Biogas upgrading through kinetic separation of carbon dioxide and methane over Rb- and Cs-ZK-5 zeolites

Abstract

Eight-membered ring (8 MR) zeolites hold large potential for industrial CO₂ separations such as biogas separation. They offer large selectivity due to the constrained environment for adsorption, especially when large cations are present in the interconnecting windows. The Rb- and Cs-exchanged ZK-5 zeolites (8 MR KFI type zeolites) were studied for kinetic CO₂/CH₄ separation. First, Rb-ZK-5 and Cs-ZK-5 were thoroughly characterized via chemical analysis, argon porosimetry, X-ray diffraction and Rietveld refinements. Afterwards, the CO₂/CH₄ separation potential of both adsorbents was assessed via the measurement of kinetic and equilibrium data (T = 261.15 - 323 K), breakthrough measurements at 303 K (P = 1 - 8 bar), and simulations of their performance. 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 at 1 bar and 303 K). Despite their very large CO₂ selectivities, the performance of Rb-ZK-5 and Cs-ZK-5 was still lower than for the benchmark 13X zeolite on a larger scale. Future research needs to further unravel the adsorption mechanism on low-silica 8 MR zeolites and their corresponding potential in separation processes such as biogas purification.