Molybdenum

Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.

Molybdenum does not occur naturally as a free metal on Earth; it is found only in various oxidation states in minerals. The free element, a silvery metal with a grey cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys, and for this reason most of the world production of the element (about 80%) is used in steel alloys, including high-strength alloys and superalloys.

Most molybdenum compounds have low solubility in water, but when molybdenum-bearing minerals contact oxygen and water, the resulting molybdate ion is quite soluble. Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications as pigments and catalysts.

are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation. At least 50 molybdenum enzymes are now known in bacteria, plants, and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain an iron-molybdenum cofactor FeMoco, which is believed to contain either Mo(III) or Mo(IV). This is distinct from the fully oxidized Mo(VI) found complexed with molybdopterin in all other molybdenum-bearing enzymes, which perform a variety of crucial functions. The variety of crucial reactions catalyzed by these latter enzymes means that molybdenum is an essential element for all higher eukaryote organisms, including humans.

Characteristics

Physical properties

In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5 and a standard atomic weight of 95.95 g/mol. It has a melting point of ; of the naturally occurring elements, only tantalum, osmium, rhenium, tungsten, and carbon have higher melting points. It has one of the lowest coefficients of thermal expansion among commercially used metals.

Chemical properties

Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. It does not visibly react with oxygen or water at room temperature. Weak oxidation of molybdenum starts at ; bulk oxidation occurs at temperatures above 600 °C, resulting in molybdenum trioxide. Like many heavier transition metals, molybdenum shows little inclination to form a cation in aqueous solution, although the Mo³⁺ cation is known under carefully controlled conditions.

Gaseous molybdenum consists of the diatomic species Mo₂. That molecule is a singlet, with two unpaired electrons in bonding orbitals, in addition to 5 conventional bonds. The result is a sextuple bond.

Isotopes

There are 35 known isotopes of molybdenum, ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring isotopes, only molybdenum-100 is unstable.

Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 10¹⁹  y and undergoes double beta decay into ruthenium-100. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium. Of the synthetic radioisotopes, the most stable is ⁹³Mo, with a half-life of 4,000 years.

The most common isotopic molybdenum application involves molybdenum-99, which is a fission product. It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine.
In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99.

Compounds

Molybdenum forms chemical compounds in oxidation states −IV and from −II to +VI. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with metal clusters, and very low oxidation states are typically associated with organomolybdenum compounds. Mo and W chemistry shows strong similarities. The relative rarity of molybdenum(III), for example, contrasts with the pervasiveness of the chromium(III) compounds. The highest oxidation state is seen in molybdenum(VI) oxide (MoO₃), whereas the normal sulfur compound is molybdenum disulfide MoS₂.

From the perspective of commerce, the most important compounds are molybdenum disulfide () and molybdenum trioxide (). The black disulfide is the main mineral. It is roasted in air to give the trioxide:

:2 + 7 → 2 + 4

The trioxide, which is volatile at high temperatures, is the precursor to virtually all other Mo compounds as well as alloys. Molybdenum has several oxidation states, the most stable being +4 and +6 (bolded in the table at left).

Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO₄²⁻). Molybdates are weaker oxidants than chromates. They tend to form structurally complex oxyanions by condensation at lower pH values, such as o₇O₂₄sup>6− and o₈O₂₆sup>4−. Polymolybdates can incorporate other ions, forming polyoxometalates. The dark-blue phosphorus-containing heteropolymolybdate P o₁₂O₄₀sup>3− is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides:
* Molybdenum(II) chloride MoCl₂, which exists as the hexamer Mo₆Cl₁₂ and the related dianion o₆Cl₁₄sup>2-.
* Molybdenum(III) chloride MoCl₃, a dark red solid, which converts to the anion trianionic complex oCl₆sup>3-.
* Molybdenum(IV) chloride MoCl₄, a black solid, which adopts a polymeric structure.
* Molybdenum(V) chloride MoCl₅ dark green solid, which adopts a dimeric structure.
* Molybdenum(VI) chloride MoCl₆ is a black solid, which is monomeric and slowly decomposes to MoCl₅ and Cl₂ at room temperature.
Like chromium and some other transition metals, molybdenum forms quadruple bonds, such as in Mo₂(CH₃COO)₄ and o₂Cl₈sup>4−. The Lewis acid properties of the butyrate and perfluorobutyrate dimers, Mo₂(O₂CR)₄ and Rh₂(O₂CR) ₄, have been reported.

The oxidation state 0 and lower are possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)₆.

History

Molybdenite—the principal ore from which molybdenum is now extracted—was previously known as molybdena. Molybdena was confused with and often utilized as though it were graphite. Like graphite, molybdenite can be used to blacken a surface or as a solid lubricant. Even when molybdena was distinguishable from graphite, it was still confused with the common lead ore PbS (now called galena); the name comes from Ancient Greek ', meaning ''lead''. (The Greek word itself has been proposed as a loanword from Anatolian Luvian and Lydian languages).

Although (reportedly) molybdenum was deliberately alloyed with steel in one 14th-century Japanese sword (mfd. ca. 1330), that art was never employed widely and was later lost. In the West in 1754, Bengt Andersson Qvist examined a sample of molybdenite and determined that it did not contain lead and thus was not galena.

By 1778 Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was (indeed) neither galena nor graphite. Instead, Scheele correctly proposed that molybdena was an ore of a distinct new element, named ''molybdenum'' for the mineral in which it resided, and from which it might be isolated. Peter Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil in 1781.

For the next century, molybdenum had no industrial use. It was relatively scarce, the pure metal was difficult to extract, and the necessary techniques of metallurgy were immature. Early molybdenum steel alloys showed great promise of increased hardness, but efforts to manufacture the alloys on a large scale were hampered with inconsistent results, a tendency toward brittleness, and recrystallization. In 1906, William D. Coolidge filed a patent for rendering molybdenum ductile, leading to applications as a heating element for high-temperature furnaces and as a support for tungsten-filament light bulbs; oxide formation and degradation require that molybdenum be physically sealed or held in an inert gas. In 1913, Frank E. Elmore developed a froth flotation process to recover molybdenite from ores; flotation remains the primary isolation process.

During World War I, demand for molybdenum spiked; it was used both in armor plating and as a substitute for tungsten in high-speed steels. Some British tanks were protected by 75 mm (3 in) manganese steel plating, but this proved to be ineffective. The manganese steel plates were replaced with much lighter molybdenum steel plates allowing for higher speed, greater maneuverability, and better protection. The Germans also used molybdenum-doped steel for heavy artillery, like in the super-heavy howitzer Big Bertha, because traditional steel melts at the temperatures produced by the propellant of the one ton shell. After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications. In World War II, molybdenum again saw strategic importance as a substitute for tungsten in steel alloys.

Occurrence and production

Molybdenum is the 54th most abundant element in the Earth's crust with an average of 1.5 parts per million and the 25th most abundant element in its oceans, with an average of 10 parts per billion; it is the 42nd most abundant element in the Universe. The Russian Luna 24 mission discovered a molybdenum-bearing grain (1 × 0.6 µm) in a pyroxene fragment taken from Mare Crisium on the Moon. The comparative rarity of molybdenum in the Earth's crust is offset by its concentration in a number of water-insoluble ores, often combined with sulfur in the same way as copper, with which it is often found. Though molybdenum is found in such minerals as wulfenite (PbMoO₄) and powellite (CaMoO₄), the main commercial source is molybdenite (Mo S₂). Molybdenum is mined as a principal ore and is also recovered as a byproduct of copper and tungsten mining.

The world's production of molybdenum was 250,000 tonnes in 2011, the largest producers being China (94,000 t), the United States (64,000 t), Chile (38,000 t), Peru (18,000 t) and Mexico (12,000 t). The total reserves are estimated at 10 million tonnes, and are mostly concentrated in China (4.3 Mt), the US (2.7 Mt) and Chile (1.2 Mt). By continent, 93% of world molybdenum production is about evenly shared between North America, South America (mainly in Chile), and China. Europe and the rest of Asia (mostly Armenia, Russia, Iran and Mongolia) produce the remainder.

In molybdenite processing, the ore is first roasted in air at a temperature of . The process gives gaseous sulfur dioxide and the molybdenum(VI) oxide:

:2MoS2 + 7O2 -> 2MoO3 + 4SO2

The oxidized ore is then usually extracted with aqueous ammonia to give ammonium molybdate:
:MoO3 + 2NH3 + H2O -> (NH4)2(MoO4)
Copper, an impurity in molybdenite, is less soluble in ammonia. To completely remove it from the solution, it is precipitated with hydrogen sulfide. Ammonium molybdate converts to ammonium dimolybdate, which is isolated as a solid. Heating this solid gives molybdenum trioxide:Sebenik, Roger F. et al. (2005) "Molybdenum and Molybdenum Compounds" in ''Ullmann's Encyclopedia of Chemical Technology''. Wiley-VCH, Weinheim.
: (NH4)2Mo2O7 -> 2MoO3 + 2NH3 + H2O
Crude trioxide can be further purified by sublimation at .

Metallic molybdenum is produced by reduction of the oxide with hydrogen:
:MoO3 + 3H2 -> Mo + 3H2O

The molybdenum for steel production is reduced by the aluminothermic reaction with addition of iron to produce ferromolybdenum. A common form of ferromolybdenum contains 60% molybdenum.

Molybdenum had a value of approximately $30,000 per tonne as of August 2009. It maintained a price at or near $10,000 per tonne from 1997 through 2003, and reached a peak of $103,000 per tonne in June 2005. In 2008, the London Metal Exchange announced that molybdenum would be traded as a commodity.

Mining

Historically, the Knaben mine in southern Norway, opened in 1885, was the first dedicated molybdenum mine. It was closed in 1973 but was reopened in 2007. and now produces of molybdenum disulfide per year. Large mines in Colorado (such as the Henderson mine and the Climax mine) and in British Columbia yield molybdenite as their primary product, while many porphyry copper deposits such as the Bingham Canyon Mine in Utah and the Chuquicamata mine in northern Chile produce molybdenum as a byproduct of copper mining.

Applications

Alloys

About 86% of molybdenum produced is used in metallurgy, with the rest used in chemical applications. The estimated global use is structural steel 35%, stainless steel 25%, chemicals 14%, tool & high-speed steels 9%, cast iron 6%, molybdenum elemental metal 6%, and superalloys 5%.

Molybdenum can withstand extreme temperatures without significantly expanding or softening, making it useful in environments of intense heat, including military armor, aircraft parts, electrical contacts, industrial motors, and supports for filaments in light bulbs.

Most high-strength steel alloys (for example, 41xx steels) contain 0.25% to 8% molybdenum. Even in these small portions, more than 43,000 tonnes of molybdenum are used each year in stainless steels, tool steels, cast irons, and high-temperature superalloys.

Molybdenum is also valued in steel alloys for its high corrosion resistance and weldability. Molybdenum contributes corrosion resistance to type-300 stainless steels (specifically type-316) and especially so in the so-called super austenitic stainless steels (such as alloy AL-6XN, 254SMO and 1925hMo). Molybdenum increases lattice strain, thus increasing the energy required to dissolve iron atoms from the surface. Molybdenum is also used to enhance the corrosion resistance of ferritic (for example grade 444) and martensitic (for example 1.4122 and 1.4418) stainless steels.

Because of its lower density and more stable price, molybdenum is sometimes used in place of tungsten. An example is the 'M' series of high-speed steels such as M2, M4 and M42 as substitution for the 'T' steel series, which contain tungsten. Molybdenum can also be used as a flame-resistant coating for other metals. Although its melting point is , molybdenum rapidly oxidizes at temperatures above making it better-suited for use in vacuum environments.

TZM (Mo (~99%), Ti (~0.5%), Zr (~0.08%) and some C) is a corrosion-resisting molybdenum superalloy that resists molten fluoride salts at temperatures above . It has about twice the strength of pure Mo, and is more ductile and more weldable, yet in tests it resisted corrosion of a standard eutectic salt ( FLiBe) and salt vapors used in molten salt reactors for 1100 hours with so little corrosion that it was difficult to measure.

Other molybdenum-based alloys that do not contain iron have only limited applications. For example, because of its resistance to molten zinc, both pure molybdenum and molybdenum- tungsten alloys (70%/30%) are used for piping, stirrers and pump impellers that come into contact with molten zinc.

Other applications as a pure element

* Molybdenum powder is used as a fertilizer for some plants, such as cauliflower
* Elemental molybdenum is used in NO, NO₂, NO_x analyzers in power plants for pollution controls. At , the element acts as a catalyst for NO₂/NO_x to form NO molecules for detection by infrared light.
* Molybdenum anodes replace tungsten in certain low voltage X-ray sources for specialized uses such as mammography.
* The radioactive isotope molybdenum-99 is used to generate technetium-99m, used for medical imaging The isotope is handled and stored as the molybdate.

Compounds (14% of global use)

* Molybdenum disulfide (MoS₂) is used as a solid lubricant and a high-pressure high-temperature (HPHT) anti-wear agent. It forms strong films on metallic surfaces and is a common additive to HPHT greases — in the event of a catastrophic grease failure, a thin layer of molybdenum prevents contact of the lubricated parts. It also has semiconducting properties with distinct advantages over traditional silicon or graphene in electronics applications. MoS₂ is also used as a catalyst in hydrocracking of petroleum fractions containing nitrogen, sulfur and oxygen.
* Molybdenum disilicide (MoSi₂) is an electrically conducting ceramic with primary use in heating elements operating at temperatures above 1500 °C in air.
* Molybdenum trioxide (MoO₃) is used as an adhesive between enamels and metals.
*Lead molybdate (wulfenite) co-precipitated with lead chromate and lead sulfate is a bright-orange pigment used with ceramics and plastics.
* The molybdenum-based mixed oxides are versatile catalysts in the chemical industry. Some examples are the catalysts for the oxidation of carbon monoxide, selective oxidation of propylene to acrolein and acrylic acid, the ammoxidation of glycerol and propylene to acrylonitrile. Suitable catalysts and process for the direct selective oxidation of propane to acrylic acid are being researched.
* Molybdenum carbides, nitride and phosphides can be used for hydrotreatment of rapeseed oil.
* Ammonium heptamolybdate is used in biological staining.
* Molybdenum coated soda lime glass is used in CIGS ( copper indium gallium selenide) solar cell

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