Iodine sequestration by thiol-modified MIL-53(Al)

A thiol-modified version of the porous metal organic framework MIL-53 is synthesised in a single step using the functionalised linker precursor 2,5-dithiol-1,4-benzenedicarboxylic acid and aluminium as the framework metal. Careful washing is needed to remove unreacted and dimerised linker from the material after synthesis, but once performed profile fitting of powder X-ray diffraction shows that thiol-modified MIL-53(Al) presents a closed, narrow-pore, structure with unit cell volume similar to 1103 Aa3 (space group C2/c). The presence of intact thiol groups is confirmed using sulfur K-edge XANES spectroscopy and IR spectroscopy, while nitrogen BET surface area analysis and krypton and xenon adsorption isotherms reveal the porosity of the material. The thiol-modified solid is capable of iodine adsorption from the vapour phase and from solution and an equilibrium uptake of similar to 325 mg per g is reached, which is higher than other reported modified forms of MIL-53. Infrared spectroscopy shows the disappearance of the S-H stretch after iodine adsorption, while sulfur K-edge XANES shows a complex spectrum, consistent with the formation of sulfenyl iodide but also oxidation of some sulfur to disulfide having occurred. We therefore propose that formation of covalent S-I bonds allows the sequestration of iodine by the porous solid, but that a proportion of the thiol groups are also in close enough proximity for the formation of disulfide links.