Molybdenum and its applications
Molybdenum compounds in catalysts
The function of a catalyst is both to speed up a chemical reaction and to steer it towards a desired product e.g. an oxygenated organic compound rather than carbon dioxide and water.
Catalysts having molybdenum as a component are listed in Table 1.
Table 1: Molybdenum compounds in catalysts
We see from Table: Molybdenum compounds in catalysts that it is Mo-O compounds which find application in high volume catalysts. Sulfide catalysts are derived from the oxides.
We see from Table: Molybdenum compounds in catalysts that it is Mo-O compounds which find application in high volume catalysts. Sulfide catalysts are derived from the oxides. From the US Patent Database we can derive an indication of the interest in catalytic applications of molybdenum compounds, see Fig. 1. We include molybdenum carbide and nitride where there is growing interest.
Fig 1: Catalytic applications of molybdenum compounds. US patents according to type of compound
Molybdenum-based catalysts have a number of important applications in the petroleum and plastics industries. A major use is in the hydrodesulfurization (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 sulfiding cobalt and molybdenum oxides on alumina. Continued use of sulfur-containing fossil fuels will require molybdenum catalysts for desulfurization. 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.
Molybdenum in hydrodesulfurization catalysts
Removal of sulfur compounds from gasoline and diesel is driven by ever more stringent legislation since during combustion the sulfur compounds oxidise to sulfur dioxide, the source of acid rain. They are also potent poisons of auto-exhaust catalysts. The process is catalytic hydrodesulfurization - removal of sulfur by reaction of the compound with hydrogen - for which molybdenum-based catalysts are essential. Recalcitrant compounds are thiophene and benzothiophenes. The reaction of thiophene with hydrogen is typical:
Fig 2: Hydrodesulfurization of Thiophene
The catalyst, see Fig. 3 , comprises cobalt (3 wt-%) and molybdenum (8 wt-%) as sulfides supported on alumina. It is prepared by impregnating gamma-alumina with ammonium heptamolybdate or dimolybdate and cobalt(II) nitrate followed by drying and calcination. The molybdenum and cobalt sulfides are formed when the catalyst is pre-sulfided or used.
Fig 3: A Co-Mo/alumina catalyst: before use, oxide form; after use or pre-sulfiding, sulfide form
The active catalyst consists of hexagonal slabs of molybdenum disulfide, sections of a molybdenum disulfide layer, Fig. 4. The utility of molybdenum in this catalysis derives from the layer structure of molybdenum disulfide. (The same is true for tungsten disulfide which also catalyses hydrodesulfurization.) The catalytic sites are at the slab edges (where cobalt atoms, not shown, are also located). A reactant molecule, thiophene, is shown in Fig. 4 adsorbed near an edge site and reacting with hydrogen atoms to give, ultimately, butane and hydrogen sulfide. We require of molybdenum disulfide the ability to dissociate hydrogen molecules, adsorb reactant molecules, and release sulfur as hydrogen sulfide reversibly.
Fig 4: Hydrodesulfurization of thiophene catalysed by molybdenum disulfide: Mo atoms lighter, S atoms darker
The active component is molybdenum disulfide, see Fig. 4. Cobalt (a promoter) is added to increase the activity. Alumina provides surface area (ca 250 m2 g -1) by dispersing the cobalt and molybdenum.
Molybdenum in selective oxidation catalysts
The technically important reactions of methanol oxidation to formaldehyde and propene to acrolein and acrylonitrile are shown in Fig. 5.
In these selective oxidations the slow step, which controls the overall reaction rate and which it is desired to catalyse, is activation of the first C-H bond. With these two-component catalysts, the first step, breaking the C-H bond, is catalysed by the more basic oxide, e.g. bismuth oxide. Molybdenum is involved in the next step - activation of a second hydrogen and insertion of an oxygen atom into the organic molecule. In this step molybdenum is reduced. Reduced molybdenum is reoxidised by oxygen from the feed. The catalysis depends on the ability of oxomolybdenum species to cycle between the VI and IV oxidation states, in the process releasing and transferring an oxygen atom:
Mo(VI)O2 Mo(IV)O + O
Note that the inserted oxygen comes from the lattice of the catalyst; there is no direct reaction between the organic compound and an oxygen molecule. The same molybdenum chemistry - reduction of molybdenum accompanied by oxygen atom transfer - also operates with oxomolybdenum enzymes, e.g. xanthine oxidase.
Fig 5: Selective oxidations over iron molybdate and bismuth molybdate catalysts. Highlights show the reactive hydrogen atoms and in the substituted atoms in the products
