Ferric molybdate

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

Coatings, Paints, and Pigments. Various slightly soluble molybdates, such as those of zinc, calcium, and strontium, provide long-term corrosion control as undercoatings on ferrous metals (90—92). The mechanism of action presumably involves the slow release of molybdate ion, which forms an insoluble ferric molybdate protective layer. This layer is insoluble in neutral or basic solution. A primary impetus for the use of molybdenum, generally in place of chromium, is the lower toxicity of the molybdenum compound. 

A conmercial catalyst frcm Harshaw was used, a 3 1 mixture of molybdenum trioxide and ferric molybdate, as well as the two separate phases. Kinetic experiments were done previously in a differential reactor with external recycle using these same catalysts as well as several other preparations of molybdenun trioxide, including supported samples. Hie steady state kinetic experiments were done in the temperature range 180-300 C, and besides formaldehyde, the following products were observed, dimethylether, dimethoxymethane, methyl formate, and carbon-monoxide. Usually very little carbon dioxide was obtained, and under certain conditions, hydrogen and methane can be produced. 

The steady state experiments showed that the two separate phases and the mixture are not very different in activity, give approximately the same product distributions, and have similar kinetic parameters. The reaction is about. 5 order in methanol, nearly zero order in oxygen, and has an apparent activation energy of 18-20 kcal/mol. These kinetic parameters are similar to those previously reported (9,10), but often ferric molybdate was regcirded to be the major catalytically active phase, with the excess molybdenum trioxide serving for mechanical properties and increased surface area (10,11,12). 

The author is thankful to U. Chowdhry for preparing the pure phase ferric molybdate. 

Formox [Formaldehyde by oxidation] A process for oxidizing methanol to formaldehyde, using a ferric molybdate catalyst. Based on the Adkins-Peterson reaction, developed by Reichold Chemicals, and licensed by that company and Perstorp, Sweden. Acquired by Dyno Industries in 1989. The process uses formaldehyde produced in this way to make formaldehyde-urea resin continuously. A plant using this process was to be built in Ghent by 1991, owned jointly by Dyno and AHB-Chemie. Licensed to 35 sites worldwide. Several other companies operate similar processes. 

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

Figure 8. Temperature Programmed Desorption of methanol from the ferric molybdate (dashed line) and the manganese pyrophosphate (solid line) catalysts determined gravimetrically.

Figure 11. Mechanistic proposal for the aamoxidation of methanol to HCN over the ferric molybdate and the manganese pyrophosphate catalysts.

FeMo04 + 3/2 02-> Fc203 + 2Fe2Mo30i2) since both ferric oxide and ferric molybdate have been detected by X-rays diffraction and Mdssbauer spectroscopy in the sample after the catalytic run (9). 

Annenkova et al. (105) studied both the physicochemical and catalytic properties of the Bi-Fe-Mo oxide system. The X-ray diffraction, infrared spectroscopic, and thermographic measurements indicated that the catalysts were heterogeneous mixtures consisting principally of ferric molybdate, a-bismuth molybdate, and minor amounts of bismuth ferrite and molybdenum trioxide. The Bi-Fe-Mo oxide catalysts were more active in the oxidation of butene to butadiene and carbon dioxide than the bismuth molybdate catalysts. The addition of ferric oxide to bismuth molybdate was also found to increase the electrical conductivity of the catalyst. 

Molybdite or molybdenum ochre forms orthorhombic crystals, and occurs with molybdenite, from which it is probably derived. It consists essentially of the trioxide MoOg, but analysis has shown that its composition is probably expressed by the formula Fe203-3Mo03.7jH20 it being, in fact, a hydrated ferric molybdate. The sample examined was of a yellow colour, possessed a fibrous structure and a silky lustre, and was pleochroic. 

Ferric Molybdate, Fe2(Mo04)j.42H20,is obtained as a j ellowish-brown precipitate when an aqueous solution of normal sodium molybdate is treated with ferric chloride. i If the di- or para-molybdate is so treated, a yellow precipitate, of composition Fe2O3.5MoO3.aq., separates rvith the tetramolybdate a pale yellow precipitate, of... 

The most selective catalysts for the oxidation of methanol to formaldehyde are molybdates. In many commercial processes, a mixture of ferric molybdate and molybdenum trioxide is used. Ferric molybdate has often been reported to be the major catalytically active phase with the excess molybdenum trioxide added to improve the physical properties of the catalyst and to maintain an adequate molybdenum concentration under reactor conditions(l,2). In some cases, a synergistic effect is claimed, with maximum catalytic activity for a mixture with an Fe/Mo ratio of l.T( 3j. A defect solid solution was also proposed( ). Aging of a commercial catalyst has been studied using a variety of analytical techniques(4) and it was concluded that deactivation can largely be account for by loss of molybdenum from the catalyst surface. 

In this paper, we will discuss results of the oxidation of methanol over a series of molybdates including solid solutions of ferric, chromium and aluminum molybdates and also over a new ferric tungstate phase. The mixed molybdates of iron/chromium, iron/aluminum and chromlum/aluminum were made for the first time in pure well-characterized forms. Results are compared with our earlier work over commercial mixtures of ferric molybdate and molybdenum tri oxide and a number of pure molybdates. ... 

All these molybdates are isostructural with ferric molybdate with an open 3-dimensional network of MO octahedra and M0O4 tetrahedra. A ferroelastic transition exists from the low temperature monoclinic form to the high temperature orthorhombic form. The transition temperature varies from 200 C for pure aluminum molybdate to 385 0 for pure chromium molybdate and 500 C for pure ferric molybdate. For the mixed molybdates, the transition temperature was found to be a linear function of composition as is illustrated in Figure 4 for the mixed iron-aluminum molybdates. 

The ferric molybdate, Fe2(Mo04)3, has a structure related to the garnet type but without cations in the 8-coordinated sites. The quadrupole splitting of 0-25 mm s at room temperature is only half that found in conventional garnets, implying that the octahedral Fe " site has a lower distortion in the molybdate structure [175]. 

Adsorption and Reactions of Methane on Ferric Molybdate using DRIFTS Technique Surajit Fuangfoo, Anand S. Chellappa and Dabir S. Viswanath ... 

The results from the infrared studies and from the GC analysis show that the reaction of methane with the ferric molybdate catalysts gives methanol, formaJdehyde, carbon dioxide, and carbon monoxide as final products. The IR spectra also indicate the formation of methoxy, surface dioxymethylene, surface formate species, and adsorbed formaldehyde. Based on these observations, a mechanism was proposed to account for all intermediates and final products and is shown in Figure 5. Since the surface structure of the catalysts is not known, the surface is represented by a straight line in the scheme. 

Figure 5. Proposed mechanism of catalytic conversion of methane over ferric molybdate catalysts.

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