High-valent cationic metal–organic macrocycles as novel supports for immobilization and enhancement of activity of polyoxometalate catalysts - 主-客体和超分子化学

Developing 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 a hexagonal terpyridine-cadmium metal-organic macrocycles (Cd-MOMs) with dodeca-valent positive charge, and subsequently employed it as a new catalyst support for immobilizing mono-transition-metal-substituted Keggin-type HPW11MO394- (PW11M, M = Ni, Co, Cu, Zn). Because of strong electrostatic interaction, the obtained polyoxometalate-metal-organic-macrocycle (PW11M-Cd-MOM or POM-MOM) catalyst exhibits high thermal stability and insolubility in common solvents. The composite catalyst was found effective for the epoxidation of cyclooctene and the sulfides 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 of epoxide. Compared with homogeneous POM and non-cyclic hybrid analogue catalysts, the conversion and selectivity of POM-MOM catalyst are both enhanced enormously, showing remarkable synergistic reactivity. Moreover, leaching tests confirmed the heterogeneity of POM-MOM catalyst and it can be recycled at least four times with no significant loss of activity.