Radiation-initiated Thermal Cracking of n-Octane Vapor

The radiation-initiated thermal cracking (RTC) of n-octane vapor was studied at temperatures from 184 to 551°C and at atmospheric pressure. Vapor-phase ionic chain reactions, which were completely suppressed by the addition of ammonia, were found to proceed simultaneously with radical chain reactions. The main ionic chain products were C3∼C5 paraffins and olefins containing quantities of branched products and β-olefins which were not formed by the thermal cracking of n-octane. For example, the G-values of isobutane and isopentane reached 82 and 78, respectively, at 524°C. The proportion of the ionic chain products to the overall products was about 10mol%. Isomerization of C8H17+ ion, its thermal decomposition, and subsequent hydride ion transfer reaction from n-octane molecule to the fragment ion are proposed for the ionic chain mechanism. Carrier ions are carbonium ions of C3∼C5.Several similarities are pointed out between the ionic products from the RTC and those from catalytic reactions over acidic sites.