Cation does matter: how cationic structure affects the dissolution of cellulose in ionic liquids
作者:Benlian Lu、Airong Xu、Jianji Wang
DOI:10.1039/c3gc41733f
日期:——
As a kind of promising solvents, ionic liquids (ILs) have been used to dissolve cellulose and great progress has been made in recent years. However, the dissolution mechanism, especially the role of cations of ILs in the dissolution of cellulose, is still in debate. In this work, 13 kinds of ILs with a fixed anion [CH3COO]− but varied cationic backbones and alkyl chains have been prepared and characterized. The solubilities of cellulose in these ILs were measured at different temperatures. This allowed us to systematically study the effect of cationic structures on the cellulose dissolution at a given temperature. In order to investigate the dissolution mechanism, Kamlet–Taft parameters of these ILs in the temperature range from 25 to 65 °C and 13C NMR spectra of 1-benzyl-3-methylimidazolium acetate ([phC1mim][CH3COO]) + cellulose systems at 90 °C were also determined. It was found that acidic protons on the heterocyclic rings of the cations are essential for the dissolution of cellulose in the ILs, but the van der Waals interaction of cation with cellulose is not important. These protons may form C–H⋯O hydrogen bonds with hydroxyl and ether oxygen of cellulose to increase cellulose solubility. Cations of the ILs may also decrease cellulose solubility by strong interaction with anions or steric hindrance effect of large size group in their alkyl chains. These interactions together with strong O–H⋯O hydrogen bonds between the anion and hydroxyl protons of cellulose resulted in the disruption of the inter- and intra-molecular hydrogen bonds and thus effective dissolution of cellulose.
离子液体(ILs)作为一种前景广阔的溶剂,近年来在溶解纤维素方面取得了长足的进步。然而,有关溶解机理,尤其是离子液体中阳离子在纤维素溶解过程中的作用仍存在争议。本研究制备并表征了 13 种阴离子[CH3COO]固定但阳离子骨架和烷基链各不相同的 IL。在不同温度下测量了纤维素在这些 IL 中的溶解度。这使我们能够系统地研究阳离子结构对特定温度下纤维素溶解的影响。为了研究溶解机理,我们还测定了这些 IL 在 25 至 65 °C 温度范围内的 Kamlet-Taft 参数,以及 1-苄基-3-甲基咪唑醋酸盐([phC1mim][CH3COO])+纤维素体系在 90 °C 时的 13C NMR 光谱。结果发现,阳离子杂环上的酸性质子对纤维素在 ILs 中的溶解至关重要,但阳离子与纤维素之间的范德华相互作用并不重要。这些质子可能与纤维素的羟基和醚氧形成 C-H⋯O 氢键,从而增加纤维素的溶解度。离子交换树脂的阳离子也可能通过与阴离子的强烈相互作用或其烷基链中大尺寸基团的立体阻碍效应来降低纤维素的溶解度。这些相互作用加上阴离子和纤维素羟基质子之间的强 O-H⋯O 氢键,导致分子间和分子内氢键的破坏,从而有效地溶解纤维素。