Pyrolysis of Di-tertiary Butyl Peroxide: Temperature Gradients and Chain Contribution to the Rate

Abstract

An investigation of the pyrolysis of di-tertiary butyl peroxide over the range 27–130 mm Hg pressure and 130–160°C has shown that the experimental stoichiometry is 3.0 rather than the previously reported 2.9. Trace amounts of t-BuOH (0.5 to 1.0%), isobutylene oxide (1–2%) and t-BuOMe (0.02%) have been identified as well as biacetonyl and small amounts of other ketones. By using spherical vessels and octopus vessels of large surface/volume ratio, it is shown that the previously reported scatter in the Arrhenius parameters can be attributed to temperature gradients established in the vessel. This is confirmed by direct measurement of the gas temperature with fine thermocouple wires in thin glass wells. The best rate constant for the first-order decomposition in the absence of temperature gradients is given by: logk=15.6–37 400/4.575T sec—1 with an uncertainty in E of about ±0.5 kcal. Assuming zero activation energy for the recombination of t-butoxy radicals, this gives the peroxide bond dissociation energy of 37.4±0.5 kcal/mole. An estimate of the standard entropy change for dtBP→2 t-BuO gives the remarkably low value of 108.2 l/mole-sec for the Arrhenius factor for the recombination of t-BuO radicals. It is shown that such a low value is needed to explain the concordance in Arrhenius parameters for liquid and gas state pyrolyses. The isobutylene oxide puts a limit to the chain contribution of about 1 to 2%. The t-BuOH and t-BuOMe yields permit independent estimates of the rate of dissociation of t-BuO radicals in fair agreement with each other and other investigations. The explosion limits for the decomposition are estimated to be about 7.5 atm (150°C). The present work confirms the importance of temperature gradients in contributing to spurious activation energies for very exothermic reactions.