Ind. Eng. Chem. Res. 2001, 40, 3139-3143
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Gas-Phase Reaction of Halon 1211 (CBrClF2) with Methane Richard Tran, Eric M. Kennedy,* and Bogdan Z. Dlugogorski Industrial Safety & Environment Protection Group, Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia
The nonoxidative gas-phase reaction of halon 1211 (CBrClF2) with methane was studied using a tubular plug-flow alumina reactor at atmospheric pressure, over the temperature range of 673-1073 K, and at residence times between 0.1 and 1.3 s. With an equimolar feed of CBrClF2 and CH4, complete halon conversion was achieved at 1073 K for all residence times considered. The initial products of the reaction are CHClF2 and CH3Br, which are replaced by C2H2F2 at elevated temperatures. We suggest C2H2F2 is produced from the direct coupling of CH3 and CF2Cl radicals, which rapidly decompose to C2H2F2 and HCl. Minor products formed during reaction include C2H3F, CHF3, C2F4, CHBrF2, and C2HBrF2. The formation of CHClF2, C2F4, CHBrF2, and C2HClF2 was observed to reach a maximium at specific residence times, and formation of soot was detected above 943 K. Possible mechanistic pathways for major and some minor species are discussed. Introduction Contribution of bromine- and chlorine-containing compounds to stratospheric ozone depletion has given rise to international regulations controlling the production and use of halon 1211 (CBrClF2), halon 1301 (CBrF3), halon 2402 (C2Br2F4), CFCs, and other ozonedepleting substances (ODS). These compounds contain bromine and/or chlorine and have atmospheric lifetimes long enough to allow them to diffuse to the stratosphere, where they are dissociated by ultraviolet radiation. The dissociation process causes the release of bromine and chlorine atoms, which are catalytically involved in the destruction of ozone. It is estimated that 1 molecule of chlorine can decompose over 100 000 molecules of ozone before being removed and that a bromine atom is 40100 times more effective than a chlorine atom in destroying ozone.1,2 Most gases regulated by the Montreal Protocol have shown a significant decrease in their net emission rates, although exceptionally the emission of halon 1211 has remained rather constant.3,4 Indeed, use of halon 1211 is still permitted but only for specific, mission-critical applications such as flight-line, landcraft, and aviation fire protection or rescue operations. This suggests that halon 1211’s emissions continue despite the ban on its production, as well as the general availability and acceptance of halon 1211 alternatives.5 With its current emission rate, halon 1211 poses a more significant ozone destruction capability than any other halocarbon.6 In recent years, thermal hydrodehalogenation (THD) has emerged as a promising halon treatment process, in which halons react with hydrogen at elevated temperatures. de Lijser et al.7 studied the THD of halon 1211 (CBrClF2) and found that the conversion of CBrClF2 commences at approximately 673 K, with the relatively weak CClF2-Br bond being homolytically broken, followed by hydrogenation reaction of the CClF2 radical with hydrogen to yield CHClF2. At higher temperatures, * To whom correspondence should be addressed. Phone: +61 2 4921-6177. Fax: +61 2 4921-6920. E-mail:
[email protected].
other products are also produced, such as CHBrF2 and CH2F2. At a temperature of 873 K, where complete conversion of halon is achieved, the methane yield starts to increase significantly. We have reported the THD of halon 1301 at atmospheric pressure.8 In these studies it was found that hydrogen has a marked influence on the conversion level of halon 1301 and that the conversion level of both halon 1301 and H2 increases with temperature and residence time. Similar to de Lijser’s analysis, the initiation reaction of THD of halon 1301 also involves the rupture of the C-Br bond, which generates the hydrogenation reaction between the CF3 radical and bath gas H2 to produce CHF3 and a hydrogen radical. Subsequently, debromination occurs when CBrF3 is reacted with H radical, resulting in the formation of a CF3 radical and HBr. This result is also in agreement with the shock tube study of the CF3Br-H2 reaction by Hidaka et al.9 and the decomposition of CCl2F-CClF2 in H2 as reported by Ritter.10 Currently, hydrodehalogenation with methane is being investigated as a potential treatment process for halons. Both the gas-phase11 and catalytic12 reactions of halon 1301 with methane have been reported. Both studies showed that methane substantially increases the conversion level of CBrF3, resulting in a range of hydrofluorocarbon products, although in both cases methane acts solely as a source of hydrogen, and little or no evidence of incorporation of carbon from methane in reaction products was observed. Here we report the gas-phase reaction of halon 1211 using methane as the reactant. Methane is available at a relatively low cost in the form of natural gas and is an excellent hydrogen source because one molecule of methane can offer more than one hydrogen atom for reaction. Experimental Section The experimental apparatus used in these studies has been described in detail elsewhere.8 A high-purity (99.99%) alumina tube (inner diameter: 0.68 cm) was chosen as the reactor, and a three-zone electrically heated furnace, surrounding the alumina reactor, was
10.1021/ie000920q CCC: $20.00 © 2001 American Chemical Society Published on Web 06/12/2001
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used to ensure an isothermal reaction environment (within 5 K) throughout the reactor. Sheathed type K thermocouple probes placed axially in the reactor tube measured the gas inlet and exit temperatures. Alumina surface reactions were investigated and found to be unimportant, as concluded by Li et al.8 for hydrodehalogenation of CBrF3. CBrClF2 with a purity of 99.4%, CH4 (99.97% purity), and N2 (BOC Gases, 99.999% purity) gas flows were metered using calibrated electronic mass flow controllers. The reactor exit was directed through a liquid trap operated at 273 K and then through a caustic scrubber (0.1 M NaOH) to remove acid productssHBr, HCl, and HFsat room temperature. The remaining gas streamflowed directly to an online MTI micro gas chromatograph (M200H GC) equipped with dual thermal conductivity detectors. Products were quantified using a 5A molecular sieve column operated at 50 °C and a Poraplot U column operated at 110 °C. Helium was used as the carrier gas, and the sample time, injection time, and run time were 20 s, 50 ms, and 160 s, respectively. For product identification, a gas chromatographmass spectrometer (GCMS-QP5000, Shimadzu) equipped with an AT-Q column (30 m × 0.32 mm) was used, operating with a temperature ramp of 60-230 °C, at 10 °C/min and using ultrahigh-purity helium (BOC Gases, 99.999% purity) as the carrier gas. The composition of the feed mixture was analyzed using the bypass line. Where possible, response factors were determined using gas standards (CH3Br, CF3H, and CHClF2). In other cases, relative molar response factor (RMR) values were estimated using the procedure of Height et al.,13 based on the following equation:
RMRi )
[
σi - σI σφ + σI
][ 2
]
Mi - MI Mi + MI
0.50
× 100
(1)
where σ is the collision diameter (Å), given by equation14
σ ) 110.2423(Tc/Pc)
1/3
(2)
In eqs 1 and 2, M is molecular weight, Tc is critical temperature (K), Pc is critical pressure (Pa), and the subscripts i, I, and φ refer to the species under consideration, the carrier gas, and benzene.15,16 The factor of 100 (units/mol) represents the arbitrary response of benzene as an internal standard. Experiments were performed at atmospheric pressure, over the range of temperatures between 673 and 1073 K, with residence time of 0.1, 0.4, and 1.3 s. Residence times were controlled by adjusting either the gas flow rate or the reactor volume. Results and Discussion The standard volumetric feed ratio of N2:CBrClF2:CH4 used throughout these experiments was 10:1:1. Figures 1 and 2 show the conversion of CBrClF2 and CH4, which began at about 773 K and increased rapidly over the temperature range of 823-973 K. As expected, reactant conversion increased with higher temperature and longer residence times. Complete conversion of CBrClF2 is observed at 1073 K for all residence times studied. At lower temperatures, there is little difference between the conversion levels of CBrClF2 and CH4. However, at higher temperatures,
Figure 1. CBrClF2 conversion as a function of temperature at various residence times.
Figure 2. CH4 conversion as a function of temperature at various residence times.
the conversion of CBrClF2 is significantly higher than that of CH4. At higher conversion levels of CBrClF2, the fractional conversion of CH4 is about 0.56 times the conversion of CBrClF2. Less CH4 is required to react with CBrClF2 because more than one C-H bond in methane can contribute to the reaction, allowing one molecule of CH4 to donate more than one hydrogen atom in the reaction. Mechanistic Considerations It is desirable to compare the experimental results obtained with reaction profiles predicted from detailed modeling, as has been performed for the reaction of H2 and CH4 with halon 1301. Unfortunately, the lack of a detailed reaction mechanism for halon 1211 precludes such a comparison. We can, however, suggest possible mechanistic pathways to explain the conversion and selectivity characteristics observed experimentally. Among the many possible processes that a radical can be initiated, (R1) is the most plausible initiation step, at least at low temperatures. The relatively weak CClF2-Br bond is homolytically cleaved, producing two reactive radicals.17 When these reactive radicals collide with a methane molecule via reactions (R2) and (R3), they abstract a hydrogen atom to produce CHClF2, HBr, and CH3 radicals. The methyl radical reacts further with CBrClF2 in the propagation step to produce CH3-
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energies of C-Br (293 kJ/mol) and C-Cl (351 kJ/mol) suggest that these products may undergo thermal decomposition or further reaction with other species at higher temperature. We have observed a similar trend when studying the reaction of halon 1301 with methane, where the production of CH3Br increased with temperature until a maximum concentration was observed; in the case of CH4 with 1301, that temperature was also approximately 970 K.11 Possible reactions resulting in the decomposition of CHClF2 are as follows:
Figure 3. Concentration of major carbon-containing species as a function of reaction temperature at 0.4 s residence time.
Br and a CClF2 radical. It is this cycle mechanism, (R2)-(R4), that is the main reaction pathway for the decomposition of CBrClF2 at low temperatures. If there were no methane molecule present, steps (R2)-(R4) would not occur, resulting in a net lower conversion level of CBrClF2.
CBrClF2 f CClF2 + Br
(R1)
CClF2 + CH4 f CHClF2 + CH3
(R2)
Br + CH4 f HBr + CH3
(R3)
CBrClF2 + CH3 f CH3Br + CClF2
(R4)
Reaction Selectivity
CHClF2 f CHF2 + Cl
(R7)
CHClF2 f CF2: + HCl
(R8)
CHClF2 f CHClF + F
(R9)
CHClF2 f CClF: + HF
(R10)
CHClF2 + H f CHF2 + HCl
(R11)
CHClF2 + Cl f CHF2 + Cl2
(R12)
CHClF2 + Cl f CClF2 + HCl
(R13)
CHClF2 + Br f CClF2 + HBr
(R14)
2CHClF2 f CHCl2F + CHF3
(R15)
Possible reactions for the decomposition of CH3Br, as suggested previously,11 are
CH3Br f CH3 + Br
(R16)
CH3Br + H f CH3 + HBr
(R17)
CH3Br + Br f CH2Br + HBr
(R18)
CH3Br + Br f CH3 + Br2
(R19)
The major carbon-containing products of the reaction are CHClF2, CH3Br, and C2H2F2. Minor species include C2H3F, CHF3, C2F4, C3H2F6, CHBrF2, C2HBrF2, C2HF3, C2H2F4, CCl2F2, C2HClF2, C2Cl2F4, C2H3F3, C3F6, C3H3F5, C2H4Cl2, C2H3BrF2, C2H2ClF, and C2H3ClF. Trace amounts of C6H3BrF2, C6H2F4, and C6H4BrF are detected at high temperature, and soot formation begins at approximately 943 K. Figure 3 portrays the major products distribution as a function of temperature at 0.4 s residence time. At low temperatures, CHClF2 and CH3Br are the two major products; however, their concentrations start to decline as they reach the maximum production rate at 943 and 973 K, respectively, at which time the C2H2F2 concentration increases and becomes the major reaction product. Reaction (R2) is considered to be the primary step in the formation of CHClF2 at low temperature because the primary hydrogen donor available under these conditions is CH4. As the temperature increases and HBr production increases, there will be hydrogen transfer competition between CH4 and HBr. With a standard enthalpy of formation of -38 kJ/mol,7 reaction (R5) plays a significant role in producing CHClF2. In addition to (R4), reaction (R6) also contributes toward the production of CH3Br. The relatively low dissociation
Note that hydrogen atoms can be abstracted from hydrogen donors such as CH4 or HX (X ) Br, Cl, F). In the consumption of CH3Br, (R16) and (R17) are believed to be of primary importance during high-temperature decomposition of this species. In their study of the inhibiting effect of CBrF3 on methane flames at 1070 K and 0.5 s residence time, Battin-Leclerc et al.18 reported that 72% of CH3Br was consumed via reaction (R16) and 24% via reaction (R17). According to Biordi et al.,19 (R18) plays a minor role in the decomposition of CH3Br. Reaction (R19) is also believed to be insignificant because its reverse reaction is relatively fast.20 The yield of C2H2F2 increases over the temperature range studied, dominating as major product species at high temperatures. There are a number of pathways that could lead to the formation of this species. As proposed by Biordi et al.19 and Battin-Leclerc et al.,18 the formation of C2H2F2 was a two-step reaction involving CH3 and CF3 radicals.
CClF2 + HBr f CHClF2 + Br
(R5)
CF3 + CH3 f CF3CH3
(R20)
CH3 + Br f CH3Br
(R6)
CF3CH3 f CF2CH2 + HF
(R21)
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Figure 4. Minor product selectivity (0-9%) as a function of reaction temperature at 0.4 s residence time. Figure 6. Minor product selectivity (0-0.4%) as a function of reaction temperature at 0.4 s residence time.
the production of C2H3F, HCl, and HF as illustrated by reactions (R24) and (R25). Furthermore, we suggest that
CHClF2 f CF2: + HCl
(R24)
CF2: + CH4 f C2H3F + HF
(R25)
the major pathway to the formation of CHBrF2 is through the elimination of HCl from CHClF2. The resulting difluorocarbene, CF2:, will then combine with HBr to form CHBrF2, as shown in (R26).7 CHBrF2
CF2: + HBr f CHBrF2 Figure 5. Minor product selectivity (0-1.6%) as a function of reaction temperature at 0.4 s residence time.
The above reaction scheme also indicates that CF3CH3 would be a byproduct, which indeed is consistent with our results, although only a very small amount of CF3CH3 was detected at high temperatures.
CClF2 + CH3 f [CClF2CH3]
(R22)
[CClF2CH3] f C2H2F2 + HCl
(R23)
Reactions (R22) and (R23) are likely to lead to the production of C2H2F2, because CClF2 and CH3 radicals are reactive and available in relatively high concentrations at higher temperatures. We suggest that this is the primary pathway for production of C2H2F2 and represents the direct coupling of major radial species produced for both of the major feed species. Figures 4-6 show the variation of other products detected during the reaction as a function of temperature at a residence time of 0.4 s. These species include C2H3F, CHF3, C2F4, C3H2F6, CHBrF2, C2HBrF2, C2HF3, C2H2F4, CCl2F2, C2HClF2, C2Cl2F4, C2H3F3, C3F6, C3H3F5, C2H4Cl2, C2H3BrF2, C2H2ClF, and C2H3ClF2. Selectivity is defined in terms of carbon-containing products, normalized for carbon number. With increasing temperature from 943 K, the concentration of CHClF2 decreases (Figure 4) with subsequent production of a range of C2 hydrofluorocarbon species. We suggest CHClF2 could act as a source for
(R26)
production appears to commence at 823 K and reaches its maximum at 973 K. Because the homolysis of the C-Cl bond in CBrClF2 is negligibly slow at low temperature [reaction (R27)], it is likely that the concentration of CBrF2 is low. For this reason, we believe reaction (R28) plays a minor role in the production of CHBrF2.
CBrClF2 f CBrF2 + Cl
(R27)
CBrF2 + HX f CHBrF2 + X
(R28)
Difluorocarbene could also undergo a series of reactions, generating other minor species.
CF2: + HF f CHF3
(R29)
2CF2: f C2F4
(R30)
CF2: + CH3 f C2H3F2
(R31)
C2H3F2 + Br f C2H3BrF2
(R32)
C2H3F2 + Cl f C2H3ClF2
(R33)
The appearance of C2HF3, C2H2F4, and C2H3F3 was detected above 943 K and was found to increase rapidly at higher temperatures. It is possible that C2HF3 was formed via reaction (R35) from the combination of CF2: and the diradical CHF:. With respect to the formation of C2H2F4, de Lijser,7 through the study of hydrodehalogenation of CBrClF2, suggested that C2H2F4 is formed through the coupling of CHF2 radicals, as illustrated in reaction (R36), and that C2H3F3 be formed from CHF2/CH2F radical coupling, as in reaction (R37). It is
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CF2: + H f CHF2
(R34)
Acknowledgment
CF2: + CHF: f C2HF3
(R35)
The authors thank the Australia Research Council for financial support of this project.
2CHF2 f C2H2F4
(R36)
CHF2 + CH2F f C2H3F3
(R37)
unlikely that reaction (R36) took place to any great extent in our experiments, because it implies that the most likely product’s conformation is CHF2-CHF2 or 1,1,2,2-tetrafluoroethane. Because the 1,1,1,2-tetrafluoroethane isomer was the primary isomer produced in these experiments, this suggests (R38) is the major contributing pathway to the formation of 1,1,1,2-tetrafluoroethane, consistent with the study made by Furutaka and Sei.21 We also suggest that C2H3F3 is
CF2CH2 + F2 f CF3CH2F
(R38)
formed through reaction (R20), because it has the 1,1,1trifluoroethane confirmation and not the 1,1,2-trifluoroethane (CHF2-CH2F) conformation, which would be produced if reaction (R37) were to take place. Pathways involving other radical interaction produce a number of chlorinated minor species.
CClF2 + Cl f CCl2F2
(R39)
CClF2 + CClF2 f C2Cl2F4
(R40)
2CH2Cl f C2H4Cl2
(R41)
Soot formation was detected above 943 K. This observation is consistent with our carbon mass balance result, which decreased from 99% ((2% error) at 943 K to approximately 87% at 1073 K and 1.3 s residence time. The produced bromine and chlorine radicals significantly suppress the concentration of ethane, increase surface growth processes, and subsequently enhance soot formation. Trace amounts of CH2BrCl are detected (
