Intracrystalline Transport Barriers Affecting theSelf-Diffusion of CH 4 in Zeolites |Na 12 |‑Aand |Na 12– x K x |‑A

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 |Na12–xKx|-A is significantly lower for samples with a high K+ content, i.e., x > 2. Nevertheless, we show, using 1H NMR experiments, that these zeolites adsorb CH4 after long equilibration times. Pulsed-field gradient NMR experiments indicated that in large crystals of zeolites |Na12–xKx|-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 μm, irrespective of the diffusion time. Also for zeolite |Na12|-A samples of three different particle sizes (∼0.44, ∼2.9, and ∼10.6 μm), the upper limit of the mean-square displacement of CH4 was 1.5 μm and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion ...