Metal supported rhenium catalysts from

In the case of alumina supported rhenium (12), the nature of the supported metal also seems highly dependent on pretreatment of the catalyst. Increasing the precalcination temperature from 500 to 700 C, evidently increased the amount of exposed, fully reduced Re shown subsequently (after reduction in hydrogen at 500 C) by adsorption of CO. Additional types of reduced Re adsorption sites were also apparently present on the sample precalcined at 700 C. 

Kirlin PS, van Zon FBM, Koningsberger DC, Gates BC (1990) Surface catalytic sites prepared from [HRe(CO)j] and [H3Re3(CO)jJ Mononuclear, trinuclear, and metallic rhenium catalysts supported on magnesia. J Phys Chem 94 8439... 

In contrast to the X-ray diffraction pattern of alumina-supported rhenium oxide, the pattern for the silica-supported samples gives diffraction lines characteristic of metallic rhenium. The metal particle size is about 75 A. Initial kinetic studies with propylene indicated that the silica-supported samples were inactive for the disproportionation reactions up to 180°C. X.p.s. studies of rhenium-supported catalysts show that the state of the initial and reduced rhenium on silica surface is quite different from that on 7-alumina and is dependent on the rhenium compound used to prepare the catalysts. Because of a stronger interaction of Re with the alumina surface, the reducibility of rhenium on alumina is much less than on silica. 

The physical and chemical nature of the rhenium in platinum-rhenium catalysts has been considered by a number of investigators. Johnson and Leroy (63) concluded that the rhenium is present as a highly dispersed oxide at typical reforming conditions. They studied a series of alumina-supported platinum-rhenium catalysts with platinum contents ranging from 0.31 to 0.66 wt% and rhenium contents ranging from 0.20 to 1.18 wt%. Their conclusions were based on measurements of hydrogen consumption during reduction of the catalysts at 482°C and on X-ray diffraction studies of the metal component of the catalyst after the alumina had been leached from the catalyst by treatment with a solution of fluoboric acid. 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

PNNL has a long history studying hydrogenolysis as a means to form value-added products from sugar alcohols including glycerol. In this paper we will report on a subset of this work, focused on rhenium-based multi-metallic catalysts supported on carbon. 

If benzene is the main product desired, a narrow light naphtha fraction boiling over the range 70 to 104°C is fed to the reformer, which contains a noble metal catalyst consisting of, for example, platinum-rhenium on a high-surface-area alumina support. The reformer operating conditions and type of feedstock determine the amount of benzene that can be produced. The benzene product is most often recovered from the reformate by solvent extraction techniques. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

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