RESEARCH NOTES Experimental and Quantum Chemical Study of the Reaction CF 2 + CH 3 T CF 2 CH 3 f CH 2 dCF 2 + H: A Key Mechanism in the Reaction between Methane and Fluorocarbons Hai Yu, John C. Mackie, Eric M. Kennedy,* and Bogdan Z. Dlugogorski Process Safety and EnVironment Protection Research Group, School of Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia The reaction of CHClF 2 with CH 3 Br 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 C 2 F 4 and CH 2 dCF 2 . The rate of formation of CH 2 dCF 2 increases with temperature and, at 1123 K, CH 2 dCF 2 becomes the dominant product. Other important products detected include CH 4 , CH 2 Br 2 ,C 2 H 2 ,C 2 H 4 , CH 3 Cl, and C 3 H 2 F 4 (CH 2 d CFCF 3 ). It is concluded that the formation of CH 2 dCF 2 occurs via the reaction of CH 3 with CF 2 . To support this conclusion, the potential energy surface for reaction between the carbene, CF 2 , and the radical (CH 3 ) 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 CF 2 and CH 3 initially produces the intermediate, CF 2 CH 3 , which then forms CH 2 dCF 2 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 CF 2 CH 3 and that the reaction between CF 2 and CH 3 leads only to the production of CH 2 dCF 2 and elemental hydrogen. Over the temperature range of 700-2000 K, the rate constant for CF 2 + CH 3 f CH 2 dCF 2 + H can be well approximated by the expression 2.1 × 10 13 T -0.207 cm 3 mol -1 s -1 . Introduction The implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer has resulted in intensive research efforts aimed at searching for alternative agents and developing technologies capable of treating the stockpiles of chlorofluo- rocarbons (CFCs), halons, and other ozone-depleting substances (ODSs). 1 Hydrofluorocarbons (HFCs), which are compounds that contain H, F, and C atoms, are viewed as acceptable alternatives to CFCs and halons, because HFCs do not contain Cl or Br atoms and, hence, have an ozone depletion potential of zero. In the last two decades, the amount of HFCs used as alternatives to CFCs and halons has been increasing dramati- cally. In the meantime, research effort for the development of technologies for the disposal of stockpiled CFCs and halon has focused on hydrodehalogenation, which is a process in which the ODSs react with hydrogen, for the purpose of converting CFCs and halon to HFCs. 2-11 Unfortunately, HFCs are themselves powerful synthetic greenhouse gases (GHGs), with long atmospheric lifetimes. For example, CHF 3 (HFC-23) finds application in the semiconductor industry and as a substitute for CFC refrigerants and halon fire- extinguishing agents. The chemical is also an unintended byproduct of the manufacture of HCFC-22 (CHClF 2 ). It exhibits an estimated atmospheric lifetime of 264 years and displays the second-highest global warming potential (GWP) among all of the known GHGs. 12 Unlike CFCs and halons, the production and consumption of HFCs had not been controlled by any international agreement until the Kyoto Protocol, which has the objective of limiting the emission of GHGs (including HFCs), came into effect in February 2005. 13 The concentration of CHF 3 in the atmosphere has been reported to be steadily increasing and is predicted to continue to do so at a rate of 5% per year. In terms of global warming, the cumulative emissions of CHF 3 up to and including the year 1995, are equivalent to 1.6 billion tonnes of CO 2 . 14 The recent implementation of the Kyoto Protocol will inevitably result in the stockpiling of vast quantities of HFCs for disposal. This will further intensify research efforts aimed at developing technologies for the treatment of not only ODSs but also HFCs. We have recently discovered that CCl 2 F 2 , CBrClF 2 , and CHF 3 can be converted to unsaturated HFCs (such as CH 2 dCF 2 ) through reaction with CH 4 in the homogeneous gas phase. 15-17 For example, a single pass yield of 63% CH 2 dCF 2 was achieved at 1173 K for a halon 1211:CH 4 feed composition of 1:2. 17 CH 2 dCF 2 is a valuable commodity; it is widely used in the fluoroelastomer and semiconductor industries, and it is the key monomer for the synthesis of a variety of products, most notably poly(vinylidene fluoride) (PVDF), Viton (which is produced by Dupont Corporation), and KEL-F (which is produced by 3M Corporation). The significance of this discovery is that a wide range of ODSs and HFCs can be converted to an unsaturated C 2 HFC, which is an environmentally benign and valuable product, through their reaction with methane. However, the fundamental chemistry involved in these novel reactions is far * To whom correspondence should be addressed. Tel.: (+61 2) 4921 6177. Fax: (+61 2) 4921 6920. E-mail: eric.kennedy@newcastle.edu.au. 3758 Ind. Eng. Chem. Res. 2006, 45, 3758-3762 10.1021/ie060221z CCC: $33.50 © 2006 American Chemical Society Published on Web 04/19/2006