Functionalized Smectites for CH4-CO2 Gas Separation in Biogas Upgrading

Description

Layered swelling clay minerals such as montmorillonite and bentonite have large internal surfaces where substantial amounts of gas can adsorb provided that the gas molecules can access the confined space of the interlayer galleries. The width of this space can be tuned by exchanging the cations in the galleries that compensate the negative charge of the aluminosilicate layers. We show that the sorption capacity for CO2 at near-ambient pressure can be increased by more than an order of magnitude upon exchanging the natural Na+ cations by bulkier Cs+ or by hydrated Ca2+ and Mg2+ ions [1]. All these three cations also lead to a selectivity of approx. 30 for CO2 vs. CH4 sorption thus enabling efficient separation of these two gases in a pressure/vacuum swing adsorption (PSA) process (see [2] and figure below). Competitive adsorption of water with CO2 and the relevance of keeping Ca2+ and Mg2+ ions hydrated highlights the relevance of controlling the humidity of the input gas in practical applications such as biogas upgrading [3].

A numerical analysis of the PSA process allows to trace the evolution of the CO2 and CH4 concentrations within the reactor with time and suggests that CH4 and CO2 recovery and purity better than 95% can be achieved in a continuous operation mode with two sorption columns in parallel in a single adsorption stage at an energy consumption of approximately 0.1 kWh/Nm3 of biomethane [4]. Higher purity levels can be achieved with two (or more) consecutive sorption stages. This makes our material an attractive alternative to currently dominant membrane-based separation technology for biogas separation from anaerobic digestion processes of various forms of biological waste.



Left: Sketch of smectite layers with cations of suitable size to permit CO2 adsorption in clay galleries and prevent CH4 adsorption. Right: measured breakthrough curves of a 50-50 CO2-CH4 mixture through a montmorillonite-filled sorption column.

References:
[1] Mendel N. et al. (2021) J Phys Chem C 49:27159-27169
[2] Mendel N. et al. (2023) Ind Eng Chem Res 62:17883-17892
[3] Mendel N. et al. (2025) J Phys Chem C 129:6953-6966
[4] Mendel N. et al. (2025) Ind Eng Chem Res (accepted for publication)

Period	16 Jul 2025 → 19 Jul 2025
Event title	18th International Clay Conference, ICC 2025: Sustaining Clays
Event type	Conference
Organisers	International Association for the Study of Clay (AIPEA), Mineralogical Society of the United Kingdom and Ireland
Location	Dublin, IrelandShow on map
Degree of Recognition	International