Elucidating the mechanism of the UiO-66-catalyzed sulfide oxidation: activity and selectivity enhancements through changes in the node coordination environment and solvent

Abstract

Benzoic acid modulators that “cap” the Zr₆-oxo-hydroxo cluster nodes in UiO-66 metal–organic frameworks can be removed to increase the number of “open” sites (i.e., those that are terminated with [μ₁-OH + μ₁-OH₂]) up to 5 per node, enabling the “decapped” materials to exhibit enhanced catalytic activity in the oxidation of methyl phenyl sulfide. Computational modeling reveals that the labile Zr-μ₁-OH groups on these open sites are likely converted into Zr-μ₁-OOH species that are active in oxidizing the sulfide as well as its sulfoxide product. In solvents such as CH₃CN and CH₂Cl₂, the sulfoxide product can additionally replace the aquo ligands of the Zr-μ₁-OH₂ moieties to increase the concentration of the sulfoxide adjacent to the active Zr-μ₁-OOH species, resulting in overoxidation to the sulfone. However, the use of CH₃OH, a solvent that can compete with the sulfoxide and suppress this binding mode, can retard the overoxidation and lead to higher selectivities for the sulfoxide product.