Energy & Fuels 2005, 19, 1971-1976
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Sulfur Speciation in Different Kerogens by XANES Spectroscopy Roger Wiltfong,† Sudipa Mitra-Kirtley,*,† Oliver C. Mullins,‡ Ballard Andrews,‡ Go Fujisawa,‡ and John W. Larsen§ Rose-Hulman Institute of Technology, 5500 Wabash Avenue, Terre Haute, Indiana 47803, Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877, and The Energy Institute, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 30, 2004. Revised Manuscript Received May 6, 2005
X-ray absorption near-edge structure (XANES) methodology has been employed to quantify the different sulfur structures present in three Type I and three Type II kerogens. Kerogens from the Green River (3), Bakken (1), Woodford (1), and Indiana limestone (1) formations were studied. Both aliphatic (sulfide) and aromatic (thiophene) forms of sulfur exist in all these kerogen samples. Except for Woodford, all of the kerogens contain oxidized functional groups. Sulfur in Types I and II kerogens mimics the carbon chemistry in that the sulfur structures are more aromatic in Type II than in Type I. It was impossible to differentiate elemental sulfur from pyrite in these samples by using K-edge XANES.
Introduction There are two reasons that the identity and quantity of sulfur functional groups in fuels are important. One reason is the undesirable products of their combustion. The other is that sulfur-sulfur and carbon-sulfur bonds are weak enough to cleave easily, thereby initiating low-temperature chemistry.1-3 The result of the weak bonds is the possible involvement of sulfur groups in kerogen maturation (petroleum formation). A complete characterization of sulfur groups is a necessary part of the chemical characterization of a kerogen that must precede attempts to understand kerogen chemistry, including maturation. For a discussion of sulfur incorporation into kerogens, see ref 3. The best way of identifying and quantifying sulfur functionalites in kerogens is XANES spectroscopy. Kerogen is the insoluble organic matter in source rock that generates petroleum upon sufficient heating for sufficient time. Bitumen, a highly viscous substance, is generally referred to as the soluble organic matter in rock. The maturity of a source rock describes the extent to which is has been heated, with concomitant diagenesis (alteration) or catagensis (dramatic chemical change with petroleum expulsion) of the kerogen. Good source rock prior to maturation contains roughly 10% or more organic carbon. Small quantities of bitumen will also * Corresponding author. Tel: 812 877 8253. Fax: 812 877 8023. E-mail:
[email protected]. † Rose-Hulman Institute of Technology. ‡ Schlumberger-Doll Research. § Pennsylvania State University. (1) Lewan, M. D. Nature 1998, 391, 164-166. (2) Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, M. A., Schoonen, M. A. A., Eds.; ACS Symposium Series 612; American Chemical Society: Washington, DC, 1995. (3) Eglinton, T.; Irvine, J. E.; Vairavamurthy, A.; Zhou, W.; Manowitz, B. Org. Geochem. 1994, 22, 781-799.
be present. This bitumen has a very different history than does bitumen in tar sands that corresponds to petroleum in shallow reserves where extensive biodegradation and loss of volatiles has occurred. Because many C-S bonds and all S-S bonds are weak, it has been suggested that sulfur functionalities play a key role in petroleum formation from kerogen.1,3 Shales and limestones are the two common source rocks. There is somewhat different chemistry associated with the two, in particular for sulfur species. For instance, it is known that petroleum sourced from shales tends not to possess H2S because the iron ubiquitously present in the shale reacts with any H2S to form pyrite. Limestones often lack iron, thus H2S is more readily contained in petroleum sourced from limestone. Understanding the chemistry of kerogen will require keeping track of the inorganic matter as well as the extent of maturation. X-ray methods have been useful for delineating the chemical structures of asphaltenes and coals. Sulfur moieties in these materials have been shown by XANES methods4-6 to consist of thiophene, sulfide, and sulfoxide. The sulfur functionality in the asphaltenes has been shown to be the same as that in the resin fraction and the oil fraction (after removal of asphaltene and resin).6 Thus, studying asphaltene structures can be generalized to the oil at times. Nitrogen XANES spectroscopy has shown that all of the nitrogen in asphaltenes is aromatic,7 with pyrrolic, followed by pyridinic, structures (4) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182-3186. (5) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53-57. (6) Mitra-Kirtley, S.; Mullins, O. C.; Ralston, C.; Sellis, D.; Pareis, C. Appl. Spectrosc. 1998, 52 (12), 1522-1525. (7) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; George, S.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115 (1), 252-258.
10.1021/ef049753n CCC: $30.25 © 2005 American Chemical Society Published on Web 07/26/2005
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dominating. Comparison of S and N XANES shows that the sulfur resonances can be treated as atomic-like, while the N resonances must be treated within a molecular orbital framework. Carbon X-ray Raman spectroscopy has recently been applied to polycyclic aromatic hydrocarbons (PAHs) and to asphaltenes.8 This work showed that the aromatic sextet-isolated double bond description of PAHs is accurate. Analysis of the asphaltene spectra showed that asphaltene PAH ring systems are largely pericondensed, most likely due to energetic considerations. XANES methods have been applied to a variety of other carbonaceous materials. Nitrogen XANES applied to kerogens showed that the nitrogen moieties in kerogens are dominated by pyrrolic nitrogen.9 The two other main nitrogen structures in these kerogens are pyridine and pyridone structures. Aromatic amines are also found in these samples. Nitrogen structures in the HF-treated kerogen do not differ significantly from HCltreated kerogen. Nitrogen XANES studies on bitumens show that pyrrole, pyridine, pyridone, and aromatic amine forms are present. Nitrogen studies on coals10,11 have revealed that pyrrole, pyridine, pyridone, and amine structures are common to all types of coals. As coals mature, pyridone forms are converted to pyridine. Nitrogen XANES on asphaltenes,7,11 which belong to a more mature stage than do kerogens, show no pyridone, confirming that with increased maturation pyridone is converted to pyridine. Sulfur XANES has been applied to coals12 and crude oil fractions, such as asphaltenes, resins, and oil fractions.6 Sulfur XANES applied to coal reveals that coals contain ample thiophene and sulfide functionality. Higher rank coals show an increased amount of aromatic sulfur than do the less mature coals. Pyrite is found in all the coals, and some sulfate is found only in low rank coals. Oxidized organic fractions have not been detected in the suite of coals studies. In contrast, sulfur XANES studies on crude oil fractions6 show that the major contribution is from thiophenic structures, followed by sulfide, and in one case, sulfoxide. Interestingly, coals are more oxidized than oils, but one crude oil asphaltene showed oxidized sulfur while coals do not. There are negligible sulfates in these samples. There have been several XANES studies of kerogens.3,13-15 All 11 of the Type I and II kerogens studied have been high-sulfur. The dominant sulfur forms are aliphatic sulfur and thiophenic. Disulfides were observed in some samples; as expected, these are thermally unstable. Oxidized forms of organic sulfur are rare. Because all of the kerogens studied to date have been high-sulfur, it is necessary to study some having (8) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Glatzel, P.; Fetzer, J.; Cramer, S. P. Chem. Phys. Lett. 2003, 369, 184-191. (9) Mitra-Kirtley, S.; Mullins, O. C.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128-1134. (10) Mullins, O. C.; Mitra-Kirtley, S.; van Elp, J.; Cramer, S. P. Appl. Spectrosc. 1993, 47 (8), 1268-1275. (11) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 72, 133-135. (12) Huffman, G. P.; Mitra-Kirtley, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5, 574-581. (13) Sarret, G.; Mongenot, T.; Connan, J.; Derenne, S.; Kasrai, M.; Bancroft, G. M.; Largeau, C. Org. Geochem. 2002, 33, 877-895. (14) Riboulleau, A.; Derenne, A.; Sarret, G.; Largeau, C.; Baudin, F.; Connan, J. Org. Geochem. 2000, 31, 1641-1661. (15) Olivella, M. A.; Palacios, Vairavamurthy, A.; del Rio, J. C.; de las Heras, F. X. C. Fuel 2002, 81, 405-411.
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more usual sulfur contents. There is no guarantee that the functional group distribution in the low-sulfur and high-sulfur kerogens will be the same. The response of the reactive sulfur functionalities to the mineral dissolution process also needs to be determined. We have used sulfur K-edge XANES to quantify the sulfur functionality of different kerogens. X-ray absorption data have been collected that provide information on the bulk samples. The major resonances in the sulfur K-edge spectra of kerogens match the energies of the 1s f 3p electronic transition resonances of some or all of the model structures: pyrite, elemental sulfur, sulfides, thiophenes, sulfoxides, sulfonates, and sulfates. These resonances therefore serve as fingerprints for identifying the different sulfur species present in kerogen samples. These different 1s f 3p resonances are overlaid on top of a preedge background and a higher energy step function, representing the electronic transitions to the continua. Each different chemical structure usually shows a single pronounced 1s f 3p resonance at a particular photon energy. With increasing sulfur oxidation number, the energy and cross section of these resonances increase. Experimental Section All the XANES studies were performed at National Synchrotron Light Source, Brookhaven National Laboratory, at X-ray beamline X-19A. This beamline is equipped with a double-crystal Si(111) crystal monochromator. High-vacuum conditions (about 10-10 Torr) were maintained along the beampipe from the front end to the experimental hutch. Fluorescence measurements were performed using a Lytle detector, with helium gas in both the sample and fluorescence chambers. The resolution of all the data collected is about 0.5 eV. A Pentium computer with CAMAC interface and NSLS Data Acquisition Software was used for data acquisition. The kerogens studied were Indiana (IN) limestone, Bakken, Woodford, Green River Shale 1 (GR-1), Green River Shale 2 (GR-2), and Green River Kerogen (GR-3). The GR samples are U.S. Department of Energy Reference Shales, which have been fully described by Owen.16 GR-1 was obtained from the Exxon Colony mine near Parachute, Colorado. The sample is part of the Mahogany zone, Parachute Creek member, Green River formation. GR-2 was obtained from an outcrop near Rock Springs, Wyoming, from the Tipton member of the Green River formation. GR-3 is from a different part of the Mahogany zone than the Exxon Colony mine. Mineral analyses of Green River Shales have been performed in the past and show a large percentage of carbonates.17 The IN sample was prepared from Indiana limestone and was obtained from Indiana Limestone Company. The Bakken and Woodford samples have been previously described.18,19 The Bakken and Woodford samples are Type II kerogens, and the Green River is Type I. To isolate the kerogen from GR1, GR2, and the Indiana limestone, the carbonates were dissolved in aqueous HCl. The resulting solids were washed with toluene. For GR3 and IN limestone, in addition, the silica was dissolved in aqueous HF following a standard procedure.20 The Bakken kerogens were isolated by (16) Owen, L. B. DOE Oil Shale Sample Bank, Quarterly Report July-September 1987, January-March, 1987, Salt Lake City, Utah, Terra-Tek, Inc. Report numbers: TR 87-89, and TR 88-14. (17) Siskin, M.; Brons, G.; Payack, J. F., Jr. Energy Fuels 1987, 1, 248-252. (18) Larsen, J. W.; Islas-Flores, C.; Aida, M. T.; Opaprakasit, P.; Painter, P. Energy Fuels 2005, 19, 145-151. (19) Zeszotarski, J. C.; Chromik, R. C.; Vinci, R. P.; Messmer, M. C.; Michels, R.; Larsen, J. W. Geochim. Cosmochim. Acta 2004, 68, 4113-4119. (20) Saxby, J. D. Chem. Geol. 1970, 6, 173-184.
Sulfur Speciation in Different Kerogens grinding the rock in a ball mill and then dissolving inorganic carbonates in aqueous HCl, followed by dissolving silicates in aqueous HF following a standard procedure. The Woodford shale was first pulverized, Soxhlet-extracted with chloroform, filtered, and dried. The sample was then treated with aqueous HCl and aqueous HF following the same standard procedure.20 The final products used in these experiments were powdery in nature, dark brown to black in color, and nonvolatile. The kerogen samples were ground up with mortar and pestle and smeared over wax paper, which was then positioned in the path of the X-ray beam in the sample chamber. The set of model compounds studied included pyrite, elemental sulfur, dibenzyl sulfide, dibenzothiophene, dibenzyl sulfoxide, 1,1′-bi-2-naphthol bis(trifluoromethanesulfonate), and ferrous sulfate. All of these model compounds were obtained from Aldrich and Alfa Aesar Chemical Co. The organic model compounds were diluted in organic solvents, and the inorganic model compounds were diluted in boron nitride, both to a sulfur concentration of 0.1 wt %. The prepared solutions were transferred to small sealed Mylar bags, which were mounted on the sample holder. The sample holders were made of poly(methyl methacrylate) and fit into the slots of the main sample chamber. All the spectra were calibrated using elemental sulfur 1s f 3p resonance at 2471.9 eV as the standard. After every beam dump, the standard was run with the new beam to monitor any shift in energy. A least-squares fitting routine has been employed to fit each spectrum. All the fitting procedures were performed using Windows-based WinXAS software.21 First, the background was subtracted from all spectra using a linear fit of pre- and postXANES contributions. The spectra were then normalized with respect to the individual step heights. Each spectrum was then fitted with a number of peaks and one or two arctangent steps. The peaks represent bound-to-bound electronic transitions, and the steps electronic transition to the continuum. PseudoVoigt profiles, consisting of part Gaussian and part Lorentzian, seemed to best fit the resonances. The model spectra were first fitted, and then the kerogen spectra were fitted using a sum of resonances, each resonance representing a particular sulfur model compound. Consistent energy position and width parameters for the model compounds were used for the fitting of all the kerogen spectra. The location of the ionization potential depends on the oxidation state of the sulfur atom. The unoxidized model spectra were fitted with one generalized arctangent step, and the oxidized models were fitted using a different arctangent step at a higher energy, somewhat similar to a previous study.12 Ideally, each different sulfur structure should have its own unique arctangent step function; however, it was found that two arctangent steps were sufficient for satisfactory fits of all the kerogen samples where the higher-energy oxidized peaks are clearly prominent. Different relative percentages of the various sulfur structures were then calculated. If a particular model spectrum had secondary contributions at signature energies of other model compounds, care was taken to subtract the secondary peak contribution before calculating the contribution from the other model structures. This methodology of identifying different quantities of sulfur structures present in complex systems has been exploited in many complex systems in the past.6,7,9-12 In some of the kerogen spectra, the fitting procedure included three peaks within the first broad resonance. In such cases, the widths and the energies of the three peaks were held constant. In these cases, there is some unavoidable overlap between the three resonances. The uncertainty in the determination of the concentration of sulfur forms is estimated to about 10% on the basis of sensitivity studies and earlier work with this technique.12 In some kerogen spectra, a preedge (21) WinXAS software, Ressler, T., Fritz-Haber-Institut der MPG, Department of Inorganic Chemistry, Faradayweg, Berlin, Germany.
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Figure 1. Sulfur K-edge XANES spectra of several Type I and Type II kerogens. feature is clearly visible, indicating the presence of an extra resonance. The position and width of the first arctangent step were preset to the same value of the corresponding step in the unoxidized model spectra. The same was done for the second step using the values from the fits of the oxidized model samples.
Results and Discussion Figure 1 shows the sulfur K-edge XANES spectra of six kerogen samples. Numerous variations in the intensities of the spectral features between the different samples are evident, even between the Green River Shale samples, which belong to the same parent formation. The spectrum of GR-3 shows a broad resonance at ∼2473 eV, probably due to a superposition of more than one resonance. The same spectrum shows a strong feature at higher energy (∼2482 eV). shows the sulfur XANES spectra of pyrite, elemental sulfur, organic sulfide, thiophene, sulfoxide, sulfonate, and sulfate. As the formal oxidation number of sulfur increases, the major resonance of the XANES spectrum shifts to higher photon energies. This correspondence between formal oxidation number of the element and the 1s f 3p transition energy has been observed before.5,6,12 The same sulfur functional group in different model compounds has been studied,6,12 and it is found that the 1s f 3p transition energy of a specific sulfur functional group is fairly insensitive to changes in molecular structure. A simple atomic explanation works very well. The major resonance in the spectrum of elemental sulfur is broader than in the other model compounds, as seen in previous studies.5 An example of the deconvolution of a kerogen XANES spectrum is shown in Figure 3. It shows the leastsquares fit of GR-3 kerogen using six model compound spectra and two arctangent steps. The resonances correspond to those of elemental sulfur, sulfide, thiophene, sulfoxide, sulfonate, and sulfate. The first generalized step corresponds to the reduced sulfur forms, and the second corresponds to oxidized sulfur forms. The
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Figure 2. Sulfur K-edge spectra of several model compounds.
widths and the positions of the resonances and the steps were held fixed for all the kerogen spectra. The intensity of the elemental sulfur or pyrite peak in this sample is significant. The proximity of this peak to the sulfide and the thiophene peaks causes a larger than usual error in the relative abundances of these sulfur structures. Figure 4 shows the least-squares fit of the GR-3 spectrum using pyrite in place of elemental sulfur. The peak positions of elemental sulfur and pyrite differ by
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only 0.5 eV, making it difficult to differentiate between the two. The earlier studies of high-sulfur kerogens did not report an examination of the possibility that there might be elemental sulfur in their samples.3,13-15 Elemental sulfur is present in many coals and is thought to originate from pyrite oxidation.22 In kerogen, it could form by oxidation of pyrite or by either thermochemical reduction or biological reduction of sulfate.2 Because the spectra are fit slightly better by using elemental sulfur than by using pyrite, we are reporting elemental sulfur contents. This is a matter of convenience. The sample could contain elemental sulfur, pyrite, or a mixture of the two. These possibilities cannot be distinguished by using K-edge XANES. In Table 1 are listed the energies of the major resonances in the XANES spectra of the sulfur model compounds and of the kerogens. The area under each separate resonance is shown within parentheses. These areas are obtained from least-squares fit of the spectra using the normalized step functions. All the kerogen samples contain elemental sulfur and/or pyrite, especially IN limestone and GR-3. Thiophene is universally present. Sulfide is present in all of the samples except in IN limestone. All the samples contain sulfoxide and sulfate. Only the Green River samples contain sulfate. The data in Table 1 reveal systematic differences between the functional groups in Type I and Type II kerogens. The outstanding difference is the much greater relative concentration of thiophenes in the Type II kerogens. This is expected because Type II kerogens are more aromatic than Type I.23 The differences in sulfur functionality parallel the differences in carbon functionality. The earlier XANES study of a Kim-
Figure 3. Least-squares fit of the GR-3 spectrum. The resonances correspond to the following sulfur structures in order of increasing photon energy: elemental sulfur, organic sulfide, thiophene, sulfoxide, sulfonate, and sulfate. The shoulder on the 2473 eV resonance clearly shows the presence of elemental sulfur in this sample.
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Figure 4. Least-squares fit of the GR-3 spectrum using pyrite in place of elemental sulfur. Table 1. Energies and Relative Areas of the Principal Peaks in the Model Compounds and in the Kerogens models - energy in eV (relative areas under resonances) kerogens IN limestone Bakken Woodford GR-1 GR-2 GR-3
sulfur
sulfide
thiophene
sulfoxide
sulfonate
sulfate
2471.9 (0.58)
2472.5 (0.50)
2473.1 (0.03)
2475.3 (0.09)
2478.7 (0.21)
2481.7 (0.20)
2471.8 (0.030) (0.010) (0.016) (0.011) (0.010) (0.027)
2472.5 (0.000) (0.008) (0.005) (0.018) (0.014) (0.001)
2473.1 (0.022) (0.029) (0.026) (0.021) (0.016) (0.009)
2475.3 (0.031) (0.021) (0.022) (0.047) (0.041) (0.023)
2479.0 (0.000) (0.000) (0.000) (0.030) (0.008) (0.026)
2481.7 (0.0290) (0.026) (0.000) (0.035) (0.023) (0.057)
Table 2. Relative Sulfur Abundances (in %) in the Kerogens kerogen (Type)
sulfur
sulfide
thiophene
sulfoxide
sulfonate
sulfate
IN limestone (II) Bakken (II) Woodford (II) GR-1 (I) GR-2 (I) GR-3 (I)
29 11 19 9 14 31
2 8 9 22 23 4
59 70 71 39 39 22
2 2 0 8 12 7
9 0 0 9 3 11
