Aerobic oxidation of formaldehyde catalyzed by polyvanadotungstates

Three tetra-n-butylammonium (TBA) salts of polyvanadotungstates, [n-Bu ₄N]₆[PW₉V₃O₄₀] (PW ₉V₃), [n-Bu₄N]₅H₂PW ₈V₄O₄₀ (PW₈V₄), and [n-Bu₄N]₄H₅PW₆V₆O ₄₀·20H₂O (PW₆V₆), have been synthesized and shown to be effective catalysts for the aerobic oxidation of formaldehyde to formic acid under ambient conditions. These complexes, characterized by elemental analysis, Fourier transform infrared spectroscopy, UV-vis spectroscopy, and thermogravimetric analysis, exhibit a catalytic activity for this reaction comparable to those of other polyoxometalates. Importantly, they are more effective in the presence of water than the metal oxide-supported Pt and/or Au nanoparticles traditionally used as catalysts for formaldehyde oxidation in the gas phase. The polyvanadotungstate-catalyzed oxidation reactions are first-order in formaldehyde, parabolic-order (slow, fast, and slow again) in catalyst, and zero-order in O₂. Under optimized conditions, a turnover number of ∼57 has been obtained. These catalysts can be recycled and reused without a significant loss of catalytic activity.