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

Abstract

The adsorption equilibria and kinetic performance of CO 2 , CH 4 , and N 2 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 CO 2 uptakes of KX, MgX, and CaX reached 4.13, 4.79, and 5.48 mmol/g, respectively. The effective binary and kinetic selectivities of CO 2 /CH 4 and CO 2 /N 2 were also calculated. Among all samples, KX showed the highest CO 2 /CH 4 and CO 2 /N 2 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 (D p ) and microporous diffusional time constant (D c /r c 2 ) 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.

Structural Characterisation of Zeolites Derived from Lithium Extraction: Insights into Channel- and Cage-Type Frameworks

“…This confirms a larger surface area for cage-type zeolites, consistent with data reported by Zeolyst International for FAU (760-885 m 2 /g) [12]. This variance can be attributed to the complexity of surface area access, influenced by pore blockage, resulting in the lower nitrogen adsorption observed in channel-type zeolites [5]. The one-dimensional pore network of mordenite structures presents more pore blockages than the open three-dimensional structural configurations found in faujasites [13].…”

“…In the Supplementary Materials (Figure S3), the N 2 adsorption isotherms of cage-and channel-type zeolites are presented. The experimental isotherm for the channel-type zeolite (LPM17) exhibits a type I isotherm with a slight slope at the end, indicating the presence of a mesoporous or external area [5]. Conversely, the adsorption isotherms for the cage-type zeolites demonstrate a rapid increase in adsorbed amount followed by a plateau at higher pressures, indicative of a limited volume of the adsorbed phase due to microporosity [12].…”

“…Gas adsorption studies are essential for comprehending processes like gas separation, purification, and storage. Factors such as Si/Al ratio, crystallinity, pore size, and surface area, alongside adsorbate properties, influence gas adsorption characteristics [4][5][6]. Therefore, elucidating the relationship between different zeolite structures, including channeland cage-like configurations, and diverse adsorbents is critical.…”

Enhancing the separation of C2H4/C2H6 in customized MOR zeolites

“…In tortuous or confined channels, the diffusion of alkane is slower, leading to more frequent collisions between the gas and zeolite frameworks. 32–34 Adjusting the aluminum content in the framework could regulate the channel environment to control the diffusion of the guest molecules. Meanwhile, to balance the negative charges resulting from the aluminium atoms, the cations are positioned close to the aluminium atoms, usually alkali or alkaline cations sourced from the synthesis.…”