| Lignocellulosic biomass is a renewable resource with a wide range of sources and a large number.It can produce a variety of fuels and chemicals through sugar platform conversion technology to alleviate energy crisis and environmental pollution.However,the resistant structure of lignocellulosic biomass limits the conversion of sugar platform,and appropriate pretreatment methods are needed to destroy this resistance.Effective pretreatment of lignocellulosic biomass is an important step to destroy the resistance of biomass and remove lignin to improve the enzymatic saccharification performance of cellulose components.The traditional pretreatment methods have the disadvantages of harsh reaction conditions,serious environmental pollution and low pretreatment efficiency.Deep eutectic solvent（DES）is a new type of pretreatment method,which has many advantages such as easy preparation,non-volatilization,low toxicity,recyclability and biodegradability.It can destroy the ether and ester bonds in lignocellulosic biomass by forming hydrogen bonds to achieve the purpose of component separation.DES can selectively dissolve lignin and hemicellulose,retain cellulose well,and destroy the three-dimensional network structure of lignocellulosic biomass,which greatly improves the efficiency of cellulase hydrolysis and shows great potential in the pretreatment of lignocellulosic biomass.Therefore,in this study,carboxylic acid DES with choline chloride as hydrogen bond acceptor and lactic acid/formic acid/acetic acid as hydrogen bond donor and alcohol DES with choline chloride as hydrogen bond acceptor and glycerol/ethylene glycol/polyethylene glycol 200 as hydrogen bond donor were designed and synthesized for the separation of lignocellulosic biomass components.The pretreatment conditions for efficient separation of biomass and improvement of enzymatic saccharification performance were explored,and the correlation mechanism between the substrate characteristics of cellulose-rich solid substrates after DES pretreatment and cellulase hydrolysis was studied.The main research contents are as follows:（1）The pretreatment of corn straw and poplar with carboxylic acid DES was carried out.The effects of carboxylic acid DES type,molar ratio of hydrogen bond donor to acceptor,solid-liquid ratio and pretreatment temperature on pretreatment efficiency were investigated.The results showed that the carboxylic acid DES pretreatment significantly improved the lignin extraction ability,cellulose retention rate and hemicellulose removal ability.The optimum conditions for the pretreatment of corn stover and poplar with carboxylic acid DES were as follows:temperature 140°C,solid-liquid ratio 1:30,pretreatment of corn stover with[Ch Cl]1[LA]5,[Ch Cl]1[FA]2,[Ch Cl]1[AA]5 for 3 h,77.6%,72.9%,73.5%of lignin and 90.3%,93.6%,90.5%of xylan were removed,respectively.After pretreatment with[Ch Cl]1[LA]4,[Ch Cl]1[FA]2 and[Ch Cl]1[AA]5 for 3 h,81.2%,78.1%,72.2%of lignin and 83.7%,82.9%,70.6%of xylan were removed,respectively.The removal of lignin and hemicellulose resulted in the loose structure of biomass,providing more accessible sites for cellulase,which was beneficial to improve the subsequent cellulase hydrolysis efficiency.In addition,the results of Fourier transform infrared spectroscopy（FT-IR）and two-dimensional nuclear magnetic resonance（2D-HSQC NMR）characterization of lignin showed that the extracted and recovered lignin was mainly composed of syringyl and guaiacyl units.（2）Three alcohol DESs were designed for the pretreatment of corn straw and poplar,and the pretreatment conditions（DES type and hydrogen bond donor-acceptor molar ratio,solid-liquid ratio and pretreatment temperature）were optimized.The results show that alcohol DES can effectively pretreat biomass.Under the optimal pretreatment conditions（solid-liquid ratio of 1:30,temperature of 200°C）,alcohol DES can efficiently dissolve hemicellulose and lignin.Due to the large removal of lignin and hemicellulose,the relative content of cellulose increased significantly.After pretreatment with[Ch Cl]1[Gly]2,[Ch Cl]1[EG]5,[Ch Cl]1[PEG]5,the glucan content of corn straw increased to 65.5%,67.4%and 68.0%.The glucan content of poplar pretreated by[Ch Cl]1[Gly]2,[Ch Cl]1[EG]4 and[Ch Cl]1[PEG]5 increased to 66.6%,74.2%,63.5%.The effective removal of lignin and xylan and the increase of cellulose relative content can create favorable conditions for subsequent cellulase hydrolysis.In addition,the results of Fourier transform infrared spectroscopy（FT-IR）and two-dimensional nuclear magnetic resonance（2D-HSQC NMR）of lignin showed that the structure of lignin extracted after DES pretreatment did not change significantly,and theβ-O-4 unit bond was substantially broken during pretreatment.（3）The enzymatic saccharification of pretreated substrates under the optimal conditions of carboxylic acid DES and alcohol DES was systematically studied.The effects of enzyme dosage and substrate concentration on enzymatic hydrolysis efficiency were investigated.Appropriate increase of enzyme dosage can improve enzymatic hydrolysis efficiency.With the increase of substrate concentration,the gradient of enzymatic hydrolysis efficiency decreases,and high substrate concentration will inhibit enzymatic hydrolysis,because the inhibitor generated under high substrate concentration will inhibit enzymatic hydrolysis.Encapsulation of cellulose by lignin and hemicellulose in lignocellulosic biomass seriously hindered enzymatic hydrolysis.Under the conditions of high enzyme dosage（15 FPU/g-cellulose）and substrate concentration of 10%,the glucose yield of untreated corn straw and poplar was 39.9%and 38.7%,respectively.After pretreatment with[Ch Cl]1[LA]5,[Ch Cl]1[FA]2,[Ch Cl]1[AA]5,[Ch Cl]1[Gly]2,[Ch Cl]1[EG]5 and[Ch Cl]1[PEG]5 for 96 h,the glucose yield increased to 93.7%,91.2%,82.7%,80.2%,89.2%and 88.3%,respectively.The glucose yield of poplar pretreated by[Ch Cl]1[LA]4,[Ch Cl]1[FA]2,[Ch Cl]1[AA]5,[Ch Cl]1[Gly]2,[Ch Cl]1[EG]4 and[Ch Cl]1[PEG]5 for 96 h increased to 94.3%,91.9%,85.7%,82.0%,89.9%and 82.3%,respectively.The results showed that DES pretreatment efficiently removed lignin and hemicellulose,destroyed the dense resistant structure of biomass,eliminated the physical barrier of enzymatic hydrolysis,provided more accessible surface area for cellulase adsorption,and also reduced the ineffective adsorption of lignin on cellulase,which provided favorable conditions for cellulase hydrolysis.The pretreated substrates rich in cellulose were characterized by Fourier transform infrared spectroscopy,scanning electron microscopy,X-ray diffractometer and contact angle.The correlation mechanism between the substrate characteristics such as morphology,crystallinity and hydrophilicity of the pretreated substrates and cellulase hydrolysis was explored.The results showed that the structure of cellulose-rich solid substrate was loose and the crystallinity increased after DES pretreatment,which provided favorable conditions for the conversion of polysaccharides into fermentable monosaccharides. |
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