Influence Of Anion Structures On Cholinium Ionic Liquid Pretreatment Of Rice Straw

Lignocellulosic biomass is the most abundant renewable resource on earth. However, itis highly recalcitrant to chemical and biological degradation due to the natural complicatedstructures. Therefore, pretreatment is essential prior to degradation. Recently, ionic liquids(ILs) have emerged as a promising class of solvents for pretreatment of lignocellulosicbiomass owing to their excellent physiochemical properties, especially the strong ability todissolve various compounds. The traditional imidazolium or pyridinium ILs have been provedto be environmental unfriendly, highly toxic and poorly biodegradable. Based on these facts, agroup of ecofriendly ILs with cholinium as the cation and fatty acids, hydroxyl acids, aminoacids and dicarboxylic acids as the anions was synthesized. And their viscosity and chemicalstructures were characterized. The solubilities of cellulose, xylan and lignin were determinedin these ILs.The efficiency of these ILs pretreatment of rice straw and enzymatic hydrolysis ofthe residues were investigated.At25°C, these cholinium ILs are liquids with the viscosity of72-2058mPa·s, except forcholinium octanoate([Ch][Oct]), cholinium malate([Ch][Mal]) and choliniumoxalate([Ch][Oxa]). There is a correlation between the viscosity and the structures andmolecular weights of the anions.These cholinium ILs show good capabilities to dissolvelignin, while the solubility of xylan is poorer, and microcrystalline cellulose is scarcelysoluble in these ILs.With fatty acids as the anions of the ILs, the lignin extractability of46-65%was obtained,and the enzymatic hydrolysis of the residues afforded glucose and xylose with the yields of58-71%and26-43%, respectively. Of this group of cholinium fatty acids ILs,[Ch][But] and[Ch][Piv] proved to be the best pretreatment solvents for rice straw. FTIR and XRD analysisof the residues revealed that the enhancement of subsequent enzymatic hydrolysis after theseILs pretreatment was attributed to the improvement of polysaccharides accessibility toenzymes because of the selective removal of lignin from rice straw. Then, the [Ch][But]pretreatment process of rice straw was optimized in detail, and the effects of pretreatmenttemperature and time on the pretreatment were explored. The optimal pretreatmenttemperature and time were90°C and6h, respectively, under which59%lignin was extractedfrom rice straw, and the yields of glucose and xylose in the subsequent enzymatic digestionwere71%and42%, respectively.With functional carboxylates as the anions, a clear dependence of the pretreatmentefficiency on the structures was found. When the carboxyl group was directly linked with an electron withdrawing group in the anions, the lignin extractability decreased significantly inthe pretreatment, thus resulting in decreases in enzymatic hydrolytic efficiency and reducingsugar yields. The decreasing order for lignin extractability of various ILs and glucose yieldswas listed as follows:[Ch][OAc]>[Ch][Ben>[Ch][Nic>[Ch][TFA]≈[Ch][Oxa]. Thesubstitution of α-H by a hydroxyl group in the anions (α-hydroxyl acids as the anions) led to aremarkable decrease in the pretreatment efficiencies of ILs. On contrary, a substantialimprovement in the pretreatment efficiencies of ILs was found when the α-H was replacedwith an amino group(α-amino acids as the anions).In addition, the blocking of free amino withthe corresponding groups (e.g., Boc and Cbz)in the anions exerted a negative effect on thepretreatment efficiencies of [Ch][amino acids]. It revealed that the presence of free aminogroup in the anions facilitated the ILs-mediated removal of lignin from lignocellulose, thusleading to the improvements in polysaccharides accessibility to enzymes and enzymatichydrolytic efficiency.When the anions were dicarboxylates, the effect of the functional groups on the ILspretreatment was similar with the above results. And the lignin extractabilities of[2Ch][dicarboxylates]were much higher than those of [Ch][dicarboxylates].The relationship of the cholinium ILs structure-function was revealed preliminarily in thepresent work, which would lay the theoretical foundation for the rational design of novel ILsfor highly efficient pretreatment of lignocellulosic biomass.