Overview of molybdenum chemistry
Molybdenum is a transition metal in Group 6 of the Periodic Table between chromium and tungsten. Although molybdenum is sometimes described as a ‘heavy metal’ its properties are very different from those of the typical heavy metals, mercury, thallium and lead. It is much less toxic than these and other heavy metals. Its low toxicity makes molybdenum an attractive substitute for more toxic materials.
Compounds of molybdenum which are commonly encountered have molybdenum in its highest oxidation state, VI, for example molybdenum trioxide, MoO3, sodium molybdate, Na2MoO4.2H2O, and ammonium di- and heptamolybdate, (NH4)2Mo2O7 and (NH4)6Mo7O24.4H2O. In aqueous solution molybdenum(VI) is present as the simple molybdate, [MoO4] 2- ion which is like sulfate or, depending on the concentration and pH as a polymeric polymolybdate ion. The lower oxidation state, IV, is found in the commonest ore of molybdenum the disulfide, MoS2. Molybdenum(IV) also forms an oxide, MoO2. The redox chemistry of molybdenum-oxygen compounds, as in selective oxidation catalysts and molybdenum oxidase enzymes, has molybdenum cycling between oxidation states (VI) and (IV).
The chemistry of molybdenum is extraordinarily versatile: oxidation states from (-II) to (VI), coordination numbers from 4 to 8 and, accordingly a very varied stereochemistry; the ability to form compounds with most inorganic and organic ligands and bi- and polynuclear compounds containing molybdenum-molybdenum bonds and bridging ligands. It is this versatility which makes the chemistry of molybdenum challenging and exciting and the actual and potential applications of its compounds many and varied. Molybdenum is the first of the transition metals to have an extensive sulfur chemistry shown, for example, having as its principal ore molybdenum disulfide, MoS2, its binding by sulfur ligands in molybdenum containing enzymes, application of MoS2 as an important industrial catalyst, and formation of many sulfur complexes some of which are used as soluble lubricating oil additives.
Molybdenum has an extensive organometallic chemistry in its lower oxidation states. These compounds contain molybdenum-carbon bonds. A well known example is molybdenum hexacarbonyl, Mo(CO)6. These compounds are difficult to prepare and may decompose on exposure to air. They have specialised small volume uses as for example catalysts in fine chemicals synthesis.
Molybdenum-based technical chemicals exploit the versatility of molybdenum chemistry in oxidation states. (VI), (V) and (IV). Materials made from molybdates are oxidation catalysts, are photoactive, and semiconducting. Many of the properties of molybdenum provide development opportunities and new commercial applications through the exploitation of its chemistry.
Molybdenum-based catalysts have a number of important applications in the petroleum and plastics industries. A major use is in the hydrodesulfurisation (HDS) of petroleum, petrochemicals and coal-derived liquids. The catalyst comprises MoS2 supported on alumina and promoted by cobalt or nickel and is prepared by sullfiding cobalt and molybdenum oxides on alumina. As the world supply of crude oil is further extended and low-sulfur crudes become less available, molybdenum-based catalysts will increase in use. Molybdenum not only allows for economical fuel refining but also contributes to a safer environment through lower sulfur emissions.
Molybdenum catalysts are resistant to poisoning by sulfur and, for example, catalyse conversion of hydrogen and carbon monoxide from the pyrolysis of waste materials to alcohols in the presence of sulfur, under conditions that would poison precious metal catalysts. Similarly Mo-based catalysts have been used in the conversion of coal to hydrocarbon liquids.
As a component of the bismuth molybdate selective oxidation catalyst molybdenum participates in the selective oxidation of, for example, propene, ammonia, and air to acrylonitrile, acetonitrile and other chemicals which are raw materials for the plastics and fibre industries. Similarly molybdenum in iron molybdate catalyses the selective oxidation of methanol to formaldehyde.
Molybdate-based pigments are used for two properties: stable colour formation and corrosion inhibition. Molybdenum oranges are prepared by co-precipitating lead chromate, lead molybdate and lead sulfate. They are light- and heat-stable pigments with colours from bright red-orange to red-yellow and are used in paints and inks, plastic and rubber products, and ceramics.
Zinc molybdate is the basis of white corrosion inhibiting pigments which are used as paint primers.
Molybdophosphoric acid is used to precipitate the dyes methyl violet and victoria blue.
Sodium molybdate has been used for many years as a substitute for chromates for the inhibition of corrosion in mild steels over a wide range of pH. Molybdates have a very low toxicity and are less aggressive oxidants than chromates toward organic additives that may be used in corrosion inhibiting formulations. A prime application is in cooling water in air-conditioning and heating systems to protect mild steel used in their construction.
Molybdates are used to inhibit corrosion in water-based hydraulic systems and in automobile engine anti-freeze.
Molybdate solutions protect against rusting of steel parts during machining.
Corrosion inhibiting pigments, primarily zinc molybdate, but also molybdates of calcium and strontium, are used commercially in paints. These pigments are white and can be used as a primer or as a tint with any other colour.
In electronic technology, wire and cable insulation represents a potential fire and smoke hazard to fire fighters and others in confined spaces of aircraft and hospitals. Ammonium octamolybdate is used with PVC to suppress the formation of smoke. These uses and other developments will increase as video, telephone and computing networks increase.
Molybdenum disulfide, the most common natural form of molybdenum, is extracted from the ore and then purified for direct use in lubrication. Molybdenum disulfide because of its layered structure is an effective lubricant. When MoS2 particles are located between moving surfaces the MoS2 layers slide over each other, permitting the surfaces of steel and other metals to move fluidly, even under severe pressures, as bearing surfaces. Since molybdenum disulfide is of geothermal origin, it has the durability to withstand heat and pressure. This is particularly so if small amounts of sulfur are available to react with iron and provide a sulfide layer which is compatible with MoS2 in maintaining the lubricating film.
Molybdenum disulfide will perform as a lubricant in vacuo where graphite fails.
A number of unique properties distinguish molybdenum disulfide from other solid lubricants:
A combination of molybdate and water soluble sulfides can provide both lubrication and corrosion inhibition in cutting fluids and metal forming materials. Oil soluble molybdenum-sulfur compounds, such as thiophosphates and thiocarbamates, provide engine protection against wear, oxidation and corrosion. Several commercial manufactures supply these additives to the lubrication industry.
Molybdenum chemicals in agriculture
Molybdenum is an essential trace element for plants and animals. It is an essential component of the enzyme nitrogenase which catalyses the conversion of atmospheric nitrogen to ammonia. Accordingly molybate is applied in fertiliser formulations.