Bismuth-tungsten-molybdenum oxide

The Structural Chemistry of Bismuth-Tungsten-Molybdenum Oxides and Bismuth-Tungsten-Niobium Oxides... 

Aluminium oxide, arsenic trioxide, bismuth trioxide, calcium oxide, chromic oxide, lanthanum oxide, lead dioxide, magnesium oxide, manganese dioxide, molybdenum trioxide, phosphorus pentoxide, stannic oxide, sulfur dioxide (explodes), tantalum pentoxide, tungsten trioxide, vanadium pentoxide. 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

Fig. 19. (a) The selectivity of propene oxidation to acrolein as a function of the composition of bismuth-molybdenum and bismuth-tungsten catalysts, (b) Variations in the electron work function (Arji in ev) vs the composition of mixed bismuth-molybdenum and bismuth-tungsten catalysts. 

Hydrogen gas is used to partially hydrogenate unsaturated oils to produce soHd fats used as shortenings and margarines, as well as to make soaps. Hydrogen is also used to convert various oxides (such as those of silver, copper, lead, bismuth, mercury, molybdenum, and tungsten) to free metals, as represented in Equation (10.12) for an M oxide ... 

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). 

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

The fust reaction takes place in the vapor phase, between 330 and 360°C, at low pressure (between 03 and 04.106 Pa absolute), in the presence of air and steam in a r-butanol/air/steam ratio of about 1 10 to 15/6 to 12, on a catalyst based on mixed oxides of molybdenum, cobalt, bismuth, iron, nickel, and additions of derivatives of alkaline metals, antimony, tellurium, phosphorus, tungsten, tin, manganese, etc. With residence times of 2 to 3 s, the molar yield of methacrolein exceeds 85 per cent for virtually total once-throngh conversion of r-butanoLThe main by-products formed are... 

A number of transition metals can be determined conveniently if their cations undergo a definite change of oxidation state see Oxidation Number) on titration with a standard solution of potassium permanganate, potassium dichromate, cerium(IV) sulfate, or ammonium hexanitratocerate(IV). Several visual indicators have been proposed, including diphenylamine and its derivatives, xylene cyanole FF, and especially A-phenylanthranilic acid and tris(l,10-phenanthroline)iron(II) sulfate ( ferroin ). Solutions of have been used in the determination of iron, copper, titanium, vanadium, molybdenum, tungsten, mercury, gold, silver, and bismuth, and standard solutions of and Sn F U, and and Mo have also... 

The enhanced performance of the Nb-containing catalyst has been attributed to the presence of the Bi3WiNb9O30 phase in which active bismuth and tungsten sites are isolated from each other by low active niobium species. Poor crystallization has not allowed the relation between behavior and phase composition for the Al-promoted catalyst to be traced. It should be emphasized that the addition of molybdenum and antimony, which are present in many selective oxidation... 

Bismuth chromate is described by Salter (1869) under bismuth yellow. Bismuth vanadate, in combination with bismuth-molybdenum-tungsten oxides, are known as yellow pigments of modem introduction (Buxbaum, 1998 see bismuth vanadate). 

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