Study On Separation Of Main Components Of Fast-Growing Hardwoods By Choline-Based Deep Eutectic Solvents And Pretreatment

Lignocellulosic resources are mainly composed by cellulose,hemicellulose,and lignin.They are the most widely distributed and abundant renewable resources on the earth,and are also considered as important raw materials for the sustainable development of industries.Therefore,the efficient separation and high-value utilization of lignocellulosic biomass have received increasingly attentions.This thesis investigated the effect of choline-based deep eutectic solvents(DES)on the separation of the hardwood main components.The chemical structures of lignin and holocellulose components were analyzed with various characterization techniques to explore the reaction mechanism.The main findings are listed as follows:The effects of functional groups in hydrogen bond donor(HBD),and the hydrogen bond acceptor(HBA)to HBD ratio on the separation of poplar main components were compared.It was found that HBD with the presence of carboxyl group presented higher separation efficiency than those with hydroxyl and an amine functional group.Increasing the acidity of HBD improved the separation efficiency of the hardwood main components.However,the degradation of cellulose occurs when the acidity is too high.At the same time,the longer alkyl chain in HBD hindered separation efficiency.Therefore,choline chloride-lactic acid(Ch Cl-LA)has been selected as an optimized DES for its high efficiency.At 1:10 HBA/HBD molar ratio,its DES-holocellulose extraction yield is 46.92%,and the DES-lignin extraction rate is 84.38%.Anion exchange was carried out based on Ch Cl-LA(1:10)DES to prepare a brandnew type of low halogen content DES,namely,choline lactate-lactic acid(Ch LA-LA).The new DES could achieve optimized separation efficiency at 120? for 12h,where the DES-holocellulose extraction yield is 58.16%,and the DES-lignin extraction rate is90.12%.Compared with Ch Cl-LA,the separation efficiency of Ch LA-LA was significantly improved,where the DES-lignin extraction rate improved by 5.74%,and the DES-holocellulose yield increased by 11.24%.In order to reveal the reaction mechanism during the DES treatment,the chemical structure of separated components has been systematically characterized.It was found that during both DES treatments,the crystalline structure of cellulose I in DESholocellulose was well-preserved.However,both DESs had strong interactions with lignin molecules,breaking the ?-O-4 linkages in lignin to generate Hibbert's ketone and reducing its molecular size.The above research results proved that DES could selectively and effectively separate lignin from hardwood with high purity,while preserving the cellulose and hemicellulose components untouched.Therefore,the choline-based DES is an excellent solvent for separating lignocellulosic main components.The study further studied the effects of different pretreatments on the dissolution of hemicellulose.In control test(hydrothermal treatment)the dissolution rate of xylose is only 22.10%.The ultrasonic assisted hydrothermal pretreatment at 1100W for 50min could improve the xylose dissolution rate to 38.01%,which is improved by 71.99%.While xylanase(200U/g)assisted hydrothermal pretreatment achieved the xylose dissolution rate of 35.99%,which is 62.85% higher than that of control test.The[Emim][Cl] assisted hydrothermal pretreatment dissolved more xylose,whose xylose dissolution rate is 40.91%,which is 85.11% higher than that of control test.