Component Elucidation And Mechanism Study On Enhancing Enzymatic Saccharification Of Eucalyptus Grandis × E.Urophylla And Energy Crops

In the process for realizing high-value utilization of all components in biomass,it is first necessary to separate them from lignocellulosic biomass.Depending on the physical and chemical characteristics of biomass,the pretreatment is conducted to separate the main components cleanly and gently.At present,various pretreatment techniques usually focus on cellulose as the major active participant,while less attention has been paid to other components,such as hemicelluloses and lignin,and they cannot be effectively utilized owing to the severe disruption of their molecular structures.The basic reason for the difficulty in effective separation of biomass components is the complexity of the cell wall structure and the heterogeneous distribution of the components in biomass.In this study,clean pretreatment techniques were used to isolate the hemicelluloses and lignin from energy plants,and their molecular structures were thoroughly investigated,which provided a theoretical basis for the construction of biomaterials.At the same time,the chemical compositions,topochemical distribution of the main components,and the ultrastructural changes in the plant cell walls were systematically studied to reveal the mechanism of enzymatic saccharification.The successive alkali treatment with the increasing alkali concentrations was employed to isolate the hemicelluloses and lignin from Eucalyptus,and their structural features were investigated comparatively.Xylose(73.15-95.34%)was the dominant sugar in all the hemicellulosic fractions,and the hemicelluloses with more linear structure were easier dissolved by the higher concentration of alkali.The hemicelluloses extracted from Eucalyptus mainly have a linear backbone of(1?4)-linked-?-D-xylopyranosyl residues decorated with branch at O-2 of 4-O-methyl-a-glucuronic acid unit.The lignin fractions were almost absence of bound polysaccharides(0.20-0.88%),and a comparatively high alkali concentration resulted in the degradation of lignin.The lignin fractions turned out to be mainly composed of ?-O-4' linkages(54.6-58.9%),followed by ?-?'(13.0%),and the low amounts of ?-5' linkages(2.4-3.4%).In addition,confocal Raman microscopy(CRM)was combined to assist visually monitor the course of hemicelluloses dissolution and delignification during the successive alkaline treatment processes.Raman spectra analysis revealed that the dissolution rate of hemicelluloses and lignin in the morphologically distinct cell wall regions was inhomogenous,and the dissolved hemicelluloses and lignin was mainly stemmed from the secondary wall layers.The most noteworthy feature of the successive alkali treatments was generating the hemicelluloses and lignin with intact structure.However,the efficiency of enzymatic hydrolysis for Eucalyptus was not significantly enhanced by this treatment.Based on the effects of five different ionic liquids(TBAA,[Amim]Cl,[Bmim]Cl,[Bmim]OAc and[Emim]OAc)on the structure and delignification of Eucalyptus cell walls,1-butyl-3-methylimidazolium chloride([Bmim]Cl,mild)and 1-butyl-3-methylimidazolium acetate([Bmim]OAc,severe)having the same cation were selected as representatives for improving the efficiency of enzymatic hydrolysis,and the hemicelluloses and lignin were isolated by an alkali post-treatment.The[Bmim]OAc pretreatment promoted the isolation of alkali-soluble hemicelluloses and lignin from the pretreated samples without obvious structural destruction.CRM revealed that the concentrations of hemicelluloses and lignin in the cell walls of raw material were decreased by the[Bmim]OAc pretreatment,and the dissolution of hemicelluloses and lignin in the subsequent alkali treatments was similar to the alone successive alkali treatments.As compared to the successive alkali treatments alone,the enzymatic digestibility was largely enhanced by the combination of ionic liquids pretreatment with successive alkali fractionations,and the glucose yield of 34.18%for the alkali treatment was increased to 90.53%.The mechanism in the improvement of the enzymatic saccharification during the process of the ionic liquid combined with successive alkali treatments was studied.The ILs pretreatment did not substantially alter the chemical composition of Eucalyptus,and only less than 6.36%of the total mass were consumed.However,ILs did change its structural features such as the swelling of cell walls,and the cellulose crystallinity was decreased from 52.6%to 44.6%([Bmim]Cl)and 26.9%([Bmim]OAc).As compared to the simplex ionic liquids pretreatment,the integrated process resulted in the significant removal of hemicelluloses and lignin,enhancing the disruption of the cell walls and increasing the exposure of cellulose,which led to a higher conversion of cellulose to glucose.The glucose yield of Eucalyptus underwent the combination of[Bmim]OAc and alkali treatments reached the maximum(90.53%),which was 6.6 times higher than that of the untreated Eucalyptus(13.71%).The chemical compositions and physical structure of plant biomass had a synergistic effect on the enzymatic hydrolysis process.Especially,the changes of cellulose crystallinity played a major role in enhancing the enzymatic digestibility of Eucalyptus in this study.The hydrothermal pretreatment as a promising clean pretreatment technology in industry was employed to effectively remove hemicelluloses(79.68-89.24%)from four energy crops(switchgrass,Miscanthus × giganteus,Hybrid Pennisetum,and Triarrhena lutarioriparia),significantly increase their efficiency of enzymatic hydrolysis with slight discrepancy(67.39-73.21%),and evenly erase the different recalcitrance of the four feedstocks.In addition,the anatomical characteristics,chemical composition,and enzymatic hydrolysis efficiency of the four energy crops were investigated comprehensively to evaluate the potential for the bioenergy production.Among the four crops,Hybrid Pennisetum had a better performance in the enzymatic hydrolysis process both before and after treatment,accompanying with generating the maximum concentration of low degree of polymerization xylo-oligosaccharides(DP 2-6)and the lowest amount of degraded products.Furthermore,the structural features of the residual lignin in the treated substrates were elaborately analyzed for deeply understanding the fundamental chemistry of lignin during the treatment.During the hydrothermal treatment,the condensation and degradation of lignin concurred simultaneously,and the depolymerization played the dominant role under the present condition,resulting the decrease of molecular weight from 6240-6690 g/mol to 4830-5700 g/mol.The present study is beneficial to the fully effective utilization of biomass components and extension of the energy sources.