THERMAL GAS-PHASE HYDRODEHALOGENATION OF BROMOCHLORODIFLUOROMETHANE

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-degrees-C with residence times of 2-3 s and CBrClF2/hydrogen molar intake ratios of ca. 10. Conversion of CBrClF2 starts at ca. 400-degrees-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-degrees-C. At temperatures above 850-degrees-C complete dehalogenation to mainly methane (yield 80%) is attained. In the temperature range 450-550-degrees-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 .CClF, 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-degrees-C HBr rather than H-2 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.