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Characterization of the Adsorption of CO2 In Metal-Organic Framework and Zeolitic-Imidazolate Framework Materials by Density Functional Theory Calculations

Rees B. Rankin, Department of Chemical and Petroleum Engineering, University of Pittsburgh (and National Energy Technology Laboratory I.A.E.S.), Benedum Hall #1123, University of Pittsburgh, Pittsburgh, PA 15261 and J. Karl Johnson, Department of Chemical and Petroleum Engineering, University of Pittsburgh, 1242 Benedum Hall, Pittsburgh, PA 15261.

Recent years have seen an increased attention in the development of advanced materials for the separation and storage of gas mixtures. Specific focus to date has been placed on novel groups of crystalline nanoporous materials such as Metal-Organic Frameworks (MOFS) and Zeolitic-Imidazolate Frameworks (ZIFS). These materials offer potentially high surface area, high purity, and can be functionally-tailored by selection and modification of the metal, ligand, or linker groups in the material structure, all at a comparatively low cost. Although MOF and ZIFS can be synthesized at relatively low cost, an experimental-screening based approach for performance characterization can be overly daunting; due to the combinatorial potential of materials that can form MOFs/ZIFs, the number of possible MOF/ZIF like materials is immense. One critical component in the design of novel MOF/ZIF like materials for selected applications is an atomic level understanding of the interactions between the framework and the gaseous adsorbate(s); specifically, the nature of changes in adsorption site and adsorption energy between similar framework materials remains largely unknown. Much attention has been recently paid to theoretical modeling of H2/MOF interactions in the literature; results reported reveal weak binding energies of 1-5 kcal/mol. However, it is suggested experimentally that some MOFs and ZIFs can potentially bond much more energetically with other gaseous adsorbates such as CO2.

To date there has significantly little research focus in the literature to apply quantum-mechanics based theoretical calculations and models to quantify the energy-structure dependence in the bonding of CO2 with these materials. In this work we present fully-periodic DFT calculations performed to quantitatively characterize and contrast the bonding of CO2 in the materials CuBTC, ZIF-68, ZIF-69, and ZIF-70. We began by calculating the fully relaxed bulk crystal structure of these materials based on the proposed structures from experimental crystallography. In most cases, minor expansions of the lattice constant, on the order of 0.5-1.0% are observed, accompanied by minor (measured by RMS analysis) atomic relaxations away from the crystallographic positions. We have also examined the effects induced on the crystal structure by defect/vacancy formation in the ideal material. We then performed calculations to examine the nature of CO2 bonding sites and bonding energy in these materials. We present a comparison of the results of these calculations to help determine the relationships between CO2 bonding in these materials associated with material purity, structure, and composition.