Molybdate

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Structure of molybdate Molybdate.png
Structure of molybdate
3D model of the molybdate ion Molybdate 3D.png
3D model of the molybdate ion

In chemistry, a molybdate is a compound containing an oxyanion with molybdenum in its highest oxidation state of +6: O−−Mo(=O)2−O−. Molybdenum can form a very large range of such oxyanions, which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state. The larger oxyanions are members of group of compounds termed polyoxometalates , and because they contain only one type of metal atom are often called isopolymetalates. [1] The discrete molybdenum oxyanions range in size from the simplest MoO2−
4, found in potassium molybdate up to extremely large structures found in isopoly-molybdenum blues that contain for example 154 Mo atoms. The behaviour of molybdenum is different from the other elements in group 6. Chromium only forms the chromates, CrO2−
4, Cr
2O2−
7, Cr
3O2−
10 and Cr
4O2−
13 ions which are all based on tetrahedral chromium. Tungsten is similar to molybdenum and forms many tungstates containing 6 coordinate tungsten. [2]

Contents

Examples of molybdate anions

Examples of molybdate oxyanions are:

The naming of molybdates generally follows the convention of a prefix to show the number of Mo atoms present. For example, dimolybdate for 2 molybdenum atoms; trimolybdate for 3 molybdenum atoms, etc.. Sometimes the oxidation state is added as a suffix, such as in pentamolybdate(VI). The heptamolybdate ion, Mo
7O6−
24, is often called "paramolybdate".

Structure of molybdate anions

The smaller anions, MoO2−
4 and Mo
2O2−
7 feature tetrahedral centres. In MoO2−
4 the four oxygens are equivalent as in sulfate and chromate, with equal bond lengths and angles. Mo
2O2−
7 can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom. [1] In the larger anions molybdenum is generally, but not exclusively, 6 coordinate with edges or vertices of the MoO6 octahedra being shared. The octahedra are distorted, typical M-O bond lengths are:

The Mo
8O4−
26 anion contains both octahedral and tetrahedral molybdenum and can be isolated in 2 isomeric forms, alpha and beta. [2]

The hexamolybdate image below shows the coordination polyhedra. The space filling model of the heptamolybdate image shows the close packed nature of the oxygen atoms in the structure. The oxide ion has an ionic radius of 1.40 Å, molybdenum(VI) is much smaller, 0.59 Å. [1] There are strong similarities between the structures of the molybdates and the molybdenum oxides, (MoO3, MoO2 and the "crystallographic shear" oxides, Mo9O26 and Mo10O29) whose structures all contain close packed oxide ions. [9]

Equilibrium in aqueous solution

When MoO3, molybdenum trioxide is dissolved in alkali solution the simple MoO2−4 anion is produced:

As the pH is lowered, condensations ensue, with loss of water and the formation of Mo–O–Mo linkages. The stoichiometry leading to hexa-, hepta-, and octamolybdates are shown: [1] [10]

[2]
[2]

Peroxomolybdates

Many peroxomolybdates are known. They tend to form upon treatment of molybdate salts with hydrogen peroxide. Notable is the monomer–dimer equilibrium:

Also known but unstable is [Mo(O2)4]2− (see potassium tetraperoxochromate(V)). Some related compounds find use as oxidants in organic synthesis. [11]

Tetrathiomolybdate

The red tetrathiomolybdate anion results when molybdate solutions are treated with hydrogen sulfide:

Like molybdate itself, MoS2−4 undergoes condensation in the presence of acids, but these condensations are accompanied by redox processes.

Industrial uses

Catalysis

Molybdates are widely used in catalysis. In terms of scale, the largest consumer of molybdate is as a precursor to catalysts for hydrodesulfurization, the process by which sulfur is removed from petroleum. Bismuth molybdates, nominally of the composition Bi9PMo12O52, catalyzes ammoxidation of propylene to acrylonitrile. Ferric molybdates are used industrially to catalyze the oxidation of methanol to formaldehyde. [12]

Corrosion inhibitors

Sodium molybdate has been used in industrial water treatment as a corrosion inhibitor. It was initially thought that it would be a good replacement for chromate, when chromate was banned for toxicity. However, molybdate requires high concentrations when used alone, therefore complementary corrosion inhibitors are generally added, [13] and is mainly used in high temperature closed-loop cooling circuits. [14] According to an experimental study, molybdate has been reported as an efficient biocide against microbiologically induced corrosion (MIC), where adding 1.5 mM MoO2−
4/day resulted in a 50 % decrease in the corrosion rate. [15]

Supercapacitors

Molybdates (especially FeMoO4, Fe2(MoO4)3, NiMoO4, CoMoO4 and MnMoO4) have been used as anode or cathode materials in aqueous capacitors. [16] [17] [18] [19] Due to pseudocapacitive charge storage, specific capacitance up to 1500 F g−1 has been observed. [17]

Medicine

Radioactive molybdenum-99 in the form of molybdate is used as the parent isotope in technetium-99m generators for nuclear medicine imaging. [20]

Other

Nitrogen fixation requires molybdoenzymes in legumes (e.g., soybeans, acacia, etc.). For this reason, fertilizers often contain small amounts of molybdate salts. Coverage is typically less than a kilogram per acre. [12]

Molybdate chrome pigments are speciality but commercially available pigments. [12] Molybdate (usually in the form of potassium molybdate) is also used in the analytical colorimetric testing for the concentration of silica in solution, called the molybdenum blue method. [21] Additionally, it is used in the colorimetric analysis of phosphate concentration in association with the dye malachite green.

Natural gems

Molybdate crystals as collected by gem enthusiasts with the world's best samples of crystalized molybdate coming from Madawaska Mine in Ontario (Canada). [22]

Related Research Articles

<span class="mw-page-title-main">Oxide</span> Chemical compound where oxygen atoms are combined with atoms of other elements

An oxide is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– ion with oxygen in the oxidation state of −2. Most of the Earth's crust consists of oxides. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 that protects the foil from further oxidation.

<span class="mw-page-title-main">Osmium tetroxide</span> Chemical compound

Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity OsO2, which is yellow-brown in colour. In biology, its property of binding to lipids has made it a widely-used stain in electron microscopy.

An oxyanion, or oxoanion, is an ion with the generic formula A
xOz−
y. Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determined by the octet rule. The corresponding oxyacid of an oxyanion is the compound H
zA
xO
y. The structures of condensed oxyanions can be rationalized in terms of AOn polyhedral units with sharing of corners or edges between polyhedra. The oxyanions adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology.

<span class="mw-page-title-main">Chromate and dichromate</span> Chromium(VI) anions

Chromate salts contain the chromate anion, CrO2−
4. Dichromate salts contain the dichromate anion, Cr
2O2−
7. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

<span class="mw-page-title-main">Polyoxometalate</span> Ion with many transition metals

In chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 or less commonly group 5 and group 7 transition metals in their high oxidation states. Polyoxometalates are often colorless, orange or red diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one or more metals, oxide, and eventually a main group oxyanion. Many exceptions to these general statements exist.

<span class="mw-page-title-main">Potassium ferrate</span> Chemical compound

Potassium ferrate is the chemical compound with the formula K2FeO4. This purple salt is paramagnetic, and is a rare example of an iron(VI) compound. In most of its compounds, iron has the oxidation state +2 or +3 (Fe2+ or Fe3+). Reflecting its high oxidation state, FeO2−4 is a powerful oxidizing agent resembling chromate (CrO2−4) or manganate (MnO2−4).

Molybdenum trioxide describes a family of inorganic compounds with the formula MoO3(H2O)n where n = 0, 1, 2. The anhydrous compound is produced on the largest scale of any molybdenum compound since it is the main intermediate produced when molybdenum ores are purified. The anhydrous oxide is a precursor to molybdenum metal, an important alloying agent. It is also an important industrial catalyst. It is a yellow solid, although impure samples can appear blue or green.

<span class="mw-page-title-main">Manganate</span> Chemical compound

In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom. However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO2−
4, also known as manganate(VI) because it contains manganese in the +6 oxidation state. Manganates are the only known manganese(VI) compounds.

<span class="mw-page-title-main">Tellurate</span> Compound containing an oxyanion of tellurium

In chemistry, tellurate is a compound containing an oxyanion of tellurium where tellurium has an oxidation number of +6. In the naming of inorganic compounds it is a suffix that indicates a polyatomic anion with a central tellurium atom.

<span class="mw-page-title-main">Pertechnetate</span> Chemical compound or ion

The pertechnetate ion is an oxyanion with the chemical formula TcO−
4. It is often used as a convenient water-soluble source of isotopes of the radioactive element technetium (Tc). In particular it is used to carry the 99mTc isotope which is commonly used in nuclear medicine in several nuclear scanning procedures.

<span class="mw-page-title-main">Sodium molybdate</span> Chemical compound

Sodium molybdate, Na2MoO4, is useful as a source of molybdenum. This white, crystalline salt is often encountered as the dihydrate, Na2MoO4·2H2O.

In chemical nomenclature, the IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds, as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in Nomenclature of Inorganic Chemistry. Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined. There is also an IUPAC nomenclature of organic chemistry.

<span class="mw-page-title-main">Potassium chromate</span> Chemical compound

Potassium chromate is the inorganic compound with the formula K2CrO4. This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate is important industrially.

In chemistry, hypomanganate, also called manganate(V) or tetraoxidomanganate(3−), is a trivalent anion (negative ion) composed of manganese and oxygen, with formula MnO3−
4.

<span class="mw-page-title-main">Chromium compounds</span> Chemical compounds containing chromium

Chromium compounds are compounds containing the element chromium (Cr). Chromium is a member of group 6 of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

<span class="mw-page-title-main">Ammonium heptamolybdate</span> Chemical compound

Ammonium heptamolybdate is the inorganic compound whose chemical formula is (NH4)6Mo7O24, normally encountered as the tetrahydrate. A dihydrate is also known. It is a colorless solid, often referred to as ammonium paramolybdate or simply as ammonium molybdate, although "ammonium molybdate" can also refer to ammonium orthomolybdate, (NH4)2MoO4, and several other compounds. It is one of the more common molybdenum compounds.

<span class="mw-page-title-main">Molybdenum blue</span> Pigment

Molybdenum blue is a term applied to:

<span class="mw-page-title-main">Manganese(II) molybdate</span> Inorganic compound

Manganese(II) molybdate is an inorganic compound with the chemical formula MnMoO4. α-MnMoO4 has a monoclinic crystal structure. It is also antiferromagnetic at low temperatures.

Ammonium dimolybdate (ADM) is the inorganic compound with the formula (NH4)2Mo2O7. It is a white, water-soluble solid. ADM is an intermediate in the production of molybdenum compounds from its ores. Roasting typical ore produces crude molybdenum(VI) oxides, which can be extracted into aqueous ammonia, affording ammonium molybdate. Heating solutions of ammonium molybdate gives ADM. Upon heating, solid ammonium dimolybdate decomposes to molybdenum trioxide:

<span class="mw-page-title-main">Potassium hypochromate</span> Chemical compound

Potassium hypochromate is a chemical compound with the formula K3CrO4 with the unusual Cr5+ ion. This compound is unstable in water but stable in alkaline solution and was found to have a similar crystal structure to potassium hypomanganate.

References

  1. 1 2 3 4 5 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  2. 1 2 3 4 Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN   0-471-19957-5
  3. ↑ V. W. Day; M. F. Fredrich; W. G. Klemperer; W. Shum (1977). "Synthesis and characterization of the dimolybdate ion, Mo
    2    O2−
    7    ". Journal of the American Chemical Society. 99 (18): 6146. doi:10.1021/ja00460a074.
  4. ↑ Guillou N.; Ferey G. (August 1997). "Hydrothermal Synthesis and Crystal Structure of Anhydrous Ethylenediamine Trimolybdate (C
    2    H
    10    N
    2    )[Mo
    3    O
    10    ]    ". Journal of Solid State Chemistry. 132 (1): 224–227(4). Bibcode:1997JSSCh.132..224G. doi:10.1006/jssc.1997.7502.
  5. ↑ B. M. Gatehouse; P. Leverett (1971). "Crystal structure of potassium tetramolybdate, K
    2    Mo
    4    O
    13    , and its relationship to the structures of other univalent metal polymolybdates". J. Chem. Soc. A: 2107–2112. doi:10.1039/J19710002107.
  6. ↑ W. Lasocha; H. Schenk (1997). "Crystal Structure of Anilinium Pentamolybdate from Powder Diffraction Data. The Solution of the Crystal Structure by Direct Methods Package POWSIM". J. Appl. Crystallogr. 30 (6): 909–913. doi:10.1107/S0021889897003105.
  7. ↑ S. Ghammami (2003). "The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI)". Crystal Research and Technology. 38 (913): 913–917. doi:10.1002/crat.200310112. S2CID   95078211.
  8. ↑ Howard T. Evans jr.; Bryan M. Gatehouse; Peter Leverett (1975). "Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo7O24]6−, in the ammonium and potassium tetrahydrate salts". J. Chem. Soc., Dalton Trans. (6): 505–514. doi:10.1039/DT9750000505.
  9. ↑ "Oxides: solid state chemistry" W.H. McCarroll, Encyclopedia of Inorganic Chemistry Ed. R. Bruce King, John Wiley and Sons (1994) ISBN   0-471-93620-0
  10. ↑ Klemperer, W. G. (1990). "Tetrabutylammonium Isopolyoxometalates". Inorganic Syntheses. Inorganic Syntheses. Vol. 27. pp. 74–85. doi:10.1002/9780470132586.ch15. ISBN   9780470132586.
  11. ↑ Dickman, Michael H.; Pope, Michael T. (1994). "Peroxo and Superoxo Complexes of Chromium, Molybdenum, and Tungsten". Chem. Rev. 94 (3): 569–584. doi:10.1021/cr00027a002.
  12. 1 2 3 Roger F. Sebenik et al. "Molybdenum and Molybdenum Compounds" in Ullmann's Encyclopedia of Chemical Technology 2005; Wiley-VCH, Weinheim. doi : 10.1002/14356007.a16_655
  13. ↑ "Open Recirculating Cooling Systems - GE Water". gewater.com.
  14. ↑ "Closed Recirculating Cooling Systems - GE Water". gewater.com.
  15. ↑ "Microbiologically Influenced Corrosion in the Upstream Oil and Gas Industry".
  16. ↑ Purushothaman, K. K.; Cuba, M.; Muralidharan, G. (2012-11-01). "Supercapacitor behavior of α-MnMoO4 nanorods on different electrolytes". Materials Research Bulletin. 47 (11): 3348–3351. doi:10.1016/j.materresbull.2012.07.027.
  17. 1 2 Senthilkumar, Baskar; Sankar, Kalimuthu Vijaya; Selvan, Ramakrishnan Kalai; Danielle, Meyrick; Manickam, Minakshi (2012-12-05). "Nano α-NiMoO4 as a new electrode for electrochemical supercapacitors". RSC Adv. 3 (2): 352–357. doi:10.1039/c2ra22743f. ISSN   2046-2069.
  18. ↑ Cai, Daoping; Wang, Dandan; Liu, Bin; Wang, Yanrong; Liu, Yuan; Wang, Lingling; Li, Han; Huang, Hui; Li, Qiuhong (2013-12-26). "Comparison of the Electrochemical Performance of NiMoO4 Nanorods and Hierarchical Nanospheres for Supercapacitor Applications". ACS Applied Materials & Interfaces. 5 (24): 12905–12910. doi:10.1021/am403444v. ISSN   1944-8244. PMID   24274769.
  19. ↑ Xia, Xifeng; Lei, Wu; Hao, Qingli; Wang, Wenjuan; Wang, Xin (2013-06-01). "One-step synthesis of CoMoO4/graphene composites with enhanced electrochemical properties for supercapacitors". Electrochimica Acta. 99: 253–261. doi:10.1016/j.electacta.2013.03.131.
  20. ↑ National Research Council (US) Committee on Medical Isotope Production Without Highly Enriched Uranium. (2009). "Molybdenum-99/Technetium-99m Production and Use". Medical Isotope Production without Highly Enriched Uranium. Washington DC: National Academies Press.
  21. ↑ "ASTM D7126 – 15 Standard Test Method for On-Line Colorimetric Measurement of Silica". astm.org.
  22. ↑ McDougall, Raymond (2019-09-03). "Mineral Highlights from the Bancroft Area, Ontario, Canada". Rocks & Minerals. 94 (5): 408–419. doi:10.1080/00357529.2019.1619134. ISSN   0035-7529. S2CID   201298402.
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