Effects of deep eutectic solvents pretreatment on coal physicochemical characteristics

Coal is a major resource in China both for chemicals production and power generation, but its impact on the environment demands attention. Solvent pretreatment is an effective method to realize cleaner utilization of coal. Shendong sub-bituminous coal was treated with three different deep eutectic solvents (DESs) – choline chloride (ChCl)-urea, ChCl-glycerol, and ChCl- ethylene glycol – at temperature as low as 120°C. Proximate/ultimate analyses, scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy were used to examine the physicochemical changes that occurred in coal after DESs treatment. Results showed that DES treatment led to coal fragmentation and decreased average particle size. Compared with raw coal (33.1%), DES-treated coal showed greater mass loss (35.2 − 37.7%), showing that the amount of low molecular mass species increased after treatment. This was confirmed by the observation that DES-treated coal had more volatile components and less minerals and functional groups that contained oxygen (especially C−O). The content of elemental sulfur also decreased from 0.26% to 0.10 − 0.15% after DES treatment, along with a reduction in aliphatic chain length and aromatic substitution degree, which were from 10.73 to 9.33−10.08 and 16.70% to 10.73−15.36%, respectively. The macromolecular structure of the coal was destroyed as a result of DES’s interactions with the weak covalent bonds and hydrogen bonds in the coal. These results suggest that DES has the potential to be used as a pretreatment solvent for coal conversion.