Elucidating the role of solvent in C=C bond hydrogenation of maleic acid at metal-solvent interface by AIMD and DFT simulations

Abstract

Ab-initio Molecular Dynamics (AIMD) and Density Functional Theory (DFT) simulations were used to understand the solvent effects in C=C bond hydrogenation using maleic acid (MAc) to succinic acid (SAc) as a model reaction on Pd (111) surface. In the presence of explicit water molecules, MAc was observed to dissociate to maleate ion (MA). The presence of hydrogen bonding network in water and charge redistribution from the Pd surface to the water molecules stabilized the reactants and intermediates and resulted in low free energy barriers (31.7 and 44.9 kJ/mol) for the first and second step of C=C bond hydrogenation. Interestingly, the C=C bond of MA in presence of water was reduced by the surface H atom rather than the solution-mediated pathway. The surface H atom, however, could be either obtained by direct H₂ dissociation or via H exchange between adsorbed H and nearby water molecules. The free energy barrier for the H exchange between nearby water molecules and surface adsorbed H atom was calculated to be 43.1 kJ/mol. In the absence of hydrogen bonding network in polar aprotic solvent like 1,4-dioxane and vacuum phase, the activation barriers were estimated to be relatively high. In general, the electronic effect of the solvents followed a linear trend, where FMO gap serves as a good descriptor of the reactivity.