Deep Eutectic Solvents-based Lignocelluloses Pretreatment,fractionation,and Valorization Process

As our country pays more and more attention to environmental pollution,energy demand,and ecological environment problems,the utilization of clean and renewable lignocelluloses to replace petroleum-based chemicals and materials has broad development prospects.In this study,deep eutectic solvents(DESs)treatment was used to improve the enzymatic digestibility and ethanol yield of lignocelluloses.And the influence of the chemical composition,structure and surface morphology of the treated materials on the conversion rate was also investigated.In order to realize high value-added utilization of the main components of lignocelluloses,the chemical structure,properties and further conversion of lignin and hemicellulose recovered from the treatment process were discussed.An efficient pretreatment of ball-milled assisted deep eutectic solvents to treat industrial xylose residue to improve enzymatic digestibility.Pretreatment with betaine/lactic acid system at 120 °C for 2 h was found to be the optimal conditions with prominently increased enzymatic digestibility(from 55.3% to 96.8%).The efficient conversion was mainly ascribed to the significant delignification efficiency of 81.6% after betaine/lactic acid pretreatment.The recoverable lignin had high purity,low molecular weight(630-2040 g/mol)and polydispersity(1.07-1.76).From the over mass balance and economic analysis,the cost of ball-milled assisted DESs pretreatment was reduced by about 38-85 times as compared with ionic liquid pretreatment.In the DESs pretreatment process,it was found that the effect of neutral deep eutectic solvents pretreatment was not significant.To improve the neutral DES(choline chloride/glycerol)pretreatment performance,three environmentally friendly heteropoly acids(phosphotungstic,phosphomolybdic,and silicotungstic acids)were used as catalysts.After treatment,the surface fiber of the Miscanthus x giganteus was peeled off,rough and porous,and the crystallinity is slightly increased.Pretreatment with silicotungstic acid at 120 °C for 3 h resulted in 97.3% of enzymatic digestibility.More importantly,80% of enzymatic hydrolysis was obtained within 12 h.Simultaneously,81.8% of ethanol yield was achieved in the semi-simultaneous saccharification and fermentation(SSSF)process,which is about 8 times that of untreated Miscanthus x giganteus.Using the same system mentioned above,three heteropoly acids with environmentally origin and economically viable were utilized as catalysts to extract nano-scaled lignin with excellent antioxidant performance.The changes of chemical structure of lignin were elucidated by HPAEC,GPC,SEM,TEM,FT-IR,elemental analysis and 2D HSQC NMR technical analysis.It was found that the silicotungstic acid-choline chloride/glycerol system resulted in a high lignin yield of 78.2%under mild conditions(120 °C,3 h).This was attributed to silicotungstic acid acting as tetrahydric acid which donated more active protons to selectively cleave ether bonds and further facilitated lignin removal.The following lignin profiling indicated that the recovered lignin had a high purity(92.7%),low molecular weight(Mw 1520-2470 g/mol),low polydispersity(PDI<1.7),excellent thermal stability,and good solubility in dimethyl sulfoxide and dimethylacetamide.Intriguingly,nano-scaled lignin was defined in the DESs fractionation procedure based on the SEM and TEM analyses.The antioxidant activity of nano-scaled lignin streams was investigated.In the DESs pretreatment process,it was found that DESs not only effectively removed lignin,but also effectively removed hemicellulose from lignocellulosic biomass.Therefore,based on the fractionation process cellulose,hemicellulose,and lignin,two new types of eutectic solvents(benzyltrimethylammonium chloride/lactic acid and benzyltriethylammonium chloride/lactic acid)were synthesized.As Compared with the previous studies on choline chloride/lactic acid eutectic solvents,the treatment time was shortened from 24 h to 2 h,which significantly improved the fractionation efficiency.Enzymatic digestibility of cellulose-enriched residues was significantly enhanced to 94.0% and approximately three-fold higher than the untreated material at an enzyme loading of 15FPU/g substrate.Furthermore,more than 95% of DES was recovered in each recycling and DESs could recycle and reuse five times without significant loss of pretreatment performance.Based on the above study,it was found that the lignin structure was damaged severely.Thus,two new class of deep eutectic solvents(tetramethylammonium chloride/lactic acid and tetramethylammonium bromide/lactic acid)integrating solvent and catalyst were synthesized to extract native-like lignin,high-purity,and uniform lignin under mild conditions.The pyrolysis behavior of native-like DESs lignin was investigated by pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS).The yield of phenolic compounds exceeded 50%,mainly including the G-type and S-type phenolic compounds,and non-phenolic aromatic compounds also had higher yields(>20%).In addition,the highest yield of furfural from the pretreatment liquid was 86.9%.Finally,the enzymatic digestibility of cellulose-enriched residues was significantly enhanced to90.0%.Through the above investigations,we obtained glucose,ethanol,native-like lignin,phenolic chemicals,and furfural by using the green solvents.This method achieved a high value-added conversion of the main components of cellulose,hemicellulose and lignin.It provides some references for the comprehensive valorization of agricultural and forestry lignocelluloses.