The effect of Ag+ cations on the micropore properties of clinoptilolite and related adsorption separation of CH4 and N2 gases

In this study, we report on the effects of Ag+ cations exchange within clinoptilolite zeolite in relation to both the physicochemical and the gas adsorption characteristics. Clinoptilolites cation-exchanged with AgNO3 to varying degrees were investigated and the influence of naturally occurring cations (Mg2+, Ca2+, K+) distributed within the material is discussed. The morphology and elemental composition of the untreated clinoptilolite and the Ag+ cation exchanged variants were characterized by XRD, EDS, FTIR, and N2 sorption experiments performed at 77 K. From the pore property analysis, we show that a slight reduction in the micropore distribution occurs for the highly exchanged Ag+ clinoptilolite compared to the untreated clinoptilolite, indicating a likely decrease in the average effective micropore size. Equilibrium adsorption isotherms of CH4 and N2 were measured at 303 K. The adsorption uptake rate of both gases was also studied and it slowed by the addition of Ag+ cations. The modified samples showed decreasing adsorption capacity for the larger CH4 gas molecule, whereas adsorption of the smaller N2 gas was less affected. This effect was most pronounced for the highly exchanged Ag+ clinoptilolite samples (AgNO3 exchange solution above 0.2M) whereby more N2 was adsorbed than CH4. At 1 atm and 303 K, the N2/CH4 equilibrium selectivity values of highly exchanged Ag+ clinoptilolite ranged from 2.8 to 7.6, which are higher than many clinoptilolite types.