Journal
MICROPOROUS AND MESOPOROUS MATERIALS
Volume 288, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.micromeso.2019.109603
Keywords
MOF; ZIF; Diffusion; PFG NMR; Gas separation
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Funding
- National Science Foundation (NSF) [1561347, CMMI-1561897]
- NSF [DMR-1157490, DMR-1644779]
- State of Florida
- NTH award [S1 OFF031637]
- Qatar National Research Fund (Qatar Foundation) [8-001-2-001]
- Div Of Civil, Mechanical, & Manufact Inn
- Directorate For Engineering [1561347] Funding Source: National Science Foundation
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Self-diffusion of pure gases including carbon dioxide, methane, ethylene, ethane, and xenon as well as selected two-component mixtures was studied in hybrid zeolitic imidazolate framework-7-8 (ZIF-7-8) crystals using pulsed field gradient (PFG) NMR. This material was formed by mixing 2-methylimidazolate (ZIF-8 linker) and bulkier benzimidazolate (ZIF-7 linker) in the same framework. The intracrystalline diffusion data measured in mixed-linker ZIF-7-8 was compared with the corresponding data in the parent ZIF-8 material. It was found that under the same or comparable experimental conditions the intracrystalline gas diffusion was always slower in ZIF-7-8 than in ZIF-8. This observation is consistent with the expected lower pore aperture size in ZIF-7-8 than in ZIF-8. At the same time, the ethane/ethylene diffusion selectivity was found to be similar in both ZIFs. It was also observed that for the pure studied gases larger than carbon dioxide the diffusivity ratios in ZIF-8 and ZIF-7-8 do not increase with increasing gas size at all loading pressures used. All these data are attributed to greater framework flexibility effects in ZIF-7-8 than ZIF-8. Such effects manifest themselves in a distortion and/or increase in the aperture size in the presence of large sorbates due to linker flexibility.
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