Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cell batteries, such as the alkaline battery and the zinc–carbon battery, although it is also used for other battery chemistries such as aqueous zinc-ion batteries. is also used as a pigment and as a precursor to other manganese compounds, such as potassium permanganate (). It is used as a reagent in organic synthesis, for example, for the oxidation of allylic alcohols. has an α-polymorph that can incorporate a variety of atoms (as well as water molecules) in the "tunnels" or "channels" between the manganese oxide octahedra. There is considerable interest in as a possible cathode for lithium-ion batteries.

Structure

Several polymorphs of are claimed, as well as a hydrated form. Like many other dioxides, crystallizes in the rutile crystal structure (this polymorph is called pyrolusite or ), with three-coordinate oxide anions and octahedral metal centres.

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 and leaving a residue of purified manganese dioxide.

:2 + 3 + 2 → 5 + + 2

The above reaction is an example of potassium permanganate reacting to make manganese dioxide.

Most reactions with potassium permanganate are known to make brown manganese dioxide as a byproduct, where potassium permanganate undergoes a Redox reaction where it reduces and oxidizes a compound with manganese dioxide byproduct.

Electrolytic manganese dioxide

Electrolytic manganese dioxide (EMD) is used in zinc–carbon batteries together with zinc chloride and ammonium chloride. EMD is commonly used in zinc manganese dioxide rechargeable alkaline (Zn RAM) cells also. For these applications, purity is extremely important. EMD is produced in a similar fashion as electrolytic tough pitch (ETP) copper: The manganese dioxide is dissolved in sulfuric acid (sometimes mixed with manganese sulfate) and subjected to a current between two electrodes. The MnO<sub>2</sub> dissolves, enters solution as the sulfate, and is deposited on the anode.

Reactions

The important reactions of are associated with its redox, both oxidation and reduction.

Reduction

is the principal precursor to ferromanganese and related alloys, which are widely used in the steel industry. The conversions involve carbothermal reduction using coke:

: + 2 C → Mn + 2 CO

The key redox reactions of in batteries is the one-electron reduction:

: + e<sup>−</sup> + → MnO(OH)

catalyses several reactions that form . 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:

:2 → 2 +

Manganese dioxide decomposes above about 530&nbsp;°C to manganese(III) oxide and oxygen. At temperatures close to 1000&nbsp;°C, the mixed-valence compound forms. Higher temperatures give MnO, which is reduced only with difficulty.

Batteries

The predominant application of is as a component of dry cell batteries: alkaline batteries and so called Leclanché cell, or zinc–carbon batteries. Approximately 500,000 tonnes are consumed for this application annually.

δ- has also been researched as the primary cathode material for aqueous zinc-ion battery systems. Such cathodes often contain additives to address structural, kinetic, and conductivity-based issues. These carbon additives can include reduced graphene oxide (rGO) and carbon nanotubes, among others.

===Organic synthesis===<!--used in ind production of p-quinone from aniline-->

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 with a Mn(II) salt, typically the sulfate. oxidizes allylic alcohols to the corresponding aldehydes or ketones:

::cis-RCH= + → cis-RCH=CHCHO + MnO +

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 to dialdehydes or diketones. Otherwise, the applications of are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.

Other potential applications

In Geobacteraceae sp., MnO<sub>2</sub> functions as an electron acceptor coupled to the oxidation of organic compounds. This theme has possible implications for bioremediation within the field of microbiology.

is used as an inorganic pigment in ceramics and in glassmaking.

See also

  • List of inorganic pigments

References

Cited sources

  • Index of Organic Synthesis procedures utilizing
  • Example Reactions with Mn(IV) oxide
  • National Pollutant Inventory – Manganese and compounds Fact Sheet
  • PubChem summary of
  • International Chemical Safety Card 0175
  • Potters Manganese Toxicity by Elke Blodgett
  • The reaction between manganese dioxide and potassium permanganate (1893) by A. J. Hopkins