SOL-GEL ROUTE TO THE TUNNELED MANGANESE OXIDE CRYPTOMELANE

The sol-gel reaction between KMnO4 and fumaric acid in a 3:1 mole rati o generates a flocculant gel that serves as a precursor to the tunnele d 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, s canning 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 birnessi te, whereas overly dilute reactions yield mixtures of cryptomelane and Mn2O3. The preference for cryptomelane over birnessite correlates wit h low potassium content in the gel. The sol-gel procedure for synthesi zing cryptomelane is not easily transferred to the preparation of anal ogous manganese oxides with different tunnel cations. Reactions that e mploy permanganates other than KMnO4 generally yield Mn2O3, with crypt omelane being a minor product at best. Thermal analyses of cryptomelan e gels indicate that calcination proceeds through a series of stages t hat involve loss of water, loss of residual organics, conversion to cr yptomelane, and finally degradation to Mn3O4. The extraction of potass ium ions from sol-gel cryptomelane by various foreign cations is minim al, with the loss of K+ being on the order of 10%.