The application of supported palladium catalysts for the liquid phase hydrogenation of benzaldehyde.

The liquid phase heterogeneously catalysed selective hydrogenation of benzaldehyde to form benzyl alcohol can be compromised by a subsequent hydrogenolysis stage that leads to toluene formation. The hydrogenation of benzaldehyde in methanol over three supported palladium catalysts (5 wt% Pd/C, 5 wt% Pd/Al2O3 and 1 wt% Pd/Al2O3) is investigated under conditions of elevated and ambient hydrogen pressure at 313 K. For the elevated pressure runs, the three catalysts afford distinct reaction profiles that range from approximately 100% selectivity to the desired product (benzyl alcohol, 1 wt% Pd/Al2O3) to 100% selectivity to the undesired by-product (toluene, 5 wt% Pd/C). Product distributions broadly correlate with the number of Pd surface atoms. However, while the hydrogenation turnover frequencies (TOFs) for the alumina-supported catalysts are comparable (22.4 ± 1 min-1), a fivefold enhancement in TOF is observed for the Pd/C sample (115 min-1). The disparity of TOF values is attributed to the morphology of the Pd crystallites on the carbon-supported catalyst being different to that encountered with the alumina-supported catalysts. Collectively, the study provides insight on how a combination of supported Pd catalyst specification and reaction operating conditions can favour benzyl alcohol selectivity. This article is part of the theme issue 'Surfaces, interfaces and heterogeneous catalysis'.