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 |Na12-xKx|-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 |Na12-xKx|-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 |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.