Selective Carbon Deposition on γ-Alumina Acid Sites: toward the Design of Catalyst Supports with Improved Hydrothermal Stability in Aqueous Media.

γ-Alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal (HT) 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 with 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 the (110) faces of γ-alumina crystals. These sites, which are responsible for the dissolution of γ-alumina crystals in water, are thus selectively protected by carbon clusters, with the rest of the surface being totally exposed and accessible to adsorbates. Under HT conditions (10 h 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 achieve similar results with more conventional carbon deposition methods.

Heterogeneous catalytic hydrogenation of biobased levulinic and succinic acids in aqueous solutions.

Supported noble-metal catalysts (Ru, Pd or Pt) and the corresponding Re-promoted catalysts exhibit a high activity for the hydrogenation of biobased carboxylic acids. Levulinic acid and succinic acid are converted into the lactones or the diols depending on the nature of the catalyst and the reaction conditions. The highest selectivity to 1,4-pentanediol of 82 % is achieved at 140 °C in the presence of the 1.9 % Ru-3.6 % Re/C catalyst.

Cellulose conversion with tungstated-alumina-based catalysts: influence of the presence of platinum and mechanistic studies.

The performances of platinum supported on tungstated alumina (Pt/AlW) in the hydrothermal conversion of cellulose at 190 °C under H2 pressure were evaluated and compared to that of Pt-free tungstated alumina (AlW). We show that the presence of Pt significantly increased the extent of conversion and led to a different product distribution with the formation of acetol and propylene glycol as the main products and a global yield of up to 40 %. Based on previous reports, we propose the formation of pyruvaldehyde on the Lewis acid sites of the tungstated alumina as a key intermediate. Pyruvaldehyde can then be transformed to acetol and propylene glycol or lactic acid depending on the presence or absence of supported Pt.

Core-shell H-ZSM-5/silicalite-1 composites: Brønsted acidity and catalyst deactivation at the individual particle level.

A combination of in situ UV-Vis and confocal fluorescence micro-spectroscopy is applied to investigate the influence of an external silicalite-1 shell on the Brønsted acidity and coke formation process of individual H-ZSM-5 zeolite crystals. Three probe reactions were used: oligomerization of styrene, methanol-to-olefin (MTO) conversion and aromatization of light naphtha (LNA) derivatives. Oligomerization of styrene leads to the formation of optically active carbocationic oligomers. Different styrene substitutions indicate the conversion ability of the catalyst acid core, a preferred alignment of the oligomers within the straight zeolite channels and a Brønsted acidity gradient throughout the zeolite crystal. Both the MTO conversion and the LNA process lead to limited carbonaceous deposition within the external silicalite-1 layer. This outer shell furthermore prevents the growth of extended coke species at the zeolite external surface. During MTO, the formation of carbonaceous compounds initiates at the center of the H-ZSM-5 zeolite core and expands towards the zeolite exterior. This coke build-up starts with a 420 nm UV-Vis absorption band, assigned to methyl-substituted aromatic carbocations, and a second band around 550 nm, which is indicative of their growth towards larger conjugated systems. Aromatization of linear and branched C5 paraffins causes negligible darkening of the zeolite crystals though it forms fluorescent coke deposits and their precursors within the H-ZSM-5 catalyst. Olefin homologues on the contrary cause pronounced darkening of the zeolite composite. Methyl-branching of these reactants slows down the coke formation rate and produces carbonaceous species that are more restricted in their molecular size.