The first catalytic active composites based on CoAl 2 O 4 nanoparticles with different size (5.5 and 2.5 nm) were successfully prepared using a simple methodology of incorporation into MIL-101(Cr) framework, CoAl-x@MIL(Cr). Characterization of CoAl-x@MIL(Cr) composites by elemental analysis, vibrational spectroscopy (FT-IR and FT-Raman), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) confirmed the successful preparation and stability of the support structure after nanoparticle immobilization. A remarkable catalytic performance was found for thioanisole oxidation under sustainable conditions (95% of conversion after 30 min of reaction) and the catalytic application of the most active composite was extended to styrene oxidation. Higher catalytic performance was achieved for the composite prepared with larger CoAl 2 O 4 nanoparticles. The recyclability and the stability of composites after catalytic use were investigated. For the CoAl-x@MIL(Cr) catalytic systems, the loading parameter instead of the nanoparticle size seemed to have a pronounced influence in the heterogeneous catalytic performance. The confinement effect promoted by MIL-101(Cr) cavities associated to the higher number of catalytic active centers (CoAl 2 O 4) is clearly more important than the size of the catalytic nanoparticles used.