The Priority Strategy Of Hemicellulose Based On Choline Chloride Treatment To Realize The Conversion Of Lignocellulose And Utilization Of Materials

Lignocellulosic feedstock is a natural carbon-neutral resource with abundant sources and large reserves,which is an ideal substitute for petroleum-based products under the"double carbon"strategy.However,due to the anti-degradation barriers formed during the evolution of plants,which resulted in the heterogeneity of lignocellulose and the complex linkage mechanism between components,the potential of lignocellulosic feedstock has not been fully realized.Therefore,in this work,the hemicellulose priority strategy of choline chloride-assisted hydrothermal treatment（ChCl/LHW）was used to destroy the anti-degradation barrier,and esterification modification technology was combined to achieve the transformation of lignocellulosic feedstock and utilization of the corresponding material.First,the effect of tiny choline chloride-assisted hydrothermal treatment on Caragana korshinskii components at different temperatures and times was investigated.Compared with hydrothermal treatment and sodium hydroxide-assisted hydrothermal treatment,tiny ChCl could promote the dissolution of xylan from Caragana korshinskii,and obtain the highest xylo-oligosaccharide（XOS）yield（36.59%）at 160℃for 120 min.Meantime,the effect of temperature on the dissolution of xylan was more significant（p<0.05）than that of time.Furthermore,the degradation products of xylan were analyzed and revealed that tiny ChCl could accelerate the shedding of glucuronic acid from side chains,resulting in a decrease in the p H value of the system,thus selectively converting xylan to XOS.The concentrations of furfural and 5-HMF in the hydrolysates were also decreased.Due to destroying the anti-degradation barrier after the removal of xylan,the conversion rate of cellulose was 41.30%after ChCl/LHW treatment when 25 FPU/g-substrate cellulase and 25 CBU/g-substrateβ-glucosidase were used for saccharification,which was 38%higher than that without treatment.The effect of ChCl concentration on the degradation of xylan in Caragana korshinskii was further investigated,and the mechanism of selective depolymerization of xylan to XOS by ChCl was elucidated.By analyzing the p H value and the contents of organic acids in the hydrolysate before and after treatment,it was found that the concentrations of ChCl could reduce the p H value of the system to varying degrees,resulting in the difference of acetyl group and glucuronic acid removal from the side chain,and affecting the distribution of xylan products.At the concentration of 1.8 wt%,the p H value of the ChCl solution was mild at 4.95,which promoted the dissolution of xylan（85.52%）and obtained the highest XOS yield（32.95%）.With the increase in ChCl concentration,the p H value of the system decreased to3.67-4.73,which aggravated the further depolymerization of XOS and increased the concentration of xylose and furfural.At the concentration of 66.0 wt%,the viscosity of ChCl solution increased and the ionization was incomplete,which had the lowest effect on the dissolution of xylan.However,it could form a deep eutectic solvent（DES）with acetic acid and glucuronic acid,which promoted the dissolution of lignin.In addition,it was found that ChCl also promoted the solubility of lignin at other concentrations.To fully realize the potential of lignocellulosic feedstocks,cellulose and lignin in the residues should be further converted.Based on ChCl/LHW treatment,combined with green iodine catalyzed esterification technology,cellulose in eucalyptus solid residue was converted into high-purity cellulose triacetate,realizing the isolation of lignin and esterification modification of cellulose.In this process,ChCl/LHW treatment significantly increased the cellulose conversion rate（90.75%）and acetylation product yield（61.34%）by increasing the specific surface area and pore size of the fiber.Due to the recondensation of lignin during ChCl/LHW treatment,95%of lignin was retained.The characterization results of the esterification products further confirmed that the acetylation product was high purity cellulose triacetate,free of lignin,with uniform molecular weight,good thermal stability,and a semi-crystalline structure.To further demonstrate the substitution potential of lignocellulosic products for petroleum-based products,synthetic cellulose triacetate（CA）was also used to enhance the performance of polylactic acid（PLA）membranes.The tensile strength and Young’s modulus of PLA-CA composites with 5%cellulose triacetate increased by 80.63%and 59.51%,respectively.The density also decreased from 1.2427g/cm³ to 1.0028g/cm³,which realized the lightweight and the enhancement of mechanical property.To recover the above catalyst iodine,a hierarchical porous UiO-66-NH₂@WCA composite was fabricated by in-situ growth of UiO-66-NH₂ on a wood-derived cellulose aerogel（WCA）with an adsorption capacity of 248 mg/g in an iodine solution.By the adsorption kinetics,thermodynamics and isothermal models,the adsorption behavior of UiO-66-NH₂@WCA was confirmed to be spontaneous,endothermic and random adsorption,which could be predicted theoretically by the pseudo-second-order kinetic model and Freundlich model.The Weber-Morris equation showed that UiO-66-NH₂ crystals growing on the surface of cellulose increased the boundary layer effect,but also provided the abundant pores and active sites,which jointly determined the adsorption rate.The adsorption mechanism could be considered as the result of hydrogen bond,halogen bond andπ-electron cloud conjugation,and cellulose on WCA matrix provided additional adsorption capacity toward iodine.A packed column system was established to demonstrate the application potential of UiO-66-NH₂@WCA composite in dealing with large volume wastewater.The recovered iodine was also reused in the synthesis of cellulose acetate without purification,showing excellent catalytic performance.To avoid the excessive condensation of lignin which increased the difficulty of utilization,acetic anhydride/choline chloride DES was used to achieve esterification modification and extract uncondensed lignin.The effect of ChCl on the yield of XOS was also investigated at low concentrations.The results showed that the yield of XOS was positively correlated with the concentrations of ChCl in the range of 0-0.25 wt%at 180℃for 30 min,and 44.34%of XOS was obtained at 0.25 wt%.Moreover,ChCl/LHW treatment promoted the fragmentation of lignin and improved the selectivity of DES for lignin extraction.Acetic anhydride based DES successfully achieved the esterification modification of lignocellulose.The esterification of lignin prevented the condensation of lignin,resulting in regenerated lignin with narrow molecular weight distribution,low molecular weight（Mn 859 and Mw 1283）,and light color.After adding acetic acid to construct ternary DES,it was found that the removal rate of lignin and the yield of DES lignin could be greatly increased when the molar ratio of acetic anhydride:acetic acid:choline chloride was 1:1:1,but the degree of esterification would be slightly reduced.Excessive addition of acetic acid greatly reduced the degree of esterification,and DES lignin was severely condensation,resulting in the darker regenerated lignin with wider molecular weight distribution,and higher molecular weight（Mn 4098 and Mw 14736）.Meantime,the esterification degree of regenerated lignin would affect the self-assembly behavior of lignin microspheres and form different morphologies.Furthermore,acetylated lignocellulose isolated from esterified lignocellulose has good interfacial compatibility with PLA,which gave the composite film good UV resistance and increased the elongation at break from 2.70%to 36.85%.The regenerated lignin microspheres could encapsulate the sun screener and improve the anti-UV performance of emulsion.