The Element Molybdenum
The element molybdenum is a transition metal belonging to group 6 of the periodic table (Other transition metals include vanadium and titanium ). This silver element is a refractory metal, meaning it is extremely resistant to high temperatures and corrosion. This metal makes up about 1.2 ppm of the Earth’s crust and is not found freely in nature. There are many important properties of molybdenum that should be appreciated, including its good thermal and electrical conductivity, ductility as well as strength, and its high density of 10.2 g/cm3. Let’s learn how useful this element is.
Cool Facts About Molybdenum
- Molybdenum primarily comes from molybdenite mineral, which was originally thought of to be lead or graphite. Simultaneously, it was thought that the “lead ore” contained sulfur. It was not until 1782 that Peter Jacob Hjelm recognized molybdenum.
- The name molybdenum comes from the Greek word “molybdos”, meaning lead-like. This name also describes galena and graphite because their natural forms are all similar structurally.
- Molybdenite is the most abundant molybdenum-containing mineral.
- Moybdenite’s applications go all the way back to ancient times. A Japanese sword from the 14th century was discovered to contain molybdenum as an alloying element.
- About 200,000 tons of molybdenum is produced annually worldwide. The largest miners of this metal are the United States, China, Chile, and Peru.
- Luna 24, a space program in Russia, discovered samples of molybdenum on the moon.
Biological Significance of Molybdenum
Molybdenum is an essential micronutrient for not just humans, but animals and plants as well. Why is it so important for the body to contain this element? Molybdenum is a cofactor of four different enzymes in our bodies: sulfite oxidase, xanthine oxidase, aldehyde oxidase, and mitochondrial amidoxime reducing component. These enzymes are responsible for aiding metabolism and breaking down harmful sulfites. Without a cofactor, these enzymes would be unable to function efficiently and toxins would build up in our bodies.
It is easy to incorporate this nutrient into our diets, as common foods, such as legumes, grains, and organ meats contain high concentrations of it. Therefore, taking molybdenum supplements are unnecessary and should even be avoided. While studies of molybdenum toxicity are limited in humans, it causes a reduction of growth, kidney failure, infertility, and diarrhea in animals. Too much of this micronutrient may also lead to gout, which can cause pain in the joints.
Additionally, molybdenum is a cofactor of nitrogenase, which is an enzyme that fixes nitrogen. Nitrogenase survives in nitrogen-fixing bacteria that take nitrogen from the air to feed to plants. Unsurprisingly, plants living in molybdenum-free soil are likely to not survive. Therefore, sodium molybdate is often used as a fertilizer for plants to ensure healthy growth and large harvest.
Molybdenum Applications in Today’s World
The primary application of molybdenum is in alloys. This metal is an amazing alloying agent because of its high melting point of 2622°C (4752°F) and the ability to increase hardness, strength, and resistance to corrosion. Additionally, the material is very ductile, making it more tolerant to harsh conditions. Products containing molybdenum alloys can withstand large temperature ranges and highly corrosive environments. Therefore, this alloy is useful for producing industrial technology.
“Moly steel” is a molybdenum-steel alloy. Adding molybdenum to steel not only increases strength and corrosion-resistance, but electrical conductivity as well. Furthermore, adding molybdenum to just small steel concentrations creates an extremely strong steel that is stable under pressures as high as 300,000 lbs/in2. Moly steel is an important component of engine manufacturing, such as missile and aircraft parts.
Molybdenum is an additive to nickel alloys. While these nickel molybdenum alloys are also highly resistant to corrosion, their ability to withstand chemical deterioration makes this material valuable in the chemical industry. Applications of this alloy include solid solution strengthening, precipitation hardening, deoxidation, desulphurization, and more.
Another molybdenum alloy is chromoly, which consists of the element chromium. This low alloy steel is significant for construction, automotive, and the oil and gas industry. The reason this alloy is so useful is that it is creep-resistant and can survive temperatures of up to 530˚C (986˚F).
One more alloy is the molybdenum tungsten alloy, also called the Mo-W alloy. What makes this material stand out is its ability to survive in molten zinc and extremely high temperatures. Some applications include making zinc and stirring tools for glass and helping to produce the coating for flat screens.
Molybdenum in the Chemical Industry
The element molybdenum is vital to the chemical industry for its function as a catalyst and lubricant. One field that utilizes this element is the petroleum industry. Not only does it serve as a catalyst in the refining of petroleum, but it powers hydrodesulfurization, which is the process of removing sulfur from natural gas.
Additionally, molybdenum is a catalyst for polymer and plastic production by initiating propen selective oxidation via the synthesis of acrolein.
History of the Element Molybdenum
Who discovered Molybdenum, and when?
Swedish scientist, Carl Wilhelm Scheele discovered the mineral molybdenite in 1768. Molybdenite is the most abundant molybdenum-containing mineral. While he was able to recognize that molybdenite contained a sulfide compound, he could not identify what element the compound was coming from. He placed the mineral in hot nitric acid to degrade it and then proceeded to burn the product in the air. This process produced a white oxide powder. Unfortunately, Scheel had to stop his experiment here, as he had no access to the right equipment to further break the powder down. Later in 1782, Peter Jacob Hjelm took Scheele’s findings and reduced the white oxide powder with carbon. This yielded a dark metal powder, which was named molybdenum.
Dissolving elemental molybdenum
If you want to dissolve metallic molybdenum, you need to use an oxidizing agent. So HF, HCl, or H2SO4 alone won’t dissolve molybdenum. If you put molybdenum powder into hot HCl, the solution turns slightly green, but that is probably due to the oxide layer on the surface of molybdenum, which serves as oxidizing agent.
If you put molybdenum powder into HCl and ammonium heptamolybdate mixture, it really starts to dissolve molybdenum and forms a green solution. Dilute HNO3 dissolves molybdenum pretty quickly and oxidizes into the +5 oxidation state. Concentrated HNO3, aqua regia or mixture of HNO3 and HBr can oxidize molybdenum in to H2MoO4, which appears as white solid on the bottom of the beaker/test tube.
White fuming nitric acid can’t dissolve molybdenum due to formation of passivation layer, but if you add few drops of water, a very vigorous reaction takes places and lot of MoO3 starts to form.
The best compound for dissolving molybdenum is hydrogen peroxide – it serves as oxidizing and also as complexing agent. You can use hydrogen peroxide alone or in mixture with acids/hydroxides/ammonia. Mo(VI)-peroxide complexes are unstable in alkaline solution and they (especially in hot solution) decompose into molybdate and singlet oxygen.
In general, molybdenum is more reactive at high temperatures. At room temperature, this metal will react with fluorine and will slightly dissolve in hydrofluoric, hydrochloric, and sulfuric acid. However, at 250˚C (482˚F), nitrate compounds and high concentrations of sulfate will easily corrode molybdenum.
Molybdenum is reactive to neither water nor oxygen at room temperature. When temperatures elevate to 500˚C (932˚F), oxidation occurs rapidly. At temperatures of 650˚C (1202˚F) and up, molybdenum evaporates into molybdenum trioxide, MoO3.
There are more reactions at elevated temperatures; At 600˚C (1112˚F), the element nitrogen starts to dissolve into molybdenum. As the heat increases, these two elements react more violently to produce a fragile compound. Moreover, it has been discovered that molybdenum is the limiting nutrient of nitrogen fixation in soil. Consequently, higher concentrations of molybdenum increase the rate of nitrogen fixation. Learn more about this reaction here.
Molybdenum oxides are the products of heating molybdenum disulfide, MoS2 in the air. These are versatile compounds that are useful in both their pure form and when combined with other elements. Oxides are insoluble in water and are thermally stable, properties useful in the glass and ceramic industries. There are a variety of molybdenum oxides in different oxidation states, making them capable of forming different crystal structures and holding various oxygen vacancies.
Molybdenum trioxide, MoO3 may be considered one of the most viable compounds, as it is the precursor to almost all other molybdenum compounds. This dense solid appears as a white powder at room temperature and melts to a dark yellow liquid, which then solidifies to a yellow crystalline. When combined with strong alkaline water, it dissolves and forms molybdate, MoO42- ions. MoO42- undergo condensation reactions to produce oxyanions. Additionally, molybdenum trioxide serves as a catalyst in the polymerization of olefins, as well as a manufacturer of molybdenum metal and alloys.
Molybdenum disulfide is a black powder that is quite inert and difficult to dissolve in acids. This compound is a transition metal dichalcogenide, which are semiconductors of transition-metal-chalcogen compounds. These metals are very sturdy, low friction, and robust, as they have a unique atomic-scale thickness and strong spin-orbit coupling.
These properties make molybdenum disulfides favorable for electronic and mechanical applications. It has low friction, making molybdenum disulfide’s most common use a lubricant. Additionally, molybdenum disulfide contains rich sulfur atoms on its surface, making it extremely absorbent and able to remove heavy metals from aqueous solutions. This compound is pretty unreactive when roasted in air, will form MoO3.
There are several oxidation states of molybdenum chlorides. Specifically, chlorides range from molybdenum (II) chloride to molybdenum (VI) chloride. To focus on just a couple, molybdenum (II) chloride, Cl2Mo is a yellow crystalline solid that is insoluble in water. It is unique because it appears more of a cluster, rather than the typical close-packed structure. This compound reacts well with other metals to form metal compounds. One form of molybdenum (II) chloride is Mo6Cl12, which is a polymeric compound. It consists of clusters of Mo6Cl84+. When molybdenum (IV) chloride, MoCl5– which serves as a catalyst- breaks down with hydrogen, it produces molybdenum (III) chloride, MoCl3. This compound exists as a dark-purple crystalline that is insoluble in water and purple colored crystals.
Cobalt (II) sulfate reacts with sodium molybdate dihydrate to produce cobalt molybdate. Check out the beautiful reaction here:
Synthesis of Molybdenum
Chart from by International Molybdenum Association
Molybdenum Oxidation States
Molybdenum is most commonly found in the +6 oxidation state, in compounds like sodium molybdate and molybdenum trioxide. Other oxidation states of molybdenum of +4 and +5 are also sometimes seen. However, its chemical compounds can have oxidation states ranging from -2 to +6. The biggest difference between the oxidation states is how it affects the compound’s applications. Lower oxidation states are associated with organomolybdenum compounds, while higher oxidation states have more biological significance.
Physical Properties of Molybdenum
- Symbol: Mo
- Melting point: 2895 K; 2622°C; 4752°F
- Boiling point: 4912 K; 4639°C; 8382°F
- Density: 10.2 g/cm3
- Atomic weight: 95.95
- Atomic number: 42
- Electronegativity: 2.16
- Classification: Transition metal
- Natural abundance in the Earth’s crust: 0.00011%
- Electron shell configuration: [Kr] 4d5 5s1
- Isotopes: 95Mo, 96Mo, 98Mo
- Found naturally in the minerals: Found primarily in molybdenite
- Toxicity: Rare
Where can I buy Molybdenum?
Amazon and specialty stores sell molybdenum metal.