Mechanism of soot initiation in methane systems

Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C2H2 produced in a chlorine catalyzed polymerization of CH4 are presented. An earlier examination of the data and mechanism leading to C2H2 supports a methyl and chloromethyl recombination path to C2 hydrocarbons. The relative yield of CH3 and CH2Cl depends on the excess of methane. In the CH4, system consumption of C2 species to ultimately form benzene is shown to proceed by a stepwise addition of CH3 radicals to CnHm species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly. A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1–3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300 K are estimated to be log [k(sec−1)] = 13.7 − 37/θ = 13.7 - 37/0 where 0 = 2.303 RT in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH3/CH2Cl radical, sequential addition mechanism. Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics.