Ultramicroporous metal-organic frameworks and porphyrin linker design toward gas-based applications

Metal-Organic Frameworks (MOFs) are a class of materials characterized by their highly porous nature. MOFs are ordered structures made up of well-defined metal ‘nodes’ that are bridged to each other through coordinating organic ‘linkers’. These frameworks have been an appealing area of research in recent years thanks to their numerous applications. Regarding gas separation in MOFs, a conventional approach is to develop a material that has a high affinity for one gas of interest, and lower affinities for other gases that may appear in a mixture. A typical example is the removal of carbon dioxide from flue gas exhaust. MOFs have been developed that can strongly and selectively bind carbon dioxide while in the presence of gases such as nitrogen, oxygen, and nitrogen oxides, and even water vapour. These separations can be challenging when the gases to be separated are low in abundance (e.g., atmospheric sequestration) or when they cannot be bound selectively over other gases. An alternative approach to separation is a method that relies on the differences in molecular size of the gases in a mixture, so-called molecular sieving. Chapter 2 describes two such MOFs (Zn2M; M = Zn or Cd), whose ultramicropores (pore width < 0.7 nm) make them capable of molecular sieving. The crystal structures of these MOFs were examined at different temperatures (100 and 273 K) and with different solvent molecules in the pores (DMSO and methanol) to help better understand the structural effects on their gas adsorption and separation properties. Critically, changing from Zn to Cd in the trimetallic node of the MOFs results in a sub-˚A change in the pore opening. At the molecular scale, this change resulted in a drastic difference in gas adsorption between the two MOFs. Zn3 only allows carbon dioxide to enter its framework, whereas Zn2Cd permits carbon dioxide, argon, nitrogen, and methane to enter the pores. The data suggest that Zn3 could be an excellent sieve for separating carbon dioxide from mixtures, even at environmental concentrations. Regarding the synthesis of MOFs, one of the ways to obtain MOFs with new topologies and unique properties is to design novel organic linkers. One class of organic molecules that lends itself well to creativity and modification is porphyrins. As the ‘pigments of life’, porphyrins are found everywhere in nature and have been used in applications ranging from catalysis to optics to therapeutics, and of course have been used as linkers in MOFs. Porphyrins in MOFs offer an additional dimension to the tuneability of the framework, as the porphyrin linker itself can coordinate a metal through the central nitrogens, changing the properties of the framework without affecting its structure. To date, porphyrin linkers have been predominantly made to coordinate to MOF nodes through substituents on their meso methine regions. Porphyrin MOF linkers where the linking moieties extend from the β-positions are as of yet unknown, leaving plenty of room for exploration. Chapter 3 discusses the synthetic methods that could give access to these linkers, as well as the progress made towards these linkers. Although ultimately the desired porphyrins could not be isolated and used in MOF synthesis due to the delays associated with the COVID-19 pandemic, Chapter 3 illustrates that the chemistry works and puzzles out the synthetic route necessary to obtain β-subsituted porphyrin linkers.

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