以离子液体为介质的纤维素均相衍生化
摘要
Cellulose derivatives are important fine chemicals with a wide range of applications in modern industries, so that cellulose derivatization has been one of hot topics for a long time. However, the existence of numerous hydroxyls in cellulose also generates a well-developed intra- and inter-molecular hydrogen bonding network, thus natural cellulose is neither meltable nor soluble in conventional solvents. Until today, the preparation method for cellulose esters begins with a heterogeneous reaction between cellulose and a large excess of the acylation reagent in the presence of pyridine or sulfuric acid as the catalyst. These heterogeneous reactions cause some problems, such as uneven distribution of substituents, side reactions, time consumption and considerable degradation of cellulose. Homogeneous esterification of cellulose in appropriate media, the alternative procedure for heterogeneous process, has been one focus in cellulose chemistry. It not only provides opportunities to obtain the effective control of degree of substitution (DS), distribution and uniformity of the functional groups along cellulose chains, but also creates more options to induce novel and functional groups. Recently, the advent of ionic liquids that can dissolve cellulose provides a new and versatile platform for the efficient and homogeneous derivatization of cellulose. A variety of well-known, novel and functional cellulose esters have been successfully synthesized in ionic liquids. Meanwhile, development of esterification techniques, utilization of agricultural residues, and pilot scheme of up-scaling esterification in ionic liquids have been in progress continuously. This review summarizes the development in homogeneous synthesis of cellulose esters in ionic liquids published in the recent decade.