X-ray Analysis of Coals Treated with Organic ... - ACS Publications

Energy & Fuels 2002, 16, 669-675

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X-ray Analysis of Coals Treated with Organic Liquids. Detailed Study of the Adduct Formed between Pyridine Molecules and Beulah Zap Lignite Stephen B. DuBose and David L. Wertz* Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 Received August 7, 2001

Wide-angle X-ray scattering methods indicate that, almost instantly, pyridine molecules penetrate between the nearest layers of the poly-cyclic units of Beulah Zap lignite to form a quasi-stable adduct. The lifetime of this adduct is several hours, and even after most of the pyridine has evaporated from the BZ/PYR gel, the expanded interlayer distance characteristic of the gel is retained.

Introduction Of the eight coals in the Argonne Premium Coal (APC) group, Beulah Zap is the member with lowest rank,1 with an average aromatic moiety of ca. nine carbons in its average poly-cyclic aromatic (PCA) unit.2 Beulah Zap (BZ) has a high moisture content and a high oxygen fraction (20.3 oxygen atoms per 100 carbon atoms). Thus, BZ has numerous molecular sites which are capable of intramolecular hydrogen-bonding. In addition, BZ has ca. 79 hydrogen atoms per 100 carbon atoms, so the simplest formula, ignoring moisture and other elements with very low abundances, for BZ is ca. C5H4O1. The dominant feature in the wide-angle X-ray scattering (WAXRS) intensity curves of coals is a broad maximum in the intensity vs 2θ scan.3-10 The average nearest interlayer distance in a coal is typically calculated from 2θ*, the angle for which the measured intensity is a maximum, by 〈d〉 ) λ/2 × sin(2θ*/2); here, λ is the wavelength of the X-rays used in the experiment.3,4,7-9 While correct, this approach has one serious limitation: using only the identification at 2θ* and its conversion to 〈d〉 precludes obtaining quantitative information about the number of atom-pairs in the adjacent layers of the coal. This limitation can be removed by using all of the pertinent experimentally measured X-ray scattering intensities and the Debye * 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) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500; Philos. Trans. Royal Soc. 1960, A252, 68. (4) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (5) Wertz, D. L.; Quin, J. L. Fuel 2000, 79, 1981-1989. (6) DuBose, S. B.; Trahan, A. D.; Turner, T. C.; Wertz, D. L. Energy Fuels 2001, 15, 1537-1538. (7) Kineda, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678. (8) Nakamura, K.; Nakanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Saada, Y. Energy Fuels 1995, 9, 1003-1010. (9) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047-1053. (10) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697-703; 1999, 13, 513-517.

transform method of data analysis.5,6,10 The Debye method utilizes the reciprocal lattice parameter, defined by q ) [4π/λ] × sin θ, so q* is the reciprocal lattice parameter at which the scattered X-ray intensity is a maximum. As noted from Bragg’s law, q* × 〈d〉 ) 2π. Since, for coals, 〈d〉 reportedly ranges from 3.3 to 3.9 Å,3-10 then this maximum may be anticipated to occur within the region corresponding to q* ) 1.6 Å-1 to 1.9 Å-1 in various coals. In BZ, the diffuse maximum which describes the interlayer distance is centered at q* ) 1.78 ( 0.05 Å-1, as seen in Figure 1. This maximum corresponds to 〈d〉 ) 3.53 ( 0.10 Å. From the quantitative intensities measured in the reciprocal lattice region of intensity in the vicinity of the interlayer peak, a structure curve may be calculated in molecular-space which describes the average distance between various atom-pairs in the two adjacent layers of the coal, i.e., the distance between atom a in layer J and atom b in layer K where J ) K + 1.11 The atompair structure curve may be modified to define the average interlayer distance between adjacent layers in the coal.5 In addition, the area under the maximum in the interlayer structure curve may be related to the number of atom-pair vectors between the atoms in layer J and layer K, and the magnitude of each atom-pair vector is the scattering power product for atoms a and b.11 The addition of selected organic liquids to coals causes differing degrees of swelling at the macromolecular level.6,10,12-17 WAXRS methods are also useful in measuring the effect(s) of the various liquids on the inter(11) Kruh, R. F. Chem. Rev. 1962, 62, 319-346. (12) Krzesinska, M. Energy Fuels 2001, 15, 324-330. (13) Takanohashi, T.; Nakamura, K.; Iino, I. Energy Fuels 1999, 13, 922. (14) Painter, P. C.; Graf, J.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 379, 384, 393. (15) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (16) Larsen, J. W.; Cheng, J. C.; Pan, C.-S. Energy Fuels 1991, 5, 57. (17) Takanohashi, T.; Nakamura, K.; Terao, Y.; Iino, M. Energy Fuels 2001, 14, 393-399.

10.1021/ef010207t CCC: $22.00 © 2002 American Chemical Society Published on Web 04/20/2002

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Energy & Fuels, Vol. 16, No. 3, 2002

Figure 1. WAXRS Scattering curves obtained for Beulah Zap and for a BZ-PYR gel obtained over the region from 0.5 Å-1 to 5.8 Å-1.

layer distance and the covalent bonding within the polycyclic layer units of the coals. It has recently been reported that the addition of pyridine to Pocahontas No. 3 coal, a coal of high rank with a complex interlayer arrangement and a very low oxygen atom/carbon atom ratio, causes minimal (if any) degradation of the average short-range structural unit of POC.6 When added to Beulah Zap lignite, pyridine causes a significant change in the WAXRS intensity, as seen in Figure 1, including both a different maximum and a different intensity in the vicinity of the maximum.10 However, there are not statistical differences in the intensity curves at q > 2.5 Å-1 for untreated BZ and for the BZ/PYR gel, indicating that pyridine does not alter the bonding within the average poly-cyclic (PC) unit of BZ to a measurable extent. For that reason, subsequent X-ray experiments and interpretations have been focused onto the reciprocal lattice region that is altered by the addition of pyridine to BZ. To better understand the interaction(s) between pyridine molecules and BZ at the molecular-level, this group has extended its wide-angle X-ray scattering studies to include the element of time in our study of BZ-PYR gels prepared by adding pyridine to Beulah Zap. Our results are presented below. Experimental Section Materials. For each sample, the -100 mesh Beulah Zap (BZ) was used “as received” from the Argonne Premium Coal Sample Program.1 Reagent grade pyridine (PYR) was used in the preparation of the gels. Preparation of Gels. Two BZ-PYR gels were prepared by adding 0.25 g of pyridine to 0.5 g of BZ, which was already mounted into our X-ray sample holder. The resulting gels were then analyzed using one of the two X-ray protocols noted below, and the mass of the gel was measured prior to several of the X-ray experiments. X-ray Experiments. Copper X-rays were used in these experiments, and a graphite crystal in the secondary X-ray beam was used to make the secondary X-ray beam monochromatic at λ ) 1.54 Å. Our micro-processor controlled diffractometer, with a fine focus slit system, was utilized in the X-ray experiments noted below.4-6,10

DuBose and Wertz

Figure 2. Percentage of pyridine retained in the BZ-PYR gel I prior to WAXRS scan 1 (0.1 h), scan 4 (1.1 h), scan 7 (1.5 h), scan 10 (2.4 h), and scan 13 (2.4 h). Shown at each data point is the stoichiometric ratio of pyridine molecules/PC units in BZ. X-ray Protocols. X-ray protocol I involves obtaining WAXRS intensities in the region of reciprocal space which includes the interlayer distances in coals.3-10 Since q* ) 2π/〈d〉, a maximum at 〈d〉 )3.5 Å in molecular space will be viewed in the experimental information at q* ) 1.8 Å-1 in reciprocal space. A wide-angle X-ray scattering intensity curve was obtained from Beulah Zap lignite, from the coal/pyr gel, and from pyridine using our high-resolution wide-angle X-ray diffractometer17 over the angular range from 2θ ) 10.00° to 2θ ) 30.00° (or from qmin ) 0.71 Å-1 to qmax ) 2.11 Å-1. For X-ray protocol I, ∆2θ ) 0.05°; and the preset for intensity accumulation was 1 s at each of the 401 angles. For protocol II, only the portion of the reciprocal space region involved with the average interlayer distance was scanned, and this region of reciprocal space was scanned rapidly. For these experiments, scattering data were accumulated from qmin ) 1.21 Å-1 to qmax ) 1.83 Å-1 with scattering intensity accumulated for 0.33 s at each preset angle. To accomplish this protocol required ca.65 s per scan. X-ray protocol II was used to obtain the WAXRS scattering curves for the fast scans discussed below. Mass Measurements. Prior to protocol I scans, the mass of the BZ/PYR gel was measured using a Mettler AE-100 balance which reports seven significant figures.

Results and Discussion Mass Measurements. The percentage of pyridine retained in the BZ-PYR gel I at the initiation of several of the WAXRS protocol I experiments is presented in Figure 2. Also shown at each data point in the figure is the ratio of pyridine molecules to poly-cyclic (PC) units in BZ. These data indicate that the rate of pyridine escape from the gel is pseudo-first order. The rate of pyridine evaporation from the sample is at least 10 times slower than the rate of pyridine evaporation for a similar sample prepared from Pocahontas No. 3 coal and pyridine.6 For gel I-21, examined several days after the preparation of the BZ/PYR gel, 99.9+% of the pyridine had evaporated from the gel. On the Rate of Formation of the PyridineBeulah Zap Adduct. The liquid-state pyridine immediately disappears after its addition to the powder BZ, causing visually observable swelling and converting the finely powdered coal into a gel. There is no evidence of liquid in the sample holder at any time during any

Adduct between Pyridine and Beulah Zap Lignite

Energy & Fuels, Vol. 16, No. 3, 2002 671

Figure 3. Fast-time WAXRS scattering curves of the BZ-PYR gel using X-ray protocol II.

of these experiments. Shown in Figure 3 are three of the very fast WAXRS scans of the in situ BZ/PYR gel made using X-ray protocol II. For comparison, the WAXRS scan of BZ, obtained under the same conditions, is also included. These results show that even at 45 s after pyridine addition to BZ, the characteristic maximum of BZ, located at ca.1.78 Å-1, is not discernible and has been replaced by the 1.45 Å-1 maximum characteristic of the molecular-level PCA-pyridine adduct.10

Figure 4. The corrected WAXRS scattering curve for untreated BZ scaled to the self-scattering, SS(q), calculated for BZ. Also shown in the phase interference curve, i(q), calculated for BZ.

The WAXRS Intensity Curve and the Phase Interference Curve. The wide-angle X-ray scattering (WAXRS) intensity curve for BZ, obtained using our new fine-focus X-ray diffractometer and X-ray protocol I, is shown in Figure 4. The phase interference curve for BZ, which describes the atom-pair distances in the average short-range structural domain of BZ, is also shown in Figure 4 along with the X-ray self-scattering curve calculated for BZ. The phase interference curve has been calculated by11,19-25

i(q) ) [I(q)/k] - SS(q)

(1)

The phase interference curve is the difference between the corrected WAXRS intensity curve measured for BZ and the self-scattering curve calculated from its chemi-

Figure 5. WAXRS scattering curves for the several scans of BZ-PYR gel I at several times after its preparation using X-ray protocol one. Scan 1 (circles), scan 4 (up-triangles), scan 7 (diamonds), scan 10 (crosses), scan 13 (x’s), and scan 21 (stars).

cal composition and conventional X-ray scattering factors for coherent and incoherent scattering.25 In eq 1, the X-ray self-scattering curve has been calculated for BZ by

SS(q) ) Σwa × Σ[f2a(q) + {Ia(q) × D(q)}]

(2)

where fa(q) is the X-ray coherent scattering factor for atom a25 (which has a fraction wa1) in the sample, Ia(q) is the incoherent scattering component for atom a,25 and D(q) is the discrimination function for the graphite monochromator for the incoherently scattered X-rays.26 The characteristic diffuse maximum of BZ, centered at 1.78 Å-1, is more clearly discernible in its phase interference curve of BZ than in the WAXRS scan of BZ. When limited to the region from 1 to 2.5 Å-1, i(q) describes the array of atom-pair distances between atoms in adjacent layers in BZ in reciprocal space; 3-10 and the phase interference curve of BZ contains one (diffuse) maximum centered at q* ) 1.78 Å-1. WAXRS Curves for the BZ-PYR Gel Obtained Using X-ray Protocol One. Shown in Figure 5 are the several WAXRS scans of the BZ/PYR gel using X-ray protocol I made over a period of several hours after the preparation of the gel. These scans show no evidence of the maximum due to BZ10 or the maximum due to liquid-state pyridine,6 but rather a new maximum, centered at 1.45 ( 0.02 Å-1, dominates these WAXRS scans of gel samples I-1, I-4, I-7, I-10, and I-13. In the WAXRS scan of I-21, the maximum is much smaller and occurs at q* ) 1.63 Å-1. The phase interference curves for the gel obtained at the various experimental times are presented in Figure 6. (18) Wertz, D. L.; Smithhart, C. B.; Wertz, S. L. Adv. X-Ray Anal. 1990, 33, 475-483. (19) Wertz, D. L.; Kruh, R. F. J. Chem. Phys. 1967, 47, 388-390. (20) Kruh, R. F.; Petz, J. I. J. Chem. Phys. 1964, 41, 890-891. (21) Wertz, D. L.; Cook, G. A. J. Solution Chem. 1985, 14, 41-48. (22) Triolo, R.; Narten, A. H. J. Chem. Phys. 1975, 63, 3624-3631. (23) Paalman, H. H.; Pings, C. J. Rev. Mod. Phys. 1963, 33, 389399. (24) Narten, A. H.; Levy, H. A. J. Chem. Phys. 1971, 55, 2263-2266. (25) Hajdu, F. Acta Crystallogr. 1971, A27, 73-76; 1972, A28, 250252. (26) Unpublished results. This laboratory, 1996.

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Figure 6. Phase interference curves calculated for the several scans of the BZ-PYR gel I. Scan 1 (circles), scan 4 (uptriangles), scan 7 (diamonds), scan 10 (crosses), scan 13 (x’s), and scan 21 (stars).

Molecular-Space Interlayer Structure Curve for Coals. As shown previously by many authors, the average interlayer distance in coals is described by the maximum which occurs in the vicinity of 〈d〉 3.5-4.0 Å,3-10 corresponding to q* ) 1.6-1.9 Å-1; where 〈d〉 has been calculated by 〈d〉 ) λ/2 × sin θ. In the calculation of the average interlayer structural distance by our Debye Fourier transform methods, the intensities of the maximum and of the minima which precede and follow the dominant maximum in the WAXRS intensity curve have also been included, i.e., all of the i(q) information from q ) 0.5-2.5 Å-1 has been used in the Fourier transforms of the phase interference curves. Each interlayer structure function has been calculated by5,6,10,11,18-24

S(d) ) [2r/π] ×

∫q × i(q) × στ2ab × M(q) ×

damp(q) × sin(q × d) × dq (3)

where d represents the distance between the scattering center of the average poly-cyclic unit in layer J and the scattering center of the corresponding unit in layer K and where layers J and K are adjacent. In this Fourier transform, M(q) is the sharpening function that converts the structure curve from an electron-electron distance distribution to a scattering center-to-scattering center distribution; and the dampening function, damp(q), ensures against over-importance being placed on the i(q) values at higher q values; and Στ2ab is the scattering power product term for the various molecular-pair distances in the average scattering center in each layer. For coals, where the predominant structural features are poly-cyclic units which lie, at least approximately, in layers, the scattering center-to-scattering center distance is comparable to the interlayer distance between adjacent layers. Since the average PC unit is approximately symmetrical, then its scattering center corresponds to its spatial center. Thus, d*, the maximum in the interlayer structure curve of a coal, is a measure of the nearest interlayer distance in the coal as determined by the Fourier transform method and corresponds to 〈d〉, which has been determined by direct application of Bragg’s law to the WAXRS intensity obtained from the coal.

DuBose and Wertz

Figure 7. The interlayer structure curve calculated for untreated BZ (squares) and for BZ/PYR gel I-21 (stars). Also shown are (2σ.

The statistical uncertainty in S(d) may also be calculated from the WAXRS intensity data for the coal.27,28 Interlayer Structure Curves for the PyridineFree Samples. Shown in Figure 7 is the interlayer structure curve calculated for BZ along with its statistical uncertainty. Also shown in Figure 7 is the interlayer structure curve calculated for gel I-21, the gel from which 99.9+% of the pyridine had evaporated. These two structure curves have statistically equivalent peak areas (543 e2 × Å for untreated BZ and 558 e2 × Å for gel I-21) but different peak maxima. For untreated BZ, d* ) 3.53 Å; and for gel I-21, d* ) 3.88 Å. The difference in the interlayer distance for these two samples indicates that, even though mass measurements indicate that all of the pyridine has been removed from gel sample I-21, its previous presence has altered the interlayer distance and the force(s) which form the bilayer unit in BZ. X-ray Scattering Power and Scattering Power Products. The X-ray scattering power of a molecular unit, τ, is given by

τ ) Σ Za

(4)

where Za is the number of electrons contained in each atom a in the repeat molecular unit.11 The area under the maximum in the interlayer structure curve is related to the scattering power products (Στ2aJ-bK) from the atom-pairs contained in the average molecular units in the short-range structural domain of the material by11

areas(d) ) Σ xa × γab γ τ2aJ-bK

(5)

where xa is the mole fraction of component a. The contribution to the area for each of the molecular-pair interactions is weighted by a counting factor, γ.11,29-31 If the moieties involved in the molecular-pair are (27) Klug, H. A.; Alexander, L. E. X-ray Analysis Procedures for PolyCrystalline and Amorphous Materials; Wiley-Interscience: New York, p 360-361. (28) Nagornyi, V. G.; Khakimova, D. K.; Deev, A. N. Sov. Phys. Crystallogr. 1976, 557-560. (29) Hill, T. L. An Introduction to Statistical Thermodynamics; Addison-Wesley Publishing Co.: Reading, MA, pp 62-63. (30) Wertz, D. L.; Steele, M. A. Inorg. Chem. 1980, 1652-1656. (31) Smith, L. S.; Wertz, D. L. J. Am. Chem. Soc. 1975, 2365-2370.



Figure 8. A schematic of the bi-layer unit proposed for BZ and for BZ/PYR gel I-21.

distinguishable, then γab ) 2; but if the moieties are indistinguishable, then γaa ) 1. The dominant repeat characteristic of the average short-range structural domain of BZ is the bi-layer unit which contains the layers of poly-cyclic carbons (PC) of this lignite. As noted above, the average distance between the scattering centers of these two layers is d* ) 3.53 Å. A molecular model of this domain is presented in Figure 8. Since the two average PC layers are indistinguishable, γPC-PC ) 1. There is one structural vector linking the two adjacent layers, so nPC-PC ) 1. In BZ, the mole fraction of aromatic carbon is ∼0.50 × 0.61 ≈ 0.30,1,2 so the mole fraction of aromatic carbon in each layer within the bi-layers is 0.30/2. Consequently, for BZ, the scattering power product is

τ2PC ) area in S(d) ÷ (xPC. × γPC-PC)

(6)

The average of the peak area in the interlayer structure curves calculated for untreated BZ and for gel I-21 is ∼550 e2, so the scattering power product for the average repeat bi-layer domain of BZ is ∼3667 e2. Then scattering power of each of the poly-cyclic layers in the bilayer of BZ may be calculated by

τPC ≈ 550 e2/(0.15 × 1) ≈ (3667 e2)1/2 ) ∼61 e

Figure 9. The interlayer structure curves calculated for the BZ-PYR gel I at several times after the preparation of the gel. Scan 1 (circles), scan 4 (up-triangles), scan 7 (diamonds), scan 10 (crosses), and scan 13 (x’s). The interlayer structure curve for gel I-21 (stars) is included for comparison purposes. Table 1. Summary of the Structure Curves for BZ and the BZ-PYR Gels sample

timea

db

peak areac

uncertaintyd

BZ Gel I-1 Gel I-4 Gel I-7 Gel I-10 Gel I-13 Gel I-21

NA 0.1 h 1.1 h 1.9 h 2.8 h 4.0 h days

3.53 Å 4.33 Å 4.17 Å 4.14 Å 4.11 Å 4.15 Å 3.88 Å

543 e2 × Å 2484 e2 × Å 1648 e2 × Å 1408 e2 × Å 1187 e2 × Å 989 e2 × Å 558 e2 × Å

(23 e2 × Å (49 e2 × Å (39 e2 × Å (36 e2 × Å (33 e2 × Å (30 e2 × Å (23 e2 × Å

a The time differential between the preparation of the BZ/PYR gel and the initiation of that WAXRS experiment. b Uncertainty is (0.03 Å. c Determined by integration. d Uncertainty is ∼(area)1/2.

(7)

Thus, for BZ, the average unit in each adjacent layer has a scattering power of ca. 61 electrons and is consistent with an elemental composition of CmOnHp, where (m × 6 e) + (n × 8 e) + (p × 1 e) ≈ 61 e. For pyridine, the scattering power may be calculated from its composition, C5H5N, by

τPYR ) (5 × 6 e) + (5 × 1 e) + (1 × 7 e) ) 42 e (8) Since N is isoelectronic with CH, then the scattering center of a pyridine molecule corresponds to its spatial center. In gel samples I-1 to I-13, pyridine is present. As noted in Figure 2 the stoichiometric ratio, nPYR-PC, of the gel ranges from 1.40/1 pyridine molecules to PC units in gel I-1 to 0.21/1 in gel I-13. Interlayer Structure Curves for the Remaining BZ Gel Samples. Shown in Figure 9 are the structure curves for the BZ/PYR gel samples I-1 to I-13 calculated from the phase interference curves noted above. An analysis of the maximum in each of the interlayer structure curves is presented in Table 1. The area under the peak in the maximum of the structure curves for gel I-1 to I-13 decreases exponentially, from 2484 e2 for gel I-1 to 989 e2 in gel I-13, as seen in Figure 10. The maximum in the S(d) values of gel I-1 occurs at 4.33 ( 0.03 Å and at 4.10-4.15 Å in the other four samples.

Figure 10. The area under the maximum in the interlayer structure curves of gel I.

The area under the maximum in the interlayer structure curve of each gel may be related to the average molecular-level adduct array present in the gel at the time of each WAXRS experiment. That the peak area increases (by 400%) and d* increases significantly in the transformation from untreated BZ to gel I-1 is consistent with the formation of new molecular-level adduct(s). That the peak area decrease parallels pyridine loss from the gel indicates that pyridine is involved in the adduct(s) at the molecular level. In gels I-4 through I-13, pyridine is present in the samples, but there is insufficient pyridine for both

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DuBose and Wertz Table 2. Correlation with the Area under the Maximum in Each Inner-Layer Structure Function with the Strucural Models One and Two Model One. nPC-PC ) 0, and Each Pyridine Molecule Bonds to One PC Unit

Figure 11. Schematic of the speculative adduct consistent with Model One.

gel I-1 I-4 I-7 I-10 I-13 I-21

stoichiometric peak area under peak area percent ratio of PYR/PC maximum in S(d) calculated differenceb 1.40/1 0.71/1 0.45/1 0.33/1 0.21/1 f0

2484 e2 1648 e2 1308 e2 1187 e2 898 e2 558 e2

1076e2 546 e2 346 e2 254 e2 161 e2 0 e2

-57% -67% -74% -79% -82% -100%

Model Two. nPC-PC ) 1, and Each Pyridine Molecule Bonds to Both PC Units stoichiometric peak area under peak areaAT percent gel ratio of PYR/PC maximum in S(d) calculated differenceb

Figure 12. Representation of the speculative adduct consist with Model Two. The spatial location of the pyridine molecule relative to the bi-layer domain of BZ cannot be determined from these experiments.

pyridine-PC molecular-pairs and for pyridine-pyridine molecular-pairs to occur. Since the pyridine molecules interact with and change the structuring within the short-range structural domain of BZ, and since there is no evidence of the WAXRS peak due to liquid-state pyridine,6 it follows that pyridine-pyridine molecular pairs are not important contributors to the measured peak areas in this gel. Thus, for these gel samples, the area under the maximum in each structure curve may be related to the molecular-pair units in the average domain of each gel sample by correlating the scattering power products of the various plausible molecular pairs to the area observed under the maximum in the structure curves of the samples of the BZ-PYR gel. Speculations about the Molecular-Level Structuring of the Average Adduct in the Gel Samples. For the structural interactions between pyridine molecules and the PC units of BZ, γPC-PYR ) 2. Peak Area Correlations. To better understand the interactions between the pyridine molecules and BZ, two structural models have been developed. In each model it has been assumed that the pyridine molecule is bonded to the PC unit(s) of BZ, at least for the lifetime of the WAXRS experiments. In Model One (Figure 11), it has been assumed that the addition of pyridine molecules to BZ causes the degradation of the bi-layer unit of BZ, so nPC-PC ) 0. It has been assumed that each of the resulting PC units is bonded to pyridine molecules to produce the PC-PYR adduct. The average distance from the center of the pyridine molecule to the center of the PC unit in each adduct is d*, as identified in the structure curve of each gel. The area attributed to Model One is consistent with

areai ) 0.30 × [(2 × nPYR-PC × 61 e × 42 e)] (9) where nPYR-PC has been determined by the mass experiments noted above. In Model Two (Figure 12), it has been assumed that the bi-layer unit of BZ remains intact and that the pyridine molecules interact with the bi-layer unit. For

I-1 I-4 I-7 I-10 I-13 I-21

1.40/1 0.71/1 0.45/1 0.33/1 0.21/1 f0

2484 e2 1648 e2 1308 e2 1187 e2 898 e2 558 e2

2702 e2 1641 e2 1242 e2 1057 e2 873 e2 550 e2

+9% f 0% -5% -11% -3% f0

a Uncertainty is (0.1. b Percent difference ) 100 × [(area Model - areaS(d)) ÷ areaS(d)]

Figure 13. Comparison of calculated areas from Model One (diamonds) and the calculated areas from Model Two (circles) with the areas measured in the interlayer structure curves of gel I (squares).

this model, the area for each gel sample is given by

areaii ) 0.15 × [(1 × 1 × 61 e × 61 e) + (2 × nPYR-PC × 61 e × 42 e)] (10) Two characteristic distances are important in Model Two: the distance between the scattering center of the pyridine molecule and the scattering center of the PC, and the interlayer distance between the PC layers of BZ. Comparisons of the calculated areas using Model One and Model Two to the measured peak areas resulting from the WAXRS experiments are also shown in Table 2 and in Figure 13. Analysis of the comparisons indicates that Model One is not consistent with the area measured for any of the gel samples, but Model Two predicts areas which are statistically equivalent to the measured peak areas in the interlayer structure curves of all of the BZ/PYR gel samples. This analysis indicates that the bi-layer layer of BZ remains intact throughout


Figure 14. Comparison of the phase interference curve measured for BZ by WAXRS experiment (solid line) and the simulated phase interference curve calculated from a bi-layer model of BZ.

the lifetime of these experiments and that the pyridine molecules interact with the bi-layer. It is beyond the scope of these one-dimensional WAXRS experiments to determine if the pyridine molecules intercalate between the layers of the bi-layer domain of BZ or if the molecules remain outside of the bi-layer domain. Simulated Phase Interference Curves Calculated from the Structural Models. To test the validity of the structural models proposed for BZ and the BZ-PYR gel, simulated phase interference curves have been calculated by the Waser-Schomaker adaption of the Debye equation; i.e.,

j(q) ) (1/2π) × Σxa × γab × τa × τb × nab × damp(q) × cos(∆q × d*) (11) where ∆q ) abs(q - q*). The simulated phase interference curve is a prediction of how the structural unit would scatter X-rays and is based only on the structural model being considered. For untreated BZ, the simulated phase interference curve for the bi-layer domain has been calculated by

j(q) )(1/2π) × [0.15 × 1 × 1 × 61 e × 61 e × cos(3.53Å × ∆q)] × damp(q) (12) In Figure 14, the simulated j(q) calculated for the bilayer model of BZ is compared to the experimentally determined i(q) for untreated BZ. In the vicinity of the first maximum, which describes the bi-layer unit, the simulated curve and the experimentally determined curve are statistically equivalent. This agreement indicates that the bi-layer model is a plausible description of the average short-range structural domain of BZ. Shown in Figure 15 are the two simulated phase interference curves consistent with gel I-1 and Model Two. The two components, the first involving the two PC units in the bi-layer and second involving the bilayer and the pyridine molecule, have been calculated by

j(q) )(1/2π) × damp(q) × 0.15 × [{1 × 1 × 61 e × 61 e × cos(3.53Å × ∆q)} + {2 × 2 × 1.4 × 61 e × 42 e × cos(4.3Å × ∆q)}] (13)


Figure 15. Comparison of the i(q) measured for gel sample I-1 (solid line) compared to the simulated phase interference curve calculated from Model II. In this model, the j(q) the PYR-PC contribution is indicated by circles and the PC-PC contribution is indicated by squares. Table 3. Average C-C Distance in Several Small PCA Units carbon atoms (m)

r*

Σ[(xaJ - xbK)2 + (yaJ - ybK)2]}1/2

〈d〉

6 7 8 9 10

4.03 Å 4.17 Å 4.26 Å 4.34 Å 4.43 Å

1.70 Å 1.97 Å 2.12 Å 2.26 Å 2.41 Å

3.36 Å 3.43 Å 3.54 Å 3.60 Å 3.67 Å

This simulated j(q) agrees, within statistical uncertainty, with the experimentally measured i(q) for gel I-1, verifying that Model Two is a plausible description of the average adduct in gel I-1. Conclusions The scattering power product for the average repeat unit in each layer of BZ is ∼61 e, so the average repeat unit may be described as CmOnHp, where (m × 6 e) + (n × 8 e) + (p × 1 e) ) 61 e. It is beyond the scope of these WAXRS experiments to determine if the carbons are aromatic and/or, aliphatic, to determine if the oxygens are phenolic, etheric, and/or carboxylcilic, and/ or to evaluate m, n, and p in the average repeat unit of BZ. The average interlayer distance in Beulah Zap lignite is rapidly increased from 3.53 Å by the addition of pyridine, suggesting that the pyridine molecules quickly interact with the bi-layer units of the short-range domain of BZ. The formation of the BZ/PYR adduct(s) is accomplished in less than 45 s after the addition of pyridine to this lignite. The disappearance (evaporation) of pyridine from the BZ/PYR gel phase occurs over a period of several hours, and the reduction in the peak area in the interlayer structure curve of the BZ/PYR gel decreases in direct proportion to the evaporation of pyridine from the sample. After 99.9% of the pyridine has evaporated from the gel, the interlayer distance remains ∼0.35 Å larger than the average interlayer distance in the untreated Beulah Zap lignite. EF010207T

X-ray Analysis of Coals Treated with Organic ... - ACS Publications

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