Microbehavior mechanism of water mediator on palladium in catalytic hydrogenation of aromatic carbonyl: Enhancement of hydrogen shuttling and modification of electronic structure

Abstract Water as the mediator is exploited to control the reaction behaviors of the aromatic carbonyl compound hydrogenation by taking advantage of its unique properties. Compared with the multifaceted effects of the aqueous solvent, the adsorbed water provides a simplified approach to gain the in-depth microbehavior mechanism of water on the palladium surface in the catalytic hydrogenation. Here we show that, with the assistance of the adsorbed water, the performance of palladium catalysts for the hydrogenation of a broad range of substrates (2-furaldehyde, benzaldehyde, anthraquinone, etc.) is enhanced. In situ experiments and DFT calculations demonstrated that hydronium ions are formed on the palladium surface via the reaction between the adsorbed water and dissociated hydrogen, accompanied by the increase in the electron density of the palladium surface via the charge separation. Mechanistic studies revealed that the former facilitates the hydrogenation of the carbonyl oxygen by promoting the hydrogen-shuttling ability, the latter favors the hydrogenation of the α-C in the hydroxyl intermediates. Meanwhile, the overstabilization of the hydroxyl intermediates by hydrogen bonds would bring about the increase in the energy barrier of the subsequent hydrogenation, resulting in the declined catalytic activity. Distinctively, the distinction in the promotional effect of water mediator for the hydrogenation of substrates is revealed to be associated with the different promotion degree of the reaction rate of the rate-determining step.