Uses of Rhenium

Rhenium, alloyed with platinum, was used in petroleum-reforming catalysts in the production of high-octane hydrocarbons, used for lead-free gasoline. This special use is expected to decline as other less-expensive catalysts become available. However, other catalytic functions of rhenium in organic chemistry and medical techniques are still important. 

Rhenium is an ideal metal for use at very high temperatures, which makes it suitable for rocket motors. Complicated geometries can be fabricated using chemical vapor deposition CVD (see section 18.6.7). Rhenium is deposited on the exterior of a mandrel, which is afterwards removed chemically. 

The compatibility of rhenium with platinum metals makes interesting combinations possible. Ultramet in the USA [29.5] developed a CVD method of depositing thick iridium films on rhenium. The combined iridium/rhenium structure permits liquid rocket operation at temperatures exceeding 2200 C. 

The thrust chamber is the heart of all liquid propellant rocket engines. It receives the propellant from the injector, burns it in the combustion chamber, accelerates the gaseous combustion products and ejects them from the chamber to provide thrust The material used in rocket thrust chambers, up to a thrust of445 N, is iridium/rhenium. The chambers are fabricated by forming an inner wall of iridium for oxidation resistance and a structural outer shell of rhenium. As rhenium has a density of 21 g/cm its volume has to be restricted. The same performance as solid iridium/rhenium can be achieved with an advanced composite. The composite consists of an inner wall of iridium and a structural outer shell of carbon/carbon. An intermediate thin layer of rhenium, 0.25-0.5 mm, binds the Ir and C/C layers together. 

Other applications are rhenium-tungsten alloys in X-ray tubes and rotating X-ray anodes. Rhenium-molybdenum alloys are superconductors at a temperature of 10 K. 

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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. 

The use of rhenium-based systems for the epoxidation of olefins has increased considerably during the last ten years [87]. In 1989, Jgrgensen stated, the catalytic... 

In the chemical process industries, nickel, cobalt, platinum, palladium, and mixtures containing potassium, chromium, copper, aluminum, and other metals are used in very large-scale dehydrogenation processes. For example, acetone (6 billion pounds per year) is made from isopropyl alcohol styrene (over 2 billion pounds per year) is made from ethylbenzene. The dehydrogenation of n-paraffins yields detergent alkylates and n-olefins. The catalytic use of rhenium for selective dehydrogenation has increased in recent years. Dehydrogenation is one of the most commonly practiced of ihe chemical unit processes. 

Savittskii, E.M. and M.A. Tylkina (editors ) The Study and Use of Rhenium AUoys (translated from the Russian), Amerind Publishing Co., Pvt. Ltd. Available through US. Bureau of the Interior, or National Science Foundation, Washington, DC. 

The dehydrogenation of //-paraffins yields detergent alkylates and //-olefins. The catalytic use of rhenium for selective dehydrogenation has increased in recent years since dehydrogenation is one of the most commonly practiced of the chemical unit processes. 

The use of organometallic rhenium complexes has found a very broad scope as oxidation catalysts as described in the previous section, making MTO the catalyst of choice for many oxidation reactions of olefins. Interestingly, MTO and related rhenium compounds have also found application in the reverse reaction, the deoxygenation of alcohols and diols. Especially in recent years, this reaction has attracted much attention due to the increased interest in the use of biomass as feedstock for the chemical industry. This section provides an overview of the use of rhenium-based catalysts in the deoxygenation reaction of renewables. 

The primary uses of rhenium are in alloys that are used at very high temperamres or exposed to a great deal of wear. 

Because of the strong coordination of sulfur to metal surfaces, sulfur-containing molecules are very effective catalyst poisons. Nevertheless, a few examples of the hydrogenation of such molecules have been reported. Thiophene can be hydrogenated to tetrahydrothiophene by use of rhenium heptasulfide [44] under harsh conditions (250 °C and 300 atm hydrogen) or with a large excess of palladium in methanolic sulfuric acid [45]. In the synthesis of biotin, stereoselective civ-hydrogenation of a tri-substituted thiophene was achieved with Pd/C in acetic acid [46]. 

It is well known that Statoil has advocated the use of rhenium promoted cobalt on alumina catalysts (Eri et al., 1987 Rytter et al. 1990). Fig. 8 illustrates how one or more promoters may influence the catalyst selectivity. All the catalysts are on the same low surface area a-alumina type support (Eri et al., 2000), with 12 wt% cobalt loading. The filled diamond symbols represent an un-promoted catalyst. It is striking that promoters not only can increase the selectivity level, but actually can enhance the catalytic properties with time on stream. This observation indicates that a beneficial structural rearrangement takes place. 

The proposed reactor uses an array of UN pellet filled pins in a triangular pitch. These pins are clad in a layer of rhenium and NblZr. The use of rhenium limits the problem of nitrogen attack on NblZr. It also has positive accident safety characteristics. These pins are similar to those developed for the SP-100 program though they are used in a different reactor setup. 

The information in Table 6.20 shows how the use of rhenium has improved operation and indicates the coke content before regeneration is necessary. The coke content shown is for the third bed in a semi-regenerative rrrrit (the first and second beds contain considerably less coke.) It has been fotmd that even 1 ppm of strUhr in the rraphtha feed reduces the cycle time with skewed and balanced catalysts by abont 25% and 35%, respectively." ... 

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