CO2 adsorption using CHA zeolite

https://doi.org/10.1039/C3RA48052F

“The zeolite chabazite (CHA – shown in Fig. 1) is one of the most studied zeolites with narrow pore windows. It has a highly accessible porous framework of the 8-ring class with exchangeable cation sites. It exists naturally but can also be synthesized.⁴¹ As the cations can very easily be exchanged, many forms of zeolite chabazite exist. Barrer and associates studied the sorption properties of natural chabazite in detail.^{8,10–12,17,41} Zeolite chabazite can occlude and separate molecules by their size. This property first shown by Barrer and Ibbiston.¹⁷ Zeolite chabazite occluded small straight chain hydrocarbons but branched hydrocarbons were completely excluded. This separation ability was due to the narrow pore windows of chabazite (0.38 × 0.38 nm).^42,43 More of zeolite chabazite’s ability to separate different gas molecules was demonstrated further by Janák et al.⁴⁴ and many of the work from Webley’s group.^27,45,46 Webley and associates observed that CO₂ adsorbed significantly more on all their zeolite chabazite samples when compared with N₂ and CH₄.²⁷ Zeolite chabazite in its K⁺ form (K-CHA) had enhanced ability to separate CO₂ from N₂ and CH₄. The CO₂ uptake of K-CHA, Na-CHA and K-CHA at 113 kPa (273 K) was around 5 times higher than the N₂ uptake (Fig. 2). Furthermore, at low pressures (1.0 kPa), this ratio (CO₂ adsorbed: N₂ adsorbed, per cavity) reached over 300 : 1 for K-CHA. They attributed this finding to the fact that CO₂ molecules could penetrate into the windows at low pressures, but the larger N₂ was essentially blocked by the big K⁺ cation.”

“In terms of the capacity to adsorb CO₂, previous literature shows that zeolites chabazite generally has a high capacity. Inui et al.⁴⁷ showed that under pressure swing adsorption (PSA) conditions, zeolite chabazite had high uptake of CO₂ (∼3.5 mmol g⁻¹) and low irreversible uptake at high pressures (up to 1.1 MPa). Watson et al.⁴⁸ demonstrated that the uptake of CO₂ of a natural version of zeolite chabazite could reach over 5 mmol g⁻¹ at a high pressure (3 MPa, 305 K). Na-CHA and Li-CHA both showed high uptake of CO₂.⁴⁵ The equilibrium uptake of CO₂ at 120 kPa (273 K) was ∼4.4 mmol g⁻¹ and 4.5 mmol g⁻¹ for Na-CHA and Li-CHA, respectively. K-CHA, Mg-CHA and Ca-CHA showed CO₂ uptake of ∼4.0 mmol g⁻¹ under the same conditions.⁴⁵ Ba-CHA showed a slightly lower uptake of CO₂ (∼3.5 mmol g⁻¹) under those conditions. The uptake of CO₂ at low and close to zero loading was higher on Ba-CHA than on Li-CHA. The high uptake at low pressures may be related to the strong cation-quadrupole interaction for Ba²⁺ cation and CO₂. This trend illustrates that the cation charge density, the electrical field gradients of the material and the interaction with the quadrupole moment of CO₂ all are important. The original study (Zhang et al.⁴⁵) gave detailed analysis into these observations. Zhang et al.⁴⁵ also examined the CO₂ isotherms of different ion exchanged chabazite materials in detail. They considered the dependence of the enthalpy of CO₂ sorption on the cation. For Li-CHA and Na-CHA, the enthalpy of CO₂ sorption increased with increased loading. Their findings agreed with the suggested explanations for the uptake dependencies on the cations, at different pressures. For Ba-CHA, Mg-CHA and K-CHA, the enthalpy of CO₂ sorption dropped at high loading. This was rationalized and related to a decrease of the cation-quadrupole interaction, and that the sorbate–sorbate interaction in these materials was not dominant. In the case of Ca-CHA, the enthalpy of CO₂ sorption stayed fairly constant with an increased loading, indicative of a balanced contribution from sorbate–sorbate interaction and cation-quadrupole interaction. These findings were corroborated by the high uptake of CO₂ observed on Ba-CHA at very low pressures of CO₂.”