Bismuth iron molybdate

Catalytic Behavior and Phase Composition of Bismuth-Iron Molybdates... 

The structure of the single phase bismuth-iron molybdate compound of composition Bl3FeMo20.2 related to the scheellte structure of Bi2Mo30-2( ). It is reported(, ) that the catalytic activity and selectivity of bismuth-iron molybdate for propylene oxidation and ammoxidatlon is not greater than that of bismuth molybdate. 

Table II. Catalytic Activity for Propylene Ammoxidation Over Bismuth-Iron Molybdate...

Bismuth iron molybdate, 27 207-209 X-ray diffraction, TlilW Bismuth molybdate, 27 184-187, 189, 191-194, 196, 199--204, 30 124-125 active site, 27 210-213 alumina supported, 27 203, 204 ammoxidation, 30 159 P phase, 27 201 catalyst... 

LoJacono et al. (108) also utilized X-ray diffraction methods to study the structural and phase transformations which occurred in the Bi-Fe-Mo oxide system. They detected two ternary compounds containing bismuth, molybdenum, and iron. One of the compounds formed when the atomic ratio Bi/Fe/Mo = 1 1 1 the other formed when the atomic ratio Bi/Fe/Mo = 3 1 2. The X-ray data indicated a close structural relationship of the bismuth iron molybdate compounds with the scheelite structure of a-phase bismuth molybdate. Moreover, their structures were similar to compound X. The structure of the Bi/Fe/Mo = 3 1 2 compound was identical to the compound reported by Sleight and Jeitschko (107). The authors proposed that the structures of both of the compounds could be viewed as resulting from the substitution of Fe3+ in the a-phase lattice. In the Bi/Fe/Mo = 1 1 1 compound, 1 Mo6+ ion is replaced by 2 Fe3+ ions one Fe3+ ion occupies a Mo6+ site the other Fe3+ ion occupies one of the vacant bismuth sites. In the Bi/Fe/Mo = 3 1 2 compound, the Fe3+ ion replaces one Mo6+ ion while the additional Bi3+ ion occupies one of the vacant bismuth sites. 

Although Wolfs indicated that the catalyst particles are covered by a skin of bismuth molybdate, Batist (112) recently found bismuth, molybdenum, and iron in the surface layers of multicomponent catalysts. Additional data are needed to determine if multicomponent catalysts gain their activity as a result of the formation of compounds such as bismuth iron molybdate, or by surface enhancement of an active component such as 7-phase bismuth molybdate, or by creation of low-energy electronic transitions. Of course, due to their complexity, all of these factors may be important. 

Many heavy metals react with dithiol to give coloured precipitates, e.g. bismuth, iron(III), copper, nickel, cobalt, silver, mercury, lead, cadmium, arsenic, etc. molybdate and tungstate also react. Of the various interfering elements, only arsenic distils over with the tin when a mixture is distilled from a medium of concentrated sulphuric acid and concentrated hydrobromic acid in a current of carbon dioxide. If arsenic is present in quantities larger than that of the tin it should be removed. 

Introduction of a support (i.e. Ti02) to these systems can induce positive effects on the active phase (FeMo), as to avoid an excessive sintering of the particles during the thermal treatment and/or modification of the reduction capacity and the acid properties. The synergy of iron molybdate and the support is therefore another way for improving the catalytic performance of these solids. Such benefitial effects have been detected in bismuth-molybdenum-titania mixed oxides prepared via sol-gel, in addition these solids resulted to be amorphous materials with a unique morphology and extraordinary dispersion of the active phase [12]. These results encouraged us to extend this field to iron molybdenum oxide catalysts. 

Bulk mixed metal oxide catalytic materials consist of multiple metal oxide components. Such mixed metal oxide catalysts find wide application as selective oxidation catalysts for the synthesis of chemical intermediates. For example, bulk iron-molybdate catalysts are employed in the selective oxidation of CH3OH to H2CO [122], bulk bismuth-molybdates are the catalysts of choice for selective oxidation of CH2=CHCH3 to acrolein (CH2=CHCHO) and its further oxidation to acrylic acid (CH2=CHCOOH) [123], selective ammoxidation of CH2=CHCH3 to acrylonitrile (CH2=CHCN) [123], and selective oxidation of linear CH3CH2CH2CH3 to cyclic maleic anhydride consisting of a flve-membered ring (four carbons and one O atom) [124]. The characterization of the surface... 

H. 8-Hydroxyquinaldine (XI). The reactions of 8-hydroxyquinaldine are, in general, similar to 8-hydroxyquinoline described under (C) above, but unlike the latter it does not produce an insoluble complex with aluminium. In acetic acid-acetate solution precipitates are formed with bismuth, cadmium, copper, iron(II) and iron(III), chromium, manganese, nickel, silver, zinc, titanium (Ti02 + ), molybdate, tungstate, and vanadate. The same ions are precipitated in ammoniacal solution with the exception of molybdate, tungstate, and vanadate, but with the addition of lead, calcium, strontium, and magnesium aluminium is not precipitated, but tartrate must be added to prevent the separation of aluminium hydroxide. 

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). 

A continuous process based on hydrodynamic cavitation can be employed to prepare a wide variety of metal oxides in grain sizes of 1 -10 nm, such as iron oxide, bismuth molybdate, perovskites, platinum-loaded zeolite, and other ceramics and superconductors [170]. The method uses a microfluidiser for mechanically generating hydrodynamic cavitation and the internal pressure of the liquid media is elevated from ambient pressure to between 1000 to 25 000 psi. 

Although this report describes the preparation of a wide variety of oxidation catalysts, only the iron modified bismuth molybdates will be described in detail. Other preparations are described in this section through an indication of the starting soluble salts and their synthesis temperatures which are the key process parameters. Other aspects of their HTAD preparations are similar to that described for the iron bismuth molybdates which follows. 

Bismuth Molybdates And Iron Substituted Bismuth Molybdates... 

Finally, Arrhenius treatments of the catalytic data were examined for the HTAD synthesized substitutional series, Bi(2-2x) 2x 030i2, and the binary bismuth molybdate series where Bi/Mo ratios were varied fi-om pure Mo oxide to pure Bi oxide. The noteworthy aspect of the oxidation results is that in the most reactive regime of x = 0-5% atom fi-action Fe, before separate phase Fe3Mo30j2 begins to dominate the catalyst composition in the iron series, the apparent activation energies were all in the range of 19-20 kcal/mol. Furthermore, the activation energies for the pure Bi-Mo series were between 19-20 kcal/mol while the activities were considerable different. Thus, the chief difference in the reactivities in both series is in the preexponential factor, i.e. the number of active sites. 

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catdysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 

Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. 

XPS has been used to characterize the three mixtures containing respectively 7,25, and 50 weight % of Bi2Mo30i2 (Table II samples J,K and L). These samples have been characterized before and after catalytic reaction (table III). Bi, Mo, Fe, Co and O have been analyzed. The Mo/0 ratio remains equal to 0.25 for all the samples, before and after catalysis which confirms that no new phase was formed since the molybdates suspected to have formed, have a much lower Mo/0 ratio (0.17 for Bi2Mo06 and Bi3FeMo20i2). Concerning the Bi/(Fe+Co) ratio, it can first be observed that before catalysis this ratio was always lower than that calculated from chemical analysis. This can be explained by the difference between the particles size of the bismuth molybdate and the iron and cobalt molybdates which is in a ratio of more than 30 as calculated from differences in surface area values, 0.3 and 9 to 22 m. g Secondly the Bi/(Fe+Co) ratio increased systematically after catalysis which could be explained by the decrease in size of the bismuth molybdate particles or by the covering of the iron and cobalt molybdate particles by the bismuth molybdate or by both effects. 

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. 

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... 

In the 1960s, a number of binary oxides, including molybdenum, tellurium, and antimony, were found to be active for the reactions and some of them were actually used in commercial reactors. Typical commercial catalysts are Fe-Sb-O by Nitto Chemical Ind. Co. (62 -64) and U-Sb-O by SOHIO (65-67), and the former is still industrially used for the ammoxidation of propylene after repeated improvements. Several investigations were reported for the iron-antimony (68-72) and antimony-uranium oxide catalysts (73-75), but more investigations were directed at the bismuth molybdate catalysts. The accumulated investigations for these simple binary oxide catalysts are summarized in the preceding reviews (5-8). 

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