KINETIC-STUDY ON THERMAL-DECOMPOSITION OF CHLOROBENZENE DILUTED IN H-2

Thermal reaction studies of dilute mixtures (0.37%) of chlorobenzene in hydrogen have been performed in tubular flow reactors at various surface to volume ratios and 1 atm total pressure. Residence times range from 0.02 to 2.5 s with temperatures between 1050 and 1275 K. HCl, benzene, and C (solids) are observed as the major products; minor products include methane, cyclopentadiene, toluene, naphthalene, and biphenyls. Chlorobenzene pyrolysis in helium yields significantly less conversion but more C (solids) for similar residence times. A detailed chemical mechanism is developed to describe this reaction system. Our modeling calculations incorporate Energized Complex QRRK analysis for accurate inclusion of temperature and pressure effects in radical addition reactions. This is a straightforward method to estimate rate constants and branching ratios as a function of both T and P. The detailed mechanism, based upon fundamental thermodynamic and kinetic principles, describes the overall reaction remarkably well. We also propose a plausible kinetic scheme describing formation of minor products. The experimental observations can be explained in terms of energized adduct formation, followed by unimolecular dissociation to low-energy exit channels. Phenyl and Cl radicals produced by the initial unimolecular decay of chlorobenzene react with H2 to form benzene and HCl + H, respectively. Hydrogen atom addition to chlorobenzene via ipso attack, with subsequent loss of Cl from the energized complex, is required to explain the faster reaction in H2 than He. QRRK rate constant analysis is presented for the following reaction systems, temperature range 300-1900 K, 10-3 to 1.0 atm, nitrogen and hydrogen bath gases: C6H5Cl + H ↔ C6H6 + Cl; C6H6 + H ↔ [CyC6H7•] ↔ products; C6H6 + CH3 ↔ C6H5CH3 + H; C6H5Cl + CH3 ↔ C6H5CH3 + Cl; C6H5Cl + C6H5• ↔ C6H5C6H5 + Cl; C6H5Cl + C6H5• ↔ C6H5C6H4Cl + H; C6H6 + C6H5• ↔ C6H5C6H5 + H. © 1990 American Chemical Society.