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The hydrogen-donor solvent (H-donor) used in direct coal liquefaction (DCL) plays a critical role in both the oil yield and distribution of the products. However, the mechanism of the reaction between H-donor and coal in the DCL is unclear. In this study, the reaction of H-donor and coal in the DCL was investigated using the model compounds tetralin (H-donor) and diphenylmethane (DPM, coal). Density functional theory calculations were employed to examine both pyrolytic radical and solvent-mediated hydrogenolysis mechanisms. Based on the computed results, a modified solvent-mediated hydrogenolysis mechanism was proposed, namely the synergistic double-hydrogen-transfer mechanism, in which the β- and α-hydrogen atoms simultaneously transfer from tetralin to the ipso- and meta-carbon atoms of DPM. The calculated energy barriers of hydrogen transfer were lower than most of the bond dissociation enthalpies for coal. The energy barriers range of the synergistic double-hydrogen-transfer reaction did not correlate with the dissociation enthalpies of Caryl−Calkyl bond, although they were consistent with the order of the hydrogen-donating abilities of different solvents. This study may provide a theoretical guidance to improve the efficiency of H-donor in the DCL.

Catalytic hydrodeoxygenation is an efficient method to upgrade the quality of low-temperature coal tar and coal liquefied oil. However, only limited researches are performed on the mechanism of hydrodeoxygenation and, therefore, innovation of more efficient and stable catalysts is less progressed and still remains a challenging subject. In this study, density functional theory calculations are carried out to get a deeper insight in the mechanism of dibenzofuran hydrodeoxygenation on the Pt(111) surface. Calculated energies and activation barriers indicate that five-membered-ring opening of tetrahydrodibenzofuran is easier than that of hexahydrodibenzofuran. Then, the generated intermediate can be preferentially hydrogenated through a non-consecutive pathway to 2-cyclohexylphenol, and would result in multiple radical species. The dehydroxylation of that intermediate formed by adding five hydrogen atoms to the phenyl ring of 2-cyclohexylphenol shows a moderate reaction barrier of 1.07 eV; whereas, dehydroxylation of cyclohexylcyclohexanol needs a higher energy barrier of 2.04 eV. For the hydrodeoxygenation of dibenzofuran on the Pt(111) surface, the target product bicyclohexane is most likely to be formed through a radical intermediate. This investigation may provide a potent theoretical guidance to design new catalyst for hydrodeoxygenation of coal to liquid.