Several polymorphs of MnO 2 are claimed, as well as a hydrated form. Like many other dioxides, MnO 2 crystallizes in the rutilecrystal structure (this polymorph is called pyrolusite or β-MnO 2), with three-coordinate oxide and octahedral metal centres.MnO 2 is characteristically nonstoichiometric, being deficient in oxygen. The complicated solid-state chemistry of this material is relevant to the lore of "freshly prepared" MnO 2 in organic synthesis. The α-polymorph of MnO 2 has a very open structure with "channels" which can accommodate metal atoms such as silver or barium. α-MnO 2 is often called hollandite, after a closely related mineral.
Naturally occurring manganese dioxide contains impurities and a considerable amount of manganese(III) oxide. Only a limited number of deposits contain the γ modification in purity sufficient for the battery industry.
Production of batteries and ferrite (two of the primary uses of manganese dioxide) requires high purity manganese dioxide. Batteries require "electrolytic manganese dioxide" while ferrites require "chemical manganese dioxide".
Chemical manganese dioxide
One method starts with natural manganese dioxide and converts it using dinitrogen tetroxide and water to a manganese(II) nitrate solution. Evaporation of the water, leaves the crystalline nitrate salt. At temperatures of 400 °C, the salt decomposes, releasing N 2O 4 and leaving a residue of purified manganese dioxide. These two steps can be summarized as:
MnO 2 + N 2O 4 ⇌ Mn(NO 3) 2
In another process manganese dioxide is carbothermically reduced to manganese(II) oxide which is dissolved in sulfuric acid. The filtered solution is treated with ammonium carbonate to precipitate MnCO 3. The carbonate is calcined in air to give a mixture of manganese(II) and manganese(IV) oxides. To complete the process, a suspension of this material in sulfuric acid is treated with sodium chlorate. Chloric acid, which forms in situ, converts any Mn(III) and Mn(II) oxides to the dioxide, releasing chlorine as a by-product.
The key reactions of MnO 2 in batteries is the one-electron reduction:
MnO 2 + e− + H+ → MnO(OH)
MnO 2catalyses several reactions that form O 2. In a classical laboratory demonstration, heating a mixture of potassium chlorate and manganese dioxide produces oxygen gas. Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water:
A specialized use of manganese dioxide is as oxidant in organic synthesis. The effectiveness of the reagent depends on the method of preparation, a problem that is typical for other heterogeneous reagents where surface area, among other variables, is a significant factor. The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated in situ by treatment of an aqueous solution KMnO 4 with a Mn(II) salt, typically the sulfate. MnO 2 oxidizes allylic alcohols to the corresponding aldehydes or ketones:
The configuration of the double bond is conserved in the reaction. The corresponding acetylenic alcohols are also suitable substrates, although the resulting propargylic aldehydes can be quite reactive. Benzylic and even unactivated alcohols are also good substrates. 1,2-Diols are cleaved by MnO 2 to dialdehydes or diketones. Otherwise, the applications of MnO 2 are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.
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