(509e) CO2 /CH4 Separation in Zeolite-Like Metal Organic Frameworks

Zeolite-like metal organic frameworks with sodalite-type framework (sod-ZMOFs) are promising candidates for natural gas (CO₂/CH₄) separation and purification applications. They have anionic frameworks and extra-framework ions in the pores of molecular dimensions increasing the interactions with guest molecules and improving their separation, storage, or ion-exchange capability.  The indium-based sod-ZMOF, with imidazoledicarboxylic acid as the organic linker, has higher CO2 adsorption capacity than ZIF-8, which has an analogous structure based on sodalite-topology. The charge-compensating extra-framework ions inside the cavities of these MOFs can be ion-exchanged with alkali metals to improve CO₂/CH₄ separation performance. Furthermore they can be used in the preparation of polyimide-based mixed-matrix membranes due their good compatibility and dispersion properties.

In this study, adsorption and diffusion of CO₂ and CH₄ gases in the sod-ZMOFs were studied using Grand Canonical Monte Carlo and Molecular Dynamics simulations. The simulations were carried out with “as-synthesized” as well as Li⁺, Na⁺ and K⁺ exchanged sod-ZMOFs. The selectivity CO₂ /CH₄  selectivity of the sod-ZMOFs were compared based the type of the cation and the ion-exchange ratio over a wide range of pressure. Adsorption and diffusion simulations showed that the selectivity of these MOFs increases two orders of magnitude when it is fully exchanged with Na⁺ ions. Furthermore, the selectivity depends not only on the cation type and ion-exchange ratio but also on the location of the cations in the framework.