Dynamic CO2 adsorption using cation-exchanged zeolites MMU-1 and CHA

MMU-1 and chabazite (CHA) zeolites were synthesised using Na and Cs cations as inorganic structure-directing templates and tested as adsorbents to produce CO 2 -free methane in semi-continuous mode from a CO 2 /methane mixture in their as made form or after ion exchange with K. Breakthrough curves showed that all cation exchanged zeolites were active in removing CO 2 with K-exchanged MMU-1 (K-MMU-1) exhibiting a similar performance to K-CHA. CO 2 adsorption capacities were 1.36 mmol g −1 for K-MMU-1 versus 1.48 mmol g −1 for K-CHA; CO 2 -free methane production times were 48.9 versus 52.2 min; and adsorption saturation times were 101 versus 94.8 min, respectively. Cs,Na-MMU-1 produces far less CO 2 -free methane than Cs,Na-CHA but nonetheless removes CO 2 continually from the biogas stream for approximately the same duration. Temperature programmed desorption (TPD) showed that CO 2 is more strongly bound to MMU-1 zeolites than CHA, and a higher bonding strength is observed for the K-forms of both zeolites compared to the Cs,Na-forms, with the effect being more pronounced for MMU-1. These zeolite framework and extra-framework cation synergistic effects in altering the adsorption properties of zeolites are intriguing where, in some cases, a seemingly small change in the zeolite may bring about a disproportionate change in activity. These findings validate the need to continue to explore how the properties and characteristics of small-pore zeolites impact the extent of gas phase reactions, including gas separation. • Zeolite MMU-1 shown to be an excellent adsorbent for biogas upgrading to pure methane in a single adsorption step. • Quantity of CO 2 -free methane produced using K + exchanged MMU-1 (K-MMU-1) comparable to K-CHA. • Adsorption of CO 2 is reversible allowing reusability of zeolite. • CO 2 adsorption activity of K-MMU-1 maintains 94.9% activity following 8 cycles of adsorbent regeneration.