Heterogenization of Phosphotungstate Clusters into Magnetic Microspheres: Catalyst for Selective Oxidation of Alcohol in Water

A polyamine-mediated bioinspired method is demonstrated to heterogenize and stabilize isolated phosphotungstic acid (PTA) Keggin units, which allows formation of a multicomponent microcapsule structure under ambient conditions. Utilizing the supramolecular aggregation property of polyamines in the presence of suitable multivalent anions, it is shown that the resulting aggregates, while facilitating the encapsulation of negatively charged Fe3O4 nanoparticles (magnetic component), further facilitate the assembly of PTA nanoclusters (catalytic component) to generate magnetic microcapsule structures (PTA@MMS). As revealed from a detailed analysis, the interaction of polyamines with PTA is the key for the Keggin units to remain stable and isolated as desirable for catalytic applications. Investigation on the catalytic activity in oxidation of benzyl alcohol with H2O2 as the oxidant in water shows that these isolated PTA units render better catalytic activity and product selectivity toward aldehyde under green conditions compared to the other reported PTA-based noble-metal-free catalysts. Moreover, the presence of magnetic nanoparticles leads to convenient catalyst separation and reuse. Therefore, the method is believed to provide ways for further developments in POM-based heterogeneous catalysts with opportunities to impart multifunctionality for sustainable organic transformations.