Natural Deep Eutectics as a "green" Cellulose Cosolvent

In this study, we report a novel, green chemistry approach for creating new cellulose solvents based on a mixture of a natural deep eutectic solvent (NADES) and a primary cellulose solvent. Because of the strong hydrogen-bond donor and acceptor ability of both the NADES and the primary cellulose solvent, the new mixed system is a cellulose solvent, with an improved cellulose dissolution capacity. We believe that this is a generic approach to prepare an entirely new class of green solvent, capable of dissolving cellulose under mild conditions. This in turn will facilitate the creation of a large amount of new cellulose-based (soft) materials. To illustrate our approach, we show that a NADES based on choline chloride + malic acid can be used as cosolvent for the industrial cellulose solvent N-methylmorpholine-N-oxide monohydrate (NMMO). The new mixed cellulose solvent system has improved cellulose dissolution capacity and has a much broader processing window, which allows working with dissolved cellulose at ambient temperatures, far below 70 °C, where NNMO monohydrate will solidify. This, in turn, can not only help to address the thermal instability issue of pure NMMO when processed at elevated temperatures, but also expands the working conditions of the lyocell process. The recyclability of the solvent system was also conducted with a recovery yield of more than 95%, validating the sustainability of this solvent system. More importantly, the cellulose dissolution capacity was still maintained after the recycling process. To highlight the potential applications, we have created highly anisotropic reconstituted cellulosic suspensions by the solvent exchange process in different antisolvents.