The pyrolysis of chlorobenzene

Abstract

A study of the thermal decomposition of chlorobenzene shows that two quite distinct mechanisms can operate according to the temperature. Between 770 and 800" C, the main products are hydrogen chloride, hydrogen and p : p'-dichlorodiphenyl ; chlorine atoms and chlorophenyl radicals appear to be the principal carriers involved in a chain cycle in which there is little or no rupture of the aromatic ring. At higher temperatures (800-850" C), extensive ring-breaking occurs and quite large yields of vinyl chloride are obtained ; here, a chain reaction is postulated in which C2HCl fragments are important intermediate species. The occurrence of two competing mechanisms results in a welldefined negative temperature coefficient of reaction rate between 830 and 850" C ; for on account of the increasing thermal instability of CzHCl fragments the ring-breaking mechanism ceases to operate and there is a consequent decrease in the rate of consumption of chlorobenzene by this process. At still higher temperatures, therefore, hydrogen chloride, hydrogen and p : p'-dichlorodiphenyl again become the main products.Pyrolysis of chlorobenzene in the presence of added hydrogen yields products qualitatively similar to those obtained in its absence. The accelerating effect of hydrogen is most marked at temperatures where the ring-stripping mechanism is predominant. It is suggested that hydrogen favours the latter process by preventing recombination of chlorine atoms at the walls, while it hinders the ring-breaking reaction by stabilizing intermediate CZHCl fragments.

A study of chlorobenzene pyrolysis

“…Cullis and Manton [24] found that the addition of hydrogen accelerated the decomposition of chlorobenzene. Louw et al [6] postulated that using H2 in excess would lead to a dechlorination of chlorinated aromatic compounds.…”

Understanding the Dechlorination of Chlorinated Hydrocarbons in the Pyrolysis of Mixed Plastics

“…When the temperature is raised up to 550 °C, the concentration of chlorobenzene in the outlet stream is similar to that at 500 °C. Cullis et al 26 reported a minimum thermal decomposition temperature of 740 °C for chlorobenzene, mainly via stripping of the aromatic rings forming benzene. Li et al 27 reported that chlorobenzene does not undergo decomposition below 600 °C in an inert gas, but there is a small extent of decomposition in hydrogen at 500 °C.…”