Liquid–Liquid Equilibrium Experiment and Mechanism Study on the System of Dimethyl Carbonate + n-Propanol + Ionic Liquids

Abstract

While synthesizing dipropyl carbonate, a mixture of dipropyl carbonate, dimethyl carbonate (DMC), and n-propanol (NPA) will be produced. DMC and NPA cannot be separated by ordinary distillation because the system has an azeotropic point; liquid–liquid extraction is a high-efficiency and energy-saving method for separating azeotropes. The liquid–liquid equilibrium (LLE) data are the basis of designing the extraction process. Ionic liquids (ILs) are green and efficient extractants with many advantages. In this study, we selected 1-ethyl-3-methyl-imidazolium dihydrogen phosphate ([EMIm][H2PO4]), 1-ethyl-3-methyl-imidazolium hydrogen sulfate ([EMIm][HSO4]), 1-ethyl-3-methyl-imidazolium nitrate ([Emim][NO3]), and 1-butyl-3-methyl-imidazolium hydrogen sulfate ([Bmim][HSO4]) as the extractants. First, the LLE data were determined at 101.32 kPa and 298.15 K and verified by the Bachman formula and Hand formula; the R2 value was all greater than 0.9782. Then, the experimental data of each system were regressed by the NRTL model, the accuracy of NRTL interaction parameters was verified through the GM/RT surface analysis, the root mean square deviation (RMSD) was calculated, and the values were all less than 0.011, which showed that the NRTL model could be used for the prediction of this system. Finally, the extraction mechanism was explored. The electrostatic potential (ESP) on the molecular surface was visually analyzed by ESP analysis; the mutual penetration distances were calculated. Combining the experiment and quantum chemical calculation can help us understand the extraction process more deeply.