Thermal gas phase hydrodehalogenation of bromochlorodifluoromethane

Abstract

The thermal hydrodehalogenation of bromochlorodifluoromethane (Halon-1211; CBrClF₂) in the gas phase has been studied using a plug flow alumina reactor at atmospheric pressure over the temperature range 400–900 °C with residence times of 2–3 s and CBrClF₂/hydrogen molar intake ratios of ca. 10. Conversion of CBrClF₂ starts at ca. 400 °C with C–Br bond homolysis followed by reaction with HX (X being Br, Cl or H) to yield CHClF₂. At higher temperatures other products arise and complete conversion of CBrClF₂ is achieved at ca. 600 °C. At temperatures above 850 °C complete dehalogenation to mainly methane (yield 80%) is attained. In the temperature range 450–550 °C the (pseudo) first-order rate constant for the overall reaction (F) was found to obey: log (k_F/s^–1)=(9.4 ± 1.5)–(150 ± 25) kJ mol^–1/2.303RT.

The thermolysis of CBrClF₂ was also studied using an excess of 2-phenylpropane (cumene) as a radical scavenger, resulting in the following Arrhenius expression for reaction (G): log (k_G/s^–1)=(15.1 ± 0.5)–(262 ± 9) kJ mol^–1/2.303RT. From these parameters the bond dissociation energy for the C–Br bond in CBrClF₂ was calculated to be 268 ± 8 kJ mol^–1, leading to a heat of formation of the ˙CClF₂ radical of –279 ± 17 kJ mol^–1. Kinetic analysis and separate experiments with H₂O₂ as an initiator for making H˙ showed that attack by H˙ is the main route for decomposition of CBrClF₂. At temperatures higher than 500 °C HBr rather than H₂ acts as a hydrogen transfer agent resulting in a fast radical chain (reactions G and L–O) with the observed Arrhenius parameters as a consequence.