聚醚离子液体在纤维素糖化过程中的亲水作用

Hydrophilic effect of PEG-linked ionic liquids in cellulose saccharification

为改善纤维素原料的亲水性,提高纤维素的可及度,将具有强亲水性的聚醚引入离子液体合成出系列聚醚功能化双核咪唑类离子液体,并将其应用于纤维素的预处理和糖化研究。相对于在咪唑类离子液体中引入烯丙基、烷基、羧酸以及乙酸乙酯等阳离子功能化基团,聚醚基团的烷氧基(CH2—CH2—O)可增大离子液体与纤维素的有效接触,并作为氢键受体促进纤维素中原有氢键的破坏;分子质量为400的聚乙二醇(PEG400)可提供足够数量和极化强度的氢原子改善亲水性;相对于单核咪唑类,双核咪唑类离子液体醚基上的次甲基氢原子的极性增强(E_T(30),61.33 kcal/mol),形成更高的氢键酸度(α,2.792);聚醚功能化双核咪唑类离子液体([PEG400-(Et-Im)_2][Cl]_2)对纤维素溶解度高达13.26%质量分数。相对于HCOO^-、Cl^-和Br^-阴离子功能化,OAc^-功能化的[PEG400-(Et-Im)_2][OAc]_2具有更大的氢键碱度(β,1.04),增加了对纤维素羟基上氢键的接受能力;聚醚的烷氧基会带来较低的黏度(63 c P,120℃),黏度表观活化能(E_η)为30.11 k J/mol,促进了离子液体与纤维素的接触与物质传输,由此获得[PEG400-(Et-Im)2][OAc]_2对纤维素的高溶解度(14.76%质量分数)。纤维素在[PEG400-(Et-Im)_2][OAc]_2溶解过程中没有发生衍生化反应,纤维素的结晶度由85.4%降至31.6%,实现了高结晶有序结构向无定型的转化。得到的再生纤维素膜具有各向同性和均一致密的性能,热力学稳定性稍有下降。酸性聚醚功能化离子液体[PEG400-(Et-Im)_2][HSO4]_2催化纤维素水解中葡萄糖得率为55.8%。

As a natural polymer,cellulose has the high-crystallinity supramolecular structure.In the cellulose processing,the poor-accessibility of raw material usually caused the decreased productivity and generated by-products.Series of polyether-functionalized ionic liquids(ILs)could be prepared and successfully utilized for improving the hydrophobic interaction.Because hydrophilic polyether functional imidazoles ionic liquids are easy to accept hydrogen atoms,good amphipathicity for improving fibers during the process of dissolving cellulose,better contact interfaces between solvent and cellulose,the solubility of cellulose can be improved.Allyl cationic functional morpholine type of ionic liquids have bipolar electronic property,which is produced by nitrogen atom in lean electronic center and oxygen atoms in rich electronic center,making positive ion+increase of supplying hydrogen bonding ability and promoting the dissolution through the increase in the activity of ionic liquids and cellulose hydroxy reaction.Choline eutectic ionic liquids formed by hydrogen bond donors have the structure of hydrogen bond network,which can accelerate the separation and recombination of hydrogen bonds on cellulose and improve the solubility.Polyethylene glycol(PEG)-group had stronger ability to accept hydrogen bonds owing to the alkoxy(CH 2-CH 2-O)in comparison with alkyl-group,carboxylic acid-group,ethyl acetate-group and allyl-group.Polyethylene glycol(PEG400)with a molecular weight of 400 could provide a sufficient number of hydrogen atoms with strong polarity to improve the hydrophilicity of cellulose.The hydrogen on methine in PEG was polarized under the influence of binuclear-imidazoles structure,and thus enhanced the polarity(E T(30),61.33 kcal/mol)and hydrogen-bond donating acidity of ILs(α,2.792).The solubility of[PEG400-(Et-Im)2][Cl]2 was 13.26 wt%.In the screening of anions,OAc-contributed more hydrogen bond basicity(β,1.04)than HCOO-,Br-and Cl-,which could promote the reaction of cellulose and ionic liquid.The low viscosity of[PEG400-(Et-Im)2][OAc]2(63 cP,120℃;Eηof 30.11 kJ/mol)also increased the accessibility between ILs and cellulose.Previously-mentioned characteristics enabled[PEG400-(Et-Im)2][OAc]2 a good solvent for cellulose 14.76 wt%.The morphology of original cellulose was dense and ordered,and then was rough and disordered with the dissolution.The crystal form was changed fromⅠtoⅡ,and the crystallinity of cellulose decreased from 85.4%to 31.6%.There were no other derivatives generated in the dissolution,and the thermodynamic stability of cellulose slightly decreased.The yield of glucose of cellulose with[PEG400-(Et-Im)2]2 as catalyst was 55.8%.