This paper aims at investigating the influences that metallic and acidic functions exert on each other in mesoporous oxide-supported catalysts, and their consequences on catalysis. Three parameters are kept constant in the systems studied: Pt particles size (smaller than 2 nm), particles surface density over the support and pore size distribution centered on the same diameter (6 nm). Siliceous SBA-15 served as a reference support. Al-SBA-15 exhibiting Brønsted acidity was prepared by post-synthesis grafting of Al(OiPr)₃ in cyclohexane. Mesoporous alumina was synthesized using glucose as an organic additive; the Pt content was 0.5 wt% instead of 1% on SBA-15 systems, due to a twice lower specific surface area.

²⁷Al NMR, FTIR of adsorbed CO and probe catalytic tests (isopropanol dehydration, cumene cracking) showed that Pt/Al-SBA-15 is less acidic than parent Al-SBA-15. Comparison with a blank system prepared without Pt suggests a redistribution of Al atoms during Pt introduction. Upon hydration of Al-SBA-15 and thermal treatment, 25% of strong and mild Brønsted sites (Si-O(H)-Al) transform into strong Lewis sites (isolated tetracoordinated Al atoms). The introduction of Pt may thus significantly influence the acidic properties of a mesoporous support.

Liquid phase hydrogenation of cinnamaldehyde to cinnamyl alcohol was used as a test reaction. Selectivities are similar on Pt/SBA-15 and Pt/Al-SBA-15 (13% at a conversion of 70%), but the presence of acidic sites on the latter leads to higher reaction rates, possibly because desorption of reactants is facilitated on particles whose electron population is depleted (blue-shift of the CO/Pt IR band on the acidic catalyst compared with Pt/SBA-15). In contrast, Pt/Al₂O₃ exhibits both high reaction rates and the highest selectivity toward cinnamyl alcohol (44% at total conversion). A direct action of Lewis acidic sites neighbouring the particles, which may favour the adsorption of the molecules by the C=O bond, is suggested.

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