Kinetic and Mechanistic Study of OH- and Cl-Initiated Oxidation of Two Unsaturated HFCs: C4F9CHCH2 and C6F13CHCH2

The kinetics and mechanisms of the OH- and Cl-initiated oxidation of two unsaturated HFCs, C 4 F 9 CH CH 2 and C 6 F 13 CH CH 2, 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 cm 3 molecule -1 s -1 ): k (OH + C 4 F 9 CH CH 2 ) = (8.5 ± 1.4) × 10 -13 exp[(139 ± 48)/ T ] and k (OH + C 6 F 13 CH CH 2 ) = (1.3 ± 0.5) × 10 -12 exp[(31 ± 124)/ T ] in the temperature range 233−372 K; and k (Cl + C 4 F 9 CH CH 2 ) = (8.9 ± 1.0) × 10 -11 and k (Cl + C 6 F 13 CH CH 2 ) = (9.1 ± 1.0) × 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 COF 2 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 (C 4 F 9 CHO and C 6 F 13 CHO) in the OH oxidation of the HFCs and ketones (C 4 F 9 C(O)CH 2 Cl and C 6 F 13 C(O)CH 2 Cl) in the Cl oxidation. This suggests that the oxy radicals, precursors of these carbonyl compounds, behave differently. The β-hydroxyoxy radicals C 4 F 9 CH(O)CH 2 OH and C 6 F 13 CH(O)CH 2 OH decompose, whereas the β-chlorooxy radicals C 4 F 9 CH(O)CH 2 Cl and C 6 F 13 CH(O)CH 2 Cl react with O 2 . 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.