| To address the crisis caused by the gradual depletion of fossil fuels, people all focus on the green and renewable biomass resources after a lot of researches and explorations. If the abundant, renewable and biodegradable biomass resources can be used as an energy or raw material alternative to fossil fuels, it will greatly ease the global energy crisis. However, the complex recalcitrant structure and the entangled and covalently cross-linked polymeric matrix of biomass materials greatly hinder their applications. Thus, it will be of positive significance for the solution of energy and chemical crisis to fractionate the main components of the biomass materials, seprate three main components from each other, and conduct the catalytic conversion of these components.In this paper, different ionic liquids (ILs) were used as selective biomass dissolving solvents, complete biomass dissolving solvents and catalysts to explore the fractionation and catalytic conversion of biomass materials. First, choline, a kind of animal biomass, and organic acids or amino acids were used to synthesize biomass-based ILs with biocompatibility and biodegradability, and the selective fractionation of biomass materials was carried out subsequently using these ILs. Secondly, biomass materials were completely dissolved, simultaneously fractionated and selectively converted in the complete biomass dissolving ILs with the polyoxometalates as catalysts. Finally, acidic ILs, combined with different alcohol-water system, were used as catalysts to selectively extract lignin from biomass meterials, and the catalytic degradation of lignin was conducted in the glycerol-acidic IL system. The main contents and conclusions of this paper are as follows:(1) Choline, a kind of animal biomass, and organic acids or amino acids were used to synthesize three biomass-based ILs (choline acetate, choline alanine and choline glycine) with low price and biodegradability. The formation of three ILs was confirmed by characterizing them, and it was proved that the resulting ILs had high purity. Biopolymer dissolution experiments showed that all three ILs can not dissolve microcrystalline cellulose. Choline acetate can selectively dissolve hemicellulose and lignin, and it had a limited impact on their chemical structure.(2) Choline acetate was used to treated and fractionate bagasse and southern yellow pine, and the effects of heating method and separation method on cellulose-rich material (CRM), hemicellulose and lignin yields, lignin content of resulting material, mass loss were investigated. The chemical structure of biomass components was characterized, and CRM was further processed to analyze its tractability. The results indicated that choline acetate can selectively dissolve the hemicellulose and lignin in bagasse and pine (total dissolved rate of 37.4% and 25.8% for bagasse and pine, respectively). The cellulose structure was not affected during the process, and CRM was separated from the IL solution after processing. Dissolution efficiency and delignification of the microwave heating method were lower than those of the conventional heating methods. Compared to the centrifugation methods, the filtration method with the addition of water was more operational, and its separation efficiency was higher. Ultrasound treatment can effectively improve the separation efficiency of hemicellulose and lignin and decrease the lignin content of hemicellulose. The characterizations of biopolymers confirmed that the main component of obtained bagasse hemicellulose was xylan, and the chemical structure of obtained bagasse lignin was similar with that of milled wood lignin. The content of syringyl (S) units in the obtained lignin was higher than that of guaiacyl (G) or p-hydroxyphenyl (H) units, and the S/G ratio was 2.04. It is also found that the crystalline structure of cellulose was not significantly affected by the choline acetate processing. Compared with the original biomass, the obtained CRM in choline acetate process was easier to further process, and its required time for complete dissolution in 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) was shortened from 17-45 h to 7-27h. After dissolution and separation in [C2mim][OAc], the lignin in CRM can be further removed with lower lignin content in CRM obtained.(3) Complete dissolution and delignification of Southern yellow pine were achieved in [C2mim][OAc] catalyzed by POM ([PV2Mo10O40]5-) with O2 at 110℃ for 6 h, and the delignification rate was up to 97.2%. CRM, hemicellulose, water-insoluble lignin, and acid-insoluble lignin could be recovered from the pine/IL solutions by adding anti-solvents in sequence. Comparison of wood processing in [C2mim][OAc]/POM solutions with or without O2 indicated that the presence of oxygen can significantly enhance the delignification rate of biomass by 2.68 times, increase the hemicellulose yield by 14 times, and decrease the lignin yield by about 50%. Both the resulting CRM and hemicellulose had high purity, and the lignin content in CRM was as low as 3.2%. The low lignin content in hemicellulose was also confirmed by characterizations of hemicellulose. POM can be only tracked in acid-insoluble lignin after the reaction, which indicated that lignin can be selectively catalyzed by POM. S units had a slightly lower content than that of G units or H units in the obtained acid-insoluble lignin. The main lignin oxidation products were extracted from the IL solution with benzene and THF, and were shown to be methyl vanillate, acetovanillone, vanillic acid, methyl 3-(3-methoxy-4-hydroxyphenyl) propionate, and methyl 4-hydroxybenzoate. A small amount of conversion products of carbohydrates, mainly dimethyl fumarate, dimethyl succinate and monomethyl succinate, were also shown in the benzene extraction solution.(4) Different alcohol-water systems were used to separate coir and poplar components with 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) as a catalyst. The effects of carbon chain length, number of hydroxyl groups, and hydroxyl group position in alcohols on the biomass processing efficiency were explored. The effect of acidic IL biomass on processing efficiency of the system was also investigated, and the resulting biomass components and catalytic conversion products were characterized. The results showed that all of the carbon chain length, hydroxyl group number and hydroxyl group location had some influence on biomass processing efficiency, and the 1,4-butylene glycol system was considered as the optimal one after a comprehensive analysis. With the catalysis of [Bmim][HSO4], lignin yield of 1,4-butylene glycol system can be as high as 79.64% and 49.42% for coir and poplar, respectively. The addition of acidic IL had greatly enhanced the lignin extraction efficiency of this system. Compared with the results of control experiment, lignin yields increased 3-6 times with the addition of acidic IL.1,4-butylene glycol-[Bmim][HSO4] system showed high delignification efficiency on coir and poplar, and the delignification rate of poplar was as high as 98%. The obtained lignin had similar structures with that obatined from the acid hydrolysis process. Coir lignin had a weight-average molecular weight of 5536.8 g/mol with a high polydispersity of 6.88. S units were the main constituents for both coir lignin and poplar lignin. Among the different alcohol systems used,1,4-butylene glycol system showed the lowest efficiency on lignin degradation and highest efficiency on lignin extraction.(5) [Bmim][HSO4] and aqueous solution of 1,4-butylene glycol were used to process coir and poplar. The effects of 1,4-butylene glycol/water ratio, IL loading, biomass loading, time, and temperature on the lignin extraction efficiency were explored. The reusability of recycled 1,4-butylene glycol-[Bmim][HSO4] system was also investigated. The resulting biomass components and catalytic conversion products were characterized. In addition, glycerol-[Bmim][HSO4] system was explored to degrade the obtained coir lignin. The results showed that all factors can affect the lignin yield of this system. Under the optimized conditions, 84.77% of the original coir lignin or 51.90% of the original poplar lignin can be isolated by the system. Recycled 1,4-butylene glycol-acidic IL system can still achieve 92.4% of the original extraction efficiency, and the resulting coir lignin yield was 78.35%. The results of component analysis showed that the system can simultaneously remove both hemicellulose and lignin from biomass in the process, and hemicellulose was degraded during processing, which cann’t be isolated out. The crystalline structure of cellulose was not significantly affected by the processing, and the relative crystallinity degree of cellulose increased after processing. Extending the processing time or increasing the temperature led to lower weight-average molecular weight and polydispersity of the obtained lignin. The results of catalytic decomposition product analysis showed that both high temperature and the absence of water can increase the lignin degradation. The degradation rate of coir lignin can reach 28.1% when treating it in glycerol-[Bmim][HSO4] system at 240℃ for 1 h. The degradation products were mainly polyhydric ether ester,2,6-dimethoxy phenol,2-methoxy-4-propyl phenol, and 4-hydroxy-3-methoxybenzoate acetone. |
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