Uranium antimonate

Shortly after the introduction of the bismuth molybdate catalysts, SOHIO developed and commercialized an even more selective catalyst, the uranium antimonate system (4). At about the same time, Distillers Company, Ltd. developed an oxidation catalyst which was a combination of tin and antimony oxides (5). These earlier catalyst systems have essentially been replaced on a commercial scale by multicomponent catalysts which were introduced in 1970 by SOHIO. As their name implies, these catalysts contain a number of elements, the most commonly reported being nickel, cobalt, iron, bismuth, molybdenum, potassium, manganese, and silica (6-8). 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

In 1966, a catalyst based on a complex uranium antimonate system was developed and brought into commercial use (4, 87). Several physical methods of analysis were used in an attempt to clarify relationships between the structure and properties of the uranium antimonate system and its catalytic properties (20, 88, 89). X-Ray diffraction and infrared analysis demonstrated that the optimum selectivity for acrylonitrile formation coincided with the maximum concentration of the USb3O10 compound. The crystal structure of USb3O10 was shown to consist of layers of heavy atoms and oxygen ions alternated by layers of oxygen ions. Measurements by ESCA indicated that the surface layers contained U5+ and Sb5+ with intensities corresponding to the USb3O10 formula. 

The most industrially significant and well-studied allylic oxidation reaction is the ammoxidation of propylene ( eq. 8 ) which accounts for virtually all of the 8 billion pounds of acrylonitrile produced annually world-wide. The related oxidation reaction produces acrolein ( eq. 9 ), another important monomer. Although ammoxidation requires high temperatures, the catalysts are, in general the same fof both processes and include bismuth molybdates, uranium antimonates (USb30j Q), iron antimonates, and bismuth molybdate based multicomponent systems. The latter category includes many of todays highly selective and active commercial catalyst systems. 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

The second generation Sohio catalyst was a uranium antimonate (USbsOio). This was more active and selective than the earher bismuth phos-phomolybdate and has been described as Phase I. Active sites in the layer strac-ture were also defect-Scheelite structures containing uranium-antimoity cation pairs. Catalysts containing USbOs, or Phase 2, were less selective. 

Margolis [203] confirms such results for antimonates and reports the existence of a surface compound containing Sb3+—O—C. Aykan and Sleight [34] examined the system U—Sb—O in air up to 1000°C by different techniques (e.g. ESR) and found the ternary components USbOs and USb3Oi0. Since USb03 is paramagnetic, the formal oxidation state of U must be 5+, hence Sb must also be in the 5+ state. The authors conclude that USb3O10 also contains pentavalent uranium. 

The uranium-antimony oxide system remains as a basis of interest for catalysts. The preparation of a new uranyl antimonate has been described and it was prepared by hydrothermal synthesis from UO3, SbsOs and KCl [46]. A detailed structural analysis was reported, but more importantly the U0sSb204 was selective for the oxidation of propylene to acrolein. 

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