Extraction Of Agroforestry Biomass Lignin Under Choline Chloride-based Deep Eutectic Solvent System And Research On The Preparation Of Nanomaterials

Lignin is the only natural aromatic compound with a variety of properties such as antibacterial,antioxidant,and UV resistance.In addition,the high carbon content,abundance,low price,good thermal stability,and biodegradability of lignin provide a wide scope for its application.However,the complex structure of lignin and the strong connection with carbohydrates lead to the extraction of lignin being a challenge.The deep eutectic solvent（DES）is a green solvent that selectively solubilizes lignin and is an ideal solvent for the pretreatment of wood fiber biomass and value addition of lignin.In this paper,by studying the relationship between DES structure and lignin solubilization,six DESs were successfully selected to solubilize lignin efficiently.based on this,pretreatment of poplar wood and maize straw was carried out,and lignin-rich pretreatment black liquor was used to prepare nano lignin and carbon quantum dots,achieving high-value utilization of lignin.The pretreatment residues were also applied to enzymatic hydrolysis to achieve the comprehensive utilization of wood fiber biomass.The specific studies are as follows:（1）Investigation of lignin solubilization pattern under the DES systemA variety of DES was successfully prepared by fixing Ch Cl as the hydrogen bond acceptor and selecting hydrogen bond donors among three major types of compounds,namely carboxyl-containing,hydroxyl-containing,and amino/amido-containing compounds,and dissolving two types of industrial lignin（alkaline lignin and sodium lignosulfonate）.It was found that the short carbon chain length and the low number of functional groups as well as the high molar ratio of hydrogen bond donors were favorable for lignin solubilization.Hydroxyl groups can promote lignin solubilization by carboxyl-containing DES.In addition,increasing temperature enhances the ability of DES to solubilize lignin.Based on this,[Ch Cl][FA]₃,[Ch Cl][LA]₃,[Ch Cl][Gly]₃,[Ch Cl][EG]₃,[Ch Cl][Urea]₂and[Ch Cl][TEOA]₃,which were effective in solubilizing lignin,were selected for subsequent studies,and their solubilities of alkaline lignin at120°C were 4.54,6.34,7.55,7.60,7.49,6.14 wt%and solubility to sodium lignosulfonate could reach 4.35,6.76,6.34,8.25,7.79,6.00 wt%,respectively.In addition,the effect of DES water content on lignin solubilization was investigated and it was found that different DES and lignin responded differently to water content,with[Ch Cl][FA]₃water content at 30 wt%being the best for alkaline lignin solubilization at8.99 wt%and reaching a maximum value of 8.92 wt%for sodium lignin sulfonate at40 wt%water.The structure of DES was also characterized by ¹H NMR to determine the effect of water on the structure of DES.The results showed that water as an additional hydrogen bond donor and hydrogen bond acceptor would participate in the intermolecular network and gradually weaken the hydrogen bonds within DES until DES was completely solvent,but within a certain range,DES could still maintain its structure.DES was used for the pretreatment of poplar and maize straw,and both showed good results for the removal of lignin.Among them,[Ch Cl][FA]₃could remove 82%and 57%of poplar lignin and maize straw lignin respectively.（2）Extraction of lignin from woody fiber materials under the DES systemBased on the exploration of the lignin solubilization pattern under the DES system,six DESs were selected for pretreatment,the optimal temperature for pretreatment was explored,four different methods of deionized water,anhydrous ethanol,acid water（p H=2）,and tetrahydrofuran were used for lignin extraction,and the structure and morphology of extracted lignin were studied.The results showed that an appropriate temperature increase could improve the pretreatment effect,and the lignin removal rate of[Ch Cl][Gly]₃was 2.61 times and 2.97 times higher than that of120°C for pretreatment of poplar and maize straw,respectively,at 200°C.Overall,DES pretreatment with carboxyl groups was more effective,with[Ch Cl][FA]₃achieving 74%lignin removal when pretreating poplar at 120°C and 69%when pretreating maize stover at 140°C.Extraction of lignin in acidic,neutral DES with acidic water（p H=2）can be as high as 99.67%;tetrahydrofuran can result in lignin purity up to 96.44%.Lignin in basic DES was extracted best with anhydrous ethanol,but the purity and extraction rate was not satisfactory.The lignin produced was found to be nanospheres with an intact benzene ring structure,with high G and S content,and higher H content in maize straw lignin than in poplar wood.The pretreatment process resulted in the cleavage ofβ-O-4,leading to an increase in hydroxyl content and a decrease in molecular weight.Among them,acidic DES was more likely to breakβ-O-4,and the molecular weight of Y-FA could reach 590 Da with a particle size of181 nm.Using the pretreatment residue for cellulase hydrolysis could increase the glucose yield by 2.1 times.（3）Preparation of CQDs using pretreated black liquor as a carbon sourceThe pretreatment black liquor was subsequently heat-treated to prepare carbon quantum dots,and the preparation temperature was investigated.High temperature could promote the formation of CQDs,and the fluorescence quantum efficiency of CQDs-Y-LA-160 and Y-LA-200 were 11.10%and 18.04%,respectively.The solvent also affected the fluorescence effect,with higher quantum yields when anhydrous ethanol was used as the solvent than deionized water.Different solvents and raw materials affect the quantum yield.The carbon quantum dot Y-EG-200 prepared from poplar wood had the highest fluorescence quantum efficiency of 25.12%,while the quantum yield of CQDs-J-Urea-200 was the highest among the corn straw-based CQDs,reaching 21.50%.The prepared CQDs can reach a minimum particle size of1.37 nm,a fluorescence lifetime of 5.92 ns,excitation dependence,and are rich in hydrophilic and oxygen-containing moieties.Their fluorescence stability was investigated and the results showed that the prepared CQDs have good thermal stability,and ionic stability and maintain good stability in weak acid and neutral media,but have average photostability.They were characterized using XPS and the results showed that the CQDs are mainly composed of C,O,and N,with traces of S elements as well.The CQDs prepared from[Ch Cl][Urea]₂have a high N content,with 12.15%and 11.81%of elemental N in CDQs-Y-Urea-200 and CDQs-J-Urea-200,respectively.