CO2 capture using zeolite A

https://doi.org/10.1039/C3RA48052F

“The high electrical field gradients of zeolite A may also be responsible to its relatively high uptake of CO₂. In an early study by Harper et al.,⁵² the capacity to adsorb CO₂ on zeolite NaA was found to be ∼6.7 mmol g⁻¹ at saturation. Those adsorption measurements were carried out at a temperature of 194 K (where CO₂ saturation occurs at atmospheric pressure). At 273 K, they observed that the capacity to adsorb CO₂ was still as high as 4.1 mmol g⁻¹ (101 kPa). Bae et al.²⁹ evaluated a range of different cation exchanged zeolite A for their CO₂/N₂ separation potential. They found that at the relevant pressure range, Ca²⁺ exchanged zeolite A (CaA) had an impressively high CO₂ uptake (∼5.0 mmol g⁻¹, 298 K) and a CO₂/N₂ selectivity of 250 (predicted by the authors using the ideal adsorption solution theory – IAST). They compared their results with Mg-MOF-74 and found that CaA had a higher volumetric uptake of CO₂ (0.15 bar CO₂, 313 K), higher working capacity (based on their TSA study) as well as a longer breakthrough time than MOF-74. Palomino et al.⁵³ tested zeolite A with high Si : Al ratios (up to 5) and observed that the capacity to adsorb CO₂ varied with the Si : Al ratio. They observed that the CO₂ uptake at 500 kPa (303 K) was the highest for an intermediate Si : Al = 2 : 1. The CO₂ uptake was lower on zeolite A with both lower and higher Si : Al ratios than for a ratio of 2. In addition, they observed that the isosteric heat of CO₂ adsorption (up to 2.5 mmol g⁻¹ loading) decreased with an increasing Si content. At high Si content, the regenerability of the zeolite A sorbent increased because of the lower heat of CO₂ adsorption.⁵³ The difference in the heat of adsorption is possibly due to the different number of cations in the zeolite, as CO₂ tends to adsorb more strongly at high energy sites close to the cations (discussed in more details later). Palomino et al.⁵³ also found that the heat of CH₄ adsorption was not significantly affected by the difference in Si content, but the CO₂/CH₄ selectivity was reduced with increasing Si content. Inui et al.⁴⁷ highlighted the high capacities to adsorb CO₂ (3–4 mmol g⁻¹, at 1.0–1.2 MPa) of zeolite NaA and CaA in an independent study. Due to the high electrical field gradients, the enthalpy of CO₂ sorption on zeolite A is high. Bae et al.²⁹ found that CaA had a noticeably higher heat of CO₂ adsorption than NaA, and MgA. The heat of CO₂ adsorption on CaA was around 60 kJ mol⁻¹ at low loading, and decreased to around 30 kJ mol⁻¹ with a loading of around 4 mmol g⁻¹. They attributed the high heat of CO₂ adsorption to the large number of accessible strong adsorption sites. Delaval and de Lara⁵⁴ showed that CO₂ physisorption on zeolite 4A had an enthalpy of around 50 kJ mol⁻¹ at zero loading. The enthalpy change reduced with increased loading down to ∼44 kJ mol⁻¹. We previously observed that the enthalpy of CO₂ physisorption on zeolite NaKA was around 37 kJ mol⁻¹ at nonzero loading.⁵⁵ The low value we observed may be due to the presence of the big K⁺ cations.

As mentioned, the window size of zeolite A can essentially be further adjusted by ion exchange. We recently demonstrated that partially K⁺ ion exchanged zeolite NaKA had pore sizes between 0.3 and 0.4 nm.²⁸ Using this feature of zeolite A, we were able to produce zeolite NaKA with 17% of the cations being K⁺, 83% being Na⁺. This zeolite, with the reduced pore size, was able to exclude N₂ from sorption onto the material (<0.01 mmol g⁻¹, 273 K, 101 kPa). The CO₂-over-N₂ relative uptake of the material reached over 200. The CO₂ capacity of this highly selective zeolite NaKA remained high (3.5 mmol g⁻¹, 273 K, 101 kPa). Mace et al.⁵⁶ suggested that the high selectivity was not solely due to the bigger cation blocking the bigger sorbates. They concluded that the difference in mobility between Na⁺ and K⁺ and the higher interaction with CO₂ allowed CO₂ to enter the pores (when the material is not fully K⁺ exchanged). Other sorbates, such as N₂, did not have the ability to do so.

The exclusion of N₂ from sorption on zeolite NaKA appeared to be related to its large effective kinetic diameter (0.36 nm). Further reducing the pore window size of zeolite NaKA with additional K⁺ ions in the 8-ring, will make the apertures too narrow for CO₂ to pass through, as the effective kinetic diameter of CO₂ is about ∼0.33 nm. However, we observed significant capacities to adsorb CO₂ also for zeolite NaKA with a high content of K⁺.²⁸ Different mechanisms have since been proposed to rationalize this unexpected phenomenon. Larin et al.⁵⁷ suggested that chemical reactions of CO₂ with the framework atoms would lead to carbonate formation on the K⁺ cations near the 8-rings (as K₂CO₃ with one other K⁺ cation). They proposed that such carbonates would reposition the K⁺ atoms away from the window aperture. This would have resulted in a wider opening for CO₂ to enter subsequently. Webley and associates,^49,50 although did not study zeolite A explicitly, proposed “molecular trap door” mechanism for CO₂ entering pores of chabazite when the material had been K⁺ exchanged. They stated in their conclusion that they expected to find a similar mechanism on zeolite LTA. The KCHA in their study also had pores that were theoretically blocked for CO₂ to enter. As discussed earlier, they proposed that CO₂ can interact and shift the position of the cation, allowing itself to enter the pores. Recently, Mace et al.⁵⁸ presented a procedure using ab initio molecular dynamics calculations to access the details of the free energy barriers for diffusion of small gas molecules through 8-ring zeolite windows. By introducing certain spatial constraints, the gas molecule could be steered towards the “rare event” of the diffusion through the pore window of interest, without losing other relevant degrees of freedom. In this work, using this procedure, the free energy barriers of diffusion for CO₂ and N₂ in zeolite NaKA were estimated, investigating the differential molecular sieving effect of the two cation types, Na⁺ and K⁺, without involving either chemisorption or explicit “molecular trap door” mechanisms. The results were in good qualitative agreement with the experimental results presented by Liu et al.²⁸ showing a drastic increase in the energy barrier for CO₂ or N₂ to pass a K⁺ blocked pore window compared to a Na⁺ blocked one, hence strongly supporting the idea of a tunable sieving effect through ion exchange.

The molecular details of CO₂ sorption on zeolite A have also been studied. Jaramillo and Chandross⁵⁹ studied CO₂ (physi-)sorption using Gibbs ensemble Monte Carlo simulations. They suggested that CO₂ sorption at low pressures occurred at a single cation Na⁺ site around the 6-ring windows. CO₂ next adsorbed on a second site where it coordinated with Na⁺ from both the 6- and 8-rings. Finally, CO₂ adsorbed on a third site where it coordinated to 3 Na⁺ cations (4-, 6- and 8-rings). Other evidence of CO₂ adsorption on different sites depending on (CO₂) pressure (or loading) was presented in a study by Delaval and de Lara,⁵⁴ as well as our recent study on nano-sized zeolite A.⁵⁵ These studies involved infrared spectroscopy and observed that the characteristic band for physisorbed CO₂ (ν₃ – asymmetric stretching vibration mode) occurred at a higher frequency (2352 cm⁻¹) and downshifted to a lower frequency up on further loading of CO₂. Density Functional Theory (DFT) studies showed that sorption of CO₂ at a low coverage (1 CO₂/α cage) occurred with CO₂ bridging between 2 or 3 cation, irrespective of the size of the cation.⁵⁵ Bae et al.²⁹ determined using a neutron diffraction technique that at low loading of CO₂, there were two adsorption sites on CaA. One of these two sites was located close to the 6-rings where CO₂ could interact with two Ca²⁺ (site A), the other site (site B) was located in the center of the 8-rings (Fig. 4).”