Solid manganese dioxide as heterogeneous oxidant of aniline in the preparation of conducting polyaniline or polyaniline/manganese dioxide composites

Abstract

Various oxidants used in the preparation of polyaniline are reviewed. Insoluble solid oxidant, manganese dioxide, has been investigated in the heterogeneous synthesis of polyaniline in detail. Aniline was oxidized in acidic aqueous medium at various oxidant-to-monomer mole ratios [MnO2]/[aniline]. At [MnO2]/[aniline] ≤ 1.25, i.e. at and below the stoichiometric composition, only polyaniline was produced, and the solid oxidant was absent in the product due to its reductive dissolution which took place during the reaction. Polyaniline prepared under such conditions did not differ with respect to conductivity from the standard polyaniline obtained with peroxydisulfate oxidant but fused polyaniline nanofibers have been produced instead of a common globular morphology. The composites of polyaniline/manganese dioxide resulted only at [MnO2]/[aniline] >1.25. Spectroscopic methods suggest that the oxidation with excess of manganese dioxide produced polyaniline at higher oxidation state. The presence of manganese dioxide in the products was confirmed by X-ray diffraction and EDAX. The results are important for the understanding of the preparation and properties of polyaniline/manganese dioxide composites that are currently used in supercapacitor electrodes. The present study also illustrates a new potential application, adsorption of organic dyes Reactive Black 5 and methylene blue, as an example of water-pollution treatment.