Kinetic and mechanistic study of OH- and Cl-initiated oxidation of two unsaturated HFCs: C₄F₉CH=CH₂ and C₆F₁₃CH=CH₂

The kinetics and mechanisms of the OH- and Cl-initiated oxidation of two unsaturated HFCs, C₄F₉CH=CH₂ and C₆F₁₃CH=CH₂, were investigated. The kinetic study was performed as a function of pressure and temperature for the OH reactions and as a function of pressure at 298 K for Cl atom reactions. The rate constants obtained are (in units of cm³ molecule^-1 s-1): κ(OH + C₄F₉CH=CH₂) = (8.5 ±1.4) x 10^-13 exp[(139 ±48)/T] and κ(OH + C₆F13CH=CH₂) = (1.3 ±0.5) x 10-¹² exp[(31 ±124)/T] in the temperature range 233-372 K; and κ(Cl + C₄F₉CH=CH₂) = (8.9 ±1.0) x 10^-11 and κ(C1 + C₆F₁₃CH=CH₂) = (9.1 ± 1.0) x 10-11 at 298 K. The OH and Cl reactions rate constants were found to be independent of pressure in the range 10-760 and 15-60 Torr, respectively. The mechanistic study was performed in air at atmospheric pressure, in the presence or absence of NO_x. CO and COF₂ have been identified as the major secondary products of both OH- and Cl-initiated oxidation of the HFCs. However, there is evidence for the formation of different primary products: aldehydes (C₄F₉CHO and C₆F₁₃CHO) in the OH oxidation of the HFCs and ketones (C₄F₉C(O)CH₂C1 and C₆F₁₃C(O)CH₂Cl) in the Cl oxidation. This suggests that the oxy radicals, precursors of these carbonyl compounds, behave differently. The β-hydroxyoxy radicals C₄F₉CH(O)CH₂OH and C₆F₁₃CH(O)CH₂OH decompose, whereas the β-chlorooxy radicals C₄F₉CH(O)CH₂C1 and C₆F₁₃CH(O)CH₂C1 react with O₂. These results are consistent with the significantly higher activation barrier for the decomposition of the β-chlorooxy, compared to that of the β-hydroxyoxy radicals.