(153d) Estimation of the H-Abstraction Reaction by a Cl Radical From Theory and Experiment: A Critical Reaction Step in Thermal Chlorination of Hydrocarbon

Understanding a chemical reaction from its incipient elementary reactions is essential for the prediction and interpretation of a chemical event happening in an industrial or laboratory scale reactor. This is important for the optimization of an existing chemical process or providing guidance to scoping and optimization of a new chemical process. Such a detailed understanding of chemical mechanism is achievable today for homogenous reactions from ab initio Quantum Mechanical (QM) calculation with high confidence limit, however, much limited for heterogeneous reaction, where lack of fundamental understanding of the heterogeneous phase (often time catalyst surface) limits the applicability. There has been an avalanche of research activity on the ab initio Quantum Mechanical (QM) approach to study chemical reactions in last few decades [1], which makes it possible to describe chemical reaction from elementary steps and develop a detailed kinetics model. Such a detailed kinetic model when apply with proper mass and energy balance can describe a chemical event a priori and optimize its performance. Chlorinated hydrocarbons are industrially important raw materials or intermediates for various application fronts such as refrigerants, industrial solvents to name a few. We have been engaged in developing detailed mechanisms for our chlorinated hydrocarbon portfolio in DOW since mid 1990s. The existing kinetics and thermodynamics in the database has been validated against various DOW processes containing C1, C2 and C3 chlorinated hydrocarbons. However, optimal design of a reactor or accurate estimation of the root cause of a production problem is critically dependent on the accuracy of the kinetic and thermodynamic parameter in the database. Often time, sensitivity of these parameters in the data base is not judged carefully, even for the simplest of the reaction and eventually led to unacceptable prediction and failure of model performance. One of the critical but simple reaction in this series is H abstraction by a Cl* radical from a hydrocarbon. This is one of the major chain carrying reactions in most of the thermal chlorination or radical polymerization processes. Even for the simplest reactions in this series ? CH4 + Cl* CH3* + HCl CH3Cl + Cl* CH2Cl* + HCl CH2Cl2 + Cl* CHCl2* + HCl CHCl3 + Cl* CCl3* + HCl reported rate constants from experiment or correlation in the literature are very scattered, and often time difficult to identify the correct one [2], as depicted in Figure 1. A systematic estimation of the reaction barrier for simple reactions is now possible using ab initio QM methodology. However, reaction energetic obtained for these series of reactions using different state of the art advanced level of theoretical methodology available today are also as scattered as the reported experimental data. As a result a very systematic estimation of the rate constant for this series of reaction is required for the development of a detailed kinetic model for any thermal chlorination process. In this work we will describe in detail the rate constant estimation using advance QM theory and experimental data for some of the key elementary reactions for the H-abstraction step of the thermal chlorination process. 1. W. J. Hehre; L. Radom; P. v. R. Schleyer and J. A. Pople, Ab initio Molecular Orbital Theory, John Wiley & Sons, NY, 1986. 2. D. L . Baulch, R. A. Cox, R. F. Hampson, Jr., J. A. Kerr, J. Troe and R. T. Watson, J. Phys. Chem. Ref. Data 13, 1259, 1984. Figure 1: Activation energy at 0K (E0) for reaction RH + Cl. R. + HCl as a function of C-H bond energy being broken at 0K.