| With the shortage of fossil resources and the increasing of energy consumption, preparation of biobased materials, energy and chemicals with the renewable lignocellulosic biomass as raw material has attracted increasing attention. Pretreatment of lignocelluloses, mainly separation of lignin and cellulose, and make full use of the two components is of vital importance for lignocellulosic biomass high-value utilization. In this study, choline chloride-based deep eutectic solvents (DESs) were used to pretreat poplar wood because of their low cost and simple preparation, moreover, esterification modification and depolymerization of lignin was also explored with DESs as solvents. Chemical methods and the advanced analytical methods were used to characterize the chemical composition and structure of biomass material before and after treatment. The main contributions of this thesis are listed as follows:(1) A process for the extraction of lignin from poplar wood using lactic acid-based DESs with different hydrogen bond acceptor was contrasted. The effects of operational parameters on the yields of the cellulosic material and the regenerated lignin, the delignification extent and the chemical compositions of the cellulosic materials were investigated with respect to several factors, such as the types of hydrogen bond acceptor (choline chloride, glycine), reaction temperature and reaction time. It was found that elevated temperature and prolonged time both can improve the delignification extent, but at the same time, the conditions that promoted the removal of lignin also decreased the yield of cellulosic material. With the removal of lignin, the cellulose I crystal structure was maintained. Compared with lactic acid/glycine process, lactic acid/choline chloride process extracted more lignin under the same reaction conditions, and the highest delignification extent was approximately 90.4%, however the maximum delignification extent of only about 58.4%was achieved with lactic acid-glycine. The extracted lignins have high purity and were typical Guaiacyl-Syringyl (GS) lignin, esterification modification of lignin was occurred during the extraction process.(2) Rapid extraction of lignin from poplar wood with succinic acid/choline chloride was proposed at high temperatures for the first time. It was found that DES prepared with choline chloride and succinic acid with a molar ratio of 1:1 could effectively pretreatment of poplar wood at high temperature (170～190℃). Delignification of 32.63% was obtained at 170℃for 15min when the ratio of solid to liquid was 10wt%. A response surface methodology was used for optimization of the processing conditions, and the most optimal condition for pretreatment of poplar wood by succinic acid/choline chloride was temperature of 190℃, 10min of pretreatment time, and a solid to liquid 5wt%. The extracted lignin was GS-lignin with high purity, S/G ratio is 2.17, and degree of polymerization is 1.63. In addition, succinylation occured during the extraction of lignin. Moreover, the extracted lignin has low molecular weight (Mw=1104g/mol) and low polydispersity (D=1.268), which means that the extracted lignin has a narrow molecular weight distribution.(3) Extraction of lignin from poplar wood with glycerol and glycerol/choline chloride DES was contrasted, it was found that lignin yield was a little higher in glycerol/choline chloride process, and the cellulose content in cellulosic material improved greatly after glycerol/choline chloride pretreatment. Compared with glycerol lignin, glycerol/choline chloride lignins remain the primary structure of lignin, and there was no derivatization reaction occurred. Glycerol/choline chloride lignin has lower molecular weight and higher thermal stability compared with glycerol lignin extracted at the same reaction condition. Molecular weight of lignin increased and thermal stability improved with extension of the extracted time. It was also found that lignin depolymerized into small compounds during the extraction, mainly phenolic compounds, besides, aldehydes compounds, ketone compounds and esters compounds were also detected. Response surface methodology was used to optimize the processing conditions over a range of temperatures (220～240℃), time (10-30min) and solid to liquid ratio (5-15wt%). The best result was obtained with a reaction temperature of 236℃, reaction time of 18min and a solid to liquid of 15wt%.(4) A novel process was developed for the acetylation and butyration of lignosulfonate (LS) with acetic anhydride and butyric anhydride in the presence of choline chloride. The effects of reaction conditions, including reaction temperature, reaction time, choline chloride dosage and the mass ratio of anhydride/LS on the degree of substitution (DS) were studied. The maximum DS of acetylated LS of 2.72 was obtained at 120℃ for 3min with 5wt% choline chloride dosage and 5/1 mass ratio of acetic anhydride/LS. The maximum DS of butyrated LS of 2.14 was obtained at 120℃ for 10min with 15wt% choline chloride dosage and 4/1 mass ratio of butyric anhydride/LS. The results showed that acetylation and butyration reaction at phenolic and aliphatic hydroxyl both occurred. It was also found that the thermal stability of the acetylated and butyrated LS decreased upon chemical modification, besides, the thermal stability of the butyrated LS decreased with increasing degree of substitution.(5) P-Toluenesulfonic acid/choline chloride acidic DES was firstly used in lignin degradation. It was found that, phenolic hydroxyl content of the regenerated lignins increased with prolonging the reaction time, while alcoholic hydroxyl group content showed little change. GPC results showed that the Mw of the regenerated lignins was decreased firstly and then increased, but were all lower than that of the original lignin. It has no obviously change of treatment efficiency with reused and fresh DES. Mainly β-O-4’ether bond linkage was destroyed during the treatment, it was proposed that depolymerization and repolymerization occurred at the same time during the reaction process. |
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