(479c) Carbon Dioxide Separation Using Highly Selective Narrow Pore Zeolite ZK-4

In recent years, zeolites were comprehensively studied for their potential as CO₂ sorbents in carbon capture and storage (CCS) processes. Zeolites offer high CO₂ capacity, selective CO₂ sorption, high thermal stability and low costs. Zeolite ZK-4, is a silicon rich version of a zeolite, which has the same framework topology as the Zeolite A. Previously, we studied and showed that the Zeolite NaKA could be tuned into a highly CO₂ selective sorbent. ZK-4 with its potential for high Si/Al ratios could potentially alleviate the disadvantages linked to the very hydrophilic nature of zeolite A. Here, we show that a zeolite ZK-4 (Si/Al =1.36) could be tuned into a highly CO₂ selective form by carefully tuning the cation ratio (K⁺/( K⁺+Na⁺))

When the cation ratio of K⁺ content was over ~ 25 atomic %, the sorbent exhibited a molecular sieving of CO₂ over N₂ and was highly selective towards CO₂ (ideal CO₂/N₂ selectivity >200). We ascribe this manifold enhanced selectivity to the specific location of the K⁺ cation in the pore windows, where their comparably larger size hinders the diffusion of N₂ more effectively than for CO₂.  The CO₂ sorption capacity remained high on the CO₂ selective materials (~ 4.1 mmol/g 273K, 101 kPa). The heat of CO₂ physisorption varied between 32 – 45 kJ/mol and was partly related to the K⁺ ion content. The rate of CO₂ physisorption was relatively fast. Infrared spectroscopy and molecular modeling gave an insight into the configuration of adsorbed CO₂ and how the cations affected the sorption of CO₂. In summary, zeolite ZK-4 offers the similar CO₂ adsorption characteristics as zeolite NaKA, but its more hydrophobic nature and the possibility for varying the Si/Al ratio allow higher flexibility for tuning this material for specific application. We are continuing these studies by increasing the Si:Al ratio of ZK-4 until the molecular sieving capacity of CO₂-over-N₂ is lost.