Separation And Extraction Of Cotton Straw Lignin By Choline Chloride Deep Eutectic Solvent

With the gradual depletion of fossil energy,it is necessary to find renewable alternative energy.Lignocellulose is the most abundant and renewable resources on the earth,among which lignin,as one of the three main components of lignocellulose.It is the only substance containing benzene ring structure and the component that is expected to replace fossil fuels.However,due to the unique structure of lignocellulose,it is difficult to separate the components,so the utilization rate of lignocellulose is greatly reduced.It is a research hotspot to select appropriate pretreatment methods to improve the utilization rate of lignocellulose.In this thesis,a series of deep eutectic solvents（DES）were prepared with choline chloride（ChCl）as hydrogen bond acceptor,formic acid,acetic acid,lactic acid,citric acid,malic acid,and oxalic acid as hydrogen bond donors.The effects of different deep eutectic solvent systems on the extraction yield of lignin from cotton stalk in Xinjiang were studied.The structure of lignin and the morphology of solid residue were characterized by chemical methods and modern instrumental analysis techniques,and the chemical composition and related properties of lignin were tested.The main research content of this thesis is as follows:（1）To solve the problem of low lignin extraction rate of deep eutectic solvent pretreatment,ultrasonic physics was introduced to assist deep eutectic solvent pretreatment cotton stalk,and its dissolution rule was studied.The effects of different hydrogen bond donors,ultrasonic power,reaction temperature and reaction time on the extraction rate of lignin and the recovery rate of solid residue were investigated.The results showed that when the molar ratio of formic acid:choline chloride was 1:1,the reaction temperature was180℃,the reaction time was 3h,and the ultrasonic power was 32k Hz,the recovery of solid residue was39.6%,the extraction rate of lignin was 87.7%,and the purity was 93.7%,which was 26.9%higher than that without ultrasonic.2D HSQC NMR showed that the main structural units of lignin extraction were G-type and S-type.The extracted lignin had higher molecular weight and PDI than that of commercial lignin.（2）Aiming at the problem of high pretreatment temperature in the previous chapter,Lewis acids were introduced to promote the separation of lignin from cotton stalk by choline chloride DES.The effects of four different Lewis acids,the molar ratio of choline chloride/formic acid/Lewis acid,reaction temperature and reaction time,on the lignin structure,extraction rate and solid residue recovery were studied.The results showed that when formic acid:choline chloride:AlCl₃=1:1:05,reaction temperature was 100℃,reaction time was 6h,the recovery of solid residue was 33%,the extraction rate of lignin was 95.6%,and the purity was 95.0%.2D HSQC NMR showed that the main structural units of lignin were G-type and S-type,but the content ofβ-O-4 bond was lower,the molecular weight of lignin was higher,and the PDI was higher than that of commercial lignin.（3）The molecular weight of lignin extracted from the first two chapters is higher and the content ofβ-O-4 bond is lower,choline chloride/oxalic acid（OA）/ethylene glycol（EG）DES was introduced to separate lignin from cotton stalk.The effects of ChCl/OA molar ratio,the amount of EG added,reaction temperature and reaction time on lignin structure,extraction rate and solid residue recovery were investigated.The results showed that when the reaction temperature was 110℃and the reaction time was 7h,the recovery of solid residue was 51.2%,the extraction rate of lignin was 47.1%,the purity of lignin was 93.6%,and manyβ-O-4bonds were preserved.2D HSQC NMR showed that the main structural units of lignin extraction were G-type and S-type,the content ofβ-O-4 bond was significantly increased,the molecular weight of lignin extraction was like that of pure lignin,the PDI was lower than that of commercial lignin,and the dispersion was good.