Polycyclic compounds, pyrolysis

A series of 10 polycyclic compounds and blends with starch, Bakelite, and hydrogenated creosote were employed as model substances to examine the effect of carbon, hydrogen, and oxygen on sulfur distribution during pyrolysis at a temperature of about 625°C. 

Polycyclic aromatic hydrocarbons (PAHs) represent a heterogeneous class of environmental contaminants formed by incomplete combustion or pyrolysis of organic matter. They comprise compounds with 2-6 fused benzene rings, most of which (4-6 ring compounds) are known human mutagens and carcinogens. 

When 2D or 3D PAHs are considered then the choice becomes much larger. An exhaustive review on the advances in the synthesis of polycyclic aromatic compounds can be found in Harvey, 2004. The most used strategies are flash vacuum pyrolysis, cross-coupling, oxidative photocyclization, Diels-Adler cycloaddition, etc. 

Head-to-tail intramolecular dimerization of nonactivated vinyl groups can also occur in compounds in which the tether is only two atoms long under conditions of flash-vacuum pyrolysis (FVP). Under such conditions strained bridged polycyclic derivatives can be prepared.78 The formation of 18 is the result of intramolecular head-to-tail cycloaddition of the Cope rearrangement intermediate derived from 1,2-divinylacenaphthene.78... 

Sharma, R. K., Chan, W. G., Seeman, J. I., and Hajaligol, M. R. (2003). Formation of low molecular weight heterocycles and polycyclic aromatic compounds (PACs) in the pyrolysis of a-amino acids. J. Anal. Appl. Pyrolysis 66, 97-121. 

Pyrolysis reactions in fuel-rich flame zones may lead, however, to emissions of polycyclic aromatic compounds (PACs) and soot. The close correlation between the concentrations of PACs and the bioactivity of flame samples is indicative of some of the health hazards involved in the emissions of PACs (Fig. 3). Fluxes of PAC species determined in fuel-rich, natural gas turbulent diffusion flames show the build up of hydrocarbons of increasing molecular weight along flames (Fig. 4). It is postulated that PACs are formed by the successive addition of C2 through C5 hydrocarbons to aromatic compounds (Fig. 5). 

Figure 11.5 shows that the functional group compositional analysis of the pyrolysis oil/waxes derived from the fixed-bed pyrolysis of PVC, PS and PET is very different from the polyalkene plastic pyrolysis oil/waxes. The spectra of the PVC pyrolysis oil/wax shows that the characteristic peaks of alkanes and alkenes are present as described for the polyalkene plastics. Since the PVC plastic polymer is based on a similar backbone structure to the polyalkene plastics, a similar degradation product oil/wax composition may be expected. However, the spectra for PVC in Figure 11.5 show that there are additional peaks in the region of 675-900 cm and 1575-1625 cm The presence of these peaks indicates the presence of mono-aromatic, polycyclic aromatic and substituted aromatic groups. Benzene has been identified as a major constituent in oils derived from the pyrolysis of PVC whilst other aromatic compounds identified included alkylated benzenes and naphthalene and other polycyclic aromatic hydrocarbons [19, 32, 39]... 

Figure 7 Carbon stable-isotope compositions of individual aromatic and polycyclic aromatic compounds, plotted against carbon number. Compounds were obtained by hydrous pyrolysis of macromolecular material from (a) Orgueil (Cl), (b) Cold Bokkeveld (CM2), and (c) by hydrous pyrolysis (closed symbols) and online pyrolysis GC-IRMS (open symbols) of Murchison (CM2) (sources (a) Sephton et al, 2000 (b) Sephton et al, 1998 (c) Sephton and Gilmour, 2001a).

Carbonaceous materials (CMs) are sometimes also named polymeric carbons. They are mostly prepared by thermal decomposition of organic precursors. One strategy is pyrolysis of gaseous or vaporized hydrocarbons at the surface of heated substrates, a second is heating (pyrolysis) of natural or synthetic polymers, both in an inert atmosphere. The latter is of special interest, and according to Miyabayashi et al. [374], precursors such as condensed polycyclic hydrocarbons, polymeric heterocyclic compounds, phenol-formaldehyde resins, polyacrylonitrile or polyphenylene are heated to 300-3000 °C for 0.15-20 h. Sometimes, a temperature/time profile is run. The temperature range must be divided into two domains, namely... 

Similarly to the study on coal, of significant importance was the evaluation of polycyclic aromatic compounds generated from the pyrolysis of oil shale. This type of study included the evaluation of PAH in shale pyrolysates, and also of sulfur containing polycyclic aromatic compounds, and nitrogen containing polycyclic aromatic compounds [41 aj. 

The pyrolytic studies on meteorites are commonly done at different temperatures. A preheating step is utilized to insure that any possible adsorbed gases on the surface of the meteorite from the terrestrial environment are eliminated. Several organic compounds are monitored in pyrolysates such as polycyclic aromatic compounds. As an example, the results on naphthalene production upon pyrolysis from several carbonaceous chondrites, normalized by the total carbon content before pyrolysis, are shown in Figure 17.2.1 [76],... 

The polyene chain further decomposes under the influence of heat, generating aromatic hydrocarbons such as benzene, styrene, naphthalene, etc. This type of reaction can be the source of some polycyclic aromatic hydrocarbons found in traces during the pyrolysis of certain vinyl polymers. The elimination of a HX molecule from a vinyl type polymer is favored by the presence of a p-double bond in a compound of the form -CH2-CHX-CH=CH-. For this reason the side reaction for vinyl polymers is slower for the intact polymer and accelerates as the polymer tends to decompose. 

A considerable number of reports regarding the formation of compounds that may represent a health hazard are related to the formation of polycyclic aromatic hydrocarbons (PAHs) during industrial pyrolysis processes (recycling of waste, incineration, etc.). This interest is particularly geared toward the study of polyolefins pyrolysis and synthetic and natural rubber pyrolysis. The formation of PAHs during polyethylene pyrolysis has been reported frequently in literature [6, 12] and is further discussed in Section 6.1. The formation of PAHs during tire pyrolysis is also of considerable concern. The concentrations of some components in the oils generated from the pyrolysis of used tires as a function of temperature are indicated in Table 5.3 1 [13]. 

The crude black, viscous pyrolysis oil product requires an upgrading step to make it suitable as a refinery feedstock. This is accomplished by high-pressure hydrogenation in a manner very similar to the upgrading step used for the coking products in tar sands processing. The influence of overall process conditions on the polycyclic aromatic compounds found in the product has been examined [62], and the supercritical water extract of the pyrolytic product has been characterized [63]. 

At low temperatures, in situations where there is relatively little 02, pyrolysis reactions (i.e. reactions where decomposition takes place as a result of heat) may cause a rearrangement of atoms that can lead to the formation of polycyclic aromatic hydrocarbons (see Section 2.7) during combustion. The most notorious of these is benzo[ ]pyrene (B[ ]P see Fig. 2.4), a cancer-inducing compound. 

Williams, P.T. Nazzal, J.M. Pyrolysis of oil shales influence of particle grain size on polycyclic aromatic compounds in the derived shale oils. J. Inst. Energy 1999, 72 (491), 48-55. 

In the mid-1970s, pyrolysis studies with several amino acids led to the isolation and identification of several additional polycyclic A-heterocyclic compounds which are reported not only to be tumorigenic to mouse skin but also to show inordinately high mutagenicity when tested in the Ames bioassay with Salmonella typhimurium. The impetus for these particular amino acid pyrolysis studies was not the attempt to define the relationship between tobacco leaf precursors and tobacco smoke components but the observation that the extracts of broiled, fried, or roasted foodstuffs (meat, fish, poultry, etc.) were highly mutagenic in the Ames bioassay Salmonella typhimurium). These A-heterocyclic... 

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