Platinum-Rhenium on Alumina

It was suggestedthat hydrogen consumption was consistent with reduction only to Re , confirmed by e.s.r. for one catalyst where the strong 

Crystallite (bulk) homogeneity unalloyed metal/phases present 

Various research groups have used the hydrogen-oxygen titration method to determine dispersion (and possibly surface composition) although oxygen and chemisorption have also been used. Menon and 

Bolivar, H. Charcosset, R. Frety, M. Primet, L. Tournayan, C. Betizeau, 

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Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

Results of some extended naphtha reforming runs illustrate further the differences between platinum-iridium on alumina and platinum-rhenium on alumina catalysts observed in the reforming of pure hydrocarbons (33). The results considered here should be regarded as illustrative only, since they are limited to the reforming of a series of Persian Gulf naphtha fractions in a particular range of operating conditions of commercial interest. The catalysts used in these runs contained 0.3 wt% platinum and 0.3 wt% of either iridium or rhenium. 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally platinum or platinum—rhenium on an alumina base. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions. Nonplatinum catalysts are used in regenerative processes for feedstocks containing sulfur, although pretreatment (hydrodesulfurization) may permit platinum catalysts to be employed. 

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. 

The selectivity to benzene is much lower for the iridium catalyst than for any of the other catalysts except rhenium on alumina. The platinum-iridium catalyst is clearly more selective than the iridium catalyst with respect to benzene formation. However, it is less selective for benzene formation than the catalyst containing platinum alone, although it is possible that this debit may disappear as the catalyst ages during a run. Also, except for the initial reaction period, the platinum-iridium catalyst is less selective for benzene formation than the platinum-rhenium catalyst. 

Catalyst contains 0.3 wt% each of platinum and rhenium on alumina. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

Bimetallic platinum-rhenium catalysts can be prepared in aqueous acid medium, under hydrogen flow, by a redox reaction between hydrogen activated on a parent platinum-alumina catalyst and the perrhenate ion ReO4. ... 

In considering the nature of platinum-rhenium catalysts, we begin with a comparison of the chemisorption properties of alumina-supported rhenium, platinum, and platinum-rhenium catalysts (40). Data on the chemisorption of carbon monoxide and hydrogen at room temperature are given in Table 4.4 for catalysts with platinum and/or rhenium contents in the range of interest for reforming applications. 

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. 

The catalysts were prepared by contacting alumina with aqueous solutions of chloroplatinic acid and ammonium perrhenate. The consumption of hydrogen during reduction corresponded to complete reduction of platinum from the +4 oxidation state to the metal and of rhenium from the +7 to the +4 state. The X-ray diffraction data on the metal residue from the leached catalysts showed no evidence for the presence of rhenium metal or a platinum-rhenium alloy. Most of the rhenium was found in the leaching solution. Finally, the authors stated that data from an electron spin resonance experiment on one of the reduced platinum-rhenium catalysts were consistent with their conclusion that the rhenium was present in the +4 state. 

Later experiments by McNicol (66) and by a group of French workers (67-72), in which the water formed during reduction was removed by a trap at 78°K, were consistent with the results of Webb in showing a change in oxidation state of rhenium from +7 to 0. These workers indicated also that the properties of alumina-supported platinum-rhenium catalysts depend on the method of preparation. 

For catalysts that were simply dried in air at 110°C after impregnation of the alumina with H2PtClfe and Re207, it was concluded that a platinum-rhenium alloy formed on reduction. This conclusion was based on the observation that the presence of platinum accelerated the reduction of oxygen chemisorbed on the rhenium and on results showing that the frequencies of the infrared absorption bands of carbon monoxide adsorbed on platinum and rhenium sites in platinum-rhenium catalysts were different from those found with catalysts containing only platinum or rhenium. However, for catalysts calcined in air at 500°C prior to reduction in hydrogen, it was concluded that the platinum exhibited much less interaction with the rhenium (66,71). 

Figure 5.4 Data on the reforming of a 99-17fC boiling range naphtha showing the temperature required to produce 100 octane number product as a function of time on stream for alumina-supported platinum, platinum-rhenium, and platinum-iridium catalysts at 14.6 atm pressure (2,7).

The studies of n-heptane and methylcyclopentane conversion provide insight into the advantages of platinum-iridium and platinum-rhenium catalysts over catalysts containing only one of the transition metal components, that is, platinum, iridium, or rhenium. If, for example, we consider an iridium-alumina catalyst for the reforming of a petroleum naphtha fraction, we find that it produces a substantially higher octane number reformate than a platinum on alumina catalyst under normal reforming conditions. The iridium-alumina catalyst will also exhibit a lower rate of formation of carbonaceous residues on the surface, with the result that the maintenance of activity with time will be much superior to that of a platinum-alumina catalyst. 

Reforming Catalytic reforming is the process of increasing the number of double bonds on a petroleum product but maintaining the same number of carbon atoms. This process is done at high temperatures in the presence of a platinum or rhenium catalyst on alumina [1],... 

D Duprez, M Hadj-Aissa, J Barbier. Effect of steam on the coking and on the regeneration of metal catalysts A comparative study of alumina-supported platinum, rhenium, iridium and rhodium catalysts. Stud. Surf. Sci. Catal., 68 (Catal. Deact. 1991), 111-118, 1991... 

EFFECT OF STEAM ON THE COKING AND ON THE REGENERATION OF METAL CATALYSTS A COMPARATIVE STUDY OF ALUMINA-SUPPORTED PLATINUM, RHENIUM, IRIDIUM AND RHODIUM CATALYSTS,... 

Kariya et al. performed dehydrogenation of methylcyclohexane and other cycloalkanes over platinum, palladium and rhodium monometallic and platinum/palladium, platinum/rhodium, platinum/molybdenum, platinum/tungsten, platinum/rhenium platinum/osmium and platinum/iridium catalysts supported on both petroleum coke active carbon and on alumina between 375 and 400 °C [279]. The platinum catalyst supported by petroleum active carbon showed the highest activity. While platinum was the most active monometallic catalyst, its activity could be increased by addition of molybdenum, tungsten and rhenium. 

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