Molybdena

Hydrodesulfurization. A commercial catalyst contains about 4 percent CoO and 12 percent M0O3 on y-alumina and is presulfided before use. Molybdena is a weak catalyst by itself and the cobalt has no catalytic action by itself. 

Effect of Catalyst The catalysts used in hydrotreating are molybdena on alumina, cobalt molybdate on alumina, nickel molybdate on alumina or nickel tungstate. Which catalyst is used depends on the particular application. Cobalt molybdate catalyst is generally used when sulfur removal is the primary interest. The nickel catalysts find application in the treating of cracked stocks for olefin or aromatic saturation. One preferred application for molybdena catalyst is sweetening, (removal of mercaptans). The molybdena on alumina catalyst is also preferred for reducing the carbon residue of heating oils. 

Beavon [Beavon Sulfur Removal] Also called BSR. A process for removing residual sulfur compounds from the effluent gases from the Claus process. Catalytic hydrogenation over a cobalt/molybdena catalyst converts carbonyl sulfide, carbon disulfide, and other... 

PHD A petroleum reforming process operated in Germany from 1940. The catalyst was molybdena on alumina. 

Standard Oil A process for polymerizing ethylene and other linear olefins and di-olefins to make linear polymers. This is a liquid-phase process, operated in a hydrocarbon solvent at an intermediate pressure, using a heterogeneous catalyst such as nickel oxide on carbon, or vanadia or molybdena on alumina. Licensed to Furukawa Chemical Industry Company at Kawasaki, Japan. 

Figure 8.12 Raman spectra of alumina- and silica-supported molybdena catalysts after impregnation of the supports with solutions of ammonium heptamolybdate, (NH4)6Mo7024 4 H20 of different pH values, and after calcination in air at 775 K. See Table 8.3 for a list of characteristic Raman frequencies of molybdate species. The sharp peaks in the spectra of the calcined MoOySiOj catalyst are those of crystalline Mo03 (from Kim el at. [43J).

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

Pines and Csicsery reported on the formation of diolefins in chromia catalyzed dehydrocyclization of Cj-Cg hydrocarbons 49). The kinetic behavior of heptadienes and heptatrienes in chromia and molybdena catalyzed aromatization of unsaturated n-Cj hydrocarbons 22, 49a) indicated that they were intermediates of the reaction. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

Stepwise Ce dehydrocyclization was observed over potassia-chromia-alumina as well as potassia-molybdena-alumina catalysts (9, 10). Higher operating temperatures (450°-500°C) of these catalysts facilitate the appearance of unsaturated intermediates in the gas phase. Radiotracer studies indicate a predominant Ce ring closure of C-labeled n-heptane over pure chromia (132,132a). 

Figure 16. Propylene conversion and product-selectivity results for the membrane-reactor measurements performed at 723 K with pure propylene as the feed. The results in panel a were for the SOFC with a Cu—ceria—YSZ anode, and the results in panel b were for the Cu-molybdena-YSZ anode. In panel a, the points are the rate of CO2 production, and the line was calculated from the current density and eq 8. In panel b, the points show the production of acrolein, and the line was calculated from eq 9. (Reprinted with permission from ref 165. Copyright 2002 Elsevier.)...

Results for the cell with a Cu—molybdena—YSZ anode, shown in Figure 16b, were very different. Unlike ceria, which is a nonselective oxidation catalyst, molybdena is a selective catalyst for the partial oxidation of propylene to acrolein (CH2=CHCHO) and is used commercially for this process. The primary product at low conversion over the Cu— molybdena—YSZ electrode was acrolein, produced according to the reaction... 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

The simplest interpretation of these results is that ceria and molybdena act as catalysts in the TPB region, as shown in Figure 17. Either molybdena or ceria are oxidized by 0 coming through the electrolyte and then subsequently reduced by the fuel. According to this picture, reaction at the TPB is a simple redox process with a nonconventional oxygen source. Because molybdena is selective for the oxidation of propylene to acrolein, while ceria is nonselective, the products formed in cells with these two catalysts are different. 

The top three spectra of Fig. 29 represent an effort to use the spectra to identify the environment of cobalt in a typical cobalt-molybdena-alumina catalyst of approximate composition 5% cobalt, 10% M0O3. The X-ray... 

Fra. 29. Spectra of cobalt-molybdena-alumina catalyst and related compositions, C oMo04, and a coprecipitated cobalt alumina catalyst (all three samples were calcined in air) also spectrum of cobalt-alumina catalyst reduced in hydrogen. 

Commercial SCR catalysts are made of homogeneous mixtures of titania, tungsta and vanadia (or molybdena). Titania in the anatase form is used as a high surface and sulfur-resistant carrier to disperse the active components. Tungsta or molybdena is employed in large amounts (10 and 6% w/w, respectively) to increase the surface acidity and the thermal stability of the catalyst and to limit the oxidation of SO2. Vanadia is responsible for the activity in the reduction of NO, but it is also active in the oxidation of SO2. Accordingly, its content is kept low, usually below 1-2% w/w. 

Additional information about the reactivity was obtained by determining the kinetic parameters during methanol oxidation for vanadia, molybdena, rhenia, and chromia on different oxide supports. For all these systems the activation energy is approximately the same, 18-22 kcal/mol. The activation energy corresponds to that expected for the breaking of the C-H bond of a surface methoxide intermediate, and should be... 

Infrared spectra of chemisorbed NO. Chemisorption of NO on partially reduced molybdena based catalysts has been shown to be a usefull technique in evaluating M0O3 exposures on such a catalyst surface (see, e. g. 16)). However, the application of this technique to a series of catalysts which differ in M0O3 loading, or even in preparation method, requires a carefull control of the degree of reduction. As NO appears to be chemisorbed mainly at Mo sites, the catalysts must be quantitatively reduced according to the reaction ... 

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