NiAg Catalysts for Selective Hydrogenolysis of the Lignin C-O Bond

Several bimetallic NiAg catalysts for the lignin hydrogenolysis reaction are evaluated. NiAg catalysts are either prepared by wet chemical reduction, in which a mixture of AgNO3 and Ni(NO3)2 or a mixture of AgOAc and Ni(OAc)2 is reduced by NaBH4 and stabilized by polyvinylpyrrolidone in water, or by the decomposition–precipitation method to obtain NiAg/SiO2. These three catalysts exhibit distinct performances in hydrogenolysis of a lignin β-O-4 model compound. For colloidal catalyst from co-reduction of AgOAc and Ni(OAc)2, separate growths of Ag and Ni nanoparticles (NPs) are observed, and the system exhibits an undesired selectivity of 31.5% toward dimer products. On the other hand, NiAg NPs are dominant after the reduction of nitrate precursors, although the NP size is not sufficiently small (6.7 nm), resulting in high selectivity but a low reaction rate (12.6% conversion with 12.1% monomers yield). Bimetallic NiAg active phase with excellent dispersion (≈1.5 nm) is obtained on NiAg/SiO2, which enables 72.7% substrate conversion and 65.6% yield of target monomer compounds. From these results, NiAg bimetallic catalyst is indeed superior to monometallic Ni in lignin hydrogenolysis, however, the formation of bimetallic NiAg catalyst is highly sensitive to the preparation conditions—proper selection of precursors, reductant, and support/stabilizer are all crucial.