Molybdena preparation

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 ... 

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. 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

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. 

A differential flow microreactor was used for the preparation of the nitrided catalyst and the TPD, TPR, and NH3-TPD measurements. Nitriding of the molybdena-alumina and alumina was carried out by temperature-programed reaction with NH3 (NH3-TPR).1719 The MoCV A1203 precursor was oxidized at 723 K for 24 h, cooled to 573 K, reacted with NH3 at 49.6 (xmols-1 from 573 to 773, 973 or 1173 K at a rate of 0.0167 Ks-1, held at the nitriding temperature for 3 h, and then cooled to room temperature (RT) in flowing NH3. The catalysts were characterized by TPD, TPR, and NH3-TPD under in situ conditions, while BET and diffuse reflectance FTIR measurements were carried out after passivation. For the diffuse reflectance FTIR study, the catalysts after NH3 treatment... 

The situation with respect to reduction of the CoMo/Al catalyst is more confusing. Various authors claim that the presence of cobalt at a low level accelerates (16), retards (27), or has no effect on (31) the reduction of the molybdena. Of course, at high Co loadings, more reduction is obtained than for the Mo/Al alone, due to reduction of the Co304 phase present, but it is difficult to assess whether the molybdena is itself affected by the cobalt reduction. It is well known that transition metals can catalyze reduction of oxides (32). It is probable that the different results obtained could be due in large part to differences in preparation or calcination temperature as pointed out earlier. 

Addition of Mo to A1203 has been reported to lower and shift the frequency of the Al-O-H band (57, 75). This has been interpreted as due to covering of the A1203 surface by a molybdena layer, or actual loss of OH by interaction during preparation or calcination of the catalyst. The latter view agrees with a suggested reaction put forth by Dufaux et al. (36),... 

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). 

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. 

Supported molybdena catalysts are prepared by impregnating alumina with ammonium molybdate, calcining in air at 500-600°C to form the oxide. 

In the absence of molybdenum, the blank dehydrated zeolites showed no CO hydrogenation activity even up to 400°C. In contrast, measurable quantities of aliphatic hydrocarbons were detected over the molybdenum-zeolite catalysts at 300°C and above. Figs. 1-2 show the time dependence of CO conversion over MOii g HY and Mo g CsY at 300°C. The conversion and product distribution were dependent on the reaction conditions, a typical set of results is illustrated in Table 1. The molybdenum-zeolites prepared by adsorption and decomposition of Mo(C0)g resembled closely the alumina-supported molybdenum catalysts prepared by decomposing Mo(C0)g on alumina (ref. 13). The results obtained presently could not match the figures reported by Brenner et aK (ref. 8), but this could be due to the significant differences in the reaction conditions used by the above authors. However, a comparison with the silica-molybdena catalyst (prepared by impregnation of ammonium molybdate) clearly indicates that the molybdenum-zeolites were more active on per molybdenum basis. The improved activity is due to the presence of zerovalent molybdenum (for LaY and HY, residual zerovalent molybdenum were responsible for the activity). 

Dehydrogenation over Molybdena—Alumina. Detailed results have been reported previously (12) for molybdena-alumina catalysts. It appears that freshly prepared molybdena-alumina catalysts are strongly acidic and exhibit very high activity but poor selectivity towards monoolefin formation initially. After about 24 hours, these catalysts become... 

The method outlined applies equally well to supported oxides of transition metals. The familiar chromia-alumina catalyst is a good example. In such cases, the degree of attenuation of the supported oxide may be much greater than in the gel oxides, which may be considered to be self-supported. All the common paramagnetic oxides have been studied in this way, on a variety of supports, and as prepared by a variety of methods. A few oxides, such as molybdena, for one reason or another do not lend themselves to this method. But for most common catalyst components, the method has proved itself to be a useful supplement to x-ray diffraction. 

The cogelled catalysts contained 0-33 % molybdena by weight and were prepared by mixing solutions of ammonium molybdate with acetic acid sols, drying, and calcining (I). For comparison catalysts containing 0-15% molybdena were prepared by impregnating dried and calcined sol with ammonium molybdate solutions. 

The concepts we have discussed show that in the preparation and use of these re-forming catalysts attention should be centered upon the conditions which produce and stabilize the active intermediate oxide. Best performance can be attained with that catalyst which combines a high surface area with a high degree of dispersion and availability of the molybdena. These factors must be balanced against operational conditions which influence the degree of reduction of the molybdena and the accumulation of sulfur. 

Butadiene can be polymerized with chromium oxide catalyst on support to form solid homopolymers. The products, however, tend to coat the catalyst within a few hours after the start of the reaction and interfere with further polymerization. Polybutadiene can also be prepared in the presence of molybdena catalyst promoted by calcium hydride. The product contains 80% of 1,4 units and 20% of 1,2 units. Of the 1,4 units, 62.5% are cis and 37.5% are transP... 

L. Wang and W.K. Hall, The preparation and genesis of molybdena-alumina and related catalyst systems, J. Catal, 77(1), 232-241, 1982. 

K. Segawa, N. Koizumi, M. Yamada, A. Nishijima, T. Kabe, A. Ishihara, T. Isoda, I. Moshida, H. Matsumoto, M. Niwa, T. Uchijima, A study on the preparation of supported metal oxide catalysts using JRC-reference catalysts. I. Preparation of a molybdena-alumina catalyst. Part 3. Drying process. Applied Catalysis A General, 170 pp. 343-357, 1998. 

Goula, M.A., et ah. Development of molybdena catalysts supported on g-alumina extrudates with four different Mo profiles Preparation, characterisation and catalytic properties. J. Catal, 137 pp. 285-305, 1992. 

Note Early naphthalene and benzene oxidation catalysts often contained alkali metal promoters. Commercial benzene oxidation catalysts were shown to contain a p-bronze phase (NaiO-V2O4 5V2O5 or NajO MoOj-SViOs) with other mixed oxide compounds such as V9Mo504o.The p-bronze could possibly stabilize the other active compounds and limit loss of molybdena during operation. M. Najbar, Preparation of Catalysts IV,, Elsevier, Amsterdam, 1987, p. 217. 

The benefical effects of phosphorus has stimulated research on its influence on molybdena based catalysts. However, most of the above mentioned literature essentially focused on the influence of phosphorus on the catalytic properties of the modified system and its effect on the dispersion on the active phase deposited on the alumina surface has not yet been investigated. Also, very few works deal with the influence of the sequence of phosphorus incorporation during the preparation step of the MoP/Al,0j catalysts on the surface acidity, dispersion and distribution of the supported phases. As the phosphate ions strongly interact with alumina, competing with molybdate ions, a factor of possible importance is the preparation procedure. 

The authors are collaborating in an extensive joint study in which the preparation, characterisation and function of a range of supported platinum catalysts is being evaluated. In part, this study will compare platinum catalysts prepared using conventional supports (e.g. silica, alumina) with those using less conventional supports (e.g. molybdena) or those prepared by less conventional methods (e.g. metal vapour deposition). The restricted object of the present paper is to compare the conventional Pt/silica prepared within this programme with the standard reference silica-supported Pt codenamed EUROPT-1 for which full preparation and characterisation details have been published (refs. 1-5). 

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