In order to investigate the reaction mechanism of pyrolysis of 1-butanethiol, low pressure pyrolysis was undertaken at about 10-3 Pa and below 1130 K (Table 1).: The parent peak intensity of the reactant observed with QPMS was found to decrease at above 950 K (Fig. 1). From the conversion of the reactant, unimolecular reaction rate constant kuni was obtained as follows, where k0 was the escape rate constant and f was a coversion of the reactant. kuni thus obtained was found to depend on the reaction temperature and the collision frequency, and not on the pressure and the collision number (Fig. 2). From the dependence of the collision frequency, this reaction was considered to be in the fall-off region under these conditions. From the independence of the pressure and the collision number, molecular collision and catalytic effect of reactor wall were considered to be negligible respectively, as expected. In proportion to the decrease of the reactant peak intensity, the intensity of some product peaks, which were assigned to ethylene, hydrogen sulfide and 1-butene, was found to increase (Fig. 3). Following stoichiometry was obtained. Compared with other experiments, which had been undertaken at higher pressure than the present experiment, and which had shown 1-butene to be the main product, the distinctive feature of low pressure pyrolysis of 1-butanethiol was considered to be the formation of ethylene. In order to explain the main product ethylene, following mechanism was proposed. Estimating the kinetic features of butyl and ethyl radicals based on the reported values, it was suggested that under these conditions butyl radical, once produced, selectively converted to two molecules of ethylene. Therefore, by assuming that 85% of initiation reaction was by means of C-S bond cleavage, the amount of ethylene produced could be explained. Based on the similar discussion, 1-butene was considered to be produced by molecular H2S elimination. Activation energy at 0 K obtained from RRKM calculation was 282~287 kj.mol-1 (assuming collision efficiency 1) or 275~281 kj.mol-1 (assuming collision efficiency dependent on temperature) (Figs. 4 and 5). These values seemed to be compatible with the above radical mechanism.
ASJC Scopus subject areas
- 化学 (全般)