Synthetic Routes to Microporous Manganese Oxides

Microporous manganese oxides with layer and tunnel structures have been synthesized by methods such as redox precipitation, thermal and hydrothermal alteration, sol-gel processing, and microwave heating. The open frameworks of these materials are composed of edge and corner shared MnO6 octahedra, with the internal pores being occupied by cations and water molecules. Birnessite-type (OL-1) manganese oxides have layered structures with interlayer spacings that depend on hydration. Hollandites (OMS-2) and todorokites (OMS-1) have tunnel structures that consist of respective 2 × 2 and 3 × 3 arrays of MnO6 octahedra. Microporous manganese oxides are of interest because of potential applications in heterogeneous catalysis, chemical absorption, and battery technology. This article describes some recent developments in the synthesis of microporous manganese oxides, with particular attention being given to birnessites. hollandites, and todorokites.