Effect of Swelling with Ionic Liquid on the Molecular Structure and Pyrolysis Behavior of Hefeng Sub-bituminous Coal

Abstract

Four ionic liquids, 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), 1-hexyl-3-methylimidazolium acetate ([HMIM][OAc]), and 1-octyl-3-methylimidazolium acetate ([OMIM]-[OAc]), were used to swell Hefeng sub-bituminous coal. The effect of swelling on the molecular structure and pyrolysis behavior of the coal was investigated by swelling degree Q, SEM, FTIR, and TG-DTG characterizations. Results showed that swelling degree by different ionic liquid was in the range of 2.0−2.4, with a higher degree of 2.4 for the coal sample swelled by [OMIM][OAc]. The swelling process could destroy the surface morphology and pore structure of the coal and increase and loosen the sample's pore channel. The results of the FTIR analysis revealed that swelling can destroy hydrogen bonds in coal, reduce the relative content of oxygen-containing functional groups, and might increase the relative amount of low-molecular-weight organics. TG-DTG profiles proposed that swelling with ionic liquid could promote weight loss and pyrolysis conversion of the coal and significantly changed its pyrolysis behavior at the temperature of 180−350 °C. The Coats−Redfern results presented that swelling increased the pyrolysis activation energy of the coal at 180−350 °C, while the energy was decreased at 350−550 °C.

Theoretical Calculation of Different Side-Chain Lengths [Cnmim]FeCl4 and Factors Influencing Coal Pyrolysis Swelling Modification

“…It was found that ILs could be used to break the weak covalent bond molecular force in various coals and pretreat coals. 18 In addition, due to their highly designable characteristics, ILs can flexibly design special ionic liquids that have a modification effect and can respond to external magnetic fields, that is, magnetic ionic liquids (MILs). 19 Compared with ordinary ionic liquids, magnetic ionic liquids can produce a certain magnetization under the action of an external magnetic field and produce a macroscopic response to an external magnetic field.…”

“…Ionic liquids are widely regarded as green, efficient solvents and ideal catalyst carriers, showing potential to replace traditional organic solvents. − Alkyl-imidazole ionic liquids are the most widely used ILs, which have unique properties different from other kinds, and their toxicity is much lower than that of conventional organic solvents. It was found that ILs could be used to break the weak covalent bond molecular force in various coals and pretreat coals . In addition, due to their highly designable characteristics, ILs can flexibly design special ionic liquids that have a modification effect and can respond to external magnetic fields, that is, magnetic ionic liquids (MILs) .…”