Investigation on the mechanism of solvothermal extraction of coals by macromolecular models

Abstract

In this study, solvothermal extraction of two coals (Fangezhuang fat coal, FGZ; Sima lean coal, SM) were conducted by N-methylpyrrolidone (NMP); the molecular models of corresponding hypercoals (HPC) were firstly constructed besides that of coals to clarify the extractions mechanism, aiming to provide a technically feasible route to investigate the mechanism of other extraction processes. The structural parameters were obtained by means of proximate analysis, ultimate analysis, carbon-13 nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Fourier Transform infrared spectroscopy. The results showed that aromatic ring types of both raw coals were mainly naphthalene and anthracene, nitrogen atoms existed in pyrrole and pyridine whilst sulfur atoms in thiophene substantially, aliphatic carbons presented in the form of methyl and methylene groups while oxygen-containing functional groups mainly consisted of phenolic hydroxyl, carbonyl, carboxyl and a few ether bonds. Thus, the macromolecular structure models were constructed, with simulated molecular formulas were C168H136N2O12S, C135H74N2O12S, C166H136N2O6S and C128H82N2O11, respectively. After modifying the macromolecular models by Materials Studio for geometric optimization, annealing dynamics calculation, density simulation and analysis of chemical bonds, these four models reflected the macromolecular structural characteristics precisely. On the other hand, both models of raw coals changed significantly in functional groups, branched chain length and quantity pre and post thermal extraction, indicated that the thermal extraction process was mainly affected by swelling behavior and pyrolysis. This study provided new theoretical support in the development of coal extraction technology and understanding of HPC on the macromolecular level by a model basis.