Alkane cracking forms double bonds due to thermal homolytic cleavage and beta-scission.
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Scientific literature on hydrocarbon pyrolysis and polymer deconstruction confirms that thermal cracking proceeds via free radical mechanisms involving homolytic bond scission and subsequent beta-scission, yielding unsaturated products containing double bonds.
Abstract Hydrocarbon pyrolysis concerns many different fields (petroleum geochemistry, refinery, fuel thermal stability, pyrocarbon deposition, etc.). It is therefore studied in a wide variety of temperature–pressure experimental conditions, which strongly affect the chemistry of hydrocarbons cracking. An experimental study of the pyrolysis of n-octane has been performed at very low reactant concentration (1 mbar diluted in inert gas − total pressure 1500 mbar − molar fraction 0.07%) in a closed reactor, at temperatures ranging between 350 °C and 450 °C, and reaction time from 1 h to 70 h. The major products of the reaction are 1-alkenes (C2H4 to C7H14), methane and ethane; other alkanes (C3H8 to C6H14) are minor products. At 450 °C and 4 h, the conversion is close to 9% and we observe, in terms of molar fractions: C2H4 > C3H6 ≈ CH4 > C4H8 > C5H10 > C6H12 ≈ C2H6 > C7H14 These experimental results are very different from those of the thermal decomposition of n-alkanes at the same low temperature but at high pressure. In particular, the cracking stoichiometric equations (for example: C8H18 = C6H14 + C2H4) are not observed since alkanes (except methane and ethane) are in very low quantities. This can be explained by the very low concentration of reactant which limits the bimolecular reactions. In our conditions, the radicals decompose several times by beta-scissions of CC bonds when it is possible (unimolecular reaction), rather than react by H-transfers (metathesis) with the reactant (bimolecular reaction) which produce alkanes. A detailed free radical mechanism (184 reactions, 16 molecules and 18 radicals— mechanism available as Supplementary material) allows modeling the experimental results.
A new route for polypropylene (PP) deconstruction through radical pathways based on thermodynamic and kinetic considerations has been investigated. Radical polypropylene (PP) deconstruction, activated through small quantities of initiators, can enable the deconstruction of waste POs into unsaturated products. We found that stirring was detrimental to the β-scission extent because it accelerates radical termination reactions through mixing. The best results were achieved by using a semibatch process that included mechanical mixing during the temperature ramp, static heating of the polymer/initiator mixture at the final temperature, and volatile product removal with N 2 . Dicumyl peroxide and alkylated dicumene initiators produced similar liquid and solid products after a 30 min thermal treatment. Terminal alkenes were formed in the liquid products of reactions at 375 and 400 °C, with about 5% of the protons in the liquid product ascribed to terminal alkenes. Yields of liquid products increased with temperature, reaching 60% w/w at 400 °C; at the same time, yields of solid products decreased with temperature to 16% w/w at 400 °C. Although radical initiators decrease PP molecular weight during the temperature ramp, at 400 °C, initiators only marginally increased the liquid product fraction compared to control experiments. Finally, the terminal double bonds in the liquid product mixture (C8–C36) provide multiple pathways for upgrading to surfactants, plasticizers, lube oils, and
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