Relative rates of radical–radical processes in the pyrolysis of ethane

Abstract

Ethane has been pyrolysed in a flow system at 900 to 963 K, at residence times from 0.5 to 50 s and with [C2H6] from 1 × 10−4 to 4 × 10−3 mol L−1. From measurements of yields of minor products at 902 K, it is concluded that the combination reaction, [5a], and disproportionation reaction, [5b], of C2H5 radicals have the rate constant ratio, k5b/k5a = 0.44 ± 0.07. It is suggested that this ratio has a curved Arrhenius plot, which is interpreted in terms of activated complex theory. The cross-combination of CH3 and C2H5 radicals becomes significant at low pressures, reaching 1.5% of the sum of the rates of [5a] and, [5b] when [C2H6] = 10−3 mol L−1 at 902 K. With [C2H6] = 10−4 mol L−1 at 963 K, the yield of CH4 was greater in the presence of an SiO2 surface than in the presence of a PbO surface, consistent with the occurrence of the atom-cracking reaction,[Formula: see text]at 15% of the combined rates of [5a] and [5b]. Rate parameters are also reported for the C—C scission in C2H6, for the abstraction of H from C2H6 by CH3, for the decomposition of C2H5, and for the reaction of C2H5 at PbO surfaces.