Improving Gas Separation Performance of ZIF-8 by [BMIM][BF4] Incorporation: Interactions and Their Consequences on Performance

Gas separation performance of the zeolitic imidazolate framework (ZIF-8) was improved by incorporating an ionic liquid (IL), 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). Detailed characterization based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed that the morphology of ZIF-8 remains intact upon IL incorporation up to 28 wt %. Thermogravimetric analysis indicated the presence of direct interactions between the IL and metal organic framework (MOF). FTIR spectroscopy illustrated that the anion of the IL was shared between the imidazolate framework and [BMIM]+ cation. Adsorption isotherms of CO2, CH4, and N2 measured for pristine ZIF-8 and IL-loaded ZIF-8 samples, complemented by grand canonical Monte Carlo (GCMC) simulations, showed that these interactions influence the gas uptake performance of ZIF-8. CH4 and N2 uptakes decreased in the whole pressure range, while CO2 uptake first increased by approximately 9% at 0.1 bar in 20 wt % IL-loaded sample and then decreased as in the case of other gases. As a result of these changes in gas uptakes occurring at different extents, the corresponding CO2/CH4, CO2/N2, and CH4/N2 selectivities were enhanced especially at the low-pressure regime upon IL incorporation. Results showed that CO2/CH4 selectivity increased from 2.2 to 4; while CO2/N2 selectivity more than doubled from 6.5 to 13.3, and CH4/N2 selectivity improved from 3 to 3.4 at 0.1 bar at an IL loading of 28 wt %. The heat of adsorption values (Qst) measured and simulated for each gas on each sample indicated that interactions between the IL and ZIF-8 strongly influence the gas adsorption behaviors. The change in Qst of CO2 upon IL incorporation was more significant than that of other gases, leading to an almost doubling of CO2 selectivity over CH4 and N2, specifically at low pressures. On the other hand, the selectivity improvement was lost at high pressures because of a strong decrease in the available pore space due to the presence of IL in ZIF-8. These results suggest that such IL/MOF combinations with tunable structures have huge potential toward high performance gas separation applications.