Rhenium Carbide

Crystal structure of this catalyst proved to be similar to a-silica structure (a = 4.90 A, c = 5.4 A), considerably distinguished from parameters of the lattice of graphite (a = 2.46 A, c = 5.7 A), and from that of rhenium carbide lattice... 

Rhenium carbides can be afforded by pyrolysis of simple rhenium carbonyl compounds. 

Ethylene oxide (qv) was once produced by the chlorohydrin process, but this process was slowly abandoned starting in 1937 when Union Carbide Corp. developed and commercialized the silver-catalyzed air oxidation of ethylene process patented in 1931 (67). Union Carbide Corp. is stiU. the world s largest ethylene oxide producer, but most other manufacturers Hcense either the Shell or Scientific Design process. Shell has the dominant patent position in ethylene oxide catalysts, which is the result of the development of highly effective methods of silver deposition on alumina (29), and the discovery of the importance of estabUshing precise parts per million levels of the higher alkaU metal elements on the catalyst surface (68). The most recent patents describe the addition of trace amounts of rhenium and various Group (VI) elements (69). 

Most chemical properties of technetium are similar to those of rhenium. The metal exhibits several oxidation states, the most stable being the hep-tavalent, Tc +. The metal forms two oxides the black dioxide Tc02 and the heptoxide TC2O7. At ambient temperature in the presence of moisture, a thin layer of dioxide, Tc02, covers the metal surface. The metal burns in fluorine to form two fluorides, the penta- and hexafluorides, TcFs and TcFe. Binary compounds also are obtained with other nonmetaUic elements. It combines with sulfur and carbon at high temperatures forming technetium disulfide and carbide, TcS2 and TcC, respectively. 

Heating elements -rhenium and rhenium alloys as [RHENIUM AND RHENIUM COMPOUNDS] (Vol 21) -silicon carbide in [CARBIDES - SILICON CARBIDE] (Vol 4) -use of silicon carbide [CARBIDES - SILICON CARBIDE] (Vol 4)... 

Because rhenium is very difficult to machine with carbide tools and other conventional methods, electrical-discharge machining (EDM), electrochemical machining (ROM), abrasive cutting, or grinding is... 

Fig. 42. [Re CO),]1-, 34, as in its Ph4P salt (76). The Re, core comprises a monocapped octahedron of rhenium atoms, with the carbide carbon in the octahedral cavity (mean Re-C = 2.13 0.02 A). The metal-metal bonds fall into several categories. Bonds from the capping atom to the capped face of the octahedron average 2.929 A those on the capped face, 2.9S5 A those between the capped face and the opposite uncapped face alternate longer (3.017 A) and shorter (2.977 A), and those in that uncapped face, average 3.080 A. There are three.terminal carbonyls on each metal atom.

For Mn2(CO)10, the ions corresponding to Mn2(CO)+ ( =8 or 9) were either not seen (247) or were of very low abundance (49,164), and Mn2(CO)7 was only apparent with less than the normal 70-V ionizing potential. All other Mn(CO)+ ( =0-5) and Mn2(CO)J (w=0-10) ions were observed, but no doubly charged ions were seen. The carbide ions MnC(CO)J (n=0-2) were observed in low abundance. Similar ions were found in the spectrum of Re2(CO)10, as also was the series Re2(CO) C+ ( = 0-5), and the doubly charged ions Re2(CO)2+ and Re2(CO) C2+ ( =0-6). Major differences in intensities in the two spectra can be attributed to the greater strength of the rhenium-rhenium bond. About 60% of the ions produced... 

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

Rhenium does not form a carbide easily, d(though it does dissolve significant carbon. An oxycarbide (MojC type) can be made by reacting the metal with CO at 470-600°C and by deeomposing Re carbonyl on graphite . A metastable cubic phase has been observed when Re and C are heated in contact at 2100°C. Application of high P (> 60 kbar) to a mixture of Re and C results in a MOC-type structure . 

Detailed discussion of bulk metal carbides would be inappropriate here, but aspects of their structures and thermochemistry are worth noting. Many metal carbides are metallic-type conductors of electricity, and have structures very similar to those of the bulk metals, with similar metal-metal distances, but with carbon atoms occupying interstitial sites (commonly octahedral holes) in the metal lattice. Thermochemical information is available on enough of them to get some insight into the relative strengths of both their metal-metal and metal-carbon bonding. Unfortunately, the metals that would be of most interest (osmium, rhenium, and rhodium) for the purpose of comparison with the molecular metal carbonyl carbides already discussed are not known to form stable binary carbide phases M cCj, and the carbides of the 3d metals in the same groups as these have very complicated structures. We therefore focus below on carbides of early transition metals, about which more is known. ... 

The data in Table 4.3 correspond to a radius for the octahedrally coordinated carbon atom that Ues in the range 0.59-0.69 A. We noted earlier that the radius of the core carbon in osmium, rhenium, and rhodium clusters lie in the range 0.59-0.62 A. It appears likely that the enthalpy change ZE(M-C), needed to cleave the six M-C bonds in these molecular carbonyl clusters, will lie in the same range (239-306 kcal moT i.e., 38-51 kcal mol per MC link) that we have now calculated for the similarly coordinated carbon atoms in these extended lattice binary carbides MC or M2C. 

Rhenium (Re) differs from the other refractory metals (Nb, Ta, Mo and W) in that it has an hep structure, and does not form carbides. Because it does not have a ductile-to-brittle transition temperature. Re retains its ductility from subzero to high temperatures. In addition, it can be mechanically formed and shaped to some degree at room temperature. It also has a very high modulus of elasticity that, among metals, is second only to those of Ir and Os. Compared with other refractory metals. Re has superior tensile strength and creep-rupture strength over a wide temperature range. 

Although most ceramics are thermal and electrical insulators, some, such as cubic boron nitride, are good conductors of heat, and others, such as rhenium oxide, conduct electricity as well as metals. Indium tin oxide is a transparent ceramic that conducts electricity and is used to make liquid crystal calculator displays. Some ceramics are semiconductors, with conductivities that become enhanced as the temperature increases. For example, silicon carbide, SiC, is used as a semiconductor material in high temperature applications. 

---