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Energy & Fuels 1997, 11, 532-538
Speciation of the Organic Sulfur Forms in a Recent Sediment and Type I and II-S Kerogens by High-Pressure Temperature-Programmed Reduction S. D. Brown, O. Sirkecioglu,† and C. E. Snape* Department of Pure & Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, U.K.
T. I. Eglinton Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1543, Received November 8, 1996. Revised Manuscript Received March 17, 1997X
Temperature-programmed reduction (TPR) with a well-swept fixed-bed reactor operating at 15 MPa of hydrogen pressure has facilitated the speciation of the organic sulfur forms in coals and petroleum source rocks through complete hydrodesulfurization with typically 75-80% of the sulfur being released as hydrogen sulfide with the remaining 20-25% occurring as primarily thiophenic compounds in the oils obtained in high yield. In this study, the technique has been applied to kerogens isolated from a series of unconsolidated Peru margin sediments, and for comparison, to type I and II-S kerogens (Go¨ynu¨k oil shale and Monterey Fm, respectively) from immature organic-rich rocks. Visual resolution in the hydrogen sulfide TPR evolution profiles has been achieved between mono- and di/polysulfidic forms with the latter being reduced below 250 °C. Indeed, di/polysulfides account for high proportions of the reduced sulfur in surficial sediment samples collected from close to the top of the sediment. In contrast, monosulfides are the dominant sulfur form in the sample analyzed from deeper in the Peru margin sediment where thiophenes account for up to ca. 35% of the organic sulfur. This distribution is remarkably similar to those found for both the type I and II-S kerogens investigated, indicating that the major changes in organic sulfur forms occur during the early stages of diagenesis.
Introduction Organic sulfur is ubiquitous to all sedimentary organic matter and fossil fuels, and the importance of characterizing organically bound sulfur forms in these systems is well recognized.1 There is much evidence to suggest that sulfur plays an important role in the formation and thermal decomposition of kerogen,2,3 as well as the generation of oil.4,5 In particular, kerogens with a high organic sulfur content are thought to produce petroleum at a lower level of thermal stress than their low sulfur counterparts due to the lower bond dissociation energies of S-S and S-C bonds in comparison with C-C bonds.6 Organic sulfur moieties in recent sediments are formed during early diagenesis by the reactions of †
Present address: Technical University of Istanbul, Faculty of Science, Department of Chemistry, Maslak 80626, Istanbul, Turkey. * Corresponding author. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Sinninghe Dampste´, J. S.; de Leeuw, J. W. Org. Geochem. 1990, 16, 1077-1101. (2) Sinninghe Dampste´, J. S.; Eglinton, T. I.; de Leeuw, J. W.; Schenk, P. A. Geochim. Cosmochim. Acta 1989, 53, 873-889. (3) Sinninghe Dampste´, J. S.; Eglinton, T. I.; Rijpstra, W. I. C.; de Leeuw, J. W. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429; American Chemical Society: Washington DC, 1990; pp 486-528. (4) Gransch, J. A.; Posthuma, J. Advances in Organic Geochemistry 1993; Tissot, B., Biennet, F., Eds.; Editions Technip: Paris, 1974; pp 727-830. (5) Lewan, M. D. Philos. Trans. R. Soc. London 1985, A315, 123134. (6) Eglinton, T. I.; Sinninghe Dampste´, J. S.; Kohnen, M. E. L.; de Leeuw, J. W.; Larter, S. R.; Prentice, R. L. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429; American Chemical Society: Washington DC, 1990; pp 529-565.
S0887-0624(96)00202-2 CCC: $14.00
inorganic sulfur nucleophiles with organic matter.1,7 This process, termed “natural sulphurization”, occurs under conditions of high bacterial sulfate reduction and low reactive iron content. Sulfur nucleophiles (e.g., HSor Sx2-) are believed to add to alkenes or other functional groups initially forming thiols8 or organic polysulfides.9-14 These groups can then either undergo intramolecular addition resulting in cyclized groups such as thiolanes or form intermolecular sulfide bridges.15 Extensive cross-linking produces complex, sulfur-rich, and increasingly less soluble macromolecular matrices.3,16 After further diagenesis, cyclized alkyl sulfides may form thiophenes and subsequent maturation is thought to cause the cyclization and aromatization of alkyl side (7) Valisolalo, J.; Perkins, N.; Chappe, B.; Albrecht, P. Tetrahedron Lett. 1984, 25, 1183- 1186. (8) Vairavamurthy, A.; Mopper, K. Nature 1987, 329, 623-625. (9) Aizenshat, Z.; Stoler, A.; Cohen, Y.; Nielsen, H. Advances in Organic Geochemistry; J. Wiley: Chichester, U.K., 1983; pp 279-288. (10) LaLonde, R. T.; Ferrara, L. M.; Hayes, M. P. Org. Geochem. 1987, 11, 563-571. (11) LaLonde, R. T. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429, American Chemistry Society: Washington DC, 1990; pp 69-82. (12) Krein, E. B.; Aizenshat, Z. J. Org. Chem. 1993, 58, 6103-6108. (13) Kohnen, M. E. L.; Sinnighe Dampste´, J. S.; ten Haven, H. L.; de Leeuw, J. W. Nature 1989, 341, 640-641. (14) Aizenshat, Z.; Krein, E. B.; Vairavamurthy, A.; Goldstein, E. P. In Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, A., Schoonen, M. A. A., Eds.; ACS Symposium Series 612; American Chemical Society: Washington DC, 1995; pp 16-37. (15) Sinninghe Dampste´, J. S.; Rijpstra, W. I. C.; Kock-van Dalen, A. C.; de Leeuw, J. W.; Schenk, P. A. Geochim. Cosmochim. Acta 1989, 53, 1343-1355. (16) Kohnen, M. E. L.; Sinninghe Dampste´, J. S.; Kock-van Dalen, A. C.; de Leeuw, J. W. Geochim. Cosomochim. Acta 1991, 55, 13751394.
© 1997 American Chemical Society
Speciation of Organic Sulfur Forms
chains affording alkylbenzothiophenes and dibenzothiophenes which predominate in mature oils and bitumens.1,6 A number of strategies have been devised to determine the distribution of organic sulfur forms in coals, kerogens, and sediments. X-rays techniques have received much attention with X-ray photoelectron spectroscopy (XPS)17-20 and K-edge X-ray absorption near edge structure (XANES)18,19,21-23 being used extensively to distinguish between thiophenic or aromatic-bound and nonthiophenic or aliphatic-bound sulfur forms. Although these techniques rely heavily on curve-fitting procedures to interpret the data, because of the relatively small binding energy differences between individual sulfur forms, they have indicated consistently for coals that the proportion of thiophenic sulfur increases with rank. Gas chromatography (GC) on solvent extracts,24,25 analytical pyrolysis-GC,26 and selective chemolysis26 have all been applied to the investigation of organically bound sulfur in sediments. While each of these approaches is capable of giving a clear indication of the different types of sulfur species present at a molecular level, they do not provide quantitative inventory describing the overall distribution of the organic sulfur present. Temperature-programmed reduction (TPR) is based on the principle that different organic sulfur forms present in fossil fuels have different characteristic reduction temperatures at which they will be converted to H2S under a hydrogen atmosphere. Initial studies27-30 yielded poor sulfur balances, with virtually all the thiophenic sulfur remaining in the char due to the low hydrogen pressures used. Additionally, little account was taken of the reduction of pyrite to pyrrhotite and retrogressive reactions including the conversion of sulfides into thiophenes. These drawbacks have recently been overcome by using well-swept reactors operating with relatively high hydrogen pressures (up to 15 MPa). Typically, over 75% of the organic sulfur is reduced to H2S with the remainder being released as thiophenic compounds in the oils.31,32 Furthermore, catalysts such as sulfided molybdenum can be used to (17) Kelemen, S. R.; George, G. N.; Gorbaty M. L. Fuel 1990, 69, 939-944. (18) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansome, M. Energy Fuels 1991, 5, 93-97. (19) Gorbaty, M. L.; Kelemen, S. R.; George, G. N.; Kwiatek, P. J. Fuel 1992, 71, 1255- 1264. (20) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 1065-1067. (21) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 945-949. (22) Huffman, G. P.; Mirta, S.; Huggins, F. E.; Shah, N.; et al. Energy Fuels 1992, 5, 574- 581 (23) Taghiei, M. M;, Huggins, F. E.; Shah, N.; Huffman, G. P. Energy Fuels 1992, 5, 293- 300. (24) Eglinton, T. I.; Irvine, J. E.; Vairavamurthy, A.; Zhou, W.; Manowitz, B. Org. Geochem. 1994, 22, 781-799. (25) Sinninghe Dampste´, J. S.; ten Haven, H. L.; de Leeuw, J. W.; Schenk, P. A. Org. Geochem. 1986, 10, 791-805. (26) Sinninghe Dampste´, J. S.; de Leeuw, J. W.; Kock-van Dalen A. C.; de Zeeuw, M. A.; de Lange, F.; Rijpstra, W. I. C.; Schenk, P. A. Geochim. Cosmochim. Acta 1987, 51, 2369-2391. (27) Attar, A. In Analytical Methods for Coal and Coal Products; Academic Press: New York, 1979; Vol. III, Chapter 56 and DOE/PC/ 30145TI Technical Report. (28) Majchrowicz, B. B.; Yeperman, J.; Reggers, G.; Gelan, J. M.; Martens, H. H.; Mullens, J.; van Poucke, L. C. Fuel Process. Technol. 1987, 15, 363-376. (29) Majchrowicz, B. B.; Franco, D. V.; Yeperman, J.; Reggers, G.; Gelan, J. M.; Martens, H. H.; Mullens, J.; van Poucke, L. C. Fuel 1991, 70, 434-441. (30) Kumar, A.; Srivastava, S. K. Fuel 1992, 71, 718-719.
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ensure complete hydrodesulfurization. A range of solidphase models have been used as calibrants for the technique,33-35 and self-consistent results have been obtained for a suite of coals and kerogens.32 Although high-pressure TPR is a thermal technique which cannot clearly discriminate oxidized sulfur functionalities and may still suffer some interferences from retrogressive chemistry involving the nonthiophenic sulfur forms, visual resolution in the H2S evolution profiles has been achieved between thiophenes and sulfides. As for XPS and XANES, high-pressure TPR has also indicated for coals that the proportion of thiophenic sulfur increases with rank.36 In this study, high-pressure TPR is used to evaluate the organic sulfur species in a recent, unconsolidated marine sediment. A series of kerogens isolated from different depths of the sulfur-rich Peru margin sediment (considered a modern precursor of type II-S kerogen)24 have been analyzed to determine the distribution of organic sulfur forms within the sediment and the transformations that occur as diagenesis progresses. Since there is the possibility that a significant amount of nonthiophenic compounds may be present in the TPR oils from labile recent sediments, information on the distribution of sulfur in the oils has been obtained from XANES. This approach has been used here in preference to gas chromatography with either mass spectrometric or sulfur-specific detection because only a small proportion of the oils are amenable and problems would be caused by the thermal instability of sulfides. The high-pressure TPR results obtained from the Peru margin core samples are compared with those for a type I and a type II-S kerogen. Comparisons are also drawn with previously published XANES results for this series of sediments.24 Experimental Section Samples. The collection and storage of the Peru margin sediments and the established procedure for isolating the kerogens (initial solvent extraction followed by a two-stage acid digestion with HCl (10%) and HF (50%) and then a final solvent extraction) have been described elsewhere.24 Six kerogens from the Peru margin sediment covering a depth range of 0-80 m were studied. The organic-rich Go¨ynu¨k oil shale (Oligocene, lacustrine; Ro % av of 0.19; Rock-Eval HI of 768 mg of HC/g TOC) from northwest Turkey is classified as an organic-rich immature type I kerogen, lamalginite being the dominant maceral.37 The preparation of the Monterey Fm type II-S kerogen has been reported previously.24 Elemental data for the kerogen samples and Go¨ynu¨k oil shale are presented in Table 1. Removal of Pyrite. All kerogens were initially washed with 0.1 M HCl to ensure that, among others, calcium minerals (31) Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E. Fuel 1993, 72, 367-371. (32) Mitchell, S. C.; Snape, C. E.; Garcia, R.; Ismail, K.; Bartle, K. D. Fuel 1994, 73, 1159-1166. (33) Ismail, K.; Mitchell, S. C.; Brown, S. D.; Snape, C. E.; Buchannan, A. C., III; Britt, P. F.; Franco, D. V.; Maes, I. I.; Yeperman, J. Energy Fuels 1995, 9, 707-716. (34) Ismail, K.; Love, G. D.; Mitchell, S. C.; Brown, S. D.; Snape, C. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (2), 551557. (35) Ismail, K.; Brown, S. D.; Sirkecioglu, O.; Snape, C. E.; Franco, D. V.; Maes, I. I.; Yperman, J. Proceedings of the 8th International Conference on Coal Science; Elsevier Science: Amsterdam, 1995; Vol. I, pp 351-354. (36) Davidson, R. M. In Organic Sulphur in Coal; IEACR/60; IEA Coal Research: U.K., 1993. (37) Love G. D.; Snape, C. E.; Carr, A. D.; Houghton, R. C. Org. Geochem. 1995, 23 (10), 981-986.
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Table 1. Elemental Compositions (Dry Basis) of the Kerogen Samples Used in This Study kerogen
%C
%H
%S
%Sorga
H/C
Sorg/C
Peru, 0-0.02 m Peru, 0-0.68 ma Peru, 2.89 m Peru, 3.74 m Peru, 6.02 m Peru, 80.11 m Monterey Fma Go¨ynu¨k oil shaleb
49.5 40.6 51.2 13.4 41.6 43.2 57.9 56.1
6.4 4.8 5.9 2.1 5.0 4.9 6.1 7.6
2.4 1.9 7.4 8.9 15.6 16.0 6.6 3.3
2.9 1.7 5.6 2.4 10.4 10.7 ndc 2.4
1.48 1.41 1.38 1.88 1.45 1.49 1.24 1.63
0.017 0.015 0.041 0.067 0.094 0.092 nd 0.013
a After CrCl treatment for pyrite removalsall S 2 org determinations and C and H contents for 0-0.68 m sediment and Monterey b kerogen. Elemental composition of whole oil shale. c nd ) not determined.
had been removed which could absorb some of the H2S produced during TPR. Pyrite was removed by treating with CrCl2 according to the method of Acholla and Orr.38 Acidic Cr(II)Cl2 was prepared by passing a 2 M CrCl3/0.5 M HCl solution through an amalgamated Zn “Jones reduction” column. An ethanol/kerogen slurry was purged with nitrogen before the CrCl2 solution and concentrated HCl (2:1 v/v) were added dropwise. The mixture was refluxed for 2 h under nitrogen which was bubbled through a 3% AgNO3 trap. On cooling, the contents of the flask were filtered and the residue was rinsed with CH3OH/CH2Cl2 before drying in a vacuum oven at 60 °C. High-Pressure TPR. The operation of the high-pressure TPR system has been described in detail previously.25 Briefly, ca. 30-50 mg of finely ground kerogen was diluted in a 1:10 w/w mixture with acid-washed sand (250-75 µm) to increase the voidage in the fixed-bed. The samples were heated from ambient to 600 °C at a rate of 5 °C min-1 with the hydrogen volumetric flow rate of 5 dm3 min-1 measured at ambient conditions corresponding to a superficial gas velocity of approximately 0.5 m s-1 through the reactor. Liquid products were collected in an ice/water cooled trap and at the end of each run were recovered in dichloromethane (DCM). The H2S and other volatiles evolved from the reactor were detected using a VG Quadrupoles 0-300 amu mass spectrometer.25 The TPR oils were reduced in volume and adsorbed onto precombusted glass fibers for XANES analysis. The procedure for acquiring the XANES spectra and subsequent data analysis has been described elsewhere.24,39 In addition, small aliquots of the oils (80 50 55 35 30 30
