Giving Carbides

Alcohols and jlkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.188 The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reducing agent. A variety of heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide dissociates on the catalytic surface to give carbides and that these are in turn hydrogenated to give surface carbenes 1 " n ... 

All the metals or their oxides react with carbon at high temperatures to give carbides MQ.15 These are ionic acetylides whose properties are discussed in Chapter 7. The carbide Mg2C316 has been obtained relatively pure by reacting Mg dust with pentane at ca. 680°C. This has Mg atoms coordinated by four linear C3 groups in a tetrahedral arrangement, three via terminal C atoms and one by a... 

There is little doubt that both the carbon monoxide and the hydrogen molecule undergo chemisorption on the catalyst surface resulting in bond dissociation to give carbide, oxo, and (monoatomic) hydrido species (Scheme 1). 

A comparative study of the thermal reactions in oxygen of the above three reactants [118], together with nickel formate, showed that the shapes of the ar-time curves were comparable with those for vacuum decompositions. The magnitudes of the Anhenius parameters (with the exception of the decomposition of the formate which does not give carbide product) were diminished. It was concluded that the initiation of reaction was unaffected by the presence of oxygen, so that the geometry of each rate process was unchanged. In the presence of Oj, interfacial reactions on the nickel oxide product (-+ COj + HjO) proceeded appreciably more rapidly by a common mechanism and all values of E, were close to 150 kJ mol. ... 

Carbon will react readily with sulfur at red heat to form carbon disulphide, CS2, but it does not react directly with nitrogen. Cyanogen, (CN)2, must be prepared by heating covalent metal cyanides such as CuCN. Carbon will also react directly with many metals at elevated temperatures to give CARBIDES. Carbides can also be ob-... 

Molecular dissociation of the carbonyl can be complicated by the secondary process of metal reaction with CO, giving carbides and oxides that contaminate the film for this reason, thermal decomposition of the carbonyl is carried out at a minimal temperature. 

Drying agents may be divided broadly into (a) those which combine with water reversibly and (6) tho.se which react chemically with water by a non-revcrsible process giving rise to a new water-free compound. Sodium, calcium carbide and phosphorus peiitoxide belong to the latter class and wih be discussed in Section 11,39. 

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

At about the same time that the Birkeland-Eyde process was developed, the Frank-Caro cyanamide process was commercialized (14). In this process limestone is heated to produce lime, which then reacts with carbon in a highly energy-demanding reaction to give calcium carbide. Reaction with N2 gives calcium cyanamide [150-62-7] which hydrolyzes to ammonia and calcium carbonate (see Cyanamides). 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

There is Htde evidence of the direct formation of sodium carbide from the elements (29,30), but sodium and graphite form lamellar intercalation compounds (16,31—33). At 500—700°C, sodium and sodium carbonate produce the carbide, Na2C2 above 700°C, free carbon is also formed (34). Sodium reacts with carbon monoxide to give sodium carbide (34), and with acetylene to give sodium acetyHde, NaHC2, and sodium carbide (disodium acetyHde), Na2C2 (see Carbides) (8). 

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