M-Gallate (M = Ni, Co) Metal-Organic Framework-Derived Ni/C and Bimetallic Ni-Co/C Catalysts for Lignin Conversion into Monophenols

Lignin is the only renewable source of aromatic compounds in nature, and high value-added monophenolic compounds can be obtained by depolymerizing lignin. This work reported a facile strategy for preparation of monometallic and bimetallic nonprecious catalysts for hydrogenolysis of lignin. Both Ni/C and bimetallic Ni-Co/C catalysts are synthesized starting from gallate-based MOFs, namely, M-gallate (M = Ni, Co), as a self-sacrificing precursor for calcination treatment. Gallic acid was used as the supporter of metal and a carbon source for carbothermal reduction of metal oxide to form well-dispersed metal nanoparticles during the calcination process. The prepared catalyst was characterized by PXRD, XPS, and STEM. The bimetallic catalyst NixCo1-x/C could form an alloy structure to produce a synergistic catalytic effect and greatly improve catalytic activity of lignin depolymerization. The highest yield of monophenols with Ni0.5Co0.5/C catalysts can reach 55.2%, and the guaiacol selectivity was as high as 70.3%. The depolymerization reaction of the lignin model compound guaiacylglycerol-beta-guaiacyl ether (GG) revealed that the beta-O-4 bond will be selectively broken, and the benzene ring will not overhydrogenated with Ni/C and the Ni0.5Co0.5/C catalyst.