Carboxylate ionic liquid solvent systems from 2006 to 2020: thermal properties and application in cellulose processing

The sufficient utilization and transformation of cellulose into cellulose derivatives has always attracted considerable attention due to its renewability, biodegradability, non-toxicity, abundance, and low cost. However, the extensive utilization of cellulose is actually hampered by its well-known insolubility in water and most organic solvents. Hence, the development of efficient solvents for cellulose has been going on to date. Among various solvents, ionic liquids (ILs), particularly 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 literature studies published recently. However, detailed overviews of the application of carboxylate ILs in cellulose dissolution and processing are scarce. Therefore, this review summarizes the dissolution performance, 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 (alpha, beta, and pi*), viscosity and conductivity with cellulose dissolution as well as microwave and ultrasonication application in cellulose dissolution are also reviewed. Moreover, the properties of the 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 their safe use. Finally, the existing problems are discussed and future prospects are given. We hope that this paper would be helpful for the design of desired carboxylate IL solvent systems and practical applications in cellulose conversion into valuable products.