Metal supported platinum-rhenium catalysts

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. 

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. 

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

Another catalyst system that could be considered in the bimetallic cluster category is supported platinum-rhenium (5), which represents still another type of system in the sense that a Group VIIA metallic element (rhenium) is incorporated with the Group VIII metal component. Platinum and rhenium have different crystal structures (fee vs. hep) (8) and do not exhibit complete miscibility in the bulk (Ref. 45, p. 820). However, these factors may have limited... 

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... 

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. 

A platinum-rhenium composite catalyst supported on the granular activated carbon (Pt-Re/C, 5 wt-Pt%, mixed molar ratio of Pt/Re = 2) [10] was prepared by a "dry-migration method" [33,34] as follows (1) The Pt/C catalyst prepared earlier (5 wt-metal%) was evacuated at 180°C for 1 h (2) The mixture (molar ratio of Pt/Re = 2) of the Pt/C catalyst and a cyclopentadienylrhenium tricarbonyl complex (Re(Cp)(CO)3) were stirred under nitrogen atmosphere at room temperature for 1 h and then heated at 100° for 1 h, with the temperature kept at a constant (3) This mixture was further stirred under hydrogen atmosphere at 240°C for 3 h and finally (4) the Pt-Re/C composite catalyst was evacuated at 180°C for 1 h. A platinum-tungsten composite catalyst supported on the granular activated carbon (Pt-W/C, 5 wt-Pt%, mixed molar ratio of Pt/W = 1) [5,6] was also prepared similarly by the dry-migration method. All the catalysts were evacuated inside the reactor at 150°C for 1 h before use. 

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. 

Based on TPR results it can be concluded that intimate contact between rhenium and platinum is provided in bimetallic alloy particles on the surface of alumina supported catalysts. Therefore, one can expect that the oxidation of the catalyst followed by reduction at moderate temperature result in the formation of platinum and/or Re-Pt metallic nanoclusters and rhenium ions in atomic closeness. 

Determination of Coke Location. The TPO technique allows the determination of the coke location on supported metal catalysts, such as naphtha reforming. Since the metal, typically platinum promoted with rhenium, iridium, tin, or germanium, has a catalytic effect for coke burning, the TPO profile displays two main peaks. The low temperature peak is due to the oxidation of the coke directly deposited on the metal particle, or in its vicinity . In this way, it is possible to study the effect of catalyst formulation and operational conditions on the formation of coke on the metal and on the support. 

Platinum—Rhenium. The Pt-Re/Al203-Cl catalyst is perhaps the most widely used in naphtha reforming. It was first patented by Chevron in 1969 (3,60). The most beneficial effect of rhenium was a substantial increase in catalyst stability. The rhenium introduced a higher resistance to deactivation by coke and stabilizes the metallic phase on the support. The disadvantage of the... 

An important commercial example of an endothermic reaction that is carried out adiabatically is the catalytic reforming of petroleum naphtha to produce high-octane gasoline. In this process, the naphtha is mixed with hydrogen and passed over a heterogeneous catalyst that contains platinum, and perhaps other metals such as rhenium or tin, on a ceramic support such as alumina. The temperature is in the region of 800-900 °F. 

With the addition of Sn as a second metal the composition was closer to that predicted for pure Cg cyclization. Rhenium had the opposite effect (107). Suppression of the acidity of the alumina support (of platinum or palladium) by incorporation of sodium nitrate (126) decreases first the C5 - Cg ring enlargement activity of the catalyst (124, 127). Potassium ions (94-94b) or iec-butylamine (70) have a similar effect. 

These heterogeneous catalysts consist of muitimetallic clusters, containing metals, such as platinum, iridium, or rhenium, supported on porous acidic oxide supports, such as alumina. The catalysts are said to be bifunctional because both the metal and the oxide play a part in the reactions. The metal is believed to carry out reversible dehydrogenation of paraffins to olefins, while the oxide is believed to carry out isomerization. 

For monometallic Pt catalysts, sulfur coverage increases with particle size while the sulfur adsorption capacity is higher for the monometallic Re catalyst. This last result is in accordance with previous work, the adsorption of an electron acceptor like sulfur being enhanced on metals of low electronic affinities [20]. Indeed, electronic affinities are 2.12 eV and 0.16 eV for platinum and rhenium, respectively. With regard to the effect of platinum particle size, the increasing 0s results fi-om the electron deficient character of small metal crystallites deposited on an acidic support [20, 21]. 

Chlorinated alumina-supported metal catalysts are the typical catalysts used today for catalytic naphtha reforming, which is performed at temperatures of480-550 °C (410). Modem versions of this type of catalyst are mul-timetaUic the catalytic properties of platinum are improved by the addition of another metal, often rhenium. Further elements that may be added are tin, silicon, germanium, lead, gallium, indium, iridium, thorium, lanthanum, cerium, cobalt, and nickel. AH these components are supported on chlorinated y-alumina (with a surface area of 150—300 m g ), which provides the acid function (411). 

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