A SINGLE PULSE SHOCK TUBE STUDY OF THE CHEMICAL MECHANISMS OF SOOT FORMATION FROM BENZENE PYROLYSIS

The thermal decomposition of vapor phase benzene has been studied using a single pulse shock tube. The reaction was studied over a temperature range of 1300 - 2300K, with initial benzene concentrations varying from (0.4 - 1.25) x 10('-6) mole cm('-3) and reaction times of 0.1 to 3.0 msec. Stable products formed were analyzed by gas chromatography, mass spectrometry and, in the case of solid products, by gravimetric determination.;Collection of solid products was achieved using a removable liner to the shock tube that could be weighed before and after the reaction. Recovery of solid and gas phase species was found to be 85-95% of the original reactant mass over the temperature range studied. Soot yields were observed to increase with increasing temperature from zero at 1300K to a plateau value of 0.8 which was maintained up to a reaction temperature of 2700K. Soot yields were shown to be substantial even when the reaction was immediately cooled. Subsequent experiments in which the reaction dwell time was varied have led to an estimated activation energy of the limiting soot formation reactions of ca 25 kJ/mole and to a frequency factor of 10('9.9) mole('-1) cm('3) sec('-1). Results of experiments in which the initial benzene concentration was varied are tenuous but suggest that over at least part of the concentration range studied, decreasing benzene concentration leads to higher yields of soot.;Benzene disappearance has been demonstrated to be second order with respect to benzene concentration and the reaction rate constant is best described by K(,II) = 4.0 x 10('14) exp(-19250/T) mole('-1) cm('3) sec('-1). Product analyses have shown that the principle products formed are acetylene and styrene. Smaller concentrations of diacetylene, methane, vinylacetylene, ethylene, and toluene were observed. Quantitative measurements of the two major products have indicated that the acetylene yields are well described by thermodynamic equilibrium calculations and are present under virtually all conditions, whereas the styrene is observed in a narrow temperature range. ('13)C labeled benzene has been used to deduce the means by which selected products are formed. This technique has demonstrated that acetylene is principally formed directly from ring scission with the remainder being formed by C(,1) fragment recombination, the C(,4) compounds observed are formed primarily via acetylene recombination and styrene is formed mainly by an acetylenic reaction with the benzene ring.