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Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites vertical bar Na-12 vertical bar-A and vertical bar Na12-xKx vertical bar-A

Artikel i vetenskaplig tidskrift
Författare N. Hedin
P. Rzepka
A. B. Jasso-Salcedo
T. L. Church
Diana Bernin
Publicerad i Langmuir
Volym 35
Nummer/häfte 40
Sidor 12971-12978
ISSN 0743-7463
Publiceringsår 2019
Publicerad vid Svenskt NMR-centrum vid Göteborgs universitet
Sidor 12971-12978
Språk en
Länkar dx.doi.org/10.1021/acs.langmuir.9b0...
Ämnesord pfg nmr diffusion, mfi-type zeolites, high-silica ddr, a-zeolites, nanoporous materials, small hydrocarbons, stimulated echo, methane, growth, co2, Chemistry, Materials Science
Ämneskategorier Kemi, Kemiteknik

Sammanfattning

Carbon dioxide must be removed from biogas or natural gas to obtain compressed or liquefied methane, and adsorption-driven isolation of CO2 could be improved by developing new adsorbents. Zeolite adsorbents can select CO2 over CH4, and the adsorption of CH4 on zeolite vertical bar Na12-xKx vertical bar-A is significantly lower for samples with a high K+ content, i.e., x > 2. Nevertheless, we show, using H-1 NMR experiments, that these zeolites adsorb CH4 after long equilibration times. Pulsed-field gradient NMR experiments indicated that in large crystals of zeolites vertical bar Na12-xKx vertical bar-A, the long-time diffusion coefficients of CH4 did not vary with x, and the upper limit of the mean-square displacement was about 1.5 mu m, irrespective of the diffusion time. Also for zeolite vertical bar Na-12 vertical bar-A samples of three different particle sizes (similar to 0.44, similar to 2.9, and similar to 10.6 mu m), the upper limit of the mean-square displacement of CH4 was 1.5 mu m and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion restriction for CH4 within the medium- and large-sized zeolite A crystals and possibly of clustering and close contact among the small zeolite A crystals. The upper limit of the long-time diffusion coefficient of adsorbed CH4 was (at 1 atm and 298 K) about 10(-10) m(2)/s irrespective of the size of the zeolite particle or the studied content of K+ in zeolites and vertical bar Na-12 vertical bar-A. The T-1 relaxation time for adsorbed CH4 on zeolites vertical bar Na12-xKx vertical bar-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH4 was hindered.

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