Solvothermal Synthesis of Co-substituted Phosphomolybdate Acid Encapsulated in the UiO-66 Framework for Catalytic Application in Olefin Epoxidation

Hybrid composites of phosphomolybdic acid@UiO-66 (PMo12@UiO-66) and Co-substituted phosphomolybdic acid@UiO-66 (PMo11Co@UiO-66) were synthesized using the direct solvothermal method. A variety of characterization results demonstrated that phosphomolybdic acid (PMo12) or Co-substituted phosphomolybdate acid (PMo11Co) clusters are uniformly dispersed in the cages of Zr-based metal-organic UiO-66 frameworks. The catalytic properties of these hybrid composites were investigated by applying the epoxidation of olefins with tert-butyl hydroperoxide as the oxidant. Compared to PMo12@UiO-66, PMo11Co@UiO-66 showed a much higher catalytic activity and was simply recovered by filtration and reused for at least ten runs without significant loss of catalytic activity. Particularly, PMo11Co@UiO-66 can efficiently convert cyclic olefins like limonenes to epoxides, and its selectivity to 1,2-limonene oxide reached 91% in the presence of a radical inhibitor such as hydroquinone. The excellent catalytic activity and stability of the hybrid composite PMo11Co@UiO-66 are mainly attributed to the uniform distribution of highly active PMo11Co units within the smaller cages of UiO-66, to the suitable surface polarity of the hybrid composite for facilitating the access of reagents and solvent, and to the strong interface-interactions between the polyoxometalate and the UiO-66 framework.