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Brown coal (lignite) is a bulk organic mixture of conjugated hydrocarbons that are complexed together via hydrogen bonds. Coal [partial] dissolution is essential to the better utilization of low-rank coal for power sources because the direct combustion of brown coal is not energy efficient. To break hydrogen bonds of low-rank coal and make it partially soluble, this project evaluates a series of ionic liquids (ILs) with specific properties as non-volatile alternatives to conventional organic solvents. A series of nitrogen- and phosphorus-based cations have been synthesized via a nucleophilic substitution reaction, the resultant bromide-based IL being converted to an acetate-based IL through an ion-exchange procedure in methanol. Water concentration and viscosity measurements, along with thermogravimetric and nuclear magnetic resonance (1H and 13C NMR) analyses, were conducted to confirm the IL structure and thermal stability. Hydrogen-bond acidity, basicity, and polarity of these ILs were measured using various dyes. We further determined the capability of these ILs for dissolving cellulose and pretreating brown coal at 100 ℃. The IL-treated coal samples were analyzed by Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Through a combined analysis of our experimental results, we concluded that hydrophilic acetate-ILs dissolve both cellulose and lignite, the latter evidenced through thorough evaluation of FTIR, XRD, and SEM analysis.


organic synthesis; ionic liquids; characterization; coal dissolution; coal pretreatment