Energy & Fuels 2001, 15, 1537-1538
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On the Molecular-Level Interactions between Pocahontas No. 3 Coal and Pyridine Stephen B. DuBose, Ashley D. Trahan, Tolecia C. Turner, and David L. Wertz* Department of Chemistry & Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received July 6, 2001. Revised Manuscript Received September 21, 2001 Pocahontas No. 3 coal is a low volatile bituminous coal1 containing, on the average, poly-cyclic aromatic (PCA) units of C20 in the x-y plane.2,3 The average interlayer distance between these PCA layers is 3.53 ( 0.02 Å;4 so the zdimension of the average short-range structural unit of POC, which contains ca. 5 layers of PCA units stacked in an approximately parallel manner, is ca.14 Å. The high aromaticity of POC suggests the possibility of extensive interlayer π-π interactions, but its very low oxygen atom/carbon atom ratio indicates that POC has few oxygen sites available for hydrogen-bonding.1-3 The effects of liquids on the molecular-level structuring of coals have been studied,4-15 and particular attention has been given to the use of pyridine.12-15 Pyridine is thought to be effective in swelling coals because of its molecular characteristicsspyridine is polar, planar, aromatic, and capable of forming hydrogen bonds. When pyridine is added to a low rank coal, the interlayer structural features of the low rank coal disappear and are replaced by X-ray scattering features which are characteristic of a molecular-level coal-pyridine adduct.5 Because the bonding between coals and pyridine (and other liquids) has not been completely elucidated, this group has initiated a study which involves low- and high-rank coals and several liquids. Presented below are the results obtained from a wideangle X-ray scattering (WAXRS) study of effects produced on the molecular-level layering in Pocahontas No. 3 coal treated with pyridine. Pocahontas-pyridine samples were prepared by adding 0.25 g of pyridine to 0.5 g of -100 mesh “as received” Pocahontas No, 3 coal from the Argonne Premium Coal Sample Program. Mixture #1 was prepared by placing the preweighed POC into a glass bottle, adding the pyridine to the POC, tightly stoppering the bottle, and allowing the mixture to equilibrate for one week. Mixture #1 was removed from the bottle, mounted onto our sample holder, and immediately examined by WAXRS methods. * Corresponding author. E-mail:
[email protected]. (1) Vorres, K. S. Energy Fuels 1990, 4, 420-425. (2) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-192. (3) Muntean, J. V.; Stock, L. S. Energy Fuels 1991, 5, 768. (4) Wertz, D. L.; Quin, J. L. Fuel 2000, 79, 1981-1989. (5) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697-703. (6) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047-1053. (7) Wertz, D. L. Energy Fuels 1999, 13, 513-517. (8) Painter, P. C.; Graf, J.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 379, 384, 393. (9) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (10) Larsen, J. W.; Cheng, J. C.; Pan, C.-S. Energy Fuels 1991, 5, 57. (11) Takanohashi, T.; Nakamura, K.; Terao, Y.; Iino, M. Energy Fuels 2001, 14, 393-399. (12) Cody, G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340344. (13) Cody, G. D.; Eser, S.; Hatcher, P.; Davis, A.; Sobkowiak, M.; Shenoy, S.; Painter, P. C. Energy Fuels 1992, 6, 716-719. (14) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4, 74-77. (15) Yun, Y.; Suuberg, E. Energy Fuels 1998, 12, 798-800.
Figure 1. WAXRS intensity curves for (a) the sample holder used in these experiments and (b) pyridine.
Figure 2. WAXRS intensity curves for (c) untreated Pocahontas No. 3 coal and (d) the POC-PYR mixture #1.
Collection of the scattered X-rays and data processing have been discussed.3-7 Throughout this manuscript, the reciprocal space parameter, defined by q ) [4π/λ] × sin θ,12 will be used in data presentations. For mixture #1, wide-angle X-ray scattering intensity was accumulated for 2 s at increments of ∆2θ ) 0.05 Å-1 from 2θ ) 5° to 2θ ) 90° or from q ) 0.363 Å-1 to 5.88 Å-1. Shown in Figure 1 are the WAXRS scans of the sample holder and of pyridine. The former exhibits no peaks in the region of reciprocal space investigated in these experiments, but the WAXRS scan of pyridine contains a sharp maximum centered at 1.36 Å-1. Shown in Figure 2 are the WAXRS scans of untreated Pocahontas No. 3 coal and of POC/PYR mixture #1. Also shown in Figure 2 is the self-scattering curve calculated for Pocahontas No. 3 coal.4 Shown in Figure 3 are the phase interference curves calculated by
i(q) ) {I(q)/k} - SS(q)
(1)
for untreated Pocahontas No. 3 coal and for POC/PYR mixture #1. In eq 1, I(q) is the corrected WAXRS intensity, k is the scaling constant which converts the corrected intensity in CPS to electron scattering units, and SS(q) is the self-scattering calculated for that sample.4-7,16 Shown in Figure 4 is the difference phase interference curve calculated
10.1021/ef0101558 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/31/2001
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Energy & Fuels, Vol. 15, No. 6, 2001
Figure 3. The phase interference curves for untreated POC
(squares) and the POC/PYR mixture #1 (circles).
Communications
Figure 7. The difference phase interference curves for mixture #2; 2a (squares), 2b (circles), 2c (triangles), and 2d (diamonds). Table 1. Comparison of the Area under the 1.37 Å-1 Peak and the Percentage of Pyridine Retained in POC/PYR Mixture #2
Figure 4. The difference phase interference curve for POC/
PYR mixture #1.
Figure 5. The four time-interval WAXRS scans for mixture
#2.
Figure 6. The four phase interference curves for POC/PYR mixture #2 compared to the phase interference curve for untreated POC (0). Squares indicate scan 2a. Circles indicate scan 2b. Triangles indicate scan 2c. Diamonds indicate scan 2d.
by ∆i(q) ) i(q)POC/PYR #1 - i(q)POC. Investigation of the ∆i(q) offers the best view of effect(s) caused by pyridine on both the intralayer and interlayer structuring of POC. At q > 2.5 Å-1, the WAXRS scans of POC and the POCPYR mixture #1 are statistically equivalent, indicating that the addition of pyridine does not measurably affect the covalent structuring within its PCA units. For coals, the average molecular-level interlayer spacing distance is described in reciprocal space by the peak which occurs at q* ) 2π/〈d〉. As previously noted,4 this peak occurs at q* ) 1.78 Å-1. In the phase interference curve of the untreated POC, the area under this peak is 25.3 ESU/Å. In the ∆i(q) for mixture #1, the decrease in the area of this peak (16) Kruh, R. F. Chem. Rev. 1962, 62, 319-346.
sample
time after preparation
peak area in ∆i(q)
pyridine retained in mixture #2
2a 2b 2c 2d
0.2 hours 0.6 hours 1.0 hours 1.4 hours
42 ESU/Å 23 ESU/Å 10 ESU/Å w0
93% 54% 28% w0
is 2.1 ESU/Å. This difference in peak area is statistically equivalent to the uncertainty in the peak area analysis, so it may be stated that addition of pyridine to Pocahontas No. 3 coal produces little, if any, structural change in the interlayer structuring of POC. In addition, the WAXRS scan of POC-PYR mixture #1 contains a second important peak, labeled q*2 and is centered at q* ) 1.37 Å-1. To determine the origin of q*2, POC/PYR mixture #2 was prepared and analyzed. For mixture #2, the preweighed POC (0.5 g) was mounted directly onto the sample holder, pyridine (0.25 g) was added to the POC sample, and the WAXRS experiments were initiated. Immediately after preparation of mixture #2 and then before each additional WAXRS scan of this sample, the mass of the POC-PYR sample was measured using a Mettler AE-100 balance. Wide-angle X-ray scattering scans of mixture #2 were made at 15 min intervals by irradiating the POC-PYR sample with copper X-rays over the interval that contains the nearest interlayer distance, 0.5-2.5 Å-1,3-7,13-16 at ∆2θ ) 0.05° at a time interval of 1 s per data point. The WAXRS scans 2a-2d of this sample are presented in Figure 5. Also shown in Figure 5 is the WAXRS scan of untreated POC. The phase interference curves calculated for POC and for scans 2a-2d of the POC-PYR sample 2 are presented in Figure 6. The difference phase interference curves, defined by ∆i(q) ) i(q)SCAN 2-x - i(q)POC, are shown in Figure 7. Comparisons indicate that the characteristic peak of untreated POC, at q* ) 1.78 Å-1 is statistically equivalent in all five of these samples, but that the peak centered at q*2 ) 1.37 Å-1 decreases with time and is, in fact, absent in scan 2d. Shown in Table 1 is a comparison of the percentage of pyridine retained in the POC/PYR mixture #2 and the area under the peak labeled q*2. The linear correlation between these two parameters is 0.9971, indicating that the peak q* ) 1.37 Å-1 is due to pyridine which has not intercalated into the interlayer structuring of POC. There is no evidence from the wide-angle X-ray scattering curves of the POC/PYR samples that a molecular-level adduct is formed. Whether the absence of hydrogen-bonding sites on POC or the strong interlayer π-π interactions between adjacent layers in the short-range structural domain of POC prevent the intercalation of pyridine molecules between nearest layers in the short-range domain cannot be determined from this study. EF0101558
