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 CO 2 could be further advanced by developing for example new adsorbents. Zeolite adsorbents can select CO 2 over CH 4 , and we here confirmed that the adsorption of CH 4 on zeolite |Na 12- x K x |-A was significantly lower for samples with a high K + content, i.e. x > 2. Nevertheless, these samples adsorb CH 4 after long equilibration times as determined with 1 H NMR experiments. To assess further the intracrystalline diffusion of CH 4 in these zeolites, pulsed-field gradient NMR experiments were performed. In large crystals of zeolites |Na 12- x K x |-A, the long-time diffusion coefficients of CH 4 did not vary with x , and the mean square displacement was about 1.5 mm irrespective of the diffusion time. Also for zeolite |Na 12 |-A samples of three different particle sizes (~0.44, ~2.9, ~10.6 mm), the mean-square displacement of CH 4 was 1.5 mm and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion restriction for CH 4 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 CH 4 was (at 1 atm and 298 K) about 1 ´ 10 –10 m 2 /s irrespective of the size of the zeolite particle or the studied content of K + in zeolites |Na 12- x K x |-A and |Na 12 |-A. The T 1 relaxation time for adsorbed CH 4 on zeolites |Na 12- x K x |-A with x > 2 was smaller than for those with x <2, indicating that the short-time diffusion of CH 4 was hindered.