Multicomponent oxide systems

Berezhnoi a. S., Multicomponent Oxide Systems (tn Russian), Naukova dumka, Kiev, 1970 (BepeacHoii A. C., MyjiTHKoMnoHeHTHbie cMCxeMbi okhcjtob, HayKosafl /(ywica, Khbb). 

The sol-gel technique has been used to prepare sub-micrometer metal oxide powders [5] with a narrow particle size distribution and unique particle shapes (e.g. AljOj, TiOj, ZrOj, Fe Oj). Uniform SiO spheres have been grown from aqueous solutions of colloidal SiO [6]. Metal-ceramic composites (e.g. Ni-Al Oj, Pt-ZrO ) can also be prepared in this manner [7]. Organic-inorganic composites have been prepared by the sot-gel route. By employing several variants of the basic sol-gel technique, a number of multicomponent oxide systems have been prepared. Some typical examples are SiO -B Oj, SiO -TiO, SiO -ZrO, SiO -Al Oj and ThO -UO. A variety of ternary and still more complex oxides such as PbTiOj, PbTij r 3 and NASICON have been prepared by this technique [1-3, 8]. 

Fab] Fabiichnaya, O.B., Saxena, S.K., Richet, R, Westram, E.F., Thermodynamic Data, Models and Phase Diagrams in Multicomponent Oxide Systems , Springer-Verlag, Berlin, Heidelberg (2004) (Review, Thermodyn.)... 

Multicomponent Silicate Systems. Most commercial glasses fall into the category of sihcates containing modifiers and intermediates. Addition of a modifier such as sodium oxide, Na20, to the siUca network alters the stmcture by cleaving the Si—O—Si bonds to form Si—0-Na linkages (see Fig. 3c). 

Investigations carried out within the past few years have revealed that multicomponent metal oxide systems may interact at interfaces by having one component form a two-dimensional metal oxide overlayer on the second metal oxide component. For example, vanadium oxide can be dispersed on Ti02, Zr02, Si02, AI2O3, and... 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

This symposium shows that research on oxidation processes constitutes a fertile field, tremendously rich in possibilities. This is particularly true of complex multicomponent chemical systems, where particularly great progress is expected. At present science is equipped with adequate experimental techniques for solving the problems. This symposium constitutes an important step in advancing chemical kinetics and will undoubtedly exert significant influence on future research on oxidation reactions and their practical applications. 

During the history of a half century from the first discovery of the reaction (/) and 35 years after the industrialization (2-4), these catalytic reactions, so-called allylic oxidations of lower olefins (Table I), have been improved year by year. Drastic changes have been introduced to the catalyst composition and preparation as well as to the reaction process. As a result, the total yield of acrylic acid from propylene reaches more than 90% under industrial conditions and the single pass yield of acrylonitrile also exceeds 80% in the commercial plants. The practical catalysts employed in the commercial plants consist of complicated multicomponent metal oxide systems including bismuth molybdate or iron antimonate as the main component. These modern catalyst systems show much higher activity and selectivity... 

Ensley, B. D., Gibson, D. T. Laborde, A. L. (1982). Oxidation of naphthalene by a multicomponent enzyme system from Pseudomonas sp. strain NCIB 9816. Journal of Bacteriology, 149, 948-54. 

As mentioned earlier, the multicomponent oxide catalysts currently commercialized contain bismuth, iron, and molybdenum, in addition to several other cations. Although few reports concerning multicomponent catalysts have appeared in the literature, there is agreement that iron affects several aspects of the catalyst system. Measurements on multicomponent catalysts by Wolfs et al. (109-111) showed that Fe3+ was partially reduced to Fe2+ after the catalytic reaction, indicating that Fe3+ ions are involved in the reaction mechanism. The observed Fe3+/Fe2+ redox couple was associated with the increased activity of the catalyst. 

One-electron oxidation systems can also generate radical species in non-chain processes. The manganese(III)-induced oxidation of C-H bonds of enolizable carbonyl compounds [74], which leads to the generation of electrophilic radicals, has found some applications in multicomponent reactions involving carbon monoxide. In the first transformation given in Scheme 6.49, a one-electron oxidation of ethyl acetoacetate by manganese triacetate, yields a radical, which then consecutively adds to 1-decene and CO to form an acyl radical [75]. The subsequent one-electron oxidation of an acyl radical to an acyl cation leads to a carboxylic acid. The formation of a y-lactone is due to the further oxidation of a carboxylic acid having an active C-H bond. As shown in the second equation, alkynes can also be used as substrates for similar three-component reactions, in which further oxidation is not observed [76]. 

A. A Malygin, Synthesis of Multicomponent Oxide Low-Dimension Systems on Surface of Porous Silicon Dioxide Using Molecular Layering Method, J. General Chem. (in Russian) 72(4), 617-632 (2002). 

---