Molybdenum preparation

This chapter aims to describe the elaboration process and refining of refractory metals by techniques involving molten salt media. Tungsten and molybdenum preparation will not be mentioned, as far for these elements, molten salt electrolysis is little repotted and moreover controversial. 

The above fusion conditions were adopted to determine oxygen in molybdenum prepared by sintering under hydrogen. The samples (diameter 15 mm, thickness 1 mm, weight 1.85 to 1.95 g) were irradiated together with the standards... 

Usually prepared by the action of NaCN on benzaldehyde in dilute alcohol. It is oxidized by nitric acid to benzil, and reduced by sodium amalgam to hydrobenzoin PhCHOHCHOHPh by tin amalgam and hydrochloric acid to des-oxybenzoin, PhCH2COPh and by zinc amalgam to stilbene PhCH = CHPh. It gives an oxime, phenylhydrazone and ethanoyl derivative. The a-oxime is used under the name cupron for the estimation of copper and molybdenum. 

Gr. molybdos, lead) Before Scheele recognized molybdenite as a distinct ore of a new element in 1778, it was confused with graphite and lead ore. The metal was prepared as an impure form in 1782 by Hjelm. Molybdenum does not occur native, but is obtained principally from molybdenite. Wulfenite, and Powellite are also minor commercial ores. 

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. 

Some molybdenum contain from 0.002% to 0.2% rhenium. More than 150,000 troy ounces of rhenium are now being produced yearly in the United States. The total estimated Free World reserve of rhenium metal is 3500 tons. Rhenium metal is prepared by reducing ammonium perrhentate with hydrogen at elevated temperatures. 

Many low molecular weight aldehydes and ketones are important industrial chem icals Formaldehyde a starting material for a number of plastics is prepared by oxida tion of methanol over a silver or iron oxide/molybdenum oxide catalyst at elevated temperature... 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Patents claiming specific catalysts and processes for thek use in each of the two reactions have been assigned to Japan Catalytic (45,47—49), Sohio (50), Toyo Soda (51), Rohm and Haas (52), Sumitomo (53), BASF (54), Mitsubishi Petrochemical (56,57), Celanese (55), and others. The catalysts used for these reactions remain based on bismuth molybdate for the first stage and molybdenum vanadium oxides for the second stage, but improvements in minor component composition and catalyst preparation have resulted in yields that can reach the 85—90% range and lifetimes of several years under optimum conditions. Since plants operate under more productive conditions than those optimum for yield and life, the economically most attractive yields and productive lifetimes maybe somewhat lower. 

Molybdenum hexafluoride can be prepared by the action of elemental fluorine on hydrogen-reduced molybdenum powder (100—300 mesh (ca 149—46 l-lm)) at 200°C. The reaction starts at 150°C. Owing to the heat of reaction, the temperature of the reactor rises quickly but it can be controlled by increasing the flow rate of the carrier gas, argon, or reducing the flow of fluorine. 

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

Reduction to Solid Metal. Metals having very high melting points caimot be reduced in the Hquid state. Because the separation of a soHd metallic product from a residue is usually difficult, the raw material must be purified before reduction. Tungsten and molybdenum, for instance, are prepared by reduction of a purified oxide (WO, MoO ) or a salt, eg, (NH2 2 G4, using hydrogen. A reaction such as... 

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. 

Molybdenum improves the corrosion resistance of stainless steels that are alloyed with 17—29% chromium. The addition of 1—4% molybdenum results in high resistance to pitting in corrosive environments, such as those found in pulp (qv) and paper (qv) processing (33), as weU as in food preparation, petrochemical, and poUution control systems. 

AMMONIUM compounds). Diammonium dimolybdate [27546-07-2] (NH 2 2 7 " ble commercially as the tetrahydrate and prepared from MoO and excess NH in aqueous solution at 100°C, has an infinite chain stmcture based on MoO octahedra. In aqueous solution the behavior of Mo(VI) is extremely pH-dependent (4). Above pH 7 molybdenum(VI) occurs as the tetrahedral oxyanion MoO , but below pH 7 a complex series of concentration-, temperature-, and pH-dependent equiUbria exist. The best known of these equiUbria lead to the formation of the heptamolybdate,... 

Biomedical Uses. The molybdate ion is added to total parenteral nutrition protocols and appears to alleviate toxicity of some of the amino acid components in these preparations (see Mineral NUTRIENTS) (97). Molybdenum supplements have been shown to reduce iiitrosarnine-induced mammary carcinomas in rats (50). A number of studies have shown that certain heteropolymolybdates (98) and organometaUic molybdenum compounds (99) have antiviral, including anti-AIDS, and antitumor activity (see Antiviral agents Chemotherapeutics, anticancer). 

Nicotinonitrile is produced by ammoxidation of alkylpyridines (11—24). A wide variety of different catalysts have been developed for this appHcation. For example, a recent patent describes a process ia which 3-methylpyridine is reacted over a molybdenum catalyst supported on siHca gel. The catalyst (PV Mo 20 ) was prepared from NH VO, H PO, and (NH Moy024. Reaction at 380°C at a residence time of 2.5 seconds gave 95% of nicotinonitrile at a 99% conversion (16). 

The catalysts are prepared by impregnating the support with aqueous salts of molybdenum and the promoter. In acidic solutions, molybdate ions are present largely in the form of heptamers, [Mo2024] , and the resulting surface species are beHeved to be present in islands, perhaps containing only seven Mo ions (100). Before use, the catalyst is treated with H2 and some sulfur-containing compounds, and the surface oxides are converted into the sulfides that are the catalyticaHy active species. 

Practices for preparation of and electroplating on Chromium (Electrodeposits) on Chromium Copper and Copper-Base Mllojs Iron Castings Eead and EeadMllojs Magnesium and Magnesium Mllojs Molybdenum and Molybdenum Mlloys MickelMlloys... 

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

Of the many molybdenum sulfides which have been reported, only MoS, M0S2 and M02S3 are well established. A hydrated form of the trisulfide of somewhat variable composition is precipitated from aqueous molybdate solutions by H2S in classical analytical separations of molybdenum, but it is best prepared by thermal decomposition of the thiomolybdate, (NH4)2MoS4. MoS is formed by heating the calculated amounts of Mo and S in an evacuated tube. The black M0S2, however, is the most stable sulfide and, besides being the principal ore of Mo,... 

This is by far the most stable and best-known oxidation state for chromium and is characterized by thousands of compounds, most of them prepared from aqueous solutions. By contrast, unless stabilized by M-M bonding, molybdenum(III) compounds are sparse and hardly any are known for tungsten(III). Thus Mo, but not W, has an aquo ion [Mo(H20)g] +, which gives rise to complexes [MoXg] " (X = F, Cl, Br, NCS). Direct action of acetylacetone on the hexachloromolybdate(III) ion produces the sublimable (Mo(acac)3] which, however, unlike its chromium analogue, is oxidized by air to Mo products. A black cyanide,... 

Reactions of the metallocene derivatives of molybdenum with pyrazole lead to the mononuclear complexes of the type 22. Structure 22 shows that it cannot be used as a ligand for the preparation of dinuclear complexes owing to geometric constraints [80JOM( 197)291 83JOM(253)53]. In acetone, an unusual complex 23 is formed [83JOM(253)53]. The bidentate ligand is the product of the reaction of pyrazole and acetone. 

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