Adsorption of sulphur dioxide in Cu(II)-carboxylate framework materials: the role of ligand functionalisation and open metal sites

The development of efficient sorbent materials for sulphur dioxide (SO2) is of key industrial interest. However, due to the corrosive nature of SO2, conventional porous materials often exhibit poor reversibility and limited uptake towards SO2 sorption. Here, we report high adsorption of SO2 in a series of Cu(II)-carboxylate-based metal-organic framework materials. We describe the impact of ligand functionalisation and open metal sites on the uptake and reversibility of SO2 adsorption. Specifically, MFM-101 and MFM-190(F) show fully reversible SO2 adsorption with remarkable capacities of 18.7 and 18.3 mmol g-1, respectively, at 298 K and 1 bar; the former represents the highest reversible uptake of SO2 under ambient conditions of all porous solids reported to date. In situ neutron powder diffraction and synchrotron infrared micro-spectroscopy enable the direct visualisation of binding domains of adsorbed SO2 molecules as well as host-guest binding dynamics. We have found that the combination of open Cu(II) sites and ligand functionalisation, together with the size and geometry of metal-ligand cages, plays an integral role in the enhancement of SO2 binding.