Effect of swelling treatment by ionic liquid on the structure and pyrolysis performance of the direct coal liquefaction residue

Direct coal liquefaction residue (DCLR) was swelled by four kinds of ionic liquids with the same anion and different organic chain length cations, EMIM MeSO₄, BMIM MeSO₄, HMIM MeSO₄ andOMIM MeSO₄, and the effects of swelling treatment with ionic liquids on swelling degree, surface morphology, functional group distribution, the main structure and pyrolysis performance of the direct coal liquefaction residue were analyzed by SEM, FT-IR and TG-DTG characterizations. The swelling results show that different chain length ionic liquid has different swelling degrees for the DCLR, andHMIM MeSO₄ presents the best swelling effect with the swelling degree of 1.78. The FT-IR results indicate that the ionic liquid could destroy C-H bond in DCLR, leading to a change in relative content of aliphatic and aromatic compounds. The TG-DTG characterization demonstrates that the pyrolysis performance of the residue is greatly affected by the different organic chain length ionic liquid. And theOMIM MeSO₄ ionic liquid is more favorable for the pyrolysis of the residue than others, with the weight loss rate of 47.5%. However, the pyrolysis performance of the residue is restrained by theBMIM MeSO₄ ionic liquid, in which the weight loss rate is lower than that of DCLR (without swelling treatment). The pyrolysis kinetic data based on Coats-Redfern method show that the pyrolysis reaction for the direct coal liquid residue and the swelled ones at low temperature (180-480 ℃) obeys a second order law, while the third and fourth order law of reaction is more suitable for the residue pyrolysis at high temperature section (480-825 ℃). In addition, the activation energy of the pyrolysis process for the DCLR is altered obviously by swelling treatment with different organic length ionic liquid, the longer the chain length, the higher the pyrolysis activation energy.