Iron antimonate

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

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... 

Shchukin et al. [288—290] confirmed that iron antimonates consist of a mixture of FeSb04 with either Sb204 or a-Fe203, and report that a maximum selectivity of 90% occurs between Fe/Sb = 0.06 and 1.7 while the activity hardly changes in this region. As with propene oxidation, a high selectivity thus requires an excess of Sb over Fe. 

Ammosov and Sazonov [21,22,24] demonstrated that for iron antimonates the initial selectivities are lower than for bismuth molybdates due to a higher rate of the parallel combustion reaction. It is proved that both selective oxidation and combustion occur by a redox mechanism. In another publication [23], the same authors report the kinetics of the butene and butadiene combustion reactions. 

The Mdssbauer spectrometer was equipped with a 10 mC 5 Co/Rh source maintained at room temperature, A Northern NS-yOO multichannel analyser was used for taking the spectra. The Mdssbauer parameters were determined by least square computer programme. Tht isomer shift (5) was calculated with reference to a-Fe. The diaracieristics of iron in various iron molybdates and iron antimonate arc given in table I. 

The allyl radical, CsHs, is an intermediate in the selective oxidation of propene to acrolein with oxygen, catalysed by bismuth molybdate and iron antimonate [69,91]... 

INS has been used to probe the binding of reactants and intermediates to an iron antimonate catalyst [96]. The spectra were assigned with reference to the spectra of compounds of known structure. 

These spectra aid the interpretation of the mode of binding of adsorbed allyl species formed in the selective oxidation of hydrocarbons over metal oxide catalysts. The INS spectra of allyl iodide adsorbed by an iron antimonate catalyst at 293 K, and after heating to 353 K, were different from the spectrum of allylpalladium chloride and consistent with the allyl binding to the catalyst through the double bond there was no evidence for a ri -allyl (3) [96]. 

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. 

The additive elements used to enhance the performance of the Fe-Sb-0 catalyst either enter the iron antimonate rutile phase to form a solid solution (49,50) or they form separate rutile phases (44). The promoter elements that produce the best performing iron antimonate-based ammoxidation catalysts are copper, molybdenum, tungsten, vanadium, and tellurium. Copper serves as a structural stabilizer for the antimonate phase by forming a rutile-related solid solution (23). Molybdenum, tungsten, and vanadium promote the redox properties of iron antimonate catalysts (51). They provide redox stability and prevent reductive deactivation of the catalyst, especially under conditions of low oxygen partial pressure (see above). The tellurium additive produces a marked enhancement of the selectivity of iron antimonate catalyst. How the tellurium additive functions to increase selectivity is not clear, but the presumption is that it must directly modify the active site. In fact, it is likely that it can actually serve as the site of selective oxidation because in its two prevalent oxidation states Te + and Te +, tellurium possesses the requirements for the selective (amm)oxidation site, a-hydrogen abstraction, and 0/N insertion (see below). 

Industrial catalysts for the synthesis of acrolein and acrylonitrile are based on these simple mixed oxides (ie, bismuth molybdate and iron antimonate) but. 

Acrolein and Acrylic Acid. Acrolein and acrylic acid are manufactured by the direct catalytic air oxidation of propylene. In a related process called ammoxida-tion, heterogeneous oxidation of propylene by oxygen in the presence of ammonia yields acrylonitrile (see Section 9.5.3). Similar catalysts based mainly on metal oxides of Mo and Sb are used in all three transformations. A wide array of single-phase systems such as bismuth molybdate or uranyl antimonate and multicomponent catalysts, such as iron oxide-antimony oxide or bismuth oxide-molybdenum oxide with other metal ions (Ce, Co, Ni), may be employed.939 The first commercial process to produce acrolein through the oxidation of propylene, however, was developed by Shell applying cuprous oxide on Si-C catalyst in the presence of I2 promoter. 

Practically complete conversion of propylene and ammonia is achieved to produce acrylonitrile in 65-70% yield. Acetonitrile and HCN are the main byproducts. The Sohio process originally used oxides of Bi, Co, and Mo, and bismuth and cobalt molybdates.898,915,941,953 Other catalysts developed later (uranyl antimonate antimony oxide-iron oxide oxides of Fe, Ce, and Mo mixed oxides of Sb and Sn)898,915,939,953,955,956 produce fewer byproducts and ensure higher yields of acrylonitrile. 

Antimony is used in alloys, with lead for storage battery plates, with lead and tin in type metals and body solders, with tin and copper in bearing or antifriction metals. Antimony occurs chiefly as the sulfide (stibnite, Sb2S3) which is produced mainly in China, only small amounts in Mexico and Bolivia. Stibnite is (1) melted and reduced to antimony by iron metal and separated from fused ferrous sulfide (See also Stibnite) (2) roasted in air, and sublimed antimonous oxide collected and reduced by heating to fusion with carbon and sodium carbonate. 

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 summary, the violet color (objects 5, 7, 8) is caused by iron in the presence of manganese and cobalt. The turquoise blue color (objects 4, 6, 9) is caused by copper in the presence of iron. These same components (copper, iron) in the presence of lead (probably as lead anti-monate) produce a green color (object 2). Lead antimonate alone causes a yellow-orange color (object 3). Cowell and Werner (I) have found also that cobalt at the 0.2% level can impart a deep blue color, that antimony at the 2% level (as calcium antimonate) can produce an opaque white glass, and that copper in a certain form see below) at the 4% level can give a deep red color. These results are in general agreement with those found by other workers (5). 

A yellowish orange colour is obtained when a dilute solution of sulpho-antimonate of sodium is treated with a mixture consisting of 1 kilo, of quicklime, dissolved in 25 litres of cold water, 1 kilo, of sulphate of iron protoxide, 1 kilo, of sulphate of zinc, and alum in amount varying according as the colour is required to be of a more yellowish or less red hue. 

Yellow Pigment that can be Vitrified.—This is produced by combining the oxide of lead with that of antimony or with the antimonate of potash, obtained by heating a mixture of 2 parts of metallic antimony and 5 parts of nitre in a crucible to red heat. The residue is washed with cold water. This, with the addition of various proportions of oxides of zinc and iron and sometimes tin, is mixed... 

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