Multifaceted Roles of Polycyclic Aromatic Compounds in Coal Liquefaction: An Integrated Analysis Combining Experimental Methods and Theoretical Calculations

Aromatics and hydrogenated aromatics constitute the primary constituents of the recycled solvent utilized in coal liquefaction. However, the presence of hydrogenated aromatics complicates the assessment of the effects of aromatic compounds during the coal liquefaction process. To eliminate the effect of hydrogenated aromatics, solvent-free experiments and experiments with specific polycyclic aromatic compounds (pyrene, fluoranthene, phenanthrene, and naphthalene) were conducted, along with utilizing density functional theory calculations. Results show that the shuttle role of naphthalene is influenced by catalyst activity. In the presence of a low-active catalyst, naphthalene acts as a shuttle agent facilitating indirect hydrogen transfer. However, with a high-active catalyst, direct transfer of hydrogen from H 2 to pyrolysis radicals occurs without involving naphthalene as an intermediary. Polycyclic aromatic compounds exhibit higher hydrogen shuttle ability (pyrene > fluoranthene > phenanthrene > naphthalene). The high hydrogen shuttle ability can be attributed to the higher reactivity in hydrogenation and lower energy barriers for hydrogen transfer. A nonradical hydrogen transfer pathway has been proposed, promoting cleavage of strong covalent bonds and enhancing the yield of asphaltene and preasphaltene. Additionally, π–π inter actions between polycyclic aromatic compounds and pyrolysis radicals have been evaluated. The interactions contribute positively to radical stability. Substances characterized by a greater number of aromatic rings display a more pronounced stabilizing effect on free radicals.