Rapid fractionation of pseudo-lignin with high yield by chemical and mechanochemical methods

The formation of pseudo-lignin (PL) is important to promote the refining of lignocellulosic biomass, but there is a problem of low yield of PL. In this study, we developed two chemical and mechanochemical approaches to efficiently extract PL using dilute acid combined with deep eutectic solvent (DES) and double ball-milling and enzymatic hydrolysis (DBEH), respectively. After dilute acid pretreatment at 220 °C for 4 h, due to the degradation of polysaccharides, a large amount of PL was formed and adhered to cellulose. Subsequently, PL was extracted using DES and DBEH, with extraction rates of 30.77% and 30.16%, respectively. There was a little difference in the structure of PL extracted by DES and DBEH, but gel permeation chromatography showed a significant difference in molecular weight between them, with the former having a weight-average molecular weight (Mw) of 1233 g/mol, and a number-average molecular weight (Mn) of 674 g/mol, and the latter a Mw of 5720 g/mol, and a Mn of 3210 g/mol. However, the polydispersity index obtained by both methods was narrow, ranging from 1.78 to 1.83. Moreover, FTIR and 2D-HSQC have provided the PL formation mechanisms. During pretreatment, holocellulose was hydrolyzed to form monosaccharides, which were further degraded to produce intermediates 5-hydroxymethylfurfural (HMF) and furfural (FF). Subsequently, FF and HMF were converted into other aromatic compounds, which then reacted with FF and HMF to form PL. Our study may pave the way for the efficient production of PL.