Unwanted Synthesis in Combustion and Pyrolysis; Soot and Dioxins

Abstract

Gas-phase thermal conversion (“pyrolysis”), important in the chemical industry, can also be very useful in the laboratory, for example for the synthesis of compounds with small rings, difficult to prepare otherwise. High temperature pyrolysis of hydrocarbons results in carbon and hydrogen, but milder reaction gives redistribution (cf cracking of mineral oils); free radicals play a major part in such reactions. Examplified by CH₄ (→ C₂H₄?) and benzene (→ biphenyl), hydrocarbon pyrolysis is briefly discussed on a thermochemical basis. A comparison is made with hydrogenolysis, and with combustion. The backgrounds of soot formation are summarized, and the fate of O, C1, etc — when present in the starting material is outlined. Combustion of C1containing materials entails generation (and emission) of toxic Cl_x-DBF’s and -“dioxins”, and chlorophenols are likely precursors. Model studies have shown that DBF’s and dioxins arise via combination of two (chloro) phenoxy radicals.