Branched alkanes, pyrolysis

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

Fig. 5. Pyrolysis of lumped component of branched alkanes C15H32. Selectivity of methyl radical and small alkenes as a function of the probability of methyl substitution.

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. 

Additional structural information can also be derived by Py-GC. Information on stereoisomerism, crystallinity, and sequence distribution data is sometimes able to be obtained. Isotactic and syndiotactic polypropylene may be identified by the ratio of oligomeric components. High- and low-density polyethylene can also be determined by the proportion of branch chain alkane pyrolysis products using a novel pyrolysis hydrogenation technique. 

The exact temperature at which the cloud point is reached depends on the total n-alkane content of the fuel, the average size of the n-alkane molecules, their size distribution and chain structure (e.g. degree of branching). Conventional diesels contain as much as 20% of long-chain n-alkanes of limited solubility in the fuel. Pyrolysis-diesels from PE feedstocks can contain more than 40% long-chain n-alkanes. Paraffins crystallize at low temperature into very thin rhombic plates which can clog filters, transfer lines, and pumps, and can lead to engine failure at low temperatures. 

Hydrogenation pyrolysis has been applied to the determination of the composition of copolymers of a-olefins, the sequence of monomer units and the manner in which they are added (head-to-head and head-to-tail) [253]. Mikhailov et al. [251] used Py—GC to investigate the structure of low- and high-density polyethylenes and copolymers of ethylene with propylene. The pyrolysis products were hydrogenated. The method made it possible to examine alkanes up to Cjo, which facilitates the investigation of the polymer chain structure. The isoalkanes identified corresponded to the branched polyethylene structure. It has been established that the ethyl and butyl side-chains occur most frequently in polyethylenes. 

THM-GC analysis of the cured Burmese lacquer results in a series of straight-chain and branched-chain alkyl benzenes ranging from toluene to dodecyl benzene, and Cg- to Ciy-alkanes and alkenes. These products are similar in composition to the products obtained by conventional pyrolysis. Compounds corresponding to peaks marked with an asterisk are FAMES that are products of the THM reaction and... 

According to Wall and co-workers [2] the pyrolysis of polyethylene proceeds by a radical chain mechanism. The products formed result from the process of random-chain cleavage, followed by intermolecular or intramolecular hydrogen abstraction. Hydrogen abstraction occurs preferentially at tertiary carbon atoms, and product formation results from homolysis of the carbon-carbon bond at the beta position relative to the radical site. The major products formed are the -alkanes and alpha-omega-diolefins. The peaks between the triplets result from chain branching. 

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