Molybdenum oxide activator

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

It was seen when studying mixed systems Pt-WOj/C and Pt-Ti02/C that with increasing percentage of oxide in the substrate mix the working surface area of the platinum crystallites increases, and the catalytic activity for methanol oxidation increases accordingly. With a support of molybdenum oxide on carbon black, the activity of supported platinum catalyst for methanol oxidation comes close to that of the mixed platinum-ruthenium catalyst. 

Fig. 6. Difference spectra between xanthine oxidase inactivated with various pyra-zolo [3, 4-d] pyrimidines and the native enzyme. The spectra are believed to represent the increase in absorption occurring when Mo(VI) of native enzyme is converted to Mo(IV) complexed with the inhibitors. Spectra were obtained by treating the enzyme with inhibitors in the presence of xanthine, then admitting air, so as to re-oxidize the iron and flavin chromophores. The extinction coefficients, de, are expressed per mole of enzyme flavin. Since some inactivated enzyme was present, extinction coefficients per atom of molybdenum of active enzyme will be about 30% higher than these values. (Reproduced from Ref. 33, with the permission of Dr. V. Massey.)...

Ruthenium on platinum was found to facilitate the oxidation of oCOad but noteoCOad interacting water, appeared to play a role of the oxygen source, replacing Pt-OH. Contrarily, tin and molybdates on platinum showed promoting effects for the oxidation of eoCOad but not AoCOad Redox couples of tin or molybdenum oxides appeared to play a role of a mediator as well as the ox en source. The active potentials were the bwest for molybdates and the second for tin. 

In the middle thirties the reactions of naphtha and certain compounds known to be present in naphtha were being studied in university and industrial laboratories. One of the problems was to find a catalyst that was capable of synthesizing an aromatic from a paraffin. It was reasoned that the hydrogenation-dehydrogenation oxide-type catalysts such as molybdenum oxide and chromium might possess suitable activity at temperatures well below those employed in thermal reforming. 

Reduction. Benzene can be reduced to cyclohexane [110-82-7], C5H12, or cycloolefins. At room temperature and ordinary pressure, benzene, either alone or in hydrocarbon solvents, is quantitatively reduced to cyclohexane with hydrogen and nickel or cobalt (14) catalysts. Catalytic vapor-phase hydrogenation of benzene is readily accomplished at about 200°C with nickel catalysts. Nickel or platinum catalysts are deactivated by the presence of sulfur-containing impurities in the benzene and these metals should only be used with thiophene-free benzene. Catalysts less active and less sensitive to sulfur, such as molybdenum oxide or sulfide, can be used when benzene is contaminated with sulfur-containing impurities. Benzene is reduced to 1,4-cydohexadiene [628-41-1], C6HS, with alkali metals in liquid ammonia solution in the presence of alcohols (15). 

Bismuth molybdenum oxide combinations that additionally contain some phosphorus were investigated by Ai and Suzuki [8,12]. The bismuth content was varied at a constant ratio P/Mo = 0.2. The effect of this variation on the overall activity for the oxidation of c/s-2-butene as a function of the temperature is shown in Fig. 5. The Bi/Mo ratio appears to have a... 

One could find that physical mixing is a simple way to prepare molybdenum oxide loaded mesoporous or silica oxide catalysts. The rate of catalyst deactivation is expressed in terms of the percentage decrease in initial conversion after 2 h of reaction. Initial and steady state activities were taken after 0.5 and 24 h of reaction on stream, respectively. Generally, the selectivity of styrene, which is the major and desired product, is at least 96% at steady state. 

In both fresh and regenerated catalysts, the MCM-41 supported catalysts are better than amorphous silica supported ones. Physically mixed molybdenum oxide catalysts with Def-MCM41 support are particularly active. The steady state activity decreases in the order Mo/DM > Mo/M > Mo/Si02. Interestingly, the rate of deactivation also seems to depend on... 

Allows a better dispersion of molybdenum trioxide from the external surface of the mesoporous support into its internal nanochannels. The active sites (possibly pairs of neighboring molybdenum cations) thus increases. As the result of better dispersion, the reduced molybdenum oxide species formed during the course of reaction through its entire surfaces and thus lowers the possibility of sintering in a reduced environment. Here, we see that the deactivation rate is the highest in Mo/Si02 catalyst due to the lowest surface area. 

The difference in reactivity of the butadiene precursor toward 02 and NzO is interesting. N20 is known to be active in selective oxidation (16). For example, on molybdenum oxide (16) and cobalt magnesium oxide (17), NzO decomposes at room temperature to form an O adsorbed species which is very active in the oxidative dehydrogenation of ethane. The results presented above suggest that the degradation of butadiene precursor on a-iron oxide requires an 02 and not an 0" species. This implies that the degradation proceeds via a peroxide intermediate. 

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