N-Methyl-2-pyrrolidone pre-treatment of lignocellulose for high lignin yield and cellulose digestibility

In the present study, we discovered a new N-methyl-2-pyrrolidone (NMP) pretreatment solvent with an excellent lignin solubility and preserved β-O-4 linkages available in the original lignin structure. The results indicate that the formation of 4-methylamino butyric acid in NMP pretreatment system increased the polarity of the solvent system. The increased polarity synergistically increased the lignin removal and reduced the amount of lignin condensation. Results indicate a ca. 94% lignin removal was achieved, which is 25.6% higher than ethanol. Meanwhile, size-exclusion chromatography analysis revealed that the residual lignin obtained from NMP pretreated Acacia confusa contained a relatively higher number average (Mn) and weight average (Mw) molecular weights, indicating its higher integrity in terms of preserving the original lignin structure, than ethanol pretreated lignin. The result corresponds to the higher amount of β-O-4 interunit linkages in NMP lignin sample and is supported via HSQC analysis. A new organic hydrolysis factor (OHF) was established and introduced as an indicator in biomass separation experiments. This method can effectively quantify the effects of temperature, time, and acid catalyst concentration. The results showed that NMP exhibited better lignin solubility under the same OHF condition. Meanwhile, the NMP system showed higher tolerance for condensed lignin. In addition, almost 100% of the cellulose conversion was obtained for the NMP pretreated Acacia confusa wood substrates after 60 h of enzymatic hydrolysis with cellulase at a loading of 15 FPU per gram of glucan.