Molybdenum

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Molybdenum, 42Mo
Molybdenum
Pronunciation/məˈlɪbdənəm/ ​(mə-LIB-də-nəm)
Appearancegray metallic
Standard atomic weight Ar°(Mo)
Molybdenum in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Cr
↑
Mo
↓
W
niobium ← molybdenum → technetium
kJ/mol
Heat of vaporization598 kJ/mol
Molar heat capacity24.06 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2742 2994 3312 3707 4212 4879
Atomic properties
Discovery
Carl Wilhelm Scheele (1778)
First isolationPeter Jacob Hjelm (1781)
Isotopes of molybdenum
Main isotopes[8] Decay
abun­dance half-life (t1/2) mode pro­duct
92Mo 14.7%
stable
93Mo synth 4839 y[9] ε
93Nb
94Mo 9.19% stable
95Mo 15.9% stable
96Mo 16.7% stable
97Mo 9.58% stable
98Mo 24.3% stable
99Mo synth 65.94 h
β−
 		99mTc
γ
 		–
100Mo 9.74% 7.1×1018 y β−β−
100Ru
 Category: Molybdenum
| references

Molybdenum is a

symbol Mo (from Neo-Latin molybdaenum) and atomic number 42. The name derived from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores.[10] 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.[11]

Molybdenum does not occur naturally as a

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

high-pressure and high-temperature applications as pigments and catalysts
.

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.[15][16] It has a melting point of 2,623 °C (4,753 °F), sixth highest of the naturally occurring elements; only tantalum, osmium, rhenium, tungsten, and carbon have higher melting points.[10] It has one of the lowest coefficients of thermal expansion among commercially used metals.[17]

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, but is attacked by halogens and hydrogen peroxide. Weak oxidation of molybdenum starts at 300 °C (572 °F); 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 to form under carefully controlled conditions.[18]

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.[19][20]

Isotopes

Main article: Isotopes of molybdenum

There are 39 known

isotopes of molybdenum, ranging in atomic mass from 81 to 119, as well as 13 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.[8]

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 4,839 years.[9]

The most common isotopic molybdenum application involves

parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine.[21]
In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99.[22]

Compounds

See also: Category:Molybdenum compounds

Molybdenum forms chemical compounds in oxidation states −4 and from −2 to +6. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with

molybdenum(VI) oxide (MoO3), whereas the normal sulfur compound is molybdenum disulfide MoS2.[23]

Oxidation
state 		Example[24][25]
−4 Na
4[Mo(CO)
4]
−2 [Mo(CO)
5]2−
[26]
−1 Na
2[Mo
2(CO)
10]
0 Mo(CO)
6
+1
C
5H
5Mo(CO)
3
+2 MoCl
2
+3 MoBr
3
+4 MoS
2
+5 MoCl
5
+6
MoF
6
Keggin structure of the phosphomolybdate anion (P[Mo12O40]3−), an example of a polyoxometalate

From the perspective of commerce, the most important compounds are molybdenum disulfide (MoS
2) and molybdenum trioxide (MoO
3). The black disulfide is the main mineral. It is roasted in air to give the trioxide:[23]

2 MoS
2 + 7 O
2 → 2 MoO
3 + 4 SO
2

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

spectroscopic detection of phosphorus.[28] The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides:[23]

Like chromium and some other transition metals, molybdenum forms quadruple bonds, such as in Mo2(CH3COO)4 and [Mo2Cl8]4−.[23][30] The Lewis acid properties of the butyrate and perfluorobutyrate dimers, Mo2(O2CR)4 and Rh2(O2CR) 4, have been reported.[31]

The oxidation state 0 and lower are possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)6.[23][25]

History

Luvian and Lydian languages).[33]

Although (reportedly) molybdenum was deliberately alloyed with steel in one 14th-century Japanese sword (mfd. c. 1330), that art was never employed widely and was later lost.[34][35] 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.[36]

By 1778 Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was (indeed) neither galena nor graphite.[37][38] 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.[17][39]

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.

froth flotation process to recover molybdenite from ores; flotation remains the primary isolation process.[44]

During

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 25 mm (1.0 in) molybdenum steel plates allowing for higher speed, greater maneuverability, and better protection.[17] The Germans also used molybdenum-doped steel for heavy artillery, like in the super-heavy howitzer Big Bertha,[45] because traditional steel melts at the temperatures produced by the propellant of the one ton shell.[46] 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.[47]

Occurrence and production

Lustrous, silvery, flat, hexagonal crystals in roughly parallel layers sit flowerlike on a rough, translucent crystalline piece of quartz.
Molybdenite on quartz

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.[17][48] The Soviet Luna 24 mission discovered a molybdenum-bearing grain (1 × 0.6 µm) in a pyroxene fragment taken from Mare Crisium on the Moon.[49] 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 (MoS2). Molybdenum is mined as a principal ore and is also recovered as a byproduct of copper and tungsten mining.[10]

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.[50]

World production trend

In molybdenite processing, the ore is first roasted in air at a temperature of 700 °C (1,292 °F). The process gives gaseous sulfur dioxide and the molybdenum(VI) oxide:[23]

The resulting oxide is then usually extracted with aqueous ammonia to give ammonium molybdate:

Copper, an impurity in molybdenite, is separated at this stage by treatment with hydrogen sulfide.[23] Ammonium molybdate converts to ammonium dimolybdate, which is isolated as a solid. Heating this solid gives molybdenum trioxide:[51]

Crude trioxide can be further purified by sublimation at 1,100 °C (2,010 °F).

Metallic molybdenum is produced by reduction of the oxide with hydrogen:

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.[23][52]

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.[53] In 2008, the London Metal Exchange announced that molybdenum would be traded as a commodity.[54]

Mining

The

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

A plate of molybdenum copper alloy

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%.[57]

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.[17][58]

Most high-strength steel alloys (for example, 41xx steels) contain 0.25% to 8% molybdenum.[10] 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.[48]

Molybdenum is also used in steel alloys for its high

superaustenitic 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.[contradictory] Molybdenum is also used to enhance the corrosion resistance of ferritic (for example grade 444)[59] and martensitic (for example 1.4122 and 1.4418) stainless steels.[citation needed
]

Because of its lower density and more stable price, molybdenum is sometimes used in place of tungsten.[48] 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 2,623 °C (4,753 °F), molybdenum rapidly oxidizes at temperatures above 760 °C (1,400 °F) making it better-suited for use in vacuum environments.[58]

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 1,300 °C (2,370 °F). 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 (

molten salt reactors for 1100 hours with so little corrosion that it was difficult to measure.[60][61]

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.[62]

Other applications as a pure element

  • Molybdenum powder is used as a fertilizer for some plants, such as cauliflower.[48]
  • Elemental molybdenum is used in NO, NO2, NOx analyzers in power plants for pollution controls. At 350 °C (662 °F), the element acts as a catalyst for NO2/NOx to form NO molecules for detection by infrared light.[63]
  • Molybdenum anodes replace tungsten in certain low voltage X-ray sources for specialized uses such as mammography.[64]
  • The radioactive isotope
    molybdenum-99 is used to generate technetium-99m, used for medical imaging[65] The isotope is handled and stored as the molybdate.[66]

Compounds

Biological role

Main article: Molybdenum in biology

Mo-containing enzymes

Molybdenum is an essential element in most organisms; a 2008 research paper speculated that a scarcity of molybdenum in the Earth's early oceans may have strongly influenced the evolution of eukaryotic life (which includes all plants and animals).[75]

At least 50 molybdenum-containing enzymes have been identified, mostly in bacteria.[76][77] Those enzymes include aldehyde oxidase, sulfite oxidase and xanthine oxidase.[17] With one exception, Mo in proteins is bound by molybdopterin to give the molybdenum cofactor. The only known exception is nitrogenase, which uses the FeMoco cofactor, which has the formula Fe7MoS9C.[78]

In terms of function, molybdoenzymes catalyze the oxidation and sometimes reduction of certain small molecules in the process of regulating

protein synthesis, metabolism, and growth.[80]

Mo is a component in most nitrogenases. Among molybdoenzymes, nitrogenases are unique in lacking the molybdopterin.[81][82] Nitrogenases catalyze the production of ammonia from atmospheric nitrogen:

The biosynthesis of the FeMoco active site is highly complex.[83]

Structure of the FeMoco active site of nitrogenase
Skeletal structure of a molybdopterin with a single molybdenum atom bound to both of the thiolate groups
The molybdenum cofactor (pictured) is composed of a molybdenum-free organic complex called molybdopterin, which has bound an oxidized molybdenum(VI) atom through adjacent sulfur (or occasionally selenium) atoms. Except for the ancient nitrogenases, all known Mo-using enzymes use this cofactor.

Molybdate is transported in the body as MoO42−.[80]

Human metabolism and deficiency

Molybdenum is an essential trace

dietary element.[84] Four mammalian Mo-dependent enzymes are known, all of them harboring a pterin-based molybdenum cofactor (Moco) in their active site: sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and mitochondrial amidoxime reductase.[85] People severely deficient in molybdenum have poorly functioning sulfite oxidase and are prone to toxic reactions to sulfites in foods.[86][87] The human body contains about 0.07 mg of molybdenum per kilogram of body weight,[88] with higher concentrations in the liver and kidneys and lower in the vertebrae.[48] Molybdenum is also present within human tooth enamel and may help prevent its decay.[89]

Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a median lethal dose (LD50) as low as 180 mg/kg for some Mo compounds.[90] Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation, infertility, low birth weight, and gout; it can also affect the lungs, kidneys, and liver.[91][92] Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.[48]

Low soil concentration of molybdenum in a geographical band from northern China to Iran results in a general dietary

squamous cell carcinoma.[96]

Molybdenum deficiency has also been reported as a consequence of non-molybdenum supplemented

urate, in much the same way as molybdenum cofactor deficiency. Since pure molybdenum deficiency from this cause occurs primarily in adults, the neurological consequences are not as marked as in cases of congenital cofactor deficiency.[97]

A congenital

urate, and neurological damage.[98][99]

Excretion

Most molybdenum is excreted from the human body as molybdate in the urine. Furthermore, urinary excretion of molybdenum increases as dietary molybdenum intake increases. Small amounts of molybdenum are excreted from the body in the feces by way of the bile; small amounts also can be lost in sweat and in hair.[100][101]

Excess and copper antagonism

High levels of molybdenum can interfere with the body's uptake of copper, producing copper deficiency. Molybdenum prevents plasma proteins from binding to copper, and it also increases the amount of copper that is excreted in urine. Ruminants that consume high levels of molybdenum suffer from diarrhea, stunted growth, anemia, and achromotrichia (loss of fur pigment). These symptoms can be alleviated by copper supplements, either dietary and injection.[102] The effective copper deficiency can be aggravated by excess sulfur.[48][103]

Copper reduction or deficiency can also be deliberately induced for therapeutic purposes by the compound

age-related macular degeneration, and other diseases that involve a pathologic proliferation of blood vessels.[105][106]

In some grazing livestock, most strongly in cattle, molybdenum excess in the soil of pasturage can produce scours (diarrhea) if the pH of the soil is neutral to alkaline; see teartness.

Dietary recommendations

In 2000, the then U.S. Institute of Medicine (now the

Adequate Intake
(AI) is used instead.

An AI of 2 micrograms (μg) of molybdenum per day was established for infants up to 6 months of age, and 3 μg/day from 7 to 12 months of age, both for males and females. For older children and adults, the following daily RDAs have been established for molybdenum: 17 μg from 1 to 3 years of age, 22 μg from 4 to 8 years, 34 μg from 9 to 13 years, 43 μg from 14 to 18 years, and 45 μg for persons 19 years old and older. All these RDAs are valid for both sexes. Pregnant or lactating females from 14 to 50 years of age have a higher daily RDA of 50 μg of molybdenum.

As for safety, the NAM sets

tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of molybdenum, the UL is 2000 μg/day. Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs).[107]

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women and men ages 15 and older the AI is set at 65 μg/day. Pregnant and lactating women have the same AI. For children aged 1–14 years, the AIs increase with age from 15 to 45 μg/day. The adult AIs are higher than the U.S. RDAs,[108] but on the other hand, the European Food Safety Authority reviewed the same safety question and set its UL at 600 μg/day, which is much lower than the U.S. value.[109]

Labeling

For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For molybdenum labeling purposes 100% of the Daily Value was 75 μg, but as of May 27, 2016 it was revised to 45 μg.[110][111] A table of the old and new adult daily values is provided at Reference Daily Intake.

Food sources

Average daily intake varies between 120 and 240 μg/day, which is higher than dietary recommendations.[91] Pork, lamb, and beef liver each have approximately 1.5 parts per million of molybdenum. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, cucumbers, and cereal grain.[17]

Precautions

Molybdenum dusts and fumes, generated by mining or metalworking, can be toxic, especially if ingested (including dust trapped in the

immediately dangerous to life and health.[115]

See also

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Bibliography

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