| In recent years, with the frequent eruption of the energy crisis and the increasingconcerns about environment pollution on earth, clean and renewable biofuels as an alternativeof fossil energy sources have attracted extensive interest. Biofuels production from abundant,cheap and renewable lignocellulosic biomass such as agricultural waste is promising.Lignocellulosic biomass is highly recalcitrant to chemical and biological degradation due toits natural complicated structure. Therefore, pretreatment is essential prior to degradation.Recently, ionic liquids (ILs) have emerged as a promising class of solvents for pretreatmentof lignocellulosic biomass owing to their excellent physiochemical properties, especially thestrong ability to dissolve various compounds. However, the traditional imidazolium orpyridinium ILs have been proved to be highly toxic and poorly biodegradable; in addition, thestarting materials for the synthesis of these ILs are non-renewable, the ILs preparationprocesses are tedious and environmentally unfriendly, and the processes of biomasspretreatment with these ILs are usually not tolerant to moisture. Recently, a type of novel andrenewable cholinium amino acids ([Ch][AA]) ILs with low toxicity and excellentbiodegradability has been synthesized by our group. To date, no work has been done on thepretreatment of lignocellulosic biomass by these new and environmentally friendly ILs. Thus,the solubility of the main components of lignocellulosic biomass (cellulose, xylan and lignin)in these [Ch][AA] ILs was determined and the effect of the dissolution process on thestructures and properties of the components was investigated in this dissertation. Then, thefeasibility of agricultural waste biomass–rice straw pretreatment with [Ch][AA] ILs wasexplored to enhance the enzymatic hydrolysis of the residues for the first time. Also, theeffects of various factors on the lignin extractability and the enzymatic hydrolysis efficiencyof the residues were examined. Furthermore, the effect of the addition of greener and cheaperwater on ILs pretreatment efficiencies was studied, and the mechanism of rice strawpretreatment using [Ch][AA] ILs aqueous solution was revealed by microscopic techniquesfor the first time. Finally, the feasibility of sugarcane bagasse pretreatment with [Ch][AA] ILswas also explored to extend the application potential of this process. Agreen, efficient, simpleand economic process of agricultural waste biomass pretreatment was developed.It was shown that most of these [Ch][AA] ILs had good abilities to dissolve lignin, andthe solubility of xylan in these ILs was relatively lower, and microcrystalline cellulose (MCC)was scarcely soluble. The enzymatic hydrolysis of MCC was improved significantly after ILspretreatment, which was attributed to the rougher and larger surface area of the pretreated MCC rather than the change of its crystal structure. Fourier transform infrared spectrum(FTIR) and thermal gravimetric (TG) analysis indicated that the skeleton structures of xylanand lignin were not damaged markedly during the dissolution in ILs, except for the breakageof partial ester bonds. The above results indicate that [Ch][AA] ILs may be a type of potentialand good solvent for highly efficient lignocellulosic biomass pretreatment by selectiveextraction of lignin.Eight [Ch][AA] ILs examined were demonstrated to be excellent solvents for rice strawpretreatment. The enhancement of enzymatic hydrolysis of polysaccharides after pretreatmentderived mainly from selective delignification, instead of changes in cellulose crystallinity.Lignin extractability and polysaccharides digestibility were significantly influenced by theanion structures and properties of the ILs, pretreatment temperature and time. The optimal IL,pretreatment temperature and time were cholinium lysinate ([Ch][Lys]),90℃and5h,respectively. Under the optimal conditions,49.7%of lignin was removed; the sugar initialrelease rates of2.35mg/(mL h) for glucose and0.51mg/(mL h) for xylose, and the sugaryields of84.0%for glucose and42.1%for xylose were achieved in the enzymatic hydrolysisof the residues. The IL [Ch][Lys] showed excellent reusability in rice straw pretreatment.It was found that the process of rice straw pretreatment with [Ch][AA] ILs was highlytolerant to moisture. The addition of cheap and green water into ILs not only led to noconsiderable decrease in the dissolution ability of the ILs to lignin, but also significantlylowered their viscosity, which made handling easier; additionally, even the cost and thepolysaccharides losses were reduced. The enzymatic hydrolysis efficiency of rice straw wasremarkably enhanced after the pretreatment with other sixteen50%[Ch][AA] ILs aqueoussolutions, with the exception of cholinium aspartate ([Ch][Asp]) and cholinium glutamate([Ch][Glu]). The mechanism of rice straw pretreatment with [Ch][Lys] aqueous solution wasinvestigated by using confocal laser scanning microscopy (CLSM), transmission electronmicroscopy (TEM) and atomic force microscope (AFM). It was revealed that duringpretreatment with50%[Ch][Lys] aqueous solution, lignin removal from cell walls of the ricestraw stem increased with the elongation of pretreatment time; swelling occurred insclerenchyma cell walls of vascular bundles near the epidermis; lignin in cell corner (CC) andcompound middle lumen (CML) of cell walls was easier to be extracted than that in thesecondary cell wall2(S2); extensive delignification resulted in the increase of xylanaccessibility; and the surface of cell walls become rougher and more hydrophilic.There is a clear correlation between the basicity of the pretreatment solvent and itsdelignification capacity, but the basicity is just one of the physiochemical properties of [Ch][AA] ILs that contribute to their excellent delignification capacities. Besides, it wasproved that lignin removal during pretreatment led to significant increases in surface area andpore volume of rice straw, which substantially improved the polysaccharides accessibility toenzymes and thus enhanced polysaccharides digestion. By carefully controlling thepretreatment severity (IL content, temperature and duration), the balance between maximizingthe polysaccharides digestibility and minimizing the polysaccharides losses could be reached,leading to high sugar yield and low energy consumption. The optimal pretreatment solvent,temperature and time were20%[Ch][Lys] aqueous solution,90℃and1h, respectively.Under the optimal conditions,37.8%of lignin was removed, and the initial sugar releasingrates of1.34mg/(mL h) for glucose and0.48mg/(mL h) for xylose, and the sugar yields of81%for glucose and48%for xylose were obtained in the enzymatic hydrolysis of theresidues.Six [Ch][AA] ILs tested were found to be effective for sugarcane bagasse pretreatment.Similarly, lignin was selectively removed from sugarcane bagasse by these ILs, thus leadingto substantial improvement of enzymatic hydrolysis of the residues. Interestingly, it wasfound that the efficiency of enzymatic hydrolysis of sugarcane bagasse after [Ch][Lys]pretreatment was independent on the substrate sizes. Also, the addition of water into [Ch][Lys]did not exert a negative effect on the subsequent enzymatic hydrolysis efficiency of theresidues, in spite of the decline in the delignification capacity. The process of sugarcanebagasse pretreatment without comminution or size reduction was successfully scaled up andsugar yields of80%for glucose and84%for xylose were obtained in the enzymatichydrolysis of the residues after pretreatment with a biomass loading of5%(w/w) by50g50%[Ch][Lys] aqueous solution at90℃for6h.[Ch][AA] ILs prepared by a simple andatom-economic approach were also found to be highly efficient for lignocellulosic biomasspretreatment. And the reactant impurities in the IL had a marginal effect on its pretreatmenteffectiveness.This study not only enriches the knowledge of ILs pretreatment of lignocellulosicbiomass, but also provides a green, highly efficient, simple and economic route foragricultural waste biomass pretreatment. |
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