Evaluation of task-specific ionic liquids applied in pervaporation membranes: Experimental and COSMO-RS studies

Abstract

The production of dimethyl carbonate (DMC) from CO2 and methanol (MeOH) is an attractive route for CO2 capture and utilization. In this process, the separation of DMC from the reaction medium is critical to maximize the conversion of CO2. However, DMC and MeOH form an azeotropic mixture that is difficult to separate. This work investigates the possibility of using task-specific ionic liquids (TSILs) in the form of supported ionic liquid membranes (SILMs) to separate and purify DMC by using pervaporation. Two tertiary amine ILs, i.e., 1-octyl-3-methylimidazolium bromide ([Omim][Br]) and 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Omim][NTf2]), and one cyclic quaternary ionic liquid, i.e., 1-octyl-1,4-diazabicyclo[2.2.2]octanium bromide ([ODABCO][Br]), were prepared. The structure and purity of the synthesized ILs were confirmed by NMR and FTIR, and the as-synthesized ILs along with one commercial quaternary ammonium IL, tetrabutylammonium bromide ([TBA][Br]), were further characterized using TGA/DSC. SEM-EDX, tensile tests and stability tests were also performed to characterize the SILMs. During the pervaporation experiments, the SILMs showed an initial decrease in DMC and MeOH permeance over time, followed by a gradual stabilization, with a relatively stable DMC/MeOH selectivity as pure liquids. In addition, an increase in temperature is shown to have negative effect on DMC and MeOH permeance because sorption, which is an exothermic process, governed the transport of molecules across the membrane. The studied SILMs exhibited higher selectivity at low temperatures, especially at 30 °C. Furthermore, the COSMO-RS model provided insights of the effect of IL structures on the separation of DMC from MeOH at molecular level.