Heterogeneous selective oxidation catalysts based on coordination polymer MIL-101 and transition metal-substituted polyoxometalates

Titanium- and cobalt-monosubstituted Keggin heteropolyanions, [PW11CoO39]5− and [PW11TiO40]5−, were electrostatically bound to the chromium terephthalate polymer matrix MIL-101. The MIL-supported polyoxometalate (POM) catalysts were characterized by elemental analysis, XRD, N2 adsorption, and FT-IR-spectroscopy. The catalytic performance of both MIL-101 and the novel composite materials M-POM/MIL-101 was assessed in the oxidation of three representative alkenes—α-pinene, caryophyllene, and cyclohexene—using molecular oxygen and aqueous hydrogen peroxide as oxidants. Ti-POM/MIL-101 demonstrated fairly good catalytic activity and selectivity in α-pinene allylic oxidation and caryophyllene epoxidation with hydrogen peroxide, whereas Co-POM/MIL-101 catalyzed α-pinene allylic oxidation by molecular oxygen. Both composite materials are stable to POM leaching, behave as true heterogeneous catalysts, and can be used repeatedly without sustaining a loss of activity and selectivity in oxidations with O2 and H2O2, provided that rather mild reaction conditions (T < 50 °C, [H2O2] < 0.2 M) are used with the latter oxidant.