High pressure pyrolysis of n-hexane, 2,4-dimethylpentane and 1-phenylbutane. Is pressure an important geochemical parameter?

Abstract

Pure hexane, 2,4-dimethylpentane and 1-phenylbutane have been pyrolysed in closed gold reactors between 290 and 365°C, in the 210–15,600 bar pressure range. Reaction products at low conversion are alkanes and alkenes (sometimes with a substituted phenyl group in the case of 1-phenylbutane) lighter than the reactant, alkanes (phenyl substituted in the case of 1-phenylbutane) heavier than the reactant and, in the case of 2,4-dimethylpentane, alkenes and cyclic compounds heavier than the reactant. The formation of these products is explained by two types of mechanism, both involving chain reactions that yield alkanes and alkenes lighter than the reactant. The first one is typical of low temperature-high pressure pyrolysis, and radicals formed by H-transfer reactions are predominant. The second one is typical of high temperature-low pressure pyrolysis, and radicals formed by decomposition reactions are predominant. Hexane pyrolysis follows the first pattern, and 1-phenylbutane pyrolysis is similar. 2,4-Dimethylpentane pyrolysis is intermediate between both mechanisms. The effect of pressure on both mechanisms is estimated. The pyrolysis of hydrocarbons following the first type of mechanism is predicted to be greatly hindered by higher pressure while the pyrolysis of hydrocarbons following the second one is slightly faster at higher pressures. It is predicted that under geochemically relevant conditions, these hydrocarbons, and similar ones, will follow a low temperature-high pressure mechanism and their pyrolysis rate will be greatly reduced by high pressures. This effect is difficult or impossible to observe under laboratory conditions, because the pyrolysis mechanism may then be in part, or totally, of the high temperature-low pressure type. In conclusion, it is suggested that high pressures probably considerably hinder the thermal evolution of geological organic matter.