Confined-space synthesis of Zr-, Ti-, and Ti-Zr-oxocluster-based hybrid nanoparticles as catalysts for H2O2-mediated oxidations

Abstract

Hybrid organic–inorganic nanoparticles were synthesized by incorporating covalently embedded monometallic (Ti or Zr) and bimetallic (Ti–Zr) oxoclusters, functionalized with polymerizable methacrylate ligands, into a polymethylmethacrylate (PMMA) matrix. Nanoparticles were synthesized via photoactivated free-radical copolymerization within direct (oil-in-water) miniemulsion droplets, serving as confined nanoreactors. This synthetic approach yielded spherical hybrid nanoparticles with an average diameter of 76 ± 50 nm. Considering the ability of d0 early transition metals to coordinate and activate the H2O2 molecule, the catalytic activity of these materials was investigated in heterogeneous oxidation reactions. Their catalytic performance was initially evaluated in the oxidation of methyl p-tolyl sulfide, representing the first example of Ti- and Ti–Zr-oxocluster-based hybrid materials as catalysts in a peroxidation reaction. Among them, the monometallic Ti-based materials demonstrated superior conversion, selectivity, and reaction rate. Subsequently, the catalysts were tested for the oxidation of benzyl alcohol and for the epoxidation of cyclooctene, demonstrating the potential application of oxocluster-based catalysts for primary alcohol and alkene oxidations. Notably, the hybrid based on Ti4O2(iOPr)6(OMc)6 displayed the highest conversions and reaction rate also for such reactions. These findings highlight the potential application of miniemulsion-synthesized oxocluster-based hybrid nanoparticles as heterogeneous catalysts for the oxidation of various organic substrates. Moreover, preliminary hypotheses regarding the formation of the active peroxometal species were formulated based on density functional theory (DFT) calculations.