Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites |Na12|-A and |Na12-xKx|-A

Removing carbon dioxide is important for the upgrading of biogas or natural gas into compressed or liquefied methane, and adsorption-driven separation of CO₂ could be further advanced by developing for example new adsorbents. Zeolite adsorbents can select CO₂ over CH₄, and we here confirmed that the adsorption of CH₄ on zeolite |Na_12-xK_x|-A was significantly lower for samples with a high K⁺ content, i.e. x > 2. Nevertheless, these samples adsorb CH₄ after long equilibration times as determined with ¹H NMR experiments. To assess further the intracrystalline diffusion of CH₄ in these zeolites, pulsed-field gradient NMR experiments were performed. In large crystals of zeolites |Na_12-xK_x|-A, the long-time diffusion coefficients of CH₄ did not vary with x, and the mean square displacement was about 1.5 mm irrespective of the diffusion time. Also for zeolite |Na₁₂|-A samples of three different particle sizes (~0.44, ~2.9, ~10.6 mm), the mean-square displacement of CH₄ was 1.5 mm 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 long-time diffusion coefficient of adsorbed CH₄ was (at 1 atm and 298 K) about 1 ´ 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.