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Synthesis of Vinylidene Fluoride via Reaction of Chlorodifluoromethane (HCFC-22) with Methane Wenfeng Han, Eric M. Kennedy,* John C. Mackie, and Bogdan Z. Dlugogorski Process Safety and EnVironment Protection Research Group, School of Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia
The gas-phase reaction of CH4 and CHClF2 (HCFC-22, R22) has been studied in an alumina tubular reactor at atmospheric pressure and in the temperature range of 673-1073 K. The motivation of the investigation is to assess this process as a potential route for the treatment of CHClF2, as well as a technology for the synthesis of CH2dCF2 (vinylidene fluoride, VDF). Under the conditions studied, the major products are C2F4, CH2dCF2, HF, and HCl. Minor products detected include C2HF3, C2H2, CHF3, C2H3F, C2H2F4, CH2F2, C3F6, CH3Cl. A mechanistic interpretation of the results is proposed, including the reactions involved in the initial decomposition of CHClF2, those contributing to the activation of CH4 and developing the pathways leading to the formation of product species. The result of changing feed ratio experiments is consistent with the reaction mechanism developed. The introduction of small amounts of O2 improves the conversion of CH4 and formation of CH2dCF2 markedly. 1. Introduction Vinylidene fluoride (CH2dCF2, VDF) is a monomer used for the preparation of a variety of fluorocarbon polymers which have excellent weathering and chemical resistance properties. It is the key component for the synthesis of a variety of fluoroelastomers products, most notably poly-B (PVDF), Viton (produced by Dupont Corporation), KEL-F (produced by 3M) and Kynar (produced by Arkema). Components fabricated from fluoroelastomers and blends thereof enhance reliability, safety, and environmental compatibility in areas such as automotive and air transportation, the chemical processing industries, and in power generation.1 A broad range of fluoroelastomer products has been developed to meet challenging performance requirements in hostile environments, and to attain fabrication characteristics comparable to other elastomers. More recently, VDF was found to be an excellent additive for the preparation of paints for high performance external architectural coatings.2 A number of processes for the manufacturing of VDF have been reported.3 However, these methods have not been entirely satisfactory either because of low yields or problems associated with sourcing the C2 starting materials. Chlorodifluoromethane (CHClF2, R22, HCFC-22) is one of the most widely used refrigerants and is a raw material for the manufacture of fluorinated compounds, such as tetrafluoroethylene (C2F4, TFE) and hexafluoropypylene (C3F6, HFP). HCFC22 is designated as a HCFC, and because of the presence of chlorine in its structure, it is a major contributor to stratospheric ozone depletion and global warming. The implementation of the Montreal and Kyoto Protocols has necessitated the development of treatment processes for these and related materials. A variety of approaches, such as high temperature decomposition,4 especially with plasma heating,5 and catalytic decomposition,6 are currently being developed to convert these compounds into environmentally acceptable products, most typically CO2 and mineral acids (HCl and HF). Other than the total destruction, an alternative approach to the treatment of these halocarbons is to convert them into useful chemicals, since they possess * To whom correspondence should be addressed. Fax: (+61 2) 4985 4422. Tel.: (+61 2) 4921 6893. E-mail:
[email protected].
valuable C-F bonds, are manufactured at high costs, and are available as relatively pure chemicals in large quantities.7 Recently, we have discovered that CBrClF2, CCl2F2, and CHF3 can be converted into VDF through reaction with CH4.8-11 At high temperatures, these materials are excellent difluorocarbene (:CF2) sources for the production of VDF after debromination, dechlorination, or dehydrofluorination, and we propose that the key elementary reaction step involves the :CF2 species reacting with CH3 (following CH4 decomposition) to form CH2dCF2 and a hydrogen atom. In this work, the reaction of HCFC-22 with CH4 was studied under conditions similar to those used for investigating the reaction of CCl2F2 with CH4.9 2. Experimental Section A tubular high purity (99.99%) alumina reactor (i.d. 7.0 mm, length 60 cm) was employed for all experiments. Flow rates of CHClF2 (>98%, Core Gas), CH4 (>99%, Linde), and dilute N2 (BOC gases, 99.99%) were controlled by mass flow controllers (Brooks) to give a total flow rate of 220 mmol h-1 (STP) with CHClF2 and CH4 accounting for 10% of the total volumetric flow. To maintain a constant residence time (0.5 s) at various temperatures, the volume of thermal zone was controlled by adjusting the position of two single-ended thermocouple sheaths (alumina, 6.0-mm o.d.) situated in the reactor. During the experiments of varying CH4/CHClF2 feed ratio from 0.3 to 1.5, CHClF2 feed rate was maintained at 12 mmol h-1 and CH4 flow rate varied depending on target ratio. HCl and HF formed during reaction was trapped by a caustic scrubber (NaOH solution) before the reactor effluent reached an online micro gas chromatograph. Carbon containing products were identified by 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. Where possible, the relative molar response (RMR) factors of halogenated hydrocarbons for thermal conductivity detection (TCD) were obtained experimentally from standard gas mixtures containing halogenated hydrocarbons diluted in nitrogen. RMR factors of species for which we had no authentic standards were estimated from published correlations.12
10.1021/ie100338j 2010 American Chemical Society Published on Web 06/02/2010
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Understanding of the conversion profile of CHClF2 can be assisted by comparing the reaction between CHClF2 and CH4 with the pyrolysis of CHClF2. The pyrolysis of CHClF2 and its mechanism have been well-explored and are dominated by the intramolecular elimination of HCl to form difluorocarbene :CF2, which then dimerizes into C2F4 (TFE).4,15-18 Indeed, this wellestablished process serves as a major industrial source of TFE. Following introduction of CH4 into the feed, the conversion pattern of CHClF2 is similar to that of pyrolysis conducted under N2 diluted conditions19 which suggests that the initial step for our target reaction of CH4 with CHClF2 is similar to pyrolysis (thermal dehydrochlorination), namely, reaction R1. CHClF2 f :CF2 + HCl
Figure 1. CH4 and CHClF2 conversion as a function of temperature under atmospheric pressure and residence time of 0.5 s. Feed: N2:CH4:CHClF2 ) 10:1:1.
The concentration of HF and HCl trapped with 0.1 M NaOH solution during the reaction were determined by an ion chromatograph (IC) (Dionex-100) equipped with an IonPAS14A column (4 × 250 mm). In reactions which involved AlF3, 0.2 g aluminum fluoride was charged into the uniform zone of the reactor and held in place by alumina chips (99.99%). Prior to reaction, aluminum fluoride was dried in situ in a nitrogen atmosphere (99.999%, Linde) for 2 h at 673 K and 1.5 h at 1073 K. Feed gases, diluted in nitrogen (99.999%, Linde), were then introduced to the reaction zone. 3. Chemical Kinetic Modeling The reaction of CHClF2 with CH4 have been modeled using the commercial software package Cosilab.13,14 During simulations, the steady state material balance for each element was performed. As all experiments were conducted under essentially isothermal conditions, energy balances were not undertaken. Successive grid tolerances for species profiles were set to 0.001 (GRAD parameter) for species concentration and to 0.01 for the concentration gradients (CURV parameter). The final grids contained 150 mesh points. 4. Results and Discussion 4.1. Conversion of CH4 and CHClF2. An equimolar feed of CHClF2 and CH4 in a nitrogen diluent was investigated with a volumetric ratio of 10:1:1 (N2:CHClF2:CH4). Figure 1 shows the conversion of CHClF2 and CH4, which commenced at about 773 and 873 K and increased rapidly over the temperature range of 850-1000 and 900-1050 K, respectively. Complete conversion of CHClF2 was observed at around 1050 K, while the corresponding conversion of CH4 was approximately 45%.
∆rHo298 ) 207.3 kJ
(R1)
However, understanding the conversion of CH4 in the process of CHClF2 is less straightforward than that of CHClF2. In our studies of the reactions of CBrF3, CCl2F2, and CBrClF2 with CH4, almost identical methane conversions were achieved compared with those of CFCs or halons.9,11,20 It is believed that reactions involving CH4 are initiated by halogen radicals, which are produced during thermal cleavage of the carbon-halogen bond from CBrF3, CCl2F2, and CBrClF2. As shown in Figure 1, the conversion levels of CH4 are far lower than CHClF2 under all the conditions studied. For example, at 973 K, the conversion level of CH4 is only 20%, while as high as 90% is achieved for that of CHClF2. Thus it is unlikely that CHClF2 forms Cl via cleavage of the carbon-chlorine bond. The cleavage of the C-Cl bond via R2, C-F bond via R3, and C-H via R4 has much higher energy barriers than the dehydrochlorination via R1. Indeed, Su et al.15 found that the thermal dissociation of CHClF2 proceeds solely via three-center molecular HCl elimination process R1. They found that the rate of C-Cl fission, R2, is negligible at temperatures below 1900 K. CHClF2 f :CF2 + HCl
∆rHo298 ) 207.3 kJ
(R1)
CHClF2 f CHF2 + Cl
∆rHo298
) 355.2 kJ
(R2)
CHClF2 f CHClF + F
∆rHo298 ) 477.3 kJ
(R3)
CHClF2 f CClF2 + H
∆rHo298 ) 424.6 kJ
(R4)
In comparison, the reaction of CH4 with CHF3 has also been investigated, as it also produces :CF2 in the absence of chlorine. Conversion of CH4 commences at a temperature of 1013 K, and a conversion level of 37.6% is obtained at 1173 K, while during the reaction of CHClF2 with CH4, conversion of CH4 commences at 873 K and the conversion level of CH4 is as high as 46.54% at a temperature only 1073 K (see Figure S1 of the Supporting Information). This indicates that Cl in CHClF2 does not play a major role for the activation of CH4, at least under the conditions studied. Because the activation energy of HF elimination from CHF3 is believed to be 295 kJ mol-1, which is much higher than that of HCl elimination from CHClF2 (229 kJ mol-1), lower CH4 conversion levels and higher reaction
Table 1. Pyrolysis of CH4 as a Function of Temperature in an Alumina Tube Reactora CH4 pyrolysis over AlF3b
CH4 pyrolysis
a
T (K)
conv (%)
C2H6 (mmol h-1)
conv (%)
C2H6 (mmol h-1)
C2H4 (mmol h-1)
973 1023 1073 1123 1173
0.049 0.024 0.018 0.14 0.31
c 0.0026 0.0018 0.004 0.013
0.11 0.26 0.68 0.37 1.1
0.0036 0.006 0.01 0.011 0.027
c c 0.002 0.005 0.009
At a pressure of 1.01 bar and residence of 0.5 s. remained unchanged. c Not detected.
b
During the experiment, 0.2 g AlF3 was packed into the reactor. Other reaction conditions
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Figure 2. Major carbon-containing product profiles as a function of temperature under atmospheric pressure and residence time of 0.5 s. Feed: N2:CH4: CHClF2 ) 10:1:1.
temperatures in the reaction of CH4 with CHF3 are most likely a result of the higher stability of CHF3 than CHClF2, which can readily produce :CF2. However, the singlet :CF2 is very nonreactive since the closed-shell singlet :CF2 (1A1) is strongly stabilized by ppbackdonation.21 In contrast, the metastable triplet :CF2 (3B1), having a rather long lifetime of about 1 s, is believed to be much more reactive, and an A factor of 1.2 × 1013 cm3 mol-1 s-1 was estimated for the reaction with CH4.22 Because the triplet lies 238.1 kJ mol-1 above the ground-state singlet, the ratio of triplet to singlet ground state populations is only 8 × 10-11; hence, the reactions of the triplet can be neglected.23 There are very few reactions occurring during pyrolysis of CH4, even at temperatures as high as 1173 K,10 while during reaction with CHClF2, CH4 conversion is observed at around 873 K. Therefore, it seems that CH4 is activated by radicals formed during decomposition of CHClF2, rather than the thermal intermolecular dissociation. The thermal decomposition of CHClF2 is dominated by reaction R1, at least at temperatures below 1173 K. Under these conditions, the major products are HCl and :CF2. It seems plausible that CH4 could be activated via its reaction with :CF2. However, the A factor for the reaction rate constant of :CF2 with CH4 was estimated to be 1.14 × 109 cm3 mol-1 s-1 using ab initio methods.22 This very low reaction rate has been confirmed experimentally by Battin-Leclerc et al.,24 who studied the reactions of :CF2 with H2, O2, CH4, and C2H4 over the temperature range 295-873 K. No reaction between :CF2 and CH4 was observed during their investigations. Yu et al.8 suggested that CH4 activation takes place as a consequence of surface reactions (fluorinated alumina reactor surface). However, this hypothesis seems to conflict with experimental results, when it was found that after packing Al2O3 or AlF3 chips into the reactor, no enhanced conversion of CH4 was observed. We suggest that CH4 is activated via a series of surfaceinitiated chain reactions. Initiation
CH4(S) f CH3 + H
Propagation H + CF2:CF2 f CHF2CF2
(R5)
∆rHo298 ) -224.0 kJ
(R6) H + CF2:CF2 f CHFCF2 + F
∆rHo298 ) 45.9 kJ (R7)
CHF2+:CF2 f H + CF2:CF2
∆rHo298 ) -11.1 kJ (R8)
CHF2CF2+CH4 f CHF2CHF2+CH3 F + CH4 f CH3 + HF
∆rHo298 ) -182.4 kJ (R9)
∆rHo298 ) -132.8 kJ
(R10) H + CH4 f CH3+H2 CH3+:CF2 f CH2)CF2 + H
∆rHo298 ) 2.6 kJ ∆rHo298
(R11)
) -81.1 kJ (R12)
To examine this hypothesis, the pyrolysis of CH4 was investigated and the results are shown in Table 1. Indeed, the conversion of CH4 (and subsequent formation of C2H6), although low, was observed with our alumina reactor. This suggests that small but significant quantities of CH4 can be activated on the surface of reactor, at least at high temperatures. Subsequently, these surface reactions act as an initiator for the above chain reactions. 4.2. Product Variation with Temperature. In addition to HCl and HF, the major products formed during the reaction were C2F4 and CH2dCF2. Minor products include C2HF3, C2H2, CHF3, C2H3F, C2H2F4, CH2F2, C3F6, and CH3Cl. Soot and solid products were deposited on the surface of reactor tube. Carbon mass balances were around 98% at low temperatures, decreasing to 68% at 1073 K and at a residence time of 0.5 s. Figure 2 illustrates the major product profiles as function of temperature. The :CF2 combining product, C2F4, dominates at temperatures below 973 K. This is consistent with the wellestablished studies of the production of TFE (C2F4)4,15,18 from the thermal decomposition of CHClF2. According to these studies, the initial step in the decomposition of CHClF2 is elimination of HCl, forming :CF2 diradical, from which a significant rate of formation of C2F4 is expected. At temperatures below 925 K, C2F4 is the dominant product with selectivity of 80% achieved, although above 925 K the rate of formation of C2F4 drops dramatically. One possible explanation to this observed declining rate of C2F4 above 925 K is that C2F4 decomposes into other products, such as C3F6 and C4F8.25,26 However, the very low A factors and high energy barriers (109 cm3 mol-1 s-1 and 34 kJ mol-1 for C3F6 and 1010 cm3 mol-1 s-1 and 96 kJ mol-1 for C4F8) suggest that these species play a minor role in the observed decreasing rate of C2F4 formation.
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We suggest that a major channel for consumption of :CF2 is via its reaction with CH3, leading to CH2dCF2 (VDF) via R12.27 It was found that the reaction between :CF2 and CH3 leads only to the production of CH2dCF2 and elemental hydrogen.27 Over the temperature range of 700-2000 K, the rate constant for R12 can be approximated by the expression of 2.1 × 1013T0.207 cm3 mol-1 s-1. The results of reaction of CHClF2 with CH3Br studied over the temperature range of 773-1123 K and atmospheric pressure yielded CH2dCF2 as a major product species, consistent with the present measurements, which the major product, CH2dCF2, was detected.28 Compared with the reaction of dimerization of :CF2 into C2F4, a much lower A factor (1010-1012 cm3 mol-1 s-1) was suggested, with the activation energy estimated to be close to zero.18,29,30 Under similar conditions, the reaction rate of R12 is at least 10 times higher than that of :CF2 dimerization. The coupling of CH3 with :CF2 will compete with dimerization of :CF2; the importance of this reaction increases with temperature, as shown in Figure 2. The high activation energy barrier for CH4 impedes higher conversion rates of CH4 and necessitates operation at elevated temperatures which are required for activation of CH4. Once the primary reactions leading to :CF2 formation are elucidated, a mechanism can be developed for minor products. Underlying the importance of :CF2 radicals, it seems likely that this species is also involved in the reactions which are responsible for the formation of minor products. The rate of formation of minor products including C2HF3, C2H2, CHF3, C2H3F, C2H2F4, CH2F2, C3F6, CH3Cl as a function of temperature are shown in Figure 3. These data indicate that, over the temperature range of 673-1073 K, there are dramatic changes in the rate of formation of C2HF3, C2H2, and C2H3F. The formation of C2H2F4, C3F6 and CH3Cl increases with temperature up to 1023 K, above which, their concentrations decrease with temperature. It is worth noting that the relatively high yield of CHF3 in the minor products is unexpected. We suggest that CHF3 is formed from the dismutation reaction of CHClF2, which is catalyzed by Al2O3 or AlF3 R13.31-33 Although numerous reactions have been proposed to explain the formation of CHF3, they usually include H abstraction by CF3 from various H sources. It seems unlikely these reactions occurred to any great extent in our experiments as CF3 concentration was very low under all the conditions studied. If this was not the case, C2F6 formation would be expected, but this is absent from the current list of product detected. Al2O3/AlF3
3CHClF2 98 2CHF3 + CHCl3
∆rH°298 ) -16.4kJ
(R13) The product of C2HF3 forms through various pathways. During the reaction of CHF3 and CH4 under conditions similar to this study, Yu et al.8 suggested that reactions leading to the production of C2HF3 include the reaction series R14-R17.
However, experimental studies of the pyrolysis of CHF2-CHF2 found that the major products were CF3CH2F (HFC 134a) and C2HF3 under condition of low conversion (2-3%) at 973 K.34 This indicates that if C2HF3 is formed via R14-R17, even larger quantities of CHF2-CHF2 and HFC 134 should be observed. In fact, as shown in Figure 3, only CHF2-CHF2 was detected during our reactions and only in trace amounts, which suggests that R14-R17 only play a minor role toward the formation of C2HF3. It is possible that C2HF3 is formed via reaction R19, via the combination of :CF2 and the diradical :CHF.11 With respect to the formation of C2H2F4 which is formed through R15, CHF2 also can be produced by the reaction of :CF2 with H, as illustrated in R20.35 CF2 + H f CHF + F
∆rHo298 ) 168.9 kJ (R18)
:CF2 + CHF f C2HF3
∆rHo298 ) -417.5 kJ (R19)
:CF2 + H f CHF2
∆rHo298 ) -283.5 kJ
CH3+CHF2 f C2H3F + HF
∆rHo298 ) -307.4 kJ (R21)
CF + CH4 f C2H3F + H
∆rHo298 ) -98.4 kJ
(R22) C2H4 + F f C2H3F + H
∆rHo298 ) -49.9 kJ
(R23) :CF2 + H f CF + HF :CF2 f CF + F
∆rHo298 ) -54.1 kJ (R24) ∆rHo298 ) 516.6 kJ
C2F4+:CF2 f C3F6
∆rHo298 ) -311.1 kJ
C2F4+C2F4 f c-C4F8
(R26)
∆rHo298 ) -170.9 kJ (R27)
The formation of C2H2 and C2H4 from CH4 and CH3 has been studied intensively, and a multistep reaction mechanism has been developed.36 In our experiments, C2H2 most probably is generated by reactions R28-R35. We suggest that R36 is the major channel for the formation of CH3Cl. CF + H2 f CH + HF
CF + H f C + HF CH + CH4 f H + C2H4
o ∆rH298 ) 65.6 kJ
∆rHo298 ) -29.6 kJ ∆rHo298 ) -248.8 kJ
CHF2+CHF2 f CHF2-CHF2
∆rHo298 ) -382.0 kJ (R15)
CH2F-CH2 + H f C2H4 + HF
∆rHo298 ) 129.7 kJ (R16)
C + CH3 f C2H2 + H
∆rHo298 ) -252.4 kJ (R17)
(R25)
Reactions between :CF2 and C2F4 also produce C3F6 and c-C4F8.
∆rHo298 ) 11.1 kJ (R14)
CHF2+CHF2 f C2HF3 + HF
(R20)
The radicals of CF and CHF2 can also undergo a series of reactions, generating C2H3F. Indeed, reaction R21 may be responsible for the formation of C2H3F at relatively low temperatures, while for R22 and R23, higher temperatures are necessary for them to play a major role because of the energy barriers for the formation of CF and F from decomposition of :CF2 via R24 or R25.
C2F4 + H f :CF2+CHF2
CHF2-CHF2 f C2HF3 + HF
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CH2:CF + H f C2H2 + HF
C2H3F f C2H2 + HF C2H4 f C2H2+H2
(R28) (R29) (R30)
∆rHo298 ) -377.0 kJ (R31) ∆rHo298 ) -379.5 kJ (R32) ∆rHo298 ) -417.8 kJ (R33) ∆rHo298 ) 89.4 kJ ∆rHo298 ) 174.3 kJ
(R34) (R35)
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Figure 3. Minor product profiles as a function of temperature under atmospheric pressure and residence time of 0.5 s. Feed: N2:CH4:CHClF2 ) 10:1:1.
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CH3 + HCl f CH3Cl + H
∆rHo298
) 80.9 kJ
(R36) 4.3. Chemical Kinetic Modeling. The NIST HFC mechanism35 is combined with Gas Research Institute GRI-Mech37 for the hydrocarbon reactions. On the basis of the above discussion, we introduce a small number of additional reaction steps involving species containing Cl which are taken directly from the literature and modify the kinetic parameters of a small number of fluorocarbon reactions already in the NIST mechanism. For those steps which are not available in the literature, kinetic parameters are estimated by referring to analogous reactions. Table 2 lists the chemical kinetic model for reaction of CHClF2 with CH4. For the purpose of brevity, only the reaction steps which are not included in NIST-GRI mechanism and modifications are listed. Figure 1 demonstrates a reasonable agreement between measured and modeled conversion of CH4 and CHClF2, especially below 900 K. As the temperature increases, the model underpredicts the measurements. We explain this difference by the dismutation of CHClF2 on the surface of reactor’s walls, as discussed earlier. This explanation has its support in significant quantities of CHF3 detected at temperatures above 900 K, indicating that the dismutation reaction amounts to a major contributor to the formation of CHF3. Therefore, we introduce a new “gas-phase” reaction (as indicated in Table 2) to mimic this surface reaction. Error of the kinetic data may also contribute to this discrepancy. For some of the reaction steps, the thermokinetic parameters have been estimated, and their further refinement may be required. To understand and improve the prediction of conversion of CH4, we performed the sensitivity analysis to the concentration of CH4, as illustrated in Figure S2 of the Supporting Information. The results show that Cl formation and some chlorine-containing species are very sensitive to the conversion of CH4. However, species such as CHCl3, CH2Cl2, and CCl4 and reaction steps involving these species are not adequately modeled using the present mechanism, and it is likely that reactions involving these species contribute the lower conversion levels of CH4 predicted by the model. Comparison of the formation rate of major carbon-containing products is presented in Figure 2, with satisfactory agreement with experimental data for C2F4 and CH2dCF2. Generally, their rates of formation are predicted satisfactorily by the proposed mechanism, although at high temperatures the discrepancies increase. We suggest that unsaturated products (such as C2F4 and CH2dCF2) tend to polymerize, especially at high temperatures, and are responsible for the observed difference between prediction and experimental measurements. As revealed by sensitivity analysis (Figure S3 and S4 of the Supporting Information), reaction R12 is most sensitive both for the yield of CH2dCF2 and C2F4. At higher temperatures, polymerization of C2F4 and CH2dCF2 is also likely to be sensitive which is absent from the mechanism and sensitivity analysis. Although the model can predict the formation of several minor species, including C2HF3, C2H3F, C2H2, CH3Cl, C3F6, CH2F2, C2H2F4, and CHF3, quantitative comparison is less satisfactory (Figure 3). The most noticeable discrepancies between experimental and modeling results are those for C2H3F and C2H2, which are observed in quite high quantities at high temperatures, while, in the model, C2H3F and C2H2 are predicted only in trace amounts.
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4.4. Effect of the CH4/CHClF2 Feed Ratio on Conversion and Reaction Selectivity. To further facilitate our understanding of the reaction between CH4 and CHClF2, the effect of changing CH4 to CHClF2 feed ratio was investigated at 1023 K, residence time of 0.5 s, and CHClF2 feed rate of 12 mmol h-1. Figures 4 and 5 show the conversion of CH4 and CHClF2, the rate of formation of C2F4, CHF3, and CH2dCF2 with feed ratio of CH4 to CHClF2 varying from 0.3-1.5. Regardless of the CH4/ CHClF2 ratio, almost 100% conversion of CHClF2 was achieved. The decomposition of CHClF2 involves the unimolecular elimination of HCl and subsequent formation of :CF2, and as a result, changing the feed ratio does not affect conversion of CHClF2 significantly. In contrast, CH4 conversion level drops dramatically as the ratio of CH4 to CHClF2 increases as shown in Figure 4. As less radicals initiated by CHClF2 decomposition are available for reactions, which are responsible for the activation of CH4, the conversion of CH4 drops. It is noted that the rate of formation of CHF3 remains approximately constant with a varying CH4/CHClF2 feed ratio. Similar to the conversion of CHClF2, this confirms that formation of CHF3 is independent of the reaction with CH4, given that the CHClF2 feed rate remains unchanged. We suggest that CHF3 is a product of CHClF2 dismutation on the surface of alumina reactor rather than free radical gas-phase reactions. Hence the concentration of other reactants has little effect on the formation of CHF3. In contrast, the rate of formation of two major products, C2F4 and CH2dCF2, change significantly with CH4/CHClF2 feed ratio. CH2dCF2 formation increases from 1.01 to 1.49 mmol h-1 when the CH4/CHClF2 ratio changes from 0.3 to around 1.5, and at the same time, C2F4 decreases from 1.01 to 0.43 mmol h-1. Since CH2dCF2 is formed via the reaction between CH3 and difluorocarbene radicals, this reaction competes with difluorocarbene dimerization which results in the formation of C2F4. 4.5. Effect of O2 on the Reaction of CH4 with CHClF2. As illustrated in Figure 1, nearly 100% conversion of CHClF2 can be achieved by increasing the temperature to above 1023 K. However, CH4 conversion remains relatively low, reflected in a small yield of CH2dCF2. In order to improve the formation of CH2dCF2, a primary investigation on the effect of O2 was undertaken. O2 was selected because it is cheap and effective to activate CH4 readily. Since the discovery of oxidative coupling of methane (OCM) in 1982,38 this process has been investigated intensively. It is believed that the initial step of OCM is the formation of CH3 radical through the abstraction of one H by surface species. Then, C2H2 and C2H6 are formed by the gas-phase coupling of CH3 radicals. In this study, we expect this process could facilitate the yield of CH2dCF2, too. The effect of O2 addition is shown in Figure 6. With the introduction of O2 (4% of total feed gas) into the feed stream, CH4 conversion levels jumped from around 15% to 35%. The conversion level of CHClF2 remains high and virtually unchanged. Once the flow of O2 was terminated (after roughly 80 min), CH4 conversion dropped significantly. Similar to the trend of CH4 conversion, the rate of formation of CH2dCF2 increased from 0.6 to about 1.5 mmol h-1 and C2F4 drops from 1.8 to 0.6 mmol h-1, accordingly, once O2 was introduced to the feed. Once O2 is switched off, the rate of formation of CH2dCF2 returns to the previous levels. Other minor products whose formation involves CH3 (C2H4, C2H2, and CH2CHF) follow the same trend. However, a distinctly different pattern of CHF3 formation and C2F4 were observed following the introduction of O2. We suggest that after the introduction of O2, the inner surface of the alumina reactor is different to that in the absence of O2, where the reactor surface is
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Table 2. Kinetic Model for the Reaction of CHClF2 with CH4a reaction CHClF2 f CF2 + HCl
replaced by
A (cm3 mol-1 s-1) 1.46 × 10
n
Ea (kJ mol-1)
∆rH °298 (kJ)
ref
15
0
CH4 + CClF2 f CHF2Cl + CH3
6.0 × 1011
0
HCl + F f HF + Cl
4.2 × 1012
0
CH2F + HCl f CH3F + Cl
5.5 × 1011
0
CHF2 + HCl f CH2F2 + Cl
5.6 × 1011
0
C2H3 + HCl f C2H4 + Cl
5.2 × 1011
0
0.83
-33.4
43
CH3 + HCl f CH4 + Cl
5.3 × 1011
0
12.89
-6.9
44
CH3 + Cl f CH3Cl
3.0 × 1013
0.3
-0.45
-275.4
45
CHClF2 + Cl f CClF2 + HCl
9.0 × 1011
2.92
21.48
-0.9
46
CH2F + HCl f CH3F + Cl
5.8 × 1011
0
10.2
4.4
42
CClF2 + HCl f CHClF2 + Cl
1.8 × 1011
0
21.6
0.8
42
H + CHClF2 f CHF2 + HCl
4.7 × 1014
0
64.2
-76.2
47
CHF2 + Cl f CHClF2
1.5 × 1014
0
0
-355.2
48
CH2F2 + Cl f HCl + CHF2
9.0 × 1012
0
13.1
-10.5
49
CH2 + HCl f CH3 + Cl
1.7 × 1012
0
-27.1
42
CF2:CF2 + CF2:CF2 f C3F6 + CF2
1.0 × 1012
0
125.5
-16.6
50
CF2:CF2 + CF2 f c-C3F6
1.9 × 1012
0
51
51
c-C3F6 f CF2:CF2 + CF2
1.8 × 1013
0
182.0
52
c-C3F6 f C3F6
6.8 × 1014
0
268.8
51
CHF2+CH3 f CF2 + CH4
3.0 × 1013
0
3.4
-155.1
CH4 + CF2 f CH3 + CHF2
1.0 × 1013
0
159.5
155.1
8
CH3 + CF2 f CH2CF2 + H
6.0 × 1012
0
14.6
-81.1
(NIST)
CH4 f H + CH3
2.1 × 1013 1.1 × 1010
-0.2 0
0 234
27 this study
3CHClF2 f 2CHF3 + CHCl3
1.0 × 1023
0
210
this study
replaced by
229 57.2
207.3 7.8
15 40b
-139.1
41
10.2
4.4
42
10.2
8.9
42b
0.07
3.62
(NIST)
a Reaction steps from the GRI-MECH hydrocarbon and NIST fluorocarbon mechanisms are not listed for the purpose of brevity. The rate coefficient of the forward reaction is k ) ATn exp(-E/RT), where A is in 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 (CHF3 + CClF2 f CHClF2 + CF3 and CH2F + HCl f CH3F + Cl).
converted to aluminum fluoride. It is possible that the presence of O2 inhibits this process. During the catalytic conversion of Halon 1211 (CBrClF2) to Halon 1301 (CBrF3) and CFC 13 (CClF3), higher activity was
observed over Al2O3 compared with AlF3.39 For the used Al2O3 catalyst, an aluminum fluoride/aluminum hydroxide hydrate phase in the X-ray diffraction (XRD) pattern was detected. It seems probable that a new phase was formed on the surface of
Ind. Eng. Chem. Res., Vol. 49, No. 13, 2010
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Figure 4. Conversion of CHClF2 and CH4 as a function of CH4/CHClF2 feed ratio under atmospheric pressure, temperature of 1023 K, and residence time of 0.5 s. The CHClF2 feed rate was 12 mmol h-1, and the CH4 flow rate varied depending on the target ratio.
Figure 5. Formation of C2F4, CHF3, and CH2CF2 as a function of CH4/ CHClF2 feed ratio under atmospheric pressure, temperature of 1023 K, and residence time of 0.5 s. The CHClF2 feed rate was 12 mmol h-1, and the CH4 flow rate varied depending on target ratio.
the reactor following the introduction of O2, which plays a major role in enhancing the rate of dismutation of CHClF2 and subsequent formation of CHF3. At the same time, this oxygencontaining reactor surface can also react with :CF2, resulting in the slow recovery of the formation rate of C2F4. Following the introduction of O2, carbon monoxide and dioxide were also detected, with the concentration levels estimated to be around 0.5% and 0.8%, respectively (not shown in Figure 6). 5. Conclusion Gas-phase reaction of CH4 with CHClF2 was performed in an alumina reactor over the temperature of 673-1073 K. At above 1023 K, we achieved nearly 100% conversion of HCFC22 and about 45% conversion of CH4. It is proposed that CH4 is activated via a series of chain reaction steps, while CHClF2 is decomposed via unimolecular elimination of HCl, providing no radicals for low-temperature activation of CH4. C2F4 was the major product at temperatures below 973 K, with its rate of formation increasing with temperature before dropping dramatically at temperatures above 950 K. Thereafter, CH2dCF2 became the dominant product at temperatures above 1000 K.
Figure 6. Effect of O2 feed on the reaction of CHClF2 with CH4 under a reaction temperature of 973 K and residence time of 0.5 s. N2:CHClF2: CH4:O2 ≈ 20:2:2:1 in the feed (CHClF2 feed rate was 12 mmol h-1).
Minor products included C2HF3, C2H2, CHF3, C2H3F, C2H2F4, CH2F2, C3F6, and CH3Cl. CH2dCF2 is formed through the combination of CH3 and :CF2 radicals, with this reaction competing with :CF2 dimerization. Improved CH2dCF2 formation was observed at higher CH4/CHClF2 ratio in the feed, with the opposite trend observed for C2F4. Introduction of O2 in feed stream considerably enhanced the conversion of CH4 and formation rate of CH2dCF2. Acknowledgment The Australian Research Council is gratefully acknowledged for financial support for this project. W.H. is indebted to the Department of Education, Science and Training (DEST) of the Australian Government and the University of Newcastle, Australia for postgraduate scholarships.
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ReceiVed for reView February 12, 2010 ReVised manuscript receiVed April 30, 2010 Accepted May 5, 2010 IE100338J
