Sol−Gel Synthesis of Layered Birnessite-Type Manganese Oxides

Abstract

Layered birnessite-type manganese oxides have been synthesized by sol−gel reactions involving KMnO 4 or NaMnO 4 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 MnO 6 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 K 0.28 MnO 1.96 (H 2 O) 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 KMnO 4 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 Mn 2 O 3 . The synthesis appears to be general for reactions of KMnO 4 with a variety of sugars as well as other polyalcohols such as ethylene glycol, glycerol, and poly(vinyl alcohol). Reactions between NaMnO 4 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.