Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent

Abstract Extracting polycyclic aromatic structures (PASs)-enriched portion from direct coal liquefaction residue (DCLR) with an energy-efficient way is significantly important for the effective utilization of DCLR. In the investigation, dipropylamine (DPA) with weak polarity was used to extract DCLR as a CO 2 -triggered switchable solvent. When the extract solution was triggered by CO 2 , the extract automatically precipitated with the yield of 22.8% of DCLR. Furthermore, adding proper amount of NaOH could successfully achieve the recovery of DPA, and the recovered solvent could be reused for extracting DCLR. According to the characterization results of the extract by multiple analytical tools, the extract mainly consisted of PASs with over 4 rings, and the average molecular model of aromatic cluster could be represented by the derivative of coronene with 8 substituent groups. The investigation proved that extraction of PASs-enriched portion using DPA from DCLR was feasible, and tremendous energetic and environmental benefits could be potentially gained given that the separation of extract and solvent as well as solvent recovery could be achieved by the non-thermal CO 2 -trigger.