On the Molecular-Level Interactions Between Pyridine and Pittsburgh

and Pittsburgh No. 8 Coal. David L. Wertz* and E. Ryan Smith. Department of Chemistry & Biochemistry and The Honors College, The University of Souther...
0 downloads 0 Views 155KB Size
Download PDF
482

Energy & Fuels 2003, 17, 482-488

On the Molecular-Level Interactions Between Pyridine and Pittsburgh No. 8 Coal David L. Wertz* and E. Ryan Smith Department of Chemistry & Biochemistry and The Honors College, The University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 Received September 24, 2002. Revised Manuscript Received January 14, 2003

The poly-cyclic units of Pittsburgh No. 8 coal (PIT), on the average, lie in short-range structural domains of ca. three layers. The species C14H9 is consistent with the average PCA unit in PIT, and its radius is ∼2.9 Å. The average distance between the PCA layers in the short-range domains of PIT is ∼3.9 Å. The addition of pyridine molecules to PIT does not alter structuring within the PCA units to an extent measured by wide-angle X-ray scattering experiments, and it appears that the interlayer structuring within the short-range structural domains in PIT is also not altered to a measurable extent. Rather, the quasi-stable adduct is formed between the three-layer domains of PIT and the pyridine molecules.

Introduction Coals are thought to contain molecular-level domains of small poly-cyclic aromatic (PCA) compounds which, at least for short distances, lie in approximately parallel layers.1-7 The effect(s) of liquids on coals have been studied by many investigators in recent years with particular emphasis placed on “coal swelling experiments” as well as on the removal of unwanted minerals from the coals.8-20 Liquids have been used as probes to alter the interlayer structuring of the coals in an attempt to gain knowledge about the noncovalent bonding which is involved between these “parallel” layers of the coal. * Corresponding author. Voice: 601-266-4701. Fax: 601-266-6075. E-mail: [email protected]. (1) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500; Philos. Trans. Royal Soc. 1960, A252, 68. (2) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (3) Wertz, D. L.; Quin, J. L. Fuel 2000, 79, 1981-89. (4) Wertz, D. L. Energy Fuels 1999, 13, 513-517. (5) Kineda, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678. (6) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Saada, Y. Energy Fuels 1995, 9, 1003-1010. (7) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047-1053. (8) Nishioka, M. Energy Fuels 2001, 15, . (9) Krzesinska, M. Energy Fuels 2001, 15, 324-330. (10) Takanohashi, T.; Nakamura, K.; Iino, I. Energy Fuels 1999, 13, 922. (11) Painter, P. C.; Graf, J.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 379, 384, 393. (12) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (13) Larsen, J. W.; Cheng, J. C.; Pan., C.-S. Energy Fuels 1991, 5, 57. (14) Takanohashi, T.; Nakamura, K.; Terao, Y.; Iino, M. Energy Fuels 2001, 14, 393-399. (15) Dzrakacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2001, 15, 1409-1415. (16) Xiong, J.; Maciel, G. Energy Fuels 2002, 16, 497-509. (17) DuBose, S. B.; Wertz, D. L. Energy Fuels 2002, 16, 669-675. (18) DuBose, S. B.; Trahan, A. D.; Turner, T. C.; Wertz, D. L. Energy Fuels 2001, 15, 1537-1538. (19) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697-703. (20) Vorres, K. S. Energy Fuels 1990, 4, 420-425.

Our previous detailed studies have involved Pocahontas No. 3 coal (APC 501) and Beulah Zap lignite (APC 801).3,4,18,19 Pocahontas No. 3 coal is the most aromatic of the Argonne Premium Coals and has both a very low moisture content and a very low oxygen content.21 Beulah Zap represents the other extreme of the coal matrix, with a very high moisture content and a very high oxygen (phenolic and carboxylic) abundance as well as a low aromaticity.21 The PCA units in BZ are, on the average, much smaller than those found in POC.22,23 The nitrogen types in the two coals also differ considerably.24 Pyridine evaporates rapidly from mixtures of Pocahontas (POC) coal,18 suggesting that no adduct is formed between the short-range structural domains of POC and pyridine molecules. In addition, pyridine does not alter either the molecular structuring within the PCA units of POC or the structuring between adjacent domains that contain “parallel” PCA layers. Pyridine is retained in a comparable pyridine/Beulah Zap (BZ) lignite mixture, and quasi-stable adduct is rapidly formed. In this adduct, the short-range structural domains of BZ are bonded to the pyridine molecules, but the structuring within the PCA units of BZ does not affect the bonding within the poly-cyclic units of BZ to a measurable extent.19,20 Because pyridine affects these two coals - the extremes of the Argonne Premium Coals - in such widely different ways, the role of pyridine as a probe to investigate molecular-level interaction(s) in more typical coals is not clearly understood. To better understand the type(s) of molecular-level interactions pyridine makes with bituminous coals, mixtures of pyridine and (21) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-192. (22) Takanoshishi, T.; Kawashima, H. Energy Fuels 2002, 16, 379388. (23) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896-906. (24) Wertz, D. L. Powder Diffraction 1988, 3, 153-156.

10.1021/ef020217+ CCC: $25.00 © 2003 American Chemical Society Published on Web 03/04/2003

Interactions Between Pyridine and Pittsburgh No. 8 Coal

Energy & Fuels, Vol. 17, No. 2, 2003 483

Table 1. Comparison of Elemental Components in Three Coals of the Argonne Premium Coals Group

a

parametera

POC

BZ

PIT

% carbon % hydrogen % oxygen % nitrogen % moisture

86.3 4.2 2.3 1.3 0.7

52.5 3.3 14.7 1.0 32.2

74.3 4.8 8.0 1.5 1.7

% represents percentage by mass in the “as received: coal.

Pittsburgh No. 8 (high volatile bituminous) coal have been prepared. Shown in Table 1 is a comparison of important molecular-level parameters of Beulah Zap lignite, Pocahontas No. 3 low-volatile bituminous coal, and Pittsburgh No. 8 coalsthe subject of this study. These three coals differ considerably not only in carbon content but also in oxygen and moistures. The PIT/PYR samples have been investigated by wide-angle X-ray scattering (WAXRS) methods. Reported below are the results of these experiments.

Figure 1. Total mass (sample bottle + mass of PIT + mass of pyridine) measured for PIT/PYR gel B over a three-week period.

Experimental Section Sample Preparation. “As received” Pittsburgh No. 8 coal (-100 mesh) was used in these experiments.21 Using the NMR studies of PIT published by Solum et al.,22 the fractions of carbon and of aromatic carbon are 0.52 and 0.75, respectively. Solum et al. have proposed that, on the average, the aromatic carbons in PIT exist as C14 units. Two samples were prepared by adding a weighed amount of pyridine (Aldrich, reagent grade) to a weighed amount of Pittsburgh No. 8 coal in a 1:2 mass ratio using a Mettler AE 100 balance. At the time of preparation, the stoichiometric ratio of moles of pyridine/moles of C14 ) 1.7. Each sample was stored in a glass bottle with a screw-on top. A 0.5 g sample of Gel A was removed from the sealed sample bottle immediately prior to and then was used for the wide-angle X-ray scattering experiments described below over the period of 1-22 days after its preparation. Sample A-1 represents this sample 1 day after its preparation. Samples A-8, A-15, and A-22 indicate this sample examined 8 days, 15 days, and 22 days after sample preparation, respectively. Gel sample B was used for the mass measurement experiments. After the conclusion of the mass/time experiments, gel B was exposed to the ambient conditions so that the maximum amount of pyridine would evaporate from the PIT/PYR sample into the atmosphere. Mass Measurements. The Mettler Model AE100 balance was used to obtain the mass of gel B each day for a 20-day period and then again after “complete” pyridine evaporation. Wide-Angle X-Ray Scattering Experiments. A 0.5 g sample of either “as received” Pittsburgh No. 8 (PIT) or a 0.5 g sample from mixture one was used for each WAXRS scan.25,26 The WAXRS methodology used for these experiments has been described in recent publications.2-4,18-20 The secondary X-rays, at λ ) 1.54 Å, were measured over the angular range from 2θ ) 5.00° to 90.00° using the fine-focus X-ray diffractometer in the Wertz laboratory. Scattering intensity was measured at the 1701 angles at increments of ∆2θ ) 0.05° for one-second intervals at each angle. The angular experimental parameter, 2θ, has been converted to the reciprocal space parameter, q, by q ) [4π/λ] × sin θ.27 (25) Wertz, D. L.; Smithhart, C. B.; Wertz, S. L. Adv. X-Ray Anal. 1990, 33, 475-483. (26) Kruh, R. F. Chem. Rev. 1962, 62, 319-346. (27) Wertz, D. L.; Kruh, R. F. J. Chem. Phys. 1967, 47, 388-390.

Figure 2. (a) WAXRS intensity scan of Pittsburgh No. 8 coal. Also shown are the self-scattering curve calculated for PIT and its phase interference curve.

Results When the pyridine is added to the powdered Pittsburgh No. 8 coal, it is immediately incorporated into the structural matrix of the coal. The resulting adduct becomes a solid and is cast into a disk, following the shape of the bottle in which it is contained. The diskshape is retained by the PIT/PYR adduct even when more than 95% of the pyridine has evaporated from the gel. Mass Measurements. Shown in Figure 1 is a graph of measured mass of PIT/PYR gel B measured over a period of 22 days. These results indicate that over this period, there is little mass change. Based on the assumption that mass loss is due to the loss of pyridine from the gel, ca. 90% pyridine remains dispersed in the coal matrix after 20 days when the sample is contained in a closed container. In the sample of gel B (exposed to the atmosphere for several days), ca. 4.5% of the pyridine initially added to the mixture is retained in the gel. The solid disk of gel B was ground to a fine powder, and a WAXRS scan of this powder was obtained. WAXRS Study of Untreated PIT. Shown in Figure 2 are the Icor(q) for PIT, its self-scattering, and its phase interference curve. The experimentally measured WAXRS intensity for Pittsburgh No. 8 coal (PIT) has

484

Energy & Fuels, Vol. 17, No. 2, 2003

Wertz and Smith

been corrected for polarization and absorption effects as well as for background by conventional methods.28-33 The corrected WAXRS scan has been fitted to the selfscattering curve calculated for PIT by

SS(q) ) Σ xa × fa2(q) + Σ xa × Ca(q) × D(q)34 In the self-scattering equation, xa is the mole fraction of atom a in the coal, fa(q) is the coherent X-ray scattering factor for atom a, Ca(q) is the Compton scattering factor for atom a, and D(q) is the monochromator discrimination factor for the Compton scattering as measured in this laboratory.35 The scale factor, k, which converts the corrected intensity in counts/second to electron scattering units, has been determined by

k ) SS(q)/Icorr(q)

27

over the region from q ) 3-6 Å-1. The monochromator discrimination35 has been determined using standard compounds with compositions similar to those of Pittsburgh No. 8 coal and pyridine. The phase interference curve for PIT has been calculated by27-33

i(q) )/[Icor(q)/k] - SS(q) The phase interference curve describes, in reciprocal space, the structuring within the average short-range structural domain of a noncrystalline material.27 When limited to the first minimum and maximum, i.e., between q values between 0.5 and 2.5 Å-1, the i(q) describes the interlayer structuring within the average short-range structural domain of the coal; i.e., between atom a in layer J and atom b in layer K where J and K are adjacent layers.2-4,18-20 Since the experimental input into the Fourier transform calculations is limited to the region between qmin ) 0.5 Å-1 and qmax ) 3.0 Å-1, then the structure curves, the atom-pair correlation functions, and the radial-distribution functions measure atom-pair distances only in the region from ∼3 Å to ∼15 Å; i.e., the spatial region that includes the interlayer distances in coals. Structural Analysis of PIT in Molecular Space. In molecular space, the atom-density function, 4πr2F(r), is related to the average atom density F of the coal.27 For PIT, the average carbon atom density (F) is equal to 0.145 atoms/Å3. Fourier transform of the phase interference curve (in reciprocal space) produces the structure curve in molecular space by27-33

S(r) ) (2r/π × ΣΣ q × i(q) × M(q) × damp(q) × sin(q × i(q)) × ∆q In the structure function calculation, M(q) is termed the sharpening function, and it “sharpens” the structure (28) Kruh, R. F.; Petz, J. I. J. Chem. Phys. 1964, 41, 890-891. (29) Wertz, D. L.; Cook, G. A. J. Solution Chem. 1985, 14, 41-48. (30) Triolo, R.; Narten, A. H. J. Chem. Phys. 1975, 63, 3624-3631. (31) Paalman, H. H.; Pings, C. J. Rev. Modern Phys. 1963, 33, 389399. (32) Narten, A. H.; Levy, H. A. J. Chem. Phys. 1971, 55, 2263-2266. (33) Hajdu, F. Acta Crystallogr. 1971, A27, 73-76; Hajdu, F. Acta Crystallogr. 1972, A28, 250-252. (34) Unpublished results, this laboratory, 1996.

Figure 3. The atom-pair correlation function calculated for PIT with peak one and peak two labeled.

function from an electron-pair distribution to an atompair distribution so that maxima in the structure function indicate distances between the numerous pairs of atoms in the short-range structural domain of PIT. The dampening function is used to prevent overimportance being given in the sharpening function at higher q values. In these calculations, the dampening function is given by damp(q) ) exp(-0.05 × q2). The structure curve provides a one-dimensional array of atom-pair distances in the material being examined by WAXRS and is frequently used to calculate the atompair correlation function, g(r), for the noncrystalline material by27-33

g(r) ) [4πr2F + S(r)]/ 4πr2F where F is the random carbon atom density in the coal. Random short-range structure is indicated by g(r) ) 1.0, and maxima in g(r) indicate clusters of atom-pair distances between atoms in different layers of a coal. In the atom-pair correlation function obtained for PIT (Figure 3), there are two statistically significant maxima, centered at 4.85 Å and at 8.31 Å, respectively. The relative intensity of the two peaks in the g(r) of PIT is ∼2:1, which is consistent with an average short-range structural domain containing three approximately parallel layers (see Figure 4). The first peak includes two sets of atom-pair distances between atoms separated by the distance 〈d〉 characteristic of the two sets of nearest PCA layers, and one structural vector of magnitude 2× 〈d〉 describes the one set of atom-pair distances between atoms in the second nearest layers. The location of each carbon atom within the shortrange structural domain of a coal may be characterized by x-, y-, and z-coordinates; i.e., the spatial coordinates of atom a in layer J are xaJ, yaJ, zaJ. The distance between atom a in layer J and atom b in layer K is given by

raJ-bK ) [(xaJ - xbK)2 + (yaJ - ybK)2 + (zaJ - zbK)2]1/2 When J and K represent nearest layers, then zaJ - zbK ) 〈d〉 for each atom-pair for atoms located in nearest PCA layers. In the three-layer structural model of the short-range structural domain of PIT, the two maxima

Interactions Between Pyridine and Pittsburgh No. 8 Coal

Energy & Fuels, Vol. 17, No. 2, 2003 485

Figure 6. The phase interference curves of the gel from 1 day (circles), 8 days (up triangles), fifteen days (diamonds), and 22 days (down triangles) compared to the i(q) obtained from untreated PIT (squares). Also shown is the i(q) for the gel B (down triangles) 2 days after pyridine had been allowed to escape (down triangles). Figure 4. A three-layer molecular model of the average shortrange structural domain of PIT with 〈d〉 ) 3.89 Å.

Figure 7. The WAXRS scan of the PIT/PYR gel B obtained after 96% of the pyridine had evaporated from the gel. For comparison, the WAXRS scan of untreated PIT is included. Figure 5. The WAXRS scans of the PIT/PYR gel obtained at 1 day (A-1) and 15 days (A-15) after its preparation. For comparison purposes the WAXRS scan of untreated PIT is also presented.

in the g(r) distances may be used to calculate the average nearest interlayer distance by

4.85 Å ) [1/n2] × {Σ (xaJ - xbK)2 + (yaJ - ybK)2} + 〈d〉21/2 and

8.31 Å ) [1/n2] × {Σ (xaJ - xbK)2 + (yaJ - ybK)2 + (2 × 〈d〉)2}1/2 where Σ {(xaJ - xbK)2 + (yaJ - ybK)2} is approximately the radius of the average PCA unit in the short-range structural domains of PIT. Solving simultaneously gives 〈d〉 ) 3.89 ( 0.05 Å; and the radius of the average PCA unit in each layer is ∼2.9 Å. WAXRS Analysis of the PIT/PYR Mixture. Shown in Figure 5 are the corrected WAXRS scans of PIT/PYR gel A obtained 1 day, 8 days, and 15 days after the

preparation of this mixture. Also shown, for comparison purposes, is the WAXRS scan of untreated PIT. The WAXRS scans of PIT and the several samples of PIT/PYR gel A are statistically equivalent to one another at q > 2.5 Å-1, indicating that the addition of pyridine to PIT does not alter the intramolecular structure of these moieties to an extent measurable by WAXRS methods. However, in all of the PIT/PYR gel A samples, a new, much sharper maximum at low q replaces the broad 1.65 Å-1 maximum characteristic of PIT. This new maximum, centered at 1.45 ( 0.02 Å-1, is shown in more detail in Figure 6, which includes the phase interference curves of PIT/PYR gel A. The 1.45 Å-1 peak is also present in the WAXRS scan of PIT/ PYR gel B (Figure 7) and in its phase interference curve (Figure 8), but its intensity in gel B is similar to the peak intensity of the corresponding maximum in the g(r) of untreated PIT. In Figure 9, the atom-pair correlation functions for the three samples of PIT/PYR gel A and for PIT/PYR gel B are presented. The maximum in the gel A samples occurs at 4.05-4.10 Å; but in gel B, the maximum is centered at 4.93 Å.

486

Energy & Fuels, Vol. 17, No. 2, 2003

Figure 8. The phase interference curve of the PIT/PYR gel B (circles) obtained after 96% of the pyridine had evaporated from the gel. For comparison, the phase interference curve of untreated PIT is included (squares).

Wertz and Smith

Figure 10. Comparison of the resolved first peak in the atom radial-distribution functions of the one-day PIT/PYR gel A-1 (circles) and the first peak in the D(r) calculated for PIT/PYR gel B with the resolved first peak in the D(r) of untreated PIT (squares).

Figure 11. Relationship between the % pyridine remaining in the gel and the area under the first peak in each RDF. Figure 9. The atom-pair correlation functions obtained from untreated PIT (squares), the PIT/PYR gel A-1 (circles), and PIT/PYR gel B (diamonds).

others27-31

Kruh and have shown that the area under the first peak in the atom-pair radial-distribution function may be related to both the short-range atom-pair distances and the number of atom-atom pairs in a noncrystalline material. The radial-distribution may be calculated from the WAXRS scans by

D(r) ) 4πr2F + S(r)27 Shown in Figure 10 is the first peak in the D(r) calculated for untreated PIT, for PIT/PYR gel A-1, and for PIT/PYR gel B. The areaa under the first peak in each of these samples and for gel samples A-8 and A-15 are presented in Table 2In the RDFs of the three gel A samples, the r*(5.07 ( 0.03 Å), and the peak areas are statistically equivalent. The r* and the peak area measured in the RDF of gel B are quite different. The peak area measured for gel B is similar to the peak area measured for untreated PIT, and its r* is intermediate between the r* measured for PIT and the r*’s measured for gel A. The decrease in the area under peak 1 in these RDFs parallels the decrease in the pyridine retained in each

Table 2. Analysis of the First Peak in the Structure Functions of Pit and the Pit/Pyr Gels peak 1 area in RDF sample

r*a

PIT Gel A-1 Gel A-8 Gel A-15 Gel B

4.85 ( 0.02 Å 5.10 ( 0.02 Å 5.08 ( 0.02 Å 5.05 ( 0.02 Å 4.93 ( 0.02 Å

areab e2

1910 2630 e2 2590 e2 2520 e2 2040 e2

uncertainty 44 e2 51 e2 51 e2 50 e2 45 e2

a The r* has been determined by differential methods. b The peak area was determined by graphical methods.

sample (see Figure 11), indicating that, in the gels, peak 1 in the RDFs describes not only the atom-pair distances between carbon atoms in nearest layers of the shortrange structural domain of PIT but also between carbon atoms in that domain and the carbon and nitrogen atoms in pyridine. Speculations about the Stoichiometry and the Structure of the PIT-PYR Adduct. The X-ray scattering power of each pyridine molecule is similar to the number of electrons in the pyridine molecule; i.e., τPYR ) ([5 × 6 e] + [6 × 1 e] + [1 × 7 e]) ) 42 e for C5H6N. Since the repeat PCA unit in PIT is not completely known, its X-ray scattering power, τPCA, is also not exactly known. The area under the first peak in each of the RDFs may be related to the average PCA unit of PIT in that moiety by

Interactions Between Pyridine and Pittsburgh No. 8 Coal

Energy & Fuels, Vol. 17, No. 2, 2003 487

areacalc ) [massPIT/(massPIT + massPYR)] × (0.69 × 0.72/3) × [(γPCA-PCA × τPCA × τPCA × 1.33) + (γPCA-PYR × τPCA × τPYR × nPYR)] In this relationship, 0.69 is the reported carbon fraction in “as received” PIT, 0.72 is the reported fraction of the carbon in PIT which is aromatic, and these aromatic carbons are found in three layers. Thus, the stoichiometric factor is related to the average PCA unit in each short-range structural domain of PIT. The statistical distinguishibility factor is denoted by γAB; and γAB ) 1 if A ) B, γAB ) 2 if A * B. In the three-layer model of PIT, the average layer has 1.33 nearest neighbor layers, i.e., nlayers ) 1.33. Since no pyridine has been added, the mass ratio term ) 1, and nYPYR ) 0. Since the three layers causing the X-ray scattering are composed of poly-cyclic units, γPCA-PCA ) 1. Equating the measured P1 area to the areacalc gives

Figure 12. Comparison of the experimental phase interference curve for untreated PIT with the simulated phase interference curve (circles) calculated from the three-layer model of PIT with 〈d〉 ) 3.89 Å.

1910 e2 ) areacalc ) 1 × [(0.69 × 0.72)/3] × 1 × 1.33 × τ2PCA. Solving yields τPCA ) 93.1; i.e., any elemental combination that totals 93.1 e for (a × 6 e) + (b × 1 e) + (c × 8 e) in the unit CaHbOc is consistent with this analysis. The C14 aromatic unit proposed by Solum et al. (∼14 carbon atoms × 6 e) along with nine hydrogen atoms (9 × 1 e) is consistent with the average poly-cyclic aromatic unit identified by the RDF of PIT. Since pyridine molecules are different from the PCA units in PIT, then γPCACA ) 2. In gel A-1, the mass ratio term has been reduced to 0.68, so the calculated area expression becomes

2630 e2 ) areacalc ) 0.68 × [(0.69 × 0.72)/3] × [(1 × 1.33 × {93.1 e} )+ (2 × 93.1 e × 42 e × nPYR)] 2

with the first contribution due to the structuring within the three-layer short-range structural domain of PIT, and the second contribution due to structural interactions between the PCA layers in PIT and adjacent pyridine molecules. Solving gives nPYR ≈ 1.5; i.e., each PCA unit is structurally related to ca. 1.5 pyridine molecules in gel A-1. Since the area and the r* of peak 1 in the Ruff’s of gel A-8 and gel A-15 are similar to those of gel A-1, it appears that the PCA-pyridine adduct is similar in all three samples of gel A examined in these experiments. In gel B, the stoichiometric factor is 0.96, so the areacalc becomes

2040 e2 ) areacalc ) 0.96 × [(0.69 × 0.72)/3] × 2

[(1 × 1.33 × 93.1 e ) + (2 × nPYR × 93.1 e × 42 e)] Solving gives nPYR ≈ 0.2; i.e., in gel B, ca. 0.2 pyridine molecules are associated with the PCA layer in the PCA-PYR adduct. Corroboration of the Molecular Models. The adequacy of structural models may be tested by using the Debye relationship to calculate a simulated phase interference curve, j(q), which is based completely on

Figure 13. Comparison between the experimentally determined phase interference curve for PIT/PYR gel A-1 and the simulated phase interference curve calculated from an adduct composed of the three-layer PIT domain and 2.4 pyridine molecules at an average distance of 4.4 Å from the center of the domain.

the structural details of the average structural unit of the scattering material. Good agreement between the experimental phase interference curve, i(q), and the simulated phase interference curve, j(q), indicates that the structural model is not inconsistent with the average structure of the species causing the X-ray scattering. For the three-layer model of the average short-range structural domain of untreated PIT, the simulated phase interference curve has been calculated by

jPIT(q) ) (1/q) × (1/k) × [(0.69 × 0.72)/3] × 1 × (93 e)2 × 1.33 × damp × cos{(q - q*) × 〈d〉} where 〈d〉 is the average distance between nearest layers in PIT, and q* ) 2π/r*. Shown in Figure 12 is the simulated phase interference curve, calculated for 〈d〉 ) 3.89 Å, compared to the experimentally measured phase interference curve for untreated PIT. The agreement is quite good, indicating that the three-layer structure of PIT is an acceptable description of the average short-range structural domain of PIT.

488

Energy & Fuels, Vol. 17, No. 2, 2003

The best structural model of the adduct between pyridine molecules and the three-layer unit of PIT is consistent with a simulated phase interference curve calculated by

adduct ) 0.68 × (1/q) × (1/k) × [(0.69 × 0.72)/3] × [1 × (93 e)2 × 1.33 × damp × cos{(q - q*) × 〈d〉} + [2 × (93 e) × (42 e) × nPYR × damp × cos {(q - q2*) × r2}] In this simulated j(q), r2 represents the average distance between the pyridine molecules and the PCA units in the three-layer domain of PIT, and q2* ) 2π/r2. The simulated phase interference curve shown in Figure 13 has been calculated for r2 ) 4.4 Å and nPYR ) 1.5 (as determined above). This value for nPYR is consistent with the stoichiometric ratio of pyridine molecules/C14 in the originally prepared PIT/PYR sample. The good agreement between j(q) calculated for the PIT/PYR adduct and the experimentally measured i(q) for sample A-1 suggests that this model is a reasonable one-dimensional description of the adduct. However, it is beyond the scope of these one-dimensional X-ray scattering experiments to determine the precise location(s) of the pyridine molecules relative to the threelayer structural domain of PIT.

Wertz and Smith

Conclusions The average short-range structural domain found in Pittsburgh No. 8 coal consists of ca. three “parallel” layers of poly-cyclic aromatic units. The structuring within this average short-range structural domain is described by a broad maximum centered at ∼1.65 Å-1. In molecular space, this maximum is equivalent to an average nearest interlayer distance of 3.89 Å. The composition of the average PCA unit is ca. C14H9 and has a radius of ∼2.9 Å. Comparison of the WAXRS data indicates that pyridine does not alter the intramolecular bonding within the PCA units of PIT to an extent that is measurable by WAXRS methods. However, when pyridine is added to Pittsburgh No. 8 coal, a new, much sharper maximum, centered at 1.45 Å-1, appears. This maximum describes not only the average interlayer distance in the short-range structural domain of PIT but also the average distance between the short-range structural domain of PIT and the pyridine molecules that forms an adduct with the domains of this coal. Taken together, these observations suggest that the pyridine molecules interact with Pittsburgh No. 8 coal at the molecular level by altering the interactions between the short-range structural domains of PIT. EF020217+