Kinetic and Isotopic Investigation on Hydrogenation of Aliphatic Aldehydes over Dispersed Ru Nanoparticle Catalysts

Kinetic assessment and isotopic studies establish a kinetic model that describes the behavior of vapor phase hydrogenation of aliphatic aldehydes over Ru/SiO2 catalysts. CO formed from decarbonylation is the most abundant surface species, which inhibits hydrogenation turnovers. Water promotes hydrogenation through a dynamic mechanistic transition in surface H-addition sequences. Without water, aldehyde hydrogenation turnovers increase linearly with butanal and H2 pressures, and shows fast C-H/C-D exchange rates, suggesting the Alkoxy pathway, initiated by quasi-equilibrated C-H bond formation before a kinetically relevant O-H bond formation. In the presence of water, rate of the first hydrogen attack is not affected, while the second hydrogen attack is greatly promoted. Theoretical calculations and kinetic isotope effects indicate the stabilization of transition state through hydrogen bonding. This work provides significant understanding in the reaction pathway of carbonyl hydrogenation under different conditions, which links and extends understanding from previous studies in the gas and liquid phase.M.A.S