Computational assessment of zeolitic imidazolate frameworks for kinetic gas separations

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Verploegh, Ross James
Sholl, David S.
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Industrial separations of light gases and hydrocarbons are currently performed with well-established energy and capital intensive distillation. Within the last decade, certain research advances have energy suppliers focused on novel separation techniques using metal-organic frameworks (MOFs) as a possible replacement for traditional distillation. Experimental groups at Georgia Tech have developed techniques for creating thin-film and mixed-matrix MOF membranes that would perform these commodity fuel and reagent separations at ambient temperature and moderate pressures. Zeolitic imidazolate frameworks (ZIFs), a family of MOFs, were shown experimentally to act as excellent molecular sieves for C1-C4 hydrocarbons and other light gases. They also have superior binding properties to polymer supports and are more easily synthesized than zeolites, enhancing large-scale manufacturability. Understanding diffusion properties of light gases and hydrocarbons in ZIFs was needed in determining which ZIFs have the most industrial promise, providing direction for future experimental efforts, and also to contribute to fundamental knowledge of diffusion processes. In this thesis, I, in collaboration with many talented researchers, established a suite of computational methods that are suited to tackling several significant challenges facing the research community studying ZIFs. ZIFs are flexible materials and this inherent material property required the use of fully flexible molecular dynamics calculations to explain hydrocarbon-ZIF framework interactions during the diffusion process. These computational methods were extended to predict loading-dependent, single-component transport diffusion coefficients of hydrocarbons and membrane permeabilities. Because there was no previous standard flexible force field for ZIF frameworks, a classical force field was developed based on Density Functional Theory (DFT) calculations capable of accurately predicting small molecule diffusivities. In a joint experimental-computational collaboration, I aided in the development of a protocol for determining the local ordering of the organic linkers in binary mixed-linker ZIFs. This structural knowledge of mixed-linker ZIFs on the unit cell level prompted the creation of a lattice-diffusion model, which was used to qualitatively explain the impact of local ordering on diffusion as well as provide quantitative predictions of diffusion through binary mixed-linker ZIFs. This work enhances scientific knowledge on molecular transport in single and mixed-linker ZIFs and provides energy suppliers with the tools to engineer new separation alternatives of light gases.
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