Energy & Fuels 1999, 13, 513-517
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Interlayer Structural Models of Beulah Zap Lignite Based on Its Wide Angle X-ray Scattering David L.Wertz† Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 Received September 8, 1998. Revised Manuscript Received November 12, 1998
Wide-angle X-ray scattering (WAXRS) in the 0.5-2.5 Å-1 region of the intensity curve of Beulah Zap (BZ) lignite indicates that the average distance between adjacent layers is ca. 3.5 Å. The resulting interlayer structural curve, calculated by Fourier transform from the WAXRS intensity, indicates that only two layers occur in the average short-range structural domain of BZ with the distances between atoms in the adjacent layers occurring between 3.4 and 5.7 Å. Both the WAXRS intensity curve (in reciprocal space) and the interlayer structure curve (in molecular space) are consistent with an average short-range structural domain in which the flexible spacers (attachments) between the polycyclic units are important contributors and where the PC units in the adjacent chains are not eclipsed.
Introduction The chemical composition of Beulah Zap (BZ) lignite is considerably different from the compositions of the more mature coals in the Argonne Premium Coals Program,1 particularly because of its high abundance of carboxylate oxygen2 and the relative importance of the attachments to its polycyclic (PC) units. In BZ, the ratio of aliphatic carbon/aromatic carbon is 0.72, which is much larger than in the more mature coals.3 Cartz et al.,4 have shown that the X-ray scattering intensity caused by irradiating a coal with Xrays may be divided into two regionssthe low-angle scattering caused by the macrostructure of the coal and the highangle scattering which is due to the coal’s short-range structural features. Using the conventional reciprocal space parameter, q, where q ) 2π/〈d〉 ) [4π/λ] sin θ, the Cartz boundary between low-angle scattering and wideangle scattering occurs at q ≈ 0.3 Å-1, i.e., scattering which describes the macromolecular structure of the coal occurs at q < 0.3 Å-1, and scattering which describes its short-range molecular level scattering occurs at q > 0.3 Å-1. They further separated the wide angle scattering region into two components: the scattering at the highest q values (which describes the structuring of the average two-dimensional unit in that coal), and the scattering which occurs in the vicinity of q ≈ 1.8 Å-1 (which describes the packing of the layers in the coal). Nakamura5 reports the interplanar distance to be ca. 4.0 Å (q ≈ 1.6 Å-1). Kineda et al.6 report that †
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[email protected]. (1) Vorres, K. S. Energy Fuels 1990, 4, 420-425. (2) Hayatsu, R.; Wynans, R. E.; Scott, R. G.; Moore, L. P.; Studier, M. H. Nature 1978, 275, 116-118. (3) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (4) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500502; Philos. Trans. R. Soc. 1960, A252, 557-600. (5) Nakamura, K.; Nakanoshashi, T.; Iono, M.; Sato, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003-1010.
the average interlayer distance varies from 3.5 to 3.8 Å (i.e., from 1.7 to 1.8 Å-1) in coals and heat-treated coals. Our recent wide-angle X-ray scattering (WAXRS) experiments coupled with molecular-structuring/modeling experiments7-10 have shown that the intensity maximum corresponding to 3.5-4.0 Å is surrounded by two minima and that all three of the structural features are due principally to the distances between the atoms in adjacent layers in the coal. The maximum and its two adjacent minima dominate the region in reciprocal space between q ∼ 0.5 Å-1 and q ∼ 2.4 Å-1. The WAXRS intensity measured for BZ exhibits these features, but the first maximum and two adjacent minima are shallow and diffuse.7,12 This group has recently used wide-angle X-ray scattering methods to show that the removal of moisture from BZ by heating in a convection oven does not alter the layering of the PC units to a measurable extent,7 but that more complete removal of water causes a small but measurable change in the WAXRS intensity in the vicinity of 1.6-1.7 Å-1.12 It has also been shown that the addition of pyridine causes the diffuse 1.8 Å-1 maximum characteristic of BZ to be replaced by a much sharper, more intense maximum centered at 1.4 Å-1 in the WAXRS scattering curve of the BZ-pyridine adduct. Taking advantage of the sharpness of the maximum and minima in the 0.5-2.5 Å-1 region of its reciprocal lattice intensity curve, a speculative molecular-level model which describes the molecular-level structure of the adduct in the pyridine-BZ gel has been presented.7 (6) Kineda, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678. (7) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697-703. (8) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, submitted for publication. (9) Wertz, D. L. Fuel 1995, 74, 1431-1435. (10) Wertz, D. L. Fuel 1998, 77, 43-53. (11) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613-617. (12) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047-1053.
10.1021/ef980183z CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
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Wertz
Figure 1. X-ray diffractogram of Beulah Zap lignite. The sharp, small peaks in the diffractogram are due to the crystalline minerals deposited in the lignite or to the aluminum sampleholder.
Figure 2. Absorption-corrected WAXRS intensity due to the organic matrix of BZ lignite (A), the self-scattering curve calculated for BZ (B), and the phase interference curve calculated for BZ (C).
The purpose of this investigation is to better understand the interlayer short-range molecular structuring in BZ lignite and to develop a plausible molecular-level model of the molecular-level structuring of this lignite which is consistent with our WAXRS measurements and other information obtained about this lignite. Our results are presented below.
several sharp, intense peaks. In the region of our WAXRD, these peaks occur at 2θ ) 39.2° and 2θ ) 44.9° and are due to diffraction from the (111) and the (200) planes of crystalline aluminum, respectively. The secondary intensity from aluminum is diminished by the BZ sample deposited onto the sampleholder. The reduction in the intensity of the diffraction peaks from the aluminum sampleholder caused by the BZ sample may be used to measure the linear absorption coefficient (µ) of the BZ sample by13
Experimental Section The sample of Beulah Zap lignite used in the intermediateangle X-ray experiment was obtained from the Argonne Premium Coal Sample Program1 and was used “as received.” A sample of BZ was mounted onto an aluminum sampleholder and mounted into our θ-2θ diffractometer, which is equipped with a sample spinner and a θ-compensating slit.7-11 The aluminum sampleholder is disk-shaped with a diameter of 2.5 cm and a depth of 1 mm. A wide-angle X-ray diffractogram (WAXRD) was obtained using Cu KR Xrays as the radiation source, and a graphite crystal monochromator in the secondary X-ray beam was obtained over the angular region from 2θ ) 5.00° to 2θ ) 75.00° (or q ) 0.36-5.00 Å-1) at increments of ∆2θ ) 0.02° for 3′ intervals at each angle. The background over the angular range of the experiment was also measured.
Results and Discussion The result of an X-ray scattering experiment is the intensity of the secondary (scattered) X-rays as measured at the detector and describes an array of distances, in reciprocal space, which have been averaged over all atom pairs included within the irradiated portion of the sample and over the time of the experiment. Shown in Figure 1 is the X-ray diffractogram, I(2θ), obtained in our laboratory for BZ lignite. Also shown in Figure 1 are the diffraction peaks due to crystalline minerals embedded in the BZ and/or the diffraction peaks due to the aluminum sampleholder. The relative standard deviation in the measured intensity (σR) may be calculated, at each angle, by σR ) 100%/ N1/2 and ranges from 2.1% in the vicinity of maximum at 25° to 2.8% at 2θ f 75°except for the sharp diffraction peaks, where σR is considerably smaller. The diffractogram of the aluminum sampleholder is characterized by low intensity (ca. 100 cps) and by
µ ) {(T sin θ)/(2m)} ln(As/Ap)
(1)
where m is the mass of the sample, T is an experimental parameter, and As/Ap is the ratio of the peak areas of the aluminum peaks in the absence and in the presence of the BZ sample.13 For BZ, µ has been determined to be 11.4 cm-1. The secondary intensity from the aluminum sampleholder that has been transmitted through the BZ sample may be calculated by
I′m(2θ) ) wmI°min(2θ)
(2)
where wm is the fraction of mineral m and I°m(2θ) is the absorption-corrected intensity for that mineral. The secondary intensity due to the organic matrix in BZ has been calculated by
IBZ(2θ) ) I(2θ) - [I′Al(2θ) + I′m(2θ) + BG]
(3)
Defining q ) (4π/λ) sin θ, the secondary intensity due to BZ has been corrected for sample self-absorption by
I*BZ(q) ) IBZ(q)µm/[Y(1 - exp{-2µm/T sin θ})] (ref 13) (4) Shown in Figure 2 is the absorption-corrected WAXRS intensity curve for BZ, IBZ(q). Also shown in Figure 2 is the self-scattering intensity curve calculated for BZ by7-11,14-16 (13) Wertz, D. L.; Smithhart, C. B.; Wertz, S. L. Adv. X-Ray Anal. 1990, 33, 475-483.
Structural Models of Beulah Zap Lignite
SS(q) )
∑wy[f2y(q) + {Iy(q)D(q)}]
Energy & Fuels, Vol. 13, No. 2, 1999 515
(5)
where fy(q) is the coherently X-ray scattering factor for atom y17 (which has a fraction wy1,18) in the sample, Iy(q) is the incoherent scattering component for atom y,17 and D(q) is the discrimination function for the graphite monochromator for the incoherently scattered Xrays.19 The self-scattering curve is a measure of how a sample of the composition of BZ would scatter Xrays if the atoms were randomly arranged, i.e., the absence of bonding and short-range structure in the sample.14,15 Also shown in Figure 2 is the phase interference curve for BZ, which has been calculated at increments of ∆q ) 0.1 Å-1 by7-11,14-16
i(q) ) [IxBZ(q)/k] - SS(q)
(6)
where k converts the experimentally measured intensity (in cps) to X-ray scattering units, i.e., e2. The phase interference curve describes the shortrange structuring within the condensed phase material and is the structurally significant portion of the scattered X-ray intensity as displayed in reciprocal space. Fourier transform of the phase interference curve (in reciprocal space) into the 1-D structure curve (in molecular space) has been accomplished by
S(r) ) (Sr/π)Sqi(q){M(q) exp(-0.02q2)} sin(qr)∆q (7) M(q) and the exponential are conventional sharpening and dampening terms used for converting the distribution function from one that describes electron-pair distances to one that describes atom-pair distances.14,15 The limits on the integral are qmin ) 0.5 Å-1 and qmax ) 2.4 Å-1, which limits the resulting structure curve to describing the interlayer atom-pair distances in BZ coals.4,7-11,14,15 The Fourier transform terminates prior to the (111) and the (200) diffraction peaks of the aluminum sampleholder. In this calculation, ∆q ) 0.1 Å-1. The statistical uncertainty, τ, has also been calculated for the structure curve by the method by Konnert and Karle.20 The region where S(r) > 0 ( 2τ describes the statistically important atom-pair distances between atoms in adjacent layers, and the region where S(r) is contained within the boundaries of 0 - 2τ and 0 + 2τ indicates the disappearance of short-range structuring. For the interlayer structure curve for BZ, τ ) 0.02. The interlayer structure curve, S(r), for BZ is presented in Figure 3. It contains only one peak, with a maximum centered at 4.3 Å and extending from 3.4 to 5.7 Å. This peak shows that all of the statistically significant distances between atoms in the short-range structural unit in BZ occur within the 3.4-5.7 Å interval and that the average interlayer atom pair distance is 4.3 Å. (14) Kruh, R. F. Chem. Rev. 1962, 62, 319-346. (15) Paalman, H. H.; Pings, C. J. Rev. Modern Phys. 1963, 33, 389399. (16) Narten, A. H.; Levy, H. A. J. Chem. Phys. 1971, 55, 2263-2266. (17) Hajdu, F. Acta Crystallogr. 1971, A27, 73-76; 1972, A28, 250252. (18) Vorres, K. S. “Users Handbook for the Argonne Premium Coal Sample Program,” ANLIPCS P-91/1, 1993; pp 17-27. (19) Unpublished results, this laboratory, 1994-present. (20) Konnert, J. H.; Karle, J. Acta Crystallogr. 1973, A29, 702-710.
Figure 3. Structure curve which describes the atom-pair distances between atoms in adjacent layers within the average structural domain of BZ lignite. The statistical deviations at 0 - 2τ and 0 + 2τ are included.
Cartz et al. first showed that the average distance between adjacent layers in coals is ca. 3.5 Å. This average interlayer distance corresponds to the shortest distance between atom pairs in the adjacent layers as measured by S(r). The recently published Mura21 model adapts the Cartz findings to lignites. All of the distances between atom a in layer J and atom b in layer K in the short-range structural domain of BZ, may be calculated by
raJbK ) {(xaJ - xbK)2 + (yaJ - ybK)2 + (zaJ - zbK)2}0.5 (8) where xaJ and xbK represent the x-coordinates of atom a in layer J and of atom b in layer K, respectively. The y-coordinates and z-coordinates are similarly defined. However, zaJ-zbK, may be replaced by the average interlayer distance. For BZ, 〈d〉 ) 3.5 Å; so
raJbK ) (xaJ - xbK)2 + (yaJ - ybK)2 + {(n‚3.5 Å)2} (9) The parameter n describes in the spatial relationship of the layers in the domain. Since, for BZ, the only maximum in its interlayer structure curve occurs in the 3.4-5.7 Å region, n is restricted to 1; i.e., J and K are adjacent layers. Speculative Models of the Layering of Adjacent Units in Beulah Zap Lignite. In order to better understand the interlayer structure curve determined for BZ, several molecular-level models have been developed. For the reasons noted above, each model has utilized only the adjacent (n ) 1) layers. In each model, the average polycyclic (PC) unit has been assumed to be C9,3 and the attachments, which serve as flexible spacers, have been included. Mura et al.21 report that the adjacent layers in lignites, held together by hydrogen bonding, are approximately parallel; but this model does not define the relative positions of the PC units and the attachments. Several of our models have assumed the “parallel” interlayer structure and are similar to the short-range domains found in crystals (21) Mura, K.; Mae, K.; Morozumi, F.-A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 209-213.
516 Energy & Fuels, Vol. 13, No. 2, 1999
Figure 4. Model A (a) with the PC unit presented as a bold straight line and the flexible spacer attachment depicted as wavy lines. Also shown are the frequencies (b) of the atompair distances calculated from model A compared to the structure curve of BZ (circles). The * indicate atom-pair distances which do not fall within the boundaries of the structure curve experimentally determined for BZ.
Wertz
Figure 6. Phase interference curve for BZ (circles) compared to the simulated phase interference curves calculated from model 1 (squares) and from model 2 (triangles), respectively.
A crucial test in evaluating the validity of any structural model involves the Fourier transform of the model’s aJ-bK atom-pair distances and frequencies, nab, into reciprocal space to produce a simulated phase interference curve, j(q), using the Debye equation, by7-11,14,15
j(q) )
∑nabfafb[qraJ bK]-1 sin(qraJ bK)
(10)
The validity of each model24 as a suitable description of the layering within the average short-range structural domain in BZ may be judged by the comparison of its j(q) to i(q) use structure correlation factor, R2, where
R2 )
Figure 5. Model B (a) with the PC unit presented as the bold straight line, and the flexible space attachment depicted as the wavy line. Also shown are the frequencies of the atompair distances calculated from model B compared to the structure curve of BZ (circles).
containing PC units similar to C9.22 Other models have assumed a nonparallel arrangement of adjacent layers. Two of the parallel layer models are presented in Figures 4 and 5. In model A (Figure 4a), the average PC units in adjacent J and K layers are eclipsed; whereas in model B (Figure 5a), the C9 unit in layer J eclipses the flexible spacer in layer K. Also shown in Figures 4 and 5 are atom-pair distances calculated for each model compared to the interplanar structure curve for BZ. Comparisons indicate that for model A, a few of the calculated atom-pair distances fall outside of the peak in S(r), while for model B, all of the calculated atom-pair distances fall within the boundaries of the peak. Intermediate “parallel layer” models, i.e., where the PC unit in layer J partially eclipses the PC unit in layer K, give intermediate results. Models where layers J and K are not at least approximately parallel predict many atom-pair distance distances which do not fall within the boundaries of the peak in the S(r) obtained for BZ. (22) Trotter, J. Acta Crystallogr. 1959, 12, 232.
∑{(q)]2/[i(q)]2
(11)
and (q) ) j(q) - i(q). The atom-pair frequency/distance array for models A and B has been Fourier transformed into reciprocal space; and the resulting simulated phase interference curve, j(q), for each model has been compared to the experimentally determined phase interference curve, i(q). This comparison is presented in Figure 6. For model A, R2 ) 0.295; and for model B, R2 ) 0.044 over the reciprocal space region from q ) 0.5 to 2.4 Å-1. For the partially eclipsed parallel layer models, R2 is intermediate between these two values. For the interlayer models which have assumed that layers J and K are not parallel, R2 . 0.295. These comparisons also indicate that, for the models considered, the simulated phase interference curve for model B best agrees with the experimentally measured phase interference curve for BZ. Conclusions The structure curve obtained by the wide-angle X-ray scattering experiment with Beulah Zap lignite contains only one statistically significant maximum, which is centered at 4.3 Å and extends from 3.4 to (23) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley Publishing Co.; Inc.: Reading, MA, 1978; p 287. (24) Clark-Baldwin, K.; Tierney, D. L.; Govindaswamy, N.; Gruff, E. S.; Kim, C.; Berg, J.; Koch, S. A.; Penner-Hahn, J. E. J. Am. Chem. Soc. 1998, 120, 8401-8409.
Structural Models of Beulah Zap Lignite
5.7 Å. The presence of one maximum in the structure curve is consistent with an average short-range structural domain which contains ca. two adjacent layers (or chains). The three-dimensional model which has the PC unit in one layer eclipsing the flexible spacer attachments in the adjacent layer is most consistent with the wideangle X-ray scattering intensity measured in the 0.52.5 Å-1 region. This model is different from the Cartz-
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Hirsch model of coal structure which specifies that the polycyclic units are eclipsed in the short-range structural domain of the coal. The diffuseness of the maximum and minima in the intermolecular portion of its reciprocal lattice intensity curve is consistent with the absence of a well-defined three-dimensional repeat unit in the average shortrange structural domain in this lignite. EF980183Z
