Oxidative conversion of lignin into monophenolic compound catalyzed by NaOH–NaAlO ₂ /γ-Al ₂ O ₃ under mild conditions

Abstract A heterogeneous catalyst NaOH–NaAlO 2 /γ-Al 2 O 3 was prepared by supporting NaOH and NaAlO 2 on γ-Al 2 O 3 , and it was further employed for the oxidative degradation of lignin. The influence of the oxidant, the molar ratio of NaOH to NaAlO 2 , and calcination temperature of the supported catalyst on the catalytic activity were investigated. Air was found to be an effective oxidant for lignin conversion in the presence of heterogeneous catalyst. The total yield of monophenolic compounds (MPCs) from dealkaline lignin was 13.0 %, which was obtained by using NaOH–NaAlO 2 /γ-Al 2 O 3 calcinated at 550 °C as the catalyst with 2:3 M ratio of NaOH to NaAlO 2 at 170 °C for 2.5 h in air. Vanillin and syringaldehyde were found to be the main products, giving yields of 6.7 and 2.9 %, respectively. The conversion of lignin derived from wheat straw was also carried out under the identical conditions. Wheat straw lignin also displayed considerable conversion to MPCs in the presence of NaOH–NaAlO 2 /γ-Al 2 O 3 , and the total yield of MPCs at 12.3 % was obtained at 180 °C. NaOH–NaAlO 2 /γ-Al 2 O 3 was recovered by filtration after the reaction and reused without further treatment, and 11.0 % total yield of MPCs was obtained after four cycles.