Routes to carbon

Concepts in Syngas Manufacture Table 5.1 Routes to carbon [389]. 

Carbon type Reaction Phenomena (Table 5.2) Critical parameters 

Whisker carbon R6-R9 Break-up of catalyst pellet low H2O/C ratio, high temperature, presence of olefins, aromatics 

Gum R50 Blocking of Ni surface low H2O/C ratio, absence of H2, low temperature, presence of aromatics 

Pyrolytic coke R49 Encapsulation of catalyst pellet, deposits on tube wall high temperature, long residence time, presence of olefins, sulphur poisoning 

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Reactions of tin(IV) halides or organotin halides with organic derivatives of more electropositive metals provide the most important general synthetic routes to carbon-tin bonds (equation 6). The most frequently used R -M compounds are those of magnesium and lithium, with sodium, zinc, and aluminum having a more limited use. Some examples of these reactions are given in equations (7-9). 

Carbonate complexes are usually synthesized from carbonate or bicarbonate anions in alkaline aqueous solution. The reaction of metal hydroxides with carbon dioxide is another route to carbonate complexes. Other methods include the oxidation of metal carbonyl complexes (equation 17), and the oxidation of metal salts in the presence of carbon dioxide. 

The addition of hydrogen cyanide to carbonyl compounds such as aldehydes or ketones leads to 2-hydroxynitriles (cyanohydrins). This reaction as depicted in Fig. 1 is remarkable in several ways it represents one of the easiest routes to carbon-carbon bond formation, and in many cases it creates a new stereocenter. 

The power of the CAB catalytic reaction for the enantioselective route to carbon-branched pyranose derivatives is also seen from the following example (Scheme 9). 

In general, nucleophilic aromatic substitution reactions are rather difficult with unsubstituted aryl derivatives or when the aromatic ring contains a strongly electron-releasing group. Formation of the chromium complex activates such aromatic compounds to nucleophilic substitution. Since the nucleophiles are carbon nucleophiles, this technique offers a route to carbon bonds that would be very difficult to form by other methods. 

As many carbonate complexes are synthesized usually in aqueous solution under fairly alkaline conditions, the possibility of contamination by hydroxy species is often a problem. To circumvent this, the use of bicarbonate ion (via saturation of sodium carbonate solution with COj) rather than the carbonate ion can often avoid the precipitation of these contaminants. Many other synthetic methods use carbon dioxide as their starting point. Transition metal hydroxo complexes are, in general, capable of reacting with CO2 to produce the corresponding carbonate complex. The rate of CO2 uptake, which depends upon the nucleophilicity of the OH entity, proceeds by a mechanism that can be regarded as hydroxide addition across the unsaturated C02. There are few non-aqueous routes to carbonate complexes but one reaction (3), illustrative of a synthetic pathway of great potential, is that used to prepare platinum and copper complexes. Ruthenium and osmium carbonate complexes result from the oxidation of coordinated carbon monoxide by dioxygen insertion (4). ... 

Nanotube theoretical and experimental research has developed very rapidly over the last seven years, following the bulk production of C(jo and structural identification of carbon nanotubes in soot deposits formed during plasma arc experiments. This review summarises achievements in nanotube technology, in particular various routes to carbon nanotubes and their remarkable mechanical and conducting properties. The creation of novel nanotubules, nanowires and nanorods containing other elements such as B, N, Si, O, Mo, S and W is also reviewed. These advances are paving the way to nanoscale technology and promise to provide a wide spectrum of applications. 

Not all possible processes are included (e.g. isotopic exchange, epimerisation, and some routes to carbon ), but all that are shown are possible in practice. 

P. Jimenez, W. K. Maser, P. Castell, M. T. Martinez, and A. M. Benito, Nanofihrilar polyaniline Direct route to carbon nanotube water dispersions of high concentration, Macromol. 

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