Acidolysis mechanism of lignin from bagasse during p-toluenesulfonic acid treatment

Abstract

Lignin was efficiently separated from lignocellulosic biomass using p-toluenesulfonic acid (p-TsOH) treatment. However, lignin depolymerization and acidolysis have not been studied to date. In this study, the mechanism of lignin acidolysis was investigated. The structure of lignin in bagasse before and after the p-TsOH treatment was analyzed. The efficiency and selectivity of lignin depolymerization were evaluated. The lignin removal rate reached 88.81%. Hemicellulose was completely removed from lignin, and cellulose destruction was inhibited. The structure of lignin in bagasse was analyzed using nuclear magnetic resonance (NMR). The results revealed that α-methoxylated β-O-4 intermediates were formed during lignin depolymerization. This indicated that the benzyl ether bonds were effectively catalyzed and broke, thereby promoting lignin dissolution. The lignin–carbohydrate complex structure was destroyed. The insoluble and soluble lignin in the hydrolysate were analyzed using NMR and gas chromatography–mass spectrometry. The results indicated that the catalyst promoted breaking of the Cβ–O bonds in the β-O-4 structures. Phenols and Hibbert ketones were formed. In addition, lignin sulfonation was confirmed by the presence of di-o-tolusulfone and di-p-tolusulfone. The low contents of β-β and β-5 bonds in the reaction products indicated that lignin condensation was inhibited. The results demonstrated that lignin acidolysis was synergistically catalyzed by H+ and p-TsOH. It provides important theoretical support for efficient and clean separation of lignin using the p-TsOH treatment.