(89e) Automated Reaction Mechanism Generation for Chlorinated Hydrocarbons: 1,1,2,3-Tetrachloropropene Production

The fluorochemical sector of the global chemical industry is currently gearing up to replace existing products with equivalents that offer the same performance characteristics but with lower global warming potential (GWP). Thus, the current generation of hydrofluorocarbon (HFC) products will be replaced with hydrofluoroolefins (HFO’s). Key to the timely commercialisation of HFO’s will be the availability of chlorinated feedstocks from which they can be conveniently prepared. 2,3,3,3-Tetrafluoropropene (1234yf) is one of the leading low GWP HFO products identified as a possible replacement for 1,1,1,2-tetrafluoroethane (134a) in mobile air conditioning applications. The key chlorinated feedstock for 1234yf manufacture is 1,1,2,3-tetrachloropropene (1230xa), which can be prepared by two reaction channels, one starting from ethylene and the other from tetrachloroethylene. Both pathways include several steps of dehydrochlorination and free chain radical chlorination reactions. Detailed kinetic modeling of these processes can be a helpful tool to better understand, design and optimize 1230xa production. In order to generate complete detailed models for these chlorination processes, an extensive set of elementary reactions with associated thermodynamic and kinetic parameters is required. Since manually generating such chemical models is effortful and error-prone, it is preferable to use computers instead. In this study, the Python version of the Reaction Mechanism Generator (RMG)¹, an open-source software, has been extended to build detailed kinetic model for 1230xa production.

RMG’s kinetics database contains various types of reaction families, with relevant reaction recipes to generate an extensive set of elementary reactions. In order to ensure that RMG is not missing any pathways for these chlorination processes, three specific reaction classes are implemented in RMG. Two of these reaction classes are related to the chlorine addition into the double bond and chlorine abstraction reactions that can take place through free radical chain mechanism. The third reaction family is the dehydrochlorination through concerted E2 elimination reactions.  Furthermore, some existing reaction families in RMG’s kinetic database are updated with chlorinated groups. Since there are not enough ex­perimental data available for the kinetics of these reactions, electronic structure calculations and Transition State Theory are used to estimate Arrhenius rate parameters and fill the kinetics database for new reaction families.

In this work, a detailed kinetic model is built for 1230xa production through both reaction channels using the extended RMG-Py and simulation results are compared with experimental data from the patent literature. Also the competition between concerted E2 elimination and Sn2 substitution reactions is investigated, and factors that can influence the selectivity and yield of 1230xa are discussed.

¹ Green, W.H., Allen, J.W. et al. RMG – Reaction Mechanism Generator, Python Version. http://rmg.mit.edu, 2013.