<p>The Keggin trimetallic polyoxometalates of [A-β-SiW₉Fe₃O₃₇]⁷⁻ and [A-β-SiW₉Co₃O₃₇]¹⁰⁻ were immobilized into the porous metal–organic framework MIL-101(Cr). The structures and properties were characterized with high-resolution transmission electron microscopies (HRTEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD)<i>,</i> nitrogen adsorption–desorption isotherm and zeta potential analysis. In this approach, the SiW₉Fe₃O₃₇@MIL-101 and SiW₉Co₃O₃₇@MIL-101 composites were synthesized with spherical particles consisting of high surface area (the BET surface area 1368.1 and 1275.8 m²/g respectively) and uniform pore size (2.10 and 2.13 nm respectively). The SiW₉Fe₃O₃₇@MIL-101 and SiW₉Co₃O₃₇@MIL-101 catalysts demonstrated exceptional performance in synthesizing 5-substituted 1<i>H</i> tetrazoles and oxidation sulfide to sulfones. Incorporating SiW₉Fe₃O₃₇ and SiW₉Co₃O₃₇ polyoxometalates into the MIL-101 matrix provided a high surface area and this unique structure allowed enhanced accessibility of reactants to the catalytic sites.</p> Graphical Abstract <p></p>