Pyrolysis kinetics for long-chain n-alkylcyclohexanes

We report new experimental data and a general, mechanism-based structure - reactivity relationship for the pyrolysis of long-chain n-alkylcyclohexanes. The relationship has as its foundation the previously deduced free-radical reaction mechanism for alkylcyclohexane pyrolysis. The model is general because it incorporates the effect of the alkyl chain length on the global kinetics by accounting for the chain-length-dependent reaction path degeneracy of the initiation, β-scission, and hydrogen-abstraction steps. The global kinetics determined from the model for pyrolyses at 400 °C and an initial reactant concentration of 0.13 mol/L are in good accord with the experimentally determined rate constants for nine different n-alkylcyclohexanes with aliphatic substituents ranging from butyl (n-C₄) to octadecyl (n-C₁₈). A sensitivity analysis reveals that the rate constants for initiation, termination, and β-scission to form a primary radical had the greatest influence on the calculated pyrolysis rate. This work provides an example of how a mechanism-based analytical rate equation, rather than some empirical reactivity index, can be used as the basis for a structure - reactivity relationship for hydrocarbon pyrolysis.