Platinum rhenium and

Perrhenate and related building blocks are constituents of several cluster compounds where they act as terminal groups in organometallic rhenium oxides such as in [(cp Re)3(//2-0)3(/U3-0)3Re03]+ (49)21 Qj. jjj heterometallic clusters such as the structurally related [(Re)3(//f dppm)3(/u -0)3Re03]+ (dppm = bis(diphenylphosphino)methane) and Pt4 P(C6H 11)3)4 (//-C0)2(Re04)2]. A series of platinum-rhenium and platinum-rhenium-mercury clusters with Pt-Re multiple bonds has been isolated from reactions of Pt3 precursors with Rc207 or perrhenate. " ... 

Table 4.4 Chemisorption Data on Platinum, Platinum-Rhenium, and Rhenium Catalysts (40)...

Tournayan, L. Charcosset, H. Frety, R. Leclercq, C. Turlier, P. Barbier, J. and Leclercq, G. "Hydrogen-oxygen titrations over platinum-rhenium and plati-... 

In commercial practice a reformer is operated to produce a constant octane number product. As the catalyst deactivates, the temperature of the system may be increased to compensate for the lower activity. In this way the octane number of the product can be maintained at the desired level. Illustrative data for alumina-supported platinum, platinum-rhenium, and platinum-iridium catalysts in this type of operation are given in Figure 5.4 (7). 

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).

Consequently, it is of interest to consider the reforming properties of platinum-rhenium and platinum-iridium catalysts in more detail. First, we consider some information obtained from studies on the reforming of pure hydrocarbons over these catalysts. This information provides us with a better understanding of the way these catalysts function in reforming. After the discussion of the studies on reforming of pure hydrocarbons, the results of some extended naphtha reforming runs on these catalysts are considered in detail. 

The reaction rates were determined at low conversion levels (7-12%) in an attempt to minimize the effects of secondary reactions. The inlet stream to the reactor contained five moles of hydrogen per mole of n-heptane. The n-heptane contained 0.5 ppm sulfur, and the reaction rates were determined after 40 hours on stream. The catalysts contained 0.9 wt% chlorine as charged. Prior to the runs the catalysts were contacted with an H2S-containing gas until H2S was detected at the reactor outlet (34). This step is routinely employed with platinum-rhenium and platinum-iridium catalysts to suppress hydrogenolysis activity (33). 

The attractive features of platinum-rhenium and platinum-iridium catalysts can be combined in a reforming operation. The data for the reactions of selected hydrocarbons considered earlier for platinum-rhenium and platinum-iridium catalysts indicate that the former catalyst is more selective for the conversion of cycloalkanes to aromatics, while the latter is more selective for the dehydrocyclization of alkanes. Since cycloalkane conversion occurs primarily in the initial part of a reforming system while dehydrocyclization is the predominant reaction after the cycloalkanes have reacted, it is reasonable to use a platinum-rhenium catalyst in the front of the system and to follow it with a platinum-iridium catalyst (32). 

The lower rate of deactivation of platinum-rhenium catalysts relative to platinum catalysts cannot be attributed to a lower rate of accumulation of carbonaceous residues on the surface. For a given time on stream, the amount of such residues on the surface is not decreased by the presence of the rhenium. This point is interesting because metallic rhenium, like metallic iridium, has much higher hydrogenolysis activity than platinum (26). It is possible that the difference between platinum-rhenium and platinum-iridium catalysts is due to the strong retention of sulfur by the former. The inhibiting effect of sulfur on hydrogenolysis activity is well known. The improved activity maintenance of a platinum-rhenium catalyst relative to a platinum catalyst is due to better tolerance of the carbonaceous residues. 

The conversion of levuUnic acid to GVL using various catalysts supported on carbon (iridium, rhodium, palladium, ruthenium, platinum, rhenium, and nickel at 5 wt % on carbon) has been reported [48]. Although high conversions of levulinic acid are seen with these catalysts, the only catalyst with a high selectivity to GVL is Ru/C. The selectivities decrease in the following order at 150°C and 800 psig for 2h ... 

L. Rhenus, Rhine) Discovery of rhenium is generally attributed to Noddack, Tacke, and Berg, who announced in 1925 they had detected the element in platinum ore and columbite. They also found the element in gadolinite and molybdenite. By working up 660 kg of molybdenite in 1928 they were able to extract 1 g of rhenium. 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

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. 

Another synthesis of pyrogaHol is hydrolysis of cyclohexane-l,2,3-trione-l,3-dioxime derived from cyclohexanone and sodium nitrite (16). The dehydrogenation of cyclohexane-1,2,3-triol over platinum-group metal catalysts has been reported (17) (see Platinum-GROUP metals). Other catalysts, such as nickel, rhenium, and silver, have also been claimed for this reaction (18). 

Selected physical properties of rhenium are summarized ia Table 1. The metal is silvery-white and has a metallic luster. It has a high density (21.02 g/cm ). Only platinum, iridium, and osmium have higher densities. The melting poiat of rhenium is higher than that of all other elements except tungsten (mp 3410°C) and carbon (mp 3550°C). 

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. 

Promoters, usually present in smaU amount, which enhance activity or retard degradation for instance, rhenium slows coking of platinum reforming, and KCl retards vaporization of CuCU in oxy-chlorination for vinyl chloride. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Reforming is the conversion primarily of naphthenes and alkanes to aromatics, but other reactions also occur under commercial conditions. Platinum or platinum/rhenium are the hydrogenation/ dehydrogenation component of the catalyst and alumina is the acid component responsible for skeletal rearrangements. 

The catalytic system used in the Pacol process is either platinum or platinum/ rhenium-doped aluminum oxide which is partially poisoned with tin or sulfur and alkalinized with an alkali base. The latter modification of the catalyst system hinders the formation of large quantities of diolefins and aromatics. The activities of the UOP in the area of catalyst development led to the documentation of 29 patents between 1970 and 1987 (Table 6). Contact DeH-5, used between 1970 and 1982, already produced good results. The reaction product consisted of about 90% /z-monoolefins. On account of the not inconsiderable content of byproducts (4% diolefins and 3% aromatics) and the relatively short lifetime, the economics of the contact had to be improved. Each diolefin molecule binds in the alkylation two benzene molecules to form di-phenylalkanes or rearranges with the benzene to indane and tetralin derivatives the aromatics, formed during the dehydrogenation, also rearrange to form undesirable byproducts. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Platinum-rhenium catalysts have been reduced in one atmosphere of flowing hydrogen and then examined, without exposure to the atmosphere, by ESCA. The spectra indicate that the Group VIII metal is present in a metallic state in the reduced catalyst and that the majority of the rhenium is present in a valence state higher than Re(0). 

LEED patterns at 0 = 1 /4, but was identified at lower coverages in islands surrounded by mobile sulfur atoms at platinum, rhodium and rhenium surfaces. Sautet and co-workers42 have analysed the statistical correlations between the intensities of sulfur features in p(2 x 2) islands on rhenium surfaces and also of streaks in areas between islands, which they attribute to sulfur atoms diffusing under the tip (Figure 10.12). 

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. 

Platinum loadings, reducing, 19 628 Platinum metals plating, 9 822-823 Platinum oxides, volatilized, 17.T80 Platinum-palladium thermocouple, 24 461 Platinum reforming catalysts, rhenium and, 21 695-696... 

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