Molybdate selective oxidation catalysis

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Results (Table I) from refinement of the powder neutron diffraction data for the Ce doped material, Bi 8 0 2 3 12 that significant structural alterations have resulcea from Ce incorporation into the solid The three Mo atoms are no longer chemically equivalent The coordination about Mo(l) is unchanged from that of the parent compound A second Mo atom contains two short (double) bonds to oxygen atoms These molybdate dioxo (dimolybdenyl) groups are believed to be an important feature for selective oxidation catalysis (7 ) The third Mo coordination sphere contains no Mo=0 bond Bond order calculations (27) about... 

In the following we will investigate the methanol oxidation process over silver, copper and heteropoly molybdates in order to identify the occurrence of the possible reaction pathways from Schemes 2 and 3 on polycrystalline surfaces and at atmospheric pressure. The main emphasis in these experiments will be on the source of active oxygen. This focus was chosen to better understand the involvement of bulk and sub-surface [10,48] chemistry in selective oxidation catalysis. Such an understanding is required when the catalytic performance is compared between series of chemically different systems which are chosen to investigate only one surface property such as acidity. These experiments will naturally only cover a small selection of the problems discussed with the reaction Schemes. 

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Villa et al. [340] have shown that the bismuth tungstates are comparable with bismuth molybdates with respect to dehydrogenation catalysis, although activities and selectivities are somewhat lower. Although the phase structures are different, interesting catalysts are formed in a similar composition range Bi/W = 2/3 to 2/1. (Note that, in case of propene (amm)oxidation, tungstates are definitely inferior to molybdates.)... 

There is growing interest in the partial oxidation of the C5 fraction of the hydrocarbon stream from naphtha steam crackers since there is no real market for them at the present time. Furthermore, the partial oxidation of lower alkanes and alkenes continues to pose challenging problems for catalysis researchers. In the case of C5 hydrocarbon oxidation to form phthalic anhydride, the challenge is even greater since the catalyst needs not only to insert oxygen selectively, but also promote the formation of C-C bonds in an oxidative medium. In recent years, several studies have been reported in the literature, focusing on Cg oxidation using catalysts such as supported vanadia, VPO catalysts, and molybdates [1-11]. ... 

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