Molecular Insight into Pt-Catalyzed Chemoselective Hydrogenation of an Aromatic Ketone by In Situ Modulation–Excitation IR Spectroscopy

Abstract

Chemoselective platinum-catalyzed liquid-phase hydrogenation of aromatic ketones is an important reaction in the production of fine chemicals and pharmaceuticals. A typical example of this class of reactions is the hydrogenation of acetophenone (AP) over a Pt/Al2O3 catalyst. We investigated the adsorption behavior of the different reaction components and their reaction pathways using in situ attenuated total reflection infrared spectroscopy in combination with modulation excitation spectroscopy and phase sensitive detection. AP adsorbed on both Pt and the alumina support. On Pt, AP adsorbed in the η1 (O) configuration prevailed, whereas on alumina, AP bound to Lewis acid sites was predominant. In the presence of hydrogen, η1 (O) AP adsorbed on Pt was hydrogenated to the main product, 1-phenylethanol (PE), with high selectivity (82.5%). The produced PE was more strongly adsorbed on the alumina support than on Pt, leading to replacement of AP and accumulation of PE on alumina. Co-adsorption experiments of AP with its products PE, 1-cyclohexylethanol, and ethylbenzene revealed that AP adsorbed in the η1 (O) configuration was always the prevalent adsorption mode of AP on Pt, which may partly explain the high selectivity to PE observed. Co-adsorption of AP and CO did not significantly affect the adsorption of AP; however, CO adsorption strongly suppressed the adsorption and dissociation of H2. The studies revealed a striking difference in the selectivity behavior between the gas-phase and liquid-phase hydrogenation. Although in the gas-phase hydrogenation of AP, a significant effect of decomposition/hydrogenolysis products on the chemoselectivity of AP hydrogenation was reported, these fragmentation reactions were barely observed in the liquid phase.