What factors determine activity of UiO-66 in H2O2-based oxidation of thioethers? The role of basic sites

Zr-based metal-organic frameworks (Zr-MOFs) have attracted significant attention as selective oxidation catalysts due to their stability in aqueous and oxidative media and high activity in H2O2 activation. Oxidation of thioethers with aqueous H2O2 over Zr-MOFs reveals extremely high selectivity toward the formation of sulfones even at low conversions. Herein, the main factors determining activity of Zr-MOF in the thioether oxidation have been investigated using zirconium terephthalate UiO-66 as a model catalyst and methyl phenyl sulfide (MPS) as a model substrate. Samples of UiO-66 differing in the particle size, number of defects, and composition have been synthesized. The particle size was evaluated based on high resolution transmission electron microscopy images. Thermogravimetric analysis (TGA), proton nuclear magnetic resonance spectroscopy (1H NMR), and inductively coupled plasma atomic emission spectroscopy (ICP-OES) were employed for quantification of defects and determination of the composition of the UiO-66 samples. The number of basic sites was determined by liquidphase adsorption of isobutyric acid. It was demonstrated that this characteristic depends on both the number of defects and specific composition of UiO-66 and can be affected by dehydration/hydration procedures. The number of basic sites turned out close to the number of terminal Zr-OH2/OH groups in the Zr-MOF defects determined by combinations of 1H NMR/TGA and 1H NMR/ICP-OES, suggesting that the basic sites are represented by Zr-OH groups at open Zr sites. Kinetic studies implicated that catalytic activity of UiO-66 in MPS oxidation is proportional to the number of basic sites provided that the particle size of the Zr-MOF does not exceed 10-20 nm. Acid additives suppress both the thioether oxidation and H2O2 dismutation, pointing to the key role of basic Zr-OH groups in these two catalytic reactions.