Thermal gas phase hydrodehalogenation of bromochlorodifluoromethane

Abstract

The thermal hydrodehalogenation of bromochlorodifluoromethane (Halon-1 21 1 ; CBrClF,) 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 CBrCIF,/hydrogen molar intake ratios of ca. 10. Conversion of CBrClF, starts a t ca. 400 "C with C-Br bond homolysis followed by reaction with HX (X being Br, CI or H) to yield CHCIF,. At higher temperatures other products arise and complete conversion of CBrClF, is achieved a t 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 (kJs9) = (9.4 k 1.5) -(150 k 25) kJ rnol-'/2.303RT.The thermolysis of CBrClF, was also studied using an excess of 2-phenylpropane (cumene) as a radical scavenger, resulting in the following Arrhenius expression for reaction (G): log (kJs-') = (15.1 k 0.5) -(262 k 9) kJ mol-'/2.303RT. From these parameters the bond dissociation energy for the C-Br bond in CBrCIF, was calculated to be 268 k 8 kJ mol-', leading to a heat of formation 'CCIF, + 'CCIF, C,Cl,F, (V) Paper 3/03922F

Gas-Phase Reaction of Halon 1301 (CBrF3) with Methane

“…Reaction R8, in particular, is very fast at high temperatures compared with other CH 3 Br consumption reactions and is one of the major hightemperature channels for production of HBr. HF formation was detected above 933 K, and its concentration increased substantially with temperature, which agrees well with previous studies (de Lijser et al, 1994;Ritter, 1994). In addition to reactions R5, R6, and R13, HF can also be generated in reactions R22-R24.…”

“…The formation of CHBrF 2 was detected above 900 K. It is possible that CHBrF 2 is formed via the combination of HBr and CF 2 , produced by reactions R5 and R22. The homolysis of C-F bond in CBrF 3 is negligibly slow at low temperatures (de Lijser et al, 1994) but may occur at high temperatures, resulting in the formation of CBrF 2 , which can then react with HBr or H 2 to form CHBrF 2 .…”

“…Nonoxidative thermal hydrodehalogenation (THD), where halons or halogenated compounds react with hydrogen or hydrogen donors at high temperature, is recognized as an emerging treatment process. De Lijser et al (1994) have investigated THD of bromochlorodifluoromethane (CBrClF 2 , halon 1211) in hydrogen (H 2 ) and have found that, at low temperature (<873 K), the et al, 1994). This mechanism is generally consistent with that suggested by Ritter (1994), who studied 1,1,2-trichlorotrifluoroethane (CCl 2 -FCClF 2 , CFC 113) decomposition in H 2 over the temperature range of 923-1098 K and at a residence time of 2 s. In this investigation, the onset of defluorination temperature occurs at 973 K, which is 100 K higher than that observed by de Lijser et al The reaction of CBrF 3 and H 2 in a shock tube was investigated at 1000-1600 K and 1.2-2.6 atm by Hidaka et al (1993).…”

Gas-Phase Reaction of Halon 1211 (CBrClF2) with Methane

“…As the temperature increases and HBr production increases, there will be hydrogen transfer competition between CH 4 and HBr. With a standard enthalpy of formation of −38 kJ/mol, reaction (R5) plays a significant role in producing CHClF 2 . In addition to (R4), reaction (R6) also contributes toward the production of CH 3 Br…”

Gas-Phase Hydrogenolysis of Benzene and Derivatives at Elevated Pressure; Methane Formation

“…RX ϩ H· Ǟ RH ϩ X· (1) Important examples of such recalcitrant RX moieties include (poly)halogenated methanes, [3] benzenes [4] and biphenyls. [5] Under the reducing conditions used, the formation of chlorinated dioxins is not possible.…”