Carboxylate ionic liquid solvent system from 2006 to 2020: Thermal properties and application in cellulose processing
Sufficient utilization and transformation into cellulose-derivatives of cellulose have always attracted great attention due to its renewability, biodegradation, nontoxicity, abundance and low cost. However, extensive utilization of cellulose is actually hampered by its well-known insolubility in water and most organic solvents. Hence, the development of the efficient solvents for cellulose has going on till now. Among various solvents, ionic liquids (ILs), especially carboxylate ILs have been found to be effective solvents for cellulose. The detailed knowledge of the application of non-carboxylate ILs in cellulose dissolution and processing has been overviewed in several literatures published recently. However, detailed overviews of of the application of the carboxylate ILs in cellulose dissolution and processing are scarely obtained. Therefore, this review summarizes the dissolution perfermance, possible dissolution mechanism and dissolution thermodynamic of cellulose in various carboxylate IL solvent systems including neat carboxylate ILs and carboxylate IL + cosolvent. Meanwhile, the relationships of Kamlet-Taft parameters (α, β and π*), viscosity and conductivity with cellulose dissolution as well as microwave and ultrasonication application in cellulose dissolution are also reviewed. Moreover, the properties of regenerated cellulose film, aerogel and fiber materials have been discussed. In addition, the thermal properties of carboxylate ILs are also concluded in this review for safe use. Finally, the existing problems are discussed and future prospects are given. We hope this paper would be helpful for the design of desired carboxylate IL solvent systems and practical application in cellulose conversion into valuable products.