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

Homo-Conjugation of Low Molecular Weight Organic Acids Competes with Their Complexation with Cu(II)

Dissolved organic matter (DOM) controls the bioavailability and toxicity of heavy metals in aquatic environments. The observation of decreased conditional binding constants with increasing DOM concentration is not well documented, which may result in significant uncertainties in heavy metal behavior modeling and risk assessment. We used eight low molecular weight organic acids (LMOC) with representative structures to simulate DOM molecules. The interactions between LMOC molecules resulted in the decreased Cu­(II)-LMOC binding with increasing LMOC concentrations, but higher pH values than theoretical calculation after mixing LMOC solutions of different pHs. We thus proposed homoconjugation between LMOC molecules through negative charge-assisted H-bond ((−)­CAHB). A mathematic model was developed to describe Cu­(II)–LMOC complexation (<i>K</i>_C) and LMOC homoconjugation (<i>K</i>_LHL). The increased competition of LMOC homoconjugation over Cu­(II)–LMOC complexation, as suggested by the increased ratios of <i>K</i>_LHL/<i>K</i>_C, resulted in the apparently decreased Cu­(II)–LMOC binding with the increased LMOC concentration. Similar concentration-dependent binding was also observed for DOM. With the identified homoconjugation between DOM molecules, some of the literature data with concentration-dependent behavior may be re-evaluated. This is the first work that quantitatively identified homoconjugation among organic molecules. Both the modeling concepts and results provide useful information in investigating the environmental roles of DOM in mediating metal bioavailability and transport