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Manganese Dioxide
Structure
MnO2 catalyses several reactions associated with the formation of O2, 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:
2H2O2 O2 + 2H2O
In molten KOH in the presence of oxygen, MnO2 reacts to give the manganate anion.
Manganese dioxide decomposes above about 530 C to give manganese(III) oxide and oxygen, while hot concentrated sulfuric or hydrochloric acid reduce the MnIV to manganese(II).
2MnO2 + 2H2SO4 2MnSO4 + O2 + 2H2O
MnO2 + 4HCl MnCl2 + Cl2 + 2H2O
This latter reaction forms the basis for 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.
MnO2 + 4NaCl + 2H2SO4 2Na2SO4 + MnCl2 + Cl2 + 2H2O
Eo (MnO2(s) + 4+ + 2 Mn2+ + 2H2O) = +1.23 V
Eo (Cl2(g) + 2 2l) = +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.
Production and synthesis
The natural occurring manganese dioxide contains impurities and a considerable amount of manganese(III) ions. Only a limited number of deposits contain the modification in high enough purity, which is the preferred material for the battery industry. The ferrite production also needs manganese dioxide with only small amounts of impurities. Therefore 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.
Chemical manganese dioxide
One of the two chemical methods starts from natural manganese dioxide and converts it with dinitrogen tetroxide (N2O4) and water to manganese(II) nitrate solution, which is purified and after evaporation of the water a crystalline solid forms. At temperatures of 400 C the reverse reaction releases the N2O4 and manganese dioxide is formed.
MnO2 + N2O4 Mn(NO3)2
Mn(NO3)2 MnO2 + N2O4
In the other chemical process, the natural manganese dioxide ore is reduced with oil or coal to the manganese 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.
In modern times, the predominant application of MnO2 is as a component of dry cell batteries, so called zincarbon batteries. Approximately 500,000 tonnes 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).
MnO2 in organic synthesis
A specialized use of manganese dioxide is as an 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 by treatment of an aqueous solution KMnO4 with a Mn(II) salt, typically the sulfate. MnO2 oxidizes allylic alcohols to the corresponding aldehydes:
cis-RCH=CHCH2OH + MnO2 cis-RCH=CHCHO + nO + H2O
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 MnO2 to dialdehydes or diketones. Otherwise, the applications of MnO2 are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.
References
^ a b Greenwood, Norman N.; Earnshaw, A. (1984), Chemistry of the Elements, Oxford: Pergamon, pp. 121820, ISBN 0-08-022057-6 .
^ Greenwood, Norman N.; Earnshaw, A. (1984), Chemistry of the Elements, Oxford: Pergamon, p. 923, ISBN 0-08-022057-6 .
^ a b c Preisler, Eberhard (1980), "Moderne Verfahren der Grochemie: Braunstein", Chemie in unserer Zeit 14: 13748, doi:10.1002/ciuz.19800140502 .
^ Reidies, Arno H. (2002), "Manganese Compounds", Ullmann's Encyclopedia of Industrial Chemistry, 20, Weinheim: Wiley-VCH, pp. 495542, doi:10.1002/14356007.a16_123, ISBN 3-527-30385-5 .
^ Cahiez, G.; Alami, M.; Taylor, R. J. K.; Reid, M.; Foot, J. S. (2004), "Manganese Dioxide", in Paquette, Leo A., Encyclopedia of Reagents for Organic Synthesis, New York: J. Wiley & Sons .
^ Attenburrow, J.; Cameron, A. F. B.; Chapman, J. H.; Evans, R. M.; Hems, B. A.; Jansen, A. B. A.; Walker, T. (1952), J. Chem. Soc.: 1094 .
Sources
Oosterhoeks Encyclopedie (Dutch)
External links
High Grade Manganese and Products Provider
REACH Mn Consortium
Index of Organic Synthesis procedures utilizing MnO2
Example Reactions with Mn(IV) oxide
National Pollutant Inventory - Manganese and compounds Fact Sheet
PubChem summary of MnO2
International Chemical Safety Card 0175
Potters Manganese Toxicity by Elke Blodgett
v d e
Manganese compounds
MnBr2 MnCO3 MnCl2 MnF2 MnF3 MnO MnO2 MnS MnSO4 MnSe2 Mn2O3 Mn2O7 Mn3O4
Categories: Manganese compounds | Manganese minerals | Oxide minerals | OxidesHidden categories: Chemboxes which contain changes to watched fields
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I am China Manufacturers writer, reports some information about lampe berger oil , acetoin.
Leo Jansen
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