The Importance of Solvent Adsorption in Liquid-Phase Reaction Kinetics: Nitrobenzene Hydrogenation in Pd (111) as a Case Study.

The catalytic hydrogenation of nitrobenzene is an essential reaction to produce aniline. This reaction can be catalyzed with noble metal-based catalysts such as Pd. In fact, hydrogenation takes place at room temperature in solution, where nitrobenzene is consecutively hydrogenated until reaching aniline. It has been empirically proven that the selection of the solvent affects the catalytic activity and selectivity of this reaction toward aniline. However, there is not a complete theoretical study that clarifies the role of solvents and their impact on the reaction mechanism. In this contribution a combined density functional theory (DFT) and experimental evaluation of the nitrobenzene hydrogenation to aniline under three different solvents (i.e., the nonpolar cyclohexane, the polar aprotic ethyl acetate, and the polar protic water) is carried out. Experimentally, the reaction with a commercial catalyst 5 wt.% Pd/C, which is modeled in DFT with a Pd (111) surface under an implicit solvation model, is carried out. The results reveal that less polar solvents compete for the catalyst adsorption sites with the reactants, producing longer induction times. On the other hand, the solvent has little effect on the reaction rate, but it does determine the pathway.