Metal-organic frameworks (MOFs) are highly crystalline compounds that consist of repeating metal ions bridged by coordinating organic linkers in order to form three-dimensional structures with high thermal and chemical stability. Their microporous nature leads to a high surface area which can enable reversible physisorption of gases, especially for frameworks with large amounts of coordinatively unsaturated metal sites which can interact strongly with guest molecules such as CH4 or H2. The nature of MOF formation provides a wealth of synthetic handles to tune the properties of the resulting framework, by modulating metal sites, linker structure, or the framework’s fundamental connectivity and topology. Taken together, these properties make MOFs ideal candidates for a variety of applications, and in particular make them attractive possibilities for automotive CH4 and/or H2 storage. We performed a targeted synthesis of Co2(fluoro-m-dobdc) (fluoro-m-dobdc4- = 5-fluoro-4,6-dioxido-1,3-benzenedicarboxylate), a variation upon the Co2(m-dobdc) framework with record-high H2 binding energies, with the goal of increasing inductive effects towards the hydrogen binding sites and further increasing the strength of the hydrogen-binding interaction. We also synthesized Zr-alkoxy-tpdc (alkoxy-tpdc2- = terphenyl-2',5'-di-R-oxy-4,4''-dicarboxylate; R = -CH3, -C2H5, -C3H7, -C4H9, -C6H13) in order to evaluate the effects of alkyl chains of varying lengths on the uptake of methane. While CH4 uptake decreased unilaterally relative to the parent Zr-tpdc framework (also known as UiO-68), the R = -C3H7 and R = -C6H13 variations displayed higher methane uptakes than the corresponding shorter-chain frameworks R = -C2H5 and R = -C4H9, respectively, indicating that other factors besides steric congestion may play a role in altering methane uptake isotherms. Another targeted framework, Co2(NDI-dobdc) (NDI-dobdc4- = N,N'-di(4,6-dioxido-1,3-benzenedicarboxylate)naphthalenediimide), was chosen because reduction of the naphthalenediimide linkers may provide a way to introduce positive counterions near the open coordination sites of the MOF-74-analogous framework, thereby increasing its capacity to bind gases. Finally, we worked toward optimizing the synthesis of the known framework Cu[Ni(pdt)2] (pdt2- = pyrazine-2,3-dithiolate), with the goal of evaluating its potential applications for olefin separations, due to its similarity to another known dithiolene system with high olefin selectivities.

Targeted synthesis of metal-organic frameworks for gas storage and hydrocarbon separations

251st National Meeting (2016)

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