Enhancing the stability of ionic liquid media for cellulose processing: acetal protection or carbene suppression?
Enhancing the stability of ionic liquid media for cellulose processing: acetal protection or carbene suppression?
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DOI：
10.1039/c6gc00027d
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发表时间：
2016-01-01
期刊：
影响因子：
9.8
通讯作者：
Welton, Tom
中科院分区：
文献类型：
--
作者：
Clough, Matthew T.;Griffith, Jeraime A.;Welton, Tom
虽然是纤维素的优秀候选溶剂，能够溶解b> = 20wt %的碳水化合物用于静电纺丝过程，但羧酸二烷基咪唑离子液体会与糖的还原端发生不良副反应，最终以2-（羟甲基）取代的咪唑“加合物”的平衡浓度终止。加入小摩尔量的良性、无毒和廉价的助溶剂，例如甘油，可以降低加合物积累的速度，从而提高昂贵的离子液体组分的长期热稳定性和可回收性。核磁共振、紫外-可见和质谱实验表明，稳定性的提高可能是由于质子共溶剂的给氢作用抑制了瞬态二烷基咪唑-2-酰基碳化合物，而不是由于碳水化合物的环缩醛保护。与纯离子液体相比，在溶剂混合物中掺入（高达）10wt %的甘油不会加剧纤维素的解聚速率，并且纤维素的高溶解度保持不变。此外，纤维素再沉淀后回收溶剂的比色比较表明，甘油不会增加有机电解质中污染物还原糖的浓度。
Although excellent candidate solvents for cellulose, capable of dissolving >= 20 wt% of the carbohydrate for electrospinning processes, dialkylimidazolium carboxylate ionic liquids undergo undesirable side reactions with the reducing end of saccharides, terminating in an equilibrium concentration of a 2-(hydroxymethyl)-substituted imidazolium 'adduct'. The addition of small molar quantities of a benign, non-toxic and inexpensive co-solvent, e.g. glycerol, reduces the rate of adduct accumulation, thereby enhancing the long-term thermal stability and recyclability of the expensive ionic liquid component. NMR, UV-vis and mass spectrometry experiments reveal that the improved stability is likely attributable to suppression of the transient dialkylimidazol-2-ylidene carbene, via hydrogen-donation by the protic co-solvent, rather than by cyclic acetal protection of the carbohydrate. The incorporation of (up to) 10 wt% of glycerol into the solvent mixture does not exacerbate the rate of cellulose depolymerisation compared to in the neat ionic liquid, and high solubility of cellulose is maintained. Furthermore, a colourimetric comparison of the recovered solvents, following cellulose re-precipitation, demonstrates that glycerol does not increase the concentration of contaminant reducing sugars in the organic electrolyte.