Thermal Transformations of Nitrogen and Sulfur Forms in Peat Related

Energy & Fuels 2006, 20, 635-652

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Thermal Transformations of Nitrogen and Sulfur Forms in Peat Related to Coalification S. R. Kelemen,* M. Afeworki, M. L. Gorbaty, P. J. Kwiatek, M. Sansone, and C. C. Walters ExxonMobil Research and Engineering Co., Annandale, New Jersey 08801

A. D. Cohen Department of Geological Sciences, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed September 23, 2005. ReVised Manuscript ReceiVed January 9, 2006

The chemical pathways for nitrogen and sulfur transformations during coalification are elucidated by comparing the chemical forms of unaltered peats, lignites, and coals and pyrolyzed peats. Nitrogen forms are characterized by a combination of X-ray photoelectron spectroscopy (XPS) and 15N nuclear magnetic resonance (NMR). In unaltered peats, the 15N NMR and XPS nitrogen (1s) spectra are consistent with the presence of amide nitrogen. When peat is pyrolyzed, the main peak in the 15N NMR spectrum broadens and shifts from -260 ppm to -245 ppm, which is consistent with the loss of some amide nitrogen and the appearance of pyrrolic nitrogen forms. The pyrolyzed peat shows a new XPS peak that appears at 398.6 eV, which is characteristic of pyridinic nitrogen. These results indicate that a thermal transformation of amide nitrogen into pyrrolic and pyridinic forms occurs after thermal stress that is roughly equivalent to lignitification. High total nitrogen levels are found in pyrolyzed peats relative to lignites and higher-rank coals, suggesting that some amides initially found in peat are lost via nonthermal pathways during coalification. Lignites contain the highest levels of quaternary nitrogen, and they are associated with protonated pyridinic structures. The relatively low levels of quaternary nitrogen in pyrolyzed peats in the presence of pyridinic nitrogen indicates that there are fewer acidic sites in pyrolyzed peats relative to lignites. Hence, most quaternary nitrogen is formed during lignitification as a result of the creation and interaction of basic nitrogen species with acidic functionalities and is lost completely during bitumenization. Sulfur X-ray absorption near-edge structure spectroscopy (SXANES) of unaltered peats detect the presence of disulfide, mercapto, aliphatic sulfide, and aromatic forms of organically bound sulfur. The level of organic sulfur in pyrolyzed peats is comparable to that in lignites and higher-rank coals, suggesting that much of the organic sulfur in coals is derived from sulfur species incorporated during peatification. XPS and S-XANES results show that the relative level of aromatic sulfur increases as the severity of peat pyrolysis increases. The relative level of aromatic sulfur increases through the selective loss of disulfide, aliphatic sulfide, and SO3 groups and through the transformation of aliphatic sulfur forms. Aliphatic sulfur is present mostly as mercapto and disulfide species in peats pyrolyzed to an equivalence vitrinite reflectance of Ro ) 0.5 and in lignites but not in higher-rank coals. These results indicate that mercapto and disulfide species are lost after lignitification. Organic sulfur in peats pyrolyzed to Ro ) 1.0 exist mainly as aromatic forms, consistent with the level of aromatic sulfur increasing with the increasing degree of coalification.

Introduction coal,1-3

Peat, which is the precursor to is a sedimentary deposit composed primarily of lignin and cellulosic materials derived from high land plants.4-12 Peat does not have a defined molecular structure, and its chemical composition is dependent * Author to whom correspondence should be addressed. Tel.: 908 730 2389. Fax: 908 730 3232. (1) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (2) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd Edition; Springer-Verlag: Berlin, 1984. (3) Durand, B. E. Kerogen; Editions Technip: Paris, 1980. (4) Alder, E. Lignin ChemistrysPast Present and Future. Wood Sci. Technol. 1977, 11, 69. (5) Hatcher, P. G. Chemical Structure Models for Coalified Wood (Vitrinite) in Low Rank Coal. Org. Geochem. 1990, 16, 959. (6) Hatcher, P. G.; Clifford, D. J. Org. Geochem. 1997, 27, 251. (7) Hatcher, P. G.; Lerch, H. E.; Verheyen, T. V. Int. J. Coal Geol. 1989, 13, 65. (8) Hatcher, P. G.; Wilson, M. A.; Vassallo, A. M.; Lerch, H. E. Int. J. Coal Geol. 1989, 13, 99.

on the nature of its plant progenitors and depositional setting. Complex biological, chemical, and thermal processes transform peat into lignite and then into higher-rank coal during diagenesis and catagenesis.5-11,13-22 (9) Hatcher, P. G.; Lerch, H. E.; Bates, A. L.; Verheyen, T. V. Org. Geochem. 1989, 14, 145. (10) Hatcher, P. G. Energy Fuels 1988, 2, 48. (11) Bates, A. L.; Hatcher, P. G. Org. Geochem. 1989, 14, 609. (12) Given, P. H.; Spackman, W.; Painter, P. C.; Rhoads, C. A.; Ryan, N. J.; Alemany, L.; Pugmire, R. J. Org. Geochem. 1984, 6, 399. (13) Rollins, M. S.; Cohen, A. D.; Bailey, A. M.; Durig, J. A. Org. Geochem. 1991, 17, 451. (14) Cohen, A. D.; Bailey, A. M. Int. J. Coal Geol. 1997, 34, 163. (15) Cohen, A. D.; Spackman, W.; Dolsen, P. Int. J. Coal. Geol. 1984, 4, 73. (16) Orem, W. H.; Neuzil, S. G.; Lerch, H. E.; Cecil, C. B. Org. Geochem. 1996, 24, 111. (17) Shearer, J. C.; Moore, T. A. Org. Geochem. 1996, 24, 127. (18) Stout, S. A.; Boon, J. J.; Spackman, W. Geochim. Cosmochim. Acta 1988, 52, 405. (19) Behar, F.; Hatcher, P. G. Energy Fuels 1995, 9, 984.

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636 Energy & Fuels, Vol. 20, No. 2, 2006

Much is known about the transformations of carbon and oxygen that occur during the conversion of peat to coal, and the main pathways have been identified by studying natural peat and coal materials at different stages of coalification. It is wellestablished that lignin-derived material selectively survive diagenesis.4-11,23,24 Lignitification of peat involves air oxidation, followed by decarboxylation and dehydration.1 The bituminization of lignite involves decarboxylation and hydrogen disproportionation.1 Laboratory studies that subject peat and lignite to simulated coalification environments also provided significant insights into the details of carbon and oxygen chemistry and accompanying petrographic changes.13,14,16-18,25-27 The hydrogento-carbon (H/C) ratio of pyrolyzed peats is higher than that of lignites and other coals at comparable levels of aromatic carbon or oxygen-to-carbon (O/C) ratio,26 indicating that there are significant differences in the carbon structural framework of these materials. The changes in carboxyl, carbonyl, phenoxy, and phenolic oxygen upon the pyrolysis of peat are very similar to the progression observed in lignites and coals at comparable levels of aromatic carbon, indicating that thermal decarboxylation/decarbonylation and demethylation pathways exist for peats26 and that similar pathways likely occur during natural coalification. Most organic nitrogen is derived from living organisms; however, decomposition processes have hampered analytical attempts to trace nitrogen in lignites and coals back to their biological origins.28-38 Furthermore, the processes that govern the initial diagenetic transformations of nitrogen forms are not well-understood. On a geological time scale, much of the nitrogen in deposited biomass is lost quickly via microbial processes. Amide groups in proteins and peptides are abundant initial sources of nitrogen but are thought to be susceptible to rapid deamination.28,29,31,34,40,41 However, numerous studies have shown that amide forms of nitrogen are present in recently (20) Tegelaar, E. W.; Leeuw, J. W.; Derenne, S.; Largeau, C. Geochim. Cosmochim. Acta 1989, 53, 3103. (21) de Leeuw, J. W.; Largeau, C. In Organic Geochemistry. Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993; pp 23-72. (22) Hatcher, P. G.; Wenzel, K. A.; Cody, G. D. Coalification Reactions of Vitrinite Derived from Coalified Wood: Transformations to Rank of Bituminous Coal. In Vitrinite Reflectance as a Maturity Parameter: Applications and Limitations; Mukhopadhyay, P. K., Dow, W. G., Eds.; ACS Symposium Series 570; American Chemical Society: Washington, DC, 1994; p 112. (23) Zang, X.; Hatcher, P. G. Org. Geochem. 2002, 33, 201. (24) Derenne, S.; Largeau, C. Soil Sci. 2001, 166, 833. (25) Freitas, J. C. C.; Bonagamba, T. J.; Emmerich, F. G. Energy Fuels 1999, 13, 53. (26) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Cohen, A. D. Energy Fuels 2002, 16, 1450. (27) Venkatesan, M. I.; Ohta, K.; Stout, S. A.; Steinberg, S.; Oudin, J. L. Org. Geochem. 1993, 20, 463. (28) Knicker, H.; Frund, R.; Ludemann, H. D. Naturwissenschaften 1993, 80, 219. (29) Knicker, H.; Almendros, G.; Gonzalez-Vila, F. J.; Ludemann, H. D.; Martin, F. Org. Geochem. 1995, 23, 1023. (30) Knicker, H.; Hatcher, P. G. Naturweissenschaften 1997, 84, 231. (31) Knicker, H.; Scaroni, A. W.; Hatcher, P. G. Org. Geochem. 1996, 24, 661. (32) Knicker, H. J. EnViron. Qual. 2000, 29, 715. (33) Knicker, H.; Schmidt, M. W. I.; Kogel-Knabner, I. Soil Biol. Biochem. 2000, 32, 241. (34) Baxby, M.; Patience, R. L.; Bartle, K. D. J. Pet. Geol. 1994, 17, 211. (35) Knicker, H.; Ko¨gel-Knabner, I. Soil Organic Nitrogen Formation Examined by Means of NMR Spectroscopy. In Nitrogen-Containing Macromolecules in the Bio- and Geosphere; Stankiewicz, B. A., van Bergen, P. F., Eds.; ACS Symposium Series 707; American Chemical Society: Washington, DC, 1998; p 339. (36) Knicker, H.; Hatcher, P. G. Org. Geochem. 2001, 32, 733. (37) Almendros, G.; Knicker, H.; Gonzalez-Vila, F. J. Org. Geochem. 2003, 34, 1559. (38) Knicker, H. Mar. Chem. 2004, 92, 167.

Kelemen et al.

deposited organic matter and peats.29-33,35-38 Generally, these studies have demonstrated that some chemical environments that contain amide groups, once created, are resistant to microbial degradation and survive through early diagenesis. Recent advances in 15N nuclear magnetic resonance (15N NMR),28,29,31,32,36-47 X-ray photoelectron spectroscopy (XPS),45,48-63 and nitrogen X-ray absorption near-edge structure Spectroscopy (N-XANES)64-67 have provided new tools to probe the chemical structure of nitrogen forms in complex solid and nonvolatile carbonaceous systems. Nitrogen in immature kerogens and oils are pyridinic and pyrrolic in nature.31,34 For coal, the distribution of nitrogen forms is dependent on rank. In low-rank coals, pyrrolic species are the most abundant forms, followed by pyridinic and quaternary nitrogen forms.48-52,57-62 Quarternary nitrogens result from pyridinic species interacting with acidic protons56 that were likely formed by oxidative processes during lignitification. Highrank coals are dominated by pyrrolic forms and an increasing content of pyridinic forms. The predominance of pyrrolic and pyridinic nitrogen forms in sedimentary organic material at all catagenetic stages and in generated oils suggests that there may be a common overriding pathway that focuses initially preserved nitrogen forms in the direction of heterocyclic nitrogen species. Lignites are obviously important for understanding the geochemical evolution of nitrogen forms, but they have not been studied as extensively as higher-rank coals. (39) Knicker, H.; Kogel-Knabner, I. Sym. Ser. #570 Vitrinite Reflectance as a Maturity Parameter 1994, 339. (40) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 1995, 9, 999. (41) Knicker, H.; Ludemann, H. D. Org. Geochem. 1995, 23, 329. (42) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. Int. J. Coal Geol. 1996, 32, 255. (43) Solum, M. S.; Pugmire, R. J.; Grant, D. M.; Kelemen, S. R.; Gorbaty, M. L.; Wind, R. A. Energy Fuels 1997, 11, 493. (44) Zang, X.; Nguyen, R. T.; Harvey, H. R.; Knicker, H.; Hatcher, P. G. Geochim. Cosmochim. Acta 2001, 19, 3299. (45) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Kwiatek, P. J.; Solum, M. S.; Hu, J. Z.; Pugmire, R. J. Energy Fuels 2002, 16, 1507. (46) McCarthy, M.; Pratum, T.; Hedges, J.; Benner, R. Nature 1997, 390, 150. (47) Knicker, H.; Hatcher, P. G.; Gonzalez-Vila, F. J. J. EnViron. Qual. 2002, 31, 444. (48) Jones, R. B.; McCourt, C. B.; Swift, P. Proc. Int. Conf. Coal Sci. 1981, 657. (49) Perry, D. L.; Grint, A. Fuel 1983, 62, 1029. (50) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Process. Technol. 1987, 15, 351. (51) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1450. (52) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100. (53) Patience, R. L.; Baxby, M.; Bartle, K. D.; Perry, D. L.; Rees, A. G. W.; Rowland, S. J. Org. Geochem. 1992, 18, 161. (54) Wilhelms, A.; Patience, R. L.; Larter, S. R.; Jorgensen, S. Geochim. Acta 1992, 56, 3745. (55) Pels, J. R.; Wojtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72, 373. (56) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 4, 897. (57) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (58) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1995, 74, 507. (59) Buckley, A. N. Fuel Process. Technol. 1994, 38, 165. (60) Buckley, A. N.; Kelly, M. D.; Nelson, P. F.; Kenneth, W. R. Fuel Process. Technol. 1995, 43, 47. (61) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J., Fletcher, T. H.; Watt, M.; Solum, M. S.; Pugmire, R. J. Energy Fuels 1998, 12, 159. (62) Kelemen, S. R.; Freund, H.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1999, 13, 529. (63) Clark, D. T.; Wilson, R. Fuel 1983, 62, 1034. (64) Vairavamurthy, A.; Wang, S. EnViron. Sci. Technol. 2002, 36, 3050. (65) Kirtley, S. M.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 74, 133. (66) Kirtley, S. M.; Mullins, O. C.; Branthaver, J.; Cramer, S. P. Energy Fuels 1993, 7, 1128. (67) Mullins, O. C.; Kirtley, S. M.; van Elp, J.; Cramer, S. P. Appl. Spectrosc. 1993, 47, 1268.

Thermal Transformations of N and S Forms in Peat

Recent advances in XPS67-74 and sulfur X-ray absorption near-edge structure spectroscopy (S-XANES)71,72,75-80 methods offer a way to investigate sulfur forms in complex carbonaceous materials (such as peats, lignites, and pyrolyzed peats) directly. In contrast with nitrogen, although some organic sulfur is present in initially deposited organic matter, most of the sulfur is incorporated into organic matter during diagenesis. Hydrogen sulfide produced by bacterial sulfate reducers reacts either with iron to form pyrite or with organic matter to form organic sulfur species. It is well-established that aliphatic and aromatic sulfur forms are present in coals and that the relative amount of aromatic sulfur increases as the coal rank increases.75-77 Lignites have not been studied as extensively as higher-rank coals, and the sulfur forms in peats are largely unstudied. Most information concerning the nature of sulfur forms in peats comes from indirect evidence.15,81 Our understanding of the main chemical reaction pathways for nitrogen and sulfur transformations during coalification is incomplete, when viewed relative to carbon and oxygen. A quantitative study is needed to determine which nitrogen and sulfur functional groups are preserved, which are added, and which are transformed during the natural coalification process. In the present study, the nitrogen and sulfur forms in peats and pyrolyzed peats are compared to those in lignites and higherrank coals, using a combination of direct characterization methods. XPS and solid-state 15N NMR are used for nitrogen. XPS and S-XANES are used for sulfur. Using this approach, it is possible to infer the thermal chemistry pathways that are active during coalification. Moreover, it is possible to identify areas where nonthermal chemical pathways must be postulated for organic nitrogen and sulfur during coalification by reconciling observations of peats and pyrolyzed peats with lignites and higher-rank coals. Experimental Section The peat samples were obtained from the University of South Carolina’s peat sample bank.82 Elemental data for the peat and pyrolyzed peat samples have been published elsewhere.82 The peats utilized in this study were selected to represent a variety of different compositions and depositional settings.82 Lignite samples were obtained from the Pennsylvania State University coal sample bank83 and are identified by their DECS (Department of Energy) and PSOC (Pennsylvania State coal) numbers. Argonne Premium (AP) coals (68) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Energy Fuels 1991, 5, 720. (69) Kelemen, S. R.; Freund, H. Energy Fuels 1990, 4, 165. (70) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939. (71) Kelemen, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991, 70, 396. (72) Gorbaty, M. L.; Kelemen, S. R.; George, G. N.; Kwiatek, P. J. Fuel 1992, 71, 1255. (73) Kelemen, S. R.; Vaugn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 5. (74) Kelemen, S. R.; Freund, H. Energy Fuels 1989, 3, 498. (75) George, G. N,; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93. (76) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, F.; Lu, F. Energy Fuels 1991, 5, 574. (77) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182. (78) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53. (79) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen, J. M. Fuel 1992, 71, 649. (80) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 945. (81) Casagrande, D. J.; Siefert, K.; Berschinski, C.; Sutton, N. Geochem. Cosmochim. Acta 1977, 41, 161. (82) Cohen, A. D.; Rollins, M. S.; Durig, J. R.; Raymond, R. J. Coal Qual. 1991, 10, 145. (83) Glick, D. C.; Davis, A. Org. Geochem. 1991, 17, 421.

Energy & Fuels, Vol. 20, No. 2, 2006 637 Table 1. Time and Temperature Conditions for Pyrolysis and Calculated Equivalent Vitrinite Reflectance (Ro) time

temperature (°C)

calculated Ro

7 days 5 min 5 min 5 min 75 min 24 h

175 350 375 400 400 400

0.3 0.5 0.6 0.7 1.0 1.5

were obtained in sealed ampules from the AP coal Sample Program.84 The peat, lignite, and coal samples were homogenized and ground into fine powders, using a Wig-L-Bug, prior to analysis. Pyrolysis was performed in a quartz tube inside a furnace. The samples were evacuated to remove moisture and pyrolyzed in helium at 1 bar. Details of the reactor system appear elsewhere.69 Pyrolysis was conducted using well-defined time and temperature conditions, which are listed in Table 1. These conditions were related to an equivalent vitrinite reflectance (Ro) using the EasyRo method.85 This method provides a estimate of thermal maturity for laboratory-pyrolyzed samples based on a kinetic model of vitrinite maturation.85 XPS results were obtained with a Kratos Axis Ultra system, using monochromatic Al KR radiation. A general description of the XPS curve resolution methods for nitrogen appears elsewhere.56,61 The use of monochromatic X-rays and a unique Kratos charge compensation system results in better energy resolution than could be achieved in our previous studies. Therefore, it is necessary to reduce the full width at half-maximum (fwhm) by ∼0.3 eV in the curve resolution of the nitrogen (1s) and sulfur (2p) spectra relative to prior studies.56,61 An energy correction was made to account for sample charging based on the carbon (1s) peak at 284.8 eV. The elemental concentrations are reported relative to carbon, calculated from XPS spectra, based on the area of the characteristic photoelectron peaks after correction for atomic sensitivity. Solid-state 15N cross-polarization magic angle spinning (CPMAS) NMR spectra were recorded using a Chemagnetics CMXII-200 spectrometer that was operating at a static magnetic field of 4.7 T (20.3 MHz, 15N). Samples were packed into a 5- or 9.5-mmdiameter zirconia rotor and spun at a frequency of 3.5 kHz. The CPMAS NMR experiments were performed at a 1H-15N crosspolarization (CP) contact time of 1 ms and a pulse repetition delay of 1 s. The 1H and 13C radio-frequency (rf) fields for CP were observed at 40 kHz, and proton high-power decoupling during data acquisition was observed at 70 kHz. The spectra are reported on a nitromethane chemical-shift scale. As in the case of XPS, it is not possible to uniquely associate individual peaks in the 15N NMR spectrum with specific chemical forms. In many cases, the peaks are relatively broad and the potential for overlap exists. Based on model compound data, different forms of nitrogen are expected to appear in different chemical-shift ranges. Pyridinic nitrogen is expected from 0 ppm to -100 ppm. Quaternary nitrogen and unsubstituted pyrrole are expected from -120 ppm to -235 ppm. Amides and pyrrolic forms appear from -235 ppm to 300 ppm, and amines are found from -300 ppm to -400 ppm. The S-XANES spectra were obtained at beamline 6-2 of the Stanford Synchrotron Radiation Laboratory (SSRL), using techniques and protocols developed previously for speciation and quantification of organically bound sulfur forms in coals.68,75 The S-XANES analysis approach starts by obtaining a sulfur X-ray absorbance spectrum of the sample, taking the third derivative of that spectrum, and curve-resolving the third-derivative spectrum using the third-derivative spectra of six model compounds with a nonlinear least-squares regression program. The values are expressed in terms of the mole percentage of total sulfur in the sample. For all of the peat samples, the S-XANES third-derivative spectra (84) (a) Vorres, K. S., Ed. The Users Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, IL, 1989; Paper No. ANL-PCSP-89-1. (b) Vorres, K. S. Energy Fuels 1990, 4, 420. (85) Sweeney, J. J.; Burnham, A. K. Bull. Am. Assoc. Pet. Geol. 1990, 74, 1559.


Kelemen et al.

Results

Figure 1. Comparison of the X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance (NMR) results for aromatic carbon of unaltered peats, pyrolyzed peats, lignites, and higher-rank coal.

could be fit using six model components: cysteine, cystine, aliphatic sulfide, aromatic sulfur, organic sulfite, and sulfate. Experiments were performed to determine the precision of the repeatability of fitting the third-derivative spectrum. The repeatability was better than (1 mol %. It is not possible to determine how accurately the spectra of these model compounds reflect the analogous local chemical environments found in peat, lignite, and pyrolyzed peat. For peats, species resembling cysteine and cystine are a major part of the non-oxidized sulfur distribution. Cysteine is a simple mercapto-amino acid (HSCH2CH(NH2)CO2H). Cystine is the disulfide form of the simple amino acid (HO2C(NH2)CHCH2SSCH2CH(NH2)CO2H).

XPS and 13C CPMAS NMR have been used to quantify the percentage of aromatic carbon in pyrolyzed peats. Figure 1 shows a comparison of XPS and 13C CPMAS NMR results for pyrolyzed peat samples for which there are available data. These results are plotted together with other data from common samples of peats, lignites, and higher-rank coals.26,86 There is generally good agreement between the percentage of aromatic carbon determined by XPS and 13C NMR. There is a tendency for the 13C NMR aromaticity values to be systematically higher than the XPS values. This tendency is outside of the range of experimental variability and has been reported previously for other complex carbonaceous materials.26 The linear relationship between 13C NMR and XPS data indicates that there is good precision for both methods, which rely on different underlying assumptions.86 These findings indicate that the carbon forms at the surfaces (determined by XPS) of these finely ground peat and pyrolyzed peat samples are close to those in the bulk composition (determined by NMR). Pyrolyzed peats have significantly higher levels of aromatic carbon than the unaltered peats and span the range of lignites and higher-rank coals. Figure 2 shows the XPS nitrogen (1s) spectrum of a Montana (DECS-25) and North Dakota (DECS-11) lignite and the results of curve resolution into different components. The XPS nitrogen (1s) spectra and curve resolution of several higher-rank AP coal samples were obtained under the same conditions of energy resolution. Lignites could be curve-resolved using three peaks at energy positions of 398.6, 400.2, and 401.4 (( 0.1) eV. These peaks correspond to pyridinic, pyrrolic, and quaternary nitrogen

Figure 2. XPS nitrogen (1s) spectra of lignites and Argonne Premium (AP) coals and curve resolution into different components.


Energy & Fuels, Vol. 20, No. 2, 2006 639

Table 2. X-ray Photoelectron Spectroscopy (XPS) and Elemental Analysis Results for Total Nitrogen and XPS Nitrogen (1s) Curve Resolution Results for Lignites Total Nitrogen (per 100 Carbons) lignite

elemental analysis

ND-B (DECS-11) Montana (DECS-25) Alaska (PSOC-1533) Texas (PSOC 1422) ND-H (PSOC 1482)

1.0 1.1 1.0 1.8 1.3

XPS Results (mol %)

XPS

398.6 eV

400.2 eV

401.4 eV

1.2 1.2 1.2 2.0 1.4

18 21 16 19 16

67 65 69 66 65

15 14 15 15 19

forms in higher-rank coals. (See Table 2.) Qualitatively, the lignites differ from higher-rank coals (i.e., Blind Canyon and Pittsburgh #8), in that they have lower amounts of pyridinic and greater amounts of quaternary nitrogen forms. The current curve resolution results for AP coal are in quantitative agreement with the curve resolution results of prior work using nonmonochromatic X-rays,56 although the pyridinic nitrogen peak must be shifted by ∼0.2 eV toward lower binding energy in the case of Upper Freeport and Pocahontas coal. Note that XPS cannot distinguish between amide and pyrrolic forms, because each display a signal at 400.2 eV. Figure 3 shows the 15N CPMAS NMR spectra of unaltered peats. Each spectrum possesses a single peak at -260 ppm. This chemical shift is within the range expected for amide nitrogen. Figure 4 shows the 15N CPMAS NMR spectrum of Montana (DECS-25) lignite. The peak is much broader than unaltered peats and is chemically shifted in the direction expected for local chemical environments of pyrrolic nitrogen. Much of the peak’s intensity occurs outside of the normal chemical shift range expected for amide nitrogen. Figure 5 shows the XPS nitrogen (1s) spectra of unaltered peats and curve resolution into different components. These spectra are dominated by a single peak at 400.2 eV. In all cases, three peak are needed to fully resolve each spectrum of unaltered peats. The quantitative results of XPS nitrogen (1s) curve resolution for nitrogen forms, on a mole percent basis in unaltered peats, are found in Table 3, along with the amount of total nitrogen per 100 carbons, from XPS and elemental analysis. Although XPS cannot distinguish amide from pyrrolic nitrogen forms, the unambiguous 398.6 eV pyridinic nitrogen peak and the 401.4 eV quaternary nitrogen peak are present in small amounts in all unaltered peat samples. Results from elemental analysis show that there is considerable variation in the total nitrogen level among the different peats. The XPS results reflect these variations; however, in most cases, XPS results show lower levels of total nitrogen. This likely indicates that the nitrogen levels at the external surfaces of peat are slightly depleted in nitrogen, relative to those in the bulk. The XPS curve-resolution results for unaltered peats are plotted in Figure 6 on a mole percent basis, together with the results for lignites and AP coals, as a function of the weight percentage of carbon. These XPS results were obtained under identical energy resolution conditions. The unambiguous pyridinic nitrogen peak clearly is present in lignites and grows in amount in higher-rank coals. Quaternary nitrogen forms are present in unaltered peats and lignites and decrease as the the weight percentage of carbon increases. The 400.2 eV peak for unaltered peaks is interpreted to be due to amide forms of nitrogen, based on 15N NMR evidence. The situation for lignites is less clear, and contributions from both pyrrolic and amide (86) Kelemen, S. R.; Rose, K. D.; Kwiatek, P. J. Appl. Surf. Sci. 1993, 64, 167.

Figure 3. 15N NMR cross-polarization magic-angle spinning (CPMAS) spectra of unaltered peats.

Figure 4.

15

N NMR CPMAS spectrum of Montana (DECS-25) lignite.

forms are likely. For higher-rank, coals the 400.2 eV peak is due to pyrrolic nitrogen; this is consistent with 15N NMR evidence.43 Significant changes appear in the 15N NMR spectra of peats upon pyrolysis. The 15N CPMAS NMR spectra of unaltered and pyrolyzed (Ro ) 0.7) Lox. Nym. peat (Figure 7) reveal that the sharp peak of unaltered peat at -260 ppm becomes much broader after pyrolysis and is chemically shifted away from the region of amide and in the direction of pyrrolic nitrogen. Most of the intensity appears at chemical shifts distinctly different than those of the unaltered peat. The 15N CPMAS NMR spectra of unaltered and pyrolyzed (Ro ) 0.5) Oke. Tax. peat, which are also displayed in Figure 7, show that the sharp -260 ppm amide peak is no longer present and the peak that appears is shifted to the range expected for pyrrolic nitrogen. Significant changes also appear in the XPS nitrogen (1s) spectrum of peat upon pyrolysis. Figure 8 illustrates the typical change that appears on peats pyrolyzed to Ro ) 1.0. The line indicates the energy where pyridinic nitrogen forms appear. There clearly is a dramatic increase in intensity at lower binding energy that reflects the appearance of pyridinic nitrogen forms. Spectra of four peats pyrolyzed to Ro ) 0.5 are shown in Figure 9, along with their curve-resolved spectra using three peaks at energy positions of 398.6, 400.2, and 401.4 ((0.1) eV (pyridinic,


Kelemen et al.

Figure 5. XPS nitrogen (1s) spectra of unaltered peats and curve resolution into different components.

Figure 6. Plot of the mole percentage of nitrogen forms from XPS versus the weight percentage of carbon for lignites and AP coals.

pyrrolic, and quaternary, respectively). These results, along with total nitrogen data, are listed in Table 4. The intensity of the 398.6 eV pyridinic nitrogen peak grows considerably upon pyrolysis, relative to that of unaltered peat. Changes in the XPS nitrogen (1s) spectra of peats pyrolyzed to levels of thermal stress equivalent to Ro values of 0.7, 1.0, and 1.5 are shown in Figures 10, 11, and 12, and the curve-resolved abundances, along with total nitrogen values from elemental analysis, are listed correspondingly in Tables 5, 6, and 7. The intensity of the 398.6 eV pyridinic nitrogen peaks grows relative to the 400.2 eV peak and the relative amount of quaternary nitrogen decreases and remains low upon further pyrolysis. For Ro ) 1.5 pyrolyzed peats, the pyridinic nitrogen peak appears at 394.4

(( 0.1) eV. This is a 0.2 eV shift to lower binding energy, relative to peats pyrolyzed at lower Ro, and coincides with the position of the pyridinic peak in higher-rank coal. The total level of nitrogen from elemental analysis differs significantly among peats; however, for a given peat, the total amount of nitrogen seems to be approximately the same in unaltered and pyrolyzed peats. The total nitrogen level from XPS analysis is in general agreement with the elemental analysis results for pyrolyzed peats, indicating that the surface composition of nitrogen in these samples is comparable to that of the bulk. A plot of the mole percent of nitrogen forms from XPS versus calculated Ro for unaltered and pyrolyzed peats, displayed in Figure 13, remarkably shows that all peats yield similar



Figure 7. 15N NMR CPMAS spectra of (left) unaltered Lox. Nym. peat and Ro ) 0.7 pyrolyzed peat and (right) unaltered Oke. Tax. peat and Ro ) 0.5 pyrolyzed peat (reproduced, in part, from ref 45). Table 3. XPS and Elemental Analysis Results for Total Nitrogen and XPS Nitrogen (1s) Curve-Resolution Results for Unaltered Peats Total Nitrogen (per 100 Carbons) unaltered peat

elemental analysis

Maine Oke. Nym. Lox. Nym. Oke. Tax. Lox. Saw. Coot Bay Min. Hem. Sh. Rv. Rhiz. NC first Col.

0.8 5.3 6.3 4.0 4.8 5.4 4.5 3.9 1.3

XPS Results (mol %)

XPS

398.6 eV

400.2 eV

401.4 eV

1.3 3.3 4.9 2.5 4 2.6 2.7 3.5 1.3

4 8 6 3 9 4 4 6 6

83 83 85 84 82 88 89 87 87

13 9 9 13 9 8 7 7 7

responses during pyrolysis. There is a sharp increase in the 398.6 eV pyridinic nitrogen peak following pyrolysis to Ro ) 0.5, then a more gradual increase with increasing thermal stress. The percent weight loss for peats pyrolyzed to Ro ) 1.0 is ∼50% (Table 8). This weight loss, when coupled with the dramatic increase of 40-50 mol % in pyridinic nitrogen forms, indicates that many of the amide nitrogen forms present in unaltered peats are converted to pyridinic nitrogen forms. This transformation occurs at thermal stresses roughly equivalent to the end of lignitification. XPS cannot determine the extent of amide conversion to pyrrolic structures; however, the 15N NMR results indicate that this reaction occurs when pyrolyzed to Ro ) 0.5. The sulfur forms present in unaltered and pyrolyzed peats were characterized using the S-XANES third-derivative analysis method. Each third-derivative S-XANES spectrum was curve fit using model compound spectra. Example spectra of unaltered Maine peat and Montana (DECS-25) lignite show that the curve fit is excellent, even though there is considerable complexity (Figure 14). Sulfur speciations are tabulated on a mole percent organic basis (inorganic sulfate excluded) for peats, lignites, and peats thermally treated to estimated Ro values of 0.5 and 0.7, respectively, in Tables 9, 10, 11, and 12. A simple

inspection of the unaltered peat spectra shows that features appear at energies expected for SO4 groups, SO3 groups, aromatic, and aliphatic sulfur forms (Figure 15). All peats have these basic features, to differing degrees. The sulfur forms in pyrolyzed peats share common features and follow progressive alteration upon increasingly severe pyrolytic conditions. This is illustrated for four peat samples in Figure 16. Clearly, the SO3 and disulfide groups are lost during the initial stages of pyrolysis and the aromatic sulfur feature is dominant following pyrolysis to Ro ) 0.7 and higher severity. Lignites appear qualitatively different than unaltered peats (Figure 17). Features are observed at energy positions expected from aromatic and nonaromatic sulfur forms. The lowest-energy feature is due to the presence of pyrite, whereas the highest-energy feature is due the presence of sulfate. There are few SO3 groups, relative to peats, and the level of sulfate, relative to organic sulfur forms in lignites, is much less than that in unaltered peats. In unaltered peats (Table 9), aliphatic sulfur forms and SO3 groups account for almost all sulfur. The level of aromatic sulfur forms is very low, relative to that in unaltered lignites (Table 10). A distinction is made among aliphatic sulfur forms as mercapto (cysteine), disulfide (cystine) and aliphatic sulfide forms. Disulfides predominate in all unaltered peat samples. Aliphatic sulfide species (e.g., dialkyl sulfides) are not present in pyrolyzed peats. The amount of SO3 groups and disulfide species decrease as the thermal stress increases, while the mercapto and aromatic sulfur species increase. In the lignites, SO3 forms are absent, the disulfide and, to a lesser extent, the aliphatic sulfur forms persist, and the mercapto and aromatic sulfur species are at a levels comparable to peats pyrolyzed to Ro ≈ 0.5. A plot of the mole percentage of organic sulfur forms determined by S-XANES versus weight percent carbon for unaltered peats, lignites, and higher-rank coals shows that the level of aromatic sulfur forms increases as the weight percentage of carbon increases, while the total level of aliphatic sulfur and


Kelemen et al. Table 4. XPS and Elemental Analysis Results for Total Nitrogen and XPS Nitrogen (1s) Curve-Resolution Results for Peats Pyrolyzed to an Estimated Ro ) 0.5 Total Nitrogen (per 100 Carbons)

XPS Results (mol %)

Ro ) 0.5 peat

elemental analysis

XPS

398.6 eV

400.2 eV

401.4 eV


1.5 5.9 6.6 3.9 5.5 6.1 4.4 4.0 1.1

1.5 5.6 6.7 4.1 6.2 5.4 4.7 4.3 1.5

34 38 33 30 32 40 29 35 40

58 53 65 61 59 55 61 57 52

8 9 2 9 9 6 10 8 8

Table 5. XPS and Elemental Analysis Results for Total Nitrogen and XPS Nitrogen (1s) Curve-Resolution Results for Peats Pyrolyzed to an Estimated Ro ) 0.7 Total Nitrogen (per 100 Carbons) Ro ) 0.7 peat

elemental analysis


1.8 5.1 6.7 4.2 5.1 4.9 3.8 4.0 1.4

XPS Results (mol %)

XPS

398.6 eV

400.2 eV

401.4 eV

2.2 5.9 5 2.7 6.1 3.1 4 4.2 1.4

34 38 37 37 39 40 35 35 38

58 53 55 58 56 55 58 57 57

8 9 8 5 5 5 7 8 5

Table 6. XPS Results and Elemental Analysis for Total Nitrogen and XPS Nitrogen (1s) Curve Resolution Results for Peats Pyrolyzed to an Estimated Ro ) 1.0 Total Nitrogen (per 100 Carbons) Ro ) 1.0 peat

elemental analysis


nd 5.2 6.6 4.5 5.1 5.3 nd 4.0 nd

XPS Results (mol %)

XPS

398.6 eV

400.2 eV

401.4 eV

2.3 3.5 5.6 3.3 4.1 3.5 3.1 2.5 1.8

37 47 43 40 45 50 43 47 41

58 49 53 57 49 45 50 48 54

5 4 4 3 6 5 7 5 5

Table 7. XPS Results for Total Nitrogen and XPS Nitrogen (1s) Curve-Resolution Results for Peats Pyrolyzed to an Estimated Ro ) 1.5 XPS Results (mol %)

Figure 8. XPS nitrogen (1s) spectra of (top) unaltered Coot Bay peat and (bottom) Ro ) 1.0 pyrolyzed Coot Bay peat.

SO3 types decrease (Figure 18). In the figure, mercapto, disulfide, and aliphatic sulfide are grouped as “aliphatic sulfur” forms and, as noted previously, SO3 groups appear only in unaltered peat. The sulfur species in unaltered peats, pyrolyzed peats, and lignites also were characterized by XPS. The sulfur (2p) spectrum from an individual species is comprised of 2p3/2 and 2p1/2 components at a 2:1 intensity separated in energy by 1.2 eV. The XPS sulfur (2p) spectra may be curve-resolved into different components characteristic of SO4, SO3, aromatic, and aliphatic sulfur for unaltered peats and lignites (Figure 19) and for pyrolyzed peats (Figure 20). Elemental analysis XPS results for total sulfur, along with the XPS sulfur (2p) curve-resolution results for unaltered peats, lignites, and Ro ) 0.7 and Ro ) 1.0

Ro ) 1.5 peat

total nitrogen per 100 carbons, via XPS

398.4 eV

400.2 eV

401.4 eV


1.3 5.1 4.4 2.8 3.9 3.5 3.4 2.9 1.0

39 48 42 41 47 53 46 45 40

55 50 52 54 48 42 45 49 55

6 2 6 5 5 5 9 6 5

pyrolyzed peats are shown, respectively, in Tables 13, 14, 15, and 16. XPS results confirm the S-XANES findings. Aliphatic sulfur is the predominant organic sulfur form in unaltered peat, followed by SO3 groups, whereas the level of aromatic sulfur is relatively low. In pyrolyzed peats, aromatic sulfur is the predominant organic sulfur form and SO3 groups are



Figure 9. XPS nitrogen (1s) spectra of Ro ) 0.5 pyrolyzed peats and curve resolution into different components.


virtually absent. There is a significant amount of total aliphatic sulfur present. The results show that organic sulfur exists almost exclusively in aromatic sulfur forms when peats are pyrolyzed to Ro ) 1.0. For lignites and higher-rank coals,

SO3 groups are absent in the XPS spectra and aromatic sulfur forms appear in greater relative abundance than in unaltered peats. Aliphatic sulfur in lignites accounts for 50%-60% of the organic sulfur. A plot of the mole percent of organic


Kelemen et al.



sulfur forms from XPS versus the weight percentage of carbon for unaltered peats, lignites, and higher-rank coals (Figure 21) yield trends that are remarkably similar to those found by S-XANES (see Figure 18). The level of aromatic

sulfur forms increases as the weight percentage of carbon increases, while the total level of aliphatic sulfur and SO3 decrease, with the SO3 essentially going to zero at ∼70 wt % carbon.



Figure 13. Plot of the mole percentage of nitrogen forms from XPS versus calculated Ro values for pyrolyzed peats. Table 8. Weight Loss for Peats Pyrolyzed to an Estimated Ro ) 1.0 Ro ) 1.0 peat

weight loss (%)


56.9 42.2 44.8 49.6 51.2 41.2 49.8 36.3 49.8

Table 9. Third-Derivative Sulfur X-ray Absorption Near-Edge Structure Spectroscopy (S-XANES) Spectrum Curve-Fitting Results for Unaltered Peats Composition (mol %)

Figure 14. Sulfur X-ray absorption near-edge spectroscopy (SXANES) third-derivative spectrum of unaltered Maine peat and Montana (DECS-25) lignite, and the results of a curve fit using spectra from model compounds.

Discussion XPS is a surface-sensitive spectroscopy technique, and the majority of the signal originates within the first 50 Å of the surface. Hence, it is possible to determine if the composition of nitrogen species at the surface is significantly different from that in the bulk by comparing the total amount of nitrogen from

unaltered peat

cysteine mercapto

cystine disulfide

aliphatic sulfide

aromatic sulfur

SO32-


18 21 14 16 9 10 9 7 26

26 26 25 25 31 24 25 33 26

13 0 8 0 23 19 15 23 0

11 14 7 10 7 9 10 7 14

32 39 45 48 30 37 40 31 35

elemental analysis with XPS. For finely ground coal, the total organic nitrogen level corresponds closely (within 10%) with that of the bulk.48,52,56,63 The XPS results for unaltered peats show that there can be significantly less (up to 50% less) total nitrogen in the near-surface region, especially for the samples with higher amounts of nitrogen. In contrast, the total amount of nitrogen for Ro ) 0.5 pyrolyzed peat closely matched the bulk. The nitrogen signal would be attenuated if nitrogen-poor organic matter preferentially and uniformly encapsulated nitrogenrich organic matter. The attenuation is dependent exponentially on the thickness of the encapsulating layer divided by the mean free path λ (λ ≈ 15 Å) of the photoemitted electrons through


Kelemen et al.

Table 10. Third-Derivative S-XANES Spectrum Curve-Fitting Results for Lignite

Table 12. Third-Derivative S-XANES Spectrum Curve-Fitting Results for Peats Pyrolyzed to an Estimated Ro ) 0.7

Composition (mol %)

Organic Composition (mol %)

cysteine cystine aliphatic aromatic mercapto disulfide sulfide sulfur SO32-

lignite ND-B (DECS-11) Montana (DECS-25) Alaska (PSOC-1533) Texas (PSOC 1422) ND-H (PSOC 1482)

45 43 26 20 27

15 24 22 33 23

3 2 9 11 16

36 31 44 37 34

0 0 0 0 0

Table 11. Third-Derivative S-XANES Spectrum Curve-Fitting Results for Peats Pyrolyzed to an Estimated Ro ) 0.5 Organic Composition (mol %) Ro ) 0.5 peat

cysteine mercapto

cystine disulfide

aliphatic sulfide

aromatic sulfur

SO32-

Lox. Nym. Oke. Tax. Lox. Saw. Coot Bay Min. Hem. Sh. Rv. Rhiz. NC first Col.

36 39 41 32 47 36 42

13 0 21 16 0 18 0

0 0 0 0 0 0 0

27 49 22 30 53 46 58

24 13 16 21 0 0 0

the organic layer. This model is applicable for λ g 1 for a uniform overlayer. An order-of-magnitude decrease in the nitrogen signal would be expected for a 15 Å encapsulating layer. Clearly, this is not the case for unaltered, high-nitrogen peat; thus, it is not possible to use this model to explain the attenuation of the total nitrogen signal quantitatively in some samples. Particle size effects offer another explanation of the XPS nitrogen results. Less nitrogen would be determined by

Figure 15. S-XANES third-derivative spectra of unaltered peats.

Ro ) 0.7 peat

cysteine mercapto

cystine disulfide

aliphatic sulfide

aromatic sulfur

SO32-

Lox. Nym. Oke. Tax. Lox. Saw. Coot Bay Min. Hem. Sh. Rv. Rhiz. NC first Col.

30 40 44 37 41 25 40

0 0 17 0 0 0 0

0 0 0 0 0 0 0

70 60 38 63 59 75 60

0 0 0 0 0 0 0

XPS, relative to the bulk average, if nitrogen exists preferentially in the larger particles of the peat. It is not possible to distinguish between these two possibilities from the current results for unaltered peats. If the former explanation is correct, then an unusual structural conformation for nitrogen may exist in the near-surface region of some unaltered peats with high nitrogen content. The vast majority of nitrogen in unaltered peats exists as amide species, and unusual structural conformations for some of the proteins and peptides in the initially deposited sediments have been offered as an explanation for their resistance to microbial degradation.87 For lignites, the total amount of nitrogen is comparable at the surface and the bulk. XPS results unambiguously show that pyridinic and quaternary nitrogen forms are present in lignites, whereas the majority of nitrogen is associated with a 400.2 eV peak attributable to either pyrrolic or amide nitrogen. The 15N NMR results indicate that much of this nitrogen is associated with pyrrolic structures. Quaternary nitrogen forms are likely associated with protonated pyridinic structures.56 The amount



Figure 16. S-XANES third-derivative spectra of unaltered peat and pyrolyzed peats.

of pyridinic nitrogen in lignite is considerably less than that in higher-rank coals and increases during bituminization. Some of these differences may be due to the subsequent conversion of quaternary nitrogen to pyridinic forms with the loss of acidic oxygen functional groups during bituminization. Another possible factor contributing to higher pyridinic levels is the conversion of amides into pyridinic and pyrrolic structures during later stages of lignitification. All unaltered peats have similar distributions of nitrogen forms and exhibit similar responses to pyrolysis. Prior 15N NMR analyses reported the presence of amide forms of nitrogen in peats,36-38,47 and a nitrogen K-edge XANES spectroscopy study measured the presence of amide as well as pyridinic forms of nitrogen in one peat sample.64 Our studies confirm that amides, by far, are the most abundant forms of nitrogen in unaltered peats. There is a significant increase in the level of pyridinic nitrogen that forms upon pyrolysis to Ro ) 0.5. The level of pyridinic nitrogen increases as the severity of pyrolysis (Ro) increases. The level of the 400.2 eV peak declines correspondingly after pyrolysis to Ro ) 0.5, and 15N NMR spectra indicate that much of the amides initially present are converted to pyrrolic nitrogen forms. The level of quaternary nitrogen in unaltered peat is relatively low and does not increase signifi(87) Tanoue, E.; Nishiyama, S.; Kamo, M.; Tsugita, A. Geochim. Cosmochim. Acta 1995, 59, 2643.

cantly upon pyrolysis to Ro ) 0.5. After pyrolysis to Ro > 0.5, the level of quaternary nitrogen in peat is approximately half that of lignite, suggesting that acidic oxygen functional groups are lost and not created during pyrolysis at these higher levels of thermal stress or that chemical specificity exists in lignites for protonating pyridinic structures. This observation is somewhat surprising, given the fact that the H/C values of pyrolyzed peats are generally greater than that of lignites at comparable levels of both the aromatic carbon and the oxygen-to-carbon (O/C) atomic ratio. During lignitification, air oxidation and other processes may result in the species necessary for protonating pyridinic nitrogen structures, and these acidic protons are likely lost during the pyrolysis of peats. Quarternary nitrogen is most abundant in lignites. The relatively low levels of quaternary nitrogen in pyrolyzed peats indicates that most quaternary nitrogen is formed during lignitification as a result of the creation of basic nitrogen species and acidic functionalities and their intimate chemical interaction. Quaternary nitrogen forms are lost during bituminization, as pyridinic forms increase. The nitrogen level in unaltered peats is highly variable. The total nitrogen level for the majority of peats is significantly greater than lignites and higher-rank coals, whereas, in two samples (Maine and NC first), the total nitrogen levels are comparable to lignites and coals. Despite these differences, the amount of nitrogen in a given peat sample remains almost


Kelemen et al.

Figure 17. S-XANES third-derivative spectra of lignites.

Figure 18. Plot of the mole percentage of organic sulfur forms from S-XANES versus the weight percentage of carbon for unaltered peats, lignites, and higher-rank coals.

constant after pyrolysis. This occurs although, between ∼35%55% of the original weight of peat is lost after pyrolysis to Ro ) 1.0, indicating that nitrogen is neither preferentially lost nor concentrated in the pyrolyzed peats. Tar was collected from the outlet of a pyrolysis unit after the pyrolysis of several peats to Ro ) 1.5 and analyzed using XPS. Both pyridinic nitrogen and a 400.2 eV peak were detected in the tar, indicating that the nitrogen that evolved from peat in the tar has been transformed to heterocylic structures. The 400.2 eV peak is interpreted as likely due to pyrrolic nitrogen. High total nitrogen levels, relative to lignites and coals, are found in unaltered and most pyrolyzed peats, suggesting that amides are lost in nature beyond the peat stage of coalification via a nonthermal pathway. We conclude this because we have shown that the total amount of nitrogen

does not change from unaltered peats and their pyrolysis residues. The hydrolysis of amide nitrogen is a likely route for nitrogen loss during lignitification. On a geochemical time scale, hydrolysis reactions occur very rapidly ( 0.5) resemble those of lignites. Mercapto species are more stable than disulfides and survive lignitification. Mercapto species are almost the exclusive nonaromatic organic sulfur form in peat pyrolyzed to Ro ) 0.7. Some mercapto species may be created from disulfides, as a result of breaking the weak disulfide bond, followed by hydrogen abstraction. This could explain the relative high abundance of this sulfur form following pyrolysis to Ro ) 0.7. Mercapto species also are present in abundance in lignite, along with disulfides; however, these species are not found in higherrank coals, where aliphatic sulfide is the predominant form.75 This observation indicates that the thermal conversion of mercapto species into aliphatic sulfides does not occur during peat pyrolysis. The presence of mercapto and disulfide sulfur in lignite but not in higher-rank coals suggests that aliphatic sulfide species arise in coals during bituminization via a mechanism not simulated by the pyrolysis conditions that have been applied in the current study. Hydrogen disulfide (H2S) is formed during the open-system TPD-MS pyrolysis of peats, and mercapto species are the only remaining nonaromatic organic sulfur forms. In a closed natural system, the decomposition of disulfides may proceed differently. Several possibilities exist to account for this. Thermal decomposition of the weak disulfide bond may result in aliphatic sulfides, in preference to H2S as the reaction product. It is also possible that the oxidation of mercapto species occurs during natural coalification leading to their loss in peats and lignites. The addition of H2S or mercaptans to olefinic bonds under acidic conditions could also lead to aliphatic sulfides. Aromatic sulfur is abundant and increases in peat pyrolyzed to Ro g 0.7 relative to unaltered peats, indicating that aromatic sulfur forms can be produced via thermal processes after a thermal stress equivalent to the onset of bituminization. XPS shows that sulfur exists almost exclusively in aromatic forms in peats pyrolyzed to Ro g 1.0. The range of total organic sulfur is 0.2-0.5 sulfur per 100 carbons for peats pyrolyzed to Ro ) 1.0, comparable to low-sulfur lignites and higher-rank coals. Illinois No. 6 coal is an exception among the AP coals, possessing significantly higher amounts of organic sulfur (>1.0

per 100 carbons) and pyrite. The relative abundance of aromatic and nonaromatic sulfur forms in Illinois No. 6 coal falls on a common rank-dependent trend line as other AP coals.75,76 S-XANES results provide an overall pattern for sulfur forms for unaltered peats, lignites, and higher-rank coals. The pattern clearly shows the progressive increase in the relative abundance of aromatic sulfur forms as coalification progresses from peats. SO3 groups are lost early during lignitification. Aliphatic sulfur forms react and are lost during bituminization and as the coal rank increases (pre-anthracitization). Interconversion of aliphatic sulfides into aromatic forms competes with their loss71,73 as the coal rank increases. XPS results confirm the S-XANES findings for unaltered peats, lignites, and coals, although XPS does not distinguish among aliphatic sulfur forms. S-XANES and XPS results, coupled with TPD-MS, show that aliphatic sulfur is lost as SO2 and H2S and some is converted to aromatic sulfur forms. Almost all aliphatic sulfur forms are lost in peats pyrolyzed to Ro ) 1.0, and the remaining sulfur exists almost exclusively in aromatic forms, comparable to that observed in higher-rank coals. The amount of aromatic sulfur is very small in unaltered peat; hence, the thermal chemistry of sulfur in unaltered peats leads to the formation of aromatic sulfur species concentrations, which are further enhanced by the loss of aliphatic species and SO3 groups. In natural coalification, the relative amount of mercapto species may be diminished by oxidative loss and by conversion to aliphatic sulfides. The present-day peats used in this study cannot be matched as exact precursors of the lignites and higher-rank coals of today, with respect to the initial biomass input or action of bacterial and fungal life cycles during peatification. Nevertheless, the comparison is instructive for identifying the basic chemical pathways that can be explained by thermal transformations and those that result from more-complex chemistries. Based on the work of van Krevelen,1 coalification does not proceed in a closed system. Some carbon and oxygen is lost while other oxygen is added. These transformations occur by way of established pathways and are illustrated on the left-hand side of Figure 22. Lignin-derived material is selectively preserved, and cellulosederived material succumbs to degradation during peatification. Oxidation and decarboxylation occurs during lignitification, along with demethoxylation. Aromatization progresses during bituminization. These are only a few familiar examples of processes that pertain to carbon and oxygen.


The present work reveals some of the chemical pathways for nitrogen and sulfur during coalification by comparing the chemical forms of unaltered peats, lignites, and coal to peats pyrolyzed at well-defined times and temperatures. We have modified Figure 22 by adding to the right-hand side some proposed chemical pathways for nitrogen and sulfur during coalification based on the results of the current study. Amides are preserved during peatification. A portion of the amides are lost during lignitification via a nonthermal pathway that likely involves hydrolysis. The remaining amides are converted to pyridinic and pyrrolic structures during lignitification. Quaternary nitrogen, as protonated pyridinic structures, appear as acidic oxygen functionalities and are created during lignitification. Quaternary nitrogen decomposes to pyridinic nitrogen structures, resulting in increasing levels of pyridinic nitrogen during bituminization. SO3 groups in peat are lost by the beginning of lignitification via a thermal pathway. Some disulfides are lost and mercapto species are formed during lignitification. Mercapto and disulfide species are converted to aliphatic sulfides following lignitification via a nonthermal pathway. Aromatic sulfur levels increase during lignitification and bituminization as a portion of aliphatic sulfides is thermally converted to aromatic sulfur forms. Conclusions (1) Amide nitrogen is the dominant nitrogen form in unaltered peat, indicating that amides are preserved during peatification but are converted to pyridinic and pyrrolic nitrogen forms after a thermal stress roughly equivalent to lignitification. Pyridinic nitrogen forms increase as the stage of coalification increases. (2) Higher total nitrogen levels are found in most unaltered and pyrolyzed peats, relative to lignites and coals, indicating that some amides are lost in nature beyond the peat stage of coalification via a nonthermal pathway, such as hydrolysis. (3) The highest quaternary nitrogen levels appear in lignites and are associated with protonated pyridinic structures. The relatively low levels of quaternary nitrogen in pyrolyzed peats,

Kelemen et al.

along with the presence of pyridinic nitrogen, indicates that lower levels of acidic sites occur in pyrolyzed peats, relative to lignites. Most quaternary nitrogen is formed during lignitification, as a result of the creation of basic nitrogen species and acid functionalities and their intimate chemical interaction. Quaternary nitrogen is lost during bituminization. (4) Sulfur X-ray absorption near-edge structure spectroscopy (S-XANES) and X-ray photoelectron spectroscopy (XPS) results from unaltered peats indicate the presence of aliphatic sulfur, aromatic sulfur, and SO3 groups. S-XANES further distinguished aliphatic sulfur as mercapto, disulfide, and aliphatic sulfide species. (5) The level of organic sulfur in pyrolyzed peats is comparable to that in low-sulfur lignites and higher-rank coals, indicating that much of the organic sulfur in lignites and higherrank coals is derived from sulfur species incorporated during peatification. (6) The level of aromatic sulfur increases as the severity of peat pyrolysis increases. Their relative increase is due to their formation from aliphatic sulfur forms and through the selective loss of disulfide, aliphatic sulfide, and SO3 species. (7) Mercapto species are found in abundance in peats pyrolyzed to an equivalent vitrinite reflectance of Ro ) 0.5 and in lignites but not in higher-rank coals, indicating that mercapto and disulfide species are lost after lignitification. Organic sulfur in peats pyrolyzed to Ro ) 1.0 exist mainly in aromatic forms and the level of aromatic sulfur increases as the degree of coalification increases. Acknowledgment. We wish to thank B. Liang for obtaining the 15N NMR data, and L. M. Kwiatek for preparing some of the peat samples. We are grateful for and wish to acknowledge the help and support received from the staffs at the Stanford Sychrontron Radiation Laboratory, The National Sychrontron Light Source at Brookhaven National Laboratory, and the Advanced Photon Source at Argonne National Laboratory. EF050307P

Thermal Transformations of Nitrogen and Sulfur Forms in Peat Related

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