Solvent-Controlled Product Distribution in Vanillin Hydrogenation over a N-Doped Carbon-Supported Nickel Catalyst

Solvent-controlled hydrogenation of vanillin to 4-hydroxymethyl-2-methoxyphenol (HMP) or 2-methoxy-4-methylphenol (MMP) is an attractive topic in the field of bio-oil utilization. Herein, a nickel-based catalyst supported on a N-doped carbon material (Ni-N-C) was prepared by facile ball milling mixing and subsequent high-temperature calcination. The as-prepared Ni-N-C800 catalyst showed more than 99% MMP yield and good stability using pure H2O as a solvent under the given conditions. By using the mixed solvent H2O/ethyl acetate with a volume ratio of 1:5, the hydrogenation was conveniently tuned to produce the intermediate product HMP. Density functional theory (DFT) calculations further showed that the intermolecular forces of HMP interacting with ethyl acetate were much stronger than those interacting with H2O. Ethyl acetate could raise the energy barrier to keep vanillin hydrogenation in the intermediate HMP stage, and H2O reduced the energy barrier to accelerate the final product MMP formation.