X-ray Analysis of Liquid-Treated Coals. 1. Effects of Pyridine on the

On the Molecular-Level Interactions between Pocahontas No. 3 Coal and Pyridine. Stephen B. DuBose, Ashley D. Trahan, Tolecia C. Turner, and David L. W...
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Energy & Fuels 1998, 12, 697-703

697

X-ray Analysis of Liquid-Treated Coals. 1. Effects of Pyridine on the Short-Range Structuring in Beulah Zap Lignite† David L. Wertz* and Jeff L. Quin Department of Chemistry & Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received October 15, 1997. Revised Manuscript Received May 4, 1998

The X-ray scattering curves are statistically indistinguishable, except for a small difference at very low angle, for a sample of “as received” Beulah Zap lignite and for the Beulah Zap which has been dried at 107 °C in a convection oven. The addition of pyridine to Beulah Zap causes a significant change in the X-ray scattering curves obtained in the 1.0-2.5 Å-1 region of reciprocal space, and this difference is also found in the resulting structure curves which have been calculated in molecular (real) space. A model that suggests pyridine is hydrogen bonded to the oxygens contained in attachments which are covalently bonded to the “planar” C9 units of Beulah Zap has been suggested, and both the specific and the average atom-pair distances calculated from this model are consistent with the structure function calculated from the X-ray scattering curve of the BZ-PYR sample.

Introduction Cartz, Diamond, and Hirsch1 showed, many years ago, that the X-ray diffractogram of a coal could be divided into three distinct segments, each of which is dominated by a specific type of scattering from the organic matrix of the coal. Of interest in this project are the high angle region, which is due to the scattering from small condensed aromatic layers,1,2 and particularly the intermediate angle, which includes the “intensity band” which is caused by the “packing of the layers of condensed aromatic units”. They1 report that the distance between the aromatic layers is ca. 3.5 Å. Recent studies of coals3 and other high-carbon materials4 have verified the Cartz and Hirsch report regarding the stacking peak distance being ca. 3.5 Å. A three-dimensional model of coal which emphasizes the small condensed aromatic units, connected by side chains, and particularly the layers of condensed aromatic units is presented in the Hirsch manuscripts.1 Cody, Davis, and Hatcher 5 report that coal is similar to rubbery materials and features entanglements of its macromolecular chains. The Carlson model6 of coal stresses the importance of both van der Waals forces and hydrogen bonding on the formation and stabilization of the macromolecular units in coals. The three† A portion of this information was presented at the 6th Chemical Sciences Coal Chemistry Conference and Workshop, Chantilly, VA, May 1997. (1) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500. Philos. Trans. R. Soc. 1960, A252, 68. (2) Wertz, D. L. Fuel 1995, 74, 1431; 1998, 77, 43. (3) Wertz, D. L.; Bissell, M. Energy Fuel 1994, 8, 613. (4) Ebert, L. B.; Scanlon, J. C.; Clausen, C. A. Energy Fuels 1988, 2, 438. (5) Cody, G. D.; Davis, A.; Hatcher, P. G. Energy Fuels 1993, 7, 455; 463. (6) Carlson, G. A. Energy Fuels 1992, 6, 771.

dimensional structural model of coal presented recently by Nakamura et al.7 provides much more structural detail. On the basis of energy minimization, this model suggests that covalently linked polycyclic aromatic (PCA) units are confined to pseudo-layers at the molecular level, with an average distance between the layers of 4.0 Å. Their model clearly illustrates that coals are anisotropic, and they propose that the degree of anisotropy parallels the aromatic character of the coal. Kidena et al.8 discuss how the three-dimensional structure of coal is rearranged by the addition of thermal energy and relate the macromolecular relaxation to cleavage of covalent bonds which cross-link the PCA units. However, they report that the d spacings between the layers of cross-linked PCA units (3.5-3.8 Å) are not greatly affected by temperatures up to 1000 °C. However, it has been suggested that the heat available at temperatures above 100 °C may alter the cross-linking in coals and thus affect liquid-coal interactions.9 Several of these structural models share similarities with the “two-phase” model10,11 which emphasizes the importance of the cross-linked macromolecular units which contain the polycyclic aromatic units as well as a “low molecular weight mobile phase” which is reported to be contained within the network of the macromolecules. (7) Nakamura, K.; Nakanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003. (8) Kineda, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672. (9) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. G. Energy Fuels 1993, 7, 384. (10) Derbyshire, F.; Marzec, A.; Schulten, H.-R.; Wilson, M. A.; Davis, A.; Tekely, P.; Delpuech, J.-J.; Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E.; Narayan, R.; Bartle, K.; Snape, C. Fuel 1988, 68, 1091. (11) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155.

S0887-0624(97)00199-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/19/1998

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None of these studies has unequivocally defined the short-range molecular-level structure of coal and/or force(s) which bind together its macromolecular units.12,13 Larsen14 has suggested that the swelling of its crosslinked macromolecular networks represents a valuable method for investigating the structure of coal. Numerous coal swelling experiments have been undertaken in an attempt to better understand the structure-bonding relationship which controls the molecular-level organization of the macromolecules in coals.5,15-26 Painter’s group has provided a detailed discussion of the thermodynamics of the swelling of coals.16 Suuberg et al.17 indicate that the electron-donating capacity of the solvent controls the swellability of coals such that pyridine is generally a far better swelling solvent for virgin coals than is benzene. Wide-angle X-ray scattering (XRS) is useful for examining the molecular-level short-range structuring in noncrystalline condensed phases. As with all other X-ray studies, the results describe the number-averaged, short-range structural unit. One limitation of XRS is that it provides only information in one spatial dimension, i.e., distance.2 An advantage is that the radiation used in our XRS studies is sufficiently penetrating so that surface effects may be ignored, and the subsurface portion of the coal is probed by the experiments. Modern X-ray analysis techniques have recently been used to investigate the structure of the average polycyclic aromatic units in Pocahontas No. 3 coal and in Pittsburgh coal.2 Our results verify the anisotropic structure of coals by proving that the PCA units and their nearest neighbor functional groups are, on the average, planar units, i.e., contained in the pseudoplanes defined by the Nakamura model. Our earlier study provided the average distance between the PCA layers for Pocahontas No. 3 coal, for Pittsburgh coal, and for several of the other Argonne Premium Coals.3 As part of our ongoing study of the short-range structure of coals, this project emphasizes relating the distance between the pseudo-layers of PCA units to the short-range structuring within the PCA layers. The second objective of this project is to measure the effect(s) of the addition of small amounts of various liquids on the average structural unit found in the coals. Beulah Zap (BZ) has been selected as the material for our first experiments because (a) it has been rather (12) Painter, P. C.; Sobkowiak, M.; Coleman, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 264. (13) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (14) Larsen, J. W.; Cheng, J. C.; Pan, C.-S. Energy Fuels 1991, 5, 57. (15) Brenner, D. Fuel 1985, 64, 167. (16) Painter, P. C.; Graf, J.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 379, 384, 393, and several other references. (17) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (18) Cai, M. F.; Smart, R. B. Energy Fuels 1994, 8, 369. (19) Takanohashi, T.; Iino, M. Energy Fuels 1995, 9, 788. (20) French, D. C.; Dieckman, S. l.; Botto, R. E. Energy Fuels 1993, 7, 90. (21) Fletcher, T. H.; Bai, S.; Pugmire, R. J.; Solum, M. S.; Wood, S.; Grant, D. M. Energy Fuels 1993, 7, 734. (22) Cody, G. D.; Eser, S.; Hatcher, P.; Davis, A.; Sobkowiak, M.; Shenoy, S.; Painter, P. C. Energy Fuels 1992, 6, 716. (23) Larsen, J. W.; Shawer, S. Energy Fuels 1990, 4, 74. (24) Antxustegi, M. M.; Mackinnon, A. J.; Hall, P. J. Energy Fuels 1993, 7, 1026. (25) Hall, P. J.; Larsen, J. W. Energy Fuels 1993, 7, 47. (26) Takanohashi, T.; Iino, M. Energy Fuels 1994, 8, 395.

Wertz and Quin

thoroughly studied27-32 and (b) its high oxygen content makes it chemically as well as structurally different from coals of higher rank. In the “as received” lignite, only ca. 45% is carbon and 15% is oxygen, while the moisture content of the coal is ca. 32%.30 Much of this oxygen is contained within carboxylic and/or phenolic functional groups.31-34 The nitrogen content in BZ is slightly lower than that found in other coals in the Argonne Premium Coal group.30 Solum, Pugmire, and Grant35 report that 58% ((4%) of the carbon in BZ is aromatic and that the average polycyclic aromatic species in BZ is ca. C9. Pyridine has been selected as the solvent because it is a polar aromatic moiety which is similar in shape to the average PCA unit in Beulah Zap lignite. In addition, it is an effective hydrogen-bonding unit. Furthermore, it has been used to study liquid-coal interactions by several other investigators. The results of our initial XRS study of Beulah Zap lignite are presented and discussed below. Experimental Section Sample Preparation. The Beulah Zap lignite is the -100 mesh sample contained in sealed ampules prepared by and sent to this group by the Argonne Premium Coal Sample Program.30 Two grams of the “as received” sample were dried at 107 °C for 24 h in a convection oven at ambient pressure to prepare the dried Beulah Zap (DBZ) sample. Each pyridinetreated sample was prepared by adding 0.25 g of the liquid to 1.00 g of Beulah Zap lignite, producing a gelatinous mixture with a pyridine/PCA molar ratio ∼ 1:1. This gelatinous mixture is designated BZ-PYR in the following discussions. This mixture was contained in a sealed sample bottle for 1 week and then removed for immediate X-ray analysis. Although the odor of pyridine could be detected before, during, and after the X-ray experiments, no liquid was visible at any point after its addition to the coal. The sample prepared by adding pyridine to dried Beulah Zap (DBZ-PYR) was prepared in exactly the same manner. In this latter sample, the molar ratio of pyridine/PCA is ∼0.7:1. The sample of BZ-PYR was examined again after a period of 3 months. At this time, the odor of pyridine from the gel was, at best, faintly noticeable. This sample is identified as BZ-PYR3MO. X-ray Analysis. A weighed portion (0.5 g) of each sample was mounted onto an aluminum sample holder and placed into our Θ-2Θ X-ray diffractometer. Intensities were accumulated at ∆2Θ ) 0.02° from 2Θ ) 5.00° to 2Θ ) 75.00°; or from qmin ) 0.3559 Å-1 to qmax) 4.967 Å-1 at ∆q ) 0.001 316 Å-1, where (27) Kumagai, H.; Matuoka, K.; Norinaga, K.; Chiba, T.; Sasaki, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 289. (28) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047. (29) Norinaga, K.; Kumagai, H.; Hayashi, J.-I.; Chiba, T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 238. (30) Vorres, K. S. Energy Fuels 1990, 4, 420. (31) Kube, W. R.; Schobert, H. H.; Bensen, S. A.; Karner, F. R. The Chemistry of Low Rank Coals; American Chemical Society: Washington, DC, 1984; Chapter 3. (32) Aidi, T.; Tsutsumi, Y.; Yoshinaga, T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 744. (33) Hayatsu, R.; Winans, R. E.; Scott, R. G.; Moore, L. P.; Studier, M. H. Nature 1978, 275, 116. (34) Tillman, D. A. The Combustion of Solid Fuels and Wastes; Academic Press: New York, 1991; p 140. (35) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.

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Figure 1. X-ray scattering curve for Beulah Zap lignite prior to (A) and after (B) convection drying at 107 °C.

Figure 3. Comparison of the corrected X-ray scattering curves for BZ (dark squares) and DBZ (open squares) shown in A and the difference curve shown in B.

Figure 2. The corrected X-ray scattering curve for Beulah Zap lignite separated into the scattering from its organic matrix (A) and the diffraction from the crystalline materials (B). q ) {4π/λ}‚sin Θ.2,36 Intensity was accumulated for a preset time of 2 s at each angle. In addition, the diffraction patterns of finely powdered graphite and of NBS #640 silicon powder were obtained and served as internal standards.

Results and Discussion

XRSC for BZ, IBZ(q), and for DBZ, IDBZ(q). Also shown in Figure 3 is the difference curve which has been calculated by:

∆(q) ) IDBZ(q) - IBZ(q)

X-ray Experiments. The statistical uncertainty (σN) in the measured intensity (N) at each angle in each X-ray scattering-diffraction experiment is given by36,37

σN ) N0.5

Figure 4. The X-ray scattering curves for BZ (A), DBZ (B), BZ-PYR (C), and DBZ-PYR (D).

(1)

Shown in Figure 1 are the diffractograms of “as received” Beulah Zap lignite and of the dried Beulah Zap. Each diffractogram consists of a few well-defined, sharp diffraction peaks due to the aluminum sample holder and/or to crystalline minerals and a series of diffuse maxima and minima due to the organic matrix of BZ. The diffraction peaks were removed by a method previously described to leave the X-ray scattering curve (XRSC) due to the organic matrix of BZ,2 as shown in Figure 2. Shown in Figure 3 is a comparison of the corrected (36) Klug, H. P.; Alexander, L. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley-Interscience: New York, 1975; p 360. (37) Kruh, R. F. Chem. Rev. 1962, 62, 319.

(2)

where q ) [4π/λ]‚sin θ. All of the data points on the difference curve fall within the interval of (2σN, i.e., the diffractograms of BZ and of DBZ are statistically indistinguishable over the range of reciprocal space from q ) 1.0-5.0 Å-1. The XRSC’s curves of BZ and of DBZ contain a small maximum in the region from 1.5 to 2.0 Å-1 (d ) 3.2-4.0 Å where d ) 2π/q* and q* is the location of the peak maximum in q). A much sharper, more intense maximum is found in this region of reciprocal space in the XRSC’s of coals of higher rank,1-3,38,39 and this peak has been attributed to the layering of the polycyclic aromatic in these more mature coals. Shown in Figure 4 are the diffractograms of BZ, DBZ, BZ-PYR, and DBZ-PYR. The latter two are similar to one another but, in the vicinity of 2θ ) 15-35°, quite different from the XRSC’s for BZ and DBZ. The sharp peak in this region of the XRSC’s of BZ-PYR and of DBZPYR is much like the first peak which occurs in this region in the diffractogram of Pocahontas No. 3 coal.2,3

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Figure 5. The difference XRSC’s for BZ-PYR (squares) and for DBZ-PYR (circles) compared to the XRSC for BZ.

Shown in Figure 5 are the difference X-ray scattering curves for the two pyridine-treated samples, each calculated by the method shown above. The large maximum in each difference curve suggests the formation of an adduct between the molecular-level coal species and pyridine molecule(s) and further suggests that this adduct is sufficiently stable to be measured by X-ray scattering methods. Reciprocal Space Structural Analysis. The result of an X-ray scattering experiment is frequently expressed in terms of the phase interference curve, i(q), which describes, in reciprocal space, the atom-pair distances in the scattering medium. The phase interference curve may be obtained from the corrected secondary X-ray intensity by37-42

i(q) ) {I*(q)/κ} -

∑xC{f2C(q) + incC(q)}

(3)

In eq 2, κ converts the corrected scattered intensity from counts per second to electron scattering units (EU), xC is the fraction of carbon in the sample, fC(q) is the X-ray scattering factor for carbon, and incC(q) is the incoherent scattering which has been corrected for the inefficiency of the monochromator. The calculated self-scattering from the organic matrix is given by ΣxC{f2C(q) + incC(q)}. Shown in Figure 6 are the corrected XRSC’s for BZ, for BZ-PYR, and for DBZ-PYR scaled to the selfscattering curve calculated for Beulah Zap. The resulting phase interference curves for these two pyridinetreated samples and for BZ are shown in Figure 7. The i(q) of each pyridine-treated sample is dominated by the large first maximum which is centered at ca. 1.4 Å-1. In this region of reciprocal space, these two XRSC’s and their resulting i(q)’s are quite different from the corresponding XRSC and the i(q) obtained for BZ. These differences are noted in Table 1. However, at q > 2.2 Å-1, the i(q)’s are statistically indistinguishable. If the structure of the average short-range species in an amorphous material is known, the Debye intensity relationship may be used to calculate the simulated phase interference curve which is based only on the known short-range structural unit.37-42 Solum et al.35 report that the average PCA unit in Beulah Zap lignite is ca. C9. A structural model of C9 has been constructed subject to the following restraints:43 (a) the bonded C-C

Figure 6. The independent atom scattering curve calculated for the carbon matrix for each sample fitted to the corrected XRSC’s for BZ (squares), for BZ-PYR (circles), and for DBZPYR (triangles).

Figure 7. The phase interference curves for BZ (squares), for BZ-PYR (circles), and for DBZ-PYR (triangles). Table 1. Summary of the First Peak in Each Phase Interference Curve sample

qmaxa (Å-1)

intensity (EU)

fwhm (Å-1)

BZ DBZ BZ-PYR DBZ-PYR

1.78 1.80 1.42 1.44

4.7 4.6 19.2 19.0

0.36 0.37 0.22 0.21

a Shown is the center of the diffuse maximum, which is not necessarily the point of highest intensity.

distances are, on the average, 1.39 Å; (b) the C-C-C bond angles are, on the average, 120°; (c) the C9 unit is planar. The C--C distances (rCC) characteristic of this twodimensional C9 model have been calculated by

rCA-CB ) {(XA - XB)2 + (YA - YB)2 + (ZA - ZB)2}0.5 (4) where A and B represent carbon atoms in the same structural plane, i.e., ZA ) ZB ) 0. From these C-C (38) Wertz, D. L.; Bissell, M. Adv. X-Ray Anal. 1994, 8, 491. (39) Wertz, D. L.; Quin, J. Fuel, submitted. (40) Wertz, D. L.; Kruh, R. F. J. Chem. Phys. 1967, 47, 388. (41) Holder, A. J.; Wertz, D. L. J. Phys. Chem. 1987, 91, 3479. (42) Narten, A. H.; Levy, H. A. J. Chem. Phys. 1971, 55, 2263. (43) Wertz, D. L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 957.

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Energy & Fuels, Vol. 12, No. 4, 1998 701

Figure 8. The simulated phase interference curve calculated for a C9 PCA unit.

Figure 9. Three three-dimensional structural models of the adduct formed between the C9 PCA unit (rectangle) and the pyridine molecule (disk). In model A, the C9 and the pyridine are approximately coplanar; in model B, they are normal; and in model C, they are approximately parallel to one another.

distances the Debye intensity, j(q), has been calculated by

j(q)C9 )

∑fC2(q) (qrCC)-1 sin(qrCC)

(5)

A structural model of pyridine has also been developed using the same methodology, and the C-C and the C-N distances have been calculated by the same method. These distances have been used to calculate the simulated phase interference curve for pyridine, j(q)PYR, from the Debye intensity equation. Shown in Figure 8 are these two simulated phase interference curves, j(q)C9 and j(q)PYR. At q > ∼2.5 Å-1, the j(q)’s obtained for the two structural models are similar to the i(q)’s obtained for the BZ sample and for the pyridine-treated BZ samples, indicating that the models upon which these calculations are based are adequate structural representations of the average intramolecular species in this sample. However, neither of these simulated curves for the PCA unit and/or the pyridine molecule predicts the very large maximum centered at ca. 1.4 Å-1 or the preceding minimum which is observed in the i(q) obtained for the BZ-PYR sample and the DBZ-PYR sample. This lack of agreement suggests that this dominant maximum and its preceding minimum are not due to intramolecular structuring from the average PCA unit in BZ and/or intramolecular structuring within the pyridine molecule. Consequently, the peak must be assigned to an intermolecular interaction which involves both the PCA unit of BZ and the pyridine molecule(s). Speculative Three-Dimensional Structural Models of the Pyridine-PCA Adduct. It is beyond the scope of this X-ray scattering study to determine the force(s) involved in the formation of the adduct between the PCA unit and the pyridine molecule. Nonetheless, because of the high fraction of carboxylate moieties attached to the PCA units in the BZ and because of the polarity of the pyridine, it is likely that the adduct between the PCA unit and the pyridine involves hydrogen bonding between the O atom of attachments (possibly carboxylates) and the N atom contained within the pyridine molecule. Several models of the adduct formed between the average PCA unit and the pyridine molecule have been considered. Shown in Figure 9 are three models of the PCA-PYR adduct which differ with regard to location

Figure 10. Frequency vs atom-pair distance graphs for the three structural models presented above. Table 2. Atom-Pair Distances (Å) Calcualted for the Three Structural Models model

characteristics

rmin

rmax

rave

A B C

pyridine and PCA are coplanar pyridine and PCA are normal pyridine is parallel to PCA

4.2 4.4 4.4

11.5 9.3 6.8

8.0 7.1 5.1

of the pyridine molecule relative to the plane of the average PCA unit. In each of these models, the Hbonded O-N distance has been assumed to be ca. 2.7 Å. The intermolecular atom-pair distances which have been calculated for each model are presented in Figure 10. The distribution of these distances is quite different, as noted in Table 2. In model A, the median distance is 8.0 Å, with the specific atom-pair distances ranging from 4.2 to 11.5 Å. In model B, the range of atom-pair distances is 4.4-9.3 Å, with the median intermolecular distance being 7.1 Å. In model C, the atom-pair distances are clustered much more closely to the median distance (5.1 Å) and range from 4.4 to 6.8 Å. The simulated phase interference curve for each model of the “intermolecular” PCA-PYR adduct has also been calculated using the methods described above. These j(q)’s, presented in Figure 11, are considerably different from one another. Only the q calculated from model C correctly predicts the large maximum at ca. 1.4 Å-1 and the preceding minimum centered at ca. 0.9 Å-1. Shown in Figure 12 is a comparison of the j(q) calculated for

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The average distance between layers may be calculated from the location of the maximum in the X-ray scattering curve by

〈d〉 ) 2π/q*

Figure 11. The j(q) calculated from model A (squares), the j(q) calculated from model B (diamonds), and the j(q) calculated from model C (circles).

where q* is the center of the maximum. For the BZPYR adduct, where q* ) 1.42 Å-1, the interlayer distance, 〈d〉, is ∼4.4 Å. For the adduct in the DBZPYR sample, q* ) 1.44 Å-1, so 〈d〉 ) 4.4 Å. In model C, the interplanar distance is calculated to ca. 4.5 Å, which is in good agreement with the average 〈d〉 measured for each pyridine-treated sample. Such a correlation cannot be made with the intermolecular species predicted by either model A or model B. Atom-Pair Structure Function. The structure function, calculated from the phase interference curve of a condensed phase sample, describes the array of atom-atom distances in the short-range scattering unit of that sample in molecular space. The structure function is averaged over the time of the experiment (ca. 2 h) and over the entire sample (0.5 g). The structure function for an amorphous material may be calculated by

∫

S(r) ) [2r/π] q i(q) M(q) P(q) sin(qrAB) ∆q

Figure 12. Comparison of the j(q) calculated from model C to the i(q) obtained for the BZ-PYR sample (circles) and the i(q) calculated for the DBZ-PYR samples..

model C and the experimentally determined phase interference curve measured for the BZ-PYR sample. Although the amplitude of the preceding minimum and of the large maximum is smaller in the j(q) calculated from model C, the simulated phase interference curve calculated from this model predicts the large maximum and the preceding minimum which are observed in the i(q) of the experimentally determined phase interference curve for the BZ-PYR sample. The j(q) calculated from model A and the j(q) calculated from model B are each significantly different from the i(q) obtained by experiment, suggesting that neither is a description of the average unit in either of the pyridine-treated BZ samples. Molecular Space Analysis. Cartz and Hirsch1 have found that the first peak in the diffractograms of coals is not due to scattering from within the PCA units but rather to the layering of the PCA units within the coal. As noted above, the large maximum in the diffractogram of the adduct formed between the average PCA unit in Beulah Zap lignite and the pyridine is consistent with the first peak in the diffractograms of coals of high rank.1-3,38,39

(6)

(7)

In eq 4, M(q) sharpens the real-space structure function from an electron-pair distribution to a distribution which describes atom-pairs, and P(q) is the dampening function which ensures that the atom-pair sharpening does not cause the higher q portion to dominate the calculation of the structure curve. For the calculations of the structure curve for the BZ-PYR sample, ∆q ) 0.10 Å-1 and the limits of the integral are qmin ) 0.5 Å-1 and qmax ) 2.2 Å-1; i.e., the region in q which is not due to scattering from within the two-dimensional PCA unit. At low r, S(r) f -1, indicating the absence of molecularlevel structure. At high r, S(r) f 0, indicating the disappearance of short-range structuring.37-42 The uncertainty (τ) in each of the atom-pair correlation functions has been calculated to be (0.02.44 Thus, the distance (r*) at which g(r) becomes contained by 1 ( 2τ is an estimate of the distance at which short-range structuring vanishes and is replaced by structural randomness. Using qmin and qmax, the resulting S(r) provides a onedimensional analysis of the “intermolecular” atom-pair distances between atoms in the PCA unit and atoms in the pyridine molecule. The intermolecular structure functions obtained for the BZ-PYR sample and for the DBZ-PYR sample are presented in Figure 13. The center of the one statistically significant maximum in each S(r) is located at ca. 5.10 Å, and the short-range structure statistically disappears at ca. 6.1 Å in each structure function. The similarities between these structure functions suggests that the same adduct is formed in each of these two samples. Also shown in Figure 13 is a comparison of each S(r) to the atom-pair frequency calculated from model C (see Figure 9). This comparison indicates that the atom-pair distances calculated from model C are consistent with those from the structure curve calculated from our X-ray (44) Konnert, J. H.; Karle, J. Acta Crystallogr. 1973, A29, 702.

X-ray Analysis of Liquid-Treated Coals

Figure 13. The structure function obtained for the BZ-PYR sample (circles) and for the DBZ-PYR sample (triangles) compared to the frequency/atom-pair graph for model C.

scattering experiment for BZ-PYR and for DBZ-PYR. Alternately, the atom-pair distances calculated from models A and B are not similar to either structure curve. Long Term Effects of Pyridine Addition. The diffractogram of the gel which had aged for 3 months no longer contains the large maximum at ca. 1.4-1 Å but rather is similar to the diffractogram of BZ, as seen in Figure 14. This similarity suggests that the adduct formed between pyridine molecules and the PCA unit, which causes the sharper, more intense first peak at 1.4 Å-1, has disappeared. Because the odor of pyridine was, at best, only faintly detected from this sample, it appears that degradation of the PCA-pyridine adduct parallels the disappearance of most, if not all, of the pyridine from the BZ-PYR sample. Both factors suggest that the BZ-PYR adduct is a kinetic rather than a thermodynamic product. Conclusions The X-ray scattering curves for Beulah Zap and for the sample of dried Beulah Zap are statistically similar,

Energy & Fuels, Vol. 12, No. 4, 1998 703

Figure 14. X-ray diffractograms of BZ (A), BZ-PYR (B), and BZ-PYR3MO (C).

suggesting that the removal of moisture from this lignite does not alter the three-dimensional molecular structuring in the lignite sample to an extent that can be measured by this method. The addition of pyridine measurably affects the threedimensional molecular-level structuring in Beulah Zap lignite by causing the formation of an adduct between the pyridine molecules and polycyclic aromatic species in BZ. This adduct is described principally by a large peak which occurs at ca. 1.4 Å-1 in the diffractograms of the pyridine-treated samples and which corresponds to an interlayer separation distance of 4.4 Å in these samples. The formation of the adduct between pyridine and BZ is a reversible process, and this adduct is a kinetically important species but is not a thermodynamic product. Hydrogen bonding between oxygens (probably from carboxylate attachments to the C9) and the nitrogen of the pyridine molecule offers an attractive explanation of the forces that are involved in the PCA-PYR moiety, but this speculation cannot be proven (or disproven) for these experiments. EF970199+