374326 Separation of Carbon Dioxide and Methane over Rb- and Cs-ZK-5 Zeolites
Eight-membered ring (8 MR) zeolites hold large potential for industrial CO2 separation such as biogas separations. They offer large selectivity due to the constrained environment for adsorption, especially when large cations are present in the interconnecting windows [1-5]. The relatively small cages and windows of these zeolites increase the interaction strength between the adsorbent and CO2. At the same time, the diffusion of the slightly larger adsorbates CH4 and N2 through the 8 MR windows can be hindered. As a result, simulations predicted the highest CO2/CH4 selectivities in 8 MR structures among the different types of zeolites.  Experimental studies have mainly focused on the RHO, LTA, CHA and KFI structures up to now [7-12].
In the present work, the Rb- and Cs-exchanged ZK-5 zeolites (8 MR KFI type zeolites) were studied for kinetic CO2/CH4 separation. ZK-5 zeolites have the KFI structure, with a slightly higher Si/Al ratio (Si/Al = 3.6). It consists of a three-dimensional network of larger α- cages (11.6 Å in diameter) and smaller γ-cages (6.6 Å x 10.8 Å). The α- and γ-cages are connected through flat eight- membered rings with a diameter of 3.9 Å. A puckered eight-membered ring with a smallest diameter of 3.0 Å connects the γ-cages with each other.
Rb-ZK-5 and Cs-ZK-5 were thoroughly characterized via chemical analysis, argon porosimetry, X-ray diffraction and Rietveld refinements. These refinements showed that a majority of the 8 MR sites were filled with cations. Afterwards, the CO2/CH4 separation potential of both adsorbents was assessed via the measurement of kinetic and equilibrium data (T = 261.15 - 323 K), adsorption and desorption breakthrough measurements at 303 K (P = 1 - 8 bar), and simulations of their performance at lab-scale and in a model pressure swing adsorption (PSA) process. The high occupation of the central 8 MR sites with large cations causes strong diffusional limitations for CH4 on Rb-ZK-5 and Cs-ZK-5. As a result, both zeolites effectively separate CH4 from CO2 with very high selectivities (α = 17). A disadvantage for Cs-ZK-5 is the occurrence of mass transfer limitations for CO2, yielding lower mass transfer coefficients on Cs-ZK-5 compared to Rb-ZK-5 and a large part of the bed being unused for separation. The global performance of both adsorbents will be compared to a benchmark 13X zeolite.
Tom Rémy and Leen Van Tendeloo acknowledge FWO- Vlaanderen for financial support. Joeri F.M. Denayer acknowledges FWO-Vlaanderen for the 1.5.280.11N research grant. Elena Gobechiya and Christine E.A. Kirschhock acknowledge the Belgian Prodex Office and ESA for financial support. Johan A. Martens and Christine E.A. Kirschhock acknowledge the Flemish Government for long-term structural funding, Methusalem.
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