Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Engineering Plastics (KLEP), Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing, 100190, China.
Phys Chem Chem Phys. 2010 Feb 28;12(8):1941-7. doi: 10.1039/b920446f. Epub 2010 Jan 19.
The dissolution mechanism of cellulose in ionic liquids has been investigated by using cellobiose and 1-ethyl-3-methylimidazolium acetate (EmimAc) as a model system under various conditions with conventional and variable-temperature NMR spectroscopy. In DMSO-d(6) solution, NMR data of the model system clearly suggest that hydrogen bonding is formed between hydroxyls of cellobiose and both anion and cation of EmimAc. The CH(3)COO(-) anion favors the formation of hydrogen bonds with hydrogen atoms of hydroxyls, and the aromatic protons in bulky cation Emim, especially the most acidic H2, prefer to associate with the oxygen atoms of hydroxyls with less steric hindrance, while after acetylation of all hydroxyls in cellobiose the interactions between cellobiose octaacetate and EmimAc become very weak, implying that hydrogen bonding is the major reason of cellobiose solvation in EmimAc. Meanwhile the stoichiometric ratio of EmimAc/hydroxyl is estimated to be between 3:4 and 1:1 in the primary solvation shell, suggesting that there should be one anion or cation to form hydrogen bonds with two hydroxyl groups simultaneously. In situ and variable-temperature NMR spectra suggest the above mechanism also works in the real system.
通过使用纤维二糖和 1-乙基-3-甲基咪唑醋酸盐（EmimAc）作为模型体系，在各种条件下使用常规和变温 NMR 光谱研究了纤维素在离子液体中的溶解机制。在 DMSO-d(6)溶液中，模型体系的 NMR 数据清楚地表明，纤维二糖的羟基与 EmimAc 的阴离子和阳离子之间形成氢键。CH(3)COO(-)阴离子有利于与羟基的氢原子形成氢键，而大体积阳离子Emim中的芳族质子，特别是最酸性的 H2，更喜欢与羟基的氧原子结合，因为其空间位阻较小，而在纤维二糖的所有羟基乙酰化后，纤维二糖八乙酸酯与 EmimAc 之间的相互作用变得非常弱，这表明氢键是纤维二糖在 EmimAc 中溶解的主要原因。同时，估计初级溶剂化壳中 EmimAc/羟基的化学计量比在 3:4 和 1:1 之间，这表明应该有一个阴离子或阳离子同时与两个羟基形成氢键。原位和变温 NMR 光谱表明，上述机制在实际体系中也适用。
