Investigation of the mechanism of solvothermal extraction of direct coal liquefaction residue by macromolecular models

Direct coal liquefaction residue (DCLR) is an industrial by-product from the direct coal liquefaction process. It has the unique characteristics of high carbon, high sulfur and high ash content, strong cohesiveness and low softening point, which makes DLCR a potential raw material. However, direct discharge of untreated DCLR can cause severe damage to environment and human body. Thus it is of great importance to utilize DCLR properly. In this study, solvothermal extraction of direct coal liquefaction residue (DCLR)was treated with N-methyl-2-pyrrolidone (NMP) and 1-methyl naphthalene (1-MN). In addition, molecular models of DCLR and two hypercoals (HPC: NMP-HPC, MN-HPC) were built to clarify the extraction mechanism, aiming to provide a technically feasible route to utilize DCLR and investigate the adaptation between extractant and raw materials. The structural parameters were obtained by means of industrial and elemental analysis, 13C NMR, XPS and FTIR. Subsequently, macromolecular structure models were constructed, with simulated molecular formulas C153H108N2O10S, C206H168N6O7 and C278H202N2O13S, respectively. After modifying the macromolecular models by Materials Studio for geometric optimization, annealing dynamics calculation, density simulation and analysis of chemical bonds and charge layout, these models reflected the macromolecular structural characteristics precisely. On the other hand, DCLR model underwent significant changes in functional groups, branched chain length and quantity pre and post solvothermal extraction, indicating that the thermal extraction process was mainly affected by swelling behavior and pyrolysis. Meanwhile the principles of the two extractants were also different.1-MN extraction was a heat-induced relaxation, while NMP extraction was heat & solvent-induced relaxation process. This study will be helpful in expanding the utilization of DCLR and understanding of mechanism of solvothermal extraction on the macromolecular level based on modeling.