Ion exchange of cations from different groups with ammonium-modified clinoptilolite and selectivity for methane and nitrogen

Abstract

The cationic exchange performances of alkali, alkaline earth metals, and other cations with clinoptilolite (CP) were investigated, in which, the dried CP was firstly converted into (Na and K)-exchanged CP and finally ammonium-exchanged CP via ammonization, and then, various cations were selected to further exchange with NH4+-CP at different times, initial concentrations and temperatures. The exchange kinetics and thermodynamic parameters were calculated, and the results demonstrated that the ion exchange behaviors were mainly thermodynamic-controlled, and the positive values of enthalpy change indicated the endothermic process. The adsorption kinetics model and diffusion model fitting were employed to evaluate their adsorption mechanism, suggesting the pseudo-second-order kinetics and intra-particle diffusion. Meanwhile, the phyico-chemical properties of before and after ion-exchanged CPs were obtained via XRD, N2 sorption isotherms, SEM and EDS, FT-IR spectra, TG-DSC profiles. The results showed that the structural parameters were strongly dependent on the exchanged cationic valence and their radii, while their textural parameters were remained intact. Finally, their CH4 and N2 adsorption capacity and selectivity were preliminary explored, showing promising adsorbents for N2 - CH4 separation. These investigates are useful in the development of more general rules to understand deeply the ion-exchanged principles of CP, and thereafter better design of new adsorbents widely employed in gas separation.