Effect of Pressure on the Thermal Cracking and Polymerization of Pentacosane (n-C25), an n-Alkane

Hydrocarbons are important carbon components in the subducting slab and play a crucial role in the Earth's deep carbon cycle. In this study, the thermal reaction of pentacosane (n-C-25), a long n-alkane, was experimentally investigated under high-pressure and high-temperature conditions at 0.5-1.5 GPa and 360-400 degrees C. The gas chromatography-mass spectrometry (GC/MS) analyses of the reaction products revealed that the radical reaction of n-C-25 proceeded above 360 degrees C at 0.5 GPa and above 380 degrees C at 1.5 GPa, while the rate constant decreased with increasing pressure. Lighter n-alkanes and heavier straight/branched alkanes were detected in the reaction products. The formation of lighter n-alkanes indicates thermal cracking progression, even at high-pressure conditions. During thermal cracking, lighter 1-alkenes were likely to form but were instead rapidly added to the initial n-C-25 to form heavier alkanes when enhanced by pressure. Thus, lighter 1-alkenes were not detected in the reaction products. As the secondary reaction, the heavier alkanes were polymerized with dehydrogenation to form amorphous carbon when the remaining percentage of the initial material became <10% and <20% at 0.5 and 1.5 GPa, respectively, while the lighter n-alkanes were detected simultaneously. Both lighter alkanes with high H/C ratios and amorphous carbon with a low H/C ratio eventually formed through the reaction of n-alkanes at high-pressure and high-temperature conditions of the deep Earth.