Green Synthesis of Thermoplastics by Transesterification of Total Lignocellulose Components of Beet Pulp Using Ionic Liquid
Shogo IWATA1, Romain MILOTSKYI2, Ryo SERIZAWA1, Tetsuo FUJIE2, Naoki WADA2, Kenji TAKAHASHI2
1Graduate School of Natural Science and Technology, Department of Biological and Technology Kanazawa University, Kanazawa, Japan
2Department of Natural System, Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan
The discovery of ionic liquids (ILs) has added a new branch of materials of choice with low environmental impact using cellulose. In general, cellulose molecules have a unique molecular structure with many hydrogen bonds and do not flow when heated, making it difficult to dissolve them in common organic solvents. However, ILs can easily dissolve cellulose and secondary wood polymers such as hemicellulose and lignin. This opens the possibility of using the lignocellulosic system, consisting of cellulose and secondary wood polymers that are abundant on earth, as a resource in a cleaner way.
Currently, commercially produced cellulose resins such as cellulose triacetate (CTA) are made from purified cellulose consisting of wood pulp and cotton linter using N-Methylmorpholine N-oxide or dimethyl sulfoxide/tetrabutylammonium fluoride as solvent systems. This series of processes from raw material procurement to resin production includes processes that may have an environmental impact, such as deforestation to secure raw materials and the environmental toxicity of solvent systems. In order to design more sustainable chemical processes, research is currently being conducted to refine cellulose and synthesize resins from agricultural waste, such as date palm trunk mesh. The use of these agricultural wastes is very important in terms of sustainability, but refining and utilizing only the cellulose component does not maximize the use of the entire lignocellulosic system.
In a previous study by our research team, we successfully used 1-ethyl-3-methylimidazolium acetate (EmimOAc), a non-toxic ionic liquid that serves as both catalyst and solvent, for the green conversion of the total lignocellulosic component of agricultural waste sugarcane bagasse into thermoplastic resin. In addition, sugar producing plants are worldwide importance from biomass refinery point of view.
In this study, we focused on beet pulp, a type of agricultural waste that is the raw material for such sugars. Beet pulp is the residue that remains after sugar beets are squeezed out of sugar, and it is valuable agricultural waste with total world production of 15 million t/y of dry matter. In addition to lignocellulosic components, beet pulp is rich in pectin. We have successfully converted all components of beet pulp into plasticizer free thermoplastic resin by transesterification reaction using EmimOAc as a catalyst. Our green synthesis approach using ILs as both solvent and catalyst, allows to perform a direct homogenous chemical modification of beet pulp components in one step. ILs are recycled and reused after use to enhance sustainability. In addition, a more concise pretreatment process resulted in a molecular weight greater than of commercial cellulose resins, exceeding 2,000,000 g/mol. Furthermore, the synthesized resin has improved thermal stability compared to the raw beet pulp. The mechanical properties were also evaluated by thermoforming and producing films. This result suggests that beet pulp can also be used as an excellent sustainable bio-based material. Resins derived from beet pulp are expected to be highly recyclable and biodegradable, so they can be used for a variety of application.