Binding of carbon dioxide and acetylene to free carboxylic acid sites in a metal-organic framework

The functionalisation of organic linkers in metal-organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible -COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO₂ and C₂H₂ molecules to the free -COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO₂ and C₂H₂ with a high selectivity for C₂H₂ over C₂H₄. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO₂ and C₂H₂ molecules with free -COOH and -OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.