Phase behavior and internal micro mechanism of separation of extracting low-carbon alcohols fuel additive with green solvents

Deep eutectic solvent (DES) has been used in chemical and pharmaceutical fields on account of their characteristics of high capacity, low toxicity and low price. Low-carbon alcohol, as a fuel additive, has the characteristics of high energy density, low corrosiveness, and high added value. Therefore, extracting low-carbon alcohols from azeotropes as fuel additives can improve the combustion efficiency of the fuel. To realize the separation of low-carbon alcohols-alkanes azeotropes, three choline-based DESs were synthesized in this work. The liquid–liquid equilibrium data of n-hexane-methanol/isopropyl alcohol-DESs were determined under 298.15 K and 1 atm. The separation efficiency of DESs for two azeotropes was compared through calculating the distribution coefficient and selectivity of DESs for low-carbon alcohols. To further explore the reasons for the different separation effects of DESs on two azeotropes, the internal mechanism of DESs extraction of two low-carbon alcohols was studied by molecular dynamics method. Based on the results of molecular dynamics simulation, the kinetic analysis results such as the interaction energies, spatial distribution function between DESs and azeotropes in the separation process were obtained. The results showed that the separation effect of three DESs on methanol-n-hexane azeotrope was better than that of isopropyl alcohol-n-hexane azeotrope, the DES1 composed of choline chloride and ethylene glycol in the molar ratio of 1:2 had the highest extraction efficiency for two low-carbon alcohols among the three DESs, and Cl- made the greatest contribution in the separation process.