Continuous Process of Cellulose Dissolution and Transesterification reaction via Homogeneous Reactive Twin-screw Extrusion Catalyzed by Ionic Liquid
Romain MILOTSKYI1, Stephanie HERNANDEZ1, Gyanendra SHARMA1, Daisuke HIROSE1, Naoki WADA1, Kenji TAKAHASHI1, Tetsuo FUJIE1
1Department of Natural System, Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan
Ionic liquids (IL) opened a new page in cellulose chemistry. As a general rule, specific molecular structure of cellulose with intensive hydrogen bonding makes the cellulose macromolecule insoluble in common single-component solvents. As a result, for long time, industrially employed conventional cellulose chemical modification (example, commercial process of cellulose acylation) has been performed through heterogeneous route which generally involves several steps. This process is often limited to the surface of cellulose and it is impossible to control the degree of substitution (DS). Moreover, use of harsh chemicals causes cellulose degradation during the process and produces large amounts of waste.
In contrast, homogeneous cellulose chemical modification helps to overcome the problems motioned above. Several efficient solvent systems such as N,N-dimethylacetamide/lithium chloride (DMAc-LiCl) or tetrabutylammonium fluoride in dimethyl sulfoxide (DMSO) were successfully applied to synthesize cellulose derivatives under homogeneous conditions with waste amount reduction. However, for example, DMAc-LiCl system is sensitive to water and impurities which makes it difficult to find an industrial application. Furthermore, the toxicity of the solvent system needs to be taken into account to design more sustainable chemical processes. In the previous research of our group a successful employment of ionic liquids which act as both catalyst and solvent allowed a direct synthesis of various cellulose thermoplastic derivatives in batch conditions. At the same time, the high viscosity of the cellulose-IL solutions causes insufficient mixing in batch reactors; thereby, favoring low cellulose concentrations with long reaction times (typically hours).
Reactive extrusion (REX) is an example of new innovation targeting cleaner and more sustainable processes. The extruder is an excellent mixing device and is particularly designed for processing highly viscous fluid. Moreover, it is possible to operate an extruder with a constant throughput which ensures flow stability and makes this process continuous.
In our group, for the first time, a one step, continuous process of cellulose monoester (laurate) and diester (acetate-propionate) synthesis catalyzed by 1-ethyl-3-methylimidazolium acetate (EmimOAc) was developed. We first examined the solubility of microcrystalline cellulose of several concentrations (10, 15, 20 wt%) in a mixture of EmimOAc/DMSO using twin screw extruder. Also, new method of measuring residence time distribution inside the extruder was developed and discussed. EmimOAc/DMSO mixture was recovered and reused. Next, transesterification of cellulose using single or mixture of acyl donors catalyzed by EmimOAc was performed. The products of high degree of substitution (DS > 2) combined with great reaction efficiency (RE) (RE > 80%) of the process as well as low E-factor (3.5) can be obtained in continuous way in short reaction time (< 10 min) using co-rotating twin screw extruder. Finally, effect of cellulose concentration on crystalline structure and mechanical properties of cellulose esters films was evaluated.
This work is funded by the Japan Science and Technology Agency (JST) (grant number JPMJPF2102) and the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant numbers 18H02253 and 22H02404)