Kinetics of the thermal decomposition of propylene, and of propylene-inhibited hydrocarbon decompositions

Abstract

The main products of the propylene decomposition, studied between 580 and 640°C, were found to be ethylene, methane and hydrogen, in the approximate ratio 2:2:1. Smaller amounts of ethane, propane, butenes, benzene, toluene and diallyl were found, but no allene was detected. The order of the reaction was 3/2, and the activation energy 56⋅7 kcal/mole. A mechanism is proposed, involving an initial split into C 3 H 5 + H and including abstractions by both CH 3 and H; the main chain-terminating step is H + C 3 H 5 . The mechanism is shown to predict the rates of formation of the individual reaction products, and to account satisfactorily for the observed activation energy. Hydrogen atom abstractions by the allyl radical were demonstrated. It is shown that the facts are consistent with the suggestion that in the propylene-inhibited paraffin decompositions there is H abstraction by C 3 H 5 and chain termination by reaction between C 3 H 6 and an alkyl radical. This proposal explains why NO and propylene give rise to the same rates for the fully inhibited reactions, and why smaller amounts of NO are required to produce a given degree of inhibition.