Photo-promoted production of a new monophenolic compound from larch lignin with polyoxometalates supported on g-C3N4 under ambient conditions

Abstract

Mild conditions and high selectivity are challenging to achieve during the oxidative depolymerization of lignin into phenolic compounds. In this study, choline chloride (ChCl) was introduced to selectively prepare monophenol compounds from the fractionation of larch lignin using a g-C₃N₄ (CN) catalyst supported by H₄PMo₁₁VO₄₀ (HPMoV). When the yield of bio-oil was 35%, the novel product 1-(4-hydroxy-3-methoxyphenyl) butan-1-one (HMB) was obtained with 11% yield under optimized conditions of 0.2 g 25% HPMoV/CN, ChCl : p-toluene sulfonic acid (PTS) (molar ratio 1 : 2), 1 mL H₂O, 6 h, 80 °C, a 220 W xenon lamp and air atmosphere. Results from different deep eutectic solvents (DESs) indicated that ChCl was essential for HMB generation. The cleavage of β-O-4 bonds was dominant in the fraction of larch lignin and five representative lignin model compounds (LMCs) under optimum conditions. In addition, distinct degradation mechanisms of native lignin and different LMCs were deduced separately. Compared with other reported products of lignin depolymerization via chemical oxidative procedures, the mechanism of HMB was different because of the conjugate addition of ˙CH₃ from ChCl accompanying the participation of H₂O. Overall, the integrative catalysis using the acidic DESs, strong oxidizing HPMoV, and visible light reported in this work provides new insights for valorizing lignocellulose into value-added aromatic products using an ambient, convenient, and photocatalytic reaction system.