Molecular simulations for adsorptive separation of CO2/CH4 mixture in metal-exposed, catenated, and charged metal-organic frameworks.

The adsorption and separation of CO2/CH4 mixture are studied using molecular simulations in a series of metal-organic frameworks (MOFs) with unique characteristics such as exposed metals (Cu-BTC, PCN-6' and PCN-6), catenation (IRMOF-13 and PCN-6), and extraframework ions (soc-MOF). Because of the strong affinity with the framework, CO2 is preferentially adsorbed over CH4 in all MOFs. Framework catenation leads to constricted pores and additional adsorption sites and enhances the interaction with the adsorbate. Therefore, catenated IRMOF-13 and PCN-6 exhibit a greater extent of adsorption, particularly for CO2, at low pressures than IRMOF-14 and PCN-6'; however, the opposite is true at high pressures. CO2/CH4 selectivity in IRMOF-1 and IRMOF-14 is almost constant at low pressures and increases with increasing pressure. As a result of a counterbalance between energetic and entropic effects, the selectivity in IRMOF-13 initially decreases at low pressures and then increases with pressure and finally approaches a constant value. Catenated MOFs have a higher selectivity than their non-catenated counterparts. The presence of electrostatic interaction between CO2 and the framework leads to an increase in CO2 adsorption and a corresponding decrease in CH4 adsorption and consequently enhanced selectivity. In charged soc-MOF, the extraframework NO3- ions are identified to be equally distributed from the nearest metal atoms and vibrate around the favorable sites. The selectivity in soc-MOF is substantially higher than in the other IRMOFs and PCNs and is the highest among various MOFs reported to date. The simulation results reveal that the selectivity of CO2 over CH4 in MOFs is enhanced slightly by exposed metals, catenation, and significantly by extraframework ions and that charged MOFs are promising candidates for the separation of CO2/CH4 mixture.