Molybdate
Chemical compound of the form –O–MoO2–O–
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Examples of molybdate anions
Examples of molybdate oxyanions are:
- MoO42−, in e.g. Na2MoO4 and the mineral powellite, CaMoO4;
- Mo2O72−, as hydrated ammonium dimolybdate. The anhydrous tetrabutylammonium salt of Mo2O7_2− is also known;
- Mo3O102− in the ethylenediamine salt;
- Mo4O132− in the potassium salt;
- Mo5O162− in the anilinium (C6H5NH3+) salt;
- Mo6O192− (hexa-molybdate) in the tetramethylammonium salt;
- Mo7O246− in ammonium heptamolybdate, (NH4)6Mo7O24·4H2O;
- Mo8O264− in trimethylammonium salt;
- Mo10O336− in silver decamolybdate.
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, Mo7O246−, is often called "paramolybdate".
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Structure of molybdate anions
The smaller anions, MoO42− and Mo2O72− feature tetrahedral centres. In MoO42− the four oxygens are equivalent as in sulfate and chromate, with equal bond lengths and angles. Mo2O72− can be considered to be two tetrahedra sharing a corner, i.e. with a single bridging O atom, as in dichromate. 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:
- in terminal non bridging M–O approximately 1.7 Å
- in bridging M–O–M units approximately 1.9 Å
The Mo8O264− anion contains both octahedral and tetrahedral molybdenum and can be isolated in 2 isomeric forms, alpha and beta.
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. 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.
Equilibrium in aqueous solution
When MoO3, molybdenum trioxide is dissolved in alkali solution the simple MoO42− anion is produced:
- MoO3 + 2 NaOH → Na2MoO4 + H2O
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:
- 6 MoO42− + 10 HCl → Mo6O192− + 10 Cl− + 5 H2O
- 7 MoO42− + 8 H+ → Mo7O246− + 4 H2O
- Mo7O246− + HMoO4− + 3 H+ → Mo8O264− + 2 H2O
Peroxomolybdates
Many peroxomolybdates are known, e.g. [Mo2O3(O2)2(H2O)2]2− and the tetraperoxide complex [Mo(O2)4]2−. They form upon treatment of molybdate salts with hydrogen peroxide.
Tetrathiomolybdate
The red tetrathiomolybdate anion results when molybdate solutions are treated with hydrogen sulfide:
- (NH4)2MoO4 + 4 H2S → (NH4)2MoS4 + 4 H2O
Like molybdate itself, MoS42− 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.
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, and is mainly used in high temperature closed-loop cooling circuits. According to an experimental study, molybdate has been reported as an efficient biocide against microbiologically induced corrosion (MIC), where adding 1.5 mM MoO42−/day resulted in a 50 % decrease in the corrosion rate.
Supercapacitors
Molybdates (especially FeMoO4, Fe2(MoO4)3, NiMoO4, CoMoO4 and MnMoO4) have been used as anode or cathode materials in aqueous capacitors. Due to pseudocapacitive charge storage, specific capacitance up to 1500 F g−1 has been observed.
Medicine
Radioactive molybdenum-99 in the form of molybdate is used as the parent isotope in technetium-99m generators for nuclear medicine imaging.
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.
Molybdate chrome pigments are speciality but commercially available pigments. 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. Additionally, it is used in the colorimetric analysis of phosphate concentration in association with the dye malachite green.
Molybdovanadate reagents contain both molybdate and vanadate ions. They are used in the determination of phosphate ion content in the analysis of wine and other fruit based products.
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).