Yield and distribution of products of lignin liquefaction catalyzed by molybdenum-vanadium-phosphorus heteropolyacids

This paper focuses on the performance of Molybdenum-vanadium-phosphorus heteropolyacids (PMo_12-xV_x) with different V-doped amounts in catalyzing the conversion of lignin to bio-oil, and the distribution of liquefied products. Molybdenum-vanadium-phosphorus heteropolyacids (PMo₁₁V₁, PMo₁₀V₂, PMo₉V₃) with different amount (0, 2.5, 5, 10 w_t%) were used to catalyze lignin liquefaction. The experimental results showed that PMo_12-xV_x could effectively catalyze the depolymerization of lignin to bio-oil, and the doping amount of the V atom in PMo_12-xV_x affects the distribution of lignin depolymerization products. The highest bio-oil yield (53.62w_t%) and conversion (57.83w_t%) could be obtained from lignin liquefaction at 2.5 w_t% PMo₉V₃. With the increase of PMo_12-xV_x V-atom doped amount, the content of aliphatic hydrocarbons increased from 3.14% to 40.58%, the content of vanillin increased from none to 8.9%. Based on the analysis of bio-oil, FT-IR data, and DFT simulation results, it was found that the selective cleavage of the C_α-C_β bond and the ring-opening of the carbon at the methyl end of lignin were the main reasons for the high yield of vanillin and aliphatic hydrocarbons during the depolymerization of lignin catalyzed by PMo_12-xV_x. Finally, a possible reaction pathway for the PMo_12-xV_x-catalyzed depolymerization of lignin to vanillin and aliphatic hydrocarbons was proposed.