Chemical transformations of n-hexane and cyclohexane under the upper mantle conditions

Alkanes are an important part of petroleum, the stability of alkanes under extreme conditions is of great significance to explore the origin of petroleum and the carbon cycle in the deep Earth. Here, we performed Raman and infrared (IR) spectroscopy studies of n-hexane and cyclohexane under high pressure up to similar to 42 GPa at room temperature (RT) and high temperature (HT). n-Hexane and cyclohexane undergo several phase transitions at RT around 1.8, 8.5, 18 GPa and 1.1, 2.1, 4.6, 13, 30 GPa, respectively, without any chemical reaction. By using resistive heating combined with diamond anvil cell at pressure up to 20 GPa and temperature up to 1000 K, both n-hexane and cyclohexane decompose to hydrogenated graphitic carbon and n-hexane exhibits higher stability than cyclohexane. Our results indicate that hydrocarbons tend to dehydrogenate in the upper mantle, and the extension of carbon chains may lead to the formation of some unsaturated compounds and eventually transfer into graphitic products.