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

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