Multicomponent Mixed Oxides

The oxidation of ethane to acetic acid is believed to proceed via the intermediate formation of ethylene (Equation Al). The catalysts are multicomponent mixed oxides, having optimized compositions for ethane oxydehydro-genation (Mo-V oxides) and ethylene oxidation (Pd, Nb oxide). Reported... 

Very different catalytic materials have been tested in the selective oxidation of propane, which includes vanadium phosphorous oxide (VPO) catalysts, industrially applied for w-butane oxidation to maleic anhydride. The other catalysts systems include Keggin structure heteropolyoxometalUc compounds (HPCs) and multicomponent mixed oxides (MMOs). These materials have different structure and properties, but have something in common they contain reducible metal 0x0 species. 

Although acrylonitrile manufacture from propylene and ammonia was first patented in 1949 (30), it was not until 1959, when Sohio developed a catalyst capable of producing acrylonitrile with high selectivity, that commercial manufacture from propylene became economically viable (1). Production improvements over the past 30 years have stemmed largely from development of several generations of increasingly more efficient catalysts. These catalysts are multicomponent mixed metal oxides mostly based on bismuth—molybdenum oxide. Other types of catalysts that have been used commercially are based on iron—antimony oxide, uranium—antimony oxide, and tellurium-molybdenum oxide. 

Calcination or dead burning is used extensively to dehydrate cements (qv) and hygroscopic materials such as MgO, and to produce a less water sensitive product. Calcination is also used to decompose metal salts to base oxides and to produce multicomponent or mixed oxide powders for... 

A number of commercial oxidation catalysts are based on V and Mo oxides. They are multicomponent materials, such as the mixed oxides Mo-Fe-O (oxidation of methanol to formaldehyde), V-P-O (oxidation of butane to maleic anhydride), Bi-Mo-O (propylene to acrylonitrile), Bi-Fe-Mo-O... 

For oxide electrocrystallization, the last condition is the most strenuous, sine many oxides are insulating non-stoichiometric compounds, however, are sufficient conductive. When two (or more) substances are codeposited, certain specific feature of the crystallization can be expressed for both cathodic and anodic processes on th basis of the thermodynamics of binary (or more complex) systems. If stabl multicomponent phases exist, then it is their deposition (not the deposition of mixture of simpler products) that preferentially proceeds in a certain potential regioi In such cases, intermetallic compounds are deposited in cathodic processes, and th deposition of mixed oxides takes place in anodic processes. These products ca represent both chemical compounds and solid solutions. 

In principle it would seem reasonable that the bulk structure and surface properties of a solid would influence the catalytic performance. Verification of this view and an assessment of its importance may be more significant than first appears since it incorporates an implication that catalytic preparation should be designed to achieve the bulk structure and surface properties that give the optimized catalytic performance. Materials which have been shown to catalyze the conversion of propylene to acrolein have included metal oxides, mixed oxides, and more lately the multicomponent catalysts. A consideration of all these solids would require the assessment of numerous data and speculation. However, the mixed oxide catalysts have been associated with many of the more recent investigations of the course of catalytic oxidation, and these catalysts therefore seem to be worthy of detailed consideration. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

In the case of multicomponent particulate gels, a primary concern is the prevention of segregation of the individual components so that uniform mixing may be achieved. Various routes have been used for their preparation, including (1) coprecipitation of mixed oxides or hydroxides, (2) mixing of sols of different oxides... 

It is also possible to precipitate multimetal ions to obtain multicomponent LDHs. For example, Morpurgo et al. synthesized several multication LDHs, such as CuZnCoAlCr- and CuZnCoCr-LDHs (192-194). This procedure suggests a general way to synthesize a number of LDHs as the precursors of mixed oxides with versatile functionalities, especially through incorporation of some rare earth metal cations (with radius from 0.085 to 0.100 nm), as has been done by Perez-Ramirez et al. (163,165). However, the overall ratio of divalent to trivalent cations should be kept in the range of 0.2-0.4. 

In the field of sol-gel derived materials, multicomponent gel evolves, after densification process and thermal annealing, to mixed oxide. However, in the synthesis of multicomponent... 

Acrylic acid has traditionally been used as the raw material for acrylic estos, polyacrylates, cross-linked polyacrylates, and copolymers. The global acrylic acid capacity was ca. 4.7 million tons in 2006, with an estimated average growth of 4%. Nowadays, acrylic acid is industrially obtained from a physically separated two-step process with propylene as the starting raw material. Firstly, propylene is selectively oxidized to acrolein at 300-350 C employing multicomponent catalysts based on metallic mixed oxides, i.e. MoBiO, FeSbO, or SnSbO. Then, the acrylic acid is obtained in a second step from acrolein oxidation at 200-260°C using multicomponent catalysts based on Mo-V-W mixed oxides. Thus, an overall acrylic acid yield of 85-90% is reached. 

Meso-Macroporous Mixed Oxides Multicomponent oxides play a central role in chemical and petrochemical processing as catalysts and as supports for catalytically active species [145]. It is known that the catalytic efficiency of metal oxides can be improved by doping them with a metal or combining them with another metal oxide [145]. The strategy based on the self-formation phenomenon to fabricate the porous hierarchy demonstrated its simplicity and superiority in the synthesis of hierarchically meso-macroporous metal oxides with multiple compositions. 

It was soon realized that commercial units would be based on the use of fluidized beds, which were then being introduced in refineries to produce gasoline. The most successful fluid bed process was introduced by Idol of Soldo in 1960 and used a bismuth phosphomolybdate catalyst supported on silica. Knapsack described a bismuth phosphomolybdate catalyst containing iron for aciylonitrile production in 1962. This rather more complex mixed oxide formulation, Fe7Bi2Moi2052, also supported on silica, foreshadowed improvements in the 1970s and the introduction of multicomponent catalysts. Since then several generations of improved catalysts have been introduced, e.g., by Sohio, as the reaction mechanism has been better understood. 

Al-Saeedi, J.N. and Guliants, V.V. (2002) High-throughput experimentation in multicomponent bulk mixed metal oxides Mo-V-Sb-Nb-O system for selective oxidation of propane to acrylic acid. Appl. Catal. A Gen, 237, 111. 

Light hydrocarbons consisting of oxygen or other heteroatoms are important intermediates in the chemical industry. Selective hydrocarbon oxidation of alkenes progressed dramatically with the discovery of bismuth molybdate mixed-metal-oxide catalysts because of their high selectivity and activity (>90%). These now form the basis of very important commercial multicomponent catalysts (which may contain mixed metal oxides) for the oxidation of propylene to acrolein and ammoxidation with ammonia to acrylonitrile and to propylene oxide. 

When multicomponent alkoxide solutions, or a single alkoxide and a soluble inorganic salt, are mixed, a multicomponent alkoxide may result. In this way, such complex oxides such as the YBCO superconductor (cf. Section 6.1.2.4) can be formed. Sol-gel processing can also be used to coat fibers for composites and to form ceramics with very fine pore sizes called xerogels. A xerogel commonly contains 50-70% porosity, a pore size of 1-50 nm, and a specific surface area exceeding 100 m /g. 

Chemical Composition. With few exceptions, inorganic pigments are oxides, sulfides, oxide hydroxides, silicates, sulfates, or carbonates (see Tables 3 and 4), and normally consist of single-component particles (e.g., red iron oxide, a-Fe203) with well defined crystal structures. However, mixed and substrate pigments consist of nonuniform or multicomponent particles. 

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