Insights into the oxidation–reduction strategy for lignin conversion to high-value aromatics

Recently, a two-step strategy involving oxidative activation and reductive depolymerization becomes very popular for lignin conversion. However, both positive and negative results are obtained, and the detailed mechanism is still unknown. This study thoroughly investigated the effects of several key factors, including hydrogen pressure, reaction temperature, and catalyst compositions, on the reductive conversion of oxidized lignin (lignin OX ) to aromatics. For lignin oxidative activation, FT-IR, 2D NMR, and TG/DTG were used to prove that hydroxyl groups at lignin side chains had been successfully oxidized to carbonyl groups by AcNH-TEMPO/HNO 3 /HCl system. For lignin OX reductive conversion, pathways and mechanisms were explored, and the applicable conditions of this strategy were determined. Under relative moderate hydrogen pressure (e.g. 2 Mpa) over metal–acid catalysts, e.g. Ru/γ-Al 2 O 3 combined with Hf(OTf) 4 ), an obvious advantage of lignin OX conversion was found. Yield of valuable product from lignin OX was 42.98% which was much higher than that (10.99%) from unoxidized lignin. While, under high hydrogen pressure or promoted by catalysts with high hydrogenation activities, lignin OX tended to be reconverted to original lignin structures with loss of its conversion advantage. 2-phenoxyacetophenone as a lignin OX model was used to verify this speculation and to predict reaction pathways over different catalytic systems. These results proved that a catalyst with both hydrogenolysis and hydrolysis activity was essential for efficient lignin OX depolymerization.