Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH ₄ in Zeolites |Na ₁₂ |-A and |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 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 m2/s irrespective of the size of the zeolite particle or the studied content of K+ in zeolites |Na12–xKx|-A and |Na12|-A. The T1 relaxation time for adsorbed CH4 on zeolites |Na12–xKx|-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH4 was hindered.