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

Abstract

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.