Vapor–liquid equilibrium in the production of the ionic liquid, 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]), in acetone

• First full experimental phase equilibria measured for a typical ionic liquid synthesis system.
• Vapor–liquid equilibrium for each of the miscible binary mixtures involved.
• Liquid–liquid equilibrium for the [HMIm][Br]-1-bromohexane system.
• Phase equilibria modeling using both activity coefficient models and equation of state with mixing rules.
• Equilibrium will also better modeling and separations development for large-scale production of ionic liquids and better life-cycle analyses.

Ionic liquids are finding a wide range of applications from reaction media to separations and materials processing. In order to provide larger quantities of ionic liquids, a sustainable synthesis method is needed which includes optimization of the separation of the reaction mixture, possibly using thermal methods. Here, the experimental vapor–liquid equilibrium (VLE) involved in the synthesis of a model ionic liquid, 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) from 1-bromohexane and 1-methylimidazole in acetone has been performed at 1.01325 bar using a modified Othmer still. The binary systems involving solvent and reactants, acetone/1-bromohexane and acetone/1-methylimidazole, possessed fairly wide equilibrium envelopes. The binary systems involving the ionic liquid, acetone/[HMIm][Br], and 1-methylimidazole/[HMIm][Br], were also measured and found to have no detectable trace of ionic liquid in the vapor phase (dew points) as expected. Isobaric liquid–liquid equilibrium (LLE) was performed for the partially miscible system of 1-bromohexane/[HMIm][Br] where 1-bromohexane was found to be moderately soluble in the IL-rich phase, but the IL is virtually insoluble in the 1-bromohexane phase. As the binary system, 1-bromohexane/1-methylimidazole reacts rapidly under the temperatures of interest, the UNIFAC activity coefficient model was used to predict the binary VLE data. All experimental data were well-correlated by the Peng–Robinson equation of state with van der Waals one-parameter mixing rule (PR-EoS VDW-1) and the non-random two liquid (NRTL) activity coefficient method.

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