Energy & Fuels 2003, 17, 1423-1428
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On the Molecular-Level Interactions between Pittsburgh No. 8 Coal and Several Organic Liquids David L. Wertz* and E. Ryan Smith† Department of Chemistry & Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received April 4, 2003. Revised Manuscript Received July 8, 2003
Pittsburgh No. 8 coal, whose average short-range structural domain contains approximately three parallel layers separated by an average inter-layer distance of 3.9 Å, forms a stable adduct with aniline. In this adduct, the inter-layer structuring characteristic of Pittsburgh No. 8 coal is not disrupted to an extent measurable by wide-angle x-ray scattering methods. No such adduct is formed when Pittsburgh No. 8 coal is treated with toluene, with triethylamine, with tetrahydrofuran, or with chlorobenzene.
Introduction The proximate analysis of Pittsburgh No. 8 high volatile bituminous coal (one of the Argonne Premium Coals) has been available for many years, as has been its elemental analysis and heat value. “As-received” Pittsburgh No. 8 coal (PIT) reportedly contains 75% carbon, of which ca. 78% is aromatic, with the average poly-cyclic aromatic (PCA) unit being ∼C15 and with a radius of ca. 2.1 Å. The average short-range structural domain of PIT contains ca. three parallel layers of PCA units, with the average nearest interlayer separation distance being 3.9 Å. The bonds (or forces) which cause the short-range alignment of the poly-cyclic rings to form the short-range structural domains in PIT (and in other coals) are not well understood. Among the Argonne Premium Coals, the nitrogen content of PIT is among the highest. The total oxygen content of PIT is among the lowest of the Ar.1-5 Both of these O-containing groups contain acidic hydrogens and are capable of hydrogen-bonding, and the carboxylate oxygens are capable of involvement with π-interactions.6 It has been proposed that the macromolecules that comprise coals form short-range structural domains composed principally of layers of poly-cyclic aromatic (PCA) units.4,7-16 The forces that cause the ordering of * Corresponding author. E-mail:
[email protected]. † The Honors College at USM. (1) Vorres, K. S. Energy Fuels 1990, 4, 420-425. (2) Botto, R. E. Energy Fuels 2002, 16, 925-927. (3) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 193-99. (4) Wertz, D. L.; Quin, J. L. Fuel 2000, 79, 1981-1989. (5) Miura, K.; Mae, K.; Hasegawa, I.; Chen, H.; Kumano, A.; Tamura, K. Energy Fuels 2002, 16, 23-31. (6) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Energy Fuels 2002, 16, 543-551. (7) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (8) Wertz, D. L. Energy Fuels 1999, 13, 513-517. (9) Takanohashi, T.; Kawashima, H.; Yoshida, T. Energy Fuels 2002, 16, 6-11. (10) Nishioka, M. Energy Fuels 2002, 16, 1109-1115. (11) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500; Philos. Trans. R. Soc. 1960, A252, 68. (12) Atria, J. V.; Rusinko, F.; Schobert, H. H. Energy Fuels 2002, 1343-1347.
the layers into domains are thought to be noncovalent. Both the size of the PCA units and the number of PCA layers in the average short-range structural domain depend on the rank of the coal.3,4,7,12 One current coal structure theory is that noncovalent forces cause the structuring between its macromolecular strands,15-17 and Nishioka17 suggests that the coal molecules are physically associated. It has been suggested that coal swelling caused by the addition of liquids represents a reorganization of such forces; i.e., a reorganization of the coal’s macromolecular structuring. Several liquids have been used as probes to study such disruptions in interlayer bonding, and pyridine5,24-32 has been commonly used for such studies. Krzesinskii28 reports that tetrahydrofuran, when used to treat a bituminous coal, has a swelling ratio that is 75% of the swelling ratio measured when the same coal is treated with pyridine. (13) Kineda, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672-678. (14) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003-1010. (15) Iino, M.; Takanohashi, T.,; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (16) Gorbaty, M. L. Fuel 1994, 73, 1819. (17) Nishioka, M. Energy Fuels 2001, 15, 1270-1275. (18) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047-1053. (19) Painter, P. C.; Graf, J.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 379-397. (20) Takanohashi, T.; Nakamura, K.; Iino, I. Energy Fuels 1999, 13, 922-926. (21) Takanohashi, T.; Nakamura, K.; Terao, Y.; Iino, M. Energy Fuels 2001, 14, 393-399. (22) Dzrakacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2001, 15, 1409-1415. (23) Sato, S.; Matsumura, A.; Saito, I.; Ukegawa, K. Energy Fuels 2002, 16, 1337-1342. (24) Larsen, J. W.; Cheng, J. C.; Pan, C.-S. Energy Fuels 1991, 5, 57-59. (25) Xiong, J.; Maciel, G. E. Energy Fuels 2002, 16, 497-509. (26) Xiong, J.; Maciel, G. E. Energy Fuels 2002, 16, 791-801. (27) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (28) Krzesinska, M. Energy Fuels 2001, 15, 324-330. (29) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697-703. (30) DuBose, S. B.; Wertz, D. L. Energy Fuels 2002, 16, 669-675. (31) DuBose, S. B.; Trahan, A. D.; Turner, T. C.; Wertz, D. L. Energy Fuels 2001, 15, 1537-1538. (32) Wertz, D. L.; Smith, E. R. Energy Fuels 2003, 17, 482-488.
10.1021/ef030076r CCC: $25.00 © 2003 American Chemical Society Published on Web 10/04/2003
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Table 1. Some Parameters for the Liquid Molecules Used as Coal Probes in This Study liquid probe parametera
CLB
molecular mass (d) vapor pressure (Torr) viscosity* (centipoise) surface tension* (dynes/cm) dipole moment* (Debye units) hydrogen-bonding capabilities (a) Lewis basicity (b) hydrogen-bond donor delocalized π-bonding
112.56 11.8 0.75 32.8 1.54
94.2 28.5 0.59 27.9 0.31
73.1 57.1 0.68 23.4 1.32
93.1 0.7 3.77 42.8 1.51
72.1 197 0.46 26.4 1.75
79.1 20.0 0.88 36.3 2.37
no no yes
no no yes
yes no no
yes yes yes
yes no no
yes yes yes
a
TOL TEA ANI THF PY*
At 25 °C.
Maciel25,26 has recently suggested that pyridine occupies “void space” in coals, and this supposition is consistent with the X-ray analysis of several coalpyridine adducts reported by this group in our studies of coal/pyridine gels.29-32 In this study, five liquids have been used as structural probes. All have some molecular parameters similar to those of pyridine. However, these liquids differ in structure, viscosity, vapor pressure, and dipole moments as seen in Table 1.33 Three of the liquids are isostructural at the molecular levelsaniline (ANI), chlorobenzene (CLB), and toluene (TOL). These molecules are aromatic, i.e., each possesses a planar structure with extensive π-delocalization about a six-membered ring. However, their dipole moments differ considerably. In addition, aniline contains sites, both acidic and basic, that are capable of hydrogen-bonding; and in the liquid state, extensive hydrogen-bonding exists between neighboring aniline molecules. Neither toluene nor chlorobenzene possesses hydrogen-bonding capabilities. The other two liquid probes, triethylamine (TEA) and tetrahydrofuran (THF), contain base sites (electron pair(s) on the heteroatom) which have hydrogen-bonding capabilities, but each has a moderate dipole moment. However, neither contains acidic hydrogen atoms. Both triethylamine and tetrahydrofuran have nonplanar shapes, and neither has π-bonding. Wide-angle X-ray scattering (WAXRS) methods have been used to study the effect(s) of the addition of each of these liquids on the structuring within the poly-cyclic aromatic (PCA) units of PIT as well as the structuring that affects the short-range structural domain of PIT. Our results and a discussion of these results are presented below. Experimental Section Sample Preparation. Two sets of samples, each with a mass ratio of 0.5 g of liquid/1.0 g of as-received Pittsburgh No. 8 coal, were prepared. For each sample, the -100 mesh asreceived coal was used, and the liquid was added to the coal in drops. The resulting sample was then mixed by shaking. The samples that were used for the WAXRS experiments were stored in 20 mL scintillation vials with screw-top caps. A portion of the sample was removed 1 day after its preparation and used as the subject of each wide-angle X-ray scattering experiment. Mass Loss Experiments. The samples used for the mass loss experiments were prepared by adding a weighed amount (33) Riddick, J. A.; Bunger, W. B. Organic Liquids. Physical Properties and Methods of Purification, 3rd ed.; Wiley-Interscience: New York, 1970.
Figure 1. Mass loss vs time for the five PIT/liquid mixtures included in this study. PIT/ANI (up triangles), PIT/CLB (squares), PIT/TOL (down triangles), PIT/TEA (diamonds), and PIT/THF (circles). of liquid to a weighed amount of PIT. Each mixture was also contained in a 20 mL scintillation vial with a screw top until the mixture reached equilibrium. At that time, the screw top was removed, and the mixture was exposed to the environment. The mass of the bottle containing each PIT/ liquid sample was measured for 10 hours at one-hour intervals using a Mettler model AE100 balance which is reportedly accurate to (0.0005 g. Wide Angle X-ray Scattering Experiments. A 0.5 g sample of each PIT/liquid mixture was mounted onto an aluminum sample holder which was placed into our fine-focus X-ray diffractometer. Copper X-rays were used to irradiate each sample, and the secondary X-rays were diffracted by a graphite crystal monochromator to allow X-rays with λ ) 1.54 Å to be diffracted onto the scintillation detector. Using θ/2θ focusing, scattering data were accumulated from each sample from 2θ ) 5.00° to 2θ ) 90.00°, or from qmin ) 0.36 Å-1 to qmax ) 5.78 Å-1, where q ) [4π/λ] × sin θ. Scattered intensities were accumulated for one second at ∆2θ ) 0.05° for each scan using a conventional crystal monochromator. The step-scan protocol was controlled by a microprocessor designed by OMNI Instruments, Inc.34
Results and Discussion When aniline (ANI) is added to PIT, the texture of the sample changes immediately from a powder to a gel which rigidly conforms to the shape of the scintillation vial. When aniline is added to PIT, the coal swells measurably. The addition of the other liquids to PIT does not result in the formation of such a gel. Mass Loss Studies. Shown in Figure 1 is a presentation of evaporation rate of the liquid from each of the five mixtures used in this study. These data indicate that (a) the mass loss from the PIT/ANI mixture f 0 over the time frame of these experiments, and (b) the mass loss of each of the other liquids is linear (but with different slopes) over the 10-hour period of the experiment. After 10 hours, all of the triethylamine (TEA) and most of the toluene (TOL) have evaporated from their respective mixtures. Chlorobenzene evaporates from the corresponding PIT/liquid mixtures much more slowly. Wide-Angle X-ray Scattering Experiments. The WAXRS scan of untreated PIT, Icorr(q), is presented in (34) Wertz, D. L.; Smithhart, C. B.; Wertz, S. L. Adv. X-Ray Anal. 1990, 33, 475-481.
Interactions between Pittsburgh No. 8 Coal and Organic Liquids
Figure 2. WAXRS scan of untreated PIT (A). Also shown is the self-scattering calculated for PIT (B) and the resulting phase interference curve (C).
Energy & Fuels, Vol. 17, No. 6, 2003 1425
Figure 3. The experimentally determined phase interference curve for PIT (squares) compared to the simulated phase interference curve (circles) calculated from the three-layer model of PIT presented in B.
Figure 2. Also shown in Figure 2 are the self-scattering curve (SS(q)) calculated for PIT and the resulting phase interference curve, i(q). The latter has been calculated from the WAXRS intensities by
i(q) ) {Icorr(q)/k} - SS(q)35-37
(1)
where k is the scaling constant that converts Icorr(q) from CPS to electron scattering units. The self-scattering term, SS(q), is defined by
SS(q) ) Σ xa × {f 2a(q) + Ca × D(q)}
(2)
where xa is the fraction of element a in the sample, fa(q) is the coherent X-ray scattering factor for element a, Ca is the Compton scattering power for element a, and D(q) is the monochromator discrimination factor for the Compton scattering. As previously noted, the WAXRS intensities measured for PIT in the 0.5-2.0 Å-1 region of reciprocal space describe the average interlayer distance between polycyclic aromatic (PCA) units in this coal. The broad maximum in the Icorr and in the i(q) of as-received PIT is located at 1.65 Å-1 (as determined by differential methods) with an uncertainty of (0.05 Å-1. The Fourier transform of this maximum into molecular space produces a radial-distribution function which is consistent with an average short-range structural domain of three layers and a nearest interlayer separation distance, 〈d〉, of 3.89 ( 0.05 Å.34 A one-dimensional model of the average short-range structural domain of PIT is presented as Figure 3B. The structural details of the average short-range structural domain of PIT may be used to simulate the phase interference curve of PIT by
j(q)SRSD ) (1/2π) × [(0.69 × 0.75)/(14 × 3)] × nlayer × τPIT × τPIT × cos (dlayer × ∆q) × [exp(-∆q2)] × ∆q38,39 (3) (35) Kruh, R. F. Chem. Rev. 1962, 62, 319-346. (36) Wertz, D. L.; Kruh, R. F. J. Chem. Phys. 1967, 47, 388-390. (37) Smith, L. S.; Wertz, D. L. J. Am. Chem. Soc. 1975, 2365-2370.
Figure 4. Comparison of the WAXRS scans of untreated PIT and the PIT/CLB1 sample.
In this equation, ∆q ) abs(q - q*) where q* is defined by q* ) 2π/dlayer, with dlayer being the average interlayer distance used in the molecular model of the average short-range structural domain of PIT. In the average short-range structural domain of PIT, there are two sets of PCA layers (nlayer ) 2) separated by the average distance (dlayer ) 3.9 Å) and the X-ray scattering power for the average PCA unit in each layer is 93 electrons.34 The latter is consistent with an average PCA repeat unit of C15, which is similar to that previously proposed for PIT.3 Presented in Figure 3A is the phase interference curve of PIT compared to the j(q) calculated from the three-layer model of PIT using the structural details noted above. The good agreement between i(q) and j(q) offers the second justification that the three-layer model is a realistic description basis of the average short-range structural domain of PIT. The PIT/Liquid Mixtures. Shown in Figures 4-7 are the WAXRS scans of PIT compared to the WAXRS scans of PIT treated with chlorobenzene (CLB), with toluene (TOL), with triethylamine (TEA), and with tetrahydrofuran (THF), respectively. For each mixture, the day-1 scan is presented. Comparisons show that when each of these liquids is added to PIT, the resulting
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Figure 5. Comparison of the WAXRS scans of untreated PIT and the PIT/TOL1 sample.
Figure 8. Comparison of the WAXRS scans of untreated PIT and the PIT/ANI1 sample.
Figure 6. Comparison of the WAXRS scans of untreated PIT and the PIT/TEA1 sample.
Figure 9. Comparison of the phase interference curves for untreated PIT and for the PIT/ANI1 sample.
Figure 7. Comparison of the WAXRS scans of untreated PIT and the PIT/THF1 sample.
WAXRS scan is statistically equivalent to the WAXRS scan of untreated PIT, both in the region from 0.5 to 2.0 Å-1 and in the region from 2 to 6.0 Å-1. This similarity indicates that the addition of any of these four liquids does not alter the structural features within the poly-cyclic aromatic units of PIT (described in the 2.06.0 Å-1 region of reciprocal space) or in the 1.0-2.0 Å-1 region (which describes the average layering in the short-range structural domain of PIT).
The Pittsburgh/Aniline Gel. Shown in Figure 8 is the corrected WAXRS scan of the Pittsburgh No. 8/aniline mixture obtained 1 day after its preparation ration; i.e., PIT/ANI1. In this figure, the Icorr for PIT has been included for comparison purposes. The measured WAXRS intensities are statistically indistinguishable in the region from q ) 2.0 Å-1 to q ) 6.0 Å-1, indicating that the addition of ANI does not alter the bonding-structuring within the poly-cyclic units of PIT to an extent measurable by WAXRS methods. However, the WAXRS scan of PIT/ANI1 is considerably different from the WAXRS scan of untreated PIT in the region from 1.0 Å to 2.0 Å-1. The phase interference curves for PIT/ANI1 and for untreated PIT are shown in Figure 9. In the i(q) of the PIT/aniline gel, the new maximum is centered at 1.4 ( 0.05 Å-1. In the WAXRS scan and in the phase interference curve of the PIT/ANI1 gel, the 1.65 Å-1 peak characteristic of untreated PIT is clearly discernible. The presence of the latter indicates that the average short-range structural domain of PIT is retained in the PIT/ANI gel. Since intermolecular distances may be estimated from reciprocal space intensities by d ) 2π/q, the atom-pair distances between atoms in adjacent layers of PCA units occur at molecular-space distances of 3-6 Å; these correspond in reciprocal space to 1-2 Å-1. Included in this distance interval is the nearest interlayer distance
Interactions between Pittsburgh No. 8 Coal and Organic Liquids
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Figure 10. A plausible model of the interactions between nearest-neighbor aniline molecules and the three-layer (average) short-range structural domain of PIT. The aniline molecules are represented as squares in this drawing.
in the average short-range structural domain of PIT (3.9 Å, which corresponds to the maximum at 1.65 Å-1in i(q) of PIT). Also included in this interval are atom-pair distance(s) between the atoms in the short-range structural domain of PIT and atoms in nearest-neighbor liquid molecules when the liquid molecules form an adduct with PIT which is stable for the time frame of the WAXRS experiment. Such an adduct has been identified in PIT/PYR mixtures34 similar in composition to these PIT/liquid mixtures and in other coal/pyridine mixtures.31,32 The new peak at 1.4 Å-1 in the i(q) of the PIT/ANI1 sample has been assigned to an adduct formed between the short-range structural domain of PIT and aniline molecules. In this adduct, the average distance between the scattering center of the PIT domain and the scattering center of the average aniline molecule may be estimated by dadd ≈ 2π/1.4 Å-1 ≈ 4.5((0.2) Å. Speculations about the ANI-PIT Adduct. A plausible model of the PIT-ANI adduct is presented in Figure 10. As noted above, the three-layer short-range structural domain of PIT causes the maximum centered at 1.65 Å-1, and the 1.4 Å-1 maximum describes, in reciprocal space, the average distance between the scattering center of the short-range domain of PIT and the scattering centers of N nearest neighbor aniline molecules. Shown in Figure 11 are two simulated phase interference curvessone calculated for the three-layer domain of PIT (see Figure 3 and eq 3 above) and the second j(q) calculated by eq 4 and based on the structural model of the PIT-ANI adduct presented in Figure 10 by
jaddt ) (1/2π) × [(0.69 × 0.75)/(14 × 3)] × nani × τSRDIT × τANI × cos(daddt × ∆qaddt) × [exp(-∆qaddt 2)] × ∆q36,37 (4) In this equation, τSRDIT ) 3 × 93 e, and τANI ) 50 e. The number of aniline molecules which, on the average, participate in the adduct with the short-range structural domain of PIT is given by nani. To produce the best agreement between the j(q) calculated to simulate the adduct peak (at 1.4 Å-1), the average center-to-center distance is 4.5 Å, and nani ≈ 2.3 molecules; i.e., on the average, each short-range structural domain of PIT has ca. 2.3((0.2) aniline nearest neighbors. Taken together, the two simulated phase interference curves (calculated by eq 3 and by eq 4) account for the
Figure 11. The simulated phase interference curve for the three PCA layer domain of PIT (squares) and the simulated phase interference curve describing the PIT-ANI adduct (circles) compared to the measured phase interference curve (solid line). For this calculation of the j(q) for the PIT-ANI adduct, 〈d〉 ) 4.5 Å, and nani ) 2.5 molecules.
Figure 12. Comparison of the WAXRS scans of PIT/ANI1 and PIT/ANI8.
experimentally determined i(q) for the PIT/ANI1 moiety in a satisfactory manner. Shown in Figure 12 is the WAXRS scan of PIT/ANI8, a sample of the gel taken 8 days after its preparation. At the time of its investigation, ∼18% of the aniline had evaporated from the gel. The WAXRS scan of PIT/ANI8 contains a well-defined adduct peak at 1.4 Å-1, but the intensity of this peak is measurably lower than the intensity of the corresponding peak in the PIT/ANI1 sample of the gel. The correlation of aniline evaporation from this gel with the reduction in intensity of the 1.4 Å-1 peak substantiates that this peak is quantitatively related to the number of aniline molecules involved in the adduct. To our knowledge, no other structural model accounts for the 1.4 Å-1 peak in the WAXRS scan of the PIT/ ANI gel in a reasonable manner. Conclusions Interlayer forces cause Pittsburgh No. 8 coal, at the molecular level, to form short-range structural domains which, on the average, contain approximately three PCA layers; and the average distance between nearest layers
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is 3.9 Åsas described by the maximum centered at in the WAXRS scan of PIT. Aniline (µ ) 1.51 D) forms an adduct with PIT, as evidenced by the formation of a solid material which is cast in the shape of the container which holds the sample. The peak in the WAXRS scan characteristic of PIT (at 1.65 Å-1) is easily identified in the WAXRS scan of the PIT-ANI adduct, suggesting that the three-layer domain of PIT is retained in the adduct. A similar adduct peak has been reported for several mixtures of Pittsburgh No. 8 coal and pyridine, and the PIT-PYR adduct is also stable for several days. It is, unfortunately, beyond the scope of the onedimensional WAXRS experiments to determine precise locations of the nearest-neighbor aniline molecules relative to the average short-range structural domain of PIT; but the structural model which is consistent with the WAXRS intensity for the PIT/ANI (and the PIT/ PYR) adducts locates the liquid molecules in the vicinity of the side chains of PIT. This location of the liquid molecules suggests that the interactions in the adduct probably involve hydrogen-bonding between the amine-N of the liquid and acidic hydrogen-bond sites on the side chains of the PIT. Pyridine forms a similar adduct with PIT. Pyridine and aniline share three common featuressextensive
Wertz and Smith
π-electron delocalization about a planar six-membered ring, a dipole moment in the 1.5-2.5 D range, and a nitrogen atom which provides a basic site for hydrogenbonding. These two liquids differ in that pyridine does not possess acidic hydrogens useful for hydrogen-bonding. Although each of the other four liquids possesses some molecular propertiessplanarity/aromaticity, high molecular polarity, and hydrogen-bonding capabilitiess found with aniline and pyridine, none of these four liquidsstoluene, chlorobenzene, tetrahydrofuran, and triethylaminespossesses all of these properties. Chlorobenzene and toluene are planar and aromatic but differ in polarity. Neither compound has hydrogenbonding capabilities. Neither compound forms a molecular-level adduct with PIT which is stable for the lifetime of these experiments. Both triethylamine and tetrahydrofuran possess a heteroatom which is capable of hydrogen-bonding and have dipole moments of 1.32 and 1.75 D, respectively. Neither has π-electrons, and both have bulky structures. Neither triethylamine nor tetrahydrofuran forms an adduct with Pittsburgh No. 8 coal which is sufficiently stable to be measured by WAXRS methods. EF030076R
