(147a) A Graph-Theoretic Moiety Hypothesis for Olefin Yields from High Temperature Pyrolyses of C8 to C12 Normal-, 2-Methyl- and 4-Methyl-Alkanes

High temperature, short residence time pyrolyses of n-octane, n-decane, n-dodecane, 2-methylheptane, 2-methylnonane, 4-methylheptane and 4-methylnonane were experimentally investigated. An isothermal flow reactor was used at temperatures from 950 to 1050 K and residence times from 30 to 200 milliseconds, using nitrogen gas at atmospheric pressure as a carrier for the hydrocarbon substrate. Initial substrate concentration was maintained constant at 1.5x10^-4 mol/L, and substrate fractional conversions varied from 0.01 to 0.89. The decomposition kinetics and the major and minor product selectivities obtained from all substrates were interpreted in terms of Rice-Herzfeld free-radical pathways. A graph-theoretic “moiety hypothesis” was proposed, according to which product selectivity S=aT+bI+cM_T+dM_I, where T, I, M_T and M_I are the numbers of substrate carbon atoms respectively contained in terminal, interior, 2-methyl and interior methyl moieties, while a, b, c, and d are the "intrinsic yields" of the product from these moieties at given pyrolysis conditions. Data from pyrolyses of all substrates at 1050 K and low conversions provided [a,b,c,d] for ethylene [24.4, 10.8, 7.5, 2.3] and propylene [10.0, 1.4, 12.5, 12.0]. The “intrinsic yield” coefficients reveal that ethylene arises mainly from the terminal and interior moieties, with smaller, but non-zero, contributions from the 2-methyl and interior-methyl moieties. Propylene arises mainly from the terminal, 2-methyl, and interior-methyl moieties, with essentially no contribution from the interior moiety. Possible applications of these results to commercial steam cracking of naphtha and hydrocracker residues are suggested.