Exploring cation gating in RHO zeolites via modification of Cs+/Na+ Ratio: Analysis of sorption isotherms for carbon dioxide, hydrogen, methane, and nitrogen

Abstract

The temperature-regulated gated adsorption behaviour of Na, Cs-RHO zeolites with varying Cs⁺/Na⁺ mass ratios (2.2, 1.0, and 0.6) was investigated for H₂, CO₂, N₂, and CH₄ at pressures up to 800 kPa and temperatures from 77.15 to 413.15 K. Zeolite RHO 2.2 exhibited a "trapdoor" effect, where adsorption was negligible at low temperatures but significantly increased above a critical temperature, particularly for H₂, CO₂, and N₂. This behaviour is attributed to Cs⁺ cations dynamically blocking or opening the pore windows, regulating gas access. In contrast, RHO 1 and RHO 0.6 followed typical physisorption without gating effects, with RHO 0.6 showing the highest CO₂, CH₄, and N₂ adsorption in the order CO₂ > CH₄ > N₂. The separation potential of zeolite RHO 2.2, 1, and 0.6 for CO₂/N₂, and CO₂/CH₄ gas mixtures was evaluated by determining the pure gas adsorption capacities. Ideal Adsorbed Solution Theory (IAST) estimated the adsorption selectivity across different conditions. The negligible CH₄ uptake by RHO 2.2 highlights its high CO₂/CH₄ selectivity, making it a strong candidate for CO₂ separation in biogas and natural gas purification. These findings provide new insights into temperature-controlled adsorption in zeolites and the tunability of gas selectivity through cation exchange.