An Efficient Enzymatic Mechanism For Cellulose Digestion Based On The Inhibition Of Lignin Condensation By Ternary Deep Eutectic Solvents

Wood represented a forest residue is available,renewable and abundant reserves,however,its utilization efficiency and added value are not satisfactory at present.Isolation and separate conversion of forest residue components is one of the strategies for efficient and high value utilization of forest resources,however,the key scientific challenges such as effective breaking of the "biomass recalcitrance" and inhibition of degradation and condensation need to be solved.It is challenging and important to explore new solvent systems that can break the"biomass recalcitrance" under mild conditions and inhibit the degradation and condensation of separated components at the same time.In this thesis,a new diol-based deep eutectic solvent system was designed to pretreat woody raw materials and effectively break the"biomass recalcitrance"while inhibiting component degradation and condensation.High purity cellulose with easy conversion and non-condensed lignin with high ether bond content were obtained.To reveal the mechanism of inhibition of lignin condensation by diol in the system by comparing different deep eutectic solvent systems,and to investigate the effect of diol on the structure and functional groups of lignin.And the effects of eutectic solvent treatment on the retention rate,structure and subsequent enzymatic hydrolysis of cellulose were investigated.This thesis provides a new idea of efficient and controllable utilization of wood resources by a diol-based deep eutectic solvent.The main research of the thesis is divided into the following three aspects:(1)Pretreatment of poplar lignocellulose with choline chloride/oxalic acid binary deep eutectic solvent at 110? for 1 h,3 h and 5 h to promote component separation and conversion.The results show that choline chloride/oxalic acid deep eutectic solvent pretreatment can effectively dissociate poplar lignocellulose,and 50.3%of cellulose solids and 10.7%of lignin were obtained by separation.With the increase of treatment time,the spherical condensed lignin on the surface of cellulose solids increased,which hindered the further enzymatic conversion of cellulose;the highest enzymatic efficiency of cellulose solids was achieved after 3 h pretreatment with choline chloride/oxalic acid,with a glucose yield of 73.3%,which is more than 3 times higher compared to the raw material.Analysis of isolated lignin after evaporation of ethanol by removal of the antisolvent method reveal that the major ether bonds in the side chain region of lignin are extensively broken and the lignin condensation rate increase to 51%.(2)On the basis of part(1),To solve the problem of lignin condensation and further improve the efficiency of enzymatic digestion of cellulose solids,this part adds ethylene glycol,which can inhibit condensation,to the choline chloride/oxalic acid deep eutectic solvent to form a ternary deep eutectic solvent for efficient and controlled dissociation of lignocellulose.The results show that the dissociation of lignocellulose is most effective when the addition of ethylene glycol in the ternary deep eutectic solvent is 1 molar ratio,54.8%of the cellulose solids are separated,which are lighter in color and more finely ground,with almost no attached lignin on the surface of the cellulose solids under microscopy,showing a more obvious cellulose fibril network structure;subsequent enzymatic digestion of the cellulose solids also reveal that the efficiency of enzymatic digestion is greatly improved,with a maximum glucose yield of 94%.8.2%of lignin was separated,and the color is also lighter compared to the above binary solvent system,and the ether bond structure in the side chain region was preserved,from 0/100Ar to 6.4/100Ar,and the lignin condensation rate was reduced from 51%to 41%.It shows that the addition of ethylene glycol can not only retain the cellulose fraction and promote cellulose enzymatic digestion,but also effectively retain the ether bond structure of lignin and inhibit the condensation,which can achieve the effective dissociation of lignin fraction and "double priority" separation transformation.(3)Based on the above-mentioned ethylene glycol-based ternary deep eutectic solvent system for the dissociation of lignocellulose,this part further expands ethylene glycol into a series of glycols,namely propylene glycol,butylene glycol and pentanediol,to form the corresponding ternary deep eutectic solvents for the dissociation of lignocellulose.The experimental results show that high purity cellulose solids were obtained with different diol-based deep eutectic solvents,and the yield of glucose by subsequent enzymatic digestion increased with the increase of diol carbon chain,with the highest glucose yield up to 99%.Different diols also inhibite the condensation of lignin,the condensation rate decreased from 51%to 13%,and the retention of lignin ether bond was up to 28.2/100Ar,and the structure was rich in aliphatic chain alcohol hydroxyl and phenolic hydroxyl groups.A comparative study of pentanediol and n-pentyl alcohol forming a ternary deep eutectic solvent reveals that the efficient removal of lignin and the inhibition of condensation mechanism are mainly due to the alkoxylation of lignin by diol molecules which inhibited the breakage of ether bonds and increased the hydrophilicity and space-site resistance of lignin.