Atmospheric chemistry of HFE-7000 (CF3CF2CF2OCH3) and 2,2,3,3,4,4,4-heptafluoro-1-butanol (CF3CF2CF2CH2OH): kinetic rate coefficients and temperature dependence of reactions with chlorine atoms

Abstract

The adverse environmental impacts of chlorinated hydrocarbons on the Earth's ozone layer have focused attention on the effort to replace these compounds by nonchlorinated substitutes with environmental acceptability. Hydrofluoroethers (HFEs) and fluorinated alcohols are currently being introduced in many applications for this purpose. Nevertheless, the presence of a great number of C-F bonds drives to atmospheric long-lived compounds with infrared absorption features. Thus, it is necessary to improve our knowledge about lifetimes and global warming potentials (GWP) for these compounds in order to get a complete evaluation of their environmental impact. Tropospheric degradation is expected to be initiated mainly by OH reactions in the gas phase. Nevertheless, Cl atoms reaction may also be important since rate constants are generally larger than those of OH. In the present work, we report the results obtained in the study of the reactions of Cl radicals with HFE-7000 (CF(3)CF(2)CF(2)OCH(3)) (1) and its isomer CF(3)CF(2)CF(2)CH(2)OH (2). Kinetic rate coefficients with Cl atoms have been measured using the discharge flow tube-mass spectrometric technique at 1 Torr of total pressure. The reactions of these chlorofluorocarbons (CFCs) substitutes have been studied under pseudo-first-order kinetic conditions in excess of the fluorinated compounds over Cl atoms. The temperature ranges were 266-333 and 298-353 K for reactions of HFE-7000 and CF(3)CF(2)CF(2)CH(2)OH, respectively. The measured room temperature rate constants were k(Cl+CF(3)CF(2)CF(2)OCH(3)) = (1.24 +/- 0.28) x 10(-13) cm(3) molecule(-1) s(-1)and k(Cl+CF(3)CF(2)CF(2)CH(2)OH) = (8.35 +/- 1.63) x 10(-13) cm(3) molecule(-1) s(-1) (errors are 2sigma + 10% to cover systematic errors). The Arrhenius expression for reaction 1 was k (1)(266-333 K) = (6.1 +/- 3.8) x 10(-13)exp[-(445 +/- 186)/T] cm(3) molecule(-1) s(-1) and k (2)(298-353 K) = (1.9 +/- 0.7) x 10(-12)exp[-(244 +/- 125)/T] cm(3) molecule(-1) s(-1) (errors are 2sigma). The reactions are reported to proceed through the abstraction of an H atom to form HCl and the corresponding halo-alkyl radical. At 298 K and 1 Torr, yields on HCl of 0.95 +/- 0.38 and 0.97 +/- 0.16 (errors are 2sigma) were obtained for CF(3)CF(2)CF(2)OCH(3) and CF(3)CF(2)CF(2)CH(2)OH, respectively. The obtained kinetic rate constants are related to the previous data in the literature, showing a good agreement taking into account the error limits. Comparing the obtained results at room temperature, k (1) and k (2), HFE-7000 is significantly less reactive than its isomer C(3)F(7)CH(2)OH. A similar behavior has been reported for the reactions of other fluorinated alcohols and their isomeric fluorinated ethers with Cl atoms. Literature data, together with the results reported in this work, show that, for both fluorinated ethers and alcohols, the kinetic rate constant may be considered as not dependent on the number of -CF(2)- in the perfluorinated chain. This result may be useful since it is possible to obtain the required physicochemical properties for a given application by changing the number of -CF(2)- without changes in the atmospheric reactivity. Furthermore, lifetimes estimations for these CFCs substitutes are calculated and discussed. The average estimated Cl lifetimes are 256 and 38 years for HFE-7000 and C(3)H(7)CH(2)OH, respectively. The studied CFCs' substitutes are relatively short-lived and OH reaction constitutes their main reactive sink. The average contribution of Cl reactions to global lifetime is about 2% in both cases. Nevertheless, under local conditions as in the marine boundary layer, tau (Cl) values as low as 2.5 and 0.4 years for HFE-7000 and C(3)H(7)CH(2)OH, respectively, are expected, showing that the contribution of Cl to the atmospheric degradation of these CFCs substitutes under such conditions may constitute a relevant sink. In the case of CF(3)CF(2)CF(2)OCH(3), significant activation energy has been measured, thus the use of kinetic rate coefficient only at room temperature would result in underestimations of lifetimes and GWPs. The results obtained in this work may be helpful within the database used in the modeling studies of coastal areas. The knowledge of the atmospheric behavior and the structure-reactivity relationship discussed in this work may also contribute to the development of new environmentally acceptable chemicals. New volatile materials susceptible of emission to the troposphere should be subject to the study of their reactions with OH and Cl in the range of temperature of the troposphere. The knowledge of the temperature dependence of the kinetic rate constants, as it is now reported for the case of reactions 1 and 2, will allow more accurate lifetimes and related magnitudes like GWPs. Nevertheless, a better knowledge of the vertical Cl tropospheric distribution is still required.