(123d) Mixed-Matrix Membranes with Zeolite-Like MOFs for CO2/CH4 Separation

The separation of CO₂ from industrially important gas mixtures like natural gas, syngas and flue gas is an enormously crucial issue in chemical industry. Zeolite-like metal organic frameworks (ZMOFs) are promising candidates for 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 charge-compensating extra-framework ions inside the cavities of ZMOFs can be ion-exchanged with alkali metals and this is expected to increase their affinity for CO₂ and improve natural gas (CO₂/CH₄) purification performance.

In the present study, sod-ZMOF crystals having a sodalite topology were synthesized under solvothermal conditions. Following characterization of the crystals (XRD, TGA, SEM analyses), they were incorporated into Matrimid^® polyimide (PI) matrix to form  dense mixed-matrix membranes (MMMs) by solvent-casting method. Although several gas adsorption studies with ZMOFs have been reported in the literature, they are used as fillers in mixed-matrix membranes and their gas separation properties are investigated for the first time in this work. SEM images of the MMMs, showed no apparent voids or defects at the filler/polymer interface and that ZMOF particles were dispersed homogenously in the polymer matrix. Physical and thermal properties of the prepared membranes (e.g. film density, T_g) were analyzed. The CO₂/CH₄ separation performances of the membranes were determined by measuring pure gas permeabilities. As-synthesized sod-ZMOF samples were also ion-exchanged with Na and K cations to examine the effect of ion-exchange on the material characteristics and gas separation properties of the MMMs. The contribution of ion-exchanged alkali cations is observed to change gas separation performance of sod-ZMOF significantly.