Molybdena supports

Zald Ml, VieUiaber B, Knozinger H (1986) Low-temperature carbon monoxide adsorption and state of molybdena supported on alumina, titania, ceria, and zirconia. An infrared spectroscopic investigatiotL J Phys Chem 90 (14) 3176-3183... 

Heracleous, E., Machli, M., Lemonidou, A. A., and Vasalos, I. A. Oxidative dehydrogenation of ethane and propane over vanadia and molybdena supported catalysts. J. Mol. Catal. A Chem. 232,29-39 (2005). 

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).

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... 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

Laboratory investigation revealed that sodium, which was present in the support to the extent of several tenths of 1%, had a profound effect on stability and activity of the moiybdena-alumina catalyst. Over a period of time it was possible to alter the procedure for preparing the support on successive occasions until the catalyst contained much less than 0.1% sodium oxide. The reduction in sodium content of the support was immediately reflected in improved catalyst life. Ultimately the life was extended to 9 to 12 months before replacement. Various forms of alumina have been used as a support, including alumina gel and a stabilized alumina gel. Moiybdena-alumina catalyst has been employed exclusively in the eight commercial plants previously referred to. Today the majority of refiners who operate hydroformers are using molybdena on alumina gel as a catalyst. The molybdic oxide content of the catalyst is somewhat below 10%. Although similar to the original catalyst as far as chemical composition is concerned, it possesses superior activity and life. 

Some catalysts, most notably W(CO)6 in a CC14 solution44,45 and systems based on molybdena 46 when irradiated, are effective in photoinduced metathesis. In fact, a photoreduced silica-supported molybdena proved to be the most active olefin metathesis catalyst.47... 

Hydrogen (Takachiho Co., 99.999%) and He (Takachiho Co., 99.999%) were dried by passing them through a Deoxo unit (SUPELCO Co. Oxysorb) and a Linde 13X molecular sieve trap prior to use. NH3 (Takachiho Co., 99.999%) was used without further purification. The alumina-supported molybdena was prepared using a mixture of hexa-ammonium molybdate and y-alumina (Nikki Chemicals Co.) and calcined in air at 823 K for 3 h. 

Molybdena catalysts have been with us for quite a long time. The term molybdena is used here to denote a composite catalyst consisting of molybdenum oxide supported on an activated support, commonly alumina. Early it was found that certain transition metals, notably cobalt and nickel, promote the molybdena catalyst for hydrodesulfurization (HDS) reactions. 

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

If the molybdenum is not observed by XRD, in what state is it The first hint of an interaction between molybdena and the alumina support came... 

Guo CS, Hermann K, Havecker M, Thielemann JP, Kube P, Gregoriades LJ, Trunschke A, Sauer J, Schlogl R. Structural analysis of silica-supported molybdena based on x-ray spectroscopy Quantum theory and experiment. The Journal of Physical Chemistry C. 2011 115(31) 15449—15458. 

Payen et al. (1986) investigated the reduction of alumina-supported molybdena and ascribed a Raman band at 760 cm-1 to reduced supported molybdenum oxide. The transformations could be reversed by reoxidization (Payen et al., 1986). Mestl and Srinivasan (1998) described some reduced phases formed from bulk molybdena, whereas reduced dispersed vanadia and chromia catalysts do not show Raman bands (Airaksinen et al., 2005 Banares et al., 2000a Gasior et al., 1988 Weckhuysen and Wachs, 1996). 

Supported dispersed metal oxides can also be reduced by alcohols. Hu and Wachs (1995) reported Raman bands of surface-reduced molybdena that was generated through contact with methanol. Zhao and Wachs (2006) recently investigated V205/Nb205 catalysts during propene oxidation to acrolein and detected a previously unknown Raman band at 978 cm-1, which was tentatively assigned to a surface V4+ species. 

Thus, heteropoly acids may not be stable under reaction conditions (Mestl et al., 2001). This statement is in line with the results of Raman investigations of supported /J-silicomolybdic acid that unambiguously demonstrated its decomposition to surface molybdena during methane oxidation (Banares et al., 1995). It was also shown that silica-supported silicomolybdic acid and silica-supported molybdenum surface oxide species with the same molybdenum loadings performed identically (Banares et al., 1995). Thus, the presence of any heteropoly acid structure during high-temperature oxidation can be ruled out. 

FIGURE 19 Changes during alcohol oxidation on supported molybdena catalysts (A) methanol oxidation (Reprinted from Journal of Catalysis 150, 407 (1994), M.A. Banares, H. Hu, I.E. Wachs, Molybdena on Silica Catalysts - Role of Preparation Methods on the Structure Selectivity Properties for the Oxidation of Methanol, copyright (1994) with permission from Elsevier). (B) ethanol oxidation (Reprinted with permission from Journal of Physical Chemistry, 99,14468 (1995) by W. Zhang, A. Desikan, S.T. Oyama, Effect of Support in Ethanol Oxidation on Molybdenum Oxide, copyright 1995, American Chemical Society). 

For most of the oxide-supported monolayer oxides (e.g. vanadia, molybdena and tungsta supported on alumina), titania, zirconia and silica surface species are... 

The properties of silica- and/or alumina-supported molybdena catalysts for propene metathesis were studied by Handzlik and coworkers. These materials could be prepared by thermal spreading of Mo02(acac)2 with well-dispersed molybdenum in a wide range of its loading. The selective metathesis activity depends on the substrate and on the surface molybdenum concentration. For example, a higher activity is found for the molybdena-alumina system at high Mo loadings. 

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