The mechanism and rate constant of hydrogen transfer from solvent radicals to coal-based model compounds in direct coal liquefaction

In direct coal liquefaction, in addition to stabilizing coal radicals, the ability of solvent-derived cyclohexadienyl radicals to hydrolyze bridge bonds in coal is a part of their hydrogen-donating activity. In this work, the density functional theory calculations and transition state theory were employed quantitatively to analyze the rate constants of hydrogen transfer from solvent-derived cyclohexadienyl radicals to coal model compounds. Results indicated that polycyclic 9,10-dihydrophenanthrene, 4,5-dihydropyrene and 4,5,9,10-tetrahydropyrene radicals have higher hydrogenolysis ability, therefore they can promote the breakage of the Caryl–Calkyl bonds more effectively. There is no obvious correlation between the rate constants of hydrogen transfer from H-donors radicals to ipso-C of different coal model compounds and the dissociation enthalpies of Caryl–Calkyl bond. However, it can be found that model compounds with higher bond dissociation energies are more susceptible to hydrogenolysis by solvent-derived cyclohexadienyl radicals. Among the coal model compounds containing the same functional group, the coal model compounds with substituent groups directly attached to aromatic rings are more easily hydrolyzed by solvent-derived cyclohexadienyl radicals.