Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH₄ in Zeolites |Na₁₂|‑A and |Na_12–xK_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 |Na_12–xK_x|-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 |Na_12–xK_x|-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 |Na_12–xK_x|-A and |Na₁₂|-A. The T₁ relaxation time for adsorbed CH₄ on zeolites |Na_12–xK_x|-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH₄ was hindered.