Sooting benzene/oxygen

The stmcture of products from sooting benzene/oxygen flames has also been studied in this context. It turned out that the sooty film precipitating on the cool parts of the apparatus consists of tubular and onion-like carbon species. Still the product obtained is rather inhomogeneous, so for the time being, combustion methods are no effective means of generating carbon onions. 

The now usual technique of preparing macroscopic quantities of fullerenes is the arc discharge in an inert atmosphere between carbon electrodes, followed by the collection of the soot produced and the removal of its soluble parts. Other methods reported are laser ablation of graphite and of polyimides, incomplete combustion of hydrocarbons in sooting flames, continuous-wave (CW) CO2 laser pyrolysis of benzene-oxygen vapor, and many not listed here [2,111,112]. It would appear that almost any kind of soot contains fullerenes and carbon nanotubes, but without the new methods of analysis, it was impossible to identify them. In some instances, the last step in the preparation involved heating of the soot over a catalytic metal, a procedure that might be called catalytic pyrolysis. 

A Raman spectrum of an extract from the soot of a benzene-oxygen flame is shown in Fig. 38 [112], All the labeled peaks were assigned to C70 fullerene except that of the highest frequency, which was assigned to Ceo- For the latest assignments, the reader is referred to the review article by Dresselhaus et al. [3]. It is important to emphasize that Ceo fullerene, because of its spherical symmetry, has fewer and stronger Raman bands than the other fullerenes. 

Figure 38 Raman spectrum of a final soot extract [112]. (Reproduced from Chemical Physics Letters, 256, Voicu, L, et aL, Laser synthesis of fullerenes from benzene-oxygen mixtures, pp. 261-268. Copyright 1996, with permission from Elsevier Science.)... 

When hydrocarbons are not completely incinerated, carbon black is obtained. Under suitable conditions this may contain not only particles of classical soot, but also fullerenes. The trace existence of fullerenes in lamp black was first proven by mass spectrometry, but after working out appropriate protocols for the reaction, smoking flames can by now be used for the production of weighable amounts. Benzene is the most common source of carbon for this process. It is mixed with oxygen and argon and burned in a laminar flame (Figure 2.18). The resulting mixture contains soot, polycychc aromatic compounds and a certain fraction of fullerenes that make up 0.003-9.0% of the soot s total mass. Other hydrocarbons like, for example, toluene or methane may be employed as well. 

Wastes containing PAHs may be effectively destroyed by various incineration processes as mentioned briefly in the above section. Weber et al. (2001) have studied the mechanisms of formation of polychlorinated diben-zofurans (PCDF), polychlorinated biphenyls (PCB), polychlorinated naphthalenes (PCN) and polychlorinated benzenes (PCBz) from the degradation of PAHs in two types of incinerators, the stoker type- and the fluidized bed incinerators. Their studies have revealed the occurrence of a sequence of steps, such as the cleavage of C-C bond in the PAHs, chlorination at these cleaved positions, further chlorination or oxygen insertion at the ortho positions to the chlorine atoms in the intermediate chlorinated species. A perylene structure in soot is proposed as the basis for the observed PCDF pattern in the fluidized bed incinerators. Polychlorinated dibenzo-dioxins (PCDD) and the polychlorinated... 

In this work the net formation rates of benzene, toluene, phenylacetylene, and styrene, and concentrations of possible precursors, were determined as a function of distance from the burner in the primary reaction zone of a low-pressure, near-sooting 1,3-butadiene-oxygen-4% argon flame. These rates are compared with estimated rates for several reaction mechanisms. 

As a general rule, the ease of oxidation of hydrocarbons and derivatives increases in the following order alkanes and isoalkanes<aromatics side chain alkylbenzenes, such as toluene and ethylbenzene, and polymethylbenzenes are more difficult to oxidize than benzene [19]. 

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