Normal alkanes, pyrolysis

Recently, researchers have detected 2,5-dimethylfuran and 2-methylfuran and normal alkanes in kerogen of the 2.7 x 109 year old Belingwe, Rhodesia stromatolites, by the method of pyrolysis/ GC/MS [26]. They concluded that although furans could probably be derived from many compounds, their probable origin is in bacterial and algal sugars, and that the alkanes are either products of decarboxylation of fatty acids or unaltered constituents of ancient organisms. 

There was a much larger variation between pyrolysis products volatile fractions for the exinites and vitrinites. The sporinite yielded mostly normal alkanes and alkenes up to approximately C19 with C16 being the most abundant product. The very low molecular weight hydrocarbons such as methane and ethane were not analyzed. Also, benzene and phenol derivatives were found as minor products. The vitrinite products were dominated by aromatics such as alkyl benzenes, alkylnaphthalenes, phenols, and naphthols. The smaller alkane/alkenes were also found. These results are most consistent with what was found in pyrolysis MS of sporinites and vitrinites (5,7). 

Figure 6. Comparison of the normal alkane distribution in shale oil generated by pyrolysis and in bitumen from geological samples with an equivalent stage of thermal...

Vacuum Pyrolysis. Fukushima (1982) applied vacuum pyrolysis at 500°C to characterize lake humic acid and humin and analyzed organic-solvent-soluble pyrolysates by GC-MS. The results showed that normal alkanes (C -C32) and normal alkenes (C14-C28) were present in the pyrolysates although their amounts were extremely small (0.001% of the initial weight for humic acid and 0.003% for humin). 

Figure 7.1 Peak areas obtained after pyrolysis of polyolefins (a) normal alkanes (b) isoalkanes. Reprinted with permission from J. van Schooten, E. W. Duck and R. Berkenbosch, Polymer, 1961, 2, 357. 1961, Elsevier [1])...

H-abstraction reactions of cyc/o-alkanes follow the same rules and apply the same reference kinetic parameters as the analogous reactions of normal and branched alkanes. For example, Fig. 6 shows the main cyclo-hexyl radical pyrolysis pathways. For simplicity s sake, most of the dehydrogenation reactions are not reported. 

Alkanes form a significant proportion of crude petroleum samples (16% of Athabasca bitumen and 32% of Maya crude [89]) and would normally be heptane soluble and part of the maltene fraction. Alkanes are also found in coal tars formed by pyrolysis without extensive secondary thermal treatment [81]. LD-MS at 337 nm does not ionize alkanes [90], and therefore, ali-phatics will not appear in the mass spectra alkanes can be ionized by silver ion adduction and LD, but this method has not been applied in work discussed here. 

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