Detection and identification of free radicals in hydrocarbon pyrolysis by an iodine trapping method

In this paper a method for simultaneous determination of several free radicals in hydrocarbon pyrolysis is described. Propane pyrolysis was carried out in a quartz flow reactor in the temperature range of 960-1180 K and at a pressure of 30 Pa. Methyl, ethyl, vinyl, propyl, and allyl radicals were trapped in a matrix of frozen iodine at 77K. After controlled warming up, the resulting alkyl iodides were analyzed in a quadrupole mass spectrometer. As natural iodine contains isotope 127 only, the radicals can be unequivocally identified. With this method semiquantitative measurements of free-radical concentrations are shown to be feasible. A reaction model was developed to evaluate the experimental results. It consists of 182 elementary reaction steps involving 25 different chemical species. The agreement between measured and calculated product distribution was reasonably good, though no attempt was made to adjust the model by fitting rate constants.