Coal Dissolution by Heat Treatments in N-Methyl-2-pyrrolidinone, 1,4,5,8,9,10-Hexahydroanthracene, and Their Mixed Solvents:  A Large Synergistic Effect of the Mixed Solvents

Heat treatments of coals of various ranks were carried out in a dipolar aprotic solvent of N-methyl-2-pyrrolidinone (NMP) and a strong hydrogen-donating solvent of 1,4,5,8,9,10-hexahydroanthracene (HHA) at 175−300 °C under N2 atmosphere. Dissolution yields were obtained using carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent (1:1 by volume) as an extraction solvent for the heat-treated coals. Considerable high dissolution yields were obtained for several coals even at temperatures as low as 175−300 °C. For the lower rank coals (C wt %, daf < 85 wt %) used, the dissolution yields in NMP and HHA increased with increasing temperature from 175 °C to 300 °C. The dissolution yields (38.2−78.1 wt %, daf) obtained in NMP were higher than those obtained in HHA. While, for Upper Freeport coal of higher rank (C% > 85 wt %, daf), a very high dissolution yield of 82.5 wt % was obtained in NMP at 175 °C, but it did not increase with increasing temperature to 300 °C and was lower than that in HHA. Similar results were observed for the other higher rank coal. NMP/HHA mixed solvents were found to show a large synergistic effect on the dissolution yields for all the coals used. An amount of 96.4 wt % of Banko coal was dissolved after the heat treatment in NMP/HHA mixed solvent (9/1 by weight) at 300 °C, which is much higher than 59.1 wt % in NMP and 44.5 wt % in HHA alone. It is also found that the extent of such synergistic effects is highly coal-rank-dependent. The mechanisms for the dissolution reaction in NMP, HHA, and NMP/HHA mixed solvents were discussed.