Catalytic Depolymerization of Lignin over B–Ti-Modified Diatomite-Supported Nickel Phosphide Catalysts Using Formic Acid-Assisted Aqueous-Phase Glycerol

Catalytic depolymerization of lignin (CDL) is a prospective technology for the efficient utilization of biomass. In this paper, novel B–Ti-modified diatomite-supported nickel phosphide catalysts (xNiP/BTD) were prepared to depolymerize lignin in a unique formic acid-assisted aqueous-phase glycerol medium. Under optimal conditions, 4NiP/BTD exhibited 100% lignin conversion and 95.4% lignin oil (LO) yield, in which the yield of petroleum ether (PE)-soluble product reached 20.6%. Based on the characterization results, the superior performance of 4NiP/BTD for CDL was attributed to the collaborative effect of metallic Ni0 centers and acidic sites within Ni3P species. Among these, metallic Ni0 sites activated formic acid and aqueous-phase glycerol to generate active hydrogen (denoted as H*); subsequently, lignin was dominantly converted into phenol and alkyl guaiacol under the comprehensive function of H*, Lewis (Niδ+ species), and Bronsted acid sites (P–OH groups). Additionally, a potential reaction pathway for CDL with 4NiP/BTD as the catalyst was suggested. This study established a feasible approach for the efficient conversion of lignin using crude glycerol and green clay resources.