Geopolymer-based materials for CO₂/CH₄ separation and H₂S capture: Current evidence, limitations, and research needs

Efficient biogas upgrading requires the removal of CO₂ and H₂S to produce biomethane suitable for downstream applications. Geopolymer-based materials have attracted interest because their aluminosilicate structures can be tailored through precursor composition, activation, curing, and secondary-phase incorporation, while waste-derived precursors offer opportunities for resource valorization. This review critically evaluates geopolymer-based CO₂/CH₄ separation and H₂S capture and examines their potential integration into a single membrane platform. The literature provides limited direct evidence of CO₂/CH₄ separation using geopolymer membranes, whereas CO₂/CH₄ adsorption selectivity has been demonstrated in geopolymer-based materials, and H₂S capture has primarily been investigated using particulate or bulk geopolymer sorbents. However, the available literature has yet to establish the integrated performance of a single geopolymer membrane for CO₂/CH₄ separation and H₂S removal under mixed CO₂/CH₄/H₂S feeds. Consequently, the existing evidence remains fragmented across adsorption, gas permeation, and H₂S sorption studies. Key research gaps include establishing the relationship between adsorption properties and membrane-level separation, controlling selective-layer thickness and defects, elucidating gas transport mechanisms, and validating performance under humid, multicomponent conditions. The review also examines membrane architecture, functionalization, composite design, and emerging metal–organic framework (MOF) and covalent organic framework (COF) systems to identify relevant design principles. Altogether, integrated CO₂/CH₄/H₂S purification using geopolymer membranes remains largely at the proof-of-concept stage. Future research should prioritize mixed-gas testing, H₂S breakthrough and regeneration studies, interface engineering, and standardized transport measurements.