Computational evaluation of RHO-ZIFs for CO2 capture: From adsorption mechanism to swing adsorption separation

Abstract

RHO zeolitic imidazolate frameworks (ZIF) are promising materials regarding chemical stabilities and CO2 capture capacities. However, their CO2 adsorption, separation properties, and applications in swinging adsorption processes have not been systematically investigated. Therefore, molecular simulations were conducted for CO2 adsorption and separation from natural gas and flue gas on three representative RHO-ZIFs (ZIF-71, ZIF-96, and ZIF-97). After identifying the most reasonable forcefield parameters for the considered gases and ZIFs (<10 % deviation), the CO2 adsorption capacities under various operating conditions were predicted. ZIF-96 was calculated to possess comparable CO2 uptake (3.42 mol/kg) with the record ZIF-69 (3.23 mol/kg) at 1 bar and 273 K due to the functionalized CN and NH2 groups. Despite containing the polar CH3OH groups, ZIF-97 showed a similar saturated CO2 adsorption as ZIF-71, which contains less-polar dichloro-imidazolate groups. Analysis reveals that the specific position of OH in ZIF-97 makes the 4-apertures more attractive to CO2 than the 6- or 8-apertures. Such narrow preferential adsorption sites limit CO2 occupancy. In addition, ZIF-96 has great CO2/CH4 and CO2/N2 selectivities over a wide range of temperatures, pressures, and CO2 concentrations, outperforming many well-known MOFs. Based on several specific separation metrics, capturing CO2 using ZIF-96 in the PTSA process is considered the most appropriate approach for natural gas purification and fuel gas treatment.