Oligomerization and carbonization of polycyclic aromatic hydrocarbons at high pressure and temperature

Abstract

We have examined the stabilities of different polycyclic aromatic hydrocarbons (PAHs) at 7GPa and 773–973K and at 16GPa and 300K. Experiments were performed using a large-volume multi-anvil apparatus. Quenched products were analyzed by matrix-assisted laser desorption/ionization (MALDI) and Raman spectroscopy. The MALDI measurements revealed the considerable oligomerization of PAHs at 7GPa and 773–873K and insignificant PAH oligomerization at 16GPa and 300K. At 7GPa and 773K, oligomers with molecular weight up to 3400Da were found, while only a small number of dimers of the starting PAHs were detected at 16GPa and 300K. PAH decomposition at 7GPa occurred from 873 to 973K, and the decomposition products consisted of nanocrystalline graphite. The determined decomposition temperatures of the PAHs (873–973K) are much lower than Earth’s geotherms and the subduction slab P–T profiles at 7GPa; therefore, PAH inclusions in mantle-derived minerals, which can be crystallized at 6–7GPa and 1600–1700K, should be secondary phases and could be formed by the successive polycondensation of simple hydrocarbon molecules under natural catalysts during eruption processes at sub-ambient pressures and temperatures.