Investigation of MOF-525 and post-metalated MOF-525 as nanofillers in mixed matrix membranes for CO2 separation

Over the past twenty years, metal-organic frameworks (MOFs) have emerged as extensively developed porous class of materials and are increasingly recognized as promising candidates for membrane-based CO2 separation. This potential primarily stems from the ability to deliberately customize their structure and functionalities to enhance interactions with guest molecules. In this study, we explore the use of MOF-525, a porphyrin-based MOF, as a nanofiller in a mixed matrix membrane (MMM) composed of 6FDA-DAM (6FDA: 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; DAM: 2,4,6-trimethyl-1,3-diaminobenzene) polymer for CO2/N2 and CO2/CH4 separations. This particular MOF is chosen because of the possibility to metalate its porphyrin ring to tailor the interaction between the CO2 molecule and the MOF framework. As a result, the CO2/N2 and CO2/CH4 separation performance of the MMM loaded with metalated MOF-525 can be significantly improved without the necessity to use a very high nanoparticle loading. When compared to the bare polymeric membrane and 2 wt% non-metalated MOF-525 MMM, around 20% improvement in the membrane permeability and selectivity can be observed for the 2 wt% metalated MOF-525 MMM. Further analysis on the gas transport property of the MMM showed that the improvement mainly results from the enhanced CO2 solubility in the MMMs and improved interaction between the metalated MOF-525 and the CO2 molecule. However, it is also found that 2 and 5 wt% are the optimum loading value, above which the interfacial defects between the MOF nanoparticles and the polymers caused by the particle agglomeration starts to appear and thus deteriorating the membrane performance. This is also confirmed through the molecular simulations where some overestimations from the Maxwell model on the membrane permeability is observed particularly at high particle loading, indicating the agglomeration and the build-up of non-selective voids. Despite this, we have successfully shown in this study the high efficacy and efficiency of using metalated porphyrin MOFs for CO2 separation in a MMM since only relatively low particle loading (around 2 wt%) is required to improve the membrane performance.