Micellar catalysis of quinquivalent vanadium oxidation of methanol to formaldehyde in aqueous medium

Abstract

The kinetics of oxidation of methanol by quinquivalent vanadium in aqueous sulfuric acid medium has been studied at 313 K under pseudo-first-order condition by UV–vis spectrophotometry. Nonfunctional sodium dodecyl sulfate (SDS) surfactant solution was used as a microheterogeneous micellar catalyst. The reaction rate and selectivity strongly depend on the chosen surfactant, and in some cases also on the surfactant concentration. The critical micelle concentration (CMC) values of the SDS surfactant in aqueous medium as well as in the presence of the substrate methanol were determined by the conductivity method, matching well with the kinetically determined CMC value. SDS was found to be an excellent catalyst for oxidation of methanol by vanadium(V) in aqueous sulfuric acid medium, leading to the corresponding oxidized product (formaldehyde), which was detected by 1H nuclear magnetic resonance (NMR). The micellar catalysis by SDS is due to strong binding of the cationic oxidant with the anionic surfactant. Formation of aggregates by the catalytic surfactant was studied using optical microscopy, and the change in shape and size of the aggregates in the reaction condition was studied by using scanning electron microscopy and the dynamic light scattering method. Mechanisms for this oxidation reaction in aqueous medium as well as with micellar catalyst are proposed, being completely supported by our experimental results.