Product distribution and mechanism of the OH- initiated tropospheric degradation of three CFC replacement candidates: CH3CF[double bond, length as m-dash]CH2, (CF3)2C[double bond, length as m-dash]CH2 and (E/ Z)-CF3CF[double bond, length as m-dash]CHF

Abstract

The OH radical initiated photodegradation of 2-fluoropropene (CH₃CF[double bond, length as m-dash]CH₂), 3,3,3-trifluoro-2-(tri-fluoromethyl)propene ((CF₃)₂C[double bond, length as m-dash]CH₂) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF₃CF[double bond, length as m-dash]CHF) has been investigated for the first time using a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air coupled with in situ FTIR spectroscopy to monitor reactants and products. The major products observed in the OH reaction were CH₃C(O)F (98 ± 5)% together with HC(O)H (89 ± 7)% as a co-product, CF₃C(O)F (103 ± 8)% together with HC(O)F (96 ± 7)% as a co-product and CF₃C(O)CF₃ (91 ± 8)% together with HC(O)H (98 ± 12)% as a co-product from the C₁-C₂ bond cleavage channel of the intermediate hydroxyalkoxy radical, formed by addition of OH to the terminal carbon of the double bond which is designated C₁ of 2-fluoropropene, (E/Z)-1,2,3,3,3-pentafluoropropene and 3,3,3-trifluoro-2-(tri-fluoromethyl)propene, respectively. The present results are compared with previous studies for the reaction of OH with the separate isomers (E) and (Z) of 1,2,3,3,3-pentafluoropropene. In addition, atmospheric implications of the reactions studied are discussed.

Fig. 1. Mechanism for the reaction of 2-fluoropropene with OH radicals via addition of OH to the double bond.

Fig. 2. Mechanism for the reaction of 3,3,3-trifluoro-2-(trifluoromethyl)propene with OH radicals via addition of OH to the double bond.

Fig. 3. Mechanism for the reaction of 1,2,3,3,3-pentafluoropropene (mixture E/Z) with OH radicals via addition of OH to the double bond.

Fig. 4. Panel A shows the infrared product spectrum obtained from a 2-fluoropropene/H₂O₂/air reaction mixture after UV irradiation after subtraction of 2-fluoropropene bands. Panels B and C show reference spectra of acetyl fluoride and formaldehyde, respectively. Panel D shows the residual product spectrum obtained after subtraction of features due to the reference spectra from the spectrum in panel A.

Fig. 5. Plots of the concentrations of the reaction products formyl fluoride and trifluoroacetylfluoride as a function of reacted 1,2,3,3,3-pentafluoropropene (mixture cis/trans) obtained from UV photolysis of 1,2,3,3,3-pentafluoropropene (mixture E/Z)/H₂O₂/air reaction mixtures.