Manganese dioxide

Template:Chembox Manganese dioxide is the inorganic compound with the formula Template:Chem. 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 Template:Chem 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.<ref name="G&E">Template:Greenwood&Earnshaw1st.</ref><ref name=":0">Template:Cite journal</ref> Template:Chem is also used as a pigment and as a precursor to other manganese compounds, such as Template:Chem. It is used as a reagent in organic synthesis, for example, for the oxidation of allylic alcohols. Template:Chem 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 Template:Chem as a possible cathode for lithium-ion batteries.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

StructureEdit

Several polymorphs of Template:Chem are claimed, as well as a hydrated form. Like many other dioxides, Template:Chem crystallizes in the rutile crystal structure (this polymorph is called pyrolusite or Template:Chem), with three-coordinate oxide anions and octahedral metal centres.<ref name=G&E/> Template:Chem is characteristically nonstoichiometric, being deficient in oxygen. The complicated solid-state chemistry of this material is relevant to the lore of "freshly prepared" Template:Chem in organic synthesis.<ref name= Cahiez/> The α-polymorph of Template:Chem has a very open structure with "channels", which can accommodate metal ions such as silver or barium. Template:Chem is often called hollandite, after a closely related mineral. Two other polymorphs, Todorokite and Romanechite Template:Chem, have a similar structure to Template:Chem but with larger channels. Template:Chem exhibits a layered structure more akin to that of graphite.<ref name=":0" />

ProductionEdit

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".<ref name="ChiuZMnO2">Template:Citation.</ref>

Chemical manganese dioxideEdit

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 Template:Chem and leaving a residue of purified manganese dioxide.<ref name="ChiuZMnO2"/> These two steps can be summarized as:

Template:Chem + Template:Chem Template:Eqm Template:Chem

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 Template:Chem. 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.<ref name="ChiuZMnO2"/>

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

2 Template:Chem + 3 Template:Chem + 2 Template:Chem→ 5 Template:Chem + Template:Chem + 2 Template:Chem

Electrolytic manganese dioxideEdit

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.<ref>Template:Cite journal</ref>

ReactionsEdit

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

ReductionEdit

Template:Chem is the principal precursor to ferromanganese and related alloys, which are widely used in the steel industry. The conversions involve carbothermal reduction using coke:<ref name=UllMn>Template:Cite book</ref>

Template:Chem + 2 C → Mn + 2 CO

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

Template:Chem + e⁻ + Template:Chem → MnO(OH)

Template:Chem catalyses several reactions that form Template:Chem. 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 Template:Chem → 2 Template:Chem + Template:Chem

Manganese dioxide decomposes above about 530 °C to manganese(III) oxide and oxygen. At temperatures close to 1000 °C, the mixed-valence compound Template:Chem forms. Higher temperatures give MnO, which is reduced only with difficulty.<ref name=UllMn/>

Hot concentrated sulfuric acid reduces Template:Chem to manganese(II) sulfate:<ref name="G&E"/>

2 Template:Chem + 2 Template:Chem → 2 Template:Chem + Template:Chem + 2 Template:Chem

The reaction of hydrogen chloride with Template:Chem was used by Carl Wilhelm Scheele in the original isolation of chlorine gas in 1774:

Template:Chem + 4 HCl → Template:Chem + Template:Chem + 2 Template:Chem

As a source of hydrogen chloride, Scheele treated sodium chloride with concentrated sulfuric acid.<ref name="G&E"/>

E^o (Template:Chem(s) + 4 Template:Chem + 2 e⁻ Template:Eqm Mn²⁺ + 2 Template:Chem) = +1.23 V

E^o (Template:Chem(g) + 2 e⁻ Template: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).

OxidationEdit

Heating a mixture of KOH and Template:Chem in air gives green potassium manganate:

2 Template:Chem + 4 KOH + Template:Chem → 2 Template:Chem + 2 Template:Chem

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

Occurrence and applicationsEdit

PrehistoryEdit

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

BatteriesEdit

The predominant application of Template:Chem 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.<ref name="Ullmann">Template:Citation</ref>

δ-Template:Chem 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.<ref>Template:Cite journal</ref>

Organic synthesisEdit

A specialized use of manganese dioxide is as oxidant in organic synthesis.<ref name= Cahiez>Template:Citation.</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>Template:Citation.</ref> The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated in situ by treatment of an aqueous solution Template:Chem with a Mn(II) salt, typically the sulfate. Template:Chem oxidizes allylic alcohols to the corresponding aldehydes or ketones:<ref>Template:OrgSynth (this procedure illustrates the use of MnO₂ for the oxidation of an allylic alcohol)</ref>

cis-RCH=Template:Chem + Template:Chem → cis-RCH=CHCHO + MnO + Template:Chem

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

Other potential applicationsEdit

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.<ref>Template:Cite book</ref>

Template:Chem is used as an inorganic pigment in ceramics and in glassmaking.