Coal liquefaction is regarded as one of the prospective ways to produce liquid transportation fuels, which is especially important in China. However, the yield of the Direct Coal Liquefaction Residue (DCLR) is as high as 30% in the direct liquefaction process, thus how to utilize the DCLR economically and environmentally is a big challenge for coal chemical industries. Due to the DCLR containing about 25% asphaltenes (poly-condensed aromatic compounds) which are the important precursors for preparing high value-added carbon materials, it is significantly vital to separate asphaltenes from DCLR and make it be the precursors of carbon materials.

Researchers from the group of ionic liquids Clean Process and Energy Saving, Institute of Process Engineering, CAS proposed an effective asphaltenes separation technology from DCLR with ionic liquids (ILs) solvents. As a new solvent, ILs are salts composed solely of ions with low melting points, and have properties of non-volatility, unique dissolubility, good thermostability and designability. In this study, a series of dialkylphosphate ILs were synthesized and used to extract asphaltenes from DCLR. The extracts were characterized by elemental analysis, fourier transfer infrared spectroscopy, solid 13C-nuclear magnetic resonance spectroscopy, and so on.

Results showed that dialkylphosphate ILs had a good ability to extract asphaltenes from DCLR, and the structures and sizes of the cations and anions of the ILs were the main factors that influence the extraction yields and the physicochemical characteristics of asphaltene products, such as atomic ratio of H/C, aromaticity and aromatic cluster size.

Compared with traditional solvents, the structures of ILs could be tuned and designed by varying the ring and substituent groups to increase the extractive efficiency and had potentials to control the structures and physicochemical properties of the extracted asphaltenes so as to meet the requirements of the precursors. It might improve evidently the economy of coal liquefaction technology by taking the extracted asphaltenes from DCLR as the raw materials to produce high value-added carbon materials.

The research results were published on Industrial & Engineering Chemistry Research (2011, 50(17): 10278-10282).