On the formation of pyridine, the first nitrogen heterocyclic ring in NPAHs

Nitrogen-containing aromatics, including nitrogen-substituted monocyclic and polycyclic aromatic hydrocarbons (NPAHs), are toxic and a specific type of combustion emissions arising from fuel-nitrogen in coal and protein-rich biomass. However, the formation chemistry of pyridine, the first nitrogen heterocyclic ring in NPAHs, is poorly understood and needs to be enhanced. In this work, we investigated the chemistry of pyridine formation based on experimental measurements and theoretical reaction pathway exploration. Three pyrolysis experiments were performed in a jet stirred reactor with reactants of acetylene, acetylene + acetonitrile, and acetylene + acrylonitrile. The large molecule products were collected offline and analyzed by comprehensive two-dimensional (2D) gas chromatogram with time-of-flight mass spectrometry (GC × GC - ToF - MS) for species identification and measurements. Guided by experimental results, four pyridine formation pathways, C2H2 + C2H2CN radical, C2H3CN + C2H3 radical, CH2CN + C3H3 radical, and HCN + n-C4H5 radical are proposed and investigated. The calculated product yields and reaction rate coefficients determined by the combination of high-level quantum chemistry and RRKM-ME theories, and the simulated mole fractions by kinetic modeling confirmed the importance of the proposed pyridine formation pathways. The unraveled pyridine formation chemistry may help explain how the first nitrogen heterocyclic ring is formed from fuel-nitrogen in biomass gas-phase combustion.