Behavior of nitrogen-containing aromatic heterocyclic compound at high-pressure and high-temperature conditions of shallow subduction zone.

Aromatic compounds are one of the largest reservoir of light elements, such as C, H, O, N, S in nature. In the Earth's surface, they represent a major organic component in sediments. When the sediments on the oceanic plate subducted from trench, organic components would also supply into the deep Earth. Aromatic compounds are found from incursions of mantle minerals in kimberlitic rocks suggesting that aromatic compounds are likely exist in high-pressure and high-temperature conditions of the deep Earth. The behavior of aromatic compounds as the reservoir of light elements in the deep Earth is controversial because the chemical reaction under high-pressure and high-temperature condition is still unclear. The oligomerization of polycyclic aromatic hydrocarbons with dehydration and new C-C bond(s) formation was observed at 500-773 K, < 3.5 GPa (Chanyshev et al. 2017). Polymerization of a nitrogen-containing heterocyclic compound at high-pressure and high-temperature was also reported (Kondrin et al. 2017) even though, details regarding the reaction mechanism and behavior of the nitrogen atom during the reaction were not revealed. In this study, behavior of a nitrogen-containing heterocyclic compound in high-pressure and high-temperature conditions are investigated and a potential role of aromatic compounds as a reservoir of nitrogen and carbon in the subduction zone are discussed.

Phthalazine (C₈H₆N₂) was used as a starting material. High pressure and high-temperature experiments using piston-cylinder type high-pressure apparatus at pressures between 0.5 and 1.5 GPa and temperatures ranging from 473 to 573 K. A distinct decrease of remaining phthalazine was observed in the samples recovered from the conditions above 523 K at 0.5 and 1.0 GPa and above 548 K at 1.5 GPa. Formation of o-xylene and o-tolunitrile accompanied a decreasing N/C ratio of the reaction products indicates decomposition of the aromatic ring and release of the nitrogen atoms. Precise analysis of the reaction products using GC/MS and MALDI/TOF-MS showed that the oligomerization of the decomposed products with the remaining phthalazine occurred to form various larger molecules. The major reaction mechanism of phthalazine is similar in 0.5-1.5 GPa, although the relative molar yield of the products has pressure dependences. Nitrogen atoms in the aromatic ring induced decomposition of the molecule and resulted in oligomerization at lower temperatures than typically reported for aromatic hydrocarbon oligomerizations. When aromatic compounds subducted in the deep earth as organics of the oceanic sediments, they could contain nitrogen as N-N bond in the aromatic ring up to 20-50 km in the hot slab and 70-80 km in the cold slab.