Ligand Effects on the Selective Hydrogenation of Nitrobenzene to Cyclohexylamine Using Ruthenium Nanoparticles as Catalysts

A series of ruthenium nanoparticles (Ru NPs) bearing different ligands on their surfaces have been used as catalysts in the selective hydrogenation of nitrobenzene to cyclohexylamine. Ru-C-60, Ru-PVP, Ru-HDA, and Ru-IPr [PVP = poly(vinylpyrrolidone); HDA = hexadecylamine, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] have been chosen because of their different electronic properties, with HDA and IPr acting as donor ligands, PVP as a polymer with small interaction with the Ru NP surface, and fullerene C-60 as an electron-attractor ligand, while their size and structure remain similar. A clear influence of the ligand on the Ru NP surface on the catalytic performances was observed. The less donor ligands promote hydrogenation of the N-phenylhydroxylamine (PHA) intermediate, which is considered to be the rate-determining step, leading to the more active catalysts, affecting also the chemoselectivity of the reaction. Density functional theory (DFT) calculations show that the hydride coverage of the Ru NPs plays a key role in the adsorption of PHA on the surface of the NPs. This adsorption is indeed unfavored for realistic coverage values, higher than 1.5H/per surface Ru atom, explaining the accumulation of this intermediate under hydrogenation conditions, as observed with all Ru NPs here studied. In addition, DFT calculations show that the adsorption of PHA is favored when fullerene C-60 is present on the Ru NP surface compared to the uncapped Ru NPs, which correlates well with the higher turnover frequency observed for Ru-C-60 catalyst compared to the other Ru NP systems bearing more donating ligands and also the higher selectivity obtained with this catalyst.