Formates degradation initiated by Cl reactions.

ARANDA, A.; DIAZ DE MERA,Y.; BRAVO, I.; MARIA EUGENIA TUCCERI; RODRIGUEZ, D.; RODRIGUEZ, A.; MORENO, E.

Manchester, Inglaterra

Simposio; The 20th International Symposium on gas Kinetics; 2008

Introduction. An important effort to replace chlorinated compounds by non-chlorinated environmentally acceptable substitutes is being carried out. Methyl formate is used as foam blowing agents replacing CFC-11 in a wide variety of applications.1 Another formate, ethyl formate, is a naturally occurring volatile compound and is being evaluated as fumigant for stored grains since it is a potential alternative to the ozone-depleting fumigant methyl bromide.2 Chlorine chemistry may play an important role in marine or coastal areas where significant chlorine atoms concentrations may be present.3 In these regions, Cl initiated processes compete with the reactions initiated by OH radicals. We present the first study of the reactions of Cl atoms with CH3OC(O)H and CH3CH2OC(O)H in the temperature range of 253-333 K, thus providing data useful to simulate the temperature profile characteristic of the troposphere: CH3OC(O)H + Cl ® Products                          (1) CH3CH2OC(O)H + Cl  ® Products                  (2) Experimental. The investigations were performed using the absolute discharge flow-mass spectrometry technique (DF-MS). Helium was used as a carrier gas. Cl atoms were produced by microwave discharge of Cl2. Chlorine atoms were followed as ClCH=CH2 during a kinetic run, after scavenging with BrCH=CH2 at the end of the reactor by the fast reaction: Cl + BrCH=CH2 → ClCH=CH2 + Br      k = 1.43±0.28x10-10 cm3 molecule-1 s-1       (3)           Ref. 4 The rate constants at 1 Torr have been determined under pseudo-first-order conditions with the organic compound in excess over Cl atom. Results and discussion. The rate coefficients measured at room temperature were k(1) = (1.01±0.15)x10-12 and k(2) = (8.78±1.22)x10-12 cm3 molecule-1 s-1 (errors are 2s+10%). Taking into account the error limits, these results obtained under low-pressure conditions in this work are in good agreement with those obtained in previous experiments by other authors. The reaction rate constants increase with rising temperature within the studied range, 253-333 K, for reactions (1) and (2) at a total pressure of 1 Torr in helium. The Arrhenius equation applies to these results. A linear least-squares analysis of the data yields the activation energy and the pre-exponential factor. From the obtained data we propose the following expressions where errors are 2s: k(1) = (1.7±1.4)x10-11exp-(814±254)/T cm3 molecule-1 s-1        k(2) = (5.5±4.8)x10-11exp-(556±268)/T cm3 molecule-1 s-1 In both cases, the obtained activation energy, Ea/R = 814±254K (6.7±2.1 kJ) and 556±268K (4.6±2.2 kJ), for reactions (1) and (2), respectively, are positive as expected for an abstraction process. The activation energy decreases as the length of the hydrogenated chain increases as shown by the data of methyl and ethyl formates. From these data, a significant decrease of the rate constants is expected within the tropospheric temperature range. The reactions 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±0.09 (error is 2σ) for reaction (1) and 0.96±0.11 (error is 2σ) for reaction (2) were obtained. Atmospheric implications. Lifetimes for both compounds are calculated using, t=1/k[X], where [X]=OH or Cl. For the average tropospheric concentrations, it is clear that OH reaction constitutes the main reactive sink for methyl and ethyl formates, (66,9 and 13,6 days, respectively) at daytime. The contribution of Cl reactions to global lifetime at daytime is about 0.6 and 1%, respectively. However, under local conditions as in the marine boundary layer and coastal regions in the early morning hours, where Cl concentrations can be as high as 1x105 molecule cm-3,3  tCl values as low as 114,6 and 13,2 days for CH3OC(O)H and CH3CH2OC(O)H are respectively expected. In these conditions, Cl oxidation competes directly with OH oxidation, and Cl reactions play a significant role as a reactive sink for these compounds in the troposphere. References. 1. U.S. Environmental Protection Agency. Clean Air Act; http://www.epa.gov/ozone/snap/foams/lists/applianc.html. 2. P. Kumar, T.R. Chauhan, R. Gera and N. Kumar, proceeding to the 2007 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions, San Diego, October, 2007. 3. C. W. Spicer, E. G. Chapman, B. J. Finlayson-Pitts, R. A. Plastridge, J. M. Hubbe, J. D. Fast and C. M. Berkowitz Nature, 1998, 394, 353-356. 4. J. Y. Park, I. R. Slagle and D. Gutman, J. Phys. Chem., 1983, 87, 1812-1818.