Micellar effect on pentavalent vanadium oxidation of formaldehyde to formic acid in aqueous acid media at room temperature

The kinetics of oxidation of formaldehyde by pentavalent vanadium in 3.0 mol dm(-3) H2SO4, at 313 K, under pseudo first-order conditions [Formaldehyde](T) a parts per thousand << [V(V)](T), have been studied by UV-visible spectrophotometry. Two representative non-functional surfactants (sodium dodecyl sulfate, SDS, and polyoxyethylene octyl phenyl ether, TX-100), at concentrations above and below their critical micelle concentrations (CMC), were used as micro-heterogeneous catalysts in this oxidation. The reaction rate and selectivity strongly depend on the surfactant used, and sometimes on surfactant concentration also. The CMC for both surfactants in aqueous media were determined by spectrofluorimetry, from the sharp change in fluorescence intensity. In contrast with TX-100, SDS was an excellent catalyst of oxidation of formaldehyde by vanadium(V) in aqueous micellar media, leading to the corresponding oxidation product. Formic acid was detected by H-1 NMR spectroscopy. Formation of aggregates by the catalytic surfactants under the reaction conditions was studied by scanning electron microscopy. Dynamic light scattering was used to characterize shape changes during oxidation, by monitoring changes in the hydrodynamic diameter (D (h) = 2R (h), where R (h) is hydrodynamic radius) of aggregates. A mechanism proposed for this micelle-catalysed slow oxidation reaction was entirely supported by our experimental results.