纤维素溶解机理研究述评

纤维素是植物细胞壁的主要组分之一,是由葡萄糖通过1,4-β糖苷键连接而成的均一聚糖,广泛应用于生产酯类、醚类等纤维素基化工产品。纤维素基产品的制备及应用与其溶解程度密切相关,探究纤维素溶解机理,寻找一种绿色高效的纤维素溶剂至关重要。综述了当前国内外学者们对纤维素溶解机理的解释(氢键破坏理论)以及可能影响溶解的因素(化学热力学、化学动力学、结晶度、纤维素两亲性、电荷数和温度),在此基础上探讨了导致纤维素水溶性差的原因,提出了在复杂体系中除了氢键作用以外,还需要综合考虑范德华力、疏水性相互作用的影响。同时,总结了纤维素在离子液体中溶解机理方面存在的争议,其中关于氢键理论还存在一定问题,动力学控制理论尚不明确,阳离子在溶解过程所起的作用被忽视,指出离子液体阳离子的两亲性是纤维素溶解的关键因素。基于纤维素的两亲性及两亲性溶剂促进纤维素溶解的重要理论,提出了应重点寻找新型两亲性溶剂的观点,为未来寻求高效、环境友好、成本低廉的纤维素溶剂指明了方向。

Research progress on mechanism of cellulose dissolution

As one of the main components in plant cell wall,cellulose is a homogeneous glycan formed by glucose through 1,4-β glycosidic bond,which can be used to produce chemical products such as cellulose-based esters and ethers.The preparation and application of these cellulose-based products are closely related to their dissolution aspects.Exploring the dissolution mechanism of cellulose is crucial to find out efficient and green solvents for the cellulose dissolution.This paper reviewed the current explanations of cellulose dissolution mechanisms and factors that may affect dissolution,such as cellulose crystallinity,temperature,and kinetic factors.Recent work has also emphasized on the role of cellulose charge and the concomitant ion entropy effects,as well as hydrophobic interactions.Generally,scholars believe that the mechanism of cellulose dissolution is based on the theory of hydrogen bond destruction.There are a large number of hydrogen bonds between and within cellulose molecules and breaking the hydrogen bonds in cellulose may cause cellulose dissolution.Despite the reasonable assumption of hydrogen bond destruction theory,there are still some unclear problems,such as the reason why cellulose is insoluble in water cannot be explained.It is suggested that,other than hydrogen bond,the Van der Waals force and hydrophobicity existed in the complex systems should be considered.The debating points related to the mechanisms of cellulose dissolved in ionic liquid were also reviewed.The theory of kinetic control indicates rather than thermodynamic control is decisive but still lack of adequate supporting data.There are many statements emphasizing the crucial role of the IL anion especially chloride ions except that of cations.Moreover,further research about other ions is needed to fully understand the dissolution mechanism.Some researchers indicated that the cationic amphiphilicity of ionic liquid was the key for dissolution.They found that almost all cations that promoted dissolution were amphiphilic.Furthermore,it is generally acknowledged that only nitrogen-containing cations work,and all the cations used that work are amphiphilic.From the previously mentioned attempt to review the literature on cellulose dissolution,it can be inferred that the amphiphilic property may be the key theory for cellulose dissolution.We therefore put forward a hypothesis that we should focus on looking for novel amphiphilic solvents in the future.It is hoped that this treatise contributes to a deeper understanding of cellulose dissolution,and in particularly,the amphiphilic theory,to help find high-efficient,environmental-friendly and low-cost solvents for cellulose dissolution.