Ultrathin, fine-tuned microporous coating modified 5A zeolite for propane/propylene adsorptive separation

Adsorptive separation of propylene from propane is very challenging because of their very similar sizes (propylene: 0.431 nm; propane: 0.446 nm) and other similar physical properties. In this study, we further explored the potential of the previously developed concept, pore misalignment, for precisely adjusting pore mouth size of 5A zeolite for propane/propylene adsorptive separation. Pore size of the ultrathin microporous TiO₂ coating on 5A zeolite was fine-tuned from 0.8 to 0.6 nm by combining molecular layer deposition (MLD) and atomic layer deposition (ALD). As a result, average pore mouth size of 5A zeolite was adjusted to be between propane and propylene and an ideal adsorption selectivity of about 22 was achieved. Meanwhile, diffusivity ratio of propylene to propane also increased drastically from about 1 for 5A zeolite to 75 for MLD/ALD modified 5A zeolite. The resulting composite 5A zeolite adsorbent, therefore, favors propylene separation from propane both thermodynamically and kinetically. Our work suggests that MLD combined with ALD might be an effective way of precisely adjusting zeolite pore mouth size for separation of propane/propylene mixtures and potentially for other mixtures composed of molecules with very small size differences.