NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures
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NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures Anne-Kristin Pusch · Tobias Splith · Lutz Moschkowitz · Shilpi Karmakar · Rajesh Biniwale · Marco Sant · Giuseppe B. Suffritti · Pierfranco Demontis · Janosch Cravillon · Evangelia Pantatosaki · Frank Stallmach
Received: 22 May 2012 / Accepted: 31 August 2012 / Published online: 20 September 2012 © Springer Science+Business Media, LLC 2012
Abstract Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by 1 H and 13 C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.
A.-K. Pusch · T. Splith · L. Moschkowitz · F. Stallmach () Faculty of Physics and Earth Sciences, University Leipzig, Linnestraße 5, 04103 Leipzig, Germany e-mail: [email protected] S. Karmakar · R. Biniwale National Environmental Engineering Research Institute (NEERI), Nagpur, India M. Sant · G.B. Suffritti · P. Demontis Department of Chemistry, University of Sassari, Sassari, Italy J. Cravillon Institute of Inorganic Chemistry, University Hannover, Hannover, Germany E. Pantatosaki School of Chemical Engineering, National Technical University of Athens, Athens, Greece
Keywords ZIF-8 · Self-diffusion · Carbon dioxide · Methane · NMR · Molecular dynamic simulation
1 Introduction Zeolitic Imidazolate Frameworks (ZIF) are a subfamily of metal organic frameworks (MOF) (Park et al. 2006). Generally, ZIF’s are composed of transition metal-clusters and imidazole as organic linkers. They are microporous crystalline materials, which resemble structure types known from inorganic zeolites. Like other MOFs, the ZIFs are characterized by a tunable pore size. For a few of the ZIF structures, high chemical and thermal stabilities of up to 550 ◦ C were reported (Park et al. 2006; Basu et al. 2011). This opens up a wide field for potential applications as high-tech material in adsorption and separation technologies (Wang et al. 2008; Liu et al. 2009, 2011; Venna and Carreon 2010; Bux et al. 2010, 2011). For an optimal characterization of porous materials for gas separation, it is crucial to know the adsorption and transport properties of molecules in the pore system (Li et al. 2009; Ferey et al. 2011; Atci et al. 2011). Experimental methods that are frequently used
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