(89b) Severe Pyrolyses of C6 to C12 Monocyclo- and Normal-Alkanes

High temperature, short residence time pyrolyses of three pairs of cyclo- and normal-alkanes, namely C6(cyclohexane, n-hexane), C10(cyclodecane, n-decane) and C12(cyclododecane, n-dodecane) were experimentally investigated using a coiled tube flow reactor with outlet temperatures from 1050 to 1175 K, residence times ~300 milliseconds, and steam/hydrocarbon ratios ~1, to simulate commercial steam cracking at severities from 1.5 to 4.  The influence of carbon number N upon monocyclo- versus normal-alkane pyrolysis pathways was discerned from the observed selectivities, Sj mol/100 mol substrate decomposed, of products j =  methane, ethylene, propylene, butadiene and benzene.  Decomposition kinetics and product selectivities were interpreted by Rice-Herzfeld types of free-radical pathways and also by graph-theoretic methods based upon the A(djacency) matrices of the substrates.  For the monocyclo-alkanes, graph theory suggested upper bounds for the selectivities of ethylene and butadiene products, respectively, S*ethylene = 100(N-4)/2 and S*butadiene = 100.  The experimentally observed selectivities for these products were always lower than theoretical, the more so with increasing N, as seen by (S/S*)ethylene ~ 0.8 and (S/S*)butadiene ~ 0.5 for N = 6, both decreasing to (S/S*)ethylene ~0.5 and (S/S*)butadiene ~ 0.33 for N = 12.  It appears likely that, as N increases, the theoretical decomposition pathway is altered by Rice-Kossiakoff isomerizations, producing interior radicals, akin to those formed during n-alkane pyrolysis, that decompose by diverse paths, diminishing the selectivities to ethylene and butadiene.