Iron-molybdate catalyst

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. 

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. 

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... 

The mechanism for the oxidation of methanol to formaldehyde on iron molybdate catalysts (illustrated in Figure 4) envisages the H-abstraction and electron transfer of surface methoxy species desorption of the products and reoxidation of metal sites complete the cycle. 

To obtain a high yield, it is important to use the right catalyst and the right material for containing the catalyst. With vanadium pentoxide in pyrex tubes, the yield is only 25 percent of the input furfural, but with a vanadium pentoxide/molybdenum trioxide/iron molybdate catalyst in nickel tubes, the yield is in the order of 75 percent. Interestingly, the best yields are obtained when the catalyst, prepared from appropriate ammonium salts, is cured with air at 300 °C in situ, and when it is then used directly at the reaction temperature of 270 °C without allowing it to cool down. 

The selective oxidation of methanol to formaldehyde is a technologically relevant [1,2] reaction carried out over metallic silver or iron-molybdate catalysts. The reaction is also suitable as a model reaction to understand the chemical principles of modifying the oxidizing potential for dissociated oxygen on various surfaces [3,4,5,6,7]. 

A COMPARISON OF IRON MOLYBDATE CATALYSTS FOR METHANOL OXIDATION PREPARED BY COPRECIPTATION AND NEW SOL-GEL METHOD... 

Many metal molybdates catalyse the reaction under consideration [4] and the active sites are widely believed to be associated with surface Mo atoms in octahedral coordination [5]. Octahedral coordination of Mo is only achieved in Fe-defective iron molybdates what is in accordance with the fact that maximum activity is obtained with catalysts with Mo/Fe atomic ratio greater than 1.5 [6]. The presence of two terminal oxygens double bonded to Mo in such coordination allows the methanol reacting molecules to be bonded simultaneously by two points. The activation of the hydrogen of the hydroxyl group produces methoxy species that are intermediates in the formaldehyde formation. The role of Fe in iron molybdates catalysts would be to act in the transfer of O2 and H2O between surface and gas phase [6] and to hinder the reduction of Mo [7]. 

To investigate of influence of preparation methods on catalytic properties of iron molybdates catalysts with controlled excess of M0O3 were prepared by coprecipitation and sol-gel techniques. Their properties and performances were compared with an industrial catalyst. 

Sol-gel techniques provide interesting routes to prepare iron molybdates catalysts for selective oxidation of methanol. Catalysts prepared by this method have higher surfiice areas than catalysts prepared by coprecipitation techniques what allows to operate at lower tempo tures with the advantage of limiting the consecutive oxidation of formaldehyde to CO. 

A kinetic study of methanol oxidation over stoichiometric iron molybdate catalyst was performed in a fixed-bed integral reactor showing kinetic influences of reaction products. In the temperature range of 548-618 K it was not possible to fit the fomoation rate data to a single power rate law. Dimethyl ether formation presents only a second order dependence with respect to methanol. CO formation seems to be inhibited by water and formaldehyde and rate data fit well to the power rate law ... 

Redispersion of M0O3 in 02" or methanol" environments has been shown to readily take place and can even be employed in the synthesis of bulk iron-molybdate catalysts from physical mixtures of M0O3 and Fc203, as well as the regeneration of spent bulk iron-molybdate catalysts. ... 

Soares A., Portela M. and Kiennemann A. (2005). Methanol Selective Oxidation to Formaldehyde over Iron-molybdate Catalysts, Catal. Rev. Sci. Eng., 47,125-174. 

Hill, C. and Wilson III, J. (1990). Raman Spectroscopy of Iron Molybdate Catalyst Systems Parti. Preparation of Unsupported Catalysts, J. Mol. Catal., 63, pp. 65-94. 

House, M., Carley, A. and Bowker, M. (2007). Selective Oxidation of Methanol on Iron Molybdate Catalysts and the Effects of Surface Reduction, J. Catal., 252, pp. 88-96. 

House, M., Carley, A., Echevenia-Valda, R., et al. (2008). Effect of Varying the Cation Ratio within Iron Molybdate Catalysts for the Selective Oxidation of Methanol, J. Phys. Chem. C, 112, pp. 4333-4341. 

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... 

Wachs, I.E. and Briand, L.E. In situ Formation of Iron-Molybdate Catalysts for Methanol Oxidation to Formaldehyde. U.S. patent No. 6,331,503 Bl, December 18,2001. 

The second commercial formaldehyde process is based on an iron molybdate catalyst. This process operates at about 450 °C, atmospheric pressure, and with a large stoichiometric excess of oxygen. Reaction (7-A) is the principal source of CH2O. The reaction temperature is so low that Reaction (7-B) is unimportant. 

Benzene oxidation to maleie anhydride. Methanol oxidation to formaldehyde (iron molybdate catalyst). 

By the 1930s, Adkins, working with the Bakehte Corporation, introdnced a mixed oxide catalyst for the direct oxidation of methanol. Dnring the development he found that pare molybdenum oxide gave about 60% conversion to formaldehyde at 400°C, although activity fell after 12-24 h to 30% conversion. Pure iron oxide, on the other hand, was not selective and prodnced only carbon dioxide. However, a mixed iron/molybdate catalyst converted more than 90% of the methanol to formaldehyde. Operation was relatively stable and by 1952 DuPont had built a plant using iron/molybdate in a process similar to that described in Adkins patent. ... 

TABLE 4.4. Silver Granule and Iron Molybdate Catalysts. 

Keywords Iron molybdate catalyst. Selective catalytic oxidation. Catalysts preparation... 

Formaldehyde, CH2O, is manufactuied by the selective oxidation of methanol over a silver [1,2] or iron molybdate catalyst [3 4]. Iron molybdate catalyst is a combination of the two oxides that produces the desired active and selective catalyst. Iron(III) oxide by itself is unselective producing carbon dioxide and water, molybdenum trioxide is selective but with low activity [5]. 

The aim of the present work is to evaluate different methods of preparation of the iron molybdate catalysts and test the activities and selectivities in the oxidation of methanol to formaldehyde by comparing with those of a conunercial catalyst. 

A.P.V. Soares, M.F. Portela, A. Kieimemaim, L. Hilaire and J.M.M. Millet (2001) Iron molybdate catalysts for methanol to formaldehyde oxidation effect of Mo excess on catalystic behaviour . Applied catalysis, 206,221-229. 

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