Rational Design of Hydrogen-Donor Solvents for Direct Coal Liquefaction

Facing the challenge of processes in direct coal liquefaction (DCL), it is vital to develop optimal hydrogen-donor solvent (H-donor) to dramatically moderate coal liquefaction conditions. Here, we propose an approach for rational design of optimal H-donor candidates based on density functional theory (DFT) calculations combining reverse searching algorithm. First, the mechanism of hydrogen transfer from H-donor to coal radical was investigated by using common model compounds. DFT calculations show that the concerted hydrogen transfer route promoted by coal radicals is the dominant pathway. The C–H bond dissociation enthalpies (BDEs) show strong correlation with intrinsic reaction barriers and rate constants (in log scale), which allow us to define a cheap metric for comparing the hydrogen-donation ability of different H-donors. Then the framework for rational design of H-donor candidates is established to seek molecules with low C–H BDEs based on inverse molecular design strategy. In the searching procedure, the chemical structure of parent molecule is varied by appropriate substituent from a predefined library (15 substituents). To reduce searching space, four empirical rules are proposed to guide the structural modifications. Finally, the H-donor candidates designed are validated by transition state calculations. It is confirmed that the inverse molecular design approach is effective for seeking candidate H-donors with lower reaction barriers and potentially higher rate of hydrogenation, which open a window for the rational design of optimal H-donors to improve the yields of the liquid products from coal under mild conditions.