On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO2 Adsorption

Published

March 9, 2017

Author(s)

Trong D. Pham, Matthew Ross Hudson, Craig Brown, Raul F. Lobo

Abstract

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.

Citation

Chemistry & Sustainability Energy & Materials

Volume

10

Issue

5

Pub Type

Journals

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Keywords

diffraction, crystallography, zeolites, carbon dioxide, adsorption

Citation

Pham, T. , Hudson, M. , Brown, C. and Lobo, R. (2017), On the Structure-Property Relationships of Cation-Exchanged ZK-5 Zeolites for CO₂ Adsorption, Chemistry & Sustainability Energy & Materials, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=922126 (Accessed April 26, 2024)