Kinetics of Hydrogen and Hydroxyl Radical Attack on Phenol at High Temperatures

Article abstract of DOI:10.1021/j100319a023

The kinetics of hydrogen and hydroxyl radical attack on phenol have been studied in single-pulse shock tube experiments.The hydrogen atoms are formed through the unimolecular decomposition of hexamethylethane (C8H18) to two tert-butyl radicals which rapidly decompose to hydrogen atoms and isobutene.The hydrogen atoms react with phenol via abstraction of the phenolic hydrogen and displacement of OH.From the yields of isobutene and benzene we derive the ratio of the rate for these two processes over the temperature range 1000-1150 K and pressures between 2.5 and 5 atm.In th e presence of added methane, benzene yield is depressed due to the reaction of hydrogen with methane.Since the rate of H atom reaction with methane is known, the rate constant ratio can be used to derive absolute rate constants.Using a rate expression for k(H + CH4 -> CH3 + H2) = 2.4*10¹¹ exp(-7000/T) L/(mol*s), we find k(H + C6H5OH -> H2 + C6H5O) = 1.15*10¹¹ exp(-6240/T) L/(mol*s) and k(H + C6H5OH -> C6H6 + OH) = 2.21*10¹⁰ exp(-3990/T) L/(mol*s).The reaction of hydrogen atoms and phenoxy radicals represents and added complication in these studies.We find k(H + C6H5O -> C6H5OH) = 2.0*10¹¹ L/(mol*s).The substitution of large quantities of carbon monoxide in place of methane leads to increased yields of benzene due to the conversion of the displaced hydroxide radicals to hydrogen atoms and their reaction with phenol.On the basis of k(OH + CO -> CO2 + H) = 7*10⁷ exp(0.00092/T) L/(mol*s), we find k(OH + C6H5OH -> C6H5O + H2O) = 6*10⁹ L/(mol*s) at 1032 K.From the equilibrium constant the rate expression for the displacement of a hydrogen atom by hydroxyl radical is k(OH + C6H6 -> H + C6H5OH) = 13.4*10⁹ exp(-5330/T) L/(mol* s).Comparisons with OH disappearance rates in high-temperature benzene systems are in agreement with the conclusion that the primary process is the abstraction of a ring hydrogen.