Solvents are usually selected based on their ability to solubilize the reactants and products, or their heat transfer properties. They can also provide additional degrees of freedom to facilitate combined reaction/separation strategies based on relative solubilities that might result in significant techno-economic advantages. Here, solvent effects are investigated for the C=C and C=O hydrogenation catalyzed by Pd/a-Al<sub>2</sub>O<sub>3</sub> in a series of different solvents by combining experimental kinetic analysis and computational calculations. In non-polar solvents, C=C compounds form ideal mixtures; thus, the hydrogenation rate follows the trend heptane (HEP) &gt; methylcyclohexane (MCHA) &gt; decalin (DL). Kinetic analysis reveals that when the solvent is a weakly interacting linear alkane (HEP), the rate determining step of the reaction is the first hydrogenation of the C=C bond, which is supported by DFT calculations. Statistical thermodynamics analysis validates the physical significance of the adsorption parameters derived from the kinetic fitting of experimental data. With solvents such as MCHA and DL, which interact more strongly with the metal surface and compete for active sites with the reactant and the surface intermediate, the rate limiting step shifts to the second hydrogenation step. In a co-solvent DL-HEP mixture, the non-ideality of the mixture drives decalin onto the catalyst surface to compete more effectively for sites with cyclohexene, leading to a larger decrease in activity than the ideal case. By contrast, in the hydrogenation of the C=O bond in benzaldehyde, the non-ideality of the reactant/solvent mixtures causes an enhanced benzaldehyde coverage reducing the competitive adsorption of the solvent, resulting in similar activities for all solvents. Likewise, the non-ideality of water-organic co-solvent enhances the adsorption of water, competing more effectively with the reactant; this explains the decreased activity for benzene hydrogenation in the presence of water. However, the H-bonded network at the liquid/solid interface provides an additional reaction pathway that facilitates the H transfer to the hydrophilic O atom in the C=O bond with a lower activation barrier, thus it promotes the C=O hydrogenation of benzaldehyde. Overall, by tuning the composition of the solvents, one can tailor the non-ideality effects of reactant-solvent mixtures to influence the coverage of surface species and consequently the reaction kinetics and catalytic performance.

Experimental and computational kinetics study of the liquid-phase hydrogenation of C=C and C=O bonds

ACS Spring 2022

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