Environ. Sci. Technol. 2005, 39, 3020-3028
Experimental and Computational Studies of the Gas-Phase Reaction of Halon 1211 with Hydrogen H A I Y U , † E R I C M . K E N N E D Y , * ,† MD. AZHAR UDDIN,‡ SIMON P. SULLIVAN,§ AND BOGDAN Z. DLUGOGORSKI† Process Safety and Environment Protection Research Group, School of Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia, and Department of Chemical Engineering, University of Cambridge New Museums Site, Pembroke Street Cambridge, CB2 3RA, U.K.
The gas-phase reaction of halon 1211 (CBrClF2) with hydrogen has been studied experimentally at atmospheric pressure in a plug flow, isothermal reactor over the temperature range of 673 to 973 K, at residence times ranging from 0.5 to 2.5 s with an input ratio of N2:H2:halon 1211 of 19:10:1. The major carbon containing products include CHClF2, CHBrF2, CH2F2, and CH4. Gas-phase reactions of CHClF2, CCl2F2, and CH2F2 with hydrogen are also investigated under the conditions similar to those for halon 1211 hydrodehalogenation, and the results are used to assist in understanding the mechanism of the reaction of halon 1211 with hydrogen. A kinetic reaction scheme involving 90 species and 430 reaction steps is developed and used to model the halon 1211 hydrodehalogenation reaction. Generally, satisfactory agreement between experimental and computational results is obtained for the production of major species. Using the software package AURORA, the reaction pathways leading to the formation of major products are elucidated. It has been found that the reaction steps involving CF2 are responsible for the formation of CH4.
Introduction Halons 1211 (CBrClF2) and 1301 (CBrF3) are highly effective fire suppressants. However, halons and chlorofluorocarbons (CFCs) have been identified as potent ozone depleting substances (1-4), and the production of halons and CFCs has been closely regulated. In accordance with the Montreal Protocol and its later amendments (5), the production of halons in developed countries ended in 2001. For developing countries, however, it will be 2010 until the phaseout of production of halons is complete. Possibly due to the continued use of halon 1211, it has recently been reported that the concentration of halon 1211 in the stratosphere is increasing despite a reported decrease in the concentration * Corresponding author phone: +61 2 4921 6177; fax: +61 2 4921 6920; e-mail:
[email protected]. † The University of Newcastle. ‡ The University of New South Wales. § University of Cambridge New Museums Site. 3020
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of CFCs (6, 7). Therefore, it is of practical significance to reduce the emission of these ozone depleting substances, to search for suitable replacements and ultimately to develop safe and economic methods for treatment and disposal of stockpiled halons. Hydrodehalogenation is a promising nondestructive treatment process in which ozone-depleting bromine and chlorine atoms are removed from the parent halon or CFC. Fluorine atoms may or may not be retained in the final product, depending on reaction conditions. The resulting products are usually a mixture of perfluorocarbons (PFCs), hydrofluorocarbons (HFCs) and hydrocarbons. HFCs can be used as replacements for CFCs and halons or as precursors for the production of other chemicals. Gas-phase reactions of halon 1301 with hydrogen, CH4 and C3H8 have been investigated recently by Li et al. (8, 9) and Yu et al. (10). They have reported that hydrodehalogenation of halon 1301 with H2 or CH4 is a promising process for production of CHF3, which can be used as the precursor for production of CF3I (11), a replacement for halon 1301. Conversely, relatively little research on hydrodehalogenation of halon 1211 has been reported in the literature. Tran et al. (12) and Uddin et al. (13) have recently studied the gas-phase reaction of halon 1211 with CH4 over the temperature range of 773-1173 K. A single pass yield of 63% C2H2F2 was achieved at 1173 K for the feed composition of halon 1211:CH4 ) 1:2 (13). Other minor species formed at elevated temperatures include several C1-C3 halocarbons. C2H2F2 is widely used in the fluoroelastomer and semiconductor industries. de Lijser et al. (14) investigated the hydrodehalogenation of halon 1211 in a large excess of hydrogen (hydrogen/halon 1211 molar ratio in the feed is ca. 10) over the temperature range of 673 to 1173 K with residence times of 2-3 s. At temperatures below 863 K, the dominant carbon containing product is CHClF2. Although yield of CH2F2, a desirable product, increases with temperature, the maximum yield obtained is only ca. 30% due to the formation of a large amount of CH4 and various other products. In a previous study, we investigated the gas-phase thermal decomposition of halon 1211, and a mechanism for lowtemperature reactions was proposed (15). In this study, we investigate the gas-phase hydrodehalogenation of halon 1211 with hydrogen both experimentally and computationally. The purpose of this investigation is to develop a detailed chemical kinetic mechanism for the gas-phase hydrodehalogenation of halon 1211 with H2, which, in turn, will facilitate the development of a process for the conversion of halon 1211 into CH2F2 or CHClF2. CH2F2 can be used as halon/CFC replacement in refrigeration applications while CHClF2 for the production of C2F4. In addition, a detailed reaction mechanism for the reaction of halon 1211 with H2 can greatly facilitate the development of a mechanism for hydrodehalogenation of halon 1211 with CH4, a more complicated system in which a high yield of C2H2F2 is obtained. Recently, catalytic hydrodehalogenation of CFCs and halons has drawn intensive research interest because the reaction takes place at relatively low temperatures and gives high selectivity to products which are environmentally benign and useful (16, 17). However it is still challenging to find a suitable catalyst which could maintain both high activity and stability under the highly corrosive conditions. Pd based catalysts are studied, which are prohibitively expensive. It could be more economically feasible to develop a process that does not use catalysts, which is based on the gas-phase reaction, even if the required temperatures for the reactions are higher than 10.1021/es049372o CCC: $30.25
2005 American Chemical Society Published on Web 03/18/2005
those for the Pd-catalyzed reactions. To this end, it is of practical significance to investigate the thermal gas-phase hydrodehalogenation with H2 or CH4 as an alternative option for the catalytic conversion of halons and CFCs.
Experimental Section The experimental facility used in this study has been described in detail elsewhere (17). Briefly, the apparatus consists of a tubular high purity (99.99%) alumina reactor (i.d. 6.8 mm) with the reactor zone maintained uniform by a three zone furnace. Carbon containing species were identified with a GC/MS (Shimadzu QP5000) equipped with an AT-Q column and quantified with a micro GC (Varian CP-2003) equipped with molecular sieve 5A and PoraPLOT Q columns. Relative molar response (RMR) factors of hydrocarbons for TCD detection were experimentally obtained from standard gas mixtures, and quantification of halogenated hydrocarbons was performed with a diluted mixture of halon 1211, CCl2F2, CHClF2, CH3Cl, CH3Br, and CH2F2 in nitrogen. For other species, RMR values were estimated from published correlations (18). Hydrogen halides (HF, HCl, and HBr) formed during reaction were trapped with 0.1 M NaOH solution, and the concentration of halides was determined with an ion chromatograph (IC) (Dionex100) equipped with an IonPac AS14A column (4 × 250 mm). For reaction of halon 1211 with hydrogen, experiments were performed over the temperature range of 673 to 973 K at atmospheric pressure with an input ratio of N2:H2:halon 1211 of 19:10:1. At each temperature, a series of 5 residence times (0.5, 1, 1.5, 2, and 2.5 s) was examined with the residence time changed by adjusting flowrates of gas mixture while maintaining the composition of mixture constant. The effect of the hydrogen/halon 1211 ratio on the hydrodehalogenation reaction was investigated at 873 K, and a fixed residence time of 2.5 s with an input ratio of hydrogen/halon 1211 varying from 1 to 10 while the concentration of halon 1211 was kept constant. Reactions of CHClF2, CCl2F2, and CH2F2 with hydrogen were also performed over the temperature range of 673-1073 K at a constant residence time of 2.5 s and an input ratio of N2:H2:halocarbon (CHClF2, CCl2F2, or CH2F2) of 19:10:1. The surface of the reactor (R-alumina) was found to have a negligible effect on the hydrodehalogenation of halon 1211 under the conditions investigated, and thus only gas-phase kinetics are considered in this study.
Chemical Kinetic Modeling The gas-phase hydrodehalogenation of CBrClF2 with hydrogen in the tube reactor was modeled using a computer code PLUG (19), which is a Fortran-based computer program for the analysis of plug flow reactors with gas-phase and surface chemistry. The reaction pathway analysis for hydrodehalogenation of halon 1211 was conducted by using a software package SENKIN (20). SENKIN is a Fortran program for predicting homogeneous gas-phase chemical kinetics with sensitivity analysis and is currently incorporated into software package AURORA (21) in Chemkin Collection 3.7.1.
Results and Discussion Figure 1 shows the variation of halon 1211 conversion with temperature at various residence times with an input ratio of N2:H2:halon 1211 of 19:10:1. As expected, an increase in either temperature or residence time results in a higher conversion level of halon 1211. The hydrodehalogenation reaction starts at 723 K at a residence time of 0.5 s, while complete conversion is obtained at 923 K and a residence time of 2.5 s.
FIGURE 1. Conversion level of halon 1211 as a function of temperature at various residence times with an input ratio of N2:H2:halon 1211 of 19:10:1. Under the conditions studied, the major carbon containing products include CHClF2, CHBrF2, CH2F2, and CH4. Minor species are detected at high temperatures, including CH3F, CH3Cl, CH3Br, CHF3, C2F4, CHF2CHF2, CH2FCF3, CHF2CH2F, C2H4, and C2H6. Fully halogenated species such as CCl2F2, CBr2F2, and C2Cl2F4 are also detected but only in trace amounts. Hydrogen halides (HBr, HCl, and HF) are also detected and quantified by IC. At temperatures below 893 K, CHClF2 is the dominant carbon containing product with a selectivity of over 80%. CHBrF2 is also detected in much smaller amounts, and the yield of CHBrF2 follows a similar trend to that of CHClF2 and increases with temperature. As shown in Figure 2, the yields of CHClF2 and CHBrF2 increase with temperature and reach maximum values at approximately 873 K. Above this temperature, the yield of both species decrease considerably with temperature especially for CHClF2, while the amounts of CH2F2 and CH4 formed increase dramatically. Although the yield of CH2F2 increases with temperature, the maximum yield of CH2F2 obtained experimentally is less than 40% (not shown in Figure 2). Increasing the reaction temperatures higher does not lead to an increase in the yield of CH2F2. It was found, however, that increasing the temperature further dramatically enhanced the formation of CH4, C2H6 and a variety of halogenated hydrocarbons. The formation of a large amount of carbonaceous species was observed on the surface of the reactor, which resulted in carbon balances of becoming far from 100% (see Table S1, Supporting Information). Similar results were obtained by de Lijser et al. in their investigation on the hydrodehalogenation of halon 1211 with hydrogen under comparable experimental conditions (14). The maximum yield of CH2F2 in their study, obtained at 973 K, was less than 30%. Figure 3 presents the halogen removal efficiency as a function of temperature under the same conditions as specified in Figure 2. Halogen removal efficiency is defined as the ratio of the amount of halogen trapped in the caustic solution (per unit time) to the amount of halogen in the feed (per unit time). Within the temperature range investigated, the amount of halogen trapped in the solution decreases in the following order
Br > Cl > F which is consistent with the bond strength in the parent molecule (halon 1211). The effect of hydrogen/halon 1211 input ratio and residence time on the conversion of halon 1211 and selectivity to major products are also investigated, and the results are VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Unreacted halon 1211 and yield of major carbon containing products as a function of temperature at a residence time of 2.5 s and with an input ratio of N2:H2:halon 1211 of 19:10:1. Unreacted halon 1211 is expressed as the ratio of the number of moles of halon 1211 (per unit time) at the outlet to the number of moles of halon 1211 (per unit time) in the feed.
FIGURE 4. Conversion level of halon 1211 and selectivity to major products as a function of the hydrogen/halon 1211 ratio at 873 K and a residence time of 2.5 s. in the reaction system, in which the bond dissociation energy decreases in the following order (22-24):
C-F (490 kJ mol-1 in CCl2F2) >
H-H (435 kJ mol-1) > C-Cl (326 kJ mol-1) > C-Br (266 kJ mol-1) CBrClF2 f CClF2 + Br (R1)
The resultant CClF2 and Br radicals either recombine (R2) and (R3) or react with the parent molecule halon 1211 or hydrogen (R4-R7).
FIGURE 3. Halogen removal efficiency as a function of temperature at a residence time of 2.5 s and with an input ratio of N2:H2:halon 1211 of 19:10:1.
presented in Figure 4 and the Supporting Information (Figure S1), respectively. With an increase in the hydrogen/halon 1211 ratio, the halon 1211 conversion level increases dramatically. Selectivity to CHClF2, CH4, and CH2F2 increases slightly, while selectivity to CHBrF2 decreases. With an increase in residence time, the conversion level of halon 1211 and selectivity to CHBrF2, CH2F2, and CH4 increases slightly, while selectivity of CHClF2 decreased marginally. Mechanistic Analysis. The initiation step of halon 1211 hydrogenation is suggested to be the homolytic cleavage of C-Br bonds (R1) because the C-Br bond is the weakest bond 3022
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CClF2 + CClF2 f CClF2CClF2
(R2)
Br +Br f Br2
(R3)
CClF2 + CBrClF2 f CCl2F2 + CBrF2
(R4)
Br + CBrClF2 f Br2 + CClF2
(R5)
CClF2 + H2 f CHClF2 + H
(R6)
Br + H2 f HBr + H
(R7)
The energy barriers for recombination of these radicals (R2) and (R3) are close to zero, but the very low concentration of these radicals makes these reactions take place so slowly that the amount of CClF2CClF2 and Br2 formed is expected to be minimal. In our previous study of the pyrolysis of halon 1211 (15), we proposed that, following the homolytic cleavage of C-Br bond in halon 1211, the resultant radicals CClF2 and Br will react with halon 1211 and act as the chain propagating steps (R4 and R5). The reaction (R4) is the primary pathway
for the consumption of halon 1211 as well as for the formation of CCl2F2, one of the major carbon containing products during the pyrolysis of halon 1211. In the absence of hydrogen, the decomposition of halon 1211 starts at 773 K, and the conversion level of halon 1211 is about 0.5% at a residence time of 2.5 s with an input ratio of N2:halon 1211 of 19:1. In the presence of hydrogen (H2/ halon 1211 ) 10), the reaction starts at lower temperatures, and a halon 1211 conversion level of 9.4% is observed at 773 K and a residence time of 2.5 s, which illustrates that introduction of hydrogen to the reaction system considerably facilitates the decomposition of halon 1211. This activating effect of the hydrogen is also highlighted by the observed effect of hydrogen/halon 1211 ratio on the hydrodehalogenation (see Figure 4). Increasing the ratio of hydrogen/halon 1211 dramatically increases the conversion level of halon 1211, which can be explained by comparing the reaction rate constants of the proposed initiation steps during the reaction of halon 1211 with hydrogen (see Table S2, Supporting Information). The initiation steps involving H2 or H radicals take place much faster than reactions where no H2 or H is involved, which is consistent with the observation that predominantly hydrogenated species are produced and fully halogenated species appear only in trace amounts. Moreover, once hydrogen radicals are generated via reactions R6 and R7, they abstract Br from halon 1211 and regenerate CClF2 (R8).
CBrClF2 + H f HBr + CClF2
(R8)
The reaction R8 takes place even faster than (R4) and (R5) and thus is expected to be the major step for the decomposition of halon 1211, as suggested by de Lijser et al. (14). Indeed, CClF2, produced from (R8), initiates radical chain reactions via (R6) and (R8), thus accelerating the decomposition of halon 1211. At low temperatures, the primary pathway for the formation of CHClF2 is via reaction R6. In addition to hydrogen, other species including hydrogen radical, HBr and HCl could react with CClF2 as hydrogen donors to produce CHClF2.
CClF2 + H/HBr/HCl f CHClF2 + Br/Cl
(R9)
HBr is believed to be a more effective hydrogen donor than H2 and HCl (14) and is produced in a large quantity especially at high temperatures. Thus we suggest that the reaction of CClF2 with HBr is the primary pathway for formation of CHClF2, at high temperatures. The formation of CHBrF2, another important product, may follow reaction pathways R10 and R11 which are similar to those proposed for the formation to CHClF2.
CBrClF2 f CBrF2 + Cl
(R13)
CF2 + HBr f CHBrF2
(R14)
Although we agree with de Lijser et al.’s conclusion, we suggest that both reaction pathways play a role in the formation of CHBrF2. At high temperatures in which concentration of CHClF2 and HBr is high, reactions R13 and R14 play a more important role, while at low temperatures, these steps are less important than the reaction pathway via reactions R12 and R11 due to the low concentration of CHClF2 and HBr and the high activation energy (221 kJ mol-1) for reaction R13. In a practical sense, it is desirable to obtain as high a selectivity to CH2F2 as possible, as it is a product of some economic value. The formation of CH2F2 is suggested, in ref 14, to proceed via the abstraction of Cl and Br from CHClF2 and CHBrF2, but no further discussion of the reaction pathway was made. We suggest that CH2F2 could be formed via the following two pathways: 1. Single-step reaction pathway (25);
CF2 + H2 f CH2F2
(R15)
2. Two-step reaction pathway: CHF2 is formed first, followed by the reactions of CHF2 with H, HBr, HCl or other hydrogen donors to produce CH2F2. The most straightforward reaction pathway to CH2F2 formation is via reaction R15, the insertion of H2 (present in a large concentration), into CF2, which primarily arises from the molecular dissociation of CHClF2 (R13). The activation energy for R15 is estimated to be between 143 and 171.2 kJ mol-1 theoretically (25, 26), which is much higher than those for the self-combination of CF2 or abstraction Br/Cl from CHBrF2/CHClF2 by H radicals, to be discussed later. It is expected that reaction R15 only plays a minor role in the formation of CH2F2. To validate this experimentally, separate experiments on the reaction of CHClF2 with hydrogen were carried out under conditions similar to those for the reaction of halon 1211 with hydrogen. The thermal pyrolysis of CHClF2 has been investigated by many authors (27-30), and it is generally accepted that CHClF2 undergoes predominantly molecular dissociation at elevated temperatures, producing CF2 and HCl, in preference to the homolytic cleavage of the C-Cl bond in CHClF2. The subsequent self-combination of CF2 (R16) produces C2F4 which is the dominant species observed experimentally:
CHClF2 f CF2 + HCl
(R13)
CF2 + CF2 f C2F4
(R16)
(R10)
CBrF2 + H/H2/HBr/HCl f CHBrF2 + H/Br/Cl (R11) Due to the high energy barrier for cleavage of the C-Cl bond in halon 1211, the amount of CBrF2 produced via reaction R10 is negligible, at least at low temperatures. One alternative pathway for the formation of CBrF2 is via the abstraction of Cl from halon 1211 by H radicals (R12):
H + CBrClF2 f HCl + CBrF2
CHClF2 f CF2 + HCl
(R12)
This step has a much lower activation energy and takes place much faster than the thermal cleavage of the C-Cl bond (see Table S2, Supporting Information) and so is expected to play a more important role in the production of CBrF2. However, de Lijser et al. (14) concluded that CHBrF2 is formed primarily via the following two steps:
In the presence of a large excess of hydrogen (H2/CHClF2 ) 10:1), a high selectivity to CH2F2 would be expected if the reaction R15 proceeded at a significant rate. However, as shown in Figure 5, the major product from the reactions of CHClF2 with hydrogen is C2F4, while selectivity to CH2F2 is very low under all conditions studied. After reaching a maximum value at temperature 923 K, the selectivity to CH2F2 decreases, while selectivity to other species, including CH4 and CHF3 increases. These results indicate that the rate of CF2 self-combination (R16) takes place much faster than that of the reaction of CF2 with hydrogen (R15) which is also consistent with Battin-leclerc et al.’s results (31). They found that, compared to reaction R1b, the reactivity of CF2 with molecular agents, such as H2, is very low. With the high concentration of HBr and HCl in the reacting system, we suggest that the formation of CH2F2 is primarily via the reaction of CHF2 with HBr, HCl and H2. CHF2 is VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Comparison of CH4/CH2F2 Selectivity Ratio and Mass Balances during Reactions of CHClF2, CCl2F2, and CBrClF2 with H2 reactant
CHClF2
CCl2F2
CBrClF2
temperature, K N2:H2:reactant in the feed residence time, s conversion, % selectivity ratio, CH4/CH2F2 C balance, % F balance, % Cl balance, % Br balance, %
973 19:10:1 2.5 72.7 1.2 76.8 83.5 97.4 naa
923 19:10:1 2.5 75.0 0.6 88.7 90.5 96.6 naa
873 19:10:1 2.5 86.4 0.3 97.9 99.4 95.6 94.7
a
FIGURE 5. Conversion level of CHClF2 and selectivity to major products as a function of temperature at a residence time of 2.5 s with an input ratio of N2:H2:CHClF2 of 19:10:1.
FIGURE 6. Conversion level of CCl2F2 and selectivity to major products as a function of temperature at a residence time of 2.5 s with an input ratio of N2:H2:CCl2F2 of 19:10:1. suggested to be formed via the abstraction of Br/Cl from CHBrF2/CHClF2 by H radicals (R17) and (R18):
CHBrF2 + H f CHF2 + HBr
(R17)
CHClF2 + H f CHF2 + HCl
(R18)
To gain a better understanding of how CHF2 is formed, we conducted experiments on the hydrodehalogenation of CCl2F2 with hydrogen, under conditions similar to those for reactions of halon 1211 with hydrogen. The results are presented in Figure 6. Since there is no C-Br bond in CCl2F2 and C-Cl bond is the weakest bond in the feed molecule, the initiation step should be the cleavage of the C-Cl bond in CCl2F2 (32). We then suggest that the reaction follows a reaction mechanism similar to CBrClF2 hydrodehalogenation and initiation steps can be described as follows:
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CCl2F2 f CClF2 + Cl
(R19)
CClF2 + H2 f CHClF2 + H
(R6)
Cl + H2 f HCl + H
(R20)
CHClF2 + H f CHF2 + HCl
(R18)
CHF2 + H/H2/HCl f CH2F2 + H/Cl
(R21)
CHClF2 f CF2 +HCl
(R13)
CF2 + CF2 f C2F4
(R16)
CF2 + H2 f CH2F2
(R15)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 9, 2005
na: not applicable.
Under the conditions studied, the primary carbon containing product is CHClF2. Other major products include CH2F2, CH4, C2F4, and CHF3. Note that the selectivity to C2F4 is quite high (>4%) compared with CBrClF2 hydrodehalogenation, where the observed selectivity to C2F4 is less than 3%. In addition, selectivity to CH2F2 is relatively low, while selectivity to CH4 and CHF3 is relatively high, which we suggest that, in the course of hydrodehalogenation of CBrClF2, CF2 produced from the dissociation of CHClF2 rapidly reacts with HBr to produce CHBrF2. The subsequent abstraction of Br from CHBrF2 by H (R17) plays a more important role in the formation of CHF2 than reaction R18, due to the facile C-Br bond and relative ease of Br abstraction by H radicals, as suggested by the reaction rate constant (see Table S2, Supporting Information). Reaction R18 needs to overcome a higher energy barrier and thus proceeds slowly. So in the case of CCl2F2, with no HBr present in the reacting system, CF2 generated from the molecular dissociation of CHClF2 (R13) is more likely to recombine or undergo F abstraction reactions, which will be explained later in this paper. As a result, the concentration of CHF2 formed via reaction R18 will be lower, and a (relatively) higher selectivity to C2F4 is observed. Compared to the reaction of CBrClF2 and CCl2F2 with H2, the generation of H radicals is unfavored during the reaction of CHClF2 with hydrogen. For the reaction of H2 with CBrClF2 and CCl2F2, the species Br, Cl and CClF2, generated from the initial reactions R1 and R10, then react with H2 via reactions R6, R7, and R20 to produce H radicals. While in the reaction of CHClF2 with hydrogen, such steps leading to the formation of H radicals are not available, and thus the abstraction of Cl from CHClF2 by H radicals (R18) is less unlikely. Instead, CHClF2 predominately undergoes a molecular dissociation to produce CF2 and HCl. CF2 then rapidly recombines to produce C2F4 as the primary product. The relatively high selectivity to CH4 in case of hydrodehalogenation of CHClF2 and CCl2F2 suggests the formation of CH4 should also include reaction steps involving CF2. As shown in Table 1, when the similar conversion level is obtained during the hydrodehalogenation of halon 1211, CHClF2, and CCl2F2, the selectivity ratio of CH4/CH2F2 decreases according to the following order:
CHClF2 > CCl2F2 > CBrClF2 This also corresponds to the order of the decreasing selectivity to C2F4 observed experimentally. The reaction pathway of formation of CH4 will be discussed later in conjunction with reaction pathway analysis. In addition to CH3F, minor species including C2F4, CHF3, and CHF2CHF2 were also observed. As mentioned previously, C2F4 is formed mainly from the self-combination of CF2. CHF2CHF2 is most likely produced from the self-combination of CHF2. Formation of CHF3 necessitates the cleavage of C-F bonds during reaction, and possible pathways include
CF2 + F f CF3
(R22)
CF3 + H/HBr/HCl/H2 f CHF3 + Br/Cl/H (R23) CHF2 + F f CHF3
(R24)
CF2 + HF f CHF3
(R25)
At high temperatures, especially over 923 K, the hydrodehalogenation reaction suffers from severe carbon and fluorine loss, and carbonaceous deposits on the wall of the reactor are observed. One possible reason for the buildup of the carbonaceous products is that carbon and fluorine are lost due to polymerization of CF2, from which species of high molecular weight are formed and then deposited on the surface of the reactor. Our results indicate that, when the comparable conversion level is achieved during the hydrodehalogenation of halon 1211, CCl2F2, and CHClF2 under the similar reaction conditions, carbon and fluorine balances decrease according to the following order (see Table 1):
CBrClF2 > CCl2F2 > CHClF2 This is consistent with selectivity to C2F4 varying in the reverse order. In addition, mass balances of Br and Cl are much closer to 100% than those of C and F (see Table 1) which also supports this explanation. Chemical Kinetic Modeling. The detailed reaction scheme used in this study comprised three reaction schemes: (1) GRI-MECH hydrocarbon mechanism (33); (2) NIST fluorocarbon mechanism (25); and (3) the reaction scheme developed in this study. Species and reactions containing oxygen are removed from GRI-MECH and NIST mechanisms since there is no oxygen involved in our study. The reaction mechanism presented here includes a halon 1211 pyrolysis mechanism (15) and a number of reaction steps involving species containing Br and Cl atoms, which, we believe, are important steps based on our previous discussion. For the purpose of brevity, only the reaction steps in scheme (3) are listed in Table 2. Whenever possible, kinetic data are taken directly from the literature in which the reaction has been studied in detail. Otherwise, kinetic parameters are estimated by referring to analogous reactions. Figure 1 shows a comparison of the halon 1211 conversion level as a function of temperature at three residence times. The predicted conversion level in both cases is slightly higher than values observed experimentally. Possible reasons for this difference include the following: (1) The possibility of back mixing of reactant species during reaction. An idealized plug flows is assumed during the simulation. However, some mixing of species in the axial direction is inevitable. (2) The kinetic data for some of the reaction steps have been estimated, and thus refinement of some parameters may be required. Since the experimental data for many of the reaction steps included in our mechanism involved are not available, estimation by referring to the analogous reactions has been made. Comparison of the yield to major carbon containing products is presented in Figure 2. The major species predicted are CHClF2, CHBrF2, CH2F2, and CH4, which is in good agreement with the experimental results obtained. Generally satisfactory prediction of the variation of the yield to these four major products as a function of temperature is given by the proposed mechanism, although, at high temperatures, the discrepancy between experimental and modeling results tends to increase. The predicted yield of CHClF2 increases with temperature and is at a maximum at 890 K, which is in good agreement with the experimental values. At temperatures above 890 K, the yield of CHClF2 is underpredicted. The predicted temperature for the maximum yield of CHBrF2
TABLE 2. Reaction Model for Hydrodehalogenation of Halon 1211 with Hydrogena reaction
A
n
CBrClF2 f CClF2 + Br CBrClF2 f CBrF2 + Cl CClF2 + CClF2 f C2Cl2F4 CClF2 + CBrF2 f C2BrClF4 CBrF2 + CBrF2 f C2Br2F4 Br + Br + M f Br2 + M Cl + Cl + M f Cl2 + M CCl2F2 + M f CClF2 + Cl + M CBrF2 + Br + M f CBr2F2 + M CBrClF2 + Cl f CBrF2 + Cl2 CBrClF2 + Cl f CClF2 + BrCl CBrClF2 + Br f CClF2 + Br2 CBrClF2 + Br f CBrF2 + BrCl CBrClF2 + CClF2 f CCl2F2 + CBrF2 CBrClF2 + CBrF2 f CBr2F2 + CClF2 CClF2 + Cl2 f Cl + CCl2F2 CBrF2 + Br2 f Br + CBr2F2 CClF2 + M f CF2 + Cl + M CBrF2 + M f CF2 + Br + M Cl + BrCl f Br + Cl2 Br + BrCl f BrCl + Br2 Br2 + Cl2 f BrCl + BrCl C2F4 + M f CF2 + CF2 + M CF2 + Cl2 f CCl2F2 CF2 + Br2 f CBr2F2 CH2F + HBr f CH3F + Br CH2F + HCl f CH3F + Cl CH3 + HBr f CH4 + Br CH3 + HCl f CH4 + Cl CH3Br f CH3 + Br CH3 + Cl f CH3Cl CH3 + Cl2 f CH3Cl + Cl CH3 + Br2 f CH3Br + Br CClF2 + H2 f CHClF2 + H H + CClF2 + M f CHClF2 + M HBr + CClF2 f CHClF2 + Br H + CBrClF2 f HBr + CClF2 H + CBrClF2 f HCl + CBrF2 H + CCl2F2 f HCl + CClF2 H + CBr2F2 f HBr + CBrF2 CHClF2 f CF2+HCl CF2 + HBr f CHBrF2 CBrF2 + H2 f CHBrF2 + H CBrF2 + HBr f CHBrF2 + Br CBrF2 + HCl f CHBrF2 + Cl CClF2 + HCl f CHClF2 + Cl CHF2 + Br f CHBrF2 CHF2 + Cl f CHClF2 H + CHClF2 f CHF2 + HCl H + CHBrF2 f CHF2 + HBr CF2 + H f CHF2 CH2F2 + Br f CHF2 + HBr CH2F2 + Cl f HCl + CHF2 CH3 + CBrClF2 f CH3Br + CClF2 CH3 + CBrClF2 f CH3Cl + CBrF2 H + Cl + M f HCl + M Cl + H2 f H + HCl Cl2 + H f Cl + HCl Br2 + H f Br + HBr Br + H + M f HBr + M Br + H2 f HBr + H Br + CH3 f CH2 + HBr CH2 + HCl f CH3 + Cl CH2 + Cl f HCl + CH CH2 + Br f CH + HBr CF2 + H2 f CH2F2
1.3E15 1.3E15 1.4E12 1.4E12 1.4E12 1.9E14 2.0E14 8.1E16 6.0E14 1.9E14 8.1E13 8.1E13 1.9E14 1.9E11 1.9E11 7.8E10 2.3E12 1.0E17 1.0E17 8.7E12 3.7E14 1.9E02 4.0E50 3.3E08 3.2E09 3.97E11 5.76E11 9.46E11 5.76E11 1.58E13 1.54E14 2.88E12 1.21E13 7.94E11 5.01E13 2.51E11 1.58E14 1.58E14 1.58E14 1.58E14 1.41E13 2.14E11 1.25E11 2.63E11 1.86E11 1.86E11 3.49E13 1.54E14 4.65E14 1.47E14 2.75E6 2.38E13 9.03E12 1.00E11 1.26E12 7.20E21 7.94E13 8.59E13 2.28E11 4.78E21 1.70E14 1.10E14 1.70E12 2.20E11 1.10E14 2.80E12
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -9.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.32 0.0 0.0 0.0 0.0 -2.0 0.0 0.0 0.0 -1.96 0.0 0.0 0.0 0.0 0.0 0.0
E
ref
262.0 15 326.4 15 0.0 15 0.0 15 0.0 15 -7.1 15 -7.5 15 263.6 15 0 15 131.4 15 105.3 15 105.3 15 131.4 15 43.5 15 43.5 15 8.0 15 2.9 15 205.4 15 205.4 15 0.0 15 25.8 15 0 15 356.9 15 8.8 15 4.3 15 4.1 {34}b 10.2 {34}b -1.6 {35}b 10.2 36 300.0 37 0.0 38 1.9 39 -1.6 40 40.0 14 16.7 {41}b 11.0 14 39.0 14 64.2 14 64.2 14 39.0 14 223.7 29 40.0 14 25.8 {42}b 10.7 {43}b 21.6 {44}b 21.6 {44}b 0.0 37 0.0 38 64.2 45 39.1 14 32.2 {25}b 69.3 46 13.1 47 26.7 48 47.3 48 0 48 22.2 49 4.9 49 1.84 50 2.1 50 80.0 14 96.4 51 3.62 51 106.4 52 96.4 {51}b 143.0 {26,30}b
a Reaction steps from the GRI-MECH hydrocarbon and NIST fluorocarbon mechanisms are not listed for the purpose of brevity except for reaction CF2 + H2 f CH2F2 whose kinetic parameters are modified to accord with the latest measurements. The rate coefficient of the forward reaction is k ) ATn exp(-E/RT), where A is in s-1, cm3 mol-1 s-1, or cm6 mol-2 s-1 as appropriate, the activation energy is in kJ mol-1, n denotes the temperature exponent, and R stands for the ideal gas constant. b {}: The estimation is made by referring to the analogous reaction in the literature.
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FIGURE 7. Reaction pathways for the formation of major products for the hydrodehalogenation of halon 1211 with hydrogen at 923 K. is about 910 K which is 30 K higher than that observed experimentally, indicating that yield of CHBrF2 is overpredicted. At temperatures below 873 K, the predicted yield of CH2F2 is in good agreement with the experimental value but increases dramatically above 873 K and is much higher than that observed experimentally. In the temperature range investigated, yield to CH4 from the model is slightly higher than the experimental value. Comparison of halogen removal efficiency between experimental and modeling results is made in Figure 3. In general, the results from the model are in good agreement with experimental results. In the low temperature range, the predicted HBr concentration is slightly higher than the experimental concentration. Likewise, the predicted halon 1211 conversion level and yield of CHClF2 are higher than the experimental values at temperatures below 825 K (see Figure 2), which indicates that the rate of C-Br bond cleavage in the model is higher than that in the experiments. The amount of HCl predicted matches well with the experiments at temperatures below 900 K. Above 900 K, the experimental values are lower, which corresponds to a lower yield of CHBrF2 in the experiments than in the model, whose formation involves the cleavage of C-Cl bonds (R12) or molecular dissociation of CHClF2 (R13). The amount of HF predicted agrees well with the experimental values in the temperature range studied. Comparison between the experimental and modeling results of the halon 1211 conversion level and selectivity of major products as a function of residence time and the hydrogen/halon 1211 ratio is also made, and generally satisfactory agreement between the experimental and modeling results is obtained (Figure 4 and Figure S1 in the Supporting Information). Although the model can predict the formation of many of minor species, including CBr2F2, CCl2F2, CClF2CClF2, CH3F, CHF3, CHF2CHF2, CH3Br, CH3Cl, C2H4, and C2H6, quantitative comparison is less satisfactory. The most noticeable discrepancy between experimental and modeling results is that for C2F4, which is observed in quite high quantities at high temperatures (see Table S1, Supporting Information), while, in the model, C2F4 is predicted only in trace amounts (yield is less than 0.1%). Reaction Pathway Analysis. To better understand the reaction mechanism involved during hydrodehalogenation of halon 1211, the software package AURORA is used to undertake a rate of production analysis. This computer code can compute rates of production and consumption for each species. Based on this information obtained from the rate of production analysis, a reaction pathway for the hydrodehalogenation of halon 1211 with hydrogen was obtained and is summarized in Figure 7. Since the product profile is dependent on the reaction conditions, pathway analysis was conducted at 923 K and a residence time of 2.5 s with an input ratio of N2:H2:halon 1211 of 19:10:1. Under these conditions, all major products and a number of minor species are present, and the amount of carbonaceous material formed was small, based on the mass balance calculations (see Table S1, Supporting Information). As shown in Figure 7, the dominant reaction pathways leading to the formation of CHClF2, CHBrF2, and CH2F2 are 3026
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FIGURE 8. Comparison of conversion level of CH2F2 and selectivity to major products between experimental and computational studies on reactions of CH2F2 with hydrogen at 1073 K and a residence time of 2.5 s with an input ratio of N2:H2:CH2F2 of 19:10:1. consistent with the pathways described qualitatively in the previous section (Mechanistic Analysis). de Lijser et al. suggested the formation of CH4 via a series of F abstraction reactions starting with CH2F2 (14). However, reaction pathway analysis suggests that, within the temperature range studied, CH4 is formed primarily via the following steps (R26-R30):
CF2 + H f CF + HF
(R26)
CF + H2 f CH + HF
(R27)
CH + H/HBr/HCl/H2 f CH2 + H + Br/Cl/H (R28) CH2 + H/HBr/HCl/H2 f CH3 + Br/Cl/H
(R29)
CH3 + H/HBr/HCl/H2 f CH4 + Br/Cl/H
(R30)
These pathways are consistent with our previous suggestions that the formation of CH4 is closely related to the presence of CF2 radicals produced in the course of hydrodehalogenation reaction. The reaction pathway from CH2F2 to CH4 is not favored under the conditions studied according to the model, owing to the higher energy barrier for the cleavage of C-F bonds in CH2F2 than that for the cleavage of C-F bonds in CF2. The activation energy estimated for CH2F2 + H f CH2F + HF is 142.7 kJ mol-1, while the activation energy for reaction R26 is only 5.2 kJ mol-1 (25). According to the model, C2F4 is produced in large amounts but is rapidly consumed via reaction R31:
C2F4 + H f CHF2 + CF2
(R31)
As a result, C2F4 is produced only in trace quantities. If reaction R31 is removed from the current chemical kinetic scheme, the C2F4 yield is then highly overpredicted, resulting in a much higher yield of C2F4 than the experimental value. It is likely that the uncertainties associated with the kinetic values for reaction R31 are responsible for the difference observed between experimental and modeling results. The uncertainties with kinetic values for other reactions could also exist, leading to the increased discrepancy between experimental and modeling results at high temperatures. In separate experiments, we conducted experiments on the reaction of CH2F2 with hydrogen at a high temperature of 1073 K and compared with modeling results (see Figure 8). The reaction of CH2F2 with H2 is a simplified case for the current reaction scheme since reactions containing Br and
Cl are not involved. In doing so, we aim to examine the validity of the GRI-MECH and NIST mechanisms in our system at high temperatures. The experimentally observed conversion level of CH2F2 is higher, and the product profile is also quite different from that predicted, which implies that the further adjustment with some reactions of the GRI-MECH and NIST mechanisms are required to adequately fit the experimental results. In addition, many minor species are detected, and a large amount of coke is deposited on the surface of the reactor during hydrodehalogenation of halon 1211 with H2 at high temperatures. Reaction pathways leading to the formation of these species are not clear and thus not considered in the current reaction scheme. In the future work, efforts will be made to elucidate the reaction pathways leading to these species and to include these reactions in the revised reaction mechanism.
Acknowledgments Australian Research Council is gratefully acknowledged for financial support of this project. H. Yu is indebted to the Department of Education, Science and Training (DEST) of the Australian Government and the University of Newcastle, Australia, for a postgraduate scholarship.
Supporting Information Available Additional experimental and modeling results on the reaction of halon 1211 with hydrogen. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review April 26, 2004. Revised manuscript received December 2, 2004. Accepted February 4, 2005. ES049372O
