Generating and optimizing the catalytic activity in UiO-66 for aerobic oxidation of alkenes by post-synthetic exchange Ti atoms combined with ligand substitution

The catalytic activity for the aerobic epoxidation of cyclooctene of UiO-66 has been introduced by post-synthetic ion exchange of Zr⁴⁺ by Ti⁴⁺ at the nodes and the performance optimized by nitro substitution in the terephthalate ligand. In this way a TON value of 16,600 (1660 considering Zr + Ti content) was achieved, comparing favorably with the highest catalytic activity reported in homogeneous for the same reaction (10,000 for γ-SiW₁₀{Fe³⁺(OH₂)}₂O₃₈ ⁶⁻). Kinetic studies have shown that the most likely reactive oxygen species involved in the oxidation is superoxide, with hydroxyl radicals also contributing to the reaction. UiO-66(Zr_5.4 Ti_0.6)-NO₂ is stable under catalytic conditions, being used six times without any change in the conversion temporal profile and in the X-ray diffractogram. The scope of UiO-66(Zr_5.4 Ti_0.6)-NO₂ promoted aerobic oxidation of alkenes was expanded by including smaller rings cycloalkenes, as well as acyclic and aryl conjugated alkenes.