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 -C 4 ) to octadecyl ( n -C 18 ). 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.