Energy & Fuels 1999, 13, 529-538
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Thermal Chemistry of Nitrogen in Kerogen and Low-Rank Coal S. R. Kelemen,* H. Freund, M. L. Gorbaty, and P. J. Kwiatek Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received October 5, 1998. Revised Manuscript Received December 10, 1998
X-ray photoelectron spectroscopy (XPS) was used to identify the forms of nitrogen present in Green River Type I and Bakken Type II kerogen concentrate samples and to follow the changes in nitrogen forms in the tars and chars produced upon pyrolysis. Pyrrolic nitrogen is the most abundant form of nitrogen, followed by pyridinic, amino, and quaternary types. XPS results show that upon devolatilization at 510 °C, the resultant kerogen tar and char contain mostly pyrrolic and pyridinic forms while amino groups are preferentially released into the tar. A portion of the quaternary nitrogen initially present in the Bakken kerogen appears in the 510 °C char and tar. Similar transformations were found for low-rank coal. These transformations occur at lower temperatures at long pyrolysis times for both kerogen and low-rank coal. Severe pyrolysis of the devolatilized kerogen char (T ) 630-810 °C) results in the appearance of an asymmetric carbon (1s) line shape indicative of very large polynuclear “graphitic-like” units. This transformation is accompanied by an increase in the relative abundance of quaternary nitrogen forms. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed kerogen chars.
I. Introduction Advances in the direct characterization of nitrogen in complex solid and nonvolatile carbonaceous systems have occurred using X-ray photoelectron spectroscopy (XPS),1-9 X-rayabsorptionnear-edgestructure(XANES),10-12 and solid-state 15N NMR spectroscopy.13,14 Recent work has established that pyrrolic, pyridinic, and quaternary nitrogen are the most abundant organic nitrogen forms in fresh coal.1-12 XPS has been used to study the nitrogen transformations during pyrolysis of coal6-9,5-17 and model compounds.16,18 The initial stages of coal (1) Jones, R. B.; McCourt, C. B.; Swift, P. Proc. Int. Conf. Coal Sci. 1981, 657. (2) Perry, D. L.; Grint, A. Fuel 1983, 62, 1029. (3) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Proc. Technol. 1987, 15, 351. (4) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 1450. (5) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100. (6) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. 24th Int. Symp. Combust. 1992, 1259. (7) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896. (8) Pels, J. R.; Wojtowicz, M. A.; Moulijn, Fuel 1993, 72, 373. (9) 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. (10) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. Fuel 1993, 72, 133. (11) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (12) Mullins, O. C.; Mitra-Kirtley, S.; Van Elp, J.; Cramer, S. P.. Appl. Spectrosc. 1993, 8, 1268. (13) Knicker, H.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 1995, 9, 999. (14) Solum, M. S.; Pugmire, R. J.; Grant, D. M.; Kelemen, S. R.; Gorbaty, M. L.; Wind, R. A. Energy Fuels 1997, 11, 493. (15) Kelly, M. D.; Buckley, A. N.; Nelson, P. F. Int. Conf. Coal Sci. 1991, 356. (16) Pels, J. R; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (17) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1995, 74, 507. (18) Stanczyk, K.; Dziembaj, R.; Piwowarska, Z.; Witkowski, S. Carbon 1995, 33, 1383.
pyrolysis involves the loss of organic quaternary nitrogen species associated with the loss of nearby or adjacent hydroxyl groups from carboxylic acids or phenols.7 Recent studies have revealed the appearance of significant levels of quaternary nitrogen following hightemperature pyrolysis.9,16-20 These quaternary nitrogen species are associated with nitrogen that has undergone significant transformation concomitant with the development of larger polynuclear aromatic carbon structures. The geological evolution pathways of nitrogen in sedimentary organic matter has been the subject of much recent discussion.21-33 Insight into these processes comes from both low-temperature and hightemperature laboratory pyrolysis experiments that have monitored gaseous nitrogen-containing reaction prod(19) Jimenez Mateos, J. M.; Fierro, J. L. G. Surf. Interface Anal. 1996, 24, 223. (20) Chambrion, P.; Suzuki, T.; Zhang, Z.; Kyotani, T.; Tomita, A. Energy Fuels 1997, 11, 681. (21) Gillaizeau, B.; Behar, F.; Derenne, S.; Largeau, C. Energy Fuels 1997, 11, 1237. (22) Boudou, J. P.; Espitalie, J. Chem. Geol. 1995, 126, 319. (23) Boudou, J. P.; Mariotti, A.; Oudin, J. L. Fuel 1984, 63, 1508. (24) Baxby, M.; Patience, R. L.; Bartle, K. D. J. Pet. Geol. 1994, 17, 211. (25) Wilhelms, A.; Patience, R. L.; Larter, S. R.; Jorgensen, S. Geochim. Acta 1992, 56, 3745. (26) Patience, R. L.; Baxby, M.; Bartle, K. D.; Perry, D. L.; Rees, A. G. W.; Rowland, S. J. Org. Geochem. 1992, 18, 161. (27) Littke, R.; Krooss, B.; Idiz, E.; Frielingsdorf, J. AAPG Bull. 1995, 79, 410. (28) Barth, T.; Rist, K.; Huseby, B.; Ocampo, R. Org. Geochem. 1996, 24, 889. (29) Bakel, A. B.; Philp, R. P. Org. Geochem. 1990, 16, 353. (30) Onen, A.; Sarac, S. J. Anal. Appl. Pyrol. 1990, 17, 227. (31) Damste, J. S.; Eglinton, T.; De LeeuW, J. W. Geochim. Cosmochim. Acta 1992, 56, 1743. (32) Harrison, W. E. Chem. Geol. 1978, 21, 315. (33) Rohrback, B. G.; Peters, K. E.; Sweeney, R. E.; Kaplan, I. R. Adv. Org. Geochem. 1981, 819.
10.1021/ef9802126 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
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ucts.21,22,27,28,31-35 There have been few XPS and XANES studies of nitrogen forms in kerogen.25,26,36 Direct analysis of nitrogen forms before and after laboratory pyrolysis of immature kerogen and low-rank coal may help to clarify the chemical pathways for nitrogen during latestage diagenesis, catagenesis, and early metagenesis of different types of organic matter. Much of the nitrogen originally present in kerogen is either retained in the solid pyrolysis product or evolves in the pyrolysate as tar. In many reactor situations, the primary pyrolysis tar is subject to secondary reactions leading to gaseous products before detection. The present work employed XPS to speciate and quantify nitrogen functionalities initially present in kerogens and low-rank coals and to follow the evolution of nitrogen species into the resultant pyrolysis chars and primary reaction pyrolysis tars produced under kinetically well-defined conditions. II. Experimental Section The Green River oil shale used in this study was from the Colony Mine, a surface mine located in Parachute Creek, CO. It was not possible to analyze the oil shale directly because of inhomogeneous sample charging during the XPS experiment, hence kerogen concentrates were studied. The Green River (Type I kerogen) and Bakken (Type II kerogen) kerogen concentrates were prepared by treatment with HCl/HF at 0 and 23 °C, respectively, to remove the inorganic carbonate and alumino-silicate mineral components.38 The kerogen, which makes up less than 20% of the sample, was thus liberated from the inorganic matrix of the oil shale. The low-rank coals were obtained from the Argonne Premium Coal Sample Program.39 Chars and tar samples from slow heat-up pyrolysis were prepared in an ultrahigh vacuum (UHV) compatible temperature-programmed decomposition (TPD) apparatus. The pressure rose to a maximum of 1 × 10-5 Torr in the reactor section during pyrolysis. A typical sample of 5 mg was loaded into a 3 mm × 15 mm ceramic vessel. A fine chromel-alumel thermocouple was inserted into the center of the sample bed. The sample vessel was indirectly heated by the surrounding tantalum resistive heating elements. A linear heating rate of 0.23 °C/s was used to produce pyrolysis tar and char samples, which were collected at 510 °C. The sample was held for 30 s upon reaching the target temperature, and then the sample was rapidly cooled (>5 °C/s) back to room temperature. These conditions resulted in approximately 85% evolution of the hydrocarbons expected for kerogen39 and coal.40 The pyrolysate from the TPD reactor was collected as thin films deposited directly onto an XPS stainless steel nub surface. The sample nub was positioned 2 cm away and directly in the line with the outlet of the TPD pyrolysis reactor. The nub served as a low-temperature cold trap (T ≈ 50 °C) for pyrolysis products. Pyrolysis experiments below 400 °C were done in a quartzlined reactor in helium at 1 atm. The longer time pyrolysis experiments were done using this reactor at 365-380 °C to (34) Oh, M. S.; Taylor, R. W.; Coburn, T. T.; Crawford, R. W. Energy Fuels 1998, 2, 100. (35) Krooss, B. M.; Littke, R.; Muller, B.; Frielingsdorf, J.; Schwochau, K.; Idiz, E. F. Chem. Geol. 1995, 126, 291. (36) Mitra-Kirtley, S.; Mullins, O. C.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128. (37) The Users Handbook for the Argonne Premium Coal Sample Program; Vorres, K. S., Ed.; Argonne National Laboratory: Argonne, IL, 1989; ANL-PCSP-89-1; Energy Fuels 1990, 4, 420. (38) Siskin, M.; Scouten, C. G.; Rose, K. D.; Aczel, T.; Colgrove, S. G.; Pabst, R. E. Proc. NATO Advanced Studies Institute on Composition, Geochemistry and Conversions of Oil Shale, Akay Turkey; Kluwer Academic Publishers: Boston, 1993. (39) Freund, H.; Kelemen, S. R. Am. Assoc. Pet. Geol. Bull. 1989, 73, 1011. (40) Kelemen, S. R.; Vaughn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 645.
Kelemen et al.
Figure 1. XPS carbon (1s) spectrum of fresh Green River kerogen before and after pyrolysis to 650 °C. achieve nearly complete (85%) evolution of volatile hydrocarbons from kerogen and low-rank coal. Higher temperature (T g 630 °C) kerogen chars were produced in the TPD apparatus. The linear heat-up rate was 0.23 °C/s. Upon reaching the desired temperature the sample was rapidly cooled (> 5 deg/ sec) back to room temperature. The time spent at the targeted temperature was 1 s. Elemental analysis of char and residue samples was carried out with XPS. Elemental concentrations are reported relative to carbon, calculated from the areas of the XPS peaks after correcting for differences for atomic sensitivity. The amount of organic oxygen was derived from the total oxygen (1s) signal by taking into account inorganic contributions. The amount of inorganic oxygen associated with silicon, aluminum, calcium, and iron were taken as SiO2, AlO1.5, CaO, and FeO1.5. A detailed analysis was not performed of the XPS carbon (1s) line shape to quantify the kinds of organic oxygen species in coal tars and chars.48 The relative amount of aromatic carbon in lower temperature (T < 630 °C) chars was determined by the XPS method of π f π* shake-up signal intensity.41 For higher temperature chars, the XPS carbon (1s) signal gradually changed from a symmetric peak to one with a pronounced asymmetry toward higher binding energies (see Figure 1). This marks the transition from small to much larger polynuclear aromatic structures.42,43 Because of this, the method π f π* shake-up signal intensity could not be used to evaluate the percentage of aromatic carbon for high-temperature chars (T g 690 °C). The XPS spectra were obtained with a Vacuum Generators (VG) ESCA lab system using either Mg K R or Al K R nonmonochromatic radiation and a five-channel detection arrangement. The Type II kerogen . Type I kerogen.46 Most of the weight loss occurs during formation of the devolatilized char at 510 °C. For all samples, as expected, there is much less relative change in the pyrolyzed char weights at progressive stages of pyrolysis. A. XPS Nitrogen Analysis of Initial Kerogen and Coal, Devolatilized Char, and Tar. Table 2 shows the results of XPS analysis for organic oxygen, aromatic carbon, and nitrogen in fresh kerogen and low-rank coal. Green River kerogen is significantly less aromatic and contains fewer organic oxygen species than Bakken kerogen or low-rank coal. Green River and Bakken kerogen concentrates were linearly heated at a rate of 0.23 °C/s up to 510 °C and held for 30 s to produce tar and char samples. The results of XPS analyses of these samples are also shown in Table 2. The aromatic carbon and organic oxygen level increases in Green River char, while they decrease significantly in the tar relative to the parent kerogen. The nitrogen level is slightly lower in the tar and higher in the char. The directional changes in aromatic carbon and organic oxygen for the Bakken kerogen char and tar samples are similar to the (49) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Gravel, D.; Baillargeon, M.; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 320. (50) Daniels, E. J.; Altaner, S. P. Am. Mineral. 1990, 75, 835. (51) Juster, T. C.; Brown, P. E.; Bailey, S. W. Am. Mineral. 1987, 72, 555. (52) Daniels, E. J.; Altaner, S. P. Int. J. Coal Geol. 1993, 22, 21. (53) Buckley, A. N.; Kelly, M. D.; Nelson, P. F.; Kenneth, W. R. Fuel Process. Technol. 1995, 43, 47. (54) Buckley, A. N. Fuel Process. Technol. 1994, 38, 165. (55) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749. (56) Stanczyk, K.; Boudou, J. P. Fuel 1994, 73, 940. (57) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710. (58) Gong, B.; Pigram, P. J.; Lamb, R. N. Int. J. Coal Geol. 1997, 34, 53. (59) Burnham, A. K.; Braun, R. L. Org. Geochem. 1990, 16, 27. (60) Tegelaar, E. W.; Noble, R. A. Org. Geochem. 1994, 22, 543. (61) Behar, F.; Tang, Y.; Liu, J. Org. Geochem. 1997, 26, 281.
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Table 2. XPS Analyses for Kerogen Chars and Tars Produced by Pyrolysis at 510 °C for 30 s
sample Beulah-Zap Beulah-Zap Beulah-Zap Wyodak Wyodak Wyodak Green River kerogen Green River kerogen Green River kerogen Bakken kerogen Bakken kerogen Bakken kerogen
fresh char tar fresh char tar fresh char tar fresh char tar
organic oxygen per 100 carbons
aromatic carbon per 100 carbons
nitrogen per 100 carbons
18.8 7.5 5.5 16.9 10 3.7 2.6 3.9 0.9 10.1 5.0 1.6
53 69 49 52 70 43 29 45 17 54 57 51
1.5 1.2 1.1 1.3 1.5 1.4 2.2 2.6 2.0 3.0 2.5 3.4
changes found for lower rank coal. The organic oxygen level of both the char and tar are lower than that in the starting sample, while the aromatic carbon level is higher in the char but lower in the tar. There is a broader variance in the changes of aromatic carbon in Green River kerogen than in Bakken kerogen. The nitrogen level in the Bakken kerogen is lower in the char but higher in the tar, opposite of what is seen for Green River kerogen. The relative amount of nitrogen present in the Beulah-Zap lignite and Wyodak subbituminous coal char and tar are similar. These levels are also similar to the amount of nitrogen in the fresh coal sample. BeulahZap and Wyodak coal tars have lower amounts of organic oxygen than do the corresponding chars. When compared to the starting coals, there is much less oxygen in both the tar and char of Beulah-Zap and Wyodak coal. This is easily explained by the evolution of oxygen species, predominantly as CO2, H2O, and CO.47-49 For these coals, there is significantly less aromatic carbon in the tar relative to the corresponding char. The relative amount of nitrogen species present in the coal char and tar samples was obtained from an analysis of the XPS nitrogen (1s) line shape. Table 3 contains the curve-resolution results for the chars and tars. XPS nitrogen (1s) spectra of the chars could be curveresolved into three peaks corresponding to the energy positions of pyridinic, pyrrolic, and quaternary nitrogen. For Beulah-Zap and Wyodak tars, it was necessary to include a small peak at the position expected for amino nitrogen in order to obtain a good fit to those spectra. The amount of quaternary nitrogen is similar in the corresponding coal chars and tars and is lower than that in the parent coal. There is a corresponding increase in the level of pyridinic nitrogen in the coal chars. The pyridinic nitrogen level in the tar is similar to the level of the parent coal. The relative amount of nitrogen species present in the coal char and tar samples was obtained from an analysis of the XPS nitrogen (1s) line shape. Figure 2 shows the XPS nitrogen (1s) spectra with the curve-resolved individual peaks for the 510 °C char and tar samples made from Green River and Bakken kerogen concentrates. Table 3 shows the curve-resolution results for these samples. The chars could be curve resolved into three peaks corresponding to the energy positions of pyridinic, pyrrolic, and quaternary nitrogen. This indicates that if amino groups are present in the kerogen
chars, they are at concentrations of
