Adjusting the proportions of extra-framework K⁺ and Cs⁺ cations to construct a “molecular gate” on ZK-5 for CO₂ removal

For the capture of CO₂ from flue gas and decarbonization of natural gas streams, an ideal adsorbent should have a high selectivity for CO₂ over N₂ and CH₄. In this work, through adjusting the proportions of extra-framework K⁺ and Cs⁺ cations and varying the Si/Al ratios of the zeolite ZK-5, the small “molecular gate” adsorbent ZK-5-n-K has been prepared. An increase in the number of K⁺ cations prevents the ingress of the larger molecules CH₄ and N₂, while allowing entry of the smaller CO₂; the net result is that CO₂ capture from CH₄ or N₂ is achieved with unusually high selectivity, of the order of 1 × 10⁵. Grand canonical Monte Carlo simulations show that the extra-framework K⁺ and Cs⁺ cations are positioned at different locations in the ZK-5; K⁺ locates near the 8-ring window, whereas Cs⁺ resides in the interior regions. The separation performances of 12 different K⁺- and Cs⁺-doped ZK-5 structures (with four different Si/Al ratios of 3.17, 3.36, 3.46, and 3.57) have been investigated. Transient breakthrough simulations have been used to quantify the CO₂ capture performances in four sets of separations: 30%/70%, 5%/95%, and 2%/98% CO₂/CH₄ mixtures at a total pressure of 1 MPa, and a 15%/85% CO₂/N₂ mixture at a total pressure of 100 kPa. In all cases, the selectivities were sufficiently high to ensure that the separation performance was primarily governed by the CO₂ uptake capacity. The overall conclusion to be drawn from this work is that ZK-5 is a suitable adsorbent for the decarbonization of natural gas.