Equilibrium and Kinetic Insights into the Comprehensive Investigation of CO2, CH4, and N2 Adsorption on Cation-Exchanged X and Y Faujasite Zeolites

The adsorption equilibria and kinetic performance of CO2, CH4, and N2 on pelletized cation-exchanged faujasite zeolites (with alkali, alkaline earth, and transition metal ions) have been investigated by an innovative volumetric apparatus simultaneously. The standard instrumental analytical techniques, including X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), and atomic absorption spectroscopy (AAS), were utilized to characterize binder-free modified zeolites. EDX and AAS analyses revealed that the ion exchange was successfully achieved. The results indicate that the type of cation present in the zeolite framework and the Si/Al ratio can have a significant impact on the adsorption capacity and kinetic performance. The obtained isotherms were determined by three isotherm models, and the Langmuir-Freundlich (Sips) model was found to show the best agreement with the experimental isotherm data for all gases. The CO2 uptakes of KX, MgX, and CaX reached 4.13, 4.79, and 5.48 mmol/g, respectively. The effective binary and kinetic selectivities of CO2/CH4 and CO2/N2 were also calculated. Among all samples, KX showed the highest CO2/CH4 and CO2/N2 selectivities of 54.46 and 91.62, respectively. Pseudo-first-, pseudo-second-order, and Avrami kinetic models were fitted to the experimental kinetic data to analyze the adsorption kinetics. Finally, the macropore diffusion coefficient (Dp) and microporous diffusional time constant (Dc/rc2) were estimated by correlating the micropore-macropore kinetic model with the experimental fractional uptake curves. Among the ion-exchanged zeolite samples, the K+ form exhibits a suitable performance in terms of kinetic behavior and adsorption capacity.