The molecular nature of the eliminating azeotropy of dimethyl carbonate–ethanol system by ionic liquid entrainer

Abstract

Ionic liquids (ILs) have unique physicochemical features, such as insignificant vapour pressure, which make ILs excellent entrainers in extractive distillation to separate the azeotrope. For better utilization of the IL entrainer in extractive distillation, the microstructure properties of dimethyl carbonate (DMC)–ethanol azeotrope mixtures, before and after the separation breaking of the azeotrope by IL, were studied by Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations with focus on the vibrational region of the ethanol hydroxyl group. Excess spectroscopy followed by two-dimensional correlation spectroscopy (2D-COS) was performed on the original IR spectra to extract the microstructural features of different mixtures step by step. Different sizes of ethanol self-aggregations, DMC–ethanol interaction complex and different IL–ethanol interaction complexes were identified in different mixtures. Specifically, the DMC–ethanol interaction complex existed in both the DMC–ethanol and IL–DMC–ethanol systems at lower IL concentrations (x(IL) < 0.125) while broken out by IL at higher IL concentrations (x(IL) > 0.125). The intrinsic cause of the azeotrope breaking by the 1-hexyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide ([HMIM][Tf2N]) entrainer was revealed to be the disappearance of the DMC–ethanol interaction complex by the IL at higher IL concentrations. The hydrogen-bonds between DMC/IL and ethanol were also identified and confirmed in the DMC–ethanol and different IL–ethanol interaction complexes. Both the oxygen atom and the hydrogen atom in the hydroxyl group of ethanol participated in the hydrogen-bond, which confirmed the sensitivity of the hydroxyl group vibrational region. The hydrogen-bonds are weak strength, closed shells and electrostatic dominant interactions.