Energy & Fuels 1997, 11, 709-715
Solid-State
13C
709
NMR of Pyridine-Swollen Coal
Hiroyuki Kawashima,*,† Yasumasa Yamashita, and Ikuo Saito National Institute for Resources and Environment, Onogawa, Tsukuba, Ibaraki, 305, Japan Received August 31, 1996. Revised Manuscript Received January 23, 1997X
We have studied the solid-state 13C CP/MAS NMR spectra of pyridine-swollen coal. The coal used in this study were Upper Freeport (UF, C 88 wt %), Wyodak (WY, C 75 wt %), and Yallourn (YL, C 65 wt %). Each coal was immersed in pyridine for 0 (momentarily) or 1 h. For UF coal, the spectra of the coal immersed in pyridine were the same as the fresh coal and pyridine peaks were not observed. So, most of the pyridine molecules in UF coal were thought to be in a mobile state. For WY and YL coal, it became clear from solid-state 13C CP/MAS NMR that pyridine connected with coal in the form of hydrogen bonding. For WY coal, R- and β-carbons’ peaks of pyridine were observed at 0 h after immersion and all carbons’ peaks were observed at 1 h after immersion. This was because the efficiency of cross polarization for γ-carbon of pyridine in coal was different from R- and β-carbons. For YL coal, all carbons’ peaks of pyridine were observed at both 0 and 1 h after immersion. The swelling of YL coal at 0 h after pyridine immersion seemed to be enough for the effective cross polarization for γ-carbon of pyridine. The T1H of pyridine-swollen coal was also measured. The T1H of UF coal decreased, while the T1H of WY and YL coal increased with the pyridine immersion. The change of the T1H of coal was followed by the change of molecular mobility of coal. WY and YL coal showed an increase in the T1H with pyridine immersion, since these coals were at around ωτc ≈ 1 originally and moved to the ωτc , 1 region with the swelling by solvent immersion. But, UF coal showed a decrease of the T1H on pyridine immersion, since the coal was at the ωτc > 1 region originally and stayed in that region even after solvent immersion.
Introduction It is well-known that solvents interact strongly with coal. Many technical processes for coal conversion such as liquefaction and pretreatment of coal are based on the interaction between the solvent molecules and coal. In the past several years, a number of papers on coalsolvent interactions, such as solvent swelling and solvent extraction of coal, have been published. It has become apparent that the noncovalent cross-links in coal are destroyed by strong coal-solvent interactions.1 Cody et al. showed that coals possess anisotropic structures and relax to their equilibrium state when transformed to a rubbery state by solvent soaking.2 Coals swell in solvents and some of coal-soluble components are released and move to the solvent phase from coals.3 However, the coal-solvent interaction, such as the mechanism of solvent penetration and the estimation of the parameter relative to solvent swelling and solvent extraction of coal, has not been completely understood. For example, it is known that solvents having similar swelling ratio (Q) show big differences in swelling rates and that swelling rates could be more sensitive to solvent viscosities and steric factors than to the affinity of solvent for coal.4 There remains a need for a better and quantitative definition of the interaction of solvent with coal, such as the effectiveness of the solvent molecules at removing the noncovalent crosslinks in coal, and the physical properties of the organic †
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[email protected]. Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538. (2) Cody, G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340. (3) Takanohashi, T.; Iino, M. Energy Fuels 1991, 5, 708. (4) Aida, T.; Squires, T. G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30, 95. X
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structures in coals of different ranks after the noncovalent interactions are removed.5 In this regard, the process by which solvent molecule interacts with coal may be significant. To reveal the coal-solvent interaction, studies on the effect of imbibed solvents on coal properties have been carried out by observing the changes in the NMR spectrum due to solvent treatment of coal. NMR is a useful method to study coal structure and the interaction between coal and the solvent.6-9 And NMR relaxation times give very useful information about the dynamics of solvent molecules in coal. It has become apparent from the 1H NMR measurement of solventadded coal that coal contains immobile and mobile phases and molecular mobility of coal increases with the addition of solvent.5,10-13 In order to further understand this coal-solvent interaction, we describe the use of solid-state 13C NMR techniques in the study of pyridine-immersed coal to clarify the process by which solvent interacts with coal at the early stage of swelling and investigated the properties of coals swollen in solvents. (5) Yang, X.; Silbernagel, B. G.; Larsen, J. W. Energy Fuels 1994, 8, 266. (6) Yokono, T.; Sanada, Y. Fuel 1978, 57, 334. (7) Jurkiewicz, A.; Marzec, A.; Pislewski, N. Fuel 1982, 61, 647. (8) Marzec, A.; Jurkiewicz, A.; Pislewski, N. Fuel 1983, 62, 996. (9) Barton, W. A.; Lynch, L. J.; Webster, D. S. Fuel 1984, 63, 1262. (10) Kamienski, B.; Pruski, M.; Gerstein, B. C.; Given, P. H. Energy Fuels 1987, 1, 45. (11) Vassallo, A. M.; Wilson, M. A. Fuel 1984, 63, 571. (12) Ripmeester, J. A.; Hawkins, R. E.; MacPhee, J. A.; Nandi, B. N. Fuel 1986, 65, 740. (13) Vassallo, A. M. In Magnetic resonance of carbonaceous solids; Advances in Chemistry Series 229; Botto, R. E., Sanada, Y., Eds.; American Chemical Society: Washington, DC, 1993; p 201.
© 1997 American Chemical Society
710 Energy & Fuels, Vol. 11, No. 3, 1997
Kawashima et al.
Table 1. Analyses of Coal Samples Ultimate Analyses (wt %, daf) coal Upper Freeport Wyodak (WY)a Yallourn (YL)
(UF)a
C
H
N
O (diff)
85.5 75.0 64.8
4.7 5.3 4.8
1.5 1.1 0.6
8.3 18.6 29.8
Proximate Analyses (wt %) coal
moisture
volatile matter
fixed carbon
ash
UF WY YL
1.1 28.1 9.9
27.1 32.2 44.7
58.8 33.4 44.9
13.0 6.3 0.5
Ash Analyses (wt %) coal
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
others
UF WY YL
44.8 28.7 17.8
24.1 15.5 2.3
17.3 10.2 28.2
4.2 15.1 13.1
1.6 3.6 15.5
0.0 1.5 4.9
8.0 25.4 18.2
a
Argonne Premium Coal Sample Program. Table 2. Content of Pyridine (Py) in Each Sample (after Suction Filtration) content of pyridine (g of Py/g of coal) sample (Py soaking time) UF + pyridine (0 h) UF + pyridine (1 h) WY + pyridine (0 h) WY + pyridine (1 h) YL + pyridine (0 h) YL + pyridine (1 h) a
a
b
0.04 0.13 0.36 0.38 0.62 0.69
0.04 0.15 0.56 0.61 1.63 2.23
Based on weight of coal + pyridine. b Based on weight of coal.
Experimental Section Coal Sample. The analyses of the coal used in this study are shown in Table 1. Upper Freeport coal and Wyodak coal were obtained from the Argonne National Laboratory Premium Coal Sample Program. The particle sizes of these coals were 1 region. In general, for polymercontaining solvents, the molecular mobilities of the polymer and the solvent increase.27 After solvent swelling, the molecular mobility of coal is known to increase.5,10-13 From the results of the experiment, WY and YL showed the increase of the T1H with pyridine immersion, so these coals were at around ωτc ≈ 1 originally and moved to the ωτc , 1 region with the swelling by solvent immersion. But this was not, however, the case for UF coal. UF coal showed the decrease of the T1H by pyridine immersion, so the coal was at the ωτc > 1 region originally and stayed in that region even after solvent immersion. Table 5 also shows the results for the T1H of pyridine in coal. The T1H of pyridine in WY and YL coal increased with soaking time or swelling. As pyridine was in ωτc , 1 region, the mobility of pyridine molecules increased with soaking time and T1H increased. The T1H of pyridine-removed samples was also measured (Table 5). For WY and YL coal, the values were returned to almost the original levels. On the other hand, for UF coal, the T1H decreased more than swelling coal. This result for UF coal indicates that the structural change was induced by pyridine immersion. Nishioka et al., Larsen et al., and Takanohashi et al. (26) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1948, 73, 679. (27) Yasunaga, H.; Ando, I. Polym. Gels Networks, 1993, 1, 83.
13C
NMR of Pyridine-Swollen Coal
studied the insolubilization behavior of coal soluble constituents by solvent treatment.3,28,29 This is because the structure of coal changed by solvent treatment. And this effect is greater for bituminous coal than for lower rank coal. For UF coal, such kind of structural changes had occurred, so the T1H decreased after pyridine removal. This was not the case for YL and WY coal, which are lower rank coal than UF coal, because the structures of WY and YL coal did not change much from the original state by removing pyridine like UF coal. Conclusion It is clarified that the process of the pyridine fixation in coal was different with coal rank. For UF coal, most of pyridine molecules in coal were thought to be in a mobile state. For WY and YL coal, it became clear from (28) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (29) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107.
Energy & Fuels, Vol. 11, No. 3, 1997 715
solid-state 13C CP/MAS NMR that pyridine connected with coal in the form of hydrogen bonding. For WY coal, a difference in the efficiency of cross polarization for each carbon was acknowledged and swelling appears to have altered the efficiency of cross polarization for γ-carbon by additional motion of pyridine. For YL coal, the swelling at 0 h after pyridine immersion seemed to be enough for the effective cross polarization for γ-carbon of pyridine. The change of the T1H of coal by pyridine immersion was followed by the change of molecular mobility of coal. WY and YL coal showed the increase of the T1H with pyridine immersion, since these coals were at around ωτc ≈ 1 originally and moved to the ωτc , 1 region with the swelling by solvent immersion. UF coal showed the decrease of the T1H by pyridine immersion, since the coal was at the ωτc > 1 region originally and stayed in that region even after solvent immersion. EF960134R
