Alumina supported molybdenum oxides

A. Christodoulakis, E. Heracleous, A.A. Lemonidou and S. Boghosian, An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane, J. 

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

The surface structure and acid sites of alumina-supported molybdenum nitride catalysts have been studied using temperature-programed desorption (TPD), and reduction (TPR), diffuse reflectance infrared spectroscopy, and X-ray diffraction (XRD) analysis. The nitride catalysts were prepared by the temperature-programmed reaction of alumina-supported molybdenum oxide (12.5% and 97.1%) with NH3 at temperatures of 773, 973, and 1173 K. TPR and XRD analyses showed that y-Mo2N was already formed at 973 K. On the basis of NH3-TPD measurements and IR spectroscopy, it was found that Lewis acid sites were predominant over Bronsted acid sites on the surface of Mo2N/A1203. 

One of the first cells that allowed exposure of the catalyst sample to reaction conditions was developed by Schrader and coworkers (Cheng et al., 1980) key feature of the design is the placement of the catalyst, which is pressed into a wafer, on a rotor. The authors investigated the structural evolution of alumina-supported molybdenum oxide catalysts during calcination. Chan and Bell (1984) reported on the formation of... 

Alumina supported molybdenum oxide catalysts are extensively used in hydrotreating reactors. Under reaction conditions the oxides are not stable because they are transformed into molybdenum sulphides. In practice this step is carried out in a so-called sulphiding step. In this step the catalyst is pretreated with a mixture containing a sulphur compound. The sulphiding step was investigated by carrying out a TPS study. TPS is strictly analogous to TPR, except for the gas mixture which is here a H2/H2S mixture. 

Chiistodoulakis, A., Heracleous, E., Lemonidou, A., etal (2006). An Operando Raman Study of Structure and Reactivity of Alumina-supported Molybdenum Oxide Catalysts for the Oxidative Dehydrogenation of Ethane, J. Catal., 242, pp. 16-25. 

Styrene, the best known representatives of the aromatic olefin class, is readily obtained by cometathesis reaction of stilbene with ethene in the presence of alumina-supported molybdenum oxide catalysts. In a two-step process [27] stilbene is first produced from toluene by catalytic dehydrogenative coupling over Pb0/Al203, followed by codisproportionation to styrene, with ethene, over M0O3/AI2O3 ... 

Catalysts used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are largely based on alumina-supported molybdenum or tungsten to which cobalt or nickel is added as a promoter [11]. As the catalysts are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2 (or by exposing the catalyst to the sulfur-containing feed). The function of hydrogen is to prevent the decomposition of the relatively unstable H2S to elemental sulfur, which would otherwise accumulate on the surface of the... 

A criterion for the suitability of a spectroscopy cell for investigations of working catalysts can be formulated as follows the activity or selectivity data and activation energy values have to be in agreement with the catalytic performance data measured with a conventional fixed-bed reactor. Table 1 is a comparison of the conversion and selectivity values characterizing an alumina-supported molybdenum-vanadium oxide catalyst during propane ODH obtained with a conventional fixed-bed reactor and with a spectroscopic cell that fulfills this requirement (Banares and Khatib, 2004). Similar considerations have also been reported earlier for other methods, such as X-ray diffraction (Clausen et al., 1991). 

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). 

The uncertainties discussed in the preceding section prompted us to look for another "chemical method" to estimate the surface of supported ceria. As for alumina-supported oxides [12-16], the adsorption of CO2 has been considered, IR spectroscopy being prefered, with the aim of finding vibrations specific of CO2 interacting either with ceria or with alumina. This method has been applied although very recent results, mainly with alumina supported molybdenum questionned the validity of CO2 adsorption to measure the coverage of alumina... 

Thomas, R. and Moulijn, J. A. A comparative study of y-alumina supported molybdenum and tungsten oxide relation between metathesis activity and reducibUity. J. Mol. Catal. 1982,15, 157-172. 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

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 catalytic toluene hydrodealkylation, toluene is mixed with a hydrogen stream and passed through a vessel packed with a catalyst, usually supported chromium or molybdenum oxides, platinum or platinum oxides, on siHca or alumina (50). The operating temperatures range from 500—595°C... 

The high-density polyethylene is linear and can be manufactured by (i) coordination polymerisation of monomer by triethyl aluminium and tritanium chloride, (ii) polymerisation with supported Metal Oxide Catalysts. Such as chromium or molybdenum oxides supported over alumina-silica bases. 

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

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... 

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