Sol−Gel Synthesis of Layered Birnessite-Type Manganese Oxides

Layered birnessite-type manganese oxides have been synthesized by sol−gel reactions involving KMnO4 or NaMnO4 with glucose. These microporous manganese oxides are designated as octahedral layer materials, K-OL-1 and Na-OL-1, because their layered structure consists of edge-shared MnO6 octahedra. The interlayer regions are occupied by alkali metal cations and water molecules. K-OL-1 and Na-OL-1 have been characterized by elemental analysis, powder X-ray diffraction, scanning electron microscopy, FT-IR spectroscopy, and Auger electron spectroscopy. The empirical formula of K-OL-1 has been determined to be K0.28MnO1.96(H2O)0.19. An interlayer spacing of 7 Å, typical of natural and synthetic birnessites, has been measured by X-ray diffraction. The sol−gel synthesis of K-OL-1 is carried out with concentrated aqueous solutions of glucose and KMnO4 in a 1.5:1 mole ratio. Diluted reaction mixtures produce flocculent gels or precipitates which yield other manganese oxide phases such as cryptomelane and Mn2O3. The synthesis appears to be general for reactions of KMnO4 with a variety of sugars as well as other polyalcohols such as ethylene glycol, glycerol, and poly(vinyl alcohol). Reactions between NaMnO4 and glucose yield two related Na-OL-1 products. A procedure analogous to the K-OL-1 synthesis generates layered sodium birnessite materials with 5.5 and 7 Å interlayer distances. The 7 Å Na-OL-1 is obtained exclusively by hydrating the mixture of products. The 5.5 Å Na-OL-1 is prepared by calcining the Na-OL-1 xerogel at 800 °C instead of the typical 400 °C temperature. Both K-OL-1 and Na-OL-1 undergo significant yet incomplete cation extraction and ion exchange with monovalent and divalent cations.