Ultra-porous materials have long been of interest for gas storage and gas separations due to their high surface areas allowing for large quantities of adsorbed gas. In particular, Metal-Organic Frameworks (MOFs), coordination complexes based upon inorganic nodes and organic linkers, offer a unique approach to this field of study due to their high tunability and crystallinity. In MOFs, both the metal clusters and the ligands are readily tunable, able to adjust not only the structure of the MOF, but also the applications under which the MOF can be utilized. These adjustments can take many forms, from altering the components used to synthesize the MOF, to performing subsequent chemical reactions on the pre-synthesized structures via post-synthetic modification. One specific type of post-synthetic modification that has been of increased scientific interest in recent years is the generation of hierarchically porous structures through structural defect engineering. Hierarchically porous structures are those structures that have not only specific, narrowly distributed pores like in traditional MOFs, but those than have a range of porosities, particularly those that have pores both in the microporous (&lt;2 nm) and the mesoporous (2-50 nm) regions. In this work we have investigated the formation of mesopores in a microporous iron-carboxylate-based MOF structure through the use of a simple thermal treatment methods. Our results show that mesopore formation is based upon the partial decarboxylation of the tetradentate ligand, with this producing defect sites while still maintaining the overall structural stability of the material. To probe this phenomenon, we have investigated the effect of metal cluster redox identity on a series of MOFs isostructural with the parent MOF, allowing us to elucidate the influencing factors and potential mechanism of the mesopore formation. Additionally, we have investigated how the formation of these mesoporous sites result in changes to both the affinity and total capacity of these structures towards both carbon dioxide and methane, providing valuable insight into how structural factors affect adsorption behavior.

Cluster redox activity influences towards thermally generated hierarchical porosity in isostructural metal-organic frameworks

ACS Spring Meeting 2021

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