Application of dual-solvent extraction for separating a low-temperature coal tar: A detailed experimental and quantum chemical study

A low-temperature coal tar (LTCT) was separated by dual-solvent systems to realize its value-added utilization. Cumulative extract yield (CEY) versus time, extraction kinetics at different stages, extracted composition, and functional group distribution were intensively studied using nonlinear fitting, linear fitting, gas chromatograph/mass spectrometer (GC/MS), and Fourier transform infrared (FTIR) spectrometer, respectively. The relationship between CEY and time conformed to logistic model, and the determination coefficient is 0.97. Among the 3 extraction stages, the slope of the kinetic fitting curves in their extraction stage using petroleum ether (the first 3 runs) in the highest extract yield is 7.5 and 5 times that using methanol and carbon disulfide extraction stage, respectively. Alkanes, arenols, and arenes can be enriched into petroleum ether, methanol, and carbon disulfide, respectively. A total of 314 compounds detected in the LTCT and extracts with GC/MS were subdivided into 85 group components. The result of FTIR analysis is consistent with that of GC/MS. Separation mechanism was expounded by theory analysis and quantum chemistry from hydrogen bonds (O-H center dot center dot center dot O) and van der Waals force. Quantum chemical calculations show that the attraction of alkane, alkene, anthracene, and naphthalene with n-hexane or carbon disulfide is mainly dispersive interaction, while the attraction of methanol with phenols is mainly electrostatic interaction. Although the bond length of O-H center dot center dot center dot O formed between methanol and phenols is 2/3 of that of C-H center dot center dot center dot O, and the bond angle is ca. 45 degrees larger, the binding energy is 9 times that of the latter, indicating that the main hydrogen bonds are O-H center dot center dot center dot O. In summary, dual-solvent extraction has considerable potential to separate alkanes, arenols, and arenes from the LTCT.