Kinetic study of the gas-phase reaction of CF3CHFCF2CH2OH with OH radicals at 230–430 K

Chemical Physics Letters 382 (2003) 277–282 www.elsevier.com/locate/cplett

Kinetic study of the gas-phase reaction of CF3CHFCF2CH2OH with OH radicals at 230–430 K L. Chen a

a,*

, K. Tokuhashi a, S. Kutsuna a, A. Sekiya a, Y. Yonei b, A. Yamamoto b

National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan b Daikin Industries Ltd., Settsu, Osaka 566-8585, Japan Received 15 May 2003; in final form 25 September 2003 Published online: 13 November 2003

Abstract The rate constants, kl , for the reaction of CF3 CHFCF2 CH2 OH with OH radicals were measured using both absolute and relative rate methods at 230–430 K. The absolute rate constants were measured at 250–430 K using flash photolysis and laser photolysis combined with laser-induced fluorescence detection. A smog chamber/FTIR technique was used for the relative rate measurements at 230–308 K with CH2 Cl2 and CHCl3 as reference compounds. The kl values obtained with the two methods were consistent within the experimental uncertainties. The temperature dependence of kl was determined to be (2.49
0.30)  1012 exp[)(880
40)/T ] cm3 molecule1 s1 . The atmospheric lifetime was estimated to be 0.34 year. Ó 2003 Elsevier B.V. All rights reserved.

1. Introduction The partially fluorinated alcohol CF3 CHFCF2 CH2 OH was developed to replace chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). The CF3 CHFCF2 CH2 OH molecule, which does not contain chlorine, has been shown to have zero stratospheric ozone depletion potential [1]. However, the carbon–fluorine bonds in CF3 CH FCF2 CH2 OH will cause absorption in the terrestrial infrared radiation region of 800–1200 cm1 [1],

*

Corresponding author. Fax: +81-298-61-8163. E-mail address: [email protected] (L. Chen).

and thus the molecule has the potential to contribute to global warming. CF3 CHFCF2 CH2 OH will be removed from the troposphere by reaction with OH radicals. The atmospheric lifetime of CF3 CHFCF2 CH2 OH depends on the rate constant for its reaction with OH radicals as well as on the temperature dependence of the rate constant. In this study, we employed absolute [2–4] and relative rate methods [3,5] to measure kl in the temperature range of 230–430 K. The kl values obtained with the two methods were compared, and the reliability of the data was determined. The atmospheric lifetime of CF3 CHFCF2 CH2 OH with respect to reaction with OH radicals was estimated.

0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.162

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2. Experimental

2.2. Relative rate method

2.1. Absolute rate method

The experimental procedures were similar to those reported previously [7]. All experiments were performed in a 1-m3 stainless steel cylindrical chamber with an inner diameter of 1.0 m interfaced to a Bomem DA8 FTIR spectrometer with a White-type optical multiple-reflection mirror system (optical path length ¼ 54 m). The inside wall of the chamber was coated with Teflon. Two 1-kW Xe short-arc lamps (Ushio Co., Japan) were used as the light sources, and the UV light was cut with two optical filters to give a wavelength >260 nm (Shima Quartz Co., Japan). OH radicals were generated by the photolysis of O3 in the presence of water vapor in 100 Torr of He. The losses of CF3 CHFCF2 CH2 OH and reference compounds were determined by FTIR and gas chromatography with flame ionization detection. The total pressure in the chamber decreased by around 0.001% when each sample for gas chromatographic analysis was collected. The reagents used were CH2 Cl2 or CHCl3 (99%), He (99.99995%), and pure O2 (99.99%).

The kinetic measurements were carried out under first-order conditions using flash photolysis (FP) and laser photolysis (LP) combined with a laser-induced fluorescence (LIF) technique to monitor the OH radical concentration [6]. A Pyrex glass reactor with an inner diameter of 25 mm and a length of approximately 40 cm was used for the FP and LP experiments. In the FP experiments, OH radicals were produced by the pulsed photolysis of H2 O by a Xe flash lamp (EG&G FX-193U, typically 800 V and 2 lF) in the presence of a large excess of argon bath gas. In the LP experiments, OH radicals were produced by the reaction O(1 D) + H2 O ! 2OH in the presence of a large excess of helium or argon bath gas, and O(1 D) atoms were generated by photodissociation of N2 O with an ArF excimer laser. Since the vapor pressure of CF3 CHFCF2 CH2 OH is low, the sample vapor could not be supplied directly to the reactor. Therefore, the gas mixture was diluted to about 1.3% with argon and then supplied to the reactor. The concentration of the gas mixture was determined by the partial pressure method. The flow rates of the gas mixture, the carrier gas, and N2 O diluted with He (for the LP method) were measured and controlled by means of calibrated mass flow controllers. The concentrations of the compounds in the reactor were determined from the flow rates of the gases, the temperature, the total pressure in the reactor, and the concentration of gas mixtures. The mixture of CF3 CHFCF2 CH2 OH diluted with argon was prepared at least several hours before use. In some cases, in order to determine the effects of mixing and the stability of the mixture, we prepared the gas mixture several days before use. The time of preparation had no detectable effects on the measured rate constants. He (99.995%) and Ar (99.995%) were used without further purification. N2 O (99.999%) diluted with He was purchased as a mixture with a N2 O concentration of around 1%. The reagent of CF3 CHFCF2 CH2 OH (97%) was obtained from Daikin Industries Ltd. Co., Japan.

3. Results and discussion 3.1. Absolute rate constants Fig. 1 shows a typical pseudo-first-order OH radical decay plot obtained with the LP method for various CF3 CHFCF2 CH2 OH concentrations. The pseudo-first-order rate constants ðkobs Þ were derived from the slope of the straight line by the least-squares method and are plotted against CF3 CHFCF2 CH2 OH concentration in Fig. 2. The rate constant for the reaction of CF3 CHFCF2 CH2 OH with OH radicals was derived as (1.55
0.14)  1013 cm3 molecule1 s1 from the slope of the line obtained by linear least-squares analysis of the data in Fig. 2. The uncertainty does not include systematic errors. We estimated the systematic errors in our experiments to be less than 10%. To minimize these errors, we repeated the experiments at intervals ranging from several days to a few months under a variety of experimental conditions. The kobs value at the intercept (zero

L. Chen et al. / Chemical Physics Letters 382 (2003) 277–282

20

impurities in the gas mixture. The kd value depended upon experimental variables such as total pressure, carrier gas, and so on. We examined the effects of impurities on the absolute measurements. The CF3 CHFCF2 CH2 OH used in these experiments (97%) was used as supplied without further purification. The major impurity was CHF2 CF(CF3 )CH2 OH (3%), and the remaining impurities amounted to less than 0.1%. Because the reaction of CHF2 CF(CF3 )CH2 OH with OH radicals probably has the same rate constant as the reaction of CF3 CHFCF2 CH2 OH, the influence of this impurity on the measured rate constants was probably negligible. Even if the OH reaction rate constants of the remaining impurities were as large as 1.0  1011 cm3 molecule1 s1 , their influence on the measured rate constants could be no larger than 8%, which is similar to the experimental uncertainty. Thus, the effects of CHF2 CF(CF3 )CH2 OH and the other impurities were assumed to be negligible. The kl data over the temperature range of 250–430 K obtained by means of the FP and LP methods are summarized in Table 1, along with the experimental conditions for the individual measurements. The OH decay shows exponential behavior, and the scatter of the points for the individual decay plots was much the same as that shown in Fig. 1 for all experiments. The plots of kobs against CF3 CHFCF2 CH2 OH concentration were similar to those shown in Fig. 2. The kl data obtained by means of the FP and LP techniques were consistent within experimental uncertainty.

10

3.2. Relative rate constants

ln([OH])

0

Time (ms) Fig. 1. Pseudo-first-order decay of OH radicals for various CF3 CHFCF2 CH2 OH concentrations. LP–LIF method. P ¼ 40 Torr, T ¼ 298 K. [CF3 CHFCF2 CH2 OH]/1014 molecule cm3 ; ðjÞ, 0; ð
Þ, 0.264; ðdÞ, 0.594; ðNÞ, 0.918; ðsÞ, 1.24; ðMÞ, 1.56; ðOÞ, 1.88; ð.Þ, 2.21; ðrÞ, 2.52; ð}Þ, 2.84. 60

50

40 -1

kobs (s )

279

30

0

0

14

1x10

14

2x10

14

3x10 -3

[CF3CHFCF2CH2OH] (molecule cm ) Fig. 2. Plot of the observed pseudo-first-order rate constant kobs against the concentration of CF3 CHFCF2 CH2 OH. LP–LIF method. P ¼ 40 Torr, T ¼ 298 K.

reactant) in Fig. 2 ðkd Þ was mainly due to the diffusion of OH radicals from the viewing zone and partially due to the reaction of OH radicals with

The relative rate constants of kl at 298 K obtained from seven and three runs based on the two reference compounds (CH2 Cl2 and CHCl3 ), respectively, are shown in Fig. 3. Eq. (I) was used to evaluate the kl /kr ratio [2,5]:   ½CF3 CHFCF2 CH2 OH0 ln ½CF3 CHFCF2 CH2 OHt    kl ½Reference0 ¼ ln ; ðIÞ kr ½Referencet

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L. Chen et al. / Chemical Physics Letters 382 (2003) 277–282

Table 1 Observed values of kl using FP and LP techniques at 250–430 K T (K)

Techniquea

1013 kl (molecule1 s1 ) Residence time (s) P (Torr) 1014 C b (molecule cm3 ) No. of experiments

250 250 273 298 298 331 375 430 430

FP–LIF LP–LIF FP–LIF FP–LIF LP–LIF FP–LIF FP–LIF FP–LIF LP–LIF

0.784
0.069 0.790
0.058 0.982
0.077 1.27
0.08 1.33
0.07 1.69
0.09 2.36
0.09 3.15
0.12 3.29
0.15

0.44–0.67 0.44–0.69 0.44–0.66 0.45–0.66 0.42–0.60 0.37–0.55 0.43–0.59 0.43–0.64 0.44–0.74

20–80 40–80 20–60 20–60 20–80 20–60 20–60 20–60 20–60

0.26–2.89 0.27–2.88 0.24–2.81 0.19–2.89 0.26–2.85 0.27–2.82 0.25–2.86 0.24–2.84 0.23–2.83

7 7 8 8 6 10 6 8 6

The quoted errors are 2 SD. FP, flash photolysis; LP, laser photolysis; LIF, laser induced fluorescence. b Concentration of CF3 CHFCF2 CH2 OH.

ln( [CF3CHFCF2CH2OH]0/[CF3CHFCF2CH2OH]t )

a

Reference þ OH ! products

1.4

CH2Cl2 x 0.5

1.2

CHCl3 1.0 0.8 0.6 0.4 0.2 0

0

0.2

0.4

0.6

0.8

1.0

1.2

" ln([Reference] /[Reference]t ) 0

Fig. 3. Loss of CF3 CHFCF2 CH2 OH vs CH2 Cl2 ð
Þ and CHCl3 ðÞ by the photolysis of a CF3 CHFCF2 CH2 OH– reference–O3 –H2 O–O2 –He gas mixture at 298 K.

where [CF3 CHFCF2 CH2 OH]0 and [Reference]0 represent the initial concentrations of CF3 CH FCF2 CH2 OH and the reference compounds; [CF3 CHFCF2 CH2 OH]t and [Reference]t represent the concentrations of CF3 CHFCF2 CH2 OH and the reference compounds at reaction time t; and kl and kr are the rate constants for reactions 1 and 2, respectively: CF3 CHFCF2 CH2 OH þ OH ! products

kl

ð1Þ

kr

ð2Þ

The slopes from the linear least-squares analysis of the data in Fig. 3 gave kl =kr . The kl (298 K) values were obtained from kl =kr and k(CH2 Cl2 ) ¼ 1.1  1013 (
40%) cm3 molecule1 s1 and k(CHCl3 ) ¼ 1.0  1013 (
20%) cm3 molecule1 s1 at 298 K [8], respectively (Table 2). Loss of CF3 CHFCF2 CH2 OH, CH2 Cl2 , and CHCl3 through reactions other than reaction with OH radicals must be taken into account quantitatively. Potential loss processes in this reaction system are UV photolysis and reactions with O(1 D) and Cl atoms. The direct photolysis of CF3 CH FCF2 CH2 OH, CH2 Cl2 , and CHCl3 was determined to be negligible. Reactions with O(1 D) were also insignificant in this system because of the large excess (60- to 1000-fold) of H2 O and O2 relative to the reactants. Cl atoms are rapidly scavenged by O3 at 1014 –1015 molecules cm3 with a reaction rate constant of 1.2  1011 cm3 molecule1 s1 [8], and the reactivity of the resulting ClO with CF3 CHFCF2 CH2 OH, CH2 Cl2 , and CHCl3 is expected to be low in comparison to that of OH radicals because CH4 reacts with ClO with a rate constant of <4.0  1018 cm3 molecule1 s1 [8]. The kl =kr ratios at different temperatures were measured over the temperature range of 230–308 K. The plots of ln([CF3 CHFCF2 CH2 OH]0 /[CF3 CHFCF2 CH2 OH]t ) vs ln([Reference]0 /[Reference]t ) obtained were similar to those shown in Fig. 3. The kl values at different temperatures obtained are summarized in Table 2.

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Table 2 Observed values kl =kr and kl using relative rate method at 230–308 K T (K)

1013 kl (cm3 molecule1 s1 )

kl =kr

230 248 278 288 298 308

CH2 Cl2

CHCl3

CH2 Cl2

CHCl3

1.53
0.06 1.38
0.03 1.18
0.06 1.13
0.05 1.04
0.04 1.08
0.03

1.59
0.14 1.52
0.10 1.31
0.08 1.29
0.03 1.22
0.03 1.23
0.06

0.659
0.026 0.800
0.017 1.03
0.05 1.11
0.06 1.14
0.04 1.32
0.04

0.636
0.056 0.807
0.053 1.03
0.06 1.13
0.02 1.19
0.03 1.32
0.06

The quoted errors are 2 SD.

The kl values at 298 K obtained with the two methods agreed within 10%, which is similar to the experimental uncertainty. Additionally, the measured kl values were consistent with the value predicted by Tokuhashi et al. [6] 1.05  1013 cm3

molecule1 s1 . We also obtained consistent OH radical rate constants for CHF2 CF2 OCH2 CF3 with the two methods used in this study [4,9]. These facts show that the kinetic data for kl obtained in this study are reliable. 3.3. Arrhenius plots

-13

5x10

-13

FP-LIF LP-LIF

4x10

CH 2 Cl 2

-1

-1

k (cm molecule s )

-13

3x10

-13

2x10

3

CHCl 3

-13

1x10

-14

5x10

2.0

2.5

3.0

3.5

4.0

4.5

-1

" 1/T (1000 K )

Fig. 4. Arrhenius plot of kl obtained from absolute and relative rate methods: ðOÞ FP–LIF; ðMÞ LP–LIF; ð
Þ CH2 Cl2 ; ðsÞ CHCl3 .

Using k ¼ AeEa =RT , we determined the Arrhenius rate parameters by nonlinear least-squares analyses of the plots in Fig. 4. The Arrhenius rate parameters and the values of kl at 298 K are listed in Table 3. The temperature dependence for kl obtained with the relative rate method was slightly lower than that obtained with the absolute method. This difference seems to be caused by the narrow temperature range of the relative rate method. Additionally, if the temperature dependencies of the reactions of CH2 Cl2 and CHCl3 with OH radicals used as references were too low, the temperature dependence for relative rate data would be lower. Therefore, although the absolute rate data show a slight curvature in the Arrhenius plot, and a curved Arrhenius plot would then rationalize the lower-temperature dependence obtained for the lower-temperature relative rate data, we could not confirm the curvature in the Arrhenius plot for kl .

Table 3 Arrhenius rate parameters of kl over the temperature range 230–430 K 1012 A (cm3 molecule1 s1 )

E/R (K)

1013 kl (298 K) (cm3 molecule1 s1 )

T (range)

Method

2.46
0.26 0.969
0.166 2.49
0.30

880
40 620
50 880
40

1.28
0.08 1.21
0.04 1.30
0.08

250–430 230–308 230–430

Absolute Relative Average

The quoted errors are 2 SD.

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L. Chen et al. / Chemical Physics Letters 382 (2003) 277–282

3.4. Atmospheric lifetime By using s ¼ ðkl (272 K)  [OH])1 , we obtained an atmospheric lifetime ðsÞ of 0.34 year for CF3 CHFCF2 CH2 OH with respect to reaction with OH radicals from a global mean value of (9.4
1.3)  105 radicals cm3 [10] for the concentration of OH radicals and a kl (272 K) value of 9.8  1014 cm3 molecule1 s1 . References [1] UNEP/WMO, Global Ozone Research and Monitoring Project: Scientific Assessment of Ozone Depletion: 1994, Report No. 37. [2] R. Atkinson, Chem. Rev. 86 (1986) 69. [3] K.-J. Hsu, W.B. DeMore, J. Phys. Chem. 99 (1995) 11141.

[4] K. Tokuhashi, A. Takahashi, M. Kaise, S. Kondo, A. Sekiya, S. Yamashita, H. Ito, J. Phys. Chem. A 104 (2000) 1165. [5] B.J. Finlayson-Pitts, S.K. Hernandez, H.N. Berko, J. Phys. Chem. 97 (1993) 1172. [6] K. Tokuhashi, H. Nagai, A. Takahashi, M. Kaise, S. Kondo, A. Sekiya, M. Takahashi, Y. Gotoh, A. Suga, J. Phys. Chem. A 103 (1999) 2664. [7] L. Chen, S. Kutsuna, K. Nohara, K. Takeuchi, T. Ibusuki, J. Phys. Chem. A 105 (2001) 10854. [8] W.B. DeMore, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, C.E. Kolb, M.J. Molina, JPL Publ. 97-4 (1997). [9] L. Chen, S. Kutsuna, K. Tokuhashi, A. Sekiya, K. Takeuchi, T. Ibusuki, Int. J. Chem. Kinet. 35 (2003) 239. [10] R.G. Prinn, J. Huang, R.F. Weiss, D.M. Cunnold, P.J. Fraser, P.G. Simmonds, A. McCulloch, C. Harth, P. Salameh, S. OÕDoherty, R.H.J. Wang, L. Porter, B.R. Miller, Science 292 (2001) 1882.