Effect of representative alkali and acid treatment on wheat straw lignin: Primary/minor units reaction behavior, thermal properties and pyrolysis products

Representative samples of wheat straw alkali lignin (WSAL), formic-acetic acid lignin (WSFAL) and p-toluene sulfonic acid lignin (WSPL) were isolated and their structures were investigated using 2D-nuclear magnetic resonance spectroscopy and other methods. WSAL, WSFAL, and WSPL underwent significant degradation, condensation, and oxidation, leading to distinct differences in their primary guaiacyl, syringyl, and p-hydroxyphenyl units. Lignin samples also exhibit significant differences in the composition and content of minor-constituents. For instance, the native structures (ferulic acid, p-coumaric acid, tricin, and cinnamyl alcohol) accounted for 5.45 %, 18.4 %, and 16.6 % of WSAL, WSFAL, and WSPL, while the newly formed structures (arylglycerol, Hibbert ketones, and vanillin/acetovanillone/vanillic acid) accounted for 5.58 %, 3.47 %, and 5.56 %, respectively. Based on the thermal degradation behavior of lignin samples, the kinetic distributed activation energy model was employed to simulate the activation energy. The activation energy decreases in the order of WSFAL (150–887 kJ/mol) > WSPL > WSAL (92–367 kJ/mol) within the conversion range of 0.2–0.9. WSPL showed the total aromatic pyrolysis products of 75.6 % which than that of WSFAL (60.4 %) and WSAL (52.4 %). This study will provide valuable insights for the structural understanding and sustainable utilization of wheat straw lignin obtained through multiple pathways.