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

Production of noncondensed lignin via lignocellulose fractionation usually shows limited yield. Promoting re-action 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 alpha-alkoxylation reaction occurred between methanol molecular and C alpha-OH group in lignin, which protected /%-O-4 linkages from condensation. After being fractionated under a relatively mild fractionation condition of 85 degrees C 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 depoly-merized 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.

Automated summary

Production of high-yield noncondensed lignin via fractionation of lignocellulose using p-toluenesulfonic acid in methanol under microwave irradiation was achieved. Microwave irradiation effectively reduced the activation energy of lignin-carbohydrate complex (LCC) bonds cleavage and facilitated lignin exfoliation from carbohydrates. The lignin obtained showed promising yield and could be selectively depolymerized into monophenols with a high yield.