Bismuth molybdate, promoted

Bismuth molybdate + promoter oxides mixed molybdates propene to acrolein... 

Bismuth molybdates promoted with cobalt and iron Pt/Al203 ... 

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). 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

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. 

A commercial iron-promoted catalyst (Sn/Sb/Fe = 1/4/0.25) was studied by Germain et al. [92,93,135,137]. Iron is reported to improve the ammoxidation qualities of the catalyst although it has no effect on the oxidation [93], The kinetics, determined in a flow reactor at 445°C and with a feed ratio C3H6/NH3/air = 1/1.2/10, are essentially similar for this catalyst and bismuth molybdate. The initial selectivity is 80% and the maximum yield is 65% (at 445°C). The initial selectivity markedly depends on the temperature (e.g. 91% at 415°C and 72% at 507°C). The effect of water is hardly significant for this catalyst the acrylonitrile formation is slightly inhibited, while some more acrolein is formed. Presumably, water and ammonia compete in the interaction with the catalyst, which is much less reactive with respect to ammonia than bismuth molybdate. The acrolein ammoxidation is very rapid (about six times the propene ammoxidation rate) and selective (86%). A comparison of the Sn—Sb—Fe—O catalyst with bismuth molybdate is presented in Table 14. 

The first set of reactions is the mainstay of the petrochemical industry 1 outstanding examples are the oxidation of propene to propenal (acrolein) catalysed by bismuth molybdate, and of ethene to oxirane (ethylene oxide) catalysed by silver. In general these processes work at high but not perfect selectivity, the catalysts having been fine-tuned by inclusion of promoters to secure optimum performance. An especially important reaction is the oxidation of ethene in the presence of acetic (ethanoic) acid to form vinyl acetate (ethenyl ethanoate) catalysed by supported palladium-gold catalysts this is treated in Section 8.4. Oxidation reactions are very exothermic, and special precautions have to be taken to avoid the catalyst over-heating. 

The structural coherence of these two phases allows for facile migration of lattice oxygen. Since the Fe-Co-Mo-0 phase is not selective for the propylene (amm)oxidation reaction, the promoting effect of the phase must be a result of its specialized function of reoxidizing the catalytically active Bi-Mo-0 phase. The criticality of specialization of functions in the complex Fe-Co-Bi-Mo-0 catalyst is further manifested in bismuth molybdate, which lacks the redox capability of Fe-Co-Mo-0 but which uniquely carries out the required steps of the surface reaction mechanism of selective alkene (amm)oxidation (see below). 

Third generation Sohio catalysts were also based on bismuth molybdates and contained nickel, cobalt, iron, and minor amounts of other promoters. It has been suggested that the Fe /Fe redox couple facilitates the adsorption and activation of molecular oxygen at the catalyst surface. 

In early patents, it was claimed that various phosphate catalysts, including calcium/nickel phosphate and bismuth/molybdate, were active. A variety of other catalysts was also introduced, including the zinc and magnesium ferrites developed by Petrotex." The best ferrite catalyst included chromium to prevent excessive catalyst reduction." PhilUps described tin oxide catalysts that were promoted with 4% phosphate and 1% lithium as well as the bismuth/phosphate catalysts that were promoted with boron or lithium compounds. 

Preliminary experiments with partial oxidation reactions (propene to acrolein) yielded promising results when using catalysts derived from the surfactant tungstates and molybdates. Moreover, it seems be to improve performance of the catalysts by including promoting metals like bismuth in the syndiesis. According to the mechanism piesrated in the introduction, such metals should easily be incorporated into the structure. 

It is conunon practice to use two reactors. The first reactor contains a conventional bismuth phosphomolybdate catalyst and is used to convert propylene to acrolein. The second reactor contains a selective vanadium molybdate catalyst promoted with tungsten, nickel, manganese or copper, " to convert acrolein to acrylic acid. Fixed bed tubular reactors ate used in both stages. Typical operating conditions ate shown in Table 4.15. 

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