The mechanism of isopropanol dehydration on amorphous silica-alumina (ASA) was unraveled by a combination of experimental kinetic measurements and periodic density functional theory (DFT) calculations. We show that pseudo-bridging silanols (PBS-Al) are the most likely active sites owing to the synergy between the Brønsted and Lewis acidic properties of these sites, which facilitates the activation of alcohol hydroxy groups as leaving groups. Isopropanol dehydration was used to specifically investigate these PBS-Al sites, whose density was estimated to be about 10 site nm on the silica-doped alumina surface under investigation, by combining information from experiments and theoretical calculations.

Alcohol dehydration is of prominent relevance in the context of biomass conversion. This reaction can be efficiently catalyzed by alumina surfaces, but the nature of active sites, the mechanisms involved, and the key parameters to tune both the activity and the alkene/ether selectivity remain a matter of debate. In the present paper, isopropanol dehydration to propene and diisopropylether over γ-alumina, δ-alumina, and sodium-poisoned γ-alumina was investigated through a combined experimental and theoretical study. The experimental kinetic study shows that dehydration occurs following the same reaction mechanism on all materials, although γ-alumina activated above 450°C exhibits the highest density of active sites and the highest global activity. Results suggest that all the reaction pathways involved in dehydration require the same set of adjacent active sites located on the (100) facets of γ-alumina. DFT transition-state calculations of the formation of propene and diisopropylether on the main terminations of alumina, (110) and (100), were also performed. The less activated pathways for both the formation of the olefin (E2 mechanism) and the formation of the ether (S N 2 mechanism) were found on a Al V Lewis acidic site of the (100) termination, with calculated activation enthalpies (125 and 112 kJ·mol −1 for propene and diisopropylether formation, respectively) in good agreement with the experimental values (128 and 118 kJ·mol −1 , respectively). The higher or lesser selectivity toward propene or ether appears to originate from significantly different activation entropies. The effect of coadsorbed sodium on the reaction is linked to the poisoning of Al sites by neighboring, Na-stabilized OH groups, but no influence of sodium on distant sites is evidenced. Reaction temperature is identified as the main key parameter to tune alkene/ ether selectivity rather than morphology effects, which in turn affect drastically the number of available active sites, and thus catalytic activity.

The role of the oxide support on the structure of the MoS2 active phase (size, morphology, orientation, sulfidation ratio, etc.) remains an open question in hydrotreating catalysis and biomass processing with important industrial implications for the design of improved catalytic formulations. The present work builds on an aqueous-phase surface-science approach using four well-defined α-alumina single crystal surfaces (C (0001), A (112̅0), M (101̅0), and R (11̅02) planes) as surrogates for γ-alumina (the industrial support) in order to discriminate the specific role of individual support facets. The reactivity of the various surface orientations toward molybdenum adsorption is controlled by the speciation of surface hydroxyls that determines the surface charge at the oxide/water interface. The C (0001) plane is inert, and the R (11̅02) plane has a limited Mo adsorption capacity while the A (112̅0) and M (101̅0) surfaces are highly reactive. Sulfidation of model catalysts reveals the highest sulfidation degree for the A (112̅0) and M (101̅0) planes suggesting weak metal/support interactions. Conversely, a low sulfidation rate and shorter MoS2 slabs are found for the R (11̅02) plane implying stronger Mo-O-Al bonds. These limiting cases are reminiscent of type I/type II MoS2 nanostructures. Structural analogies between α- and γ- alumina surfaces allow us to bridge the material gap with real Al2O3-supported catalysts. Hence, it can be proposed that Mo distribution and sulfidation rate are heterogeneous and surface-dependent on industrial γ-Al2O3-supported high-surface-area catalysts. These results demonstrate that a proper control of the γ-alumina morphology is a strategic lever for a molecular-scale design of hydrotreating catalysts.

The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the supportdependent genesis of MoS 2 on four planar, single crystal -Al 2 O 3 surfaces with different crystal orientations (C (0001), R , M and A). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, the Mo is introduced by impregnation from an aqueous solution, in order to mimic the standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. AFM, XAS, TEM and XPS show that the -Al 2 O 3 orientation has a clear impact on the strength of metal-support interactions at the oxide state with consequences on the sulfidation, size, stacking and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0001) plane suggesting weak metal-support interactions leading to high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R plane implies stronger metal-support interactions leading to a low sulfidation degree and shorter MoS 2 slabs. Both A and M facets, of similar crystallographic structure, display an intermediate behaviors in terms of sulfidation rate and MoS 2 size in line with intermediary metal-support interactions. Polarization-dependent Grazing-Incidence-EXAFS experiments as well as HR HAADF-STEM analysis allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. A predominant basal bonding is suggested on the C (0001) plane in agreement with the existence of weak metal-support interactions. Conversely, a random orientation (edge and basal-bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed based on the comparison of the surface structure of the various model -Al 2 O 3 orientations used in this work and the predominantly exposed -Al 2 O 3 surfaces ((110), (100) and (111)).

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Successfully modeling the behavior of catalytic systems at different scales is a matter of importance not only for a fundamental understanding but also for a more rational design of catalysts and a more precise definition of the kinetic laws used as inputs in chemical engineering. We have developed here a multiscale modeling of the dehydration of isopropyl alcohol to propene and diisopropyl ether on γalumina catalysts, which clearly evidences and explains the central character of cooperative effects between coadsorbates in the kinetic network. The evolution of partial pressures with contact time was simulated using an original DFT-based microkinetic model based on a "macro site" centered on the main active site located on the (100) planes of alumina and comprising several neighboring adsorption sites. The formation of isopropyl alcohol−isopropyl alcohol or water−isopropyl alcohol dimers on the surface was required to correctly simulate the production of the minor product, diisopropyl ether, and the evolution of the product partial pressures at high conversion. DFT calculations were used to identify the structure of these dimers. In addition to entropic effects, the selectivity to ether is ruled by (i) stabilizing interactions between coadsorbed isopropyl alcohol or water molecules and the nucleophilic alcohol molecule reacting with the alcoholate intermediate, (ii) the formation of alcoholate−water dimers that selectively inhibit the formation of propene and increase the selectivity to ether at low conversion, and (iii) the reverse transformation of diisopropyl ether into propene and isopropyl alcohol that consumes ether at high conversion. The analytical expression of the reaction rate derived from this model and based on the existence of ensembles of interacting isopropyl alcohol and water molecules leads to a satisfactory modeling of the experimental kinetic measurements at all conversions.

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