Energy & Fuels 1989,3, 200-205
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Investigation of the Molecular Structure of Organic Sulfur in Coal by XAFS Spectroscopy G. P. Huffman,*'? F. E. Huggins,? Sudipa Mitra,? N. Shah,? R. J. Pugmire,i B. Davis,g F. W. Lytle," and R. B. Greegor" T h e Consortium for Fossil Fuel Liquefaction Science, University of Kentucky, 233 Mining & Mineral Resources Building, Lexington, Kentucky 40506, Fuel Science Department, University of Utah, Salt Lake City, Utah 84112, Center for Applied Engineering Research, University of Kentucky, Lexington, Kentucky 40512, and T h e Boeing Company, Seattle, Washington 98124 Received October 11, 1988. Revised Manuscript Received November 22, 1988
X-ray absorption fine structure (XAFS) spectroscopy has been used to investigate the molecular structure of organic sulfur in a suite of maceral separates and in several biodesulfurized and extracted coal specimens. For most samples, the X-ray absorption near-edge structure (XANES) exhibits sharp peaks just above the absorption edge that are characteristic of s p transitions of compounds containing an aromatically bound sulfur atom and a broad, structured maximum at somewhat higher energies. The latter maximum is believed to arise from resonant backscattering of photoelectrons by carbon atoms 3.5-4.1 8, from the sulfur atom and possibly from s p transitions of sulfur bonded to oxygen. The radial structure functions derived by Fourier analysis of the EXAFS exhibit peaks at distances that are compatible with the first three neighbor shells surrounding an aromatically bound sulfur atom.
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Introduction X-ray absorption fine structure (XAFS) spectroscopy can provide information on the electronic bonding state and atomic environment of dilute (-100 ppm to 1%) elements in complex samples. Consequently, it is ideally suited to the investigation of impurity elements in coal. Recently, it has been demonstrated that XAFS is an excellent method for investigating sulfur in While a number of techniques exist for determining the structure of the inorganic sulfur-bearing minerals in coal and coal derivative^,^ very few methods exist for direct, nondestructive analysis of the molecular structure of organic sulfur. In this paper, we present the results of an XAFS investigation of organic sulfur in a suite of maceral separates, several coal extracts and biodesulfurized coals, and a variety of standard compounds. The results demonstrate that XAFS is an excellent technique for investigating the molecular structure of organic sulfur. Experimental Procedures T h e XAFS experiments were conducted during a dedicated run a t the Stanford Synchrotron Radiation Laboratory on wiggler beam-line VII-3. Electron energies were 3 GeV and beam currents were typically 40-80 mA. A Si(ll1) double-crystal monochromator was used to vary the X-ray energy from approximately 100 eV below to 600 eV above the sulfur K-shell absorption edge (2472 eV). To minimize absorption of these relatively soft X-rays, an all-helium pathway from the beam pipe to the sample and detector was constructed and thin (6 pm) Mylar windows were used wherever possible. The experments were done in the fluorescent mode, using a fluorescent ionization detector described elsewhere? Most of the samples examined were maceral separates prepared by density gradient centrifugation (DGC). Discussions of the DGC methods and a description of the chemical and physical properties of the whole coals and maceral separates are given el~ewhere."~ 'The Consortium for Fossil Fuel Liquefaction Science, University of Kentucky. *University of Utah. f Center for Applied Engineering Research, University of Kentucky. '1 The Boeing Co. 0887-0624/89/2503-0200$01.50/0
Table I. Summary of the Source Coals from Which Maceral Separates, Biodesulfurized Specimens, and Extracts Were Preoared rank location sample PSOC 733 HVAB PA, Appale UT SO-W. PSOC 1111 MVB PSOC 1110 UT SO-W. SUBC PSOC 1108 lignite UT SO-W. W. Ky. No. 9, 71094 W. KY HVAB W. Ky. No. 9, 71095 W. KY HVAB W. Ky No. 11,91182 W. KY HVCB Boghead coal, Soxhlet extract cannel (bituminous) KY wv Bakerstown coal, HVAB NMP extract Here, we will only note the density ranges of the separates: exinite, 1.17-1.20 g/cm3; vitrinite, 1.28-1.305 g/cm3; inertinite, 1.31-1.355 g/cm3. XAFS measurements were made on exinite, vitrinite, and inertinite separates from coals of several ranks. In addition to maceral separates, several coals from which pyrite had been removed by biological desulfurization7 and pyrite-free extracts prepared by using a supersolvents were examined. A list of the coals from which the maceral separates and other specimens examined in this study were prepared is given in Table I. The (1)Spiro, C. E.; Wong, J.; Lytle, F.; Greegor, R. B.; Maylotte, D.; Lampson, S. Science 1984, 226, 48. (2) Huffman, G. P.; Huggins, F. E.; Shah, N.; Bhattacharyya, D.; Pugmire, R. J.; Davis, B.; Lytle, F. W.; Greegor, R. B. Prepr. Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33(1), 200. (3) Huffman, G. P.; Huggins, F. E.; Shah, N.; Bhattacharyya, D.; Pugmire, R. J.; Davis, B.; Lytle, F. W.; Greegor, R. B. In Processing and Utilization of High Sulfur Coals II; Chough, Y. P., Caudle, R. D., Eds.; Elsevier: Amsterdam, 1987; p 3. (4) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, E. C.; Wong, J.; Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods 1988,226, 542. ( 5 ) Karas, J.; Pugmire, R. J.; Woolfenden, W. R.; Grant, D. M.; Blair, S. Int. J. Coal Geol. 1985, 5, 315. (6) Keogh, R.; Poe, S.; Chawla, B.; Davis, B. In Coal Science and Technology; Vol.II, Moulijn, J. A., Nater, K. A., Chemin, H. A. G., Eds.; Elsevier: Amsterdam 1987; Vol. 11, p 289. (7) "Cooperative Research in Coal Liquefaction Infratechnology and Generic Technology Development"; Final Report, DOE Contract No. DE-FC22-86PC90017, Consortium for Fossil Fuel Liquefaction Science: Lexington, KY, May 10, 1988. (8) Renganathan, K.; Zondlo, J. W.; Mintz, E. A.; Kneisl, P.; Stiller, A. H. Fuel Process. Technol. 1988, 18, 273.
0 1989 American Chemical Society
Organic Sulfur in Coal
Energy & Fuels, Vol. 3, No. 2, 1989 201 XANES of Macoral SOparl1.S
XANES of Sulfur Standards
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ENERGY
XAFS specimens were prepared in the form of pellets by pressing the coal powder into a boric acid cylinder or, in some cases, adding an epoxy binder.
Results and Discussion XAFS spectroscopy determines the electronic structure and atomic environment of an element by analysis of the fine structure associated with an X-ray absorption edge of that element. The spectra are normally divided into two regions. The region within about 20-50 eV of the absorption edge is called the X-ray absorption near-edge
Figure 4. Least-squares fit of a typical coal maceral XANES.
structure, or XANES (see Figures 1,3, and 4). The peaks and other structure in this region are derived primarily from two sources: photoelectron transitions to vacant, bound levelsgJOand low-energy scattering (9) Wong, J.; Ltyle, F. W.; Messmer, R. P.; Maylotte, D. H. Phys. Reo. B 1984, 30, 5596. (10) Kutzler, F. W.; Natoli, C. R.; Misemer, D. K.; Doniach, S.; Hodgson, K. 0. J. Chem. Phys. 1980, 73, 3274.
Huffman et al.
202 Energy & Fuels, Vol. 3, No. 2, 1989 600
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Figure 6. Typical radial structure functions of maceral separates. The labels denote the peaks arising from the first, second, and third nearest-neighbor shells. The XANES spectra are quite sensitive to the detailed nature of the electronic bonding and can frequently serve as fingerprints to identify different compounds or types of binding. The extended X-ray absorption fine structure, or EXAFS, is the oscillatory structure that begins at 30-50 eV above the edge and extends to fairly high energies (500-1000 eV). These oscillations arise from interference between the outgoing and backscattered photoelectron wave functions. They can be subjected to a Fourier transform analysis to yield a radial structure function (see (11) Sette, F.; Stohr, J.; Hitchcock, A. P. Chem. Phys. Lett. 1984, 110, 517. (12) Bianconi, A.; Fritsch, E.;Calas, G.; Petiau, J. Phys. Reu. B 1985, 32, 4292. ( 1 3 ) Lytle, F. W.; Geegor, R. B.; Panson, A. J. Phys. Reu. E 1988, 37, 1550.
Figures 5 and 6) from which interatomic distances and coordination numbers for the atomic neighbor shells of the absorbing atoms can be determined.l4J5 XANES Analysis. Sulfur K-shell XANES of several standard compounds appear in Figure 1. The zero of energy is taken at the first peak in the differential of the spectrum of elemental sulfur. The first large peak in the XANES, the so-called "white line", arises from a transition of the photoelectron from the Is level to hybridized p levels-3p/3d-4~ for pyrite and 3p/2p for the remaining compounds. The subsequent broader peaks, located from approximately 5 to 30 eV above the white line are believed to arise primarily from low-energy scattering resonanc(14) Sayers, D. E.; Lytle, F. W.; Stern, E. A. Phys. Reu. Lett. 1971,27, 1204 ___.
(15) Lee, P. A,; Citrin, P. H.; Eisenberger, P.; Kincaid, B. M. Reu. Mod. Phys. 1981, 53, 769.
Organic Sulfur in Coal
Energy & Fuels, Vol. 3, No. 2, 1989 203
Table 11. Positions &0.2 eV) and Area Percentages of the Three Principal XANES Peaks Observed for Coal Samplesa peak 1 peak 2 peak 3 coal name position, eV area, % position, eV area, % position, eV area, % PSOC 733 (e) 2.8 47 4.9 7 11.3 46 2.9 43 4.7 6 11.3 51 PSOC 733 (v) 41 11.6 PSOC 733 (i) 2.9 4.9 3 56 41 5.1 23 10.2 36 PSOC 1111 (e) 3.3 24 10.0 37 PSOC 1111 (VI 39 3.3 5.0 24 PSOC 1111 (i) 43 3.3 5.0 9.9 33 46 3.0 4.8 20 9.8 34 PSOC 1110 (e) 26 4.8 3 9.9 71 PSOC 1110 (v) 3.1 21 9 10.1 68 PSOC 1108 (v) 3.1 4.8 41 4 W.Ky. 9, 71094b (e) 2.9 4.8 10.8 55 57 W. Ky. 9, 71094 (v-1)' 38 5 2.8 4.8 10.6 W. Ky. 9, 71094 (v-2)' 40 5 2.9 5.0 10.8 55 5 11.1 38 2.9 4.8 57 W. Ky. 9, 71094 (v-3)' 11.3 W. Ky. 9, 71094 (i-lId 50 5 45 2.9 4.8 W. Ky. 9, 71094 (i-2)d 3 11.2 63 34 2.9 4.9 10 11.2 46 44 2.9 4.9 W. Ky. 9, 71071 (v) 11.2 46 2.9 W. Ky. 9, 71095 (v) 6 48 4.9 11.7 Boghead coal, Soxhlet extract 3 56 40 2.8 4.9 11.5 40 Bakerstown Coal, NMP extract 5 55 2.8 4.8 4 11.7 64 W. Ky. No. 11, biodesulfurized, run 5 32 2.9 4.9 11.5 57 37 W. Ky. No. 11, biodesulfurized, run 6 6 2.9 4.9 O e , v, and i denote the exinite, vitrinite, and inertinite fractions prepared by DGC. bNumber refers to a location in the W. Ky. No. 9 seam.' 'Subdivision of vitrinite density range: v-1 = 1.279-1.287 g/cm3, v-2 = 1.291-1.296 g/cm3, and v-3 = 1.300-1.306 g/cm3. dSubdivision of inertinite density range: i-1 = 1.309-1.315 g/cm3 and i-2 = 1.320-1.355 g/cm3.
es.*l-13 It is evident that the increase in valence of the sulfur ions bonded to oxygen in sulfosalicylic acid and ferrous sulfate causes a significant positive shift in the white line and other XANES features. This occurs because the transfer of valence electrons from sulfur to oxygen decreases the screening of the nuclear charge more for the 1s level than for the p levels, resulting in a net increase p transition energy. in the s The scattering resonance features in the XANES appear to arise primarily from single scattering events for which the wavelength of the photoelectron matches exactly the interatomic distance from the sulfur atom to a neighboring shell of atoms.12J3If the average potential energy between atoms is ignored and the photoelectron is treated as a free electron, this corresponds to
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where E is the resonance energy in eV and R is the interatomic spacing in A. In Figure 2, a plot of resonance energies from the XANES of organic compounds in which sulfur is bonded to carbon versus the inverse square of the interatomic spacings out to approximately 6 A is shown. The compounds used to derive this correlation included dibenzothiophene, benzo[b] thiophene, dibenzyl disulfide, dibenzyl sulfide, thianthrene, diphenyl disulfide, thiophene-3-carboxylic acid, thiophene-2-acetic acid, and thioacetamide. In order to avoid valence effects, we have not included any compounds in which sulfur is bonded to oxygen or any inorganic compounds in this correlation. As seen, the relationship of eq 1 is fairly well obeyed. While these results are encouraging, it is nevertheless clear that systematic studies of a wider range of standard sulfur compounds are desirable. Furthermore, it should be emphasized that detailed theoretical treatment of the XANES region of XAFS spectra and the transition to the EXAFS region is a topic that is currently undergoing rapid development."j XANES spectra of several maceral separates from a high-volatile bituminous coal (PSOC 733, HVAB, Appale, (16) Proceedings of XAFS V , The Fifth International Conference on X-ray Absorption Fine Structure, Seattle, WA, Aug 21-26,1988; to be published as a special issue of Physica B.
PA) and from vitrinite separates from coals of several ranks are shown in Figure 3. Approximate least-squares analyses of the coal XANES were made by assuming an arctangent edge step and three Lorentzian peaks as illustrated in Figure 4. The results are summarized in Table 11. Similar results for standard compounds are given in Table 111. In these tables, the edge fit analysis results are summarized by indicating the percentage of the total Lorentzian peak area contributed by each peak. It is seen that the XANES of all of the coal specimens examined were qualitatively similar, consisting of an intense (-25450%) sharp peak at approximately 3 eV, a sharp, relatively small (-5-20%) satellite peak a t approximately 5 eV, and a broad maxim (-3040%) at approximately 1&12 eV. Although we have fit this latter maximum by a single, broad Lorentzian for easy inter-sample comparison, it is evident from Figures 3 and 4 that it consists of several overlapped contributions. It is seen from the figures and Tables I1 and I11 that the sharp peaks observed in the coal XANES at 3 and 5 eV are very close in energy and have intensities similar to those of corresponding peaks in the XANES of dibenzothiophene (DBT) and benzo[ blthiophene. Presumably, they represent s p transitions characteristic of an aromatically bound sulfur atom. Although the peak at 5 eV could arise from low-energy photoelectron scattering by atoms approximately 5.5 A from the sulfur, the sharpness of the peak indicates that it is probably an s p transition to a crystal field split level. Thianthrene, however, which contains two bridging sulfurs between benzene rings, exhibits only the first of these two sharp peaks, at approximately 3 eV. This could reflect the elimination of a crystal field split level a t 5 eV associated with the increase in symmetry of the sulfur sites in thianthrene. The XANES of compounds containing single thioether or disulfide linkage sulfur (dibenzyl sulfide (DBS), dibenzyl disulfide (DBDS), and diphenyl disulfide) are qualitatively similar to those of DBT and benzo[b]thiohene, but the p transition occurs a t slightly lower energies main s (1.7-2.4 eV) while the second peak is noticeably broader (Figure 1)and also occurs at lower energies. It is seen in Table I1 that nearly all of the coal specimens examined exhibited s p peaks within the standard deviation (*0.2 eV) of those exhibited by DBT and benzo[b]thiophene.
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204 Energy & Fuels, Vol. 3, No. 2, 1989
Table 111. Position (+0.2 eV) and Area Percentages of the XANES Peaks Observed for a Variety of Standard S u l f u r Compounds peak 1 peak 2 peak 3 peak 4 peak 5 peak 6 position, area, position, area, position, area, position, area, position, area, position, area, comDou nd eV % eV % eV 90 eV 90 eV 9i eV % 8.3 18 11.4 20 14.1 20 dibenzothiophene 2.8 36 4.8 6 2.8 36 4.9 2 7.5 16 10.0 28 14.8 18 benzo[b] thiophene thianthrene 3.2 64 8.0 6 9.8 30 12.1 47 dibenzyl sulfide 2.0 47 4.9 6 2.4 35 4.4 4 1 8.1 7 10.7 17 13.6 36 6.6 dibenzyl disulfide 4.5 5 9.4 14 13.4 34 22.2 13 diphenyl disulfide 1.7 24 3.3 10 0.0 19 2.4 6 11.3 26 18.9 25 4.9 3 5.8 21 thioacetamide 1 6.5 7 10.5 43 2.2 20 3.3 29 5.0 thiophene-2-acetic acid 22 1 7.0 3 9.7 25 13.1 30 5.1 2.5 19 3.3 thiophene-3-carboxylic acid phenylmethanesulfonamide 8.2 28 10.0 37 16.1 3 19.1 32 sulfamic acid 8.9 41 11.5 31 18.0 28 10.2 72 16.9 15 22.7 13 sulfosalicyclic acid 11.4 38 14.9 2 20.9 9 26.5 26 29.2 26 CaS04 11.4 35 18.7 5 24.5 29 29.2 31 FeS04 11.4 26 15.2 4 18 29.1 37 17.3 6 23.2 9 25.6 K2S04 pyrite 1.5 94 8.6 3 11.9 3 pyrrhotite -0.5 7 6.8 32 12 54
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This does not rule out the presence of other forms of sulfur Table IV. Distances (8,)from the S u l f u r Atom to Its in the coals, since the peaks in question are 1-2 eV in Neighbor Shells i n Relevant Standard Compoundsa and a Variety of Coal SDecimensb width. It does seem to indicate that aromatic sulfur is dominant. It may be possible to make quantitative measample shell 1 shell 2 shell 3 surements of the amounts of aromatic and other forms of dibenzothiophene 1.74 2.71 4.02 sulfur by using electron yield XAFS spectroscopy, which benzo[b]thiophene 1.74 2.7 3.93 thianthrene‘ 1.76 2.72 exhibits negligible self-absorption effects. Such mea3.85 dibenzyl sulfide 1.75 2.75 3.9 surements will be carried out in future experiments. dibenzyl disulfide‘ 3.8 1.87 2.84 The broad, structured maximum a t 10-12 eV was ob4.02 PSOC 733 (e) 1.75 2.75 served for all coal samples examined. This feature is be4.02 1.74 2.74 PSOC 733 (v) lieved to have two origins. First, from the correlation 4.02 PSOC 1111 (e) 1.74 2.74 shown in Figure 2, it is evident that carbon or other atoms 1.74 PSOC 733 (i) 2.73 that are approximately 3.5-4.1 A from the sulfur can give 4.05 PSOC 1111 (e) 1.74 2.75 PSOC 1111 (i) 4.02 1.74 2.73 rise to resonant scattering peaks in this energy range. PSOC 1110 (e) 1.74 2.76 Undoubtedly, there are a number of atoms located at these PSOC 1110 (v) 1.74 2.75 sorts of distances from the sulfur atoms in the coal matrix. PSOC 1108 ( V I 1.74 2.75 These include both atoms in the same molecular layer as W. Ky. No. 9 (e) 1.78 2.74 the sulfur and atoms in neighboring molecular layers. It W. Ky. No. 9 (v) 1.75 2.74 is worth noting that this feature is a broad peak in the coal W. Ky. No. 9 (i) 1.75 2.82 W. Ky. No. 9 (v) 1.75 2.73 XANES, while the relevant standard compounds (Figure W. Ky. No. 9 71095 1.75 2.75 lb) exhibit a number of sharper peaks in the same energy W. Ky. No. 3 (v) 1.75 range. This probably reflects the fact that coal is an W. Ky. No. 5 (v) 2.77 1.75 amorphous structure with a distribution of interatomic W. Ky. No. 6 (i) 2.80 1.74 distances, while the standard compounds have ordered W. Ky. No. 9, run 6, biodesulfirized 2.75 1.75 structures with a finite set of discrete interatomic spacings. 2.88 Boghead Coal, Soxhlet extract 1.75 3.99 Bakerstown, NMP extract 1.70 2.69 3.95 Second, it is seen in Figure 1and Table I1 that the strong “white line” of both organic and inorganic standard comThe distances for standard compounds are from crystallopounds in which sulfur is bonded to oxygen occurs a t apgraphic data. b e , v, and i denote exinite, vitrinite, and inertinite proximately this location. Consequently, sulfur bound to separates prepared by DGC. Only carbon shells are shown. The distances to the nearest sulfur shell are 3.18 8, for thianthrene and oxygen may be partially responsible for this feature. 2.04 A for dibenzyl disulfide. However, s p transitions normally contribute sharp rather than broad peaks to the XANES. Furthermore, the 6. Figure 5 shows the RSFs of DBT and pyrite. The peaks current samples are believed to contain very little inorganic labeled C-1 to C-3 represent the contribution of the first, sulfate, and thiophenic structures are not expected to and third neighbor carbon shells to the RSF of oxidize significantly in the maceral separation p r o c e s ~ . ~ ~ ~second, J~ DBT. In the RSF of pyrite, the first major feature is the For these reasons, we believe that resonant photoelectron overlapped contribution of the nearest-neighbor S and Fe scattering makes the dominant contribution to the broad shells, while the peaks at larger distances arise primarily XANES peak at 10-12 eV. Nevertheless, it is possible that from Fe neighbor shells. a small amount of organic sulfones and sulfoxides may be Radial structure functions (RSFs) produced by Fourier present in the original source coal, and the feature in transformation of the EXAFS of maceral separates are question would include the s p transition peaks of such shown in Figure 6. It appears that two atomic shells, and sulfur. possibly three, can be resolved. Assuming that the atoms EXAFS Analysis. Typical radial structure functions surrounding the sulfur in coal are primarily carbons, a (RSF) obtained by Fourier transformation of the EXAFS back-transform a n a l y ~ i s ’ ~ofJ the ~ RSF peaks was carried region of the absorption spectra are shown in Figures 5 and out by using an empirical S-C phase shift determined from the EXAFS data for DBT. The interatomic distances (17) Wender, Irving. Unpublished research, personal communication. determined in this manner are summarized in Table IV.
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In general, they seem quite reasonable and are compatible with the distances to first, second, and third neighbor carbon atoms for aromatically bound sulfur (see inset, Figure 6). Sulfur to carbon distances for other sulfur compounds such as DBS are also close to the values observed for the coal samples. Again, the results indicate primarily aromatic sulfur but do not rule out the presence of other forms of sulfur. Preliminary attempts to determine coordination numbers for different neighbor shells indicate that more accurate EXAFS over a wider k range, possibly from low-temperature measurements, is desirable for conclusive results. Coal Derivatives. While our research has concentrated primarily on maceral separates, we have also investigated other types of specimens derived from coal that contain little or no inorganic sulfur. The data for these coal extracts8 and biodesulfurized coals' are included in Tables I1 and 111. Both the XANES and RSFs of the sulfur in these coal derivatives are qualitatively similar to those exhibited by the sulfur in maceral separates. It will be of interest in future studies to compare the structure of the organic sulfur in such coal derivatives to that in the parent coals. Future Work. Future studies will concentrate on clarifying remaining ambiguities in the interpretation of the XAFS data and on developing XAFS spectroscopy as a tool for monitoring the molecular structure of sulfur during coal desulfization or conversion processes. First, a larger suite of standard compounds will be investigated to develop better systematization of sulfur XAFS data. While correlations such as that shown in Figure 2 are encouraging, they are not unambiguous, and it is important to determine how general such relationships are for a wider range of structures and valences. Second, it may be possible t o obtain better resolution of spectral features by employing different types of XAFS measurements. Electron yield XAFS will give inherently better XANES line widths because of negligible self-absorption effects, while the possible directionality of certain bonds may be investigated by employing oriented XAFS measurements. Low-temperature measurements will be made on selected samples in order to obtain improved EXAFS with the goal of analyzing RSFs to determine coordination numbers as well as interatomic spacings. Finally, XAFS spectroscopy should be capable of determining the alterations in the molecular structure of sulfur produced by various coal desulfurization and conversion processes. Such studies will include not only the investigation of pretreated samples
Energy & Fuels, Vol. 3, No. 2, 1989 205
but also in situ XAFS measurements during pyrolysis, oxidation, and hydrogenation.
Summary and Conclusions The current results demonstrate that XAFS spectroscopy is an excellent method for direct, nondestructive investigation of the molecular structure of organic sulfur in coal and coal derivatives. The spectra obtained from a suite of maceral separates and coal-derived samples containing little or no inorganic sulfur were qualitatively similar. The XANES region of the spectra exhibited sharp peaks characteristic of s p transitions of a single, aromatically bound sulfur atom and a broad, structured maximum believed to arise primarily from photoelectron scattering by atoms 3.5-4.1 8, from the sulfur. The broad maximum would also contain the s p transitions of any sulfur atoms bonded to oxygen; however, oxidized forms of sulfur are believed to be relatively minor for the current samples. The RSFs derived from the EXAFS exhibited several peaks consistent wtih carbon first, second, and third nearest-neighbor shells of aromatic sulfur atoms. Future research will concentrate on eleminating remaining ambiguities in the interpretation of the XAFS spectra and on developing XAFS spectroscopy as a tool for monitoring alterations in the molecular structure of sulfur that occur during the coal desulfuization and conversion processes. -+
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Acknowledgment. This research was sponsored by the Department of Energy under DOE Contract No. DEFG22-86PC90520 and under DOE Contract No. DEFC22-86PC90017. We would also like to acknowledge the staff of the Stanford Synchrotron Radiation Laboratory, sponsored by DOE. The coal extracts were provided by Prof. John Zondlo of West Virginia University, and the biodesulfurized coal was provided by Prof. Dibakar Bhattacharyya of the University of Kentucky. Prof. J. E. Penner-Hahn and G. S. Waldo of the University of Michigan provided us with XAFS data for a number of critical standard compounds. Registry No. CaS04, 7778-18-9; FeS04, 7720-78-7; K2S04, 7778-80-5; benzo[ blthiophene, 95-15-8; thianthrene, 92-85-3; dibenzyl sulfide, 538-74-9; dibenzyl disulfide, 150-60-7; diphenyl disulfide, 882-33-7; thioacetamide, 62-55-5; thiophene-2-acetic acid, 1918-77-0;thiophene-3-carboxylic acid, 88-13-1; phenylmethane sulfonamide, 4563-33-1; sulfamic acid, 5329-14-6; eulfosalicyclic acid, 97-05-2; pyrite, 1309-36-0; pyrrhotite, 1310-50-5; dibenzothiophene, 132-65-0.
