Metal-Organic Frameworks for Carbon Dioxide Capture: Design Principles and Performance

Separating carbon dioxide from flue gas and from air is an adsorption problem with unusually demanding constraints: the target is dilute, the competing components are abundant, water is always present, and the energy spent regenerating the sorbent determines whether the process is worth operating. Metal-organic frameworks address these constraints through structural control that is difficult to achieve in other porous solids, since the metal node and the organic linker can be varied independently and the pore environment adjusted afterwards. This review sets out the three binding mechanisms these materials use, namely physisorption in tailored micropores, coordination at open metal sites, and chemisorption at appended amines, and explains why each produces a characteristic isotherm shape. Performance is then assessed against the criteria that govern practical use, which are working capacity between adsorption and regeneration conditions, selectivity in the presence of nitrogen and water vapour, the heat of adsorption, and stability over repeated cycles. Materials that combine a stepped isotherm with moderate regeneration temperature offer the clearest advantage over aqueous amine scrubbing. Recent work has shifted from record surface areas towards water tolerance, mechanical robustness and inexpensive synthesis, and one framework has now been produced at industrial scale.