Double cations modified clinoptilolite enhancing the adsorption separation of CH4 and N2

Abstract

Coal bed methane is an important supplement to natural gas, but its application is constrained by low CH₄ concentration. Zeolite is most promising adsorbent for coal bed methane purification, its adsorption and separation performance still need to be further improved. In terms of this issue, we successfully synthesized a Na⁺ and Ag⁺ double cations modified clinoptilolite inspired by that Na ⁺ had an enhancing effect on N₂ adsorption capacity of clinoptilolite, while Ag ⁺ had a negative effect on CH₄ adsorption capacity. The elemental composition and morphology of the cation exchanged clinoptilolite were characterized by SEM, EDS, XRD and BET method. Adsorption capacity of CH₄ and N₂ were measured by saturation method and isothermal adsorption lines were plotted using the Langmuir-Freundlich model. The results show that the double cations modification can bring both the modification effect of two cations, in which Na ⁺ improves N₂ adsorption and Ag ⁺ reduces CH₄ adsorption. It leads to a great improvement in the separation performance of clinoptilolite. The N₂/CH₄ adsorption selectivity and the N₂ adsorption capacity of the cation exchanged clinoptilolite reaches 9.17 and 0.33 mmol/g at 0.1 MPa, respectively. We successfully combined the modifying effects of Na⁺ and Ag⁺ on clinoptilolite, resulting in better gas separation performance compared to a single cation-modified clinoptilolite.