Novel 1,3-Dimethylimidazolium Dimethyl Phosphate Ionic Liquid for Efficient Separation of the 1,3,5-Trioxane/Water Azeotropic System by Extractive Distillation: Molecular Insights and Process Enhancement

With the increasing emphasis on sustainable development, people have realized that energy transformation is the key to solving the global energy and climate crisis. 1,3,5-Trioxane (TOX), as a crucial chemical raw material, is an important substitute for anhydrous formaldehyde, and the production of bulk industrial products through TOX is a significant way to extend the C1 industry chain. However, in the industrial synthesis process of TOX, coexisting water (H2O) and TOX form an azeotrope. This paper proposes for the first time a novel method of using green and environmentally friendly 1,3-dimethylimidazolium dimethyl phosphate ([MMIM][DMP]) ionic liquid as an entrainer to separate TOX and H2O through extractive distillation. Based on the COSMO-RS model, [MMIM][DMP] was screened and understood to be the most promising entrainer. Vapor–liquid equilibrium (VLE) experiments demonstrated that [MMIM][DMP] can obviously break the azeotropic point of the (TOX + H2O) system, the critical concentration for breaking the azeotropic point is about 1 mol/kg. The TOX + H2O + [MMIM][DMP] ternary VLE data were regressed by the NRTL model, e-NRTL model, Wilson model, and UNIQUAC model, and the results showed that the NRTL model achieved the best regression accuracy. In addition, the separation mechanism of [MMIM][DMP] breaking the azeotropic point of the TOX + H2O system was evidenced by the density functional theory and the COSMO-RS model. Finally, by conducting experiments on the extractive distillation pilot device, it was further verified that under the conditions of solvent ratio 2 and reflux ratio 1, TOX could be completely separated from H2O by using [MMIM][DMP] as an entrainer, and the TOX product with a mass fraction greater than 99.5% was obtained. Compared with the traditional use of organic solvents as extractants to separate TOX, this method can avoid the challenges of volatility and toxicity, demonstrating its enormous potential in promoting sustainable development.