On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO2 Adsorption
Trong D. Pham, Matthew Ross Hudson, Craig Brown, Raul F. Lobo
Molecular understanding of guest-host interactions is critical for improving adsorbent performances in carbon capture and storage applications. The CO2 adsorption properties of cation-exchanged Li-, Na-, K-, and Mg-ZK-5 zeolites is measured at 283 K, 303 K, 323 K, and 343 K up to 1 bar pressure, and correlated to the molecular structures determined by Rietveld refinement of the dehydrated zeolites and the CO2 dosed samples using synchrotron powder X-ray diffraction. Li-ZK-5 exhibited the highest adsorption capacity at 1 bar with with 32 mol CO2/uc at 283 K. Li-, K-, and Na-SK-5 all exhibit high heats of adsorption at low CO2 coverage, with Na-ZK-5 having the highest initial isosteric heat of adsorption, 49 kJ/mol. Mg-ZK-5 has lower overall uptake capacity. It is determined that Mg2+ is located at the center of the zeolite hexagonal prism, with the cation inaccessible to CO2, leading to a much lower initial isosteric heat of adsorption (~30 kJ/mol). Multiple CO2 adsorption sites are identified at a given CO2, loading amount for all four cation-exchanged ZK-5 adsorbents. 'Site A', at the flat 8-membered ring windows, and 'site B/B*' in the γ-cages, are the primary adsorption sites in cation-exchanged Li- and Na-ZK-5 zeolites. Relatively strong dual-cation adsorption sites contribute significantly to an enhanced electrostatic interaction for CO2 in all the ZK-5 samples. This interaction gives rise to a migration of Li+ and Mg2+ cations out from their original locations at the center of the hexagonal prisms, towards the α-cages where they interact more strongly with the adsorb CO2.
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On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO<sub>2</sub> Adsorption, Chemistry & Sustainability Energy & Materials, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=922126
(Accessed June 25, 2021)