Effect of Methanol on the Gas-Phase Reaction of Trifluoromethane

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Ind. Eng. Chem. Res. 2010, 49, 8406–8414

Effect of Methanol on the Gas-Phase Reaction of Trifluoromethane 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 reaction of CHF3 with CH4 in the presence of small amounts of CH3OH (5% of CHF3 concentration) was investigated in a tubular alumina reactor at temperatures between 900 K and 1150 K. The presence of CH3OH has a significant influence on both the conversion and selectivity of the reaction. Increasing the CH3OH concentration enhances the conversion of CHF3 by a factor of 1-4 times at temperatures between 873 K and 1123 K, although the rate of formation of the major products (CH2dCF2) does not increase. A reaction scheme, based on NIST HFC and GRI-Mech mechanisms, is developed to model the reactions. Satisfactory agreement between experimental results and predictions is achieved for the conversion of reactants and formation of products. Based on experimental measurements and modeling results, a detailed reaction mechanism is proposed, and insight into how CH3OH influences the reaction of CHF3 with CH4 is presented. Introduction Trifluoromethane (CHF3, HFC 23) is a potent greenhouse gas (GHG), and it has the second-highest global warming potential (GWP) among all the GHGs. It has been reported that the atmospheric concentration of CHF3 has been steadily increasing since at least 1978, and it is presently increasing at an estimated rate of ∼5% per year.1 Together with other hydrofluorocarbons (HFCS), it is an important contributor to the global emission of GHGs.2 Recently, during the Copenhagen Climate Change Summit, countries such as the United States and the European union (EU) acknowledged that the strategy adopted to facilitate phaseout of CFCs and HCFCs should be considered for HFC phaseout. Although a binding agreement was not achieved at the meeting, efforts to reduce the emissions of HFCs will ultimately result in the stockpile of HFCs for eventual destruction. However, few technologies are available to treat HFCs such as CHF3.3-5 The process most widely adopted, as outlined by the United Nations Framework Convention on Climate Change (UNFCC) under the clean development mechanism (CDM)6 uses temperatures as high as 1473 K, together with liquid petroleum gas (LPG) and combustion air, to decompose this extremely stable compound. According to data reported by UNFCCC, ∼25 projects have been registered under the CDM, which aims to reduce the emission of CHF3, thereby reducing the emission of an equivalent of 82.6 million tonnes of CO2.7 As reported in the EU Council with regard to the net GHG emission the EU position on the Copenhagen Climate Conference, other incentives for CHF3 destruction should be found, while strong support for existing CDM projects should continue. We have discovered that a variety of chlorine and bromine containing fluorochemicals such as CCl2F2, CBrClF2, and CHClF2 can be converted to unsaturated hydrofluorocarbon, CH2dCF2, via their reactions with CH4 or CH3Br.8-11 The product, CH2dCF2 is a monomer for the synthesis of poly(vinylidene difluoride) (pVDF), which is a thermoplastic that is inert to various solvents, oils, and acids and shows low permeabilities to gases and liquids.12 A detailed review about the applications of VDF-containing polymers and copolymers has recently been published.13 As reported elsewhere, CH2dCF2 can also be produced via the gas-phase reaction of CHF3 with CH4 at temperatures above * To whom correspondence should be addressed. Tel.: +61 (2) 4985 4422. Fax: +61 (2) 4921 6893. E-mail: [email protected].

1023 K with C2F4, HF, C2H2, C2H3F, C2HF3, and C3F6 as byproducts.9 However, a relatively low yield of CH2dCF2 was observed. It was argued that the reaction between CH3 and CF2 is the major elementary reaction channel for the formation of CH2dCF2 with almost zero activation energy; thus, high CH3 and CF2 concentrations are necessary to enhance the rate of formation of CH2dCF2.14 Generation of CF2 via decomposition of CHF3 has been widely investigated at temperatures above 1023 K.15-19 During the reaction of CHF3 with CH4, it has been hypothesized that reactions occur on the surface of the reactor (R-Al2O3) and CH4 activation takes place as a result of these surface reactions,9 although this hypothesis conflicts with recent experimental results, where it was found that, after packing Al2O3 or AlF3 chips into the reactor, no enhanced conversion of CH4 was observed. More recently, it has been suggested that CH4 is likely to be activated via a series of chain reactions that involve a range of intermediate fluorine-containing species.19 Based on kinetic modeling, these chain reactions are initiated by H and CH3 radicals formed on the surface of reactor (R-Al2O3) via decomposition of CH4. This discovery prompted the current investigation, which aims to examine the effect of initiators on these chain reactions given that the limiting concentration of CH3 radical appears to be the major obstacle to a high rate of CH2dCF2 formation. In the work presented here, CH3OH and C2H6 are employed as initiators for the reaction of CHF3 with CH4, and an assessment of their influence on the reaction is investigated using various chemical kinetic modeling techniques. CH3OH and C2H6 are selected because they are known to release H, CH3, and other radicals readily under the conditions used to study the reaction of CHF3 with CH4. Experimental Section The apparatus used in this study consists of a tubular highpurity (99.99%) alumina reactor with dimensions of 7.0 mm (i.d.) and 60 cm (length). Prior to experiments, the thermo uniform zone was determined (∼15 cm). To maintain reactions taking place in this uniform zone, the volume of reaction zone and residence time was controlled by adjusting the position of two single-ended thermocouple sheaths (alumina, 6.0-mm o.d.) situated in the reactor. Carbon-containing products were identified using a gas chromagraphy/mass spectroscopy (GC/MS) system (Shimadzu Model QP5000) equipped with an AT-Q

10.1021/ie100349x  2010 American Chemical Society Published on Web 08/06/2010

Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010

column and quantified with a micro GC apparatus (Varian Model CP-2003) equipped with a 5A molecular sieve and PoraPLOT Q columns. Where possible, the relative molar response (RMR) factors of halogenated hydrocarbons for TCD detection 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.20 The feed gas was composed of N2 (99.999%, BOC), CH4 (99.97%, Linde), CHF3 (>98%, Coregas), CH3OH (99.5%, Sigma-Aldrich), and C2H6 (99%, Coregas). N2, CH4, CHF3, and C2H6 were metered with calibrated mass flow controllers (Brooks). CH3OH was introduced by bubbling the feed gas through CH3OH liquid at 273 ( 1 K controlled by an ice bath. Prior to the effluent gas reaching the micro GC, mineral acids and moisture were captured by a caustic scrubber and a sodium hydroxide trap. The concentration of trapped hydrogen halides formed during the reaction was determined with an ion chromatograph (Dionex-100) equipped with an Ion-Pac AS14A column (4 mm × 250 mm). This experimental facility has been described in detail elsewhere.21,22 Chemical Kinetic Modeling. The reactions involving CHF3, CH4, C2H6, and CH3OH are modeled using the computer code, “Plug-Flow Reactor”, available with the commercial software package Cosilab. It is a comprehensive tool for numerical simulation of a variety of chemically reactive flows.23 During modeling, all reaction steps are treated as reversible and the kinetics of the reverse reactions is determined by the thermo data and the forward reaction kinetics. Typically, a detailed reaction scheme that is composed of numerous gas-phase reactions forms the basis of the mechanism, and surface reactions are included where appropriate. In the present study, the energy balance equations were neglected, because all experiments were conducted under essentially isothermal conditions. In separate experiments, R-alumina chips (same material used in the reactor) were charged into the reactor. It is found that R-alumina chips had a negligible effect on the reaction, and therefore, no surface reactions were included in the overall mechanism. The elementary reaction mechanism used in this study is based on the NIST mechanism,24 as well as GRI-Mech 3.0 mechanism,25 in combination with thermokinetic parameters obtained from other literature.9,19,26 The mechanism developed and applied in the current study includes 790 reactions and 90 species for the prediction of reaction of CHF3 and CH3OH with CH4. Experimental and Modeling Results Experimental Results. Figure 1 illustrates the influence of methanol on the conversion of CH4 and CHF3. The conversion of CH4 and CHF3 increases significantly following the introduction of relatively small amounts of CH3OH. Although CH3OH makes up only 5% of CHF3 in the feed, the conversion levels of CH4 are almost doubled at 1073 and 1093 K although the influence of CH3OH on conversion is somewhat diminished at 1133 K. To a less extent, conversion levels of CHF3 are increased by 50% at 1073, 1093, and 1133 K upon the addition of CH3OH. Under the conditions studied, the major carbon-containing products include CH2dCF2, C2HF3, and C2F4. Minor species are detected at high temperatures, including C2H2, CH2F2, C2H3F, C3F6, CHF2CHF2, CH3CF3, C2H4, and C2H6. In the presence of CH3OH, CO and CO2 are also detected, although CO2 occurs only in trace amounts. HF is also detected and

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Figure 1. Effect of methanol on the reaction of CHF3 with CH4: (a) conversion of CH4 and (b) conversion of CHF3. Reaction conditions: pressure ) 1 bar, residence time ) 0.5 s, and CHF3:CH4 ) 1. CH3OH was introduced in the ratio of CH3OH:CHF3 ) 0.05.

quantified by IC. At temperatures of >1023 K, some unidentified products are also observed in small quantities and during quantification, their RMR factors are assumed to be the same as that of CH2dCF2. The major product profiles at 1073, 1093, and 1133 K are illustrated in Figure 2. In the presence of CH3OH, CH2dCF2 is the dominant carbon-containing product under all conditions studied. In the absence of CH3OH, the rate of formation of C2F4 is roughly equal to CH2dCF2 (139% of CH2dCF2 at 1073 K, 102% at 1093 K), until at 1133 K, where the rate of formation of CH2dCF2 dominates (rate of C2F4 is only 35% of that of CH2dCF2). Following the introduction of CH3OH, C2F4 formation is suppressed dramatically, down to trace levels. Conversely, the rate of formation of C2H2, C2H6, and CH2F2 increases considerably, especially at temperatures of 1073 and 1093 K. The rates of formation of C2H2 increase from 0.026 mmol h-1 to 0.076 mmol h-1 and from 0 mmol h-1 to 0.088 mmol h-1. Similarly, the rates of formation of C2H6 increase from 0.002 mmol h-1 to 0.069 mmol h-1 and 0.003 mmol h-1 to 0.008 mmol h-1. The concentration of other minor products such as C3F6, C2HF3, and CH2F2 changes slightly with temperature, as shown in Figure 2. Table S1 in the Supporting Information shows the elemental balances of carbon and fluorine during the reaction of CHF3 with CH4 in the presence of CH3OH as a function of temperature at a residence time of 0.5 s, feed ratio of N2:CHF3:CH4 of 10: 1:1, while the CH3OH feed is ∼5% of the flow rate of CHF3. At temperatures of ∼1023 K, elemental balance of C is >95% and gradually drops with temperature. Only 79% of C is recovered at 1113 K. The elemental balance for fluorine follows a similar trend to carbon balance. At temperatures of 973 K and the conversion of CHF3 increases dramatically with temperature. In the presence of CH3OH, the conversion of CHF3 is observed at temperatures as low as 723 K and increases gradually with temperature. This observation is consistent with the comparative activation energies of reactions R2 and R26, as presented in Table 1. The activation energy of reaction R2 is reportedly 1073 K also can be attributed to reaction R3, and, as a result of reaction R3, CF3 is converted back to CHF3 via reaction with CH4 or CH3, which are formed via the decomposition of CH3OH, even though CH4 was not introduced in the feed. Reaction Channels to Product Formation. The primary channel contributing to the formation of CH2dCF2, the reaction of CF2 with CH3 (reaction R27 in Table 1), has been well-

Figure 4. Experimental and modeling results of the rate of formation of major products as a function of temperature for the reaction of CHF3 with CH4 and with CH3OH at a residence time of 0.5 s and with an input ratio of CHF3:CH4 ) 1. CH3OH was introduced in the ratio of CH3OH:CHF3 ) 0.05.


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Figure 6. Ratio of the amount of CH4 in the outlet to the amount in the feed, as a function of temperature at a residence time of 0.5 s and with a CHF3:CH4:CH3OH input ratio of 1:1:0.5. Here, nout denotes the flow rate of CH4 in the outlet of reactor, while nin represents the flow rate of CH4 in the feed.

Figure 7. Comparison of the conversion of CHF3 between reaction of CHF3 with CH3OH and pyrolysis of CHF3, as a function of temperature at a residence time of 0.5 s and a pressure of 1 bar.

Figure 5. Effect of CH3OH:CHF3 feed ratio on the conversion of CHF3 and rates of formation of major carbon-containing products for the reaction CH4 with CHF3 at a residence time of 0.5 s and with an input ratio of CHF3:CH4 ) 1: (a) conversion of CHF3, (b) conversion of CH2dCF2, and (c) conversion of C2F4.

established, both experimentally and theoretically.14 In addition to this reaction, the reaction of CF3 with CH3 (reactions R4, R5, and R6) also leads to the formation of CH2dCF2.43-45 CH3 + CF3 f CH3CF3

(R4)

CH3CF3 f CH2dCF2 + HF

(R5)

CH3 + CF3 f CH2dCF2 + HF

(R6)

In the absence of CH3OH, the contribution from reactions R4-R6 is negligible, because of the low concentration of CF3. This is consistent with the absence of CH3CF3 formed during

the reaction of CH4 with CHF3. In contrast, the presence of CH3OH enhances the rate of formation of CF3 and CH3CF3 is subsequently detected during the reaction (see Table S1 in the Supporting Information), especially at low temperatures. At elevated temperatures, CH3CF3 rapidly undergoes dehydrofluorination, producing CH2dCF2 and a subsequent decrease in the concentration of CH3CF3 is observed with increasing temperature. However, it is suggested that reactions R4-R6 do not play a critical role for the formation of CH2dCF2, because of the low concentration of CH3OH in the feed. It is evident that another role of CH3OH in enhancing the rate of formation of CH2dCF2 is to provide H, CH3, and OH radicals and increase the concentration of CF3 radicals. The major channel for the formation of C2F4 is the dimerization reaction involving CF2 (reaction R7), which is produced by the decomposition of CHF3. However, the rate of reaction R7 is almost 10 times slower than that of reaction R27,46 and thus reaction R27 dominates the consumption of CF2. As a consequence, the rate of formation of CH2dCF2 increases by 100% at 1073 and 1093 K, and, to a lesser extent at 1133 K, whereas the rate of formation of C2F4 decreases by a factor of 7-10, as shown in Figure 2. Note that C2F4 formation is overpredicted considerably by the GRI-NIST mechanism (see Figure 4). One explanation for this is the fact that the GRI-

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NIST mechanism does not contain any reaction steps, leading to the formation of C3F6 from C2F4, and soot as well. In fact, significant quantities of carbonaceous deposits were found in the reactor following reaction, while virtually no soot is predicted by this mechanism. In the modified mechanism, several reaction steps were introduced (reactions R30-R33) and, as a result, good agreement between predictions and experimental measurements are achieved, as shown in Figure 4 and Figure S1 in the Supporting Information. CF2 + CF2 f C2F4

(R7)

Although reactions R30-R33 in Table 1 can predict the formation of C3F6 satisfactorily, there are many uncertainties in elucidating its formation pathways. The reaction of CF2 with C2F4 was studied using the hybrid density functional theory method of BB1K, which showed that the addition of CF2 to C2F4 invariably leads to the formation of c-C3F6.33 However, in the present study, no c-C3F6 was detected. Similarly, no c-C3F6 was observed during the pyrolysis of C4F10.33 The rate of formation of minor products, such as C2HF3, C2H3F, C2H2, and CH2F2, although only present in trace amounts, are also overpredicted by GRI-NIST mechanism significantly. Similar to the argument proposed for C2F4, the absence of reaction steps, which leads to the formation of C3 fluorocarbons, polymers, and carbonaceous deposits, is most likely to be responsible for the observed overprediction of these species, because it is likely that these species undergo further reactions under the conditions studied. Based on the results of pathway analysis, the major channels for these minor products can be outlined as follows: CH3 + CH3 f C2H6

(Formation of C2H6)

C + CH3 f C2H2 + H

(R8)

(Formation of C2H2)

(R9) CHF2 + CH4 f CH2F2 + CH3

(Formation of CH2F2) (R10)

CHF3 + CHF2 f CH2F2 + CF3


2CHF2 f CH2F2 + CF2


CH4 + CF2 f C2H3F + HF

(Formation of C2H3F) (R13)

CH4 + CF f C2H3F + H


CH3 + CHF2 f C2H3F + HF


CH2dCF2 + H f C2H3F + F


CHF2 + CHF2 f CHF2sCHF2

(Formation of C2H2F4) (R17)

CHF2sCHF2 f C2HF3 + HF

(Formation of C2HF3) (R18)

CHF2 + CHF2 f C2HF3 + HF

(Formation of C2HF3) (R19)

Comparison of CH3OH and Ethane. Ethane was selected for comparison with CH3OH, because it does not contain an O

Figure 8. Conversion of CHF3 and CH4 for the reaction of CHF3 with CH4, reaction in the presence of small amounts of ethane and reaction with small quantities of CH3OH, as a function of temperature at a residence time of 0.5 s and a pressure of 1 bar. The feed compositions are CH4:CHF3 ) 1:1, CHF3:CH4:C2H6 ) 1:1:0.2, and CHF3:CH4:CH3OH.

atom in its structure. The comparison enables a better understanding of the role of CH3OH in enhancing the rate of reaction of CHF3 with CH4. As shown in Figure 8, the conversion levels of CH4 and CHF3 in the presence of CH3OH are higher than that of ethane, although the C2H6:CHF3 ratio is close to 0.2, while the ratio of CH3OH:CHF3 is only 0.05. The main reactions involved in the thermal decomposition of ethane can be summarized as reactions R20-R25:47,48 C2H6 f CH3 + CH3

(R20)

C2H6 f C2H5 + H

(R21)

C2H5 f C2H4 + H

(R22)

CH3 + C2H6 f CH4 + C2H5

(R23)

H + C2H6 f C2H5 + H2

(R24)

C2H5 + C2H5 f C4H10

(R25)

It is generally accepted that the decomposition mechanism of C2H6 consists of a series of free-radical chain reactions (reactions R21-R25), which is probably initiated by unimolecular decomposition of C2H6 (reaction R20).49 Since the activation energy of reaction R20 is as high as 370 kJ mol-1, the formation of the CH3 radical from C2H6 is relatively insignificant.50 By contrast, large quantities of H radicals are produced via reactions


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techniques. In the presence of relatively small amounts of CH3OH, the conversion levels of CH4 and CHF3 increase by 50% and 100%, as does the rate of formation of CH2dCF2. Further increasing the proportion of CH3OH, up to a CH3OH/ CHF3 ratio to 0.5, enhances the conversion of CHF3 at temperatures between 873 K and 1123 K, although the rate of formation of CH2dCF2 does not increase. Generally, a reaction mechanism based on the NIST HFC mechanism and GRI-Mech mechanism can predict the trends of conversion of reactants and rates of formation of products, although there are some notable discrepancies. The inclusion of new reactions and modifications of a small number of key elementary reactions improve the predictions markedly. Mechanistic analysis shows that CH3OH provides H and CH3 (enhancing the conversion of CH4 and rate of formation of CH2dCF2) and other radicals, such as OH (enhancing the conversion of CHF3 and the rate of formation of CH2dCF2) to the reaction pool. Comparatively, the promotion of ethane is not as notable, in comparison with CH3OH, although a relatively high concentration of ethane is introduced. Acknowledgment The Australia Research Council is gratefully acknowledged for financial support for this project. W.F.H. is indebted to the Department of Eduction, Science and Training (DEST) of Australia Government and the University of Newcastle, Australia for postgraduate scholarships.

Figure 9. Rate of formation of major carbon-containing products for the reaction of CHF3 with CH4, reaction in the presence of ethane and CH3OH as a function of temperature at a residence time of 0.5 s and a pressure of 1 bar. The feed compositions are CH4:CHF3 ) 1:1, CHF3:CH4:C2H6 ) 1:1: 0.2, and CHF3:CH4:CH3OH ) 1:1:0.05.

R21-R25. In the presence of these reactions steps, the activation of CH4 is enhanced and, subsequently, the rate of formation of CH2dCF2 is augmented (see Figure 9).19 However, conversion of CHF3 seems to be dependent solely on the rate of CHF3 dehydrofluorination (reaction R26), which is inert to attack by H and CH3 radicals.19 Conversion of CHF3, in the presence of ethane, is very similar to that observed during the gas-phase reaction of CHF3 with CH4. Unlike C2H6, which decomposes to produce H, C2 H5, C2H3, and comparatively small amounts of CH3 radicals during decomposition, CH3OH produces numerous oxygen-containing radicals, such as OH, CH2OH, HCHO, and CO. The results in Figure 8 highlight the important role of OH, CH2OH, and HCHO radicals on CHF3 conversion. In the presence of ethane, even when the concentration of C2H6 is relatively high, the rate of formation of CH2dCF2 is lower than that in the presence of CH3OH, as shown in Figure 9. In the presence of CH3OH, reactions R4-R6 contribute to the formation of CH2dCF2, in addition to the major channel of reaction R27. This is the most significant difference between ethane and CH3OH, with respect to their influence on the rate of formation of CH2dCF2. Conclusion The effect of CH3OH on the equimolar gas-phase reaction of CHF3 with CH4 at temperatures between 900 K and 1150 K has been explored using both experimental and modeling

Supporting Information Available: Conversion of CH4 and CHF3, reaction product profiles and elemental balances as a function of temperature (Table S1); experimental and modeling results of rates of formation of minor products as a function of temperature for the reaction of CHF3 with CH4 in the presence of CH3OH (Figure S1), experimental and modeling results for the pyrolysis of CH3OH (Figure S2), and reaction steps included in the modeling mechanism (Scheme S1). This material is available free of charge via the Internet at http://pubs.acs.org/. Literature Cited (1) McCulloch, A.; Lindley, A. A. Global emissions of HFC-23 Estimated to Year 2015. Atmos. EnViron. 2007, 41, 1560. (2) Velders, G. J. M.; Fahey, D. W.; Daniel, J. S.; McFarland, M.; Andersen, S. O. The Large Contribution of Projected HFC Emissions to Future Climate Forcing. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10949. (3) Feaver, W. B.; Rossin, J. A. The catalytic Decomposition of CHF3 over ZrO2-SO4. Catal. Today 1999, 54, 13. (4) Onoda, H.; Ohta, T.; Tamaki, J.; Kojima, K. Decomposition of Trifluoromethane over Nickel Pyrophosphate Catalysts Containing Metal Cation. Appl. Catal., A 2005, 288, 98. (5) Moon, D. J.; Chung, M. J.; Kim, H.; Kwon, Y. S.; Ahn, B. S. Pyrolysis of Trifluoromethane to Produce Hexafluoropropylene. Ind. Eng. Chem. Res. 2002, 41, 2895. (6) UNFCCC, AM0001: Incineration of HFC 23Waste Streams, http:// cdm.unfccc.int/methodologies/index.html, 2006. (7) UNFCCC, Projects of thermal oxidation of synthetic greenhouse gases registered under CDM program of UNFCCC, http://unfccc.int/ 2860.php/, Data accessed on Jan. 8, 2010. (8) Yu, H.; Kennedy, E. M.; Mackie, J. C.; Dlugogorski, B. Z. Simultaneous conversion of CHClF2 and CH3Br to CH2CF2. Chemosphere 2007, 68, 2003. (9) Yu, H.; Kennedy, E. M.; Mackie, J. C.; Dlugogorski, B. Z. An experimental and kinetic modeling study of the reaction of CHF3 with methane. EnViron. Sci. Technol. 2006, 40, 5778. (10) Uddin, M. A.; Kennedy, E. M.; Dlugogorski, B. Z. Gas-phase reaction of CCl2F2 (CFC-12) with methane. Chemosphere 2003, 53, 1189. (11) Tran, R.; Kennedy, E. M.; Dlugogorski, B. Z. Gas-phase reaction of halon 1211 (CBrClF2) with methane. Ind. Eng. Chem. Res. 2001, 40, 3139.

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ReceiVed for reView February 15, 2010 ReVised manuscript receiVed July 5, 2010 Accepted July 19, 2010 IE100349X

Effect of Methanol on the Gas-Phase Reaction of Trifluoromethane

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