(734g) Understanding Solvent Effects over Supported Metal Catalysts: Ketone Hydrogenation over Ru/SiO2

Ketone hydrogenation over supported Ru catalysts is frequently reported to possess strong solvent effects, where different solvents are found to enhance the hydrogenation rate to varying extents. While there is no consensus as to the order in which different functionalities will enhance the rate through solvation, water is consistently reported as the solvent that provides the greatest promotional effect. Several qualitative explanations have been proposed for the observed solvent effects, where the majority propose that the kinetics of the hydrogenation are rendered more favorable in the presence of a solvent. To evaluate this claim quantitatively the kinetics of ketone hydrogenation in the absence of solvents must first be established, which can be better studied in the absence of any solvent.

To this end, the kinetics of the gas phase hydrogenation of C₃-C₅ ketones over Ru/SiO₂ catalysts was examined over a range of partial pressures and temperatures ranging from 303-476 K. A single thermodynamically consistent microkinetic model is developed to fit the observed experimental trends over all conditions and chain length, where a step-wise addition of hydrogen Horuiti-Polanyi mechanism is adopted. The reaction is found to proceed through a reactive alkoxide intermediate on the surface, where the addition of hydrogen to the alkoxide forming the alcohol on the surface is found to be the rate determining step. The developed model is then expanded to include the presence of multiple solvent functionalities on the surface including water, alcohols, alkanes and ethers. Multiple side reactions due to the presence of the solvents were observed, which lead to erroneous solvent effect observations if their presence is not accounted for. With the exception of water, no substantial promotional effect on the rate in the presence of any solvent was observed. Solvents are found to inhibit the rate through competitive adsorption on the surface; active sites are occupied which would otherwise facilitate ketone hydrogenation. Water is found to provide a promotional effect only at relatively high water/ketone activity ratios, which is thought to be due to the water itself participating in the reaction as a hydrogen donor. The consequence of ignoring thermodynamic non-idealities and catalyst stability are also examined in the context of observing an apparent solvent effect.