Boosting CO2 separation in porphyrinic MOF-based mixed matrix membranes via central metal atom integration

Abstract

As atmospheric CO₂ levels continue to rise, contributing to the climate crisis, there is an increasing urgency to separate this gas from others and to expedite related research. Metal-Organic Frameworks (MOFs), known for their porosity and tunability, have already made significant impacts in this field, particularly to be used as part of a membrane material. This study introduces a novel method to enhance the CO₂ separation capabilities of MOFs-based mixed matrix membranes (MMMs). Instead of taking the traditional approach by functionalizing the MOF's ligands or varying the metal or metal-oxo MOF nodes, we harness the properties of metal atoms by integrating them as central elements within porphyrinic MOF linkers through a simple post-metalation method. As a result, by incorporating the post-metalated MOF-525 as fillers into the 6FDA-DAM (6FDA: 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; DAM: 2,4,6-trimethyl-1,3-diaminobenzene) polymer to fabricate MMMs, we effectively demonstrate improved CO₂/N₂ and CO₂/CH₄ gas separation capabilities of around 20 % without the necessity to use a very high MOF loading (only 2 wt%). Further analysis on the gas transport reveals that such a performance improvement mainly comes from the enhanced CO₂ solubility, which might be attributed to the presence of the metal atoms in the post-metalated MOF 525. Lastly, in order to get a more comprehensive understanding, we also carry out a computational study as a tool to validate and predict the experimental results of our MMMs. This study then opens up the possibility to further investigate the efficacy of introducing various metal atoms in other porphyrinic MOFs when they are used as fillers to significantly boost the CO₂ separation performance of MMMs.