Critical role of solvent-modulated hydrogen-binding strength in the catalytic hydrogenation of benzaldehyde on palladium

Solvents not only disperse reactants to enhance mass transport in catalytic reactions but also alter the reaction kinetically. Here, we show that the rate of benzaldehyde hydrogenation on palladium differs by up to one order of magnitude in different solvents (dioxane < tetrahydrofuran < water < methanol). However, the reaction pathway does not change; the majority of turnovers occurs by stepwise addition of sorbed hydrogen to sorbed benzaldehyde, first to the carbonyl oxygen and then to the carbon atom of the formyl group, forming benzyl alcohol. An analysis of the solvation energies shows that both ground and transition states are destabilized by the solvents compared to those at the gas–solid interface. The destabilization extent of the reacting organic substrates in both states are similar and, therefore, compensate each other, making the net kinetic effects inconsequential. Instead, the marked reactivity differences arise only from the differences in the solvation of sorbed hydrogen. Solvent effects play major roles in determining the mechanism of catalytic reactions, but their understanding remains often qualitative. Here, the authors provide a quantitative analysis of the effect of solvents on the catalytic hydrogenation of benzaldehyde on palladium, revealing the solvents’ crucial role in modulating the hydrogen-binding strength.