Microwave-assisted fractionation of poplar sawdust into high-yield noncondensed lignin and carbohydrates in methanol/p-toluenesulfonic acid

Abstract

Production of noncondensed lignin via lignocellulose fractionation usually shows limited yield. Promoting reaction conditions benefits lignin production but has negative effect on structural condensation and thus inhibits its value-added applications in depolymerization. To address this issue, a unique fractionation method was proposed using p-TsOH (p-toluenesulfonic acid) in methanol under microwave irradiation to produce high-yield noncondensed lignin. It was found that microwave irradiation efficiently decreased the activation energy of lignin-carbohydrate complex (LCC) bonds cleavage from 102.1 to 63.8 kJ/mol and thereby facilitated lignin exfoliation from carbohydrates. And an effective α-alkoxylation reaction occurred between methanol molecular and Cα-OH group in lignin, which protected β-O-4 linkages from condensation. After being fractionated under a relatively mild fractionation condition of 85 ℃ for 30 min, a promising lignin yield of 87.9 % can be obtained from poplar sawdust, with 91.8 % of ether bonds preserved. The obtained lignin can be selectively depolymerized into monophenols with a high yield of 42.0 %, which was 74.9 % to the theoretical yield. Besides, 94.3 % of cellulose was retained in solid part after fractionation, which can be efficiently converted into glucose with a yield of 98.5 % via enzymatic hydrolysis.