Transfer hydrogenation of cinnamaldehyde with 2-propanol on Al2O3 and SiO2–Al2O3 catalysts: role of Lewis and Brønsted acidic sites

Abstract

Transfer hydrogenation of α,β-unsaturated aldehyde is an important reaction (MPV reaction) for the synthesis of allyl alcohol because of its high selectivity for the hydrogenation of the carbonyl group. This reaction can proceed over many oxide catalysts but the role of acidic and basic sites on the oxide catalyst surface has remained an issue of debate. We report herein the catalytic data of serial Al₂O₃ and SiO₂–Al₂O₃ catalysts for the MPV reaction of cinnamaldehyde at 150 °C using 2-propanol as the H-donor. The surface acidity of the catalysts is adjusted by changing the calcination temperature between 400 and 1000 °C and quantified to correlate with the catalyst performance. Surface Lewis acidic sites that are poisoned by pyridine but not by hindered pyridine (lutidine) under catalyst working conditions are identified as the catalytic sites for the MPV reaction. Lutidine-sensitive surface Brønsted acidic sites, which are among the spectator sites during the MPV reaction, are shown to serve as the catalytic sites for cascade cross-etherification reaction between cinnamyl alcohol and 2-propanol. The MPV reaction is also conducted under high-pressure CO₂, the results of which exclude possible involvement of surface basic sites during the reaction.