View source for Manganese dioxide

{{chembox
| ImageFile = Manganese(IV) oxide.jpg
|   ImageName = Manganese dioxide
| IUPACName = Manganese dioxide
|   OtherNames = [[Pyrolusite]]<br/>Manganese(IV) oxide
| Section1 = {{Chembox Identifiers
|   InChI = 1/Mn.2O/rMnO2/c2-1-3
|   StdInChI = 1S/Mn.2O
|   InChIKey = NUJOXMJBOLGQSY-MQJJFXMEAL
|   StdInChIKey = NUJOXMJBOLGQSY-UHFFFAOYSA-N
|   CASNo = 1313-13-9
|    CASNo_Ref = {{cascite}}
|   EC-number = 215-202-6
|   ChemSpiderID = 
|   PubChem = 14801
|   RTECS = OP0350000
  }}
| Section2 = {{Chembox Properties
|   Reference = <ref>{{RubberBible62nd|page=B-118}}.</ref>
|   Formula = MnO<sub>2</sub>
|   MolarMass = 86.937 g/mol
|   Appearance = black solid
|   Density = 5.026 g/cm<sup>3</sup>
|   Solubility = insoluble
|   MeltingPt = 535 °C decomp.
  }}
| Section4 = {{Chembox Thermochemistry
|   Reference = <ref>{{Jolly2nd|page=586}}.</ref>
|   DeltaHf = −519.5 kJ/mol
|   Entropy = 53.0 J&thinsp;K<sup>−1</sup>&thinsp;mol<sup>−1</sup>
  }}
| Section7 = {{Chembox Hazards
|   Reference = <ref>{{CLP Regulation|index=025-001-00-3|page=420}}</ref>
|   ExternalMSDS = {{ICSC-small|01|75}}
|   EUIndex = 025-001-00-3
|   GHSPictograms = {{GHS07|Acute Tox. (oral, inhal.) 4}}
|   GHSSignalWord = WARNING
|   HPhrases = {{H-phrases|302|332}}
|   PPhrases = {{P-phrases|264|270|261|271|301+312|330|304+340|501}}
|   FlashPt = non-flammable
  }}
| Section8 = {{Chembox Related
|   OtherAnions = [[Manganese disulfide]]
|   OtherCations = [[Technetium dioxide]]<br/>[[Rhenium dioxide]]
|   OtherFunctn = [[Manganese(II) oxide]]<br/>[[Manganese(II,III) oxide]]<br/>[[Manganese(III) oxide]]<br/>[[Manganese heptoxide]]
|    Function = [[manganese]] [[oxide]]s
  }}
}}

'''Manganese dioxide''' is the [[inorganic compound]] with the [[chemical formula|formula]] MnO<sub>2</sub>. This blackish or brown solid occurs naturally as the mineral [[pyrolusite]], which is the main ore of [[manganese]]. It is also present in [[manganese nodule]]s. The principal use for MnO<sub>2</sub> is for dry-cell [[battery (electricity)|batteries]], such as the [[alkaline battery]] and the [[zinc–carbon battery]].<ref name="G&E">{{Greenwood&Earnshaw1st|pages=1218–20}}.</ref> In 1976 this application accounted for 500,000&nbsp;[[tonne]]s of pyrolusite. MnO<sub>2</sub> is also used for production of [[Potassium permanganate|MnO<sub>4</sub><sup>−</sup>]].  It is used extensively as an [[redox|oxidizing agent]] in [[organic synthesis]], for example, for the oxidation of [[allylic]] [[alcohol]]s.

==Structure==
[[File:Resinoid crystal of manganese dioxide.jpg|thumb|200px|left|a resinoid crystal of the manganese dioxide generated in the crack of the limestone of mud from [[Solnhofen]], [[Bayern]], [[Germany]].]]
A number of [[polymorphism (materials science)|polymorphs]] of MnO<sub>2</sub> have been identified. The most common is β-MnO<sub>2</sub>,([[pyrolusite]]) which has the [[titanium dioxide|TiO<sub>2</sub>]], rutile structure.<ref name=Wells>{{Wells5th}}.</ref> Others are  α-MnO<sub>2</sub>, γ-MnO<sub>2</sub>, ε-MnO<sub>2</sub>, and λ-MnO<sub>2</sub>.<ref name=Wells/><ref name=Fong>{{citation | last1 = Fong | first1 = G. C. | last2 = Kennedy | first2 = B. J. | last3= Elcombe | first3 = M. M. | title = A powder neutron diffraction study of λ- and γ-manganese dioxide and of LiMn<sub>2</sub>O<sub>4</sub> | journal = Z. Kristallogr. | yesr = 1994 | volume = 209 | issue = 12 | pages = 941–45}}.</ref> The structures of all of these forms are closely related and are built from linked MnO<sub>6</sub> octahedra and "tunnels" capable of holding cations are formed in all except β-MnO<sub>2</sub>.<ref name=Wells/><ref name=Fong/> Interest in these [[polymorphism|polymorphs]] is due to their use in [[lithium ion]] batteries.

==History and chemical reactions==
The chemical properties of MnO<sub>2</sub> have been well examined since relatively pure materials have long been readily found or prepared.  MnO<sub>2</sub> was used for production of [[chlorine]] in the eighteenth century, before being displaced by electrolytic methods. The manganese dioxide was subsequently recovered by the [[Weldon process]].

MnO<sub>2</sub>  catalyses several reactions associated with the formation of O<sub>2</sub>, reminiscent of the role of Mn in the oxygen evolving center.  In a classical laboratory demonstration, a mixture of [[potassium chlorate]] and manganese dioxide is heated and the oxygen gas collected over water.  Manganese dioxide also catalyses the decomposition of [[hydrogen peroxide]] to oxygen and [[water (molecule)|water]]:
:2H<sub>2</sub>O<sub>2</sub> → O<sub>2</sub> + 2H<sub>2</sub>O
In molten KOH in the presence of oxygen, MnO<sub>2</sub> reacts to give the [[manganate]] anion.

Manganese dioxide decomposes above about 530&nbsp;ºC to give [[manganese(III) oxide]] and oxygen, while hot concentrated [[Sulfuric acid|sulfuric]] or [[hydrochloric acid]] reduce the Mn<sup>IV</sup> to manganese(II).<ref name="G&E"/> 
:2MnO<sub>2</sub> + 2H<sub>2</sub>SO<sub>4</sub> → 2MnSO<sub>4</sub> + O<sub>2</sub> + 2H<sub>2</sub>O
:MnO<sub>2</sub> + 4HCl → MnCl<sub>2</sub> + Cl<sub>2</sub> + 2H<sub>2</sub>O
This latter reaction forms the basis for [[Carl Wilhelm Scheele|Scheele's]] first isolation of [[chlorine]] gas in 1774: Scheele used nascent [[hydrogen chloride]], prepared from the reaction of [[sodium chloride]] with concentrated sulfuric acid.<ref>{{Greenwood&Earnshaw1st|page=923}}.</ref>
:MnO<sub>2</sub> + 4NaCl + 2H<sub>2</sub>SO<sub>4</sub> →  2Na<sub>2</sub>SO<sub>4</sub> + MnCl<sub>2</sub> + Cl<sub>2</sub> + 2H<sub>2</sub>O
::''E''<sup><s>o</s></sup> (MnO<sub>2</sub>(s) + 4 H<sup>+</sup> + 2 e<sup>−</sup> {{eqm}} Mn<sup>2+</sup> + 2H<sub>2</sub>O) = +1.23&nbsp;V
::''E''<sup><s>o</s></sup> (Cl<sub>2</sub>(g) + 2 e<sup>−</sup> {{eqm}} 2 Cl<sup>−</sup>) = +1.36&nbsp;V
The reaction would not be expected to proceed, based on the [[standard electrode potential]]s, but is favoured by the extremely high [[acidity]] and the evolution (and removal) of gaseous chlorine.

==Production==
The natural occuring manganese dioxide contains impurities and a considerable amount of manganese(III) ions. Only a limited number of deposits contain the γ modification in high enough purity, which is the prefered material for the battery industry. The ferrite production also needs manganese dioxide with only small amounts of impurities. Therfore the production of synthetic manganese dioxide is important. Two groups of methods are used, yielding chemical manganese dioxide (CMD) and electrolytical manganese dioxide (EMD). The CMD is mostly used for the production of ferrites, while the EMD is used for the production of batteries.<ref name="ChiuZMnO2">{{citation | last = Preisler | first = Eberhard | title = Moderne Verfahren der Großchemie: Braunstein | journal = Chemie in unserer Zeit | year = 1980 | volume = 14 | pages = 137–48 | doi = 10.1002/ciuz.19800140502}}.</ref>

===Chemical manganese dioxide===
One of the two chemical methods starts from natural manganese dioxide and converts it with [[dinitrogen tetroxide]] (N<sub>2</sub>O<sub>4</sub>) and water to manganese(II) nitrate solution, which is purified and after evaporation of the water a crystaline solid forms. At temperatures of 400&nbsp;°C the reverse reaction releases the N<sub>2</sub>O<sub>4</sub> and manganese dioxide is formed.<ref name="ChiuZMnO2"/>
:MnO<sub>2</sub> +  N<sub>2</sub>O<sub>4</sub>  → Mn(NO<sub>3</sub>)<sub>2</sub>
:Mn(NO<sub>3</sub>)<sub>2</sub> →  MnO<sub>2</sub> +  N<sub>2</sub>O<sub>4</sub>

In the other chemical process the natural manganeses dioxide ore is reduced with oil or coal to the managanese oxide. The [[manganese(II) oxide]] is dissolved in [[sulfuric acid]] and after purification the manganese(II) is precipitated as carbonate by adding [[ammonium carbonate]]. The carbonate is heated with air and a manganese oxide forms, which is a mixture between manganese(II) and manganese(IV) oxide. To complete the reaction the oxide is suspended in sulfuric acid and [[sodium chlorate]] is added. The [[chloric acid]] oxidizes the manganese oxide and chlorine is formed as by-product.<ref name="ChiuZMnO2"/>
:2MnO<sub>2</sub> +  C  →  2MnO + CO<sub>2</sub>
:MnO + H<sub>2</sub>SO<sub>4</sub> →  MnSO<sub>4</sub> + H<sub>2</sub>O
:MnSO<sub>4</sub> + (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>   →  MnCO<sub>3</sub>↓ + (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>
:MnCO<sub>3</sub> + O<sub>2</sub> → MnO<sub>1.85</sub> + CO<sub>2</sub>
:MnO<sub>1.85</sub> + 2n HClO<sub>3</sub> → MnO<sub>1.85</sub> + ''n''Cl<sub>2</sub> + ''n''H<sub>2</sub>O

===Electrolytical manganese dioxide===
The electrochemical method starts from the manganese(II) sulfate solution, which is obtained by similar processes mentioned in the previous method, but some producers use hydrogen instead of coal and oil for the reduction. Iron is oxidized by addition of manganese dioxide and precipitates at a pH of 5 to 6, while other ions like cobalt, nickel and copper are precipitated as sulfides. [[Electrolysis]] is than used to oxidize the manganese. During that process the manganese dioxide forms a layer on the [[anode]], which is made from [[titanium]], [[lead]] or [[graphite]].<ref name="ChiuZMnO2"/>
 :MnO<sub>2</sub> +  H<sub>2</sub>  →  MnO + H<sub>2</sub>O
:MnO + H<sub>2</sub>SO<sub>4</sub>  →  MnSO<sub>4</sub> + H<sub>2</sub>O
:Mn<sup>2+</sup> + H<sub>2</sub>O → MnO<sub>2</sub> + 2e<sup>−</sup> + 2H<sup>+</sup>

==Applications==
Ancient [[Cave painting|cave painters]] and later the ancient Egyptians used MnO<sub>2</sub> as a black or brown pigment.  One of its first uses was in glass making. Egyptian and Roman glasswares have been shown by analyses to contain over 2% manganous oxide. Pliny mentions the use of manganese oxide under the name of "magnes" among the Romans for decolorizing glass. He considered it a variety of lodestone or magnetic iron ore.<ref>{{citation | title = Manganese Deposits of the United States  with Sections on Foreign Deposits, Chemistry, and Uses | first = Edmund Cecil | last = Harder | series = Bulletin of the United States Geological Survey 427 | publisher = Government Printing Office | location = Washington, DC | url = http://www.archive.org/details/manganesedeposi00hardgoog | year = 1910}}.</ref>  Eighteenth-century British chemists referred to MnO<sub>2</sub> simply as ''manganese.''{{Fact|date=March 2009}}

In modern times, the predominant application of MnO<sub>2</sub> is as a component of dry cell batteries, so called zinc–carbon batteries.  Approximately 500,000&nbsp;[[tonne]]s are consumed for this application annually.  Two distinct synthetic forms of the dioxide are used for batteries, chemical manganese dioxide (CMD) and electrolytic manganese dioxide (EMD).<ref name=Ullmann>{{citation | first = Arno H. | last = Reidies | contribution = Manganese Compounds | title = Ullmann's Encyclopedia of Industrial Chemistry | publisher = Wiley-VCH | location = Weinheim | year = 2002 | doi = 10.1002/14356007.a16_123 | volume = 20 | pages = 495–542 | isbn = 3-527-30385-5}}.</ref>

===MnO<sub>2</sub> in organic synthesis===
A specialized use of manganese dioxide is as an oxidant in [[organic synthesis]].<ref>{{EROS | last1 = Cahiez | first1 = G. | last2 = Alami | first2 = M. | last3 = Taylor | first3 = R. J. K. | last4 = Reid | first4 = M. | last5 = Foot | first5 = J. S. | contribution = Manganese Dioxide}}.</ref> 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.<ref>{{citation | last1 = Attenburrow | first1 = J. | last2 = Cameron | first2 = A. F. B. | last3 = Chapman | first3 = J. H. | last4 = Evans | first4 = R. M. | last5 = Hems | first5 = B. A. | last6 = Jansen | first6 = A. B. A. | last7 = Walker | first7 = T. | journal = J. Chem. Soc. | year = 1952 | page = 1094}}.</ref> The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated by treatment of an aqueous solution KMnO<sub>4</sub> with a Mn(II) salt, typically the sulfate.  MnO<sub>2</sub> oxidizes allylic alcohols to the corresponding aldehydes:
:''cis''-RCH=CHCH<sub>2</sub>OH + MnO<sub>2</sub> → ''cis''-RCH=CHCHO + “MnO” + H<sub>2</sub>O

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<sub>2</sub> to dialdehydes or diketones. Otherwise, the applications of MnO<sub>2</sub> are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.

===Precursor to permanganate===
The green salt [[potassium manganate]] is obtained in minutes when a solution of MnO<sub>2</sub> in molten KOH or NaOH is treated with [[oxidizing agent]]s such as [[potassium nitrate]] (KNO<sub>3</sub>), [[potassium perchlorate]] (KClO<sub>4</sub>), or even oxygen gas.<ref name="Ullmann" />
:2MnO<sub>2</sub> + 4OH<sup>−</sup> + O<sub>2</sub> → 2MnO<sub>4</sub><sup>2−</sup> + 2H<sub>2</sub>O

Potassium manganate converts into purple [[potassium permanganate]] in [[aqueous]] [[acid]]ic solution:
:3MnO<sub>4</sub><sup>2−</sup> + 4H<sup>+</sup> → 2MnO<sub>4</sub><sup>−</sup> + MnO<sub>2</sub> + 2H<sub>2</sub>O

==References==
{{reflist}}

==External links==
*{{ICSC|01|75}}
*{{OrgSynth preps|id=33430|name=manganese dioxide}}
* [http://www.npi.gov.au/database/substance-info/profiles/52.html National Pollutant Inventory - Manganese and compounds Fact Sheet]
* [http://www.mn-consortium.org/ REACH Mn Consortium]
* [http://www.organic-chemistry.org/chemicals/oxidations/manganese(IV)oxide.shtm Example Reactions with Mn(IV) oxide]
*[http://web.archive.org/web/20041124065033/ceramic-materials.com/cermat/education/139.html Potters Manganese Toxicity by Elke Blodgett]

[[Category:Manganese compounds]]
[[Category:Oxides]]
[[Category:Reagents for organic chemistry]]

{{Imported from Wikipedia|name=Manganese dioxide|id=309074527}}