Structure and reactions of oil shale kerogen: Insights from covalent bond composition and free radical evolution

Taking Fushun oil shale kerogen as the research object, this study investigates its structural characteristics and pyrolysis reaction mechanism from a synergistic perspective of covalent bond composition and radical generation. First, the structural characteristics of kerogen were analyzed based on 13 C NMR and XPS characterization results, and the distribution of nine major types of covalent bonds in kerogen was quantified. Subsequently, 9,10-dihydroanthracene (DHA) was employed as an efficient hydrogen donor to indirectly quantify the generation of active radicals during kerogen pyrolysis, and by comparison with 9,10-dihydrophenanthrene (DHP) results, the radicals were classified into highly active and less active types. Finally, the variation patterns of concentration, g-value, and line width of stable radicals were analyzed by ESR to reveal the characteristics of radical condensation reactions. The results indicate that kerogen is dominated by aliphatic structures, featuring long bridged methylene chains and relatively large aromatic ring systems. The yield of active radicals increases significantly with temperature and time, while the generation rate follows a negative exponential decay trend, with temperature being the dominant factor. Further analysis combined with the bond energy distribution of covalent bonds reveals that, at low temperatures (≤340 °C), weak bonds such as β-O-C al , β-C al -C al , β-O-C ar , and C al -O-C al preferentially break, whereas conventional C al -C al bond cleavage is gradually introduced at higher temperatures. Highly active radicals are more likely to remain in the products and form stable radicals, whereas low-activity aliphatic carbon radicals tend to undergo complete coupling.