Keggin-Type (Mo–V–P) Heteropolyacid-Catalyzed Selective Cleavage of β-O-4 Bond in Cotton Stalk Lignin for High Bio-Oil Yield

Depolymerization of cotton stalk lignin through catalyzing cleavage of the β-O-4 structure is mostly utilized for bio-oil production, while the formation of benzylic carbocations (C_α+) after the cleavage of the β-O-4 structure generally leads to repolymerization of monomers, resulting in low depolymerization efficiency and nonselective chemical production. Keggin-type (Mo–V–P) heteropolyacids can provide both H⁺ to catalyze the depolymerization of lignin into monomers and the subsequent methoxylation of α-OH with methanol and vanadium (V) to oxidize α-OH into α-ketones, subsequently avoiding the formation of C_α+ for repolymerization of medium products. On this basis, H₅PMo₁₀V₂O₄₀ (HPMoV₂) is first used to catalyze the decomposition of cotton stalk lignin in a methanol and water mixture. Results indicate that an optimum bio-oil yield of 58.8% with an alkane content of 30.6% is finally obtained at a temperature of 140 °C, time of 4 h, and a methanol/water ratio of 9:1. HPMoV₂-catalyzed methoxylation and oxidation of α-OH in 2-phenoxy-1-phenylethanol (PP-ol) is confirmed by the formation of 1, 2-dimethoxyethylbenzene and methyl benzoate, avoiding the formation of C_α+ with the subsequent condensation of free radicals. O₂ favors the regeneration of HPMoV₂ (V) through oxidation of HPMoV₂ (III), consequently leading to higher bio-oil yield. Moreover, addition of water is beneficial to both the dissolving of lignin and the following ring-opening and decarboxylation reactions of methyl benzoate, finally leading to a high alkane production. This work illustrates that HPMoV₂ (V) is favorable to the depolymerization of lignin with high selectivity in a methanol–water mixture, providing an efficient strategy for high-quality bio-oil production through cotton stalk lignin.