Polyoxomolybdic Cobalt Encapsulated within Zr-Based Metal-Organic Frameworks as Efficient Heterogeneous Catalysts for Olefins Epoxidation

The encapsulation of polyoxomolybdic cobalt (CoPMA) and polyoxomolybdic acid (PMA) within the Zr-based metal-organic frameworks (Zr-MOFs) of UiO-bpy (connected by 2,2′-bipyridine-5,5′-dicarboxylic acid linkers) and UiO-67 (connected by 4,4′-biphenyldicarboxylic acid linkers) has been achieved by direct solvothermal synthesis. Relatively high content of polyoxometalate (POM) clusters (ranging from 12 to 15 wt % loading) could be introduced to the cages of Zr-MOFs to form uniform hybrid composites of POM@Zr-MOFs. The catalytic properties of these composites were investigated for the olefins epoxidation with H₂O₂ or molecular O₂ as oxidant. Among them, the catalyst CoPMA@UiO-bpy showed the highest catalytic activity and stability for cyclooctene epoxidation with H₂O₂ as oxidant and could also act as efficient heterogeneous catalyst for the oxidation of styrene and 1-octene with O₂ as oxidant and tert-butyl hydroperoxide (t-BuOOH) as initiator. The excellent catalytic performance of the hybrid composite CoPMA@UiO-bpy should be mainly attributed to the uniform distribution of POM clusters within the size-matched cages of Zr-MOFs, as well as the multiple interactions between the CoPMA clusters and the functional groups (bipyridine and Zr-OH) located in the framework of UiO-bpy.