High-valent cationic metal-organic macrocycles as novel supports for immobilization and enhancement of activity of polyoxometalate catalysts

Developing a new strategy for preparing highly active and recyclable heterogeneous polyoxometalate (POM) catalysts has attracted much attention in the past decades. In this work, we prepared hexagonal terpyridine-cadmium metal-organic macrocycles (Cd-MOMs) with a dodeca-valent positive charge, and subsequently employed them as new catalyst supports for immobilizing mono-transition-metal-substituted Keggin-type HPW₁₁MO₃₉⁴⁻ (PW₁₁M, M = Ni, Co, Cu, Zn) polyoxometalates. Because of strong electrostatic interactions, the obtained polyoxometalate-metal-organic macrocycle (PW₁₁M-Cd-MOM or POM-MOM) catalysts exhibit high thermal stability and insolubility in common solvents. The composite catalysts were found effective for the epoxidation of cyclooctene and sulfide oxidation with hydrogen peroxide. In the epoxidation reaction of cyclooctene, PW₁₁Ni-Cd-MOM shows the highest catalyst performance with 96.4% conversion and 95.7% selectivity for epoxide. Compared with homogeneous POM and non-cyclic hybrid analogue catalysts, the conversion and selectivity of POM-MOM catalysts are both enhanced enormously, showing remarkable synergistic reactivity. Moreover, leaching tests confirmed the heterogeneity of POM-MOM catalysts and they can be recycled at least four times with no significant loss of activity.