THERMAL DECOMPOSITONS OF 1,1,1-TRIFLUOROETHANE AND PENTAFLUOROETHANE IN A TURBULENT FLOW REACTOR

Abstract

The thermal decompositions of 1,1,1-trifluoroethane (CH₃CF₃) and pentafluoroethane (CHF₂CF₃) were studied by using a turbulent flow reactor at atmospheric pressure over the temperature ranges of 1213–1333 K and 1273–1373 K. The rate coefficients for these thermal decompositions were determined from the first-order decays of the reactants to be k(CH₃CF₃) = 10^12.9±1.2 exp[−(276 ± 29) kJ [sdot] mol⁻¹/RT] and k(CHF₂CF₃) = 10^13.9±1.4 exp [−(324 ± 36) kJ [sdot] mol⁻¹/RT]s⁻¹. To examine the reaction paths, we identified the decomposition products using gas chromatography-mass spectrometry (GC-MS) and performed ab initio MO calculations. These results showed that the sequential HF elimination reactions, CH₃CF₃ → CH₂CF₂ + HFand CH₂CF₂ → CHCF + HF, were favorable for the CH₃CF₃ pyrolysis. On the other hand, four initial steps: CHF₂CF₃ → CF₃CF + HF, CHF₂CF₃ → CF₂CF₂ + HF, CHF₂CF₃ → CHF₃ + CF₂ and CHF₂CF₃ → CHF₂ + CF₃, were possible for the CHF₂CF₃ pyrolysis. Such a difference in the reaction paths between the CH₃CF₃, and CHF₂CF₃ pyrolyses can be explained by fluorine hyperconjugation and by repulsion between the fluorine atoms on the 1- and 2-carbons of each hydrofluoroethane.

Keywords:

• Chemical kinetics
• Hydrofluorocarbons
• Pentafluoroethane
• Product channels
• Rate coefficients
• Thermal decomposition
• Turbulent flow reactor
• 1,1,1-Trifluoroethane

The authors wish to express our gratitude to Professor Frank Scott Howell S. J. for his valuable comments. This work was supported in part by Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C) No. 16560194.