Photo-Promoted Production of a New Monophenolic Compound from Larch Lignin with Polyoxometalates Supported on g-C3N4 under Ambient Conditions

Mild condition and high selectivity are challenging in oxidative depolymerization of lignin to phenolic compounds. In this study, choline chloride (ChCl) was introduced to selectively prepare monophenol compounds from the fractionation of larch lignin with the catalyst of g-C3N4 (CN) supported by H4PMo11VO40 (HPMoV). When the yield of bio-oil was 35%, the novel product of 1-(4-hydroxy-3- methoxyphenyl) butan-1-one (HMB) was obtained with the 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, 220 W xenon lamp and air atmosphere. Results from different deep eutectic solvents (DES) 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 in depolymerizing lignin by chemical oxidative procedures, the presentation of HMB was different because of the conjugate addition of ·CH3 from ChCl accompanying the participation of H2O. Overall, the integrative catalysis from acidic DES, strong oxidizing HPMoV, and visible light in this work provides new insights for valorizing lignocellulose into value-added aromatic products in an ambient, convenient, and photocatalytic reaction system.