Carbon and hydrogen isotope effects during the thermocatalytic production of hydrocarbons in laboratory simulation experiments

Abstract

The molecular compositions and stable carbon and hydrogen isotope fractionations associated with the production of lower-molecular-weight hydrocarbons by pyrolysis of normal octadecane were determined as a function of time, temperature and catalysis. Chromatographic studies indicated that at the beginning of the pyrolysis of n-octadecane at 500°C, a complex spectrum of hydrocarbons is produced which becomes skewed towards the lowest molecular weight components with increasing time. At 400 and 500°C there are carbon isotope fractionations during methane production of 27.9 and 25.4‰ respectively, with carbon-12 enrichment in the methane. Hydrogen isotope measurements indicate a deuterium depletion in methane of 170‰ at 400C and, depending on how the data are extrapolated, a deuterium depletion of from 140 to 178‰ at 500°C. The liquid nitrogen condensable fraction, consisting primarily of the C 2C 6 hydrocarbons, shows similar but less pronounced trends in carbon isotope fractionations. The initial methane formed in pyrolysis experiments carried out in the presence of a clay mineral shows little, if any, carbon isotope fractionation. This remarkable behavior is apparently due to a different mechanism of formation, the carbonium ion mechanism induced by the clay mineral rather than the thermal free radical mechanism operable in previous experiments.