(479c) Carbon Dioxide Separation Using Highly Selective Narrow Pore Zeolite ZK-4
AIChE Annual Meeting
Wednesday, November 6, 2013 - 1:06pm to 1:24pm
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.