manganese dioxide

Several polymorphs of {{chem|MnO|2}} are claimed, as well as a hydrated form. Like many other dioxides, {{chem|MnO|2}} crystallizes in the rutile crystal structure (this polymorph is called pyrolusite or {{chem|β-MnO|2}}), with three-coordinate oxide anions and octahedral metal centres. {{chem|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" {{chem|MnO|2}} in organic synthesis. The α-polymorph of {{chem|MnO|2}} has a very open structure with "channels", which can accommodate metal ions such as silver or barium. {{chem|α-MnO|2}} is often called hollandite, after a closely related mineral. Two other polymorphs, Todorokite and Romanechite {{chem|MnO|2}}, have a similar structure to {{chem|α-MnO|2}} but with larger channels. {{chem|δ-MnO|2}} exhibits a layered structure more akin to that of graphite.

Naturally occurring manganese dioxide contains impurities and a considerable amount of manganese(III) oxide. 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".{{citation | last = Preisler | first = Eberhard | title = Moderne Verfahren der Großchemie: Braunstein | journal = Chemie in unserer Zeit | year = 1980 | volume = 14 | issue = 5 | pages = 137–48 | doi = 10.1002/ciuz.19800140502}}.

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 {{chem|N|2|O|4}} and leaving a residue of purified manganese dioxide. These two steps can be summarized as:

:{{chem|MnO|2}} + {{chem|N|2|O|4}} {{eqm}} {{chem|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 {{chem|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.

Lastly, the action of potassium permanganate over manganese sulfate crystals produces the desired oxide.Arthur Sutcliffe (1930) Practical Chemistry for Advanced Students (1949 Ed.), John Murray – London.

:2 {{chem|KMnO|4}} + 3 {{chem|MnSO|4}} + 2 {{chem|H|2|O}}→ 5 {{chem|MnO|2}} + {{chem|K|2|SO|4}} + 2 {{chem|H|2|SO|4}}

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 MnO₂ dissolves, enters solution as the sulfate, and is deposited on the anode.{{cite journal | url=https://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra05892a | doi=10.1039/C5RA05892A | title=Electrolytic manganese dioxide (EMD): A perspective on worldwide production, reserves and its role in electrochemistry | date=2015 | last1=Biswal | first1=Avijit | last2=Chandra Tripathy | first2=Bankim | last3=Sanjay | first3=Kali | last4=Subbaiah | first4=Tondepu | last5=Minakshi | first5=Manickam | journal=RSC Advances | volume=5 | issue=72 | pages=58255–58283 | url-access=subscription }}

The important reactions of {{chem|MnO|2}} are associated with its redox, both oxidation and reduction.

{{chem|MnO|2}} is the principal precursor to ferromanganese and related alloys, which are widely used in the steel industry. The conversions involve carbothermal reduction using coke:{{cite book |doi=10.1002/14356007.a16_077|chapter=Manganese and Manganese Alloys |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2000 |last1=Wellbeloved |first1=David B. |last2=Craven |first2=Peter M. |last3=Waudby |first3=John W. |isbn=3527306730 }}

:{{chem|MnO|2}} + 2 C → Mn + 2 CO

The key redox reactions of {{chem|MnO|2}} in batteries is the one-electron reduction:

:{{chem|MnO|2}} + e⁻ + {{chem|H|+}} → MnO(OH)

{{chem|MnO|2}} catalyses several reactions that form {{chem|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:

:2 {{chem|H|2|O|2}} → 2 {{chem|H|2|O}} + {{chem|O|2}}

Manganese dioxide decomposes above about 530 °C to manganese(III) oxide and oxygen. At temperatures close to 1000 °C, the mixed-valence compound {{chem|Mn|3|O|4}} forms. Higher temperatures give MnO, which is reduced only with difficulty.

Hot concentrated sulfuric acid reduces {{chem|MnO|2}} to manganese(II) sulfate:

:2 {{chem|MnO|2}} + 2 {{chem|H|2|SO|4}} → 2 {{chem|MnSO|4}} + {{chem|O|2}} + 2 {{chem|H|2|O}}

The reaction of hydrogen chloride with {{chem|MnO|2}} was used by Carl Wilhelm Scheele in the original isolation of chlorine gas in 1774:

:{{chem|MnO|2}} + 4 HCl → {{chem|MnCl|2}} + {{chem|Cl|2}} + 2 {{chem|H|2|O}}

As a source of hydrogen chloride, Scheele treated sodium chloride with concentrated sulfuric acid.

::E^o ({{chem|MnO|2}}(s) + 4 {{chem|H|+}} + 2 e⁻ {{eqm}} Mn²⁺ + 2 {{chem|H|2|O}}) = +1.23 V

::E^o ({{chem|Cl|2}}(g) + 2 e⁻ {{eqm}} 2 Cl⁻) = +1.36 V

The reaction would not be expected to proceed, based on the standard electrode potentials, but is favoured by the extremely high acidity and the evolution (and removal) of gaseous chlorine.

This reaction is also a convenient way to remove the manganese dioxide precipitate from the ground glass joints after running a reaction (for example, an oxidation with potassium permanganate).

Heating a mixture of KOH and {{chem|MnO|2}} in air gives green potassium manganate:

:2 {{chem|MnO|2}} + 4 KOH + {{chem|O|2}} → 2 {{chem|K|2|MnO|4}} + 2 {{chem|H|2|O}}

Potassium manganate is the precursor to potassium permanganate, a common oxidant.

Excavations at the Pech-de-l'Azé cave site in southwestern France have yielded blocks of manganese dioxide writing tools, which date back 50,000 years and have been attributed to Neanderthals . Scientists have conjectured that Neanderthals used this mineral for body decoration, but there are many other readily available minerals that are more suitable for that purpose. Heyes et al. (in 2016) determined that the manganese dioxide lowers the combustion temperatures for wood from above 350°C (662°F) to 250°C (482°F), making fire making much easier and this is likely to be the purpose of the blocks.{{Cite web |title=Neandertals may have used chemistry to start fires |url=https://www.science.org/content/article/neandertals-may-have-used-chemistry-start-fires |access-date=2022-05-30 |website=www.science.org |language=en}}

The predominant application of {{chem|MnO|2}} 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.{{citation |last=Reidies |first=Arno H. |title=Ullmann's Encyclopedia of Industrial Chemistry |volume=20 |pages=495–542 |year=2002 |contribution=Manganese Compounds |location=Weinheim |publisher=Wiley-VCH |doi=10.1002/14356007.a16_123 |isbn=978-3-527-30385-4}}

δ-{{chem|MnO|2}} 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.{{Cite journal |last=Azmi |first=Zarina |last2=Senapati |first2=Krushna C. |last3=Goswami |first3=Arpan K. |last4=Mohapatra |first4=Saumya R. |date=September 2024 |title=A Comprehensive Review of Strategies to Augment the Performance of MnO₂ Cathode by Structural Modifications for Aqueous Zinc Ion Battery |url=https://linkinghub.elsevier.com/retrieve/pii/S0378775324007687 |journal=Journal of Power Sources |language=en |volume=613 |pages=234816 |doi=10.1016/j.jpowsour.2024.234816|url-access=subscription }}

===Organic synthesis===

A specialized use of manganese dioxide is as oxidant in organic synthesis.{{citation | last1 = Cahiez | first1 = G. | last2 = Alami | first2 = M. | last3 = Taylor | first3 = R. J. K. | last4 = Reid | first4 = M. | last5 = Foot | first5 = J. S. |doi=10.1002/047084289X.rm021.pub4 | contribution = Manganese Dioxide | title = Encyclopedia of Reagents for Organic Synthesis | pages = 1–16 | editor-first = Leo A. | editor-last = Paquette | year = 2004 | publisher = J. Wiley & Sons | location = New York| isbn = 9780470842898 }}. 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.{{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. | title = A synthesis of vitamin a from cyclohexanone | journal = J. Chem. Soc. | year = 1952 | pages = 1094–1111 |doi= 10.1039/JR9520001094 }}. The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated in situ by treatment of an aqueous solution {{chem|KMnO|4}} with a Mn(II) salt, typically the sulfate. {{chem|MnO|2}} oxidizes allylic alcohols to the corresponding aldehydes or ketones:{{OrgSynth | author = Paquette, Leo A. and Heidelbaugh, Todd M. | title = (4S)-(−)-tert-Butyldimethylsiloxy-2-cyclopen-1-one | |collvol = 9 | collvolpages = 136 | year = | prep = cv9p0136}} (this procedure illustrates the use of MnO₂ for the oxidation of an allylic alcohol)

::cis-RCH={{chem|CHCH|2|OH}} + {{chem|MnO|2}} → cis-RCH=CHCHO + MnO + {{chem|H|2|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 {{chem|MnO|2}} to dialdehydes or diketones. Otherwise, the applications of {{chem|MnO|2}} are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.

In Geobacteraceae sp., MnO₂ functions as an electron acceptor coupled to the oxidation of organic compounds. This theme has possible implications for bioremediation within the field of microbiology.{{cite book |doi=10.1016/S0065-2911(04)49005-5|title=Dissimilatory Fe(III) and Mn(IV) Reduction |series=Advances in Microbial Physiology |year=2004 |last1=Lovley |first1=Derek R. |last2=Holmes |first2=Dawn E. |last3=Nevin |first3=Kelly P. |volume=49 |pages=219–286 |pmid=15518832 |isbn=9780120277490 }}

{{chem|MnO|2}} is used as an inorganic pigment in ceramics and in glassmaking.

• List of inorganic pigments

{{Reflist|30em}}

• {{RubberBible99th}}

{{Commons category|Manganese dioxide}}

• [https://web.archive.org/web/20050218190527/http://www.orgsyn.org/orgsyn/chemname.asp?nameID=33430 Index of Organic Synthesis procedures utilizing {{chem|MnO|2}}]
• [https://www.organic-chemistry.org/chemicals/oxidations/manganese(IV)oxide.shtm Example Reactions with Mn(IV) oxide]
• [https://web.archive.org/web/20060301151751/http://www.npi.gov.au/database/substance-info/profiles/52.html National Pollutant Inventory – Manganese and compounds Fact Sheet]
• [https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=14801 PubChem summary of {{chem|MnO|2}}]
• [https://www.inchem.org/documents/icsc/icsc/eics0175.htm International Chemical Safety Card 0175]
• [https://web.archive.org/web/20041124065033/http://ceramic-materials.com/cermat/education/139.html Potters Manganese Toxicity by Elke Blodgett]
• [https://www.gutenberg.org/ebooks/74154 The reaction between manganese dioxide and potassium permanganate] (1893) by A. J. Hopkins

{{Manganese compounds}}

{{Oxides}}

{{DEFAULTSORT:Manganese Dioxide}}

Category:Manganese(IV) compounds

Category:Inorganic pigments

Category:Transition metal oxides

Category:Glass dyes