively. The content of sulfur and nitrogen in the feedstock and hydrogenated product was determined by elemental analysis on a Multi EA 5000 analyzer (analysis error ± 0.1 ppm), Analytik

Jena. The content of mono-, bi-and polycyclic aromatic hydrocarbons was determined by HPLC on an LC-20 Prominence chromatograph, Shimadzu. All catalysts showed steady state activity after 24 h of continuous testing. As mentioned earlier, some samples were also retested under straight run gas oil hydrotreating conditions to confirm the results.

The polycyclic aromatic hydrocarbons (PAH) hydrogenation and hydrodenitrogenation degrees over the catalysts were calculated according to the equations: 𝐻𝑌𝐷 100% and 𝐻𝐷𝑁 100%

where 𝐶 and N0 are PAH and nitrogen content in the feedstock, respectively; 𝐶 and N are PAH and nitrogen content in the hydrogenation products, respectively.

The apparent HDS reaction order for middle distillates was calculated based on experimental results from [34], [35] according to the following equation:

n = 0.2156S0 + 1.2823 (8)
The apparent reaction order in present work was equal to ~1.4 which is typical for a straightrun gas-oils and consistent with the literature data [34], [36], [37].

Apparent HDS rate constant was calculated as follows [35]:

𝑘 100% ( 9
)
where LHSV is the feed hourly space velocity (h -1 ); n is the apparent reaction order; 𝑆 and 𝑆 are the concentrations of sulfur in the hydrogenation product and in the initial feedstock (wt. % of sulfur), respectively

Fig. 1 . 1

 11 Fig. 1.1 Projected consumption of energy sources by various sectors [adapted from 11]

Fig. 1 . 2

 12 Fig. 1.2 Process scheme of a typical petroleum refinery. [adapted from 10]

Fig. 1 . 3

 13 shows the various types of sulfur containing compounds present in a typical Kuwait straight-run light gas oil analyzed by GC-SCD.

Fig. 1 . 3

 13 Fig. 1.3 GC-SCD chromatograms of sulfur compounds distribution in Kuwaitstraight-run light gas oil[8]

Fig. 1 . 4 .

 14 Fig. 1.4. Reaction network of the HDS of DBT adapted from Ref.

Fig 1 . 5 Fig. 1 . 5

 1515 Fig. 1.5 Relative reactivity of alkyl-substituted BTs and DBTs [9]

Fig. 1 . 6

 16 Fig. 1.6 Directions of phenanthrene hydrogenation reactions.

Fig. 1 .

 1 Fig. 1.11 Experimental HDS activity of X-MoS2 catalysts vs. adsorption energy of thiophene. Adapted from [55] Toulhoat and Raybaud [56] also reported that for HDT reactions the sulfur-metal bond energy (EMS), correlates well with the activity via a volcano curve. Nikulshin et al. [57] obtained similar dependences not only for the reaction of the HDS reaction, but also for the HYD reaction in the presence of supported XMo6(S)/Al2O3 (where X= Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga) catalysts. It was found that the Co and Ni atoms can optimize the electron density on the anti-bonding d-orbital of Mo

1. 4 . 1 Fig. 1 . 1 Fig. 1 .

 41111 Fig. 1.12 Thiophene HDS activity as a function of the sulfur metal bond energy calculated on the (a) Mo(W)-edge and (b) S-edge. The reference in energy is chosen for the CuMoS active phase.Dashed volcano curves are only guide for the eyes[START_REF] Thomazeau | Predictive approach for the design of improved HDT catalysts: γ-Alumina supported (Ni, Co) promoted Mo1-xWxS2 active phases[END_REF]

1. 4 . 2 Heteropolyanions as effective precursors of hydrotreating catalysts 1 . 4 . 2 . 1

 421421 Type, composition and structures of heteropolyanionsWorks concerning synthesis of hydrotreating catalysts via heteropolyanions (HPAs) have been known since the middle of 1980s. Instead of the use of traditional Mo and W precursors as AHM, AHT, AMT and others inorganic heteropolyanions are more and more common in the literature as precursor of active phase[START_REF] Kraleva | Support effect on the properties of ironmolybdenum hydrodesulfurization catalysts[END_REF][START_REF] Toulhoat | Prediction of optimal catalysts for a given chemical reaction[END_REF][START_REF] Pope | Heteropoly and isopoly oxometalates[END_REF]. For hydrotreating catalysts preparation, HPAs with the Keggin[START_REF] Trong On | Perspectives in catalytic applications of mesostructured materials[END_REF][START_REF] Kozhevnikov | Heterogeneous acid catalysis by heteropoly acids: Approaches to catalyst deactivation[END_REF] and Anderson[START_REF] Kozhevnikov | Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions[END_REF][START_REF] Keggin | The structure and formula of 12-phosphotungstic acid[END_REF][START_REF] Damyanova | Surface properties of titania-supported 12molybdophosphoric acid hydrodesulphurization catalysts[END_REF][START_REF] Lizama | Highly active deep HDS catalysts prepared using Mo and W heteropolyacids supported on SBA-15[END_REF] structures and their derivatives [104, 105] (Fig. 1.14) are the most frequently used.

Fig. 1 .

 1 Fig. 1.14 The most commonly used heteropolyanions (I -Keggin structure; II -Anderson structure; III -substituted Anderson derivative) for a synthesis of HDT catalysts. Color scheme: Mo (blue); O (red); P (pink); Co (green) [91]

Fig. 1 . 16 .

 116 Fig. 1.16. HAADF images of sulfide Mo 3 W 9 -GS (a), Mo 3 W 9 -LS (b) catalysts (adapted from [119, 120]) (GS -gas phase sulfidation; LS -liquid phase sulfidation)

Fig. 1 . 18 .

 118 Fig. 1.18. 3D diagram for rate constant kHDS as a function of average length and average stacking number. (The values near the catalyst marker show the nitrogen content wt. %) [123].

Fig

Fig. 1 .

 1 Fig. 1.19 Variation of the catalytic properties of unsupported fully sulfided CoMo and NiW catalyst [133]. It was found that the optimum HDS activity for bulk catalysts based on Mo and W is achieved with the addition of a promoting metal (Me) corresponding to an atomic ratio Me/(Me + Mo(W))] ≈ 0.3. Later similar dependencies were published by Inamura et al. [135]. Inamura and Prins discovered interesting dependencies in the study of the promotion of bulk molybdenum disulfide. Promoted MoS2 were prepared by impregnation of pure MoS2 with solution of Co(NO3)2 ‧6H2O in acetone. The HDS of thiophene and the consecutive hydrogenation of butene properties indicated that the Co ions are preferentially situated at the edges of the MoS2 crystallites below Co/(Co + Mo) ≈ 0.08 and promote the HDS reaction. Further addition of Co leads to the segregation of Co9S8, which acts as a support for highly dispersed Co-promoted MoS2 crystallites. As a result of this structural effect, the Co-MoS2 crystallites become better accessible and the catalytic activity increases again above Co/(Co + Mo) ≈ 0.2. Structural models have been proposed for various Co/(Co + Mo) ratio (Fig 1.20).

Fig. 1 .

 1 Fig. 1.20 (a) the rate constant k"hds normalized to the mos2 content versus the co/(co+mo) atomic ratio, with standart deviations given by error bars. (b) schematic models for the co sulfideand the mos2 crystallite phases[START_REF] Inamura | The role of Co in unsupported Co-Mo sulfides in the hydrodesulfurization of thiophene[END_REF]

  1.21).

Fig. 1 .

 1 Fig. 1.21 (A) The resulting model interface between the layered MoS2 (010) plane and the Co9S8 (111) plane. The atoms are colored as follows: cobalt (blue), sulfur (yellow) and molybdenum (green), (B) portion of the interface called "seed" (red dashed line) used for DFT calculations, (C) zoom on a cobalt cubic cluster arrangement connected to Mo atoms, (D) another view of (B) [137]. Modeling of the interface showed the creation of open latent vacancy sites on half of the Mo atoms interacting with cobalt and formation of Co-Mo bonds. Strong electron donation from Co to

Chapter 1 Ni7S6

 1 , while monometallic MoS and WS species were located in the interlayer space (Fig. 1.22).Wang and co-workers[START_REF] Wang | Hydrodeoxygenation of p-cresol on unsupported Ni-W-Mo-S catalysts prepared by one step hydrothermal method[END_REF] used the bulk NiMoWS in hydrodeoxygenation of p-cresol.The catalysts with W/Mo molar ratio from 0 to 0.7 were prepared by one step hydrothermal method. Among these, the MoW catalyst with W/Mo=0.5 exhibited the highest activity in the HDO of p-cresol. The reaction rate constant was almost twice higher than that of the bimetallic NiMo analog. Thanks to the Extended Hückel calculations, Olivas et al. [142] discovered a synergistic effect in a three-metal NiMoW system. A higher synergistic effect of Ni in (MoW)S2 than in Ni(MoS2) and Ni(WS2) was found. The authors explain this by the fact that the combination of Mo and W atoms in forming binary (Mo-W)S2 compounds changes the semiconductor character of MoS2 or WS2 increasing the metallic character and addition of Ni increases the availability of electrons over the Fermi level. At the same time, two research groups, Huirache-Acunã et al. [143] and Nava et al. [144], studied the effect of ex situ and in situ activation, respectively, for trimetallic (Ni -Mo -W) precursors containing alkyl fragments. Results obtained by Nava et al. showed that, after in situ decomposition of alkylthiometallates with the production of unsupported NiMoW catalysts, they had large specific surface areas (33.8-108.1 m 2 /g), whereas after ex situ activation of tetraalkylammonium thiometallates (R4N)2Mo(W)S4, the materials had lower SSA values (3.9-11.3 m 2 /g). By ex situ activation no clear relationship could be observed between the alkyl group length, specific surface areas and the catalytic activity. But the opposite situation was observed in the case of in-situ activation. The alkyl group present in the tetraalkylammonium precursor has a direct influence on specific area and HDS activity [144]. .

Fig. 1 . 1 1. 5 . 4

 1154 Fig. 1.22 SEM image and elemental analysis by EDX of the NiMoW sulfide catalyst and b) HRTEM image of the NiMoW sulfide catalyst.

Fig. 1 .

 1 Fig. 1.23 Hydroprocessing catalyst development history [145] The NEBULA's catalysts line was adapted not only for the diesel hydrotreatment process, but also for hydrocracking of heavy oil fractions (Fig. 1.24) [149].

Fig. 1 . 1

 11 Fig.1.24 The positive aspects of using catalyst NEBULA.

Fig. 1 .

 1 Fig. 1.25 Relative activity of Nebula, Celestia and supported NiMo catalysts [150]

4

 4 SiMo n W 12-n O 40 Keggin-type heteropolyacids 2.2.2.1 IR and Raman analysis IR and Raman spectra of all synthesized HPAs are presented in Figs. 2.1 and 2.2, respectively. IR spectra of monometallic and mixed HPAs are very close and present the typical peaks of Keggin structure. For all samples, characteristic peaks assigned to the Keggin structure are observed at 913-930 and 957-982 cm -1 for Si-O and Mo(W)-Ot vibrations stretching mode respectively together with lines at 867-882 and 773-780 cm -1 corresponding to Mo(W)-Ob-Mo(W) and Mo(W)-Oc-Mo(W)

Fig. 2 . 1 .

 21 Fig. 2.1. IR-spectra for SiMonW12-nHPAs.

Fig. 2 . 2

 22 Fig. 2.2 Raman spectra for SiMonW12-nHPAs.

  Mo(W)S2 active phase. The TEM micrographs are shown in Fig. 2.7. The observed black thread-like slabs correspond to Mo(W)S2 crystallites with 0.65 nm interplanar distances matching characteristic (002) basal planes of crystalline MoS2. In TEM, only the slabs parallel to the electron beam can be visualized and the observed contrast does not allow to distinguish MoS2, WS2 or MoWS2 particles. Only global morphological information will be obtained, through the distributions in stacking degree and length of the Mo(W)S2 slabs, presented together with the corresponding average values in

Fig. 2 . 8 .

 28 Fig. 2.8. HAADF images of sulfided MoW catalysts, with intensity profile corresponding to the row of atoms identified by the arrow on mixed Mo3W9/Al2O3. * adapted from [10]. Moreover, as HAADF allows discriminating Mo and W atoms, it is possible to visualize their relative positions inside the slabs. In the case of the Mo6W6/Al2O3 sample, as for the Mo3W9/Al2O3, mixed slabs are formed in which the molybdenum atoms are concentrated in a core, surrounded by tungsten atoms. The Mo9W3/Al2O3 sample was characterized by a disordered structure of Mo and W atoms inside the slabs, where the two-dimensional foils can be represented as MoS2 particles with small inclusions of W atoms. Perhaps this can be explained by the different kinetics of sulfidation of Mo and W atoms and an excess of the former in the structure. According to the data obtained by

2 . 9 )

 29 . The Mo3d spectra contain three doublets: 229.0 eV and 232.0 eV correspond to Mo3d5/2 and Mo3d3/2 of Mo 4+ (MoS2 species), doublet at about 230.0 and 233.5 eV is related to Mo 5+ (MoOxSy species) and doublet at 232.5 and 235.7 eV is associated with Mo 6+ oxide species.

Fig 2 . 9 ,

 29 W5p3/2 and Mo4p ranges (non splitted level) overlap the W4f levels. The contributions of these peaks were excluded when calculating the true content of W4f fractions.

Fig. 2 . 9

 29 Fig. 2.9 XPS Mo 3d and W 4f spectra recorded for sulfided Mo3W9/Al2O3, Mo6W6/Al2O3 and Mo9W3/Al2O3 catalysts; in blue: Mo(W) 6+ oxide contributions; in pink: Mo(W)SxOy contributions; in black: Mo(W)S2 contributions: in green S contributions.

11 .

 11 Dependence of DBT HDS (a) and naphthalene HYD (b) rate constants on total number of edge sites This allows to estimate in a global way the number of active centers without taking into account their nature. Since the values of dispersion of the active phase for all catalysts were approximately

Fig. 3 . 1 3 to

 313 Fig. 3.1 HRTEM micrographs of sulfided NiMo(W)/Al 2 O 3 catalysts

3 . 5 . 3 [ 16 ]

 35316 Ni(Mo)W/Al 2 O 3 catalysts HAADF technique allows to visualize the sulfide NiMoW slabs with basal planes oriented perpendicularly to the electron beam. HAADF images of sulfided NiMo3W9/Al2O3 and Ni(Mo3+W9)/Al2O3 catalysts are presented in Fig. The sulfide particles of Ni-promoted catalysts had irregular shapes, as observed in the case of unpromoted samples, which is attributed to the interaction with alumina surface. On HAADF micrographs, Mo and W atoms are easily discriminated due to their Z difference: heavy tungsten atoms are imaged as brighter spots than lighter molybdenum atoms. We can thus see that NiMo3W9/Al2O3 images show the formation of mixed (Ni)MoWS slabs, in which small agglomerates of Mo atoms were surrounded by W ones. Even if mixed MoW slabs are clearly observed in the case of this promoted catalyst, the atomic repartition in a disulfide slab differs from the core-shell structure of unpromoted Mo3W9/Al2O3 catalyst (sulfided with the same gas phase procedure) containing a whole Mo core [15]. The distribution of Mo and W atoms in the particles of NiMo3W9/Al2O3 is more similar to the one that has been observed for unpromoted Mo3W9/Al2O3 catalyst sulfided under liquid phase by DMDS Chapter . The active phase of Ni(Mo3+W9)/Al2O3 sulfided under gas phase consisted mainly of homogeneous (Ni)MoS and (Ni)WS slabs (Fig 3.5). However, the presence of some mixed (Ni)MoWS slabs was also evidenced. Thus, the structure of sulfide particles in Ni(Mo3+W9)/Al2O3

Fig. 3 . 5

 35 Fig. 3.5 HAADF images of sulfided NiMo3W9/Al2O3 and Ni(Mo3+W9)/Al2O3 catalysts with intensity profiles corresponding to the row of atoms identified by the arrow on NiMo3W9/Al2O3.

3. 4 . 2 Fig. 3 . 6

 4236 Fig. 3.6 compares the magnitude of k 3 -weighted Fourier transformed (FT) EXAFS spectra for bimetallic NiMo12/Al2O3 (NiW12/Al2O3) and trimetallic NiMo3W9/Al2O3, and Ni(Mo3+W9)/Al2O3 catalysts. The best-fit parameters for EXAFS data are gathered in Table 3.4. At both Mo K-and W

3

 3 Fig. 3.6 k 3 -weighted EXAFS at the Mo K-edge and W L3-edge, and the corresponding Fourier transforms of sulfided NiMo(W)/Al2O3 catalysts.

Fig. 3 . 7

 37 Fig. 3.7 Ni K-edge XANES spectra comparison (A): (a) Ni/Al2O3 and bimetallic samples, (b) Ni/Al2O3 and trimetallic samples, (c) NiMo12/Al2O3 and trimetallic samples, (d) Ni(Mo3+W9)/Al2O3 and bimetallic catalysts; Comparison of k 3 -weighted EXAFS spectra of sulfided catalysts (B): (a) Ni/Al2O3 and bimetallic samples, (b) Ni/Al2O3 and trimetallic catalysts

Fig. 3 .

 3 Fig. 3.8a shows that the HDS rate constants of all trimetallic NiMonW12-n/Al2O3 catalysts based on mixed HPAs are higher than that of their Ni(Mon+W12-n)/Al2O3 counterparts and then the values calculated by additive way using the data for NiMo12/Al2O3 and NiW12/Al2O3 samples (dotted line). The most active NiMo9W3 catalyst even surpasses the NiMo12 one. Ni(Mo+W) catalysts show values close to the additive ones. Trimetallic catalysts prepared using mixed SiMoW

Fig. 3 .

 3 8c shows that the introduction of one molybdenum atom in the catalysts contributes to a shift in the reaction to the route of pre-hydrogenation, particularly sharp in the case of Ni(Mo1+W11)/Al2O3. With a further increase of the Mo/(Mo+W) atomic ratio from 0.08 to 0.75, a gradual decrease in SHYD/DDS selectivity occurs, except for NiMo9W3/Al2O3 catalyst, showing a slight increase in selectivity. The NiMo12/Al2O3 catalyst exhibits the lowest SHYD/DDS selectivity. Regarding hydrogenation, bimetallic NiMo12/Al2O3 sample was the most active in naphthalene HYD reactions (Fig. 3.8b). The conversion of naphthalene HYD almost does not change with an increase in the Mo/(Mo+W) atomic ratio in the catalysts based on mixed HPAs and varies in the range of 37-39 %. The hydrogenating character of Ni seems to level any other effect, while for Ni(Mon+W12-n)/Al2O3 catalysts, the activity passes through a minimum at a ratio of 0.25.As in the DBT HDS, mixed NiMonW12-n/Al2O3 catalysts are superior to reference samples prepared from a mixture of monometallic HPAs in HYD of naphthalene. The higher activity of the mixed NiMonW12-n/Al2O3 catalysts in comparison with the samples prepared from the mixture of

Fig. 3 . 9

 39 Fig. 3.9 Dependence of reaction rate constants in DBT HDS (a) naphthalene HYD (b), quinoline HDN (c) and selectivity route of DBT HDS (d) on Mo/(Mo+W) atomic ratio in NiMo(W)/Al2O3 catalysts (circles correspond to NiMonW12-n/Al2O3 catalysts; squares are Ni(Mon+W12-n)/Al2O3 catalysts; dotted line indicates additive values between NiMo12/Al2O3 and NiW12/Al2O3).

  tungsten atom by molybdenum leads to a decrease in the content of sulfur and nitrogen, by 35% and 28% for S and N contents on Ni(Mo1+W11)/Al2O3 and by 57% and 46% for S and N contents on NiMo1W11/Al2O3. When increasing the Mo content, performance in HDS and HDN evolve in a similar way. As previously observed in the hydrotreatment of model feeds, mixed HPAs based NiMonW12-n/Al2O3 catalysts demonstrated higher activity during hydrotreating of SRGO than their corresponding counterparts, which can be related to the presence of mixed NiMoWS phase in NiMoW solids. The optimum Mo/(Mo+W) atomic ratio, corresponding to the lowest content of both residual sulfur and nitrogen, corresponds to 0.25 for both types of catalysts (Fig 3.11).

Fig. 3 .

 3 11 Dependence of reaction rate constants kHDS (a), HYD (b) and HDN (c) activities in HDT of SRGO on Mo/(Mo+W) atomic ratio in NiMo(W)/Al2O3 catalysts (dotted line indicates additive values between NiMo12/Al2O3 and NiW12/Al2O3).

1 NiMo

 1 Mo/(Mo+W) atomic ratio k HDS ×10 4 , h -1 (wt.% S) n-In the present chapter, NiMoW catalysts supported on alumina were prepared by coimpregnation of nickel carbonate, citric acid and mixed SiMonWn-12 (n = 1, 3, 6 and 9) HPAs and characterized by different techniques. Trimetallic reference Ni(Mon+W12-n)/Al2O3 catalysts were also synthesized from a mixture of two monometallic SiMo12 and SiW12 HPAs with the same Mo/W ratio as in the mixed HPAs based catalysts. HAADF and XAS characterizations performed after gas phase sulfidation of trimetallic solids with a (Mo/Mo+W) ratio equal to 0.25 have allowed to evidence the formation of promoted mixed MoWS2 slabs, where Mo islands are distributed inside the WS2 slabs for the catalyst prepared from SiMo3W9 HPA while the active phase of the catalysts based on two separate HPAs consisted mainly of monometallic sulfide slabs. The behavior of the promoted samples towards the formation of the sulfide slabs is similar to that observed with unpromoted ones and presence of nickel does not hinder the formation of mixed clusters in HPA based catalysts. Liquid sulfided NiMo(W)/Al2O3 catalysts were characterized by XPS and TEM. The dispersion values obtained by the TEM results indicate that the type of precursor as well asMo/(Mo+W) ratio does not affect the distribution of the NiMo(W)S2 active phase particles over the support surface. Tungsten and molybdenum sulfidation degrees in the NiMonW12-n/Al2O3 catalysts were higher compared to the corresponding counterparts, which can be related to the simultaneous sulfidation of Mo and W due to their proximity in the initial MoW-heteropolyanion precursor, as observed in the non-promoted solids. Higher quantity of NiMoWS phase was also measured on the NiMonW12-n/Al2O3 catalysts, where the metals thus appear better sulfided and better promoted than in the Ni(Mo+W) catalysts.

Fig. S3 .

 S3 Fig. S3. Mass spectra obtained by TOF-SIMS analysis of MonW12-nS2 catalysts.

1 .

 1 Nitrogen adsorption/desorption isotherms (a) and pore size distributions (b) of Al2O3, SBA-15 and COK-12 Nitrogen sorption curves and pore size distribution are shown in Fig. 5.1. Mesostructured silicas demonstrate typical type IV isotherms exhibiting hysteresis loops of type H1, with parallel adsorption and desorption branches due to the regular array of cylindrical pores, according to the classification

Fig. 5 . 2 .

 52 Fig. 5.2. TEM micrographs of sulfided MoW/Sup catalysts

Fig. 5 . 3

 53 Distribution of slab length (a) and of stacking number (b) (rel. %)

Mo 3 W 9 /Al 2 O 3 Mo 3 W 9 /COK- 12 Mo 3 W 9 /SBA- 15 Fig. 5 . 4

 3933912391554 XPS Mo 3d and W 4f spectra recorded for sulfided Mo3W9/Sup catalysts; in blue: Mo(W) 6+ oxide contributions; in pink: Mo(W)SxOy contributions; in black: Mo(W)S2 contributions: in green S contributions.

Fig. 5 . 5 .

 55 In both reactions, all the MoW catalysts present improved catalytic performances by comparison with the monometallic ones showing a clear synergetic effect between Mo and W whatever the Mo/(Mo+W) ratio. Moreover, the maximum activity is achieved at Mo/(Mo+W) atomic ratio equal to 0.75 corresponding to Mo/W = 9/3 in both reactions. It is noticeable that for a same metallic ratio, rate constants of catalysts prepared by mixed HPA and mixture of HPA are similar in both reactions. At the same time, similar selectivity SHYD/DDS are also reported (Fig. 5.5c) For comparison purposes, catalytic results of alumina supported MoW studied in Chapter 2 have been plotted on the same figure. Both types of catalysts possess the same percentage of metals.

5

 5 Dependence of reaction rate constants in DBT HDS (a), naphthalene HYD (b), and selectivity route of DBT HDS (c) on Mo/(Mo+W) atomic in MoW/SBA-15 (circle) and MoW/Al2O3 (triangle) catalysts (filled dots correspond to mixed MoW catalysts prepared from mixed SiMonW12-nHPA; open dots correspond to mixed Mo+W catalysts prepared from a mixture of SiMo12 and SiW12HPAs.).

Fig. 5 . 6

 56 Fig. 5.6 shows plots of kHYD and the contribution of the HYD pathway to DBT HDS versus the relative amounts of the sulfide phase. The content of the active phase on the surface of SBA-15 was calculated on the basis of XPS data. The obtained data on the hydrogenating activity are linearly correlated with the content of the active phase (Fig. 5.6). It was found that the highest content of the active phase is formed at the Mo/(Mo+W) ratio equal to 0.75 for Mon+W12-n/SBA-15, 0.5 and 0.75 for MonW12-n/SBA-15. The selectivity of DBT HDS increased almost linearly from 0.70 to 1.96 with

Fig. 5 . 6

 56 Fig. 5.6 Dependence of the hydrogenating activity on content of Mo(W)S2 active phase of Mo(W)/SBA-15 catalysts

Fig. 1

 1 Fig. 1 Processes of energy state change under Rayleigh and Raman distributions

Fig. 3

 3 Fig. 3 Bragg diffraction. A diffraction pattern is obtained by measuring the intensity of scattered waves as a function of scattering angle. Very strong intensities known as Bragg peaks are obtained in the diffraction pattern at the points where the scattering angles satisfy Bragg condition. Single crystal XRD data for H4[SiMo6W6O40]ꞏ36H2O and H4[SiMo9W3O40]ꞏ36H2O have been collected using a Bruker Apex Duo diffractometer with a Mo-IμS microfocus tube (λ=0.71073 Å). It should note that the poor stability of the crystals in air at ambient temperature, even more pronounced for the H4[SiMo6W6O40] × 36H2O, polycation for which a single crystal had to be isolated very rapidly and collected at 100K in a cold nitrogen flow to prevent from degradation by moisture into an amorphous solid. For the H4[SiMo9W3O40] hydrate a simple protection of the crystals by vacuum grease allows full data collection at room temperature. The intensity data have been extracted and corrected from Lorentz Polarization using the program SAINT-Plus 8.27b. Multiscan absorption

1. 6

 6 High-resolution transmission electron microscopy (HRTEM)HRTEM works on the principle of transmitted electron beams, in contrast to the metallographic light microscope, in which the image is formed by reflected light beams. The light source in the electron microscope is replaced by an electron source (gun), instead of glass optics, electromagnetic lenses are used (to refract electron beams) (Fig.4)[13].The electron source (gun) creates a stream of electrons that passes through the electromagnetic lens of the capacitor to form an electron beam that illuminates the sample. The beam strikes the specimen and a part of it gets transmitted through it. This transmitted beam is focused by the objective lenses into an image, which is then fed down the column through the projector lenses enlarging the image, depending upon the set magnification. Electrons going through thicker areas of the material or material with higher Z undergo more scattering and the corresponding areas in the sample therefore appear darker in the image, while thinner areas or lighter elements allow more electrons to pass through and appear lighter.HRTEM was carried out using a Tecnai G2 20 electron microscope with LaB6 cristal with a 0.19 nm lattice-fringe resolution and an accelerating voltage of 200 kV. The samples were dispersed in ethanol. The suspension was collected on carbon films supported on copper grids and 10 -15 representative micrographs were obtained for each catalyst in high-resolution mode. Typically, the length and the stacking of at least 500 slabs were measured for each catalyst. ImageJ free software developed for direct qualitative analysis of images. The distribution in length and the stacking of the

Fig. 4

 4 Fig. 4 Scheme of transmission electron microscope

Fig. 5

 5 Fig. 5 HAADF-STEM scheme with an example of a photomicrograph of MoWS2

  the spectra were charge-corrected to provide the C 1s spectral component of adventitious carbon (C-C, C-H) at 284.8 eV. In addition to the survey photoelectron spectra, narrow spectral regions (Al 2p, S 2p, Mo 3d, W 4f, C 1s and O 1s) were recorded. The binding energy (BE) scale of the spectrometer was preliminarily calibrated using the position of the peaks for the Au 4f7/2 (83.96 eV) and Cu 2p3/2 (932.62 eV) core levels of pure metallic gold and copper. The pass energy of the analyser was 160 eV for the survey spectra and 40 eV for the narrow scans. The individual spectral regions were analysed to determine the BE of the peaks, identify the chemical state of the elements and calculate the relative ratios of the elements on the catalyst surface.

7 )

 7 makes it possible to determine the valence of atoms, the distribution of electron density on them and the structure of electronic bands; this information is of fundamental importance for studies of superconductivity, catalysis and other phenomena. The study of an extended fine structure extending in the energy range from 30 to ∼1500 eV above the absorption edge (EXAFS region) makes it possible with a sufficiently high accuracy to determine the distances, coordination numbers and the type of atoms in the local environment of atoms, at the absorption edge of which the studies are carried out.

Fig. 7

 7 Fig. 7Normalized XAS Spectrum of Pd foil, Eo = 24.350 keV[24]

Fig. 8 .

 8 Fig. 8. Scheme of laboratory hydrotreating bench-scale flow unit.

  k are the pseudo-first-order reaction constants for the DBT HDS and naphthalene HYD (mol g -1 h -1 ), respectively, DBT x and Naph x are the conversions (%) of DBT, and naphthalene, respectively, DBT F and Naph F are the reactant flows in moles (mol h -1 ) and W is the weight of the catalyst (g).

2 Product

 2 Only traces of hydrogenated tetrahydro-and hexahydrodibenzothiophenes were observed. The HYD/DDS selectivity was calculated according to the reaction network for DBT HDS: CCHB, CBCH and CBP are the concentrations (mol. %) of CHB, DCH and BP in the reaction products, respectively.

  flows (mol h -1 ), DBT x and Naph x are the conversions (%) of DBT and naphthalene, respectively; W is the weight of the catalyst (g); content of W and Mo, respectively, in MonW12-nS2 species (wt. %); D is the dispersion of MonW12-nS2 species; W Ar and Mo

  The composition and structure of the active phase 1.3.2 On active phase and active sites of unpromoted hydrotreating catalysts 1.3.3 Relations between active phase composition and catalytic properties 1.3.4 The influence of the support on the catalytic activity 1.3.5 Mesostructured silica as a promising support for hydrotreating catalysts 1.4 Supported MoWS catalysts 1.4.1 Methods for the synthesis of supported mixed MoWS hydrotreating catalysts 1.4.2 Heteropolyanions as effective precursors of hydrotreating catalysts 1.4.2.1 Type, composition and structures of heteropolyanions 1.4.2.2 Heteropolyanions as precursor for hydrotreating catalysts 1.4.2.3 Mixed MoW-heteropolyanions as interesting precursors for hydrotreating catalysts

	1. Literature review
	1.1 Introduction
	1.2 The development of hydrotreating processes
	1.2.1 The role and purpose of hydrogenation processes in oil refining
	1.2.2 Main compounds and target reactions of hydrotreating
	1.3 Hydrotreating sulfide catalysts
	1.3.1 1.5 Unsupported (bulk) sulfide hydrotreating catalysts
	1.5.1 Methods for the synthesis of unsupported sulfide hydrotreating catalysts
	1.5.2 Promoted unsupported sulfide catalysts
	1.5.3 Unsupported mixed (Ni)MoW catalysts
	1.5.4 The bulk catalysts: from science to industry
	1.6 Conclusion
	References
	Chapter 2
	2. Unpromoted MoWS hydrotreating catalysts supported on alumina
	2.1 Introduction
	2.2 Synthesis and characterization of H4SiMonW12-nO40 Keggin-type heteropolyacids
	53B2.2.1	Synthesis of H4SiMonW12-nO40 Keggin-type heteropolyacids
	2.2.1.1 Synthesis of monometallic H4SiMo12O40 and H4SiW12O40 heteropolyacids
	60B2.2.1.2

  type heteropolyacids Comparison of the catalytic properties of supported mixed MoW catalysts based on alumina and mesostructured silica (SBA-15 and COK-12).

		Introduction
	6B3.2	Synthesis of NiMo(W)/Al2O3 catalysts
	7B3.3
	Introduction	107
	20BFull
		Introduction	125
	25B5.2			126
	26B5.2.1	Supports and catalysts preparation	126
	62B2.2.2.1 27B5.2.2 Characterization of MoW/Sup catalysts IR and Raman analysis	127
	63B2.2.2.2 28B5.2.3	Single-crystal XRD analysis

Table 2 . 1

 21 Crystal data, data collection and structure refinement parameters for H4[SiMo6W6O40]•36H2O and H4[SiMo9W3O40] 36H2O

	Crystal data

R 1 (F) all , [I>3σ(I)] = ∑�|F O | -|F C |�/ ∑|F O | (%)

Table 2 . 2

 22 The oxidic catalyst precursors after maturation were dried at 60°C (4 h), 80°C (2 h) and 100°C (4 h) in air atmosphere without further calcination. The chemical compositions of the prepared catalysts are given in Table2.2. All the obtained samples had approximately the same textural characteristics, regardless of the type of precursor. Composition and textural characteristics of sulfided Mo(W) catalysts.

	Catalyst	d(Me) at nm -2	Content (wt. %) Mo W	Textural characteristics SBET a (m 2 g -1 ) Vp b (cm 3 g -1 )	D c (nm)
	Mo12/Al2O3 *		18.0	-	215	0.53	7.6
	Mo9W3/Al2O3 Mo9+W3/Al2O3		13.1	7.1	204 210	0.56 0.59	7.7 7.5
	Mo6W6/Al2O3 Mo6+W6/Al2O3 Mo3W9/Al2O3 * Mo3+W9/Al2O3 *	3.9	8.5 4.2	13.8 20.1	191 204 210 206	0.58 0.59 0.53 0.53	7.9 7.8 7.6 7.6
	Mo1W11/Al2O3 Mo1+W11/Al2O3		1.4	24.2	205 208	0.53 0.55	7.5 7.6
	W12/Al2O3 *		-	26.2	208	0.54	7.6

a SBET is the surface area, b Vp is the pore volume, and c D is the pore diameter. * adapted from

[10]

Table 2 .3. Mo 6 W 6 /Al 2 O 3 Mo 6 +W 6 /Al 2 O 3 Mo 9 W 3 /Al 2 O 3 Mo 9 +W 3 /Al 2 O 3 Fig

 2

. 2.7. TEM micrographs of sulfided MoW catalysts.

10 nm 10 nm 10 nm 10 nmTable 2 . 3 .

 23 Morphological characteristics of sulfided Mo(W) catalysts.The average particle length and stacking number ranged from 3.6 to 4.2 nm and from 1.2 to 2.3, respectively. The average length appears to be similar regardless of the Mo/W ratio, between 3.5 and 3.8 nm for Mon+Wn-12/Al2O3 catalysts and between 3.8 and 4.0 nm for MonWn-12/Al2O3 catalysts, the highest value (4.2 nm) being observed on W12/ Al2O3 catalyst. The highest stacking value corresponds

		Average	Average	Dispersion		Distribution of slab length (rel. %)		Distribution of stacking number
	Catalyst	length L̅	stacking	of MoS2							(rel. %)
	(nm) to Mo3+W9/Al2O3 catalyst (2.3). The increase in molybdenum percentage led to a decrease of the number N̅ particles D <2 nm 2-4 nm 4-6 nm 6-8 nm >8 nm	1	2	3	>4
	Mo12/Al2O3 stacking number of Mo(W)S2 particles, from 2.3 to 1.2 for Mon+Wn-12/Al2O3 catalysts, from 2.0 to 3.8 1.2 0.31 3 61 30 5	1	79		2	1
	Mo9W3/Al2O3 1.5 for MonWn-12/Al2O3 catalysts. The dispersion of the active phase varies between 0.28 and 0.34. 4.0 1.5 0.29 2 49 40 8	1	57		4	2
	Mo9+W3/Al2O3	3.6	1.2	0.32	6	62	29	2	1	82		2	1
	Mo6W6/Al2O3	3.9	1.4	0.30	3	57	34	5	1	70		5	1
	Mo6+W6/Al2O3	3.8	1.3	0.31	3	58	31	6	2	73		2	1
	Mo3W9/Al2O3	3.8	2.0	0.33	4	56	36	3	1	34		14	10
	Mo3+W9/Al2O3	3.5	2.3	0.29	6	68	23	2	1	29		30	12
	Mo1W11/Al2O3	3.9	2.1	0.30	3	56	37	4	1	32		19	5
	Mo1+W11/Al2O3	4.1	2.2	0.29	2	51	40	7	2	31		22	4
	W12/Al2O3	4.2	1.2	0.28	2	46	42	8	2	84		2	-

a MoS2 dispersion calculated from HRTEM results.

Table 2 . 4 .

 24 Metal distribution for Mo and W species present at the surface of sulfided Mo(W) catalysts.

		Mo percentage (rel. %)	W percentage (rel. %)	Number of edge sites (10 20 at g -1 )
	Catalyst	MoS2 MoSxOy	Mo 6+	WS2	WSxOy	W 6+	𝑀𝑀𝑀𝑀 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐼𝐼𝐼𝐼	𝑊𝑊 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐼𝐼𝐼𝐼	∑𝑀𝑀𝑀𝑀 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐼𝐼𝐼𝐼	+ 𝑊𝑊 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐼𝐼𝐼𝐼
	Mo12/Al2O3 *	70	15	15	-	-	-	0.88	-	0.88
	Mo9W3/Al2O3	91	7	2	85	3	12	0.82	0.60	1.42
	Mo9+W3/Al2O3	83	11	6	71	6	23	0.87	0.59	1.46
	Mo6W6/Al2O3	91	7	1	76	5	19	0.61	0.80	1.40
	Mo6+W6/Al2O3	81	16	3	62	23	23	0.63	0.80	1.43
	Mo3W9/Al2O3 *	81	14	5	64	18	18	0.23	0.94	1.17
	Mo3+W9/Al2O3 *	90	8	2	77	11	12	0.48	0.74	1.22
	Mo1W11/Al2O3*	85	13	2	51	7	42	0.10	1.02	1.12
	Mo1+W11/Al2O3*	89	2	9	57	8	35	0.11	0.98	1.09
	W12/Al2O3 *	-	-	-	51	16	33	-	0.98	0.98

* -Adapted from

[10]

Table 2 . 5 .

 25 Catalytic properties of MoW/Al2O3 catalysts in the hydrotreating of DBT and naphthalene.

	Catalyst	Conversion (%)	Reaction rate constant (×10 5 mol h -1 g -1 )	SHYD/DDS
		DBT HDS	Naphthalene HYD	kHDS	kHYD
	Mo12/Al2O3 *	51.9	40.5	45	157	1.73
	Mo9W3/Al2O3	59.2	55.2	55	244	2.83
	Mo9+W3/Al2O3	44.3	39.6	36	153	2.51
	Mo6W6/Al2O3	61.3	57.3	59	258	3.16
	Mo6+W6/Al2O3	47.6	42.2	40	166	2.33
	Mo3W9/Al2O3 *	52.9	47.5	46	195	3.18
	Mo3+W9/Al2O3 *	32.1	27.3	24	97	1.76
	W12/Al2O3 *	22.2	23.8	15	82	2.61
	* -Adapted from [10]
	The reactants conversions varied in a wide range, from 22.2 to 61.3% for HDS of DBT and
	from 23.8 to 57.3% for HYD of naphthalene over all prepared catalysts. The W12/Al2O3 sample

demonstrated the lowest activities in DBT HDS (22.2 %) and in naphthalene HYD (23.8 %).

Table 3 . 1 .

 31 Composition of the dried solids and textural characteristics of sulfided NiMo(W)/Al2O3 catalysts.

	Catalyst	Mo	Content (wt. %) W	Ni	Textural characteristics SBET (m 2 g -1 ) Vр (cm 3 g -1 )	D (nm)
	NiW12	-	20.8	3.3	240	0.58	8.4
	NiMo1W11	0.9	19.0	3.4	202	0.47	8.4
	NiMo3W9	2.8	15.9	3.4	233	0.49	8.4
	NiMo6W6	5.5	10.5	3.4	205	0.48	8.2
	NiMo9W3	8.4	8.7	3.5	225	0.48	8.4
	Ni(Mo1+W11)	0.9	19.0	3.4	230	0.51	8.4
	Ni(Mo3+W9)	2.8	15.9	3.4	210	0.48	8.4
	Ni(Mo6+W6)	5.5	10.5	3.4	216	0.50	8.4
	Ni(Mo9+W3)	8.4	8.7	3.5	233	0.53	8.6
	NiMo12	12.0	-	3.7	236	0.42	8.4
	3.3

Characterization of sulfided NiMo(W)/Al 2 O 3 catalysts

Table 3 . 2 .

 32 Morphological characteristics of the NiMo(W)S active phase species.

	Catalyst	Average length L̅ (nm)	Average stacking number N̅	Dispersion of NiMo(W)S particles D
	NiW12	3.9	2.1	0.30
	NiMo1W11	3.9	2.1	0.30
	NiMo3W9	3.6	1.8	0.32
	NiMo6W6	3.7	2.0	0.32
	NiMo9W3	3.6	1.9	0.32
	Ni(Mo1+W11)	3.7	1.9	0.32
	Ni(Mo3+W9)	3.6	1.4	0.32
	Ni(Mo6+W6)	4.1	1.9	0.29
	Ni(Mo9+W3)	3.8	1.7	0.31
	NiMo12	3.4	1.6	0.34

Table 3 . 3 .

 33 Relative metal fractions measured by XPS for Mo and W species present at the surface of sulfided NiMo(W)/Al2O3 catalysts.

		Ni percentage (rel. %)	Mo percentage (rel. %)	W percentage (rel. %)
	Catalyst										Ni(Mo+W)edge
		NiMo(W)S	NiS	Ni 2+	MoS2 MoSxOy Mo 6+	WS2	WSxOy	W 6+
	NiW12	30	56	14	-	-	-	64	7	29	0.74
	NiMo1W11	37	51	12	72	28	0	69	10	21	0.82
	NiMo3W9	42	54	4	74	26	0	74	8	18	0.82
	NiMo6W6	40	50	10	71	26	4	73	7	20	0.88
	NiMo9W3	43	49	8	69	25	6	77	8	15	0.92
	Ni(Mo1+W11)	35	55	10	70	30	0	59	9	33	0.79
	Ni(Mo3+W9)	36	43	21	67	19	14	59	7	34	0.87
	Ni(Mo6+W6)	37	44	19	63	22	15	61	10	29	0.97
	Ni(Mo9+W3)	39	43	18	60	23	17	64	9	27	0.99
	NiMo12	62	27	11	73	15	12	-	-	-	0.90

Table 3 . 4 .

 34 Structural parameters obtained by multi-edge fitting from the Fourier-analysis of Mo Kand W L3-edges EXAFS spectra of sulfided NiMo(W)/Al2O3 catalysts.

	Scattering path	N	R (Å)	σ² x 10 3 (Ų)	E0 (eV)	R-factor
			NiMo 12 /Al 2 O 3
	Mo-S	5.4 ± 0.5	2.41 ± 0.01	3.2 ± 0.7
	Mo-Mo	3.7 ± 0.7	3.18 ± 0.01	4.0 ± 0.9	1.9 ± 0.8	0.0057
			NiW 12 /Al 2 O 3
	W-S	5.8 ± 0.7	2.41 ± 0.01	3.0 ± 0.9
	W-W	4.3 ± 1.1	3.16 ± 0.01	4.3 ± 1.0	7.7 ± 1.6	0.0105
			NiMo 3 W 9 /Al 2 O 3
	Mo-S	5.5 ± 0.5	2.42±0.01	3.3 ± 0.8
	Mo-Mo	1.5 ± 0.6	3.21±0.02	4.8 ± 2.3	3.0±1.0
	Mo-W	2.8 ± 1.2	3.20±0.01	4.8 ± 2.3		0.010
	W-S	6.1 ± 0.7	2.41±0.01	3.0± 0.8
	W-W	2.8 ± 1.6	3.17±0.01	4.8 ± 2.3	9.0 ±1.7
			Ni(Mo 3 +W 9 )/Al 2 O 3
	Mo-S	5.9 ± 0.9	2.42 ± 0.01	3.4 ± 1.0
	Mo-Mo	3.3 ± 0.8	3.19 ± 0.01	4.4 ± 1.1	2.9 ± 2.0
	Mo-W	1.6 ± 0.8	3.19 ± 0.03	4.4 ± 1.1		0.0058
	W-S	5.6 ± 0.4	2.41 ± 0.01	2.9 ± 0.5
	W-W	3.5 ± 0.8	3.17 ± 0.01	4.4 ± 1.1	9.2 ± 1.3

At the W L3-edge of trimetallic samples, a lower W-W coordination number is observed, compared to that of NiW12 catalyst. The morphology of WS2 slabs could thus be affected by the

Table 3 . 5 .

 35 Catalytic properties of NiMo(W)/Al2O3 catalysts in HDT of a mixture of DBT and naphthalene.

		Conversion (%)	k×10 4 (mol h -1 g -1 )
	Catalysts	DBT HDS	Naphthalene HYD	kHDS	kHYD	SHYD/DDS
	NiW12 a	49.0 ± 1.5	39.6 ± 0.6	16.4 ± 0.7 60.2 ± 2.3	0.32
	NiMo1W11 a	60.1 ± 1.8	36.8 ± 1.1	22.3± 1.1 56.3 ± 2.1	0.34
	NiMo3W9 a	73.2 ± 2.2	38.9 ± 1.2	31.9 ± 2.0 60.2 ± 2.7	0.19
	NiMo6W6	71.6	37.3	30.6	56.8	0.21
	NiMo9W3	75.4	38.9	34.2	60.0	0.24
	Ni(Mo1+W11) a	49.0 ± 1.0	35.0 ± 0.7	16.4 ± 0.5 52.5 ± 1.3	0.49
	Ni(Mo3+W9)	42.0	32.1	13.3	47.0	0.36
	Ni(Mo6+W6)	60.4	33.0	22.5	48.8	0.24
	Ni(Mo9+W3)	72.7	37.6	31.6	58.9	0.20
	NiMo12 a	70.8 ± 2.1	49.2 ± 1.4	29.9 ± 1.8 79.2 ± 3.3	0.13

(Test conditions: T=280℃, LHSV= 40 h -1 , KH=500 NL L -1 , Р = 3.0 MPa) a samples have been tested twice

Table 3 . 6 .

 36 Catalytic properties of NiMo(W)/Al2O3 catalysts in HDT of a mixture of DBT, naphthalene and quinoline.

	Catalysts	DBT HDS	Conversion (%) Naphthalene HYD	Quinoline HDN b	HDS	k×10 4 (mol•g -1 •h -1 ) HYD	HDN	SHYD/DDS
	NiW12 a	65.0 ± 2.0	11.0 ± 0.3	10.2 ± 0.4	25.5 ± 1.4 14.2 ± 0.5	2.0 ± 0.1	0.24
	NiMo1W11 a	77.9 ± 2.3	14.2 ± 0.4	21.2 ± 0.6	36.8 ± 2.7 18.6 ± 0.6	4.4 ± 0.1	0.03
	NiMo3W9 a	72.0 ± 2.2	7.2 ± 0.3	19.4 ± 0.7	31.0 ± 1.9	9.1 ± 0.4	4.0 ±0.1	0.02
	NiMo6W6	70.2	6.4	17.0	29.5		8.0		3.4	0.01
	NiMo9W3	73.2	6.0	14.3	32.0		7.5		2.9	0.01
	Ni(Mo1+W11) a	70.0 ± 1.4	6.9 ± 0.3	15.0 ± 0.5	29.3 ± 1.1	8.8 ± 0.4	3.0 ± 0.1	0.08
	Ni(Mo3+W9)	60.0	4.0	14.5	22.3		5.0		2.9	0.05
	Ni(Mo6+W6)	68.0	4.8	13.8	27.8		6.0		2.7	0.01
	Ni(Mo9+W3)	71.3	5.4	13.2	30.3		6.7		2.6	0.01
	NiMo12 a	79.6 ± 2.4	6.4 ± 0.3	11.8 ± 0.4	38.8 ± 2.9	8.0 ± 0.3	2.3 ± 0.1	0.09
	(Test conditions: T=280℃, LHSV= 40 h -1 , KH=500 NL L -1 , Р = 3.0 MPa)

a samples tested twice; b indicates the depth of nitrogen removal (%).

Table 3 .

 3 7 HDS rate constants (kHDS) and composition of products obtained in the process of SRGO hydrotreating

	over the synthesized NiMo(W)/Al2O3 catalysts
	Content of products
	Catalyst

Table 5 . 1

 51 Metal loadings and textural properties of MoW/Sup catalysts after sulfidation

	Catalyst	Content, wt % d(Mo+W) at nm -2 MoO 3 WO 3	Textural characteristics SBET a (m 2 g -1 ) Vp b (mL g -1 ) D c (nm)
	Al2O3	-	-	-	240	0.91	7.6
	Mo3W9/Al2O3 Mo3+W9/Al2O3	4.2	20.1	3.9	210 206	0.53 0.53	7.6 7.6
	COK-12	-	-	-	720	1.27	7.5
	Mo3W9/COK-12 Mo3+W9/COK-12	3.5	16.7	1.2	587 523	1.02 0.92	7.2 7.1
	SBA-15	-	-	-	850	1.18	5.6
	Mo3+W9/SBA-15 Mo3W9/SBA-15	4.1	20.7	1.2	452 447	0.78 0.64	5.6 5.6

a SBET is the surface area, b Vp is the pore volume, and c D is the pore diameter.

Table 5 . 2

 52 Morphological characteristics of MoW/Sup catalysts

			Morphological characteristics
	Catalyst	Average length L̅	Average stacking	Dispersion of Mo(W)S
		(nm)	number N̅	particles D
	Mo3W9/Al2O3	3.8	2.0	0.29
	Mo3+W9/Al2O3	3.5	2.3	0.33
	Mo3W9/COK-12	3.7	2.3	0.31
	Mo3+W9/COK-12	4.0	2.5	0.29
	Mo3+W9/SBA-15	3.9	2.3	0.30
	Mo3W9/SBA-15	4.1	2.7	0.29

Table 5 . 3 .

 53 Metal distribution for Mo and W species present at the surface of sulfided Mo(W) catalysts.

	Catalyst	Mo percentage (rel. %) MoS2 MoSxO y Mo 6+	W percentage (rel. %) WS2 WSxO y W 6+
	Mo3W9/Al2O3	89	2	9	58	8	34
	Mo3+W9/Al2O3	90	8	2	77	11	12
	Mo3W9/COK-12	94	6	0	78	15	7
	(Mo3+W9)/COK-12	99	1	0	83	9	8
	Mo3W9/SBA-15	80	20	0	71	20	9
	(Mo3+W9)/SBA-15	82	18	0	75	18	7

Table 5 . 4 .

 54 Catalytic properties of MoW/Sup catalysts in hydrotreating of DBT and naphthalene.

		Conversion (%)	k×10 5 (mol h -1 g -1 )
	Catalyst	DBT HDS	Naphthalene HYD	DBT HDS	Naphthalene HYD	SHYD/DDS
	Mo3W9/Al2O3*	52.9	47.5	46.1	195.4	3.18
	Mo3+W9/Al2O3*	32.1	27.3	24.2	97.1	1.76
	Mo3W9/COK-12	23.4	17.1	14.8	52.3	1.53
	Mo3+W9/COK-12	22.4	14.6	14.1	44.0	1.32
	Mo3W9/SBA-15	52.2	40.9	64.6	230.8	2.48
	Mo3+W9/SBA-15	47.8	38.6	56.9	214.1	2.78

Table 5 . 5 .

 55 Composition, textural characteristics and morphology of sulfide particles of prepared Mo(W)/SBA-15 catalysts

		Content, wt. %.	Textural characteristics		Morphological characteristics
	Catalyst	MoO 3	WO 3	SBET a (m 2 g -1 )	Vp b (cm 3 g -1 )	D c (nm)	Average length L̅ (nm)	Average stacking number N̅	Dispersion of Mo(W)S2, D
	SBA-15	-	-	850	1.18	5.6	-	-	-
	W12/SBA-15	-	26.4	453	0.55	5.6	4.4	2.6	0.27
	Mo3W9/SBA-15	4.2	20.1	452	0.78	5.6	3.9	2.3	0.30
	(Mo3+W9)/SBA-15	4.2	20.1	447	0.64	5.6	4.1	2.7	0.29
	Mo6W6/SBA-15	9.1	14.7	424	0.61	5.4	3.0	2.0	0.38
	(Mo6+W6)/SBA-15	9.1	14.7	469	0.69	5.6	3.1	2.1	0.37
	Mo9W3/SBA-15	14.1	7.6	450	0.66	5.6	3.4	2.1	0.34
	(Mo9+W3)/SBA-15	14.1	7.6	455	0.65	5.7	3.2	2.1	0.36
	Mo12/SBA-15	18.1	-	492	0.68	5.6	4.1	2.7	0.29

Table 5 . 6 .

 56 Metal distribution for Mo and W species present at the surface of sulfided Mo(W)

	catalysts.
	Catalyst	Mo percentage (rel. %) MoS2 MoSxOy Mo 6+	W percentage (rel. %) WS2 WSxOy W 6+
	W12/SBA-15				61	14	25
	Mo3W9/SBA-15	80	20	0	71	20	9
	(Mo3+W9)/SBA-15	82	18	0	75	18	7
	Mo6W6/SBA-15	83	17	0	69	16	15
	(Mo6+W6)/SBA-15	84	14	2	57	24	19
	Mo9W3/SBA-15	86	12	2	55	28	17
	(Mo9+W3)/SBA-15	88	8	4	62	13	25
	Mo12/SBA-15	74	13	13
	5.3.4 Mo(W)/SBA-15 catalysts in hydrotreating of DBT and naphthalene
	A series of SBA-15 based catalysts, similar to the samples presented in Chapter 2, was obtained
	to determine the optimal atomic ratio Mo/(Mo+W). Catalysts were tested in co-hydrotreatment of the
	mixture of DBT with naphthalene and toluene (as a solvent). Experimental conditions are similar to
	those described in Chapter 2. The conversion varied from 24.6 to 60.8% for DBT and from 19.5 to
	54.9% for naphthalene, respectively (

Table 5 .

 5 7)showing large effect of the Mo/(Mo+W) ratio.

Table 5 . 7 .

 57 Catalytic properties of Mo(W)/SBA-15 catalysts in hydrotreating of DBT and naphthalene.

		Conversion (%)	Reaction rate constant
	Catalyst	DBT HDS	Naphthalene HYD	(×10 5 mol h -1 g -1 ) kHDS kHYD	SHYD/DDS
	W12/SBA-15	24.6	19.5	24.7	95.2	2.66
	Mo3W9/SBA-15	52.2	40.9	64.6	230.8	2.48
	(Mo3+W9)/SBA-15	47.8	38.6	56.9	214.1	2.78
	Mo6W6/SBA-15	55.9	50.1	71.6	305.1	2.52
	(Mo6+W6)/SBA-15	57.5	51.2	74.9	314.9	2.82
	Mo9W3/SBA-15	58.3	52.3	76.5	324.9	1.92
	(Mo9+W3)/SBA-15	60.8	54.9	82.0	349.5	2.12
	Mo12/SBA-15	49.3	39.7	57.7	222.0	1.10
	Based on the conversions obtained, the reaction rate constants were calculated and are reported
	in

H4SiMo12O40 (SiMo12) HPA was obtained according to the method described by Sanchez et al. [11]. Sodium molybdate dihydrate (58.1 g) was dissolved in 240 ml of distilled water (1 mol L -1 ions MoO4 2-) and then 74 ml of concentrated nitric acid (HNO3) was added to the solution under stirring.

After that, 100 ml of sodium silicate solution (0.2 mol L -1 SiO3 2-) was added dropwise under stirring.

To transform the polyanion from β to α form, the solution was heated and kept at 80℃ for 40 min.

Then the solution was cooled down to room temperature and extraction was performed with ether [12,13]. The cooled solution was transfused into a 500 ml separatory funnel. Then, in equal portions (30 ml each), concentrated hydrochloric acid and diethyl ether were added. The mixture was stirred, the pressure being periodically released. The heavy phase containing the ether-polymolybdate complex was isolated piece by piece by addition of new portion of acid and ether. The isolated ether complex was mixed with distilled water in a ratio of 2/1 and poured into a Petri dish. The resulting solution was placed in a fume hood to remove ether vapors and transferred to a fridge for crystallization at a temperature of 4℃.

H4[SiW12O40] (SiW12) HPA was prepared according to [12]. 182 g оf sodium tungstate dihydrate was dissolved in 300 ml of hot distilled water. 240 ml of 3 M aqueous HCl were added by fractions under vigorous stirring in order to redissolve the local precipitate of tungstic acid. Then, in a separating funnel, 100 ml of sodium silicate solution (0.5 M) was added by dropping and following by 50 ml of hydrochloric acid solution. The resulting mixture was heated and kept at 90℃ for 1 h.

The acid was isolated from the solution by the ethereal method, similar to that used in the synthesis of H4SiMo12O40 acid.

Synthesis of mixed H 4 SiMo n W 12-n O 40 (n = 1 and 3) heteropolyacids using lacunar salts

Mixed H4SiMo3W9O40 and H4SiMo1W11O40 HPAs were prepared by multistage synthesis using the corresponding lacunar structures [9]. The lacunar salt α-K8[SiW11O39] was obtained by the following procedure: 11 g of sodium silicate pentahydrate was dissolved in 100 ml of distillate water followed by the addition of 4 M solution of hydrochloric acid (50 ml); the second solution was prepared by dissolving 182 g of sodium tungstate dihydrate in 230 ml of distilled water followed by the addition of 4 M solution of hydrochloric acid (145 ml). Then, the previously obtained sodium silicate solution was added dropwise to the second solution with stirring. The final solution was boiled at 65℃ for 1 h and then potassium chloride (75 g) was added to precipitate K8[SiW11O39] separated by filtration [14]. The corresponding mixed α-K4[SiMo1W11O40] salt was obtained according to the Chapter 3 Binding Energy (eV)

Ni-promoted MoWS hydrotreating catalysts supported on alumina

Chapter 3

presence of Mo. The same evolution of the coordination numbers was also observed for the corresponding unpromoted catalysts showing that the behavior does not depend on the Ni promotion. The second coordination shell of trimetallic samples contains one heteroatomic metalmetal contribution. It should also be noticed that tungsten concentration was three times higher than molybdenum. Thus, the obtained distribution of the contributions may be indicative of a greater presence of tungsten atoms surrounding small molybdenum islands, which have more W neighbors in this case. That is in agreement with the results of HAADF characterization.

Determination of the local environments of the Ni atoms in the sulfided catalysts from the EXAFS data is hampered. In sulfided catalyst nickel could be part of various compounds, such as Ni3S2, NiSx, and NiMo(W)S phase. Such multicomponent system presents different scattering paths overlapping in the R-range, which leads to instability of the fitting procedure and correlations of the fitting parameters.

The Ni K-edge XANES and EXAFS spectra of sulfided catalysts are shown in Fig. 3.7A and B, respectively. For comparison purposes, Ni/Al2O3 catalyst was also synthesized using impregnating solutions prepared from a mixture of nickel carbonate and citric acid with molar ratio citric acid/Ni = 1/1. This sample was gas-phase sulfided under the same conditions as Ni(Mo)W/Al2O3 catalysts and its XAS spectra were also recorded.

All Ni K-edge EXAFS spectra are similar in phase (Fig. 3.7B). The oscillations of the Ni-S scattering will dominate in the lower k-region with a characteristic maximum around 6 Å -1 corresponding to the maximum of the scattering amplitude of S backscatters, whereas Mo and W backscatters will contribute at the higher k, leading when they are coordinated as second neighbors of promotors to a second relative maximum beyond 10 Å -1 [18,19]. For the bimetallic and trimetallic catalysts, this second relative maximum is observed in agreement with the decoration of the sulfide slabs by nickel atoms. It is noteworthy that both trimetallic catalysts (Fig. 3.7B (b)) display a maximum at the higher k, which is markedly lower in intensity than the ones observed for the bimetallic catalysts (Fig. 3.7B (a)). This decrease of signal in the higher k-region is in agreement with the simultaneous presence of W and Mo, that is due to the destructive interference of the individual scattering paths of these atoms, which are out of phase [17,20].

The Ni K-edge XANES spectra of the NiMo(W)/Al2O3 catalysts differ from Ni/Al2O3 due to the changes in the electronic structure by the appearance of Mo and/or W neighbors (Fig.

Chapter 3

The values of the degree of inhibition were calculated taking into account the values of the reaction rate constants before and after the addition of quinoline in model feed. The introduction of quinoline into the model mixture promoted a slight increase in HDS activity, as evidenced by the obtained values of the inhibition effect (Fig. 3.10a). This effect is pronounced for bimetallic NiW12 and trimetallic catalysts with high W percentages in the catalysts. The mechanism of the effect of nitrogen-containing compounds on the hydrogenolysis of DBT was previously described in the literature [25], [26]. It was shown that the tetrahydroquinoline formed as a main product of the The highest selectivity in the DBT HDS is observed on a bimetallic NiW12/Al2O3 catalyst due to the lesser susceptibility to the action of quinoline. With the introduction of molybdenum into the structure of the active phase, a sharp decrease in selectivity is observed for both types of trimetallic catalysts. Kinetic studies using model compounds presented in [27,28] also confirm the assumption that nitrogen-containing heterocyclic compounds inhibit to a greater extent the HYD route of the DBT HDS.

The inhibition of the reaction by the N-containing component is more intense for naphthalene HYD, which is consistent with literature data [29]. The inhibitory effect of naphthalene HYD ranges Chapter 3 inhibition resistance being observed at a low Mo content, the Mo/(Mo+W) ratio in the catalysts should be selected depending on the content of N-containing compounds in the feed. For petroleum fractions with a high nitrogen content, NiMoW catalysts with a high tungsten content are most preferred due to their higher resistance to inhibition effect.

Chapter 4

Bulk mixed MoWS hydrotreating catalysts

Supporting Information for Chapter 5

Mixed MoW catalysts supported on mesostructured silica

Annex the electrons of neighboring atoms. The outgoing photoelectron wave will be reflected from a neighboring atom, thereby creating an incoming electron wave. The final state is the sum of the outgoing and all incoming waves, one from each neighboring atom [25]. Depending on the type of neighboring atoms and the degree of their remoteness, the amplitude and frequency of sinusoidal oscillations μ vs E. The EXAFS function, χ (k), is described by the following formula [25]:

where 𝑆 𝑘 is the amplitude reduction factor due to many-body effects such as shake up/off processes at the central atom (denoted by i), Fj (k) is the backscattering amplitude from each of the Nj neighboring atoms of the j th type with a Debye-Waller factor of σj (to account for thermal vibration (assuming harmonic vibration) and static disorder (assuming Gaussian pair distribution) and at a distance between the absorbing atom and a neighbor rj away), 𝜙ij(k) is the total phase shift experienced by the photoelectron, and 𝑒 / is due to inelastic losses in the scattering process (due to neighboring atoms and the medium in between) with 𝜆 𝑘 -the photoelectron mean free path.

The Mo K-edge and W L1,2,3 -edges EXAFS spectra for bulk MoWS catalysts were collected at BM31 beamline of the ESRF (Grenoble, France) [26]. The sample powder was diluted with boron nitride, pressed into a pellet and sealed with Kapton tape. All spectra were recorded at room temperature in transmission mode with simultaneous collection of molybdenum and tungsten metal foils for energy calibration. The energy was selected with a double-crystal Si [START_REF] Nikulshina | Enhancing the hydrodesulfurization of 4,6-dimethyldibenzothiophene through the use of mixed MoWS2 phase evidenced by HAADF[END_REF] monochromator in the continuous scanning mode. EXAFS data analysis (normalization, background removal, energy alignment, extraction of χ(k) signal and Fourier analysis) was performed in the Demeter software package [27], FEFF6 [28] was used to calculate theoretical phases and amplitudes. The following structural parameters were set as variables during the fit: interatomic distances (R), coordination , respectively. Amplitude reduction factors (S0 2 ) were obtained by fitting bulk MoS2 and WS2 references. For direct comparison, the same fitting strategy was applied for the analysis of the data previously collected for the supported catalysts [29].

For NiMoW/Al2O3 catalysts, quick XAS was employed to follow in situ sulfidation of Mo, W and Ni in alumina supported trimetallic hydrotreating catalysts. XAS measurements were carried out at the ROCK beamline at the SOLEIL synchrotron [30]. Oxide catalysts were ground and loaded in the cell [31], where they were gas-phase sulfided in situ. Spectra were recorded in transmission mode.

The use of the remotely-controlled edge jumping capability of the beamline allows to record XAS spectra alternately at the Ni and Mo K-edges and W L1-, L2-, L3-edges on the same sample during one sulfidation procedure [30]. At the end of sulfidation, the cell was cooled down at room temperature and 1180 spectra at each edge for the as-sulfided catalysts were recorded and merged. Extended Xray absorption fine structure (EXAFS) spectra were background-subtracted with Athena and fitted with Artemis, which is an interface to IFEFFIT [27].

The following structural parameters were determined during the fit: interatomic distances (R),

Evaluation of catalytic activities in model reactions

The catalytic activity was determined in a flow high-pressure fixed-bed microreactor (Fig. 8).

Prior to testing, the catalysts were sulfided in a flow of H2S/H2 (10 vol. %) at atmospheric pressure and 400 °C for 2 h. 0.4 g of catalyst (0.25 -0.50 mm) was diluted with of low-surface-area sieved carborundum (0.2-0.4 mm) in a ratio of one to one and placed in isothermal zone of the reactor.

Catalysts were tested under the following conditions: 320°C, 3.0 MPa of hydrogen, 10 h -1 liquid hourly space velocity (LHSV) and a 500 NL L -1 volume ratio of hydrogen to feed. For evaluation of HDS and hydrogenation performances, a mixture of DBT (1000 ppm S), naphthalene (3 wt. %), hexadecane (as an internal standard, 1 wt. %) and toluene (as a solvent) was used. The liquid product compositions of the samples collected every hour were determined using a Crystall-5000 Gas

Chromatograph equipped with a 30 m OV-101 column.

For bulk catalysts the same unit was used. MonW12-nS2 catalysts (0.2 g) were diluted with 0.6 cm 3 of low-surface-area carborundum (0.2-0.4 mm) and placed in the center of the reactor. A toluene solution of DBT (Aldrich, 1500 ppm of S), naphthalene (Aldrich, 3 wt. %) and hexadecane (as an internal standard, 1 wt. %) was used as a model feedstock. The catalytic test was performed under the same conditions as for the supported catalysts but at a higher LHSV (40 h -1 ), since bulk catalysts are significantly superior in activity.