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'.
Nadia et al. (Thu,) studied this question.