Influences of the composition and structure of extractants on their extraction behavior for coal direct liquefaction residue

During the direct coal liquefaction process, coal direct liquefaction residue (CDLR), which accounts for about 30% of the coal input, will be produced and the polycyclic aromatic hydrocarbons can be extracted from it by extraction to prepare high-value carbon materials. The influences of composition and structure of extractants on the extraction rate and properties of extracted products were systematically summarized. The extraction rate of the extractants which have aromatic structure or nitrogen and oxygen atom can reach more than 50%. The extracts in alkane organic solvents mainly contain condensed aromatic structure with 2–4 benzene rings and have a lower molecular weight and heteroatom content. The extracts in heteroatoms containing solvents are mainly composed of condensed aromatic structure with 4–7 benzene rings and have a larger molecular weight. And they are rich in heteroatoms, such as nitrogen, oxygen and sulfur, and have a high atomic ratio of C/H. The extracts of pyridinium-based ionic liquids have higher atomic ratio of C/H and aromaticity. And the ash content of the extracts of ionic liquids containing organic acid ions is close to 0. When coal liquefaction oil or coal tar distillate are used as extractants, the extraction rate can reach 60%, which are suggested for industrial applications of CDLR extraction.