Rational Design of Hydrogen-Donor Solvents for Direct Coal Liquefaction

Abstract

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.

Theoretical Study on the Mechanism of Hydrogen Donation and Transfer for Hydrogen-Donor Solvents during Direct Coal Liquefaction

“…[29]. Hence, scientists select model compounds, such as 4,5-dihydropyrene, 9,10-dihydroanthracene, 9,10-dihydrophenanthrene, or tetralin, to study the hydrogen donation mechanism of H-donor solvents [23,28,30]. Among these candidates, tetralin is the most popular due to its low cost, simple structure, and high performance.…”

“…It is believed that C 1 -H of tetralin would be donated first during the DCL process [28]. To better understand the stepwise mechanism, the influence of temperature and pressure on the BDE of the C 1 -H bond of tetralin, which has the highest possibility of donating a hydrogen atom, was further researched.…”

“…Hou et al, using model compounds, compared the stepwise and concerted mechanisms and concluded that the concerted mechanism is more favorable than the stepwise mechanism [28]. However, they did not provide the detailed hydrogen donation pathways for model compounds of H-donor solvents and did not consider the reaction condition.…”

Theoretical Study on the Hydrogenation Mechanisms of Model Compounds of Heavy Oil in a Plasma-Driven Catalytic System

“…The calculation results showed that the thermodynamic obstacle was easy to be overcome under cold plasma conditions. A DFT study has also been used to study the interaction of plasma species with the catalyst surface to reveal the effect of plasma and explain the reaction mechanism under plasma conditions [21][22][23].The hydrogenation mechanisms of heavy oil on traditional catalysts were also widely investigated using DFT [24][25][26]. Due to the complex of reaction and reactants, model compounds, such as thiophene pyridine, 2,6-dimethylpyridine and benzyl radical, were selected to represent the heavy oil.Thus, there is a lack of theoretical studies about the hydrogenation mechanism of heavy oil in a plasma-driven catalytic system.…”

“…The hydrogenation mechanisms of heavy oil on traditional catalysts were also widely investigated using DFT [24][25][26]. Due to the complex of reaction and reactants, model compounds, such as thiophene pyridine, 2,6-dimethylpyridine and benzyl radical, were selected to represent the heavy oil.…”