Molybdenum Compounds as Efficient Additives to Lubricants (pp. 181-200)
Authors: (O.P. Parenago, Institute of Petrochemical Synthesis, RAS, Moscow, Russia)
Abstract: Molybdenum is one of the unique transition metals of Periodic Table playing significant role in modern technical equipment. Metal molybdenum is used widely as constructional material in electric lamps, electric vacuum equipment and nuclear reactors production. Molybdenum wire and tape are used as heating elements for high-temperature furnaces. Molybdenum is also used in metallurgy for steel doping letting one to get heatproof and acid-resistant steels.
Molybdenum also plays the significant role in biologic processes including humans. All molybdenum enzymes include sulfur atoms at that . Molybdenum actively promote activity of antioxidants including vitamin C. Molybdenum also hastens synthesis of amino acids, improves nitrogen accumulation being part of active centers of some ferments such as nitrogenase which is ferment for nitrogen binding.
Thanks to rather wide range of molybdenum valency +2 to +6 (coordination number is 12), its compounds are used in a number of chemical processes and in many valuable products industry processes. For example, molybdenum compounds as a part of pigments are used in dye production. Various compounds of this metal are widely used as catalysts of many chemical processes. In petrochemistry and oil-refining molybdenum is included in aluminum – nickel (cobalt) – molybdenum catalysts used for hydro refining of oil fractions from sulfur compounds, and for hydrogenation of aromatic hydrocarbons as well. Sulfur-containing molybdenum compounds are active catalysts of crude oil and oil fractions hydrodesulfurization. Molybdenum halides in polymerization processes are parts of catalytic systems for stereospecific of butadiene polymerization and for processes of olefines metathesis and cycloolefines polymerization with ring opening.
Molybdenum compounds also play significant role as additives to lubricants. Molybdenum compounds actively affect on lubricants oxidation stability, increase lubricating efficiency and improve antiwear properties of rubbing metal surfaces.
Almost all modern machines and mechanisms contain various friction units where there are interactions of rubbing metal surfaces. Lubrication oils and greases contain usually special compounds – additives improving all the major properties of lubricants. They are widely used to prevent metal surfaces friction and to decrease wear. Additives are especially of great importance in transportation where their utilization results not only in decrease of metal details wear but also lets one to economize fuels due to decrease of friction loss. Saving of oil products (fuels and oils) results in improvement of the ecology and of environmental engineering. Additives influencing in friction and wear processes are called tribological active ones.
The great variety of organic, hetero-organic and metal complex compounds synthesis and investigations are described in scientific and technical and patent literature as such additives. Sulfur containing molybdenum compounds are in merited important place among these additives being investigated for several decades due to high lubricating and antioxidant properties. These compounds are synthesized in a number of companies such as Vanderbilt (USA), Chemtura (USA), and they are included in commercial packages of lubricating oils additives.
Interest to sulfur containing molybdenum complexes was induces by the challenge of fuel consumption decrease during fuel and energy crisis in the mid 70es of the last century [2, 3]. The matter is that these compounds decrease significantly the friction coefficient of rubbing surfaces playing role of antiwear additives. Sometimes they are also called friction modifiers. Doping of these compounds to transmission oils results in significant fuels economy with simultaneous decrease of details wear and oil consumption.