Investigations into physicochemical changes in thermal coals during low-temperature ionic liquid treatment

Two Australian thermal coals were treated with four different ionic liquids (ILs) at temperatures as low as 100 °C. The ILs used were 1-butylpyridinium chloride ([Bpyd][Cl]), 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCM]), 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), and 1-butyl-3-methylimidazolium tricyanomethanide ([Bmim][TCM]). Visual comparisons were made between the raw and IL-treated coals via optical microscopy. Changes in thermal behavior of these treated coals were compared against raw coals via pyrolysis experiments in a thermogravimetric analyzer (TGA). Changes in functional group composition in the treated coals were probed via Fourier transform infrared (FTIR) spectroscopy. The recovered ILs were also analyzed via FTIR and nuclear magnetic resonance (NMR) spectroscopies to observe any changes after recovery. Low-temperature IL treatment of each of the coals resulted in fragmentation and fracturing, reducing the average particle size. An increase in mass loss in the treated coals was also observed when compared to each raw coal, indicating an increase in lower molecular weight fragments after treatment. This was corroborated by a large increase in aliphatic hydrocarbons being observed in the treated coals, along with a decrease in oxygenated functional groups and mineral matter in one coal. The recovered ILs were shown to be unchanged by this treatment process, indicating their potential recyclability. These results indicate the potential for ILs to be implemented as solvent treatments for coal conversion processes.