Post-synthetic ligand exchange in zirconium-based metal-organic frameworks: beware of the defects!

Post-synthetic ligand exchange in the prototypical zirconium-based metal-organic framework UiO-66 was investigated by in situ solution 1H nuclear magnetic resonance. Samples of UiO-66 having different degrees of defectivity were exchanged using solutions of several terephthalic acid analogues in a range of conditions. Linker exchange only occurred in defect-free UiO-66, whereas monocarboxylates grafted at defective sites were found to be preferentially exchanged with respect to terephthalic acid over the whole range of conditions investigated. A 1:1 exchange ratio between the terephthalic acid analogue and modulator was observed, providing evidence that the defects had missing-cluster nature. Ex situ characterisation of the MOF powders after exchange corroborated these findings and evidenced that the physical-chemical properties of the MOF depend on whether the functionalisation occurs at defective sites or on the framework. N2 sorption analysis showed that, upon exchange, defective samples underwent significant decrease of surface area and disappearance of large pores, associated with the presence of missing-cluster defects in the starting material. CO2 sorption studies displayed the different impact of pure defect functionalisation and pure framework functionalisation on isosteric heat of adsorption and CO2/N2 selectivity.