Effect of framework Si/Al ratio on the CO₂ adsorption mechanism on cesium-exchanged phillipsite zeolites

Abstract

Small-pore zeolites are of technological relevance to selectively adsorb CO2 from fossil fuel flue gas, as well as from natural gas. Very recently, we have shown that CO2 adsorption on the cesium-exchanged form of the channel-based small-pore zeolite phillipsite with a framework Si/Al ratio of 2.5 (Cs-PHI-2.5) operates by a new mechanism that includes Cs+ ion crowding and subsequent dispersal at a critical CO2 pressure. Here we compare the CO2 adsorption behavior at 25 - 45 degrees C up to 1.0 bar of four Cs-PHI zeolites with different Si/Al ratios (2.1-3.6) and their dehydrated and CO2-loaded structures to elucidate the effect of framework Al (or extraframework Cs+ ion) content on this unprecedented adsorption mechanism. Structural analysis shows that when the number of Cs+ ions per PHI unit cell with 16 tetrahedral atoms lies between 4.9 (Si/Al = 2.1) and 4.5 (Si/Al = 2.5), the CO2 adsorption properties of the resulting zeolites can be described by the cation crowding mechanism. However, when the number of Cs+ ions is 4.0 or smaller (Si/Al >= 3.1), CO2 adsorption is already dominated by the cooperative cation gating-framework breathing mechanism. We provide evidence for the existence of a threshold number of Cs+ ions per t-phi cage in Cs-PHI to manifest the cation crowding mechanism.