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... 

Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect to acrolein. Acetone is the chief product at low temperature (200°C), whereas carbon oxides, besides some acrolein, predominate at higher temperatures [182,257], Firsova et al. [112,113] report that adsorption of propene on iron molybdate (Fe/Me = 1/2) at 80—120°C causes cation reduction (Fe3+ -> Fe2+) as revealed by 7-resonance spectroscopy. Treatment with oxygen at 400°C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related to the low selectivity of iron molybdate. 

Selective oxidation of methanol is the industrial route to formaldehyde. In practice, two types of process are used, differing with respect to the catalyst and process conditions. Silver is a very active catalyst at 600— 700°C and requires a high methanol/oxygen ratio for a good selectivity, while iron molybdate catalysts are already active at 350° C and may be used with low methanol/oxygen ratios. 

Iron molybdate and other metal oxide catalysts... 

The use of iron molybdate in industrial plants started about I960. Yields of about 90% are reported for this process, applying either excess air or excess methanol and recirculation of the latter. Carbon dioxide is the chief by-product. 

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due to Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. 

With regard to iron molybdates, the correlation is less clear. Trifiro and Pasquon [318] defend the view that Mo=0 is also of importance for iron molybdates and state that pure Fe2(Mo04)3 is inactive because of the absence of such oxygen species. However, Carbucicchio and Trifiro [75] have recently reported that no differences in selectivity and specific activity exist between iron-deficient molybdate and the pure compound, although Mo=0 oxygen is only detected in the former. 

Iron molybdates were investigated by several authors. It is generally observed that iron is reduced first (Fe3+ - Fe2+), while deeper reduction is required to reduce the molybdenum ions as well. Both cations occur in partially reduced states during the reaction with butene. Pernicone [254] concludes from his ESR work that under stationary reaction conditions the iron ions stay in the reduced state and that the redox process only involves Mo6+ and Mos+. However, Trifiro and Pasquon [318] and Matsu ura and Schuit [207] are of the opinion that reoxidation initially may lead to Fe3+ which in turn (rapidly) oxidizes the Mos+ ions at the hydrocarbon reaction sites of the catalyst. However, direct evidence is not provided. 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

Iron oxide is an important component in catalysts used in a number of industrially important processes. Table I shows some notable examples which include iron molybdate catalysts in selective oxidation of methanol to formaldehyde, ferrite catalysts in selective oxidative dehyrogenation of butene to butadiene and of ethylbenzene to styrene, iron antimony oxide in ammoxidation of propene to acrylonitrile, and iron chromium oxide in the high temperature water-gas shift reaction. In some other reactions, iron oxide is added as a promoter to improve the performance of the 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. 

Firsova et al. (136) also investigated a cobalt molybdate catalyst containing a small amount of Fe3+, after exposure to a reaction mixture of propylene and oxygen. The authors observed the valence change of Fe3+ to Fe2+ and the formation of a surface complex between the hydrocarbon and the iron (Fe—O—C—). In contrast to pure iron molybdate which also forms a surface Fe—O—C— complexes, the electronic transitions in the cobalt iron molybdate were reversible. The observed valence change showed that iron ions increase the electronic interaction between ions in the catalyst and the components of the reaction mixture. 

Methanol Oxidation. - The selective oxidation of methanol to formaldehyde by vanadium-containing catalysts has been widely studied even though in practice only iron molybdate and Ag catalysts are used for this reaction. 

This process is performed very successfully on an industrial scale using either macrocrystalline silver or an iron molybdate type of catalyst. The... 

Depending on the kinetics of the different elementary processes involved in the formation of the precipitate, a temperature increase might lead to an increase in crystallite size, as was observed for the crystallization of pseudoboehmite [24] or iron molybdates [25]. However, in other cases no influence of the precipitation temperature on the crystallite size of the final catalyst was reported [26], or a decrease was reported, as for the ZnO system [27],... 

Iron molybdate Fe(Mo04)3, M0O3 methanol to formaldehyde... 

A good example of the latter case is the reaction of iron with silica in silica-supported iron molybdate, a fact which made impossible the use of silica as a support in the case of catalysts used for the oxidation of methanol to formaldehyde. Some of the methods to overcome these difficulties in catalyst preparation are the subject of this section. 

Figure 7. Formation of gels from iron molybdate precipitates [5]. A very fine colloidal precipitate forms in all cases but undergoes different changes suggested by the arrows, according to concentration and Mo Fe proportions, as well as other conditions mentioned in the figure. In the upper part domain, the colloidal precipitate seems to dissolve to form a solution whose viscosity increases until formation of a gel. In the right-hand domain, the gel directly forms from the colloidal system but can become transparent if the reaction takes place in the temperature range 293-313K.

WAXS/XANES and UV-vis Methanol to formaldehyde over iron-molybdate catalysts [55]... 

O Brien MG, Beale AM, Jacques SDM, Weckhuysen BM. A combined multi-technique in situ approach used to probe the stability of iron molybdate catalysts during redox cycling. Top Catal. 2009 52 1400. 

Raman investigation by Hill and coworkers (Wilson et al., 1990) of bulk and supported iron molybdate catalysts during methanol oxidation to formaldehyde. This Raman reaction cell was designed to minimize void... 

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