466660 Understanding Solvent Effects over Supported Metal Catalysts: Ketone Hydrogenation over Ru/SiO2
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.
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