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Selective carbon deposition on γ-alumina acid sites: towards the de- sign of catalyst supports with improved hydrothermal stability in aqueous media

Abstract : γ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal conditions, resulting in strong modification of its intrinsic properties. Most of the strategies that have been proposed to prevent or at least minimize its transformation into oxy-hydroxides consist in covering the alumina surface by a hydrophobic carbon layer, making it less sensitive to modifications induced by water. However, such methods necessitate high carbon contents, which significantly modifies structural and chemical properties of alumina. Here, we propose a new method based on a series of adsorption/pyrolysis cycles using sorbitol molecules previously adsorbed on specific hydration sites of (110) faces of γ-alumina crystals. Those sites, which are responsible for the dissolution f γ-alumina crystals in water, are thus selectively protected by carbon clusters, the rest of the surface being totally exposed and accessible to adsorbates. Under hydrothermal conditions (10 hours in water at 200°C), the formation of hydroxides is almost totally suppressed by covering less than 25% of the surface with only 7 wt. % carbon, which is far below the amount necessary to get similar results with more conventional carbon deposition methods.  INTRODUCTION Molecules derived from existing biomass treatment processes such as fermentation and hydrolysis (sugars, alcohols, polyols, carboxylic acids, etc.) are platform molecules for bio-based products manufacture. They consist of oxygen-rich, highly functionalized and generally water soluble molecules and reactions to convert them into valuable bio-products in aque-ous media may require relatively high temperatures and sometimes high pressures. These particular conditions, which combine water and temperature, are called hydrothermal conditions (HT) and they represent a serious challenge in the field of today's heterogeneous catalysis. 1,2 Indeed, conventional catalysts used in the petroleum refining industry generally consist of an active metal phase deposited onto an inorganic support and they may not be appropriate for applications involving water as solvent under such HT conditions. For exam-γ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides by a dissolution/re-precipitation process when placed in a water-rich medium at moderate temperature, resulting in strong modification of its intrinsic properties. 3,4,5-8 The stability of alumina materials in water has been a matter of great interest during the last years and many strategies have been proposed to prevent or at least minimize their transformation into hydroxides. The approach generally consists in modifying the alumina surface in order to make it less sensitive to the modifications produced by water under hydrother-mal conditions. The benefits of organic additives (such as carbon 9,10), inorganic additives (such as silicon 11,12 or phosphorus 13) and metallic additives 14 on the HT stability of alumina have been reported. However, the main parameters that govern the dissolution of alumina remain poorly known, which makes difficult the development of rational methods to improve the stability of this material. Ravenelle and coworkers first noticed that alumina transformation did not occur when polyols were present in the aqueous medium during a hydrothermal process. 15,16 The effect is supposed to result from the chemisorp-tion of the organic molecules on alumina, which avoids direct contact between water molecules and the surface and significantly improves water resistance. 17 This result can be compared with the work of Pham et al. who increased alumina stability in water by covering its surface with an oxygenated carbonaceous layer resulting from sucrose pyrolysis at 400°C. 9 They showed that 10 wt. % of carbonaceous deposit on the alumina surface was enough to prevent the transformation of the solid into boehmite. In another study, the same group used methane as a carbon source to produce graphitic carbon on alumina surface by gas phase carbonization. In this case, a higher amount of carbon was required to effectively protect alumina (35 wt. %), leading to a complete modification of its original properties (textural properties and surface chemistry). 10 Despite a growing number of articles dedicated to the improvement of alumina based materials in hydrothermal conditions, rationalization of predominant parameters that control hydrothermal stability remain unclear. In a previous work, we proposed a mechanism of the first steps of alumina dissolution based on experimental and theoretical approaches. 18 This mechanism involves some specific surface sites located on basal (110) planes of alumina platelets which are particularly reactive towards liquid water. We also showed
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Submitted on : Tuesday, May 5, 2020 - 10:44:36 AM
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Etienne Girel, Amandine Cabiac, Alexandra Chaumonnot, Michèle Besson, Alain Tuel. Selective carbon deposition on γ-alumina acid sites: towards the de- sign of catalyst supports with improved hydrothermal stability in aqueous media. ACS Applied Materials & Interfaces, Washington, D.C. : American Chemical Society, 2020, 12 (11), pp.13558-13567. ⟨10.1021/acsami.0c01646⟩. ⟨hal-02553402⟩

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