Effect of cation exchange in the sorption of CO2, CH4 AND N2 and mixtures on binder-free faujasite zeolite Y

Ion-exchange was performed on commercial binder-free NaY zeolite with alkali metal and alkaline earth metal cations to produce binder-free beads containing 23, 58 and 95% of potassium, as well as 56 and 71% of calcium exchanged from the bare samples. These cation-exchanged faujasites were studied by adsorption of carbon dioxide (CO2), methane (CH4), and nitrogen (N2) through single, binary, and ternary fixed bed breakthrough experiments, covering the temperature range between 308 and 348 K and pressure up to 350 kPa. The single and multi-component breakthrough apparatus that was used to study the fixed bed adsorption of CO2, CH4, and N2 and their binary/ternary mixture, is illustrated in Figure 1. The dynamic equilibrium loading is calculated by integrating the molar flow profiles of the breakthrough curves, as explained in previous works. The adsorption equilibrium data was then modelled by the extended dual-site Langmuir model, and the breakthrough curves were numerically simulated using Aspen Adsorption v10. Adsorption equilibrium measurements of CO2 on each of the studied material can reveal different behaviours and trends based on the modification of the intracrystalline environment through ion-exchange. Factors such as cation size, surface basicity, number and location of exchangeable cations, and strength of electric field can all have a great impact on the performance of the adsorbent. Figure 2 shows a comparison of the CO2 isotherms between NaY, K(23)Y, K(58)Y, K(95)Y, Ca(56)Y, and Ca(71)Y, collected at 308 K. A trend in the order of adsorption at low pressure (between 0 and 50 kPa) is observed: Ca(71)Y < Ca(56)Y < NaY < K(23)Y < K(58) < K(95)Y. As the exchange rate from Na+ to K+ increases, the CO2 adsorption capacity increases; the opposite is observed (decrease of adsorption uptake) when the rate changes from Na+ to Ca2+. At 25 kPa, the loading of binder-free Na(100)Y is equal to 4.05 mol/kg, compared to 4.29 for K(23)Y, 4.57 for K(58)Y, 4.97 for K(95)Y, 2.63 for Ca(56)Y and only 2.02 mol/kg for Ca(71)Y. This indicates a good response between the acidic CO2 to the basic properties of the zeolites containing larger monovalent cations at low pressure. Bigger cations such as K+ exhibit strong interaction with CO2, since they are both preferentially exchanged in the supercages; while smaller cations such as Na+ have less molecular interaction with the adsorbate molecules, since they are spread around the zeolite framework accessing narrow locations such as the sodalite cages, where CO2 cannot reach due to its size. Moreover, the CO2 loading of Ca(71)Y is significantly lower than all the rest (around half of that of NaY), which is due to the decrease of the amount of exchangeable cations between the divalent Ca2+ cations and the adsorbate molecules. For partial pressures above 200 kPa, K(23)Y and NaY are characterized with the highest adsorption capacity followed by K(95)Y and K(58)Y, then Ca(56) and finally Ca(71)Y, as shown in Figure 2. These trends are explained by the reduction of the basic strength and the electropositivity of exchangeable cations in larger ions, since they accept less charge transfer from the neighboring lattice oxygen atoms when compared to smaller cations. This leads to the weakening of the electric field induced by the exchangeable cations and so the adsorption capacity is reduced. It is also explained by the volume occupied by the large cations, which reduces the space available for adsorption of CO2 when the pores are reaching saturation. The studied binary experiments consist of 15% CO2 / 85% N2 (vol.%) mixture, representing a typical post-combustion stream. Figure 3a shows the adsorption breakthrough curves in binder-free K(95)Y for the binary mixture at 313 K. Figure 3b displays the breakthrough curves for ternary mixtures feeds of CO2 /CH4/ N2 (20/20/20 vol.% balanced with He) on binder-free zeolite KY, under conditions in the range used for biogas upgrading regarding the removal of CO2. As can be seen in Figure 3c, the binary experiment show a selectivity of CO2 over N2 around 105 at 313 K; the ternary system resulted in a selectivity of CO2 over CH4 and over N2 of around 14 and 32 at 313 K, respectively. These results indicate that binder-free K(95)Y works best in the low-pressure region and therefore, is a promising adsorbent for the recovery of CO2 from post-combustion streams. Numerical simulations were performed with a model implemented in Aspen Adsorption simulator, allowing to predict accurate breakthrough curves for dynamic experiments carried out in a fixed bed adsorption system, as shown in Figure 3. Briefly, most of the studied ion-exchanged materials show a lot of potential for the capture of CO2 from CO2/N2 and CO2/CH4/N2 mixtures. Nevertheless, each adsorbent differs from one another and can only reach its full potential under specific conditions. Therefore, it is possible to tune the adsorptive properties of zeolites by ion exchange, to optimize the most suitable material that enriches substantially the CO2 adsorption for a specific process.