Adsorption versus Diffusion in FAU Zeolite

Despite the burgeoning research and development activities on novel metal-organic frameworks (MOFs) for applications in several separation applications, cation-exchanged zeolites remain viable contenders for use as adsorbents in a number of industrially important separations. For post-combustion CO2 capture, a Na+ cation exchanged FAU (= faujasite) zeolite, NaX, also commonly known by its trade name 13X, with Si/Al  1.2, is considered to be the benchmark adsorbent, with the ability to meet the U.S. Department of Energy (US-DOE) targets for CO2 purity and recovery.
Li+ cation exchanged FAU (= faujasite) zeolite, LiX, is commercially used for separation of N2/O2 mixtures. For supplying medical grade oxygen to prevent hypoxemia-related complications related to COVID-19, portable medical oxygen concentrators commonly use LiLSX (LS = low silica; Si/Al  1) in order to achieve high N2/O2 adsorption selectivities, ensuring enhanced rejection of purified O2, the desired product.
For separation applications using pressure swing adsorption (PSA) technology, consisting of adsorption/desorption cycles, there is often a mismatch between the requirements of strong adsorption and ease of desorption. For example, NaX has a very strong affinity for CO2 but the regeneration requires application of deep vacuum.
In the design and development of PSA technologies employing cation-exchanged zeolite adsorbents, we also require data on the intra-crystalline diffusivities of guest molecules. Most commonly, diffusion limitations cause distended breakthrough characteristics and reduction in the purities of the desired products. Diffusivity data are also of vital importance in the development of zeolite membrane constructs for mixture separations in which cation-exchanged zeolites are used as thin layers or as fillers in mixed-matrix configurations. In this presentation, I stress the anti-synergy between adsorption and diffusion in cation-exchanged FAU zeolites.

The background material is provided in my paper
Krishna, R.; Van Baten, J. M. Highlighting the Anti-Synergy between Adsorption and Diffusion in Cation-Exchanged Faujasite Zeolites. ACS Omega 2022, 7, 13050-13056. https://doi.org/10.1021/acsomega.2c00427.