768
Energy & Fuels 2003, 17, 768-773
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 Chunqi Li, Toshimasa Takanohashi,* and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 3058569, Japan
Masashi Iino Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 9808577, Japan Received November 19, 2002. Revised Manuscript Received April 1, 2003
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 acidtreated 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.
Introduction al.1
reported that the addition of lithium Takahashi et halides (LiCl, LiBr, LiI) increased the CS2/N-methyl-2pyrrolidinone (NMP) mixed solvent (1/1 by vol) extraction yields for several bituminous coals at room temperature, but had little effect on yields for low-rank coals. Acid-base interactions between coal acidic sites and halogenide anions1 or breaking of the π-cation interactions in coal2 were reported to be responsible for the increases in coal extraction yield. While, on heat treatment at temperatures from 175 to 300 °C in the dipolar solvent NMP, the addition of lithium halide had a much greater effect on the dissolution yield for lowrank than for high-rank coals. For example, for low-rank * Corresponding author. E-mail:
[email protected]. (1) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141. (2) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Energy Fuels 2002, 16, 543.
Banko 97 coal (%C: 70.0%), the dissolution yield at 300 °C increased by 41.4%, from 50.4% in NMP alone to 91.8%, with the addition of LiCl at 2.4 mmol/g-coal.3 The mechanism of the effect of lithium halide addition on the dissolution of low-rank coal thus appears to be different from that of high-rank coal extraction at room temperature reported by Takahashi et al.1 In the present study, structural changes in low-rank coals that were heat-treated in NMP with addition of LiCl under various conditions were investigated by FTIR spectrometry. The effects on coal dissolution of adding LiCl were found to be due to the release of the noncovalent bonds in the coals. This finding was confirmed by the results for acid-treated coals. The detailed mechanism of the dissolution of low-rank coals in NMP with the addition of LiCl is discussed. (3) Li, C.; Takanohashi, T.; Saito, I.; Iino, M. Energy Fuels 2003, 17, 762-767.
10.1021/ef0202790 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/29/2003
Coal Dissolution at Temperatures up to 300 °C in NMP
Energy & Fuels, Vol. 17, No. 3, 2003 769
Table 1. Ultimate Analysis and Ash Content of Coal Samples Used coal sample Beulah-Zap Beulah-Zap acid-treated Banko 97 Banko 97 acid-treated a
ultimate analysis (wt %, daf) ash C H N S Oa (wt%, db) 71.6 72.0 70.0 70.4
4.8 5.0 5.3 5.4
1.0 1.1 1.3 1.2
0.9 0.8 0.3 0.4
21.7 21.1 23.1 22.6
9.6 3.0 2.4 0.8
By difference.
Experimental Section Materials. The results of ultimate analyses and ash contents for one Argonne Premium Beulah-Zap coal (
