Influence of anion size and electronic structure on the gas separation performance of ionic liquid/ZIF-8 composites

We investigated the influences of the changes in the electronic structure and size of the anion of an imidazolium ionic liquid (IL) on gas adsorption and separation performance of the IL/ZIF-8 (zeolitic imidazolate framework) composites. We studied four different imidazolium ILs having the same cation, 1-n-butyl-3-methylimidazolium, [BMIM]+, with anions having structures allowing a systematic comparison of the changes in the electronic structure and size. To examine the influence of changes in the electronic structure, we considered anions representing the fluorination on the anion, methanesulfonate, [MeSO3]−, and trifluoromethanesulfonate, [CF3SO3]−. To investigate the change in the anion size, methyl sulfate, [MeSO4]−, and octyl sulfate, [OcSO4]−, were studied. Characterization of IL/ZIF-8 composites demonstrated successful incorporation of each IL in ZIF-8 without causing any detectable changes in the crystal structure and morphology of ZIF-8. Thermogravimetric analysis and infrared (IR) spectroscopy indicated the presence of direct interactions between ILs and ZIF-8, which directly control gas separation performance of the composite. Gas adsorption measurements illustrated that incorporation of ILs significantly improves the gas separation performance of the pristine ZIF-8. [BMIM][MeSO4]/ZIF-8 composite had 3.3- and 1.8-times higher CO2/N2 and CH4/N2 selectivities compared to ZIF-8, respectively, at 1 bar. When the IL has a fluorinated anion, CO2/CH4 selectivity improved 3-times compared to its non-fluorinated counterpart. Upon the incorporation of IL with a small anion, IL/ZIF-8 composite showed higher CO2/N2 and CH4/N2 selectivities compared to the composite having an IL with a bulky anion. These results will contribute in guiding rational design of IL/MOF composites for different gas separations.