Sol−Gel Route to the Tunneled Manganese Oxide Cryptomelane

Abstract

The sol−gel reaction between KMnO 4 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 K 0.12 MnO 2.0 (H 2 O) 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 Mn 2 O 3 . 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 KMnO 4 generally yield Mn 2 O 3, 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 Mn 3 O 4 . 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%.