Selective hydrogenation of phenol to cyclohexanone by SiO₂-supported rhodium nanoparticles under mild conditions

A silica-supported rhodium catalyst for the selective hydrogenation of phenol to cyclohexanone under mild conditions has been developed. As the Rh concentration on the catalyst increased from 0.5 to 15 wt%, the conversion (at phenol/Rh mole ratio 100/1) dropped whereas the initial selectivity to cyclohexanone increased. The direct hydrogenation to cyclohexanol occurred in parallel with partial hydrogenation to cyclohexanone. The negative correlation between selectivity and Rh dispersion suggests that direct hydrogenation occurs at low coordination sites whereas dissociation of phenol to phenoxy followed by hydrogenation to cyclohexanone takes place at higher coordinated terrace sites. DFT calculations revealed that the activation barrier for O–H bond cleavage is lower for phenol adsorbed on a Rh(1 1 1) flat surface than on small particles. By blocking the low coordination edge and step sites through grafting with (3-mercaptopropyl)trimethoxysilane, the cyclohexanone selectivity was improved from 82 to 93% at 100% conversion. The catalyst is active at room temperature and 1 atm H₂ pressure and can be easily activated by in-situ reduction.