Experimental and quantum chemical study of the reaction CF2+CH3⇆CF2CH3→CH2=CF2+H:: A key mechanism in the reaction between methane and fluorocarbons

The reaction of CHClF2 with CH3Br was studied over the temperature range of 773-1123 K at atmospheric pressure in an alumina tubular reactor. At temperatures <923 K, the major products are C2F4 and CH2=CF2. The rate of formation of CH2=CF2 increases with temperature and, at 1123 K, CH2=CF2 becomes the dominant product. Other important products detected include CH4, CH2Br2, C2H2, C2H4, CH3Cl, and C3H2F4 (CH2=CFCF3). It is concluded that the formation of CH2=CF2 occurs via the reaction of CH3 with CF2. To support this conclusion, the potential energy surface for reaction between the carbene, CF2, and the radical (CH3) has been investigated by quantum chemical techniques, using density functional (B3LYP//6-31G(d)) and MP2//6-31G(d) methods. Stationary points on the surface have been computed at a high level of theory, using G3B3 methods. Recombination between CF2 and CH3 initially produces the intermediate, CF2CH3, which then forms CH2=CF2 and elemental hydrogen. Simulations using the MultiWell suite of programs indicate that, over the temperature range of 700-2000 K, there is essentially no stabilization of CF2CH3 and that the reaction between CF2 and CH3 leads only to the production of CH2=CF2 and elemental hydrogen. Over the temperature range of 700-2000 K, the rate constant for CF2 + CH3 --> CH2=CF2 + H can be well approximated by the expression 2.1 x 10(13) T-0.207 cm(3) mol(-1) s(-1).