(395aa) Ion-Exchanged Sod-ZMOFs for CO2 Capture Via Adsorption

Separation of CO₂ from streams containing CH₄ and N₂ is a still challenging problem in the era where low-energy consuming processes are studied extensively. CO₂ present in natural gas causes decrease in energy content and corrosion in aqueous medium. Therefore, removal of CO₂ from natural gas is still remaining a major problem in petroleum industry. Similarly, the discharge of CO₂-containing flue gas from coal-fired power plants is also a serious concern due to global warming,  and to decrease CO₂ emission into the atmosphere remains a serious problem.

Amine absorption have been conventionally useds to capture CO₂ from CO₂/CH₄ and CO₂/N₂ mixtures. However, separation of CO₂ via adsorption or membrane-based separation processes, are less-energy demanding and environmentally friendly alternatives. Zeolites are widely used as solid adsorbents in adsorption processes, or as fillers in mixed-matrix membranes but they have limited separation capacity and selectivity. Recently, due their high free volumes and tunable porosities, metal organic frameworks (MOFs) have been proposed to replace zeolites in gas separation processes. A subclass of MOFs, zeolite-like metal organic frameworks (ZMOFs) have anionic structure and charge compensating extraframework cations, such as alkali earth metals.

In this study, separation of CO₂/CH₄ and CO₂/N₂ gas mixtures was studied in ZMOFs of sodalite topology (sod-ZMOFs) via Monte Carlo simulations. Gas separation performances of “as-synthesized” and partially and completely Li⁺-, Na⁺- and K⁺-exchanged ZMOFs were evaluated on the basis of adsorption selectivities and working capacities at different operating pressures. The working capacity is an important parameter in evaluating adsorbents as a gas separation material and economics of gas separation processes such as pressure swing adsorption (PSA).

Results showed that both the adsorption selectivities and working capacities were enhanced by increasing the ion-exchange ratio. Thus, completely ion-exchanged sod-ZMOFs, particularly K⁺-exchanged ZMOF showed the best performance in capturing CO₂ from CO₂/CH₄ and CO₂/N₂ mixtures.