Sol−Gel Route to the Tunneled Manganese Oxide Cryptomelane

The sol−gel reaction between KMnO4 and fumaric acid in a 3:1 mole ratio generates a flocculant gel that serves as a precursor to the tunneled manganese oxide, cryptomelane. The elemental composition of sol−gel cryptomelane has been determined to be K0.12MnO2.0(H2O)0.09. Further characterization has been performed using powder X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. The sol−gel process is heavily dependent on reactant concentration. Solutions that are too concentrated produce the layered manganese oxide birnessite, whereas overly dilute reactions yield mixtures of cryptomelane and Mn2O3. The preference for cryptomelane over birnessite correlates with low potassium content in the gel. The sol−gel procedure for synthesizing cryptomelane is not easily transferred to the preparation of analogous manganese oxides with different tunnel cations. Reactions that employ permanganates other than KMnO4 generally yield Mn2O3, with cryptomelane being a minor product at best. Thermal analyses of cryptomelane gels indicate that calcination proceeds through a series of stages that involve loss of water, loss of residual organics, conversion to cryptomelane, and finally degradation to Mn3O4. The extraction of potassium ions from sol−gel cryptomelane by various foreign cations is minimal, with the loss of K+ being on the order of 10%.