Development of Magnetic Nanoparticles as Microwave-Specific Catalysts for the Rapid, Low-Temperature Synthesis of Formalin Solutions

A series of heterogeneous catalyst materials possessing good microwave absorption properties were investigated for their activity as oxidation catalysts under microwave irradiation. These catalysts, a series of nanoscale magnetic spinel oxides of the composition MCr₂O₄ (M = Cu, Co, Fe), were irradiated in aqueous methanol solution (1:1 MeOH:H₂O v:v). This resulted in rapid conversion of methanol to formaldehyde, directly generating aqueous formalin solutions. The catalytic reaction occurred under relatively mild conditions (1 atm O₂, 60 °C), with irradiation times of 80 min converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly, reactions run under identical conditions of concentration, time, and temperature using traditional convective heating yielded dramatically lower amounts of conversions; specifically, 1.0% and 0.21% conversions were observed with Cu and Co spinels, and no observable thermal products were obtained from the Fe spinels. This work provides a clear demonstration that microwave-driven catalysis can yield enhanced reactivity and can afford new catalytic pathways