Pyrolysis kinetics for long-chain n-alkylcyclohexanes

We report new experimental data and a general, mechanism-based structure-re activity 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 bec ause it incorporates the effect of the alkyl chain length on the global kin etics by accounting for the chain-length-dependent reaction path degeneracy of the initiation, beta -scission, and hydrogen-abstraction steps. The glo bal kinetics determined from the model for pyrolyses at 400 degreesC and an initial reactant concentration of 0.13 mol/L are in good accord with the e xperiment ally determined rate constants for nine different n-alkylcyclohex anes with aliphatic substituents ranging from butyl (n-C-4) to octadecyl(n- C-18). A sensitivity analysis reveals that the rate constants for initiatio n, termination, and beta -scission to form a primary radical had the greate st influence on the calculated pyrolysis rate. This work provides an exampl e of how a mechanism-based analytical rate equation, rather than some empir ical reactivity index, can be used as the basis for a structure-reactivity relationship for hydrocarbon pyrolysis.