Coal Dissolution by Heat Treatment at Temperatures up to 300 °C in N-Methyl-2-pyrrolidinone with Addition of Lithium Halide. 2. Elucidation of Mechanism by Investigation of the Structural Changes of Heat-Treated Coals

Abstract

FT-IR spectrometry was used to investigate structural changes in coals that were heat-treated in N-methyl-2-pyrrolidinone with the addition of LiCl under various conditions. The intensity of the peak corresponding to carboxyl groups in low-rank Banko 97 coal (%C: 70.0%) decreased, while the intensities of carboxylate group peaks increased after heat treatment of the coal at 300 °C. Banko 97 and Beulah-Zap (%C: 71.6%) coals were acid-treated in organic methoxyethoxy acetic acid before heat treatment at 300 °C with addition of LiCl. The acid-treated Banko 97 coal showed a dissolution yield similar to that of raw coal, whereas the dissolution yield for acid-treated Beulah-Zap coal was 90.6%, which was much higher than that of the corresponding raw coal (51.8%). FT-IR spectra showed that there were no structural changes in Banko 97 coal after acid treatment. However, for Beulah-Zap coal, the intensities of peaks corresponding to carboxylate groups decreased, while the intensity of the carboxyl group peak markedly increased after acid treatment. Furthermore, the increase in the intensity of the carboxyl group peak decreased after heat treatment with the addition of LiCl. The mechanism of the effect of the addition of LiCl on the dissolution of low-rank coal is suggested to be as follows. Heat treatment of low-rank coals at 175−300 °C in NMP with the addition of LiCl results in the formation of a complex between lithium and n molecules of NMP with chloride anion, [Li·nNMP]+ Cl-. Due to its high basicity, the Cl- anion releases the aggregation of the coal by weakening the hydrogen bonds formed between carboxyl groups, resulting in the observed high dissolution yields.