Tailoring Montmorillonite Interlayer Microporosity for Highly Selective CO2/CH4 Separation

Cation-exchanged montmorillonite clays with tunable interlayer microporosity were investigated as adsorbents for CO2/CH4 separation. A series of materials was prepared by substitution of native interlayer Na+ charge-balancing cations with Cs+ and a series of methylammonium cations of increasing size (CH3NH3+, (CH3)2NH2+, (CH3)3NH+, and (CH3)3N+), enabling progressive modulation of the interlayer spacing within the range 0.25-0.43 nm. Structural and textural characterization confirmed the development of gas-accessible microporosity correlated with cation size. CO2 and CH4 adsorption equilibria were measured up to 100 kPa pressure, at temperatures ranging from -13 to 50 °C and simulated using dual-site Langmuir and Henry models, respectively. The determined adsorption heats were found to lie between 20 and 30 kJ·mol-1 for CO2 and 8 to 24 kJ·mol-1 for CH4. The results reveal a strong dependence of adsorption behavior and separation performance on interlayer pore width. The CH3NH3+-exchanged clay exhibits the most favorable balance, combining high CO2 uptake with minimal CH4 adsorption, leading to exceptional CO2/CH4 selectivity exceeding that of reference zeolites. This performance is attributed primarily to the steric effects or appearance of new adsorption sites rather than enhanced adsorption energetics. Water adsorption measurements demonstrated a significantly lower hydrophilicity compared to zeolite 13X, suggesting potential improvement of material robustness under Vacuum Pressure Swing Adsorption (VPSA) process operation conditions for biomethane upgrading. Process-relevant performance indicators, including working CO2 adsorption capacity, Adsorbent Figure of Merit, Sorbent Selection Parameter, and Adsorbent Performance Indicator, confirm the strong potential of this material for VPSA CO2/CH4 separation.