| Wheat straw is one of the most common agro-forestry biomass and agricultural by-products.It has the characteristics of wide source,easier to obtain,lower cost and more renewable.It has great potential for application in straw return to the field for fertilization,paper pulp replacement,biomass fuel,and lignocellulose-related chemical feedstock preparation.The composition of wheat straw is intricate,and it is significant to clarify the main chemical composition of wheat straw,distinguish its similarities and differences with other biomass such as wood,simplify the procedure of lignin separation and purification of wheat straw,and transform the low-cost straw into a bulk industrial raw material to achieve high-value and efficient utilization of wheat straw.In this experiment,wheat straw was used as the test material.The main cross-linked structures of straw lignin were clarified by UPLC-MS/MS combined with the nuclear magnetic resonance method based on the analysis of the main chemical components of wheat straw.The geometrical structures of different ratios of choline chloride and lactic acid low eutectic solvents were calculated theoretically and verified by NMR using the artificial bee colony algorithm and density flooding theory calculation method.The hydrogen bond strength and lignin depolymerization efficiency of different ratios of low eutectic solvents were clarified,and the principle of lignin depolymerization of wheat straw by lactic acid and choline chloride low eutectic solvents were explored.Through designing a temperature gradient test and different ratios of deep eutectic solvents to investigate the depolymerization performance of wheat straw.The experimental results provided an effective and relatively simple treatment method for the lignin separation and purification of wheat straw.The main findings are as follows.1.Analysis of the main chemical components of wheat strawQuantitative analysis showed that the content of hemicellulose,nitrocellulose and ash in the wheat straw component was relatively high,of which cellulose was type I natural crystal with a crystallinity of 52.4%.The lignin content in wheat straw was slightly lower than that in wood biomass,and the contents of acid-insoluble lignin and acid-soluble lignin were 20.24%and 1.59%.High crystallization of straw cellulose will reduce the accessibility of enzymes to wheat straw.The differences in the components of different biomass materials lead to further testing and raw material ratio and optimization when using lignin and cellulose.Reasonable pretreatment of raw materials is beneficial to improve the utilization efficiency.2.Quantitative analysis and structural characterization of lignin in wheat strawThe ~1H,13C,HSQC and HMQC spectra of nuclear magnetic resonance indicated that straw lignin was mainly composed of syringyl units,guaiacyl units,p-hydroxyphenyl monomers,p-coumaric acid and ferulic acid.The connecting bonds between lignin units are mainly composed ofβ-O-4（84.1%）,β-βresinol（12.78%）,phenylcoumaran structure（2.65%）and a small amount of resin alcohol,α,β-Consisting of diaryl ethers and spirodienones.The high content of guaiac group and high content ofβ-O-4 bonds make the primary structure of lignin show the characteristics of high"linearity"and"less branching".The HMQC spectrum showed that there were more triglycone monomers in the wheat straw.It is also linked to guaiac throughβ-O-4,which is cross-linked in lignin macromolecules.3.Analysis of low eutectic solvent system and lignin separation principleBy using AB Cluter 3.0 software and Gaussian 09 software to optimize the three ratios of the deep eutectic solvent choline chloride and lactic acid molar ratio 1:1,1:2,1:3.The analysis showed that as the ratio of lactic acid increased,the mutual attraction between lactic acid and choline chloride increased.This leads to more hydrogen protons leaning towards Cl-,causing the delocalization of hydrogen protons,increasing the acidity of the deep eutectic solvent and the depolymerization ability of lignin.NMR spectra showed that the DES dissolved lignin by breaking C-O ether bonds,and the main lignin units extracted were syringyl and guaiac-based units.The guaiac group is easier to separate than the syringyl group,and the deep eutectic solvent has a"demethoxylization"effect on the syringyl unit,thus reducing the S/G.4.Effect of temperature and low eutectic solvent component ratio on the separation and purification of ligninThe temperature gradient test showed that the relatively good conditions for the extraction of lignin from wheat straw were:130℃,the molar ratio of choline chloride and lactic acid was1∶3（purity 91.6%,yield 90.32%）. |
