(553e) Ion-Exchange in FAU-Y Binder-Free Zeolites for Post-Combustion CO2 Capture

Ion-exchange on zeolites can offer enhanced adsorption mechanisms with impact for post-combustion CO₂ capture (PCC) applications. Zeolites exhibit substantial promise in the realm of carbon capture due to their highly favorable CO₂ adsorption isotherms, rapid kinetics, non-toxicity, and cost-effectivness. In comparison to alternative materials such as activated carbons (ACs), carbon molecular sieves (CMS), and metal-organic frameworks (MOFs), zeolites offer a more economically viable solution, with an average price at approximately $1.5∙kg^-1, compared to $1.6∙kg^-1, $8∙kg^-1, and $500∙kg^-1, respectively.

Accordingly, here we investigate the effect of ion-exchange (K⁺ and Ca²⁺) on the bare NaY zeolite for PCC applications. The resulting binder-free adsorbents include K(23)Y, K(56)Y, K(95)Y, Ca(56)Y, and Ca(71)Y were also compared with commercial binder-free 13X. The studies include the measurement of CO₂ and N₂ adsorption isotherms and respective selectivity, via single and binary fixed bed breakthrough experiments between 306 K to 344 K and pressures up to 350 kPa. Figure 1 shows the experimental breakthrough apparatus employed for the study. The adsorption equilibrium data was modeled using the extended dual-site Langmuir (DSL) model, and numerical simulations of breakthrough curves conducted using Aspen Adsorption v10.

Figure 2 shows the CO₂ and N₂ adsorption isotherms along with the respective selectivities at 306 K including for the bare binder-free 13X (Si/Al=1.18) and NaY (Si/Al=2.5). The CO₂ isotherm (Figure 2a) highlights the superior adsorption capacity of binder-free 13X compared to all binder-free Y zeolites. Higher Al content in 13X leads to greater heterogeneity and intensified electrostatic fields resulting in higher adsorption capacities. At 150 kPa and 306 K, the CO₂ loading for 13X is 6.48 mol∙kg^-1, surpassing that of NaY by 5.43%, K(23)Y by 2.04%, K(58)Y by 7.75%, K(95)Y by 5.93%, Ca(56)Y by 33.9%, and Ca(71)Y by 59.2%.

However, despite its potential for high CO₂ adsorption capacity, 13X zeolite exhibits poor moisture tolerance due to its lower Si/Al ratio, leading to reduced CO₂ capture efficiency in the presence of water. This is because polar H₂O molecules are preferentially adsorbed onto exchangeable cations, weakening the electric field and promoting bicarbonate formation. Increasing the Si/Al ratio, as seen in Y zeolites, enhances catalytic and hydrothermal stability, as well as under flue gas and adsorption operation conditions. Y-zeolites have found industrial applications due to the stability of their crystal structure, as well as substantial available pore volume and surface area.

In Y zeolites, the observed sorption hierarchy at low pressures (up to 50 kPa) is: Ca(71)Y < Ca(56)Y < NaY < K(23)Y < K(58)Y < K(95)Y; across all studied temperature ranges, as illustrated in Figure 2a. For instance, at 25 kPa and 306 K, the CO₂ loading for bare NaY is 4.05 mol∙kg^-1, increasing to 4.29 for K(23)Y, 4.59 for K(58)Y, and peaking at 4.72 for K(95)Y, representing a 16% improvement compared to bare NaY. Conversely, Ca(56)Y exhibits a loading of 2.63 mol∙kg^-1, and Ca(71)Y records a lower loading of 2.01 mol∙kg^-1, indicating that Ca(71)Y adsorbs less than half the amount compared to commercial NaY. At pressures above 200 kPa, K(23)Y and NaY demonstrate the highest CO₂ loadings, succeeded by K(58)Y and K(95)Y. Smaller exchangeable cations positively impact CO₂ adsorption uptake, establishing stronger ion-quadrupole interactions with CO₂.

The N₂ isotherms, displayed in Figure 2b, exhibit a more linear behavior compared to CO₂ and follow a sorption hierarchy similar to that of CO₂, especially at low pressures, but with considerably lower recorded loadings. At 150 kPa and 306 K, the observed N₂ loadings are as follows: 0.63 mol∙kg^-1 for 13X, 0.43 for K(95)Y, 0.39 for K(58)Y, 0.37 for NaY, 0.35 for K(23)Y, 0.32 for Ca(56)Y, and 0.28 mol∙kg^-1 for Ca(71)Y. This trend suggests that Y-type zeolites containing smaller monovalent cations, such as binder-free K(23)Y and NaY, exhibit slightly lower N₂ uptake compared to other K⁺ exchanged adsorbents, potentially contributing to higher CO₂/N₂ selectivities and enhancing CO₂ separation efficiency in industrial applications.

Analysis of the CO₂/N₂ selectivity profile in Figure 2c reveals consistent superior performance by binder-free zeolite K(23)Y across various conditions. For instance, at 308 K and 50 kPa, the CO₂/N₂ selectivity for K(23)Y is 41, surpassing those of 13X (25), NaY (38), K(58)Y (35), K(95)Y (28), Ca(56)Y (26), and Ca(71)Y (24). This sustained elevation highlights K(23)Y’s notable proficiency in preferentially adsorbing CO₂ over N₂, suggesting potential applicability in PCC processes.

Binary breakthrough experiments were conducted on all binder-free FAU zeolites under standard PCC conditions, using a gas mixture comprising 15% CO₂ and 85% N₂ (mol. %) at 101.3 kPa and 308 K. The CO₂ separation efficiency was evaluated through various adsorbent metrics, including CO₂ loadings, selectivities, and working capacities. The results indicate the highest binary CO₂ loading on binder-free 13X with 4.69 mol∙kg^-1. However, the CO₂/N₂ selectivities are higher for K⁺ ion-exchange samples, with binder-free K(23)Y showing the highest binary selectivity with 101, followed by K(58)Y with 97, compared to 89 for NaY and 70 for 13X.

Furthermore, working capacity values for CO₂ were calculated across a range of Vacuum Swing Adsorption (VSA) PCC processes, considering regeneration pressures of 3, 10, and 15 kPa, relative to a feed pressure of 101.3 kPa, as shown in Figure 3. In these calculations, a CO₂ molar fraction of 15% in the feed and 90% during regeneration was assumed to align with realistic operational conditions. The results demonstrate superior performance of K⁺ exchanged adsorbents, notably binder-free K(23)Y, over bare 13X and NaY across most conditions. In the pressure range of 3 to 101.3 kPa, binder-free K(23)Y exhibits the highest CO₂ working capacity at 2.37 mol∙kg^-1, surpassing NaY by 15.2%, 13X by 24.9%, K(58)Y by 7.17%, K(95)Y by 5.06%, Ca(56)Y by 46.8%, and Ca(71)Y by 55.3%. These findings highlight the potential of ion-exchange to enhance adsorption processes, with binder-free K(23)Y emerging as a promising adsorbent, particularly when compared to the bare commercial 13X for PCC applications.

Figure 4 presents the simulated and experimental breakthrough curves and temperature fronts for binary CO₂/N₂ adsorption in binder-free K(23)Y at 306 K. The simulated data with Aspen Adsorption effectively reproduces the dynamic data for both concentration and temperature profiles, being a valuable tool for the design of cyclic adsorption processes using FAU-Y zeolites in PCC applications.