M El Doukkali
email: m.eldoukkali@usms.ma

S Paul

F Dumeignil

New insights in single-step hydrodeoxygenation of glycerol to propylene by coupling rational catalyst design with systematic analysis

Keywords: Rational catalyst design, Mo δ+ active-sites, in-situ/ex-situ analyses, single-step hydrodeoxygenation, glycerol-to-propylene routes

Glycerol-to-propylene routes involve overlapped hydrodeoxygenation (HDO) events on the active sites of heterogeneous catalysts which stability in aqueous media is a challenge. Herein, we proposed a rationalized approach to develop such catalysts by using hydrophobic-inert silica supports to protect highly reactive sub-2 nm Mo particles against leaching and sintering. Propylene yield from 84.1% to 65.6% were obtained on MoOx nanoparticles at space-velocity ~1.7 h -1 and H2-pressure ~50 bar. Carburizing MoOx nanoparticles to β-Mo2C leads to an extremely efficient HDO catalyst, featuring, unprecedented TOF !"#!$%&'& ranging from ~153.1 h -1 to 226.4 h -1 , while η-MoC phase gave better hydrogenation activity. Coupling in-situ XPS and kinetic data reveal that Mo 5+ , Mo 3+ and Mo 2+-C species play a critical role in the HDO events. Glycerol-topropylene progresses over MoOx via successive Mo 5+ /Mo 6+ and Mo 3+ /Mo 4+ cycles, while over β-Mo2C/η-MoC, it occurs mostly on carbidic-oxycarbide surface. Surface accumulation of CxHyOz species caused Mo δ+ re-oxidation, which hinders the HDO events.

INTRODUCTION

The development of sustainable technologies for transforming biomass into useful chemicals with improved energetic and environmental qualities, in substitution to the fossil-based ones, has been the driving force of many recent researches in heterogeneous catalysis. [START_REF] Lin | Emerging heterogeneous catalysts for biomass conversion: studies of the reaction mechanism[END_REF], [START_REF] Sudarsanam | Functionalised heterogeneous catalysts for sustainable biomass valorisation[END_REF] Many surface-catalyzed reactions such as dehydration, [START_REF] Xu | Recent catalytic routes for the preparation and the upgrading of biomass derived furfural and 5-hydroxymethylfurfural[END_REF] hydrogenation, [START_REF] Fang | Recent advances in heterogeneous catalytic transfer hydrogenation/hydrogenolysis for valorization of biomass-derived furanic compounds[END_REF] hydro-deoxygenation (HDO) [START_REF] Kim | Recent advances in hydrodeoxygenation of biomass-derived oxygenates over heterogeneous catalysts[END_REF] or including C-C/C-H breaking, [START_REF] Nabgan | Catalytic biohydrogen production from organic waste materials: A literature review and bibliometric analysis[END_REF] have been explored in attempts to approximate the properties of the target biobased compounds to those conventionally obtained from petrol-based resources. Catalytic HDO reaction, which involves overlapped deoxygenation-hydrogenation events per-site, is a well-suited strategy as it aims simultaneously at increasing the H/C ratio of the target biomolecules. In addition, oxygen removal by C-O cleavage is thermodynamically favored under typical HDO conditions (temperature ≤ 400ºC and H2-pressure ≤100 bar), [START_REF] Liu | Ambient-pressure and low-temperature upgrading of lignin bio-oil to hydrocarbons using a hydrogen buffer catalytic system[END_REF], [START_REF] Hulteberg | Process for preparing lower hydrocarbons from glycerol[END_REF] which allow avoiding biomass degradation to coke, CO and CO2 actually prominent at higher temperatures (≫ 450 ºC). [START_REF] Zakaria | A perspective on catalytic conversion of glycerol to olefins[END_REF] Therefore, such moderate conditions with less-energy consumption benefit also the techno-economic feasibility and environmental impact of the target HDO process. [START_REF] Lin | Emerging heterogeneous catalysts for biomass conversion: studies of the reaction mechanism[END_REF], [START_REF] Sudarsanam | Functionalised heterogeneous catalysts for sustainable biomass valorisation[END_REF] However, it is yet particularly challenging to develop efficient HDO catalysts able to work in H2O-rich hydrothermal reaction medium for producing acceptable yield of the desired products without excessive H2 consumption. This requires new design paradigm that can be built based on mechanistic and kinetic explorations of pertinent HDO model reactions to understand better how the oxygen can be removed with simultaneous hydrogenation in multioxygenated molecule without breaking the length of their carbon chain.

The HDO of glycerol that involves three -OH groups seems to be an ideal model reaction for such studies, which is encouraged by the great potential of crude bioglycerol upgrading to promote the economic competitiveness of biodiesel industry. [START_REF] Lari | Environmental and economical perspectives of a glycerol biorefinery[END_REF] It offers stimulating opportunities to obtain value-added compounds such as ethanol, [START_REF] Kostyniuk | One-step synthesis of ethanol from glycerol in a gas phase packed bed reactor over hierarchical alkali-treated zeolite catalyst materials[END_REF] 1-propanol, [START_REF] Wang | Catalytic transformation of glycerol to 1-propanol by combining zirconium phosphate and supported Ru catalysts[END_REF], glycidol, [START_REF] Kostyniuk | One-step synthesis of glycidol from glycerol in a gas-phase packed-bed continuous flow reactor over HZSM-5 zeolite catalysts modified by CsNO3[END_REF] allyl-alcohol, [START_REF] Konaka | Conversion of glycerol into allyl alcohol over potassium-supported zirconia-iron oxide catalyst[END_REF], [START_REF] Kostyniuk | Allyl alcohol production by gas phase conversion reactions of glycerol over bifunctional hierarchical zeolite-supported bi-and tri-metallic catalysts[END_REF], acrolein, [START_REF] Katryniok | Towards the Sustainable Production of Acrolein by Glycerol Dehydration[END_REF],

hydrogen, [START_REF] Doukkali | A comparison of sol-gel and impregnated Pt or/and Ni based γ-alumina catalysts for bioglycerol aqueous phase reforming[END_REF], [START_REF] Doukkali | Bioethanol/glycerol mixture steam reforming over Pt and PtNi supported on lanthana or ceria doped alumina catalysts[END_REF] C3-diols, [START_REF] Doukkali | Enhanced catalytic upgrading of glycerol into high valueadded H2 and propanediols: Recent developments and future perspectives[END_REF], [START_REF] Vasiliadou | Glycerol transformation to value added C3 diols: reaction mechanism, kinetic, and engineering aspects[END_REF], lights olefins, [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] etc. For example, converting glycerol-topropylene (GTP) can contribute to balance the demand/supply ratio of propylene whose market is frequently suffering from shortage. [START_REF] Monai | Propane to olefins tandem catalysis: a selective route towards light olefins production[END_REF] Also, obtaining a massively used product through this eco-friendly GTP route (with practically null carbon footprint) will undoubtedly mitigate not only the arising environmental issues, but also those related with the mismanagement of crude glycerol. Currently, GTP route is being investigated either via high-temperature processes, [START_REF] Zakaria | A perspective on catalytic conversion of glycerol to olefins[END_REF], [START_REF] Lima | Catalytic conversion of glycerol to olefins over Fe, Mo, and Nb catalysts supported on zeolite ZSM-5[END_REF] multi-steps reactions; including triple staged reactors (ZSM-5, Pd/Al2O3 and H-BEA catalysts), [START_REF] Blass | Conversion of glycerol to light olefins and gasoline precursors[END_REF] separate double-reactors (Ir/ZrO2 and ZSM-5), [START_REF] Yu | Propylene from renewable resources: catalytic conversion of glycerol into propylene[END_REF] Tandem-processes (Pt/ ZSM-5 coupled ZSM-5), [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] dual-bed catalysts (Cu-WOx/Al2O3 coupled with silica-alumina) [START_REF] Sun | Efficient production of propylene in the catalytic conversion of glycerol[END_REF] or (MoNi2P/Al2O3 coupled with ZSM5). [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF] Nevertheless, the viability of these strategies is questionable [START_REF] Chieregato | Olefins from Biomass[END_REF] because they involve: i) Intensive energy consumption for high-temperature reactions, ii) extensive H2 consumption particularly when noble metals are used, iii) complex catalysts combination with undefined active-sites, of which the catalytic stability is a major challenge, making difficult their pre-activation and postuse regeneration. Furthermore, the use of such catalysts mixtures is challenging under H2 co-feeding because, as shown in Scheme 1, they can, instead of completely remove oxygen from glycerol to selectively form propylene, only contribute to partial C-O scission to form C3-oxygenates, breaks C-C and C-H bounds to produce syngas (COx + H2) and compounds with shorter carbon chain length, or at the limit, saturate the C=C bound of propylene to yield propane.

To the best of our knowledge, there are only three recent works on single-step GTP reaction in the gas [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] and mixed gas-liquid [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF] phases. The latter shed interesting early insights using reduced MoFe elements supported on Activated Carbon (AC) that allowed reaching high propylene selectivity (~76.1%) at nearly full glycerol conversion. These incipient studies, however, only reported initial single-step GTP activities using large-residence times of highly diluted glycerol aqueous solutions (2 wt. %) in batch-reactor loaded with a high mass of catalyst (1.5 g). In addition, these catalysts were tested without adopting an approach that allows understanding the factors affecting the stability of partially reduced Mo 5+ sites; hypothesized as active-sites for GTP reaction. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Other researchers report only Mo 4+ states as preferred catalytic site for propylene formation, [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] which leaves some controversy. In addition, operating in batchreactor generated diffusion restrictions, particularly, when high loading of catalyst was used. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Further, it did not provide solutions for the problematic sintering and leaching of Mo and Fe metals revealed in these works. [START_REF] Kostyniuk | Allyl alcohol production by gas phase conversion reactions of glycerol over bifunctional hierarchical zeolite-supported bi-and tri-metallic catalysts[END_REF] This panoramic review reflects many gaps to progress towards the development of efficient enough HDO catalysts for single-step GTP route.

Mo oxides and carbides can be particularly relevant for C-O scission and C-C hydrogenation because

of their multi-oxidation states and noble metal-like catalytic functions, respectively. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF]- [START_REF] You | Understanding the effect of Mo2C support on the activity of Cu for the hydrodeoxygenation of glycerol[END_REF] They have been used as impregnated or bulk phases for GTP routes, [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] but suffered from leaching, sintering and low surface per volume ratio, particularly, when high metals content is used. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF]In situ time-resolved X-ray diffraction study of the synthesis of Mo2C with different carburization agents[END_REF] Thus, the utilization of Mo element lacks of effective experimental protocols that allows stabilizing high population of Mo species with intrinsic HDO reactivity and stability in hydrothermal medium that usually induce aging tendencies. [START_REF] Chen | Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts[END_REF] As for the design scheme, a limited number of approaches have been explored in HDO catalysis of glycerol that encompasses randomized trials of solid catalysts, [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Lima | Catalytic conversion of glycerol to olefins over Fe, Mo, and Nb catalysts supported on zeolite ZSM-5[END_REF], [START_REF] Zakaria | Catalyst screening for conversion of glycerol to light olefins[END_REF] building steps based on ab initio and DFT calculations [START_REF] You | Understanding the effect of Mo2C support on the activity of Cu for the hydrodeoxygenation of glycerol[END_REF], [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF], [START_REF] Wan | Controlling reaction pathways of selective C-O bond cleavage of glycerol[END_REF] or purely ex-situ analyses. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF] The first approach is based on an indiscriminate screening of a large number of materials. The time and cost invested in this approach may limit their utilization even when high-throughput experimentations are used. The strategies based on numerical modelling, which aim at predicting the ideal architectural schemes for experimental catalysts design, are revolutionizing the studies of catalyzed reactions. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF], [START_REF] Wang | Towards rational catalyst design: boosting the rapid prediction of transition-metal activity by improved scaling relations[END_REF] Even though the growing progresses of artificial intelligence and the use of machine learning can vastly reduce the calculations time, it is still unfeasible to obtain every single detail for each reaction intermediate over each site. Furthermore, the conceptual prediction is mainly evaluated based on linear scaling from ideal single-site to real catalyst with complex physiochemistry, [START_REF] Calle-Vallejo | Introducing structural sensitivity into adsorption-energy scaling relations by means of coordination numbers[END_REF] causing in many times that the predictions does not fit well the real behavior under reaction conditions. The adjustment of the catalyst's properties based on ex-situ characterization is the most reported approach in the literature. It consists of finding correlations between the physicochemical properties of as-synthesized or post-functionalized samples and/or of the used ones, and their performance in the reaction. This relatively logical principle and easy experimental exploration allow great progresses in catalysis; [START_REF] Murugappan | Operando NAP-XPS unveils differences in MoO3 and Mo2C during hydrodeoxygenation[END_REF], [START_REF] Zichittella | Quantification of Redox Sites during Catalytic Propane Oxychlorination by Operando EPR Spectroscopy[END_REF] however, excessive ex-situ characterization of catalysts introduces uncontrolled changes in their characteristics, what upset accurate interpretations.

In an attempt to overcome the aforementioned hurdles, our research aims at combining a rationalized synthesis-functionalization tasks of Mo oxides and carbides-based catalysts, which will involve in-situ and ex-situ manipulations of sub-2 nm Mo particles and systematic mechanistic-kinetic tests to reach relevant progresses in single-step catalytic GTP route. The idea is not to only develop and compare efficient HDO catalysts that are better than those of the state-of-the art reports, but also to project a series of well-justified research actions that will be meticulously articulated within a novel and powerful approach to reach further improvements in single-step GTP route. The proposed approach enables qualitive and quantitative modulation of Mo properties, which offers therefore a potential versatility to serve as bases for building new design schemes destined to HDO of complex biobased molecules.

In this way, we will take advantage of early insights in partial reduction of Mo 6+ to Mo δ+ intermediates, which are suggested to be critical for C-O bounds cleavage, [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF] Murugappan | Operando NAP-XPS unveils differences in MoO3 and Mo2C during hydrodeoxygenation[END_REF], [START_REF] Borowiec | An acrolein production route from ethanol and methanol mixtures over FeMo-based catalysts[END_REF] to expand on how the population of these Mo species onto silicas supports can be intensified without penalizing their diffusion ability. So, three types of Ordered Mesoporous Silicas (OMS) were first prepared using hydrothermal synthesis, in which the loading of sub-2 nm Mo particles as controlled by melt-infiltration backed by in-situ DSC, X-ray scattering, and HRTEM-STEM/EDX mapping. Two requirements reinforce this synthesis combination: the hydrothermal method enables tailoring the OMS channels as desired, [START_REF] Konaka | Conversion of glycerol into allyl alcohol over potassium-supported zirconia-iron oxide catalyst[END_REF] while meltinfiltration allows the liquid droplets of transition metals precursor to diffuse before its decomposition, [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] allowing a direct immobilization of thin Mo nanoparticles. This in-pores hosting strategy can protect Mo nanoparticles against leaching and agglomeration phenomena. [START_REF] Konaka | Conversion of glycerol into allyl alcohol over potassium-supported zirconia-iron oxide catalyst[END_REF] OMS are chosen as supports due to their inert surface for C-C breaking [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] and relatively hydrophobic character expecting to provide more watertolerant ability. [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] Determining the limit of maximal Mo loading will allow keeping the integrity of the 3D structure of OMS and their pores accessibility, and thereby, preserving their capacity to let the reactant and products diffuse properly in their porous network. In addition, it will avoid an excess of Mo loading that can remain on the outer surface of OMS, which can be subjected to sintering or leaching by hot water. Subsequent reduction or reductive-carburization, which will lean on literature to find the pertinent treatment conditions [START_REF]In situ time-resolved X-ray diffraction study of the synthesis of Mo2C with different carburization agents[END_REF], [START_REF] Wang | Synthesis and characterization of molybdenum carbides using propane as carbon source[END_REF] are expected to stabilize new Mo δ+ -C bounds at partially reduced state (δ + <+6). Thus, three Mo phases; MoOx, β-Mo2C and η-MoC, will be further studied in bulk and dispersed forms. Parallelly, extensive kinetic tests in single-step HDO of glycerol/water mixture were carried out at different temperatures, H2-pressures and weight-hourly-space-velocity (WHSV) to evaluate the performance of the studied systems in C-O removing, C-C hydrogenation and/or C-C breaking. All these actions allow determining the main factors affecting the single-step GTP reaction under gas and liquid-phase conditions. Thus, considering all the mentioned issues, we discuss herein the utility of the adopted research approach to go beyond the merely progress on the singlestep GTP route to exhibit a versatile re-design paradigm of HDO catalysts able to be used for a wide range of HDO reactions of other industry-relevant biobased molecules.

EXPERIMENTAL PROCEDURES

All the used reagents, of research-lab grade purity, are listed in Table S1.

Catalyst preparation

First, three type of OMS materials; SBA-15, MCM-41 and KIT-6, were prepared by modifying the procedures reported by Zhao, [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF] Kresge, [START_REF] Kresge | Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism[END_REF] and Kleitz,[START_REF] Kleitz | Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes[END_REF] respectively. Table 1 summarizes the main synthesis conditions that were chosen on purpose from existing literature [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF] , [START_REF] Kresge | Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism[END_REF] , [START_REF] Kleitz | Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes[END_REF] in order to obtain silica materials with the target 3D structure and meso-porosity.

In a typical synthesis, the desired structure-directing agent (P123, CATB and/or BuOH, whose description is included in table S1) was mixed with deionized water, where pH of medium was adjusted by adding HCl or NH4OH agent under stirring (500 rpm) at 38 ºC or 25 ºC, depending on the case, up to a complete dissolution. Then, the desired quantity of tetraethylorthosilicate (TEOS) source was added dropwise to the solution which was maintained under the same conditions during the ripening time chosen for each synthesis. The resulting hydrolysis mixture was then transferred into a closed Teflon-lined autoclave and put in an oven at the aging temperature and time set for each synthesis. Afterwards, the solid were recovered by filtration and washed with deionized water until obtaining a filtrate with neutral pH. Finally, the solid was dried under static air at 95 ºC for one night, and about 100 mg of each as-prepared OMS were calcined under static air at 500 ºC (2 ºC/min) for 3 h to determine the yield of SiO2 expected to obtain from each OMS's synthesis.

Secondly, the Mo-based OMS catalysts were prepared by dry melt-infiltration without submitting the as-prepared OMS samples to template removal. This strategy has been recently reported by our group lab [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] as an efficient route to improve the diffusion of transition metals inside porous materials. Thus, an increasing amount of H3P(Mo3O10)4,12H2O salt, used as the Mo precursor, was added to uncalcined SBA-15 (chosen as support model) to occupy between 3 and 93% of its total pore volume at calcined state. This loading was calculated by considering the apparent density of H3P(Mo3O10)4,12H2O salt (1.62 g/cm 3 ) and the yield of SiO2 (previously determined by calcination of SBA-15). After this, the quantity of Mo to be incorporated in MCM-41 and KIT-6 supports was estimated based on the one determined as optimal for MoOx@SBA-15 catalyst (see section 3.1 for further preparation details). Then, the target amount of Mo salt was manually mixed with the corresponding mass of each OMS (SBA-15, MCM-41 or KIT-6) by active grinding for 30 min to obtain a homogeneous yellow-beige powder. Then, each mixture (Mo salt + OMS support) was melt-infiltrated at 85 ºC using closed-vessels to avoid the salt dehydration because this latter can strongly inhibit the diffusion of Mo salt. Finally, in attempt to check the most convenable thermal treatment for decomposing the salt and template of the melt-infiltrated Mo@OMS systems, the as melt-infiltrated Mo@SBA-15 (chosen as model)

was divided in two equal portions; i) one to be calcined under static air at 500 ºC (2 ºC/min) for 3 h, and ii) second to be thermally reduced under a dynamic flow of 14.5 mL(STP)•min -1 .g -1 of pure H2 at 500 ºC (2 ºC/min) for 3 h. Thus, the name of pre-calcined samples was termed by C (i.e. MoOx@SBA-15C), while that of pre-reduced was termed by R (i.e. MoOx@SBA-15R).

For comparison purposes, MoFe-impregnated activated carbon (AC) was chosen as reference catalyst from Zacharopoulou's [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF] and Mota's [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] works. It was prepared following the same procedures as reported by the authors. The corresponding Mo and Fe loadings were also maintained at the same reported values; 19.3 wt.% and 2.8 wt.%, respectively, estimated based on the fully reduced state of metals (Mo 0 and Fe 0 ).

Finally, Bulk MoOx and Fe/MoOx were also prepared to highlight the role of the individual and combined metals because Fe species considered as stabilizing element for Mo species. [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] For this, about 16 g of H3P(Mo3O10)4,12H2O salt were finely dispersed in a minimal quantity of ethanol and then dried at 95 ºC up to a complete ethanol evaporation. Finally, it was calcined under static air at 500 ºC (2 ºC/min) for 3 h to yield MoOx. Then, about 1.02 g of Fe (NO3)3,9H2O was dissolved in 5 mL of deionized water and impregnated on 1.45 g of the prepared MoOx at 25 ºC for 24 h. The Mo/Fe molar ratio was set at 4, which is identical to that of the MoFe/AC sample. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF] After water evaporation, the solid was dried at 95 ºC for one night and calcined under static air at 500 ºC (2 ºC/min) for 3 h.

Catalyst carburization

The carburization conditions (T, H2 flow, carburizing agent, etc.) applied before each tests were chosen from relevant literature. [START_REF]In situ time-resolved X-ray diffraction study of the synthesis of Mo2C with different carburization agents[END_REF], [START_REF] Wang | Synthesis and characterization of molybdenum carbides using propane as carbon source[END_REF] Thus, chosen mass of Mo-based catalyst; previously sieved at 100-200 µm and dried overnight at 95 ºC under static air, was mixed with SiC (1/3 wt. ratio) to avoid hot-spots and, then, carburized in the reactors according to the following protocols:

-Non-topotactic route to form β-Mo2C, which was performed by heating under 180 mL (STP).min -1 .g - 1 of CH4/H2 (18/82 vol. %) up to 350 ºC (5 ºC/min) and then up to 700 ºC (1 ºC/min). The sample was held at this temperature 1 h to remove possible coke, and then cooled down under the H2 flow alone.

-Topotactic route to form η-MoC, which was performed by heating under 180 mL (STP).min -1 .g -1 of C3H8/H2 (10/90 vol. %) up to 350 ºC (5 ºC/min) with a plateau for 7 h in order to facilitate the atomic exchange between oxygen and carbon at low temperature. Subsequently, the sample was heated up to 590 ºC (1 ºC/min)

where the subsequent held and cooling were maintained the same as for Non-topotactic route.

Finally, the samples destined to further ex-situ characterization were softly passivated using 300 mL(STP)..min -1 .g -1 of O2/He (1/99 vol. %) for 2 h in order to avoid aggressive bulk re-oxidation. Immediate XRD analyses confirmed the formation of the target phases.

Characterization procedures

The physicochemical properties of the catalysts (i.e., composition, texture, redox/acid sites, particles location and distribution, chemical states, etc.) were further studied by a set of complementary techniques. The analytical procedures, which leans on previous works [START_REF] Doukkali | A comparison of sol-gel and impregnated Pt or/and Ni based γ-alumina catalysts for bioglycerol aqueous phase reforming[END_REF], [START_REF] Wang | Synthesis and characterization of molybdenum carbides using propane as carbon source[END_REF], [START_REF] Kleitz | Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes[END_REF]- [START_REF] Jana | Progress in Pore-Size Control of Mesoporous MCM-41 Molecular Sieve Using Surfactant Having Different Alkyl Chain Lengths and Various Organic Auxiliary Chemicals[END_REF] were described in supplementary information.

Testing conditions and performance evaluation

All catalysts were tested in the single-step HDO of glycerol using a high-throughput micro-activity setup (Flowrence®, Avantium) of the REALCAT platform. [START_REF] Paul | REALCAT: A New Platform to Bring Catalysis to the Lightspeed[END_REF] This compact system integrates 4 reactionblocks each one containing 4 continuous-flow reactors. About 65 mg of each catalyst (sieved between 100-200 μm) was placed between two wool plugs of SiC. Prior to test, the carburized catalysts were previously prepared according to the treatment confirmed by XRD analyses (section 2.2), while the free-carburized samples were in-situ activated under 180 mL(STP).min-1.g -1 of pure H2 up to 350 ºC (5 ºC/min) followed by immediate increasing of temperature up to 500 ºC (2 ºC/min) and held for 3 h. Then, the systems were cooled under H2 down to the reaction temperature (318 ºC or 264 ºC), and the backpressure regulator was set at the desired value (50 bar or 30 bar) by passing 693-1386 mL(STP).min -1 g -1 of pure H2. Once the system reached its stability, 0.26-0.36 mL(STP).min -1 g -1 of liquid C3H8O3/H2O mixture (10/90 wt. %) was introduced using a HPLC pump to reach a WHSV of 1.7 h -1 , 2.35 h -1 or 4.7 h -1 . The molar ratio H2/C3H8O3 was set at 78 or 98.

Time-on-stream (TOS) between 4.9 h and 14.5 h were studied. The gas and liquid products were separated at 10 ºC using vapor-liquid condenser through which 35-70 mL(STP).min -1 g -1 of He were injected as internal standard for estimating the gas-flow composition. The exhausted gases were sampled every 145 min using an online GC (Agilent 7890). The instrument is equipped with four columns: Haysep Q as pre-column, Molsieve 5A for H2 and CO separation, Poraplot-Q for CO2, C1-C3 alkanes and C2-C3 alkenes separation, and

Stabilwax for the volatile C1-C3 oxygenated hydrocarbons (OHCs) separation. To assess the carbon balance (CB), the condensed phase containing heavy OHCs and non-converted glycerol were also analyzed every 145 min by another offline GC-FID-2010 Plus (Shimadzu) equipped with the same Stabilwax column. The performance of catalyst was evaluated in terms of:

-Total glycerol conversion (TGC )*+. ) and carbon yield of each product i, (Y -) calculated according to eq. 1 and 2, respectively: Additionally, the performance of catalysts, in term of deoxygenation, hydrogenation and reforming activities, was also evaluated as:

TGC )*+. (%) = F )*+. .'
-Glycerol conversion by C-O scission, noted as GC 470 and calculated as in eq. 5, which reflects the C-O bonds broken in glycerol:

GC 470 (%) = ∑ Mγ 2,4 -γ 2,0 N. F (2) 01/%&/ (mol/min) < - 3. MF )*+.
.'%&/ (mol/min)N x 100 (9:. O)

where γ 2,0 stands the number of oxygen atoms in product (i), as in the case of carbon. GC 470 (%) can reach a maximum of 100 % when all C-O bonds in glycerol were broken to form only alkenes and/or alkanes.

-Glycerol conversion by C-C hydrogenation, noted as GC 474 and calculated as in eq. 6, which was estimated based on the H atoms observed in the products and the hydrogenation events required to fully convert glycerol in propane:

GC 474 (%) = ∑ Pγ 2,= - 2 3 γ 2,4 R < -
. F (2) 01/%&/ (mol/min) 6. MF )*+.

.'%&/ (mol/min)N x 100 (9:. T)

where γ 2,= is the number of H atoms in product (i). GC 474 (%) can reach a maximum of 166.6% when all glycerol was converted into CH4. A value of 100% indicates that all glycerol is converted into C3H8.

-Glycerol conversion by C-C breaking, noted as GC 47/74 and calculated as in eq. 7, which reflects the C-C bands broken in gas and liquid products:

GC 47/74 (%) = ∑ P1 - γ 2,4 3 R . F (2) 01/%&/ (mol/min) < -
2. MF ?%$ .'%&/ (mol/min)N x 100 (9:. U) GC 47/7 (%) can reach a max value of 100% when all C-C bonds were broken to only C1 compounds.

Moreover, the stability of catalyst in term of the loss of each activity-type was also evaluated, as in eq.

:

-lnMC @ N = K @ . t (9:. W)

where K @ is the pseudo-first order deactivation constant, and

C @ = 4 ! 4 "
is the advances of a conversion type (C / ) at time t regarding the initial value of the same conversion (C A ) calculated at the first recorded data by GC (considered as t0 time). j denotes the type of conversion activity (total conversion, C-O scission, C-C hydrogenation or C-C breaking). The K @ value was every time calculated at R 2 coefficient superior to 0.83.

-The carbon balance (CB) was calculated as in eq. 9:

CB(%) = ∑ γ 2,4 . F (2) 01/%&/ (mol/min) ' 2
3. MF )*+.

.'%&/ (mol/min)N x 100 (9:. Y)

where n, is the number of all compounds detected at outlet including unreacted glycerol.

RESULTS AND DISCUSSIONS

Pre-design and pre-characterization for catalyst preparation

As aforementioned, three OMS (SBA-15, MCM-41 and KIT-6) were firstly synthesized. Their respective mass yield in SiO2 (after calcination at 500 ºC) were 71.5%, 59% and 56.9%, as indicated by TGA analysis (Fig. S1 A). The conditions of preparation were adapted from the literature so that the total volume of pores (Vp) follows this order: SBA-15 (~1.49 cm 3 /g)> MCM-41 (~1.06 cm 3 /g) > KIT-6 (~ 0.77 cm 3 /g), which has been measured by N2 physisorption (Fig. S1 B-C). The objective was to examine the capacity of different OMS for hosting controlled-size nanoparticles of Mo species, because oxides of transition metals are mainly suffering from low surface per volume ratio, sintering and leaching, particularly when high loading of metals was used in hydrothermal medium. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF]In situ time-resolved X-ray diffraction study of the synthesis of Mo2C with different carburization agents[END_REF], [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] Besides, existing nanosized porosity in OMS can lead to a good dispersion of Mo particles and more uniform sizes distribution. This architectural approach offers to evaluate how further geometrical exposure of Mo particles affects both the population of redox sites and their surface acidity, if actually relevant.

Secondly, the optimal content of Mo salt (H3P(Mo3O10) Furthermore, the diffusion process of Mo salts is facilitated by the hydrophilic character of internal pores of SBA-15 that possess some Si-OH groups, [START_REF] Brutchey | Influence of Surface Modification of Ti-SBA15 Catalysts on the Epoxidation Mechanism for Cyclohexene with Aqueous Hydrogen Peroxide[END_REF] while its outer surface and bulk framework are dominantly hydrophobics. [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] So, the maximum pore-fillings corresponded to 18 vol. % of filled pore volume from SBA-15 which has a total volume of 1.49 cm 3 .g -1 , as calculated by N2-physisorption (Table in Fig. S1 B). This approximation was done considering that the intra-porous Mo salts maintain the same crystalline phase with an apparent density of 1.62 g.cm -3 . Beyond this practical limit of pore-filling, it is unnecessary to increase the metals loading, since it will remain agglomerated and weakly anchored on the limited external surface of SBA-15, which will be easy to drag during the envisaged use in hydrothermal reaction medium.

To check the evolution of 3D structure of SBA-15 after pore-filling, small-angle X-ray diffraction was applied to bare SBA-15 and hybrid 7.7Mo@SBA-15, 14.8Mo@SBA-15 and 28.8Mo@SBA-15 samples, which have been previously melt-infiltrated at 85 ºC for 3 successive days. This temperature of diffusion was just slightly higher than the experimental melting point of Mo salt (81 ºC), but below its decomposition temperature (~95 ºC), as experimentally determined (Fig. 1 A). Besides, The diffusion time was optimized by in-house preliminary experiments to be three days to promote a homogeneous filling of SBA-15 pores (data not shown), which is in good agreement with the data reported by our group for similar metals. [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] The obtained small-angle X-ray diagrams are plotted in Fig. 2, where the (100), ( 110) and (200) reflections were clearly observed for all the samples. This is an indication that the hexagonal symmetry (p6mm) of ordered SBA-15 remained intact including at Mo loading higher than the limit suggested by in-situ DSC analyses.

This suggests that the pore-walls of the prepared SBA-15 are sufficiently strong to support high Mo loading.

It is also observed that intensity of the main peak (100) decreased as the Mo loading increased, which confirms the filling of pores by Mo particles. This pores' filling by metal provoke an increasing of the density of pores initially empty that becomes near to that of SiO2 framework, thereby, suppressing the SBA-15 orderperiodicity and diminishing the intensity of X-ray peaks. [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] In comparison with parent SBA-15, the main (100) peak is slightly shifted to lower 2θ in the melt-infiltrated Mo@SBA-15 samples, while the corresponding d100 (nm) spacing increased as Mo amount increase. This may reflect some expanding in the framework of silica since the pores filling effectively introduces a certain alteration in the local order of SBA-15 but without destructing its 3D structure. This suggests that the hybrid Mo@SBA-15 samples are stable and maintain a similar structure to that of the parent silica.

As a conclusion, small-angle X-ray diffraction and in-situ DSC data well-complement each other;

precisely in confirming the good stability of silica framework and effective pores filling unveiling the maximal limit of Mo amount to incorporate onto OMS. This latter was determined at a level of occupation of 18% of the OMS total pores volume. Based on these preliminary insights, it was decided to incorporate a Mo loading equivalent to occupy only 3/4 of the available pores volume of SBA-15, MCM-41 and KIT-6 materials. This partial value was chosen to avoid any risk of blocking all the accessible porosity by an excess of Mo. This maximal loading of Mo corresponds to nominal values of 14.8 wt. %, 11 wt.% and 8.7 wt.%, which is estimated based on the fully reduced state of Mo 0 to incorporate in the total pore volume of each calcined OMS that has been previously measured by N2 sorption (Fig. S1B-C).

Mo oxide-based catalysts and their physiochemistry

Based on the aforementioned preliminary insights, the target loading of MoOx was incorporated onto SBA-15, MCM-41, and KIT-6 systems to compare them with impregnated MoFe/AC (chosen as a reference [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF]), bulk MoOx and Fe/MoOx catalysts. In some cases, some samples were pre-calcined under air or pre-reduced under H2 to check if MoOx strongly interacts with SiO2. Thus, the name of precalcined samples was termed by C (i.e. MoOx@SBA-15C), while that of pre-reduced was termed by R (i.e.

MoOx@SBA-15R). The corresponding XRD diagrams (Fig. S2) indicate that both treatments did not produce any silico-molybdates that are less reactive. However, calcination caused a pronounced sintering of MoO3

(JCPDS Nº 05-0508), leading to particles sizes between 27 nm and 61 nm, as estimated by the Scherrer model (eq. 3). Also, it provokes sintering of the Fe2(MoO4)3 phase (JCPDS Nº 83-1701) formed by the interaction of FeOx and MoO3. After reduction, both Fe/MoOx and MoOx samples feature XRD diagrams with a predominant MoO2 phase (JCPDS 86-0135), of which the sintered particles are between 42 nm and 56 nm, respectively.

However, the reduction of the MoOx@SBA-15R catalyst stabilizes a well-dispersed Mo particle (with sub-2 nm size) that is invisible in the XRD profile, while the pre-calcination of MoOx@SBA-15C strongly sintered the Mo species (up to 27 nm). Furthermore, TGA analysis (Fig. S3) revealed that the pre-treatment of the Mobased SBA-15 catalyst under a H2 atmosphere formed carbonaceous species (~ 6.1 wt.%) by polymerizing the structure-directing agent used during the synthesis. These carbon fragments seem to slow down the mobility of transition metals towards agglomeration [START_REF] Wu | In Situ FT-IR Spectroscopic Studies of CO Adsorption on Fresh Mo2C/Al2O3 Catalyst[END_REF]. In fact, as we will see further on, the rest of the Mobased OMS catalysts were pre-reduced under H2 to improve the Mo dispersion. Table 2 shows detailed information in terms of elemental composition and textural, redox, as well as acid-sites properties of the studied Mo oxide-based catalysts.

ICP data show that the real mass loading of metals in all pre-calcined catalysts was in good agreement with the predicted nominal values. The deviation does not exceed ± 6%, which is in the range of the usually observed experimental error. However, the pre-reduced systems, MoOx@SBA-15R, MoOx@MCM-41R, and MoOx@KIT-6R, present some more deviation (up to 18 %), which could be attributed to the deposition of the carbon fragment during the reduction; 6.1 wt.%, 8.1 wt.%, and 7.3 wt.%, respectively (Fig. S3). The presence of such species was also confirmed by CHNS measurements, where their content is between 3.51 wt.% and 4.95 wt.%. The difference between the mass values of the CHNS and TGA analyses suggests that these species are polymerized CxHy compounds rather than a purely graphitic-like carbon.

In terms of the textural properties (Table 2 and Fig. S4), bulk MoOx and Fe/MoOx feature a N2 isotherm of intragranular materials, where their surface and pore volume do not exceed 83 m 2 /g and 0.08 cm 3 /g, respectively. This is in line with the XRD insights, indicating the existence of sintered MoO3 particles. It is well-reported that these oxides suffer from a low surface area and porosity. [START_REF]In situ time-resolved X-ray diffraction study of the synthesis of Mo2C with different carburization agents[END_REF] However, the texture of the MoFe/AC catalyst is mostly set by the AC support, disposing of well-developed laminar structures. In turn, each MoOx@OMS-based catalyst showed a N2 isotherm similar to that of the bare OMS family (SBA-15, MCM-41, KIT-6) because the texture of the supported catalysts is mainly imposed by the support-type. [START_REF] Doukkali | A comparison of sol-gel and impregnated Pt or/and Ni based γ-alumina catalysts for bioglycerol aqueous phase reforming[END_REF] According to the IUPAC classification, MoOx@OMS systems show a type IV (or mixed type IV + type I) N2 isotherm, indicating the existence of remarkable mesoporosity with a minor part of micropores. Each isotherm shows a pronounced H2 hysteresis, which is characteristic of OMS, [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] confirming that meso-porosity is maintained after the incorporation of the Mo element. The catalysts based on SBA-15 and MCM-41 feature only mesopores, because during the synthesis of these supports, a single template; P123 for SBA-15 and CATB (with C19) for MCM-41, was used. Also, the crystallization of SBA-15 at temperature (in our case 105 ºC), which is higher than the cloud point of the P123 surfactant (90 ºC), seems to have modified the mesophase of P123 surfactant. Such temperature decreases the strength of the interaction between the P123 surfactant and the inorganic matter, resulting in a densification of the walls and the formation of larger mesopores. [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF] For MCM-41, we have used CTAB as surfactant (with alkyl chain of C19 atoms) that led to the formation of typical mesopores of 2-3 nm. [START_REF] Jana | Progress in Pore-Size Control of Mesoporous MCM-41 Molecular Sieve Using Surfactant Having Different Alkyl Chain Lengths and Various Organic Auxiliary Chemicals[END_REF] To form micropores in MCM-41, organic surfactant with lower alkyl chain length (mainly with C<16) should be used. [START_REF] Jana | Progress in Pore-Size Control of Mesoporous MCM-41 Molecular Sieve Using Surfactant Having Different Alkyl Chain Lengths and Various Organic Auxiliary Chemicals[END_REF] Moreover, comparing the texture of MoOx@OMS catalysts (Table 1 and Fig. S4 C-D) with that of the bare OMS support (Fig. S1 B-C), we observe that only the SBA-15-based catalyst suffered from some decrease in the porosity. In fact, the distribution of their pore-size becomes bimodal and less intense due to the pore filling by Mo nanoparticles. This decrease is more pronounced on the pre-calcined MoOx@SBA-15C, which may be due to the blockage of mesopores by larger MoOx particles formed during the calcination, as revealed by XRD analysis. All of these insights thus confirm once again the existence of some pores filling by Mo particles for the SBA-15 catalyst, as initially revealed by in-situ DSC and X-ray scattering. However, the monomodal pore-size distribution of MCM-41 centered at 2 nm and the bimodal one centered at 3.7 nm and 5.1 nm for the KIT-6 support remain practically unchanged after Mo incorporation. This suggests that Mo species particles remain mostly located outside the pores of KIT-6 and MCM-41 materials.

In addition to these elemental and morphological differences, NH3-TPD and in-situ CO-IR data from selected catalysts, MoOx, Fe/MoOx, MoOx@SBA-15C, and MoOx@SBA-15R, also indicate remarkable differences in their acidity and redox properties.

As shown in Table 1 and Fig. S5, these four catalysts exhibit weak and mild acidity through two overlapped peaks of NH3 desorption between 115-495 ºC. MoOx and Fe/MoOx systems provide a low amount of acid-density, 0.14 mmol/gmetal and 0.11 mmol/gmetal, respectively. It is well reported [START_REF] Borowiec | An acrolein production route from ethanol and methanol mixtures over FeMo-based catalysts[END_REF], [START_REF] Shozi | Hydrogenolysis of Glycerol to Monoalcohols over Supported Mo and W Catalysts[END_REF] that MoOx and FeOx tend to exhibit mainly Brønsted sites related to their Mo-OH and/or Fe-OH termed. The small difference observed between both values may originate from the deposition of impregnated FeOx species that block more acid Mo-OH centers. [START_REF] Borowiec | An acrolein production route from ethanol and methanol mixtures over FeMo-based catalysts[END_REF], [START_REF] Shozi | Hydrogenolysis of Glycerol to Monoalcohols over Supported Mo and W Catalysts[END_REF] The blockage of pores on Fe/MoOx has been observed in the BJH profile (Fig. S4 B). On the other hand, the density of acid-sites was noticeably increased when MoOx was dispersed onto SBA-15, i.e., 1.65 mmol/gMo for MoOx@SBA-15R and 2.16 mmol/gMo for MoOx@SBA-15C.

This acidity increase might be firstly caused by a more geometrical exposure of highly defected MoOx species because the well-developed 3D structure of SBA-15 mainly induce further dispersion of metals. [START_REF] Gonçalves | Effect of the support on the hydrodeoxygenation of m-cresol over molybdenum oxide based catalysts[END_REF] Secondly, this acidity is more pronounced on the pre-calcined MoOx@SBA-15C catalyst, which implies that the precalcination under air might be responsible for further increasing the interfacial electrons transfer between MoOx and SiO2. This is supported by the fact that the SiO2 matrix is inert towards NH3 chemisorption, [START_REF] Gonçalves | Effect of the support on the hydrodeoxygenation of m-cresol over molybdenum oxide based catalysts[END_REF] which means that the increase in the NH3 uptake is essentially coming from the vacancy of SiO2/Mo electron transfers.

In turn, in situ CO-IR data (Fig. S6 and Table 1) indicate that both bulk MoOx and dispersed MoOx@SBA-15C and MoOx@SBA-15R catalysts feature carbonyl species coordinated to Mo B (3 < φ ≤ 5)

sites, exhibiting band frequencies between 2203-2257 cm -1 that are higher than those of free or physiosorbed CO at 2138 cm -1 . [START_REF] Wu | In Situ FT-IR Spectroscopic Studies of CO Adsorption on Fresh Mo2C/Al2O3 Catalyst[END_REF] This large frequency distribution is a signal of various oxidation-reduction degrees of Mo species. Quantitively, MoOx@SBA-15R features the highest density of CO-sites (1.98 mmol/gMo), which is practically 2 times higher than that of MoOx@SBA-15C (1.07 mmol/gMo) and 4.6 times higher than that of bulk MoOx (0.43 mmol/gMo). This high density of CO sites can be attributed not only to the double reduction treatments that mainly create Mo defects with an unsaturated oxygen coordination, acting as redox-sites, [START_REF] Prasomsri | Effective hydrodeoxygenation of biomass-derived oxygenates into unsaturated hydrocarbons by MoO3 using low H2 pressures[END_REF] but also to the improved geometrical exposure of Mo nanoparticles, as previously evidenced by the XRD data.

The carbon clusters on MoOx@SBA-15R seem to significantly prevent the sintering of Mo nanoparticles. The existence of carbon has also been confirmed by the MS detector during the NH3-TPD measurements as its thermal decomposition lead to overlapped peaks of COx and H2O observed between 495-700 ºC (Fig. S5).

Mo carbide-based catalysts and their physiochemistry

In view of the promising properties of MoOx@SBA-15R, it was used to obtain the dispersed carbide phases, such as β-Mo2C@SBA-15R and η-MoC@SBA-15R, in order to compare with bulk β-Mo2C and η-MoC systems. The aim was to unveil how forming new Mo-C bounds affect the redox and acid characteristics of Mo species. The XRD diagrams (Fig. S7) of representative bulk carburized samples, prepared as described in section 2.2, confirmed the formation of the β and η phases. The carbothermal treatment under CH4/H2 mixtures effectively leads to a well-dispersed β-Mo2C phase (JCPDS 00-011-0608), while the treatment under C3H8/H2 forms a η-MoC phase (JCPDS 01-089-4305) with a minor presence of MoO2 (JCPDS 86-0135). In turn, β-Mo2C@SBA-15R and η-MoC@SBA-15R exhibited broad XRD diagrams (not shown), which suggests that the β-Mo2C and η-MoC phases are still invisible as sub-2 nm crystallites. The content of Mo and C in carbide-based catalysts and their main texture and the most significant redox and acid properties are summarized in Table 3 and illustrated in Fig. S8-10.

Thus, the mass loading of Mo was increased in the carburized bulk samples due to the exchange of heavy O atoms by light C atoms that represent less than 11.3 wt.%. However, this Mo content was not greatly changed in the dispersed Mo carbides onto SBA-15 because SiO2 is the major component (> 82 wt.%). The surface areas and meso-porosity were significantly increased for bulk β-Mo2C (457 m 2 /g, 0.45 cm 3 /g) and bulk η-MoC (220 m 2 /g, 0.25 cm 3 /g) samples in comparison with bulk MoO3 samples (83 m 2 /g, 0.08 cm 3 /g). This is in good agreement with the XRD diagrams (Fig. S7), reflecting a dispersed carbide phase with broad diffraction peaks. Delporte et al. [START_REF] Delporte | Physical characterization of molybdenum oxycarbide catalyst; TEM, XRD and XPS[END_REF] reports that a reducing atmosphere mainly creates a structural defect in Mo species, which increases their surface area. Besides, the dispersed Mo carbides feature globally similar N2 isotherms and BJH profiles to those of the MoOx@SBA-15R counterpart. This is an indication that the initial well-developed porosity and 3D structure are still maintained despite the carburization treatment at high temperatures (590-700 ºC) for extended times.

As a consequence of this further geometrical exposure, β-Mo2C@SBA-15R exhibits a significant increase in the population of NH3 sites (1.41 mmol/gMo) compared to bulk β-Mo2C (0.17 mmol/gMo), as shown in Table 3 and Fig. 9. This acidity is also related to the creation of oxygen vacancies that act as acid-sites [START_REF] Prasomsri | Effective hydrodeoxygenation of biomass-derived oxygenates into unsaturated hydrocarbons by MoO3 using low H2 pressures[END_REF] and the relatively more acidic characteristic of Mo-C bands. In turn, the population of CO sites follows the same trend. Thus, the dispersed β-Mo2C and η-MoC phases inside SBA-15 contain about 1.15 mmol/gMo and 1.35 mmol/gMo, respectively, while the bulk β-Mo2C shows just 0.58 mmol/gMo (Table 3, Fig. S10). Bulk β-Mo2C shows two vibration bands at 2157 cm -1 and 2138 cm -1 , corresponding to partially reduced Mo-CO and physically adsorbed CO, respectively, [START_REF] Wu | In Situ FT-IR Spectroscopic Studies of CO Adsorption on Fresh Mo2C/Al2O3 Catalyst[END_REF] while β-Mo2C@SBA-15R and η-MoC@SBA-15 feature additional shoulders at 2206 cm -1 and 2175 cm -1 (Fig. S10). This difference in bandings and number of CO-sites, which depend on the dispersion and carbide phases, suggests the presence of Mo sites with a different chemical environment. Stellwagen et al. [START_REF] Stellwagen | Structure-performance relations of molybdenum-and tungsten carbide catalysts for deoxygenation[END_REF] reported that sub-10 nm particles of Mo carbides increase their oxyphilic surface. In fact, the high dispersion of our sub-2 nm Mo particles might not only increase the population of CO-sites through more geometrical exposure but also induce an even greater oxyphilic characteristic of Mo-C bands. This aspect is more pronounced on the η-MoC@SBA-15R catalyst, carburized via a transition topotactic route; MoO3 →MoO2→MoOxCy and η-MoC. The existence of MoOxCy in carbidic samples has been experimentally revealed, [START_REF] Mo | Effect of carburization protocols on molybdenum carbide synthesis and study on its performance in CO hydrogenation[END_REF] while DFT calculations indicated that these intermediates with submonolayer oxygen feature high oxophilicity.

[64]

Glycerol HDO on Mo oxide-based catalysts

As described earlier, all Mo oxide-based catalysts were firstly tested in a single-step HDO of glycerol/H2O (10 wt.%) at 318 ºC and a pressure of 50 bar or 30 bar for TOS between 4.8 h and 14.5 h.

Preliminary tests with a long time on stream (TOS) were performed first to check if it is possible to overtake the initial transition state of the catalyst to reach its stationary state.The molar ratio of H2/glycerol and WHSV were set at 98 and 1.7 h -1 , respectively. These conditions correspond to a fully gas-phase reaction, as calculated by Aspen software (Fig. S11). Blank tests with an excess mass of bare SBA-15 mixed in inert SiC were also performed and repeated two times. For these blank tests, the conversion never exceeded 9.1%, where the mol of glycerol converted per mol of SiC/SiO2 mixture during 1 h was much less than 1, which suggests a thermoconversion of glycerol rather than a proper catalytic act. This confirms the inactivity of SiC and SiO2 species used as the bed-diluent and support, respectively.

As for Mo oxide-based catalysts, they show a high catalytic reactivity, where the distribution of the arising products depends on the characteristics of the catalysts themselves and their testing conditions. The main products detected in the gas phase are propylene and propane, as well as volatile OHCs such as propionaldehyde, 1-propanol, acrolein, allyl alcohol, acetone, acetaldehyde, ethanol, and methanol. Very small amounts of ethylene, ethane, methane, and carbon dioxides were also detected. The condensed phase consisted of trapped volatiles and heavy OHCs such as hydroxyacetone, 1.2-propanediol, 1.3-propanediol, propanoic acid, ethylene glycol, and non-converted glycerol. Table 4 summarizes the performance of all Mo oxide-based catalysts in terms of total glycerol conversion, deoxygenation, hydrogenation, and C-C breaking activities, as well as in terms of propylene yield and carbon balance. As these parameters sometimes decrease gradually after several hours in the reaction, the pseudo-first order constant of their deactivation rate was also included. In turn, Fig. 3 plots the temporal evolution of the total glycerol conversion and propylene and OHC yields of MoOx@SBA-15R and bulk MoOx catalysts in order to put in evidence their significant behavior compared to those of MoFe/AC (chosen as reference) and Fe/MoOx catalysts.

For the MoFe/AC catalyst (with ~ 2.9 wt.% Fe and ~ 18.6 wt.% Mo), the conversion of glycerol decreases with time from 92.4% to 51.1%, leading to a positive constant of deactivation (~ 63.8 10 -3 h -1 ), while the propylene yield also decreases from 14.3% to 4.2%. The deactivation constants of the deoxygenation, hydrogenation, and reforming activity are also positive values; 68.6 10 -3 h -1 , 75.4 10 -3 h -1 , and 110.5 10 -3 h -1 , respectively. This tendency may be due to a diminution of active-sites mainly caused by easy sintering of metals impregnated on AC, as reported previously by Zacharopoulou et al. [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF] It is a reasonable explanation because the surface of carbon contains less charges and develop low interaction with transition metals, thereby evolving to form larger particles that are easy to be agglomerated.

Bulk MoOx catalysts provide full glycerol conversion and stable propylene yield at around 59.6 %, where the main co-products are 1-propanol, propionaldehyde, acetone, propanoic acid, and propane. They also feature high and stable activity in deoxygenation (80.9%) and hydrogenation (66.9%) and a negligible activity in reforming (0.8%) to close the carbon balance at 95.7%. This high HDO activity is related to the high Mo content in bulk Mo oxides (~ 66.6 wt.% Mo), where H2 reduction leads to the creation of unsaturated MoO3-x sites. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] The stable behavior may be attributed to the larger initial particles of MoO3 (~ 61 nm, Fig. S4) that are already thermodynamically stable and do not undergo further agglomeration. In turn, Fe/MoOx catalysts (~ 8.8 wt.% Fe and ~54.6 wt.% Mo) provide just 26% propylene yield, which is 2.3 times lower than that of MoOx catalysts, despite their full glycerol conversion. They also feature minor activities in deoxygenation (58.9%) and hydrogenation (68.7%) that forms rather more partially deoxygenated C3 molecules such as 1-propanol, acetone, and propanoic acid. Wan et al. [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] reported that the addition of FeOx can reduce the oxophilicity of MoOx species, which consequently minimalizes their tendency to interact with oxygenate intermediates. This can be related to the formation of [Fe2MoO3]4 crystallites, as indicated by our XRD data (Fig. S2), whereas the H2 treatment was not sufficient in completely reducing the FeOx nanoparticles, thus subtracting the certain availability of Mo sites for HDO events. In summary, an adequate availability of Mo sites with rich-vacancy defects is necessary for improving the HDO of glycerol and its intermediates to form more propylene rate.

Thus, the MoOx@SBA-15R catalyst (with a high dispersion of just 14.8 wt.% of Mo) features an unprecedented almost quantitative yield of fully deoxygenated C3 molecules (~84% propylene and ~15%

propane) at full glycerol conversion, while the yield of liquid oxygenates is insignificant. However, despite the fact that the conversion of glycerol remains total, the yield of propylene and propane slightly decreases with TOS progresses down to 65.6% and 9.1% after under 14.5 hours stream, respectively. This decreasing tendency was accompanied with a proportional increase in the yield of 1-propanol, propanoic acid, and acetone. This proportional exchange in the yield of in products does not cause a significative decrease in its deoxygenation and hydrogenation activities, whereas the deactivation rates stay moderate at 14.9 10 -3 h -1 and 7 10 -3 h -1 , respectively. However, an appreciable increase in the activity of C-C breaking to form C1-C2 molecules, mainly acetaldehyde, ethylene, ethane, and very little CO2, was observed, which manifests itself through a high negative value of the rate constant (-110.7 10 -3 h -1 ). This slight deviation in the reaction route may be related to a certain change in the redox sites that represses the deoxygenation and hydrogenation events while concurrently amplifying the acidic cracking rate. The NH3-DTP profile of the used MoOx@SBA-15R sample (Fig. S.12) revealed that the density of acid-sites increased up to 1.94 mmol/gMo, which is 17.7% higher than that of the fresh sample, i.e., 1.65 mol/gMo. This acidity increase usually arises from the appearance of new -OH groups over MoOx species after interacting with the H2O of the medium or oxygenates intermediates. Nonetheless, the GTP route largely remains the main catalytic route.

In turn, the pre-calcined MoOx@SBA-15C catalyst features a glycerol conversion of 96.3% with notably less activity in deoxygenation (43.5%) and hydrogenation (40.1%, Table 4). Propionaldehyde is the main product whereas the yield of propylene decreased from 30.8% to 12%, as shown in Fig. S13. In fact, the deactivation rates for C-O cleavage (56.10 -3 h -1 ), C-C hydrogenation (56.6 10 -3 h -1 ), and C-C breaking (164.6

10 -3 h -1 ) are more pronounced compared to those of the MoOx@SBA-15R catalyst. This behavior may be related to the decreased availability of Mo sites with HDO abilities. The CO chemisorption data support this explanation because the density of redox-sites is practically two times lower in MoOx@SBA-15C than in MoOx@SBA-15R. It seems that the pre-calcination under air, which caused some sintering of the Mo surface (see XRD data in Fig. S2), induces further electron transfer between MoOx and SiO2. This interaction may contribute to lowering of the reducibility of Mo oxides, which usually occurs with transition metals. [START_REF] Gao | State-of-art modifications of heterogeneous catalysts for CO2 methanation -Active sites, surface basicity and oxygen defects[END_REF] Also, the NH3-TPD data (Table 1 and Fig. S9) indicate the high acid density on MoOx@SBA-15C that may increase the formation of oxygenates by direct dehydration (i.e., glycerol to acrolein, etc.). For example, the gradual increase in the acrolein yield during the last hours of reaction (Fig. S13), which is accompanied by a proportional decrease in the propionaldehyde yield, can also be related to some suppression of hydrogenation events of acrolein to propionaldehyde. However, further details about the contribution of the redox/acidity ratio could be the object of a separate study in the future.

Regarding the MoOx@MCM-41R and MoOx@KIT-6R catalysts, they showed the worst stability despite their good initial catalytic activity and selectivity towards propylene (Table 4 and Fig. S13). At the beginning of the test, they featured a total glycerol conversion of 100% and 96%, respectively, but after 14.5 h of reaction, both values were drastically decreased to only 24.6% and 53.5%, respectively. Their respective propylene yield follows the same trend, decreasing from 48.5% and 52.1% to only 2.1% and 7.1%, respectively. Also, the deoxygenation, hydrogenation, and reforming activities follow the same trend, where the corresponding constants of deactivation provide high positive values of 242.4 10 -3 h -1 , 202.4 10 -3 h -1 , and 155.6 10 -3 h -1 , respectively, for the MoOx@MCM-41R catalyst, and 182.6 10 -3 h -1 , 164.510 -3 h -1 , and 195.310 - 3 h -1 , respectively, for the MoOx@KIT-6R catalyst. Consequently, the yield of oxygenates, mainly hydroxyacetone, propionaldehyde, allyl alcohol, and ethylene glycol, also decreased, which suggests that both the surface quality and number of sites were greatly changed during the reaction. These changes could emerge from a possible evolution of Mo species or of the silica support itself.

Normally, MCM-41 is well known for its narrow mesopores (~2 nm) [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] that could have further prevented the melt-infiltration of Mo particles inside the pores, thus, leaving them on the outer surface suffering from a quick agglomeration. Also, it is well-known that MCM-41 features a nonexistent interconnectivity between their neighboring meso-channel, [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] which makes them unstable materials in a hydrothermal medium. Concerning the MoOx@KIT-6R catalyst, comparative high-resolution TEM-HAADF images of selected fresh and used samples (Fig. 4) effectively confirm the evolution of the MoOx and KIT-6R

morphologies. The fresh sample shows more superficial dispersion of Mo nanoparticles over KIT-6R, where the histogram of their average size distribution is monomodally centered at 1.4 nm. However, this good Mo dispersion was strongly changed after reaction towards larger agglomerates, resulting in a broad geometrical distribution of Mo particles with bimodal histograms centered at around 6.5 nm and 19.9 nm. This could be caused by a partial structural fall of the KIT-6 framework during the reaction because some distortion in the used KIT-6 structure was observed (Fig. 4). KIT-6 aged at a low 85 °C habitually develops a thin and insufficient interconnectivity between its meso-channels, which gives it a frail silica framework; vulnerable to being destroyed in a H2O-rich medium. As consequence of all these changes, the number of accessible active sites might have significantly decreased on MoOx@MCM-41R and MoOx@KIT-6R catalysts to convert glycerol and oxygenates, thus resulting in an evident deactivation.

The MoOx@SBA-15R catalyst features, however, very good morphological and structural stabilities, as confirmed by TEM-HAADF images of fresh and used samples (Fig. 4). Thus, sub-2 nm MoOx particles are clearly observed in the fresh sample, where their size distribution is symmetrically centered at around 1.7 nm.

EDX mapping clearly showed that these thin particles were mostly formed inside pores and arranged along uniform meso-channels, which is characteristic of SBA-15. The size of these channels (~ 8 nm) is of the same order of magnitude as that already indicated by the BJH model (Table 1 and Fig. S4). The visual filling of these mesopores confirms our preliminary insights obtained from in-situ DSC and X-ray scattering. Moreover, the TEM-HAADF images of used MoOx@SBA-15R showed that both the Mo particle sizes (~2.1 nm) and their monomodal distribution remain similar to those of the fresh sample. Also, the hexagonal array of SBA-15 remains unchanged after the reaction. This good morphological stability of MoOx@SBA-15R can be explained by the synthesis protocol. The aging of the starting mixture for the SBA-15 synthesis at around 105 ºC usually makes the as-synthesized SBA-15 develop an optimal compromise between the opening of the meso-pores and the resistance of the silica walls/thickness. [START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF], [START_REF] Ciotonea | Improved dispersion of transition metals in mesoporous materials through a polymerassisted melt infiltration method[END_REF] This silica with enough strength enables the hosting of a high loading of Mo nanoparticles and facilitates the diffusion of the reactant and products, which explains, in a certain way, the better stability of the MoOx@SBA-15R catalyst, including under harsh hydrothermal conditions. The slight decrease in the yield of propylene along TOS (Fig. 3) could be related to some re-oxidation of partially reduced Mo δ+ species, as we will see further on. However, the small evolution of the Mo particle sizes from 1.7 nm to 2.2 nm does not seem to significantly change the surface of the catalyst.

So, seeing that MoOx@SBA-15R and MoOx@SBA-15C catalysts dispose of inversed redox and acid properties, they were also tested at a lower total pressure (30 bars). As shown in Fig. 5, the MoOx@SBA-15R catalyst features better HDO performance than that of MoOx@SBA-15C, although it is less than those obtained at 50 bars. MoOx@SBA-15R maintains full conversion and yields more hydrogenated molecules, mainly propylene, propane, and 1-propanol, which are a result of its high deoxygenation and hydrogenation activities, 74% and 70.5%, respectively. However, MoOx@SBA-15C provides just 79% of the total glycerol conversion, which is mainly to less hydrogenated OHCs such as propionaldehyde, acrolein, allyl alcohol, hydroxy-acetone, 1.3-propandiol, and unconverted glycerol. The difference in their HDO activity can be correlated to the density of redox and acid sites in each catalyst. However, their lower catalytic performance compared to that obtained at 50 bars can be explained by the lower H2 availability at reduced total pressure of 30 bars. Actually, when decreasing the total pressure from 50 to 30 bars, the partial H2-pressure decreases consequently from 35 to only 18 bars. Thus, H2 adsorption becomes insufficiently activated, which negatively affects the regeneration of redox-sites that are necessary to further accomplish the hydrogenation events and maintain the high C-O cleavage activity. Recent DFT calculations [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] indicated an effectively high energy barrier for H-H dissociation over MoO3, whose values are 1.46-1.63 eV depending on the configuration of the -OH formed. In fact, the creation of the oxygen vacancy through the H* reaction with neighboring surface O is kinetically decelerated despite the energy of the global process being only 0.30 eV. This lower regeneration of the oxygen vacancy could be offering a greater possibility of the intervention of acid-sites, thereby deviating the reaction towards dehydration pathways. This effect is more pronounced in MoOx@SBA-15C, which provide a greater NH3-site/CO-site ratio (~2.02) due to its pre-calcination under O2/N2 (20/80 vol.%). In fact, this acid catalyst forms more OHCs and exhibits very lower selectivity towards further hydrogenated molecules such as propylene and propane. This is exactly the opposite for the MoOx@SBA-15R catalyst (with a low NH3-site/Co-site ratio of ~0.83), which was pre-reduced under pure H2, that shows a significant selectivity towards further hydrogenated molecules (i.e., propylene, propane, and 1-propanol). In conclusion, the high HDO rate of the MoOx@SBA-15R catalyst can be ascribed to its robust activity in C-O breaking and C-C hydrogenation induced by its high density of redox-sites, while the extent of the deoxygenation-hydrogenation of the MoOx@SBA-15C catalyst seems to be repressed by its pronounced acidic characteristic.

Regarding the lower performances of both catalysts at 30 bars compared to those at 50 bars, the thermodynamics may also be playing an unfavorable role, thus negatively affecting the propylene yield at a lower total pressure. According to the Le Châtelier rules, [START_REF] Liu | Application of the Le Chatelier principle on gas reactions[END_REF] when the total pressure is decreased, the reaction will shift towards more molecules in the gas phase. In our case, it will favor the reaction pathways that repress the consumption of H2 or at least those that consume less moles of H2 (used in excess) to maintain the high number of moles of gas, thus partially rewarding the effect of decreasing the pressure. Concretely, HDO of glycerol theoretically needs two or three moles of H2 to be fully deoxygenated in one mole of propylene or propane, respectively. However, certain partially dehydrated compounds (i.e., acrolein, C3 diols, allyl alcohol, etc.) can be formed via dehydration without external H2 consumption or via hydrogenolysis with just one mole of H2, which maintain high number of moles in gas-phase at lowing the total pressure. It is thus concluded that increasing the H2 pressure is evidently preferred to favor the pathways of single-step GTP. However, a reasonable ratio of H2/glycerol vis-a-vis glycerol/H2O ratio should be optimized because: i) excessive H2pressure can further hydrogenate the C=C band of propylene into C-C bands in propane, ii) high partial pressure of H2O would cause excessive re-oxidation of partially reduced Mo δ+ sites, and iii) use a minimal of external H2 is also good for the economy of GTP process.

Finally, it should be noted that a carbon balance close to 100% (Table 4) was observed on the catalysts, MoOx@SBA-15R and MoOx, that feature high HDO activity in forming more hydrogenated gaseous products.

However, a slightly lower carbon balance between 75.6% and 91.2% was observed on the remaining Mo oxide-based catalysts that strenuously form more liquid oxygenates. This may be due to the chemical instability of these OHC intermediates (i.e., acrolein, propionaldehyde, etc.) that rapidly undergo further oligomerization to undetectable compounds. Furthermore, the trapped solution provided by some catalysts was initially yellow to slightly brown. Its successive injection in the GC, separated by various days, indicated effectively a slight evolution of their concentration but without detecting any oligomers, which can be due to incompatible GC's column (Stabliwax) used for these experiments.

Glycerol HDO on Mo carbide-based catalysts

To study the effect of carburization on the stability of highly reactive Mo nanoparticles, the performance of Mo carbide-based catalysts was evaluated under more relevant HDO conditions. Thus, the temperature was set depending on the case at 264 ºC or 318 ºC and total pressure at 50 bars, which corresponds to a fully liquid or gas-phase reaction, respectively (Fig. S11). Also, the molar ratio of H2/glycerol was decreased to 78, while the space-velocity was increased to 2.35 h -1 and then to 4.7 h -1 . Table 5 summarizes the kinetic data estimated at the end of TOS for bulk β-Mo2C, β-Mo2C@SBA-15R, and η-MoC@SBA-15R catalysts. The performance of the bulk MoOx catalyst tested at 4.7 h -1 was also included for comparison purposes.

Thus, despite the fact that the space-velocity of the test was increased to 2.35 h -1 and the molar ratio of H2/glycerol was decreased to 78, all tested carbides, i.e., bulk β-Mo2C and dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R, feature full glycerol conversion and relatively stable activities in C-O deoxygenation (63.9%, 74.6%, and 76.8%, respectively) and C-C hydrogenation (53.8%, 64.1%, and 62.7%, respectively), while their activity in C-C breaking is insignificant. Any decreasing rate was not observed, and thus, the pseudo-first order deactivation constant is null. This performance resulted in a relatively stable propylene yield not only on bulk β-Mo2C (at 49.1%) but also on the dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts at about 54% and 55%, respectively (Fig. 6). This suggests that the dispersed carbidic Mo sites are more catalytically stable than oxidic Mo sites dispersed on MoOx@SBA-15R (Fig. 3). Besides, the dispersed Mo carbides feature robust HDO activity, where the distribution of products on β-Mo2C@SBA-15R and η-MoC@SBA-15R includes mostly fully deoxygenated products (i.e., propylene and propane) with a total yield of 71% and 65.1%, respectively, which are much higher than those observed on bulk β-Mo2C (50.3%).

The latter bulk carbide trends parallelly to form appreciated yields of oxygenates mainly; 1-propanol, acetone, and propanoic acid. This trend is in close agreement with literature, [START_REF] Stellwagen | Structure-performance relations of molybdenum-and tungsten carbide catalysts for deoxygenation[END_REF] which reported that nanoparticles of Mo carbides tend to have an oxyphilic character, thus strongly interacting with the oxygen of glycerol and intermediates. The good hydrogenation activity, which is more pronounced in the η-MoC@SBA-15R catalyst, can be ascribed to the presence of a residual monolayer of MoOxCy, [START_REF] Lee | Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo 2 C-Catalyzed Vapor Phase Anisole Hydrodeoxygenation[END_REF] which has been justified previously in section 3.2.

When the space-velocity was further increased to 4.7 h -1 , bulk β-Mo2C, β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts are being providing full glycerol conversion, however, bulk MoOx catalyst reaches just 93% of conversion. The three catalysts also show a good activity in deoxygenation (53.4%, 50.8%, and 43.9%, respectively) and hydrogenation (40.8%, 45.3%, and 39.9%, respectively), which are much better than those of bulk MoOx catalyst (Table 5). Comparing the performance of bulk MoOx and β-Mo2C catalysts, it is clear that the carburization of Mo-O to Mo-C bands improves both the extent of HDO activities and the resistance to deactivation. In fact, β-Mo2C catalyst provides constant propylene yield at 43.2%, which is 2 times higher than that of MoOx catalyst. The propylene yield of this later decreases progressively to reach only 21.1%. This can be ascribed to the high density of redox-sites on bulk β-Mo2C (0.58 mmol/g Mo), which seem emerge from the increased surface area of β-Mo2C (457 m 2 /g) compared to that of MoOx (83 m 2 /g). In turn, the further HDO activities of dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts (with just 9.3 wt.% and 12.2 wt.% of Mo content, respectively) are manifested through the good initial yield of propylene that slightly decreases with TOS to reach a steady-state value of 31.0% and 32.1% after 9.7 h under stream, respectively. The relatively low values of the pseudo-first order deactivation constant of C-O cleavage and C-C hydrogenation activities for the β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts indicate that the loss in HDO events does not severely accentuated.

To better visualize the aforementioned observations and to accurately identify what is the most active

Mo phase for glycerol HDO and propylene formation, the specific activity in terms of TOF, namely, the mole of glycerol converted to propylene and each co-product per hour per CO-site on MoOx, β-Mo2C, β-Mo2C@SBA-15R, and η-MoC@SBA-15R catalysts, is plotted in Fig. 7.

For bulk MoOx catalyst, the TOF of propylene and of propionaldehyde decreases gradually with time from 68.2 h -1 to only 33.7 h -1 , whereas those of allyl alcohol and acrolein trend to increase proportionally.

This tendency is an indication that both the quality and the robustness of the catalytic site are changing with TOS to have less hydrogenation capacity under these conditions. Some researchers [START_REF] Liu | From glycerol to allyl alcohol: iron oxide catalyzed dehydration and consecutive hydrogen transfer[END_REF] report direct generation of allyl alcohol from glycerol, claiming a glycerol transfers hydrogen to acrolein to form allyl alcohol, which becomes incapable of further hydrogenation into propionaldehyde and then to propylene.

However, for β-Mo2C, the catalytic behavior becomes notably stable to form more hydrogenated products, mainly propylene and 1-propanol, and their TOF remains practically constant at 40.6 h -1 and 6.7 h -1 , respectively. This stability per-site is an indication that no preferential loss of reactivity is occurring with time, which suggests that the carbidic site is catalytically different than oxidic sites. This can be related to the increase in the almost metallic character of Mo-C bands on the β-Mo2C catalyst because there is a consensus that the hybridization of s/p-orbitals of the C atom with the d-orbital of the Mo atom tends to expand the dband, approaching the electronic behavior of noble metals. [START_REF] Chen | Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts[END_REF] When β-Mo2C or η-MoC was dispersed onto SBA-15, the TOF of propylene was multiplied to reach, at steady-state, 153.1 h -1 and 98.2 h -1 , respectively, which are at least 3.8 and 2.4 times, respectively, higher than those provided both by the bulk β-Mo2C or bulk MoOx catalyst. Besides, the carbide Mo-C sites, particularly those on β-Mo2C@SBA-15R to a lesser extent those on η-MoC@SBA-15R, feature robust activities in C-O deoxygenation and C-C hydrogenation under these conditions. More specifically, the Mo-C sites of dispersed β-Mo2C phase is more active and selective towards propylene formation, while those of the dispersed η-MoC one seems to have a slightly better hydrogenating character that is much better than those of Mo-O sites. This difference in their HDO robustness is clearly evidenced via the difference in their capacity to form fully deoxygenated and more hydrogenated compounds (i.e., propylene, propane) per site.

The improved HDO efficiency of the β-Mo2C@SBA-15R and to a lesser extent η-MoC@SBA-15R catalysts, compared to those of bulk β-Mo2C and MoOx, can be also related to their high density in term of redox-sites, as indicated by IR-CO chemisorption (Table 3 and Fig. 10). However, the total TOF of propylene + propane tends to slightly decrease with time for both catalysts from 236.8 h -1 to 155.8 h -1 on β-Mo2C@SBA-15R and from 154.2 h -1 to 103.9 h -1 on η-MoC@SBA-15R. In parallel, a proportional increasing in propionaldehyde yield; from 31.6 h -1 to 84.7 h -1 and from 0.9 h -1 to 1.5 h -1 , respectively, was observed. This suggests that the carbide sites on dispersed catalysts, β-Mo2C@SBA-15R and -MoC@SBA-15R, also undergo

Given that the β-Mo2C@SBA-15R catalyst provides a promising TOF in propylene formation, it was also tested under liquid-phase HDO of glycerol at 264 ºC and 50 bars because it is interesting from an economic point of view. Fig. 8 plots the temporal evolution of the conversion of glycerol, deoxygenation and hydrogenation activities, as well as the yields distribution of propylene and of the main co-products. Despite these unfavorable thermodynamic conditions, β-Mo2C@SBA-15R remains highly reactive, as shown by its capacity to convert glycerol which starts from 94.3% and reaches 84.5% at steady state. It features low deoxygenation and hydrogenation activities at 27.6%, forming mainly mono-and bi-OHCs such as propionaldehyde, allyl alcohol, acrolein, hydroxy-acetone, and C3 diols, while the propylene yield does not exceed 8.8%. An unrepresentative yield (< 0.1%) of C1-C2 co-products (i.e., acetaldehyde, ethylene glycerol, ethanol, and ethylene) was observed without COx detection, which confirms that the β-Mo2C@SBA-15R catalyst is inactive towards C-C breaking, including under liquid-phase conditions. However, a low carbon balance (64%) was obtained in this test, which can be related to the instability of highly reactive intermediates or undetectable compounds.

In view of the low yield of propylene at the high glycerol conversion of the β-Mo2C@SBA-15R catalyst, it seems that this lower propylene selectivity is related to the unfavored effect of thermodynamics rather than to the lower reactivity of the catalyst. At 264 ºC and 50 bars, both water and glycerol is in a full liquid phase, as evidenced by the Aspen calculation (Fig. S11), which affects the solubility of H2 [START_REF] Vasiliadou | Kinetic study of liquid-phase glycerol hydrogenolysis over Cu/SiO2 catalyst[END_REF] that is necessary to accomplish the hydrogenation events to form more hydrogenated propylene. Thus, we conclude that to work in a fully liquid phase, the H2 pressure should be increased at an optimized temperature. This will allow the suppression of the reactions pathways that lead to the formation of partially deoxygenated coproducts. However, this increase in pressure should be proportional to the temperature of the reaction to avoid excessive saturation of C=C of propylene bands to C-C of propane. Detailed optimization of the operating reaction conditions will be separately addressed in future work.

So far, no research has made a comparison of the single-step GTP reaction in the liquid and gas-phase as it is still particularly challenging to catalyze a hydrogenation reaction in a fully liquid phase and under a continuous-flow of H2-pressure. However, it is very interesting to down with this defiance to highlight the advantages of liquid single-step GTP route regarding the other reaction strategies. To mention a few: i) working in a liquid phase using a single fixed-bed reactor can greatly reduce the installation volume and cost, ii) a low-temperature reaction limits both the degradation of biomolecules and the energy consumption due to unnecessary water evaporation, iii) a moderate pressure could be easily used in-situ for subsequent separation of propylene using well-developed membrane technologies or pressure swing adsorption. These beneficial characteristics could greatly improve the techno-economic viability of producing green propylene from bioglycerol.

Causes of HDO activity loss and active-site requirement for GTP reaction

In an attempt to investigate the causes of the slight decreasing HDO activity and propylene yield with time over the dispersed Mo-based catalysts, which were observed particularly at increasing space-velocity, chosen fresh and post-reaction catalysts (with relevant catalytic behavior) were examined by HRTEM-STEM/EDX and in-situ XPS techniques. H2O-rich reaction medium (90 wt.%). EDX spectra (Fig. S14) recorded on this zone of pores confirms a Mo loading of ~12.9 wt.%, which is in the same order as the nominal value (14.8 wt.%) and to the real content measured by ICP (9.3 wt.%), thus confirming that Mo leaching did not take place. The monomodal histogram of the Mo particle size distribution centered at 2.7 nm indicates that the geometry of particles does not suffer significant changes compared with that of the fresh MoOx@SBA-15R catalyst initially centered at 1.7 nm (Fig. 4). The stability observed both for Mo nanoparticles and SBA-15 support rules out the sintering of Mo or the destruction of the 3D framework of SBA-15. "These tangibles findings demonstrated the effectiveness of searching for hydrophobic OMS; specifically, SBA-15 with highly resistant silica walls and mesopores, to repel excessive H2O adsorption, thus, protecting the hosted Mo nanoparticles against leaching and sintering, usually occurring under such hydrothermal reaction medium (H2O ~ 90 wt. %, temperature and pressure). [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF]. However, it remains to give further insights on how the different reduction-oxidation degree of Mo species, previously reveled by in-situ CO-IR spectra, have evolved along the reaction.

Thus, Fig. 10 shows a comparison between the Mo3d3/2-Mo3d5/2 spin-orbit components of in-situ H2reduced (fresh) and post-reaction MoOx, β-Mo2C, β-Mo2C@SBA-15R, and η-MoC@SBA-15R catalysts (used). Table 6 also summarizes the banding energy (BE) and chemical states of the most existing Mo δ+ (2 ≤ δ+ ≤ 6) species, which were enumerated based on the reported surface science [START_REF] Murugappan | Operando NAP-XPS unveils differences in MoO3 and Mo2C during hydrodeoxygenation[END_REF], [START_REF] Choi | XPS study of as-prepared and reduced molybdenum oxides[END_REF]. The Mo composition and C/Mo and O/Mo atomic ratios were also included in an effort to find a correlation between the surface change and loss of HDO performance.

As mentioned in the characterization procedure (section 2.3), Mo 6+ , Mo 5+ , Mo 4+ , and Mo 3+ were deconvoluted as one set of doublet peaks. However, Mo 2+ -C consists of three sets of doublets, where the first peak is strongly asymmetric, and the seconds are minors that correspond to an unscreened environment. The change of BE, intensity, and area of the Mo3d peaks from fresh and post-reaction samples were compared with those of the C1s, O1s, and Si2p core levels peaks. These spectral treatments and their interpretation offered a consisted deduction of the most active species for selective HDO of glycerol to propylene. Novel explanation for the loss of the HDO performance during the reaction were discussed.

Thus, the fresh MoOx catalyst, which was in-situ reduced, features 3d5/2 and 3d3/2 doublets that are broad and non-symmetrical due to the overlapping of peaks related with the presence of Mo 4+ at 229.6 eV (50%), Mo 5+ at 231.2 eV (48%), and Mo 6+ species at 232.1 eV (2%). After the reaction, the MoOx sample features BEs related to the existence of Mo 3+ at 228.9 eV (20%), Mo 4+ at 229.7 eV (30%), Mo 5+ at 231.5 eV (17%), and Mo 6+ species at 232.5 eV (33%). This assignment of BEs are consistent with the values reported by Choi et al. [START_REF] Choi | XPS study of as-prepared and reduced molybdenum oxides[END_REF] The emergence of appreciated amount of new Mo 3+ species (30%) in the used MoOx, which is accompanied by a significant diminution of Mo 4+ species, suggests that the latter underwent an over H2reduction to Mo 3+ . The population of Mo 5+ also decreases from 48% to only 14%, while the population of Mo 6+ increased up to 33%, suggesting also a re-oxidation of Mo 5+ to Mo 6+ . Even so, the bulk MoOx catalyst features a stable catalytic behavior in the GTP reaction, where the propylene yield remains constant at ~ 59.6% as function of TOS. This suggests that the change in the Mo 4+ and Mo 6+ population does not affect the HDO activity; thus, it does not play a significant role in propylene formation. The decrease in the Mo 5+

population could be recomposing by the emergence of new Mo 3+ species that seems to be more catalytically active.

In turn, bulk β-Mo2C, which is pre-carburized and in-situ reduced, features 3d5/2 and 3d3/2 signals with a more complex shape due to the overlapped peaks at Mo Moreover, the used sample features other two peaks at 286.4 eV and 288.8 eV, corresponding to C-O and C═O species, respectively, [START_REF] Wang | In Situ Preparation of Mo2C Nanoparticles Embedded in Ketjenblack Carbon as Highly Efficient Electrocatalysts for Hydrogen Evolution[END_REF] which are ascribed to carbonaceous species in the form CxHyOz. Also, the chemical states and distribution of Mo δ+ species on the used β-Mo2C sample suffered great changes. As a consequence, the 35% of Mo 4+ at 229.8 eV completely disappeared, while an equivalent proportion of Mo 6+

(38%) appeared at 232.9 eV. However, this surface modification did not affect either the deoxygenation and hydrogenation of glycerol or the yield of propylene, which remain constant at 63.9%, 53.8%, and 49.1%, respectively (Table 5 and Fig. 6). This confirms once more that these Mo 4+ and Mo 6+ states are not catalytically active for the GTP reaction. The slight decrease of the Mo 5+ species (at 231.7 eV) may be compensated by the formation of the Mo 3+ state, which cannot be distinguished from the Mo 2+ -C state, appearing at the close BEs (228-229 eV [START_REF] Oshikawa | Total conversion of glycerol, deoxygenation and hydrogenation activities, and yields of propylene and main OHCs obtained over MoOx@SBA-15C and MoOx@SBA-15R catalysts, which were estimated at 4.8 hours of reaction at; 318 °C, 30[END_REF]). For this, some increase in the population of Mo 2+ -C was observed, knowing the hydrothermal conditions cannot cause a re-carburization of the sample. The prevalence of the Mo 2+ -C states may be primarily responsible for the glycerol HDO and propylene formation over the carbide and/or oxycarbide surface.

When β-Mo2C was dispersed onto SBA-15, the fresh sample also shows Mo 2+ -C at 228.1 eV (23%), Mo 4+ at 229.3 eV (34%), and Mo 5+ at 230.8 eV (43%). The formation of the Mo-C band in this catalyst was also proven by the C1s spectra (Fig. S15 B), which features peaks at 282.1 eV and 284.8 eV, corresponding to C-Mo bands and adventitious carbon, respectively. After the test, the used sample features only BE related to the presence of Mo 4+ at 229.6 eV (31%), Mo 5+ at 230.8 eV (45%), and Mo 6+ species at 232.8 eV (25%).

The amount of Mo 4+ and Mo 5+ on the used sample is very similar to that of the fresh sample (31-34% and 43-45%, respectively). However, the Mo to 153.1 h -1 ) observed for the β-MoC@SBA-15R catalyst (Fig. 7), as we will further corroborate later.

For the fresh η-MoC@SBA-15R catalyst, it features the Mo3d spectra that are relatively similar to those of the β-Mo2C@SBA-15R catalyst. Oshikawa et al. [START_REF] Oshikawa | Total conversion of glycerol, deoxygenation and hydrogenation activities, and yields of propylene and main OHCs obtained over MoOx@SBA-15C and MoOx@SBA-15R catalysts, which were estimated at 4.8 hours of reaction at; 318 °C, 30[END_REF] also reported that the nature of Mo species revealed by XPS tend to be nearly the same regardless of β-Mo2C, η-MoC, or α-MoC1-x. In fact, the two nonsymmetrical shoulders were also deconvoluted into Mo 2+ -C at 228.3 eV (16%), Mo 4+ at 229.1 eV (50%), and Mo 5+ at 230.9 eV (34%). However, the η-MoC@SBA-15R catalyst contains a low amount of Mo 2+ -C and a predominant percentage of Mo 4+ than of the β-Mo2C@SBA-15R catalyst. This agrees with the XRD data (Fig. S7), indicating that the samples carburized under C3H6/H2 usually contain the MoO2 phase. The C1s spectrum (not presented) also indicates the existence of the carbidic C-Mo band at 282.3 eV that completely disappeared in the used η-MoC@SBA-15R sample, suggesting a re-oxidation of Mo 2+ -C. Thus, the sum of the Mo 2+ -C and Mo 5+ proportions (50%) decreased to 43% for the used η-MoC@SBA-15R, while the new Mo 6+ species (with a significant proportion ~ 20%) reappeared at 232.9 eV. This re-oxidation tendency of partially reduced Mo δ+ species, particularly Mo 2+ and Mo 5+ , may be responsible for the TOF decreasing of the glycerol HDO to propylene (Fig. 7).

Otherwise, comparing the atomic ratios of C/Mo and O/Mo, we observe that the values are much higher in each used sample compared to its fresh counterpart, which confirms our prior insights, suggesting a deposition of oxygenated carbonaceous species on Mo sites. This also justifies the low intensity of the 3d5/2 and 3d3/2 signals in all used samples compared to their fresh counterparts, knowing that sintering of Mo was completely discarded by HRTEM analysis. These carbonaceous species seem to be effectively deposited as CxHyOz rather than a pure graphite. Our observation is in line with those reporting also carbon deposition over Mo2C during the HDO of acetic acid [START_REF] Schaidle | Experimental and Computational Investigation of Acetic Acid Deoxygenation over Oxophilic Molybdenum Carbide: Surface Chemistry and Active Site Identity[END_REF] and anisole, [START_REF] Lee | Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo 2 C-Catalyzed Vapor Phase Anisole Hydrodeoxygenation[END_REF] but the authors could not put in evidence the accumulation of oxygen on the spent catalyst. Thus, the oxygen accumulation from adsorbed CxHyOz species, which cannot be differentiated to those from H2O of the medium, appears to be re-oxidizing the Mo species, thereby, leading to loss of HDO activity. Normally, SBA-15 support has hydrophobic characters, [START_REF] Serrano | Adsorption and Hydrophobic Properties of Mesostructured MCM-41 and SBA-15 Materials for Volatile Organic Compound Removal[END_REF] which could contribute to H2O repelling. Nevertheless, the dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts (with an excellent initial deoxygenation activity) accumulate more oxygen than bulk MoOx and β-Mo2C catalysts. These make us attribute the slight deactivation of Mo based catalyst to oxygen accumulation rather than to carbon itself, contrary to the previous authors. [START_REF] Schaidle | Experimental and Computational Investigation of Acetic Acid Deoxygenation over Oxophilic Molybdenum Carbide: Surface Chemistry and Active Site Identity[END_REF], [START_REF] Lee | Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo 2 C-Catalyzed Vapor Phase Anisole Hydrodeoxygenation[END_REF] Our explanation is supported by i) the emergence of new C-O and C═O bands (at BE of about 286.4 eV and 288.8 eV, respectively) in the C1s corelevel of all used samples (see for example Fig. S15 A-B), ii) significant simultaneous increase in the oxygen and carbon content in the surface of the used catalysts, and iii) no significant breaking of the C-C band during the single-step GTP reaction, which is the main precursor of graphitic coke formation. In addition to this, the high amount of oxygen cannot originate from passivation treatment by O2/He mixture (1/99 vol. %) because it is well known that this treatment forms just an atomic oxidized monolayer (~ 0.073 nm), while XPS beam usually provide the composition of a minimum interface of ~ 5 nm. In fact, the proportion of passivation oxygen does exceed 1.46 % in poor of cases. Thus, the surface accumulation of oxygen from CxHyOz species seem to be a justified cause for Mo δ+ re-oxidation, and thus, causing slight loss in HDO performance of the dispersed Mo-based systems.

To summarize, we can conclude that the HDO of glycerol to propylene is developed over the MoOxbased catalyst via successive transition reduction-oxidation cycles of Mo 6+ /Mo 5+ and Mo 4+ /Mo 3+ species, as illustrated in Scheme 2 A. The creation of undercoordinated Mo defects, arising from either the H2-reduction of MoO3 to MoO3-x and MoO2 to MoO2-x, could be acting as an oxygen vacancy susceptible to interactions with the oxygen of glycerol and intermediates. However, Mo 6+ and Mo 4+ states by themselves do not seem to be catalytically active in the single-step GTP route, which updates the early insights of Mota et al., [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] who reported that the direct reduction of Mo 6+ to Mo 4+ is responsible for propylene formation. Zacharopoulou et al. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] also attributed a positive effect to the presence of Mo 5+ and, to a lesser extent, Mo 4+ for propylene formation. Zacharopoulou et al. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] could not identify Mo 3+ species as potential active-sites for GTP route.

As just revealed by our research, Mo 4+ species do not directly contribute the to the catalytic act itself.

In contrast, the Mo carbide-based catalysts feature specific Mo 2+ -C species, where their relative population overlapped with those of possible Mo 3+ formed probably from Mo 4+ over-reduction, which somehow justifies the improved stability of β-Mo2C and the high initial HDO rates of β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts. However, after the tests, Mo 2+ -C species completely disappeared on the disposed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts, which was accompanied by an increase in the relative population of Mo involves an oxygen-rich sub-monolayer when are exposed to an oxidant medium. [START_REF] Prasomsri | Effective hydrodeoxygenation of biomass-derived oxygenates into unsaturated hydrocarbons by MoO3 using low H2 pressures[END_REF], [START_REF] Schaidle | Experimental and Computational Investigation of Acetic Acid Deoxygenation over Oxophilic Molybdenum Carbide: Surface Chemistry and Active Site Identity[END_REF] Consequently, the mentioned divergence in the surface quality is manifested as a difference in their HDO reactivity and stability.

A carbidic or oxycarbidic surface tends to have more hydrogenator characteristics including at high spacevelocity. The relative decrease in the performance of the Mo-based systems seems to be caused by the reoxidation tendency of Mo δ+ to form MoO2 and/or MoO3 that are more thermodynamically stable than MoO2-

x and MoO3-x ones.

Finally, it should be noted that to fully resolve the evolution of the most catalytically active-species for the single-step GTP route, it would be more relevant to combine in-situ ERP and operando NAP-XPS analyses with further kinetic measurements. This will allow more precise information about the real-time evolution of the active species as a function of the progress of main deoxygenation and hydrogenation events to be gathered. Even so, our study showed clear responsibility of termed Mo 3+ -O, Mo 5+ -O, and Mo 2+ -C species in selective HDO of glycerol to propylene. Thus, further studies on the optimal adjustment of the surface oxygen content in highly dispersed Mo oxycarbides and Mo carbide nanoparticles are necessary to develop stable Mo-based catalysts with robust HDO reactivity. The adjustment of chemical states of Mo species can be archived by controlling the initial reduction and carburization and subsequently remodeling the operating conditions, particularly the H2-pressure and temperature vis-a-vis the H2O-partial pressure under which the GTP reaction is taking place.

CONCLUSIONS

In this work, we provide a systematic structure-properties-performance exploration to further improve the single-step HDO of glycerol to high propylene yield. The strategy consists of modulating the physicochemical properties of Mo-based catalysts for use in an H2O-rich hydrothermal medium. Thus, we have successfully built and used a novel re-designing approach, providing qualitative and quantitative insights on how to get high loadings of highly reactive sub-2 nm Mo particles onto neutral ordered mesoporous silicas (OMS), which allowed overcome the problematic sintering and leaching of Mo species mainly prominent in H2O-rich reaction medium. The proposed paradigm offers a good dispersion of MoOx, β-Mo2C, and η-MoC species onto OMS, featuring remarkable HDO activities and practically null C-C breaking tendency, which allows the preservation of the high-carbon content along the GTP route.

At a space-velocity of 1.7 h -1 , the pre-reduced MoOx@SBA-15R catalyst (with a low acid/redox site ratio) features a very high propylene yield between 84.1% and 65.6%, while pre-calcined MoOx@SBA-15C

(with a high acid/redox site ratio) mainly yields propionaldehyde (52%). A simple thermal pre-treatment seems to offer a good tool for fine-tuning the selectivity of MoOx@SBA-15 catalysts towards specific products. However, MoOx-based MCM-41 or KIT-6 showed the poorest thermal stability despite their good initial GTP performance. At increasing space-velocity (up to 4.5 h -1 ), the HDO capacity of Mo-O sites tends to be suppressed. However, highly dispersed β-Mo2C and η-MoC species maintain good intrinsic HDO activities, providing at steady-state, the highest propylene TOF of 153. Fig. 10. High-resolution XPS spectra of Mo3d core-level and their deconvolution for fresh and used bulk MoOx (A) and β-Mo2C (B), and dispersed β-Mo2C@SBA-15R (C) and η-MoC@SBA-15R (D) catalysts. All systems were tested at same glycerol/water mixture (10/90 wt. %), T=318 °C and P=50 bar. However, H2/C3H8O3 molar ratio, WHSV and TOS were; 98, 1.7h -1 and 14.5 h, respectively, for bulk MoOx; 78, 2.35 h -1 and 9.7h, respectively, for bulk β-Mo2C; and 78, 4.7 h -1 and 9.7h, respectively, for dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts. Table 6. Banding energy of the most existing Mo δ+ species and their relative composition, and the interfacial atomic ratio of C/Mo and O/Mo elements for in-situ reduced (fresh) and post-reaction MoOx, β-Mo2C, β-Mo2C@SBA-15R and η-MoC@SBA-15R (used) catalysts. All systems were tested at same glycerol/water mixture (10/90 wt. %), T=318 °C and P=50 bar. However, H2/C3H8O3 molar ratio, WHSV and TOS were; 98, 1.7h -1 and 14.5 h, respectively, for bulk MoOx; 78, 2.35 h -1 and 9.7h, respectively, for bulk β-Mo2C; and 78, 4.7 h -1 and 9.7h, respectively, for β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts.

Fig. 9

 9 Fig. 9 displays high-resolution HAADF images, which include the corresponding STEM/EDX mapping and the histogram of the Mo particle size distribution, for the post-reaction β-Mo2C@SBA-15R catalyst tested at 318 °C, 50 bars, and WHSV = 4.7 h -1 for 9.7 h. These images clearly visualize a high density of longitudinal arrangement of Mo nanoparticles mostly located inside the pores and on the walls, where the hexagonal symmetry of SBA-15 remains intact despite the thermo-carburization at 700 ºC and the test in a

  2+ -C at 228.3 eV (32%), Mo 4+ at 229.8 eV (35%), and Mo 5+ at 231.3 eV (33%). The BEs of these peaks are consistent with the assignment of Murugappan et al. for similar Mo carbides.[42] However, these BEs and the percentage of Mo states indicate that the sample was not completely carburized despite the fact that the formation of the carbidic band has been confirmed. The deconvolution of the C1s core-level confirms the presence of the C-Mo band at 282.1 eV and of the adventitious carbon C-C/C═C at 284.8 eV for both fresh and post-reaction β-Mo2C samples (Fig. S15 A).

  2+ -C species (23%) completely disappeared, leading to the emergence of an equivalent proportion of Mo 6+ (25%). The absence of the carbide phase after the reaction was proven by the C1s spectrum (Fig.S15 B), which features only bands at 284.8 eV, 286.3 eV, and 288.5 eV, corresponding to adventitious carbon, C-O, and C═O, respectively. The re-oxidation of Mo 2+ -C may have occurred on the highly dispersed Mo carbide particles that are more reactive with the hydrothermal medium or the oxygen of the deposited CxHyOz species. This may be responsible for the decreasing TOF of propylene (from 226.4 h -1

  6+ states, thus leading to a gradual diminution in their GTP yield. This substantial difference in the surface of Mo carbides regarding Mo oxide-based catalysts suggests that most HDO sites in carbides must be different to those of MoOx-based catalyst. Probably, Mo 2+ -C species are catalytically acting as individual carbide-sites versus oxycarbide sites (Mo 3+ -Mo 5+ states), as illustrated in Scheme 2 B. Normally, oxycarbide phases are shown to exist on Mo carbides because they are extremely oxophilic species and usually

51 Scheme 2 .Fig 1 .Fig. 2 .Fig. 3 .Fig. 4 .

 5121234 Scheme 2. Simplified graphical illustration of the progress of successive deoxygenation and hydrogenation of glycerol and main C3 intermediates to form propylene over Mo 5+ /Mo 6+ and/or Mo 3+ /Mo 4+ redox cycles (A) and Mo 2+ -C carbidic sites or oxycarbide surface (B)

Fig. 6 .

 6 Fig. 6. Evolution of total glycerol conversion and of yields of propylene and co-products as function of time on stream for; A) bulk η-MoC@SBA-15R , B) β-Mo2C@SBA-15R, and C) β-Mo2C catalysts tested in single-step HDO of glycerol/H2O (10/90 wt.%) at 318 °C, 50 bar, H2/C3H8O3 (mol.)= 78, WHSV= 2.35h -1 , and TOS of 9.7 h.

Fig. 7 .Fig. 8 .

 78 Fig.7. Evolution of TOF of propylene and co-products as function of time on stream for; A) bulk MoOx, B) β-Mo2C, C) β-Mo2C@SBA-15R and D) η-MoC@SBA-15R catalysts tested in single-step HDO of glycerol/H2O (10/90 wt. %) at 318 °C, 50 bar, H2/C3H8O3 (mol.)=78 and WHSV=4.7h -1 .

Fig. 9 .

 9 Fig. 9. Representative TEM-HAADF images at scale between 20 nm (right) and 60 nm (left), which include the corresponding maps of STEM/EDX scanning and the histogram of Mo nanoparticles sizes distribution for used β-Mo2C@SBA-15R catalysts tested at 318 °C, 50 bar, C3H8O3/H2O = 10 wt. %, H2/C3H8O3 (mol)=78, WHSV=4.7 h -1 and TOS = 9.7 h.

  1 h -1 and 98.2 h -1 , respectively, which are at least 3.8 and 2.4 times, respectively, higher than those of bulk β-Mo2C and bulk MoOx systems that have never been reported for state-of-the-art GTP route. Globally, the good geometrical exposure of Mo species improves both C-O removal and C-C hydrogenation without significantly affecting C-C breaking, while the carburization of Mo-O to Mo-C seems to increase these catalytic characteristics and relatively balance the stability. Furthermore, in-situ XPS analyses coupled to kinetic data revealed the existence of partially reduced Mo 5+ , Mo 3+ , and Mo 2+-C species that play an active catalytic role for accomplishing the HDO events in glycerol and intermediates to produce fully deoxygenated and more hydrogenated compounds (i.e., propylene, propane). The whole of these analyses reveals that the single-step GTP route seems to progress; i) over Mo oxide-based catalysts via successive redox cycles of Mo 5+ /Mo 6+ and/or Mo 3+ /Mo 4+ states, and ii) over Mo carbides seems to occur mostly over Mo 2+ -C carbidic and oxycarbide surfaces that dispose of more hydrogenator character. The re-oxidation of partially reduced Mo δ+ states seems to be responsible for the certain loss of HDD activity, which arises from the surface accumulation of oxygen as deposited carbonaceous species. Graphitic coke formation, Mo sintering and/or leaching were completely discarded for these catalysts.In summary, good control of the Mo physicochemical properties (i.e., particles sizes, chemical states, interaction with inert SBA-15 support, acid/redox sites ratio, etc.) at the synthesis and functionalization steps under a controlled atmosphere (air, hydrogen, or carburizing agent) offers interesting tools to enhance the robustness of HDO catalysts, thus reaching relevant advances in glycerol HDO into propylene. The discussed research approach could be expanded to other types of reactions, particularly those occurring under an aggressive hydrothermal medium. This could drive the development of the next-generation of HDO catalysts and upgrades of many industry-relevant biomass-derived molecules.

	Selective/complete C-O scission	Propylene	+H 2	Propane
		OH	+H 2
	HO
	Glycerol	OH
			CO x , H 2 ,CH 4 or C 1-2 H y O z
	Scheme 1. Main transformation tendencies of glycerol under H2 co-feeding.

Table 1 .

 1 Molar composition of the starting reactants mixtures and, operating conditions of the hydrothermal synthesis

		25000
			2500
															5000	1000
		20000	MoO x _U
	(a.u.) Intensity	10000 15000	A)								Raw Data Fitted Mo 3+ Mo 4+ Mo 5+ Mo 6+ Background	(a.u.) Intensity	4000 3000 2000	Mo 2 C@SBA-15R_U C)					Raw data Fitted Mo 2+ -C Mo 4+ Mo 5+ Mo 6+ Background
		5000
			MoO x _R											1000	Mo 2 C@SBA-15R
		224	226	228	230	232	234	236	238	240	242	244		224	226	228	230	232	234	236	238	240	242	244
							Banding Energy (eV)											Banding Energy (eV)
		25000	2500 b-Mo 2 C_U										6000	1000						h-MoC@SBA-15R_U
	Intensity (a.u.)	10000 15000 20000	B)								Raw data Fitted Mo 2+ -C Mo 4+ Mo 5+ Mo 6+ Background	Intensity (a.u.)	3000 4000 5000	D)							Raw Data Fitted Mo 2+ -C Mo 5+ Mo 6+ Background Mo 4+
		5000																					h-MoC@SBA-15R
			b-Mo 2 C_R										2000
			224	226	228	230	232	234	236	238	240	242	244			224	226	228	230	232	234	236	238	240	242	244
							Banding Energy (eV)											Banding Energy (eV)

Table 2 .

 2 Nomenclature of Mo oxides-based catalysts and their main physicochemical properties .% → Nominal weight loading of elements, while (*) stands their real loading determined by ICP or CHNOS. SBET → Surface area estimated by BET; Vp(Tot)→ Total pore volume obtained at P/Pº = 0.95; Dp(large) and Dp(narrow)→ Average width of large and narrow mesopores, respectively, estimated by BJH model; Vμ → Micropore volume estimated by "t-plot" method. Acid-sites → Total number of acid-sites quantified from NH3-TPD profiles. Redox-sites→ Number of partially reduced sites estimated by CO adsorption. -→ Not determined.

	Catalyst	Loading			SBET Vp(Tot) Dp(Large) Dp(Narrow) Vμ	Acid-sites	Redox-sites
		wt.% (*)
		Mo	Fe	C	m 2 /g cm 3 /g	nm	nm	cm 3 /g	mmol(NH3)/gMo	mmol(CO)/gMo
	FeMo/AC	19.3(18.6) 2.8(2.9) (78)	1285 1.21	-	3.8	0.14	-	-
	MoOx	66.6 (-)	-	-	83	0.08	-	3.5	0	0.14	0.43
	Fe/MoOx	58.1	8.9(8.8) -	81	0.08	-	3.7	0	0.11	-
		(54.6)
	MoOx@SBA-	14.8	-	-	733 0.86	7.1	3.7	0	2.16	1.07
	15C	(13.9)
	MoOx@SBA-	14.8	-	(3.51)	772 1.10	7.5/6.4	-	0	1.65	1.98
	15R	(12.1)
	MoOx@MCM-	11 (9.5)	-	4.48)	1070 0.98	0	2.6	0	-	-
	41R
	MoOx@KIT-6R 8.7 (7.3)	-	(4.95)	735 0.74	5.0	3.7	0.07	-	-

wt

Table 3 .

 3 Nomenclature of Mo carbides-based catalysts and their main physicochemical properties Wt.% * → Real weight loading of Mo determined by ICP and of C determined by CHNOS. SBET → Surface area estimated by BET; Vp(Tot)→ Total pore volume obtained at P/Pº = 0.95; Dp(large) and Dp(narrow)→ Average width of large and narrow mesopores, respectively, estimated by BJH model; Vμ → Micropore volume estimated by "t-plot" method. Acid-sites → Total number of acid-sites quantified from NH3-TPD profiles. Redox-sites→ Number of partially reduced sites estimated by CO adsorption. -→ Not determined.

	Catalyst	Weight loading	SBET SBET Vp(tot) Dp(Large) Dp(Narrow)	Acid-sites	Redox-sites
		Wt.% *
		Mo	C	m 2 /g cm 3 /g	nm	nm	mmol(NH3)/gMo	mmol(CO)/gMo
	β-Mo2C	88.6	11.34	457	0.45	-	3.8	0.17	0.58
	β-Mo2C@SBA-15R	9.3	4.97	941	1.12	5.2	3.8	1.41	1.15
	η-MoC	88.7	11.27	220	0.25	-	3.9	-	-
	η-MoC@SBA-15R	12.2	4.83	1319 1.52	5.1	3.7	-	1.35

Table 4 .

 4 Total glycerol conversion, and deoxygenation, hydrogenation and reforming activities, and their pseudo-first order deactivation constant, as well as propylene yield and carbon balance provided by Mo oxides-based catalysts tested in single-step HDO of glycerol. Test conditions: 318 °C, 50 bar, C3H8O3/H2O = 10/90 wt. %, H2/C3H8O3 (mol)=98.7 and WHSV=1.7h -1 for TOS of 14.5 h. Parameters calculated at steady-state by taking an average value of GC analysis data (at last 1.5-3 hours of test).

		#3
	(%)*	(%) *

Catalyst

!"# !"#. (%) * [) %&' !"#. . +, ( -.

)* /] "# ')+ (%) * [ ) &' %&' . +, ( -. )* /] "# ')' (%) * [) &' %&% . +, ( -. )* /] "# ')/)' (%) * [ ) ')/)' . +, ( -. )* /]

1 -( . ) *

Table 5 .

 5 Total glycerol conversion, and deoxygenation, hydrogenation and reforming activities, and their pseudo-first order deactivation constant, as well as propylene yield and carbon balance provided by carbides-based catalysts tested in single-step HDO of glycerol. Test conditions: 318 °C, 50 bar, C3H8O3/H2O = 10/90 wt. %, H2/C3H8O3 (mol)=78, WHSV=2.35 h -1 and 4.7 h -1 for TOS of 9.7 h. %&' !"#. . +, ( -. )* /] "# ')+ (%) * [ ) &' %&' . +, ( -. )* /] "# ')' (%) * [) &' %&% . +, ( -. )* /]

	Catalyst	WHSV (h -1 )	!"# !"#. (%) * [)

  () → Surface atomic percent of different Mo δ+ species. C/Mo and O/Mo → Interfacial atomic ratio of carbon and oxygen per molybdenum calculated based on the relative sensitivity factors of Wagner. 2 ≤ δ+ ≤ 6 → Stands the oxidation state of Mo species.

	Catalyst	Mo δ+ species *Atomic ratio	B.E. (eV)_Fresh B.E. (eV)_Used Mo 3d5/2 (eV) Mo 3d5/2
		Mo 2+ -C	-	-
		Mo 3+	-	228.9 (20)
		Mo 4+	229.6 (50)	229.7 (30)
	MoOx	Mo 5+	231.2 (48)	231.5 (17)
		Mo 6+	232.1 (2)	232.5 (33)
		C/Mo	0.01	7.67
		O/Mo	2.82	3.71
		Mo 2+ -C	228.3 (32)	228.4 (40)
		Mo 3+	-	-
		Mo 4+	229.8 (35)	-
	β-Mo2C	Mo 5+	231.3 (33)	231.7 (22)
		Mo 6+	-	232.9 (38)
		C/Mo	0.52	10.95
		O/Mo	2.23	5.92
		Mo 2+ -C	228.1 (23)	-
		Mo 3+	-	-
		Mo 4+	229.3 (34)	229.6 (31)
	β-Mo2C@SBA-15R	Mo 5+	230.8 (43)	230.8 (45)
		Mo 6+	-	232.8 (25)
		C/Mo	1.10	25.42
		O/Mo	31.19	50.79
		Mo 2+ -C	-228.3 (16)	-
		Mo 3+	-	-
		Mo 4+	229.1 (50)	229.2 (37)
	η-MoC@SBA-15R	Mo 5+	230.9 (34)	230.8 (43)
		Mo 6+	-	232.9 (20)
		C/Mo	1.29	73.57
		O/Mo	26.75	54.00

Acknowledgements:

This research is part of CatGTP project, which is an MSCA grant, funded by the EU's H2020 program (agreement No. 101030636). The project leaders thank the FRAPPE program of region "Hauts-de-France" for its support. The REALCAT platform is benefitting from a state subsidy administrated by the French National Research Agency (ANR) within the frame of the 'Future Investments' program (PIA), with the contractual reference 'ANR-11-EQPX-0037'. The European Union, through the ERDF funding administered by the Hauts-de-France Region, has co-financed the platform. Centrale Lille, the CNRS, and University of Lille as well as the Centrale Initiatives Foundation, are thanked for their financial contributions to the acquisition and implementation of the equipment. The authors would also like to thank the technical staff of UCCS's services for their help during the implementation of experiments; high-speed catalytic tests at REALCAT platform (S. Heyte), ICP-OES and CHNS (J. Thuriot-Roukos), in-situ DSC (N. Djellal), in-situ XPS (P. Simon), gas sorption (J. Jezequel, O. Gardoll, and J. Charles), HRTEM-STEM/EDX (M. Marinova) and X-ray diffraction-scattering (L. Burylo and F. Blanchard). The discussions with prof. Royer and Dr. Dhainaut and their help with TGA analysis are gratefully acknowledged.

certain deactivation and thus become less hydrogenator for propionaldehyde to 1-propanol and then to propylene and finally to propane. This trend is less pronounced on the site of the η-MoC@SBA-15 catalyst, which features a more robust hydrogenation capacity to form more saturated propane, as mentioned above.

From this analysis, we can conclude that the reductive-carburization of Mo-O sites to Mo-C significantly improves their catalytic activity and stability in C-O cleavage and C-C hydrogenation under hydrothermal conditions, including when the high space-velocity of glycerol was used. The stability improvement is more pronounced on the bulk β-Mo2C catalyst, while the HDO robustness is remarkably higher on the dispersed β-Mo2C@SBA-15R and η-MoC@SBA-15R catalysts, generating further propylene and propane per site.

To provide a comparative summary with the most relevant GTP performance reported so far, data from recent literature, [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF], [START_REF] Lima | Catalytic conversion of glycerol to olefins over Fe, Mo, and Nb catalysts supported on zeolite ZSM-5[END_REF]- [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF], [START_REF] Zacharopoulou | Novel process for propene production from biomass: catalytic glycerol hydro-deoxygenation[END_REF] are shown in table 7. It appears that the configuration of the GTP process is evolving from combined multi-steps reactions to continuous single-step HDO strategy. This latter seems to be more suitable for mass production of propylene. Thus, considering the relevance of GTP operating conditions particularly reaction steps, space-velocity and/or H2/glycerol molar ratio, it seems that our single-step GTP performance over Mo-based catalysts is more promising than the majority of the previous works [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF], [START_REF] Lima | Catalytic conversion of glycerol to olefins over Fe, Mo, and Nb catalysts supported on zeolite ZSM-5[END_REF]- [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF], [START_REF] Zacharopoulou | One-step propylene formation from bioglycerol over molybdena-based catalysts[END_REF], [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF], [START_REF] Zacharopoulou | Novel process for propene production from biomass: catalytic glycerol hydro-deoxygenation[END_REF] and at least comparable with those of Fadigas et al., [START_REF] Mota | Green propene through the selective hydrogenolysis of glycerol over supported iron-molybdenum catalyst: The original history[END_REF] The propylene yield obtained in our study can reach values up to ~84.1 %, depending on the space-velocity (1.7 h -1 to 4.7 h -1 ) and the molar ratio of H2/glycerol (78/1 to 98/1). However, Fadigas et al. used higher H2/glycerol molar ratios (~120:1) to reach ∼90% propylene selectively and ∼100 % glycerol conversion at space-velocity ~5.4 h -1 . The difference in propylene formation at similar space-velocity can also due to the high content of Mo and Fe metals (~28.7 wt.%) in AC support, while our Mo-based catalyst contains only ~14.8 wt.% of Mo. Regarding the other GTP performance, for example Wan et al. [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF] and Sun et al. [START_REF] Sun | Efficient production of propylene in the catalytic conversion of glycerol[END_REF], reported propylene selectivity of 88% and 84.8%, respectively, at H2/glycerol (mol.) ∼ 100:1, but they used space-velocity of only 1 h -1 and 0.07 h-1, respectively. Moreover, both studies combined a complex mixture of catalysts; WO3(9.3 wt.%)/T317 + SiO2-Al2O3 [START_REF] Sun | Efficient production of propylene in the catalytic conversion of glycerol[END_REF] and MoO3/Ni2P (20 wt.%)/Al2O3 + ZSM-5, [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF] respectively, in a double-bed reactor to reach such propylene selectivity. These complex mixtures of catalysts with less defined active-sites would make difficult their in-situ regeneration, since the mentioned selectivity is reported for only 2-5 h. According to the employed configuration, which involves two tandem reaction zones, [START_REF] Yu | Propylene from renewable resources: catalytic conversion of glycerol into propylene[END_REF]- [START_REF] Wu | MoO3 modified Ni2P/Al2O3 as an efficient catalyst for crude glycerol to propylene[END_REF] the global GTP route is occurring through successive reactions with different kinetic rates. Thus, if the GTP reaction is extended for a longer time, some active sites may be deactivated before others. These drawbacks would negatively affect the operability of such multi-step strategy, which converts the single-step GTP route over Mo-based catalyst in a promising alternative. Finally, it should be noted that a detailed discussion on GTP catalysis will be included in our next review, where the last mechanistic and engineering GTP's findings will be systemically confronted and contextualized within state-of-the art GTP literature to fill existing gaps and postulate future research trends.

Table 7. Summary of a selection of the most recent GTP performance as a function of operating conditions, catalyst type and reactor configuration. * Separated three-step reactions, ** Combined three-staged reactors, Conv.→ conversion of glycerol, Sel. →selectivity of propylene, Yield → Yield of propylene → wt. → weight content, T→ temperature, P → pressure, Fi→flow of gas, gly. → glycerol, WHSV→ weight hourly space velocity, CT→ contact time, TOF → Turnover-Of-Frequency, TOS→ time-on-stream.

Ref.; year Catalyst type and beds configuration

Main operating conditions Conv.

(%)

Sel. (%)

Yield (%) [START_REF] Blass | Conversion of glycerol to light olefins and gasoline precursors[END_REF] ; 2014 Three staged fixed-beds; H-ZSM-5 (1 g), Pd (1 wt.%)/γ-Al2O3 (1 g) and H-BEA (0.5 g) T~ 500 ºC, 300 ºC, 500 ºC, P~ 1 bar, FH2~ 0.