Revealing the regularity of hydrogenation and ring-opening via hydrodenitrogenation on the CaO surface of pyrolysis conditions – Taking pyrrole and pyridine as examples

Abstract

Removing nitrogen-containing substances during low-rank coal and sludge co-pyrolysis is crucial for the resource utilization and environmental protection. We have intensively investigated the intramolecular hydrogen transfer denitrification and hydrodenitrogenation (HDN) reaction pathways of pyrrole and pyridine on the CaO(001) surface using density functional theory (DFT) method. Compared to hydrogen transfer, hydrogenation significantly promotes nitrogen removal. We obtained the mechanism of partial hydrogenation pathways in which the unsaturated β-C position radical is formed to break C−N bond via Hofmann elimination (E2) mechanism. On this basis, we found the universality of nitrogen-containing heterocycle compounds HDN on CaO surface: the delocalized conjugated radical intermediates are more stable than the localized radical intermediates, which is the decisive factor for the cleavage of the C−N bond and the C−C bond; the strong nucleophilicity and weak steric hindrance mean the high hydrogenation activity, which influences the hydrogenation sequence; the acidity or alkalinity of the nitrogen-containing compounds determines their adsorption modes on the CaO surface, which is a factor affecting the distribution of the products. The microkinetic analysis result shows that the HDN reaction rates of pyrrole and pyridine significantly increase when the temperature reaches to 950 K and the main denitrification product is NH₃. Our research provides new insights into more nitrogen-containing heterocycle compounds HDN.