Characterization of Organically Bound Oxygen Forms in Lignites

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Characterization of Organically Bound Oxygen Forms in Lignites, Peats, and Pyrolyzed Peats by X-ray Photoelectron Spectroscopy (XPS) and Solid-State 13C NMR Methods S. R. Kelemen,* M. Afeworki, and M. L. Gorbaty ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801

A. D. Cohen Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Received February 28, 2002

A combination of XPS and solid-state 13C NMR techniques have been used to characterize organic oxygen species and carbon chemical/structural features in peats, pyrolyzed peats, lignites, and other coals. Both the 13C NMR and XPS results show the same ordering for the amount of aromatic carbon, higher ranking coals > lignites > peats. In general the value for H/C decreases as the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher than lignites and other coals of comparable levels of aromatic carbon. This is likely due to significant differences in the carbon structural framework of these materials. A van Krevelen plot, based on elemental H/C data and XPS derived O/C data, shows the well-established pattern for peats, lignites, and other coals. In general, O/C decreases as the percent of aromatic carbon increases, with the expected magnitude ordering, peats > lignites > higher ranking coals. Most of the H/C values of pyrolyzed peats are higher than coals at comparable O/C. A range of O/C levels (0.230.13) were produced from pyrolysis of peats; however, these data, when plotted versus the percent aromatic carbon, fall below the values for lignites and other coals. These results indicate that simple pyrolysis does not appear to fully capture the chemical transformations encountered during the natural formation of coals. Both XPS and 13C NMR results are sensitive to the basic difference in the kinds of organic oxygen species found in peats and coals. The advantages of using a combination of XPS and 13C NMR along with the pitfalls of using a single technique for organic oxygen speciation are discussed. For peats, pyrolyzed peats, lignites, and other coals, XPS results for the total amount of organic oxygen fall between upper and lower limit estimates based on 13C NMR derived parameters associated with different oxygen species. For lignites and other coals, there is a sharp drop in the number of carbonyl and carboxyl groups near 60% aromatic carbon. The amount of carbon- oxygen single-bonded species reflected in the NMR parameters falO and faOCH3 and the XPS parameter C-O oxygen, decrease as the percent aromatic carbon increases. The highest levels of phenolic and phenoxy oxygen are found near 60% aromatic carbon. NMR results show that the amount of phenolic and phenoxy carbon (faP) and aliphatic carbon-oxygen single-bonded species (falO) are very similar for pyrolyzed peats, lignites, and other coals at comparable levels of aromatic carbon. These results indicate that thermal decarboxylation/ decarbonylation and demethoxylation pathways exist for peat and suggest that similar pathways occur during natural coalification processes.

I. Introduction Peat is a sedimentary deposit composed primarily of plant derived material and is a precursor to coal.1-3 There have been many recent studies dealing with the petrographic, chemical, and physical structure of coal and coal precursors.4-34 Additional studies deal with * Corresponding author. (1) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (2) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (3) Durand, B. E. Kerogen; Technip: Paris, 1980. (4) Hatcher, P. G.; Clifford, D. J., Org. Geochem. 1997, 27, 251.

aspects of the chemistry that occur during early stages of the coalification processes.4-20 The chemical transformations of organic oxygen functionalities are an (5) Hatcher, P. G.; Lerch, H. E.; Verheyen, T. V. Int. J. Coal Geol. 1989, 13, 65. (6) Hatcher, P. G.; Wilson, M. A.; Vassallo, A. M.; Lerch, H. E. Int. J. Coal Geol. 1989, 13, 99. (7) Hatcher, P. G.; Lerch, H. E.; Bates, A. L.; Verheyen, T. V. Org. Geochem. 1989, 14, 145. (8) Hatcher, P. G. Energy Fuels 1988, 2, 48. (9) Bates, A. L.; Hatcher, P. G. Org. Geochem. 1989, 14, 609. (10) Rollins, M. S.; Cohen, A. D.; Bailey, A. M.; Durig, J. A. Org. Geochem. 1991, 17, 451. (11) Cohen, A. D.; Bailey, A. M. Int. J. Coal Geol. 1997, 34, 163.

10.1021/ef020050k CCC: $22.00 © 2002 American Chemical Society Published on Web 10/09/2002

Organically Bound Oxygen Forms

integral part of this process. Thermal chemistry and other pathways contribute to the alterations of the deposited materials that are the precursors of coals. Tools that give the ability to quantitatively track organic oxygen species in greater detail would facilitate future work and understanding in this area. Past work has demonstrated that XPS and solid-state 13C NMR are viable direct characterization probes to quantify carbon structural features and organic oxygen functionalities in coal. However, there have been no recent attempts to examine comprehensively the major forms of oxygen and carbon in lignites and peats using a combination of these direct characterization methods. Oxygen is the most abundant heteroatom in peats, lignites, and higher ranking coals; nevertheless, the direct quantification of the organic oxygen species has historically been a formidable analytical challenge. There are well-known problems associated with different techniques for quantifying the total amount of organic oxygen and discrete oxygen functionalities. Total organic oxygen is usually determined by difference.35,36 However, much work has been done to develop alternative methods for organic oxygen determination. Fast neutron activation analysis (FNAA) results corrected for inorganic forms and other modified by-difference formulas have been examined as an alternative to the ASTM oxygen by difference method.35 Other methods employ quantification of high temperature pyrolysis products of coals, by completely converting the CO, CO2 and H2O products to either CO or CO2 and subsequent (12) Cohen, A. D.; Spackman, W.; Dolsen, P. Int. J. Coal. Geol. 1984, 4, 73. (13) Orem, W. H.; Neuzil, S. G.; Lerch, H. E.; Cecil, C. B. Org. Geochem. 1996, 24, 111. (14) Shearer, J. C.; Moore, T. A. Org. Geochem. 1996, 24, 127. (15) Behar, F.; Vandenbroucke, M.; Teermann, S. C.; Hatcher, P. G.; Leblond, C.; Lerat, O. Chem. Geol. 1995, 126, 247. (16) Hatcher, P. G. Adv. Org. Geochem. 1989, 16, 959. (17) Faulon, J.; Carlson, G. A.; Hatcher, P. G. Org. Geochem. 1994, 21, 1169. (18) Stout, S. A.; Boon, J. J.; Spackman, W. Geochimica. Cosmochim. Acta 1988, 52, 405. (19) Behar, F.; Hatcher, P. G. Energy Fuels 1995, 9, 984. (20) Buchanan, A. C.; Britt, P. F.; Struss, J. A. Energy Fuels 1997, 11, 247. (21) Bailey, A. M.; Cohen, A. D.; Orem, W. H.; Blackson, J. H. Chem. Geol. 2000, 166, 287. (22) Venkatesan, M. I.; Ohta, K.; Stout, S. A.; Steinberg, S.; Oudin, J. L. Org. Geochem. 1993, 20, 463. (23) de Leeuw, J. W.; Largeau, C. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993, pp 23-72. (24) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. J. Anal. Appl. Pyrolysis 2000, 54, 153. (25) Britt P. F.; Buchanan, A. C., III; Martineau, D. R. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 283. (26) Tooke, P. B.; Grint, A. Fuel 1983, 62, 1003. (27) Aida, T,; Nishisu, A.; Yoneda, M.; Yoshinaga, T.; Tsutsumi, Y.; Yamasishi, I,; Yoshida, T. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2001, 46, 325. (28) Ignasiak, B. S.; Nandi, N.; Montgomery, D. S. Anal. Chem. 1969, 41, 1676. (29) Kuehn, D. W.; Snyder, R. W.; Davis, A.; Painter, P. C. Fuel 1982, 61, 682. (30) Riesser, B.; Starsinic, M.; Squires, E.; Davis, A.; Painter, P. C. Fuel 1984, 63, 1253 (31) Payne, D. F.; Ortoleva, P. J. Org. Geochem. 2001, 32, 1073. (32) Given, P. H.; Spackman, W.; Painter, P. C.; Rhoads, C. A.; Ryan, N. J., Alemany, L.; Pugmire, R. J. Org. Geochem. 1984, 6, 399. (33) Wilson, M. A.; Pugmire, R. J.; Grant, D. M. Org. Geochem. 1983, 5, 121 (34) Dereppe, J.; Boudou, J. P.; Mreaux, G.; Durand, B. Fuel 1983, 62, 575. (35) Ehmann, W. D.; Koppenall, D. W.; Hamrin, C. E., Jr.; Jones, W. C.; Prasad, M. N.; Tian, W. Z. Fuel 1986, 65, 1563. (36) Gluskoter, H. J.; Shimp, N. F.; Rucch, R. R. In Chemistry of Coal Utilization; Wiley-Interscience: New York, 1981; Chapter 7, p 384.

Energy & Fuels, Vol. 16, No. 6, 2002 1451

quantification of either CO28 or CO2.37 Other approaches in this area capitalize on recent analytical advances38 and directly quantify the CO, CO2, and H2O pyrolysis products.39 All of these pyrolysis approaches rely on complete elimination of organic oxygen forms during pyrolysis and minimal inorganic contributions to the evolved oxygen-containing gases for an accurate determination of organic oxygen in coals and other types of sedimentary organic matter. XPS is a surface sensitive technique that enables the direct quantification of oxygen as well as other elements. Organic oxygen can be determined using methods that take into account the inorganic contribution to the total oxygen signal. Although XPS is a surface sensitive technique, numerous XPS studies of coals and other complex carbonaceous materials have shown that the amount of surface organic oxygen can be comparable to bulk analysis when samples are homogenized by finely grinding the sample.40 If bulk data is available, comparisons with XPS data for oxygen can ensure surface homogeneity or reveal important information about differences in surface physical properties and chemistry. These materials have been studied extensively in the past by other techniques and valuable data on oxygen functionalities exist, often qualitative in nature, which were either obtained by wet chemical or indirect characterization methods. In addition to the changes in organic oxygen functionalities during coalification, significant changes occur to the hydrocarbon chemical and structural framework. Because of this, peats, lignites and higher ranking coals are ideal, well-defined, complex materials to use for further development of 13C NMR and XPS methods for quantitative analysis of organic oxygen species. The ability to differentiate the kinds of organic oxygen is important for understanding both the formation and the utilization processes of complex carbonaceous materials. Infrared (IR) spectroscopy has been used for many years to get qualitative pictures of organic oxygen species.29,30,41 Wet chemical methods are another established way to get information about organic oxygen functionalities.27,42,43 Solid-state (SS) 13C NMR analysis31-34,43-54 and XPS40,50-53,55,56 have been used to gain quantitative information about the kinds of organic oxygen species in coals and other carbon(37) Culmo. R. Microchim. Acta 1968, 811-815. (38) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R., Grave, D., Baillargeon, M,; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 319. (39) MacPhee, J. A.; Charland, J. P.; Giroux, L.; Price, J. T.; Hutny, W. P.; Khan, M. A. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2001, 46, 340. (40) Kelemen, S. R.; Kwiatek. P. J. Energy Fuels 1995, 9, 841. (41) Dyrkacz, G. R.; Bloomquist, C. A.; Solomon, P. R. Fuel 1984, 63, 536. (42) Blom, L.: Edelhausen, L.; van Krevelen, D. W. Fuel 1957, 36, 135. (43) Marata, S.; Hosokawa, M.; Kidena, K.; Nomura, M. Fuel Process. Technol. 2000, 67, 231. (44) Mathews, J. P.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 2001, (45) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (46) Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68, 547. (47) Pugmire, R. J.; Solum, M. S.; Grant, D. M.; Critchfield, S., Fletcher, T. H. Fuel 1991, 70, 414. (48) Perry, S. T.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Proc. Combust. Inst. 2000, 28, 2313.

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Table 1. Elemental Data for Lignites and Higher Ranking Coals

sample

source of sample

wt % (daf) carbon

(per 100 C) hydrogen

(per 100 C) nitrogen

(per 100 C) sulfur

(per 100 C) oxygen by difference43

North Dakota-B (lignite) Montana (lignite) Alaska (lignite) Texas (lignite) North Dakota-H (lignite) Beulah Zap (lignite) Wyodak (coal) Illinois No. 6 (coal) Blind canyon (coal) Pitt. No. 8 (coal) Lewiston (coal) U. Freeport (coal) Pocahontas (coal) Buck Mountain (coal)

DECS-11 DECS-25 PSOC-1533 PSOC-1422 PSOC-1482 AP AP AP AP AP AP AP AP PSOC-1468

74.2 75.6 69.9 74.9 72.2 75.1 76.0 80.7 81.3 85.0 85.5 88.1 91.8 96.2

73 82 84 74 76 79 86 77 86 77 76 66 59 16

1.2 1.2 1.2 2.0 1.4 1.4 1.3 1.5 1.7 1.7 1.6 1.6 1.3 0.8

0.4 0.2 0.1 0.4 0.4 0.4 0.3 1.3 0.2 0.4 0.3 0.3 0.2 0.2

20.2 17.6 26.0 18.2 22.4 19.4 16.7 9.4 10.0 6.1 5.9 4.0 1.4 0.9

Table 2. Elemental Data for Peats

sample

wt % (daf) carbon

(per 100 C) hydrogen

(per 100 C) nitrogen

(per 100 C) sulfur

(per 100 C) oxygen by difference

Minn. Hem. (peat) Me. Sph. (peat) Lox. Nym. (peat) Lox. Saw. (peat) Sh. Rv. Rhiz. (peat) Oke. Tax. (peat) Oke. Nym. (peat) Coot Bay (peat) N. C. 1st Col. (peat)

58.6 52.5 58.1 58.8 52.8 58.8 58.9 44.3 63.5

117 131 130 111 118 121 122 131 107

4.5 0.8 6.3 4.8 3.9 4.0 5.3 5.4 1.3

0.2 0.1 0.5 0.7 2.8 0.2 0.3 2.6 0.1

42.1 58.9 39.1 40.1 50.4 41.0 39.4 32.3 35.0

aceous materials. The carbon signal from 13C NMR and carbon (1s) XPS are sensitive to chemical shifts caused by bonding to oxygen. Both techniques take advantage of this fact in developing methods to quantify different oxygen functionalities. Different underlying assumptions go into the interpretation of 13C NMR and XPS data for the bound oxygen’s effect on the carbon signal. A comparison of results from 13C NMR and XPS have the potential to reveal different aspects of organic oxygen functional groups in complex materials. In the present study, a combination of XPS and solid-state 13C NMR data has been used to quantify the organic oxygen species in peats, lignites and higher ranking coals. Both 13C NMR4-9,33,34,45-54,57-59and XPS50-53,60 are capable of providing information about the amount of aromatic carbon present in complex carbonaceous materials. In the past, there has been considerable debate (49) Genetti, D.; Fletcher, T. H.; Pugmire, R. J. Energy Fuels 1999, 13, 60. (50) Kelemen, S. R.; Siskin, M.; Homan, H. S.; Pugmire, R. J.; Solum, M. S. SAE Technical Paper Series 982715; 1998. (51) Kelemen, S. R.; Siskin, M.; Most, W. J.; Kwiatek, P. J.; Pugmire, R. J.; Solum, M. S. SAE Technical Paper Series 982716; 1998. (52) Edwards, J. C.; Choate, P. J. SAE Technical Paper Series 932811; 1993. (53) Kelemen, S. R.; Siskin, M.; Avery, N. L., Rose, K. D., Solum, M., Pugmire, R. J. SAE Technical Paper Series 2001-01-3583; 2001. (54) Freitas, J. C. C.; Bonagamba, T. J.; Emmerich, F. G. Energy Fuels 1999, 13, 53. (55) Clark, D. T.; Wilson, R. Fuel 1983, 62, 1034. (56) Grint, A.; Perry, P. L. Fuel 1983, 62, 1029. (57) Solum, M. S. Encyclopedia of NMR; John Wiley and Sons Ltd.: New York, 1996; p 5047. (58) Solum, M. S.; Pugmire, R. J.; Jagtoyen, M.; Derbyshire, F. Carbon 1995, 33, 1247. (59) Orendt, A. M.; Solum, M. S.; Sethi, N. K.; Pugmire, R. J.; Grant, D. M. In Advances in Coal Spectroscopy; Meuzelaar, H., Ed.; Plenum Press: New York, 1992; Chapter 10, p 215. (60) Kelemen, S. R.; Rose, K. D.; Kwiatek, P. J. Appl. Surf. Sci. 1993, 64, 167.

concerning the quantitative nature of 13C NMR for determining the percentage of aromatic carbon in coal.46 In the present study, both 13C NMR and XPS have been used to track the level of aromatic carbon. Use of both techniques to check aromatic carbon is desirable since each approach has a different set of underlying assumptions and associated uncertainties. 13C NMR is also a powerful way to extract other hydrocarbon chemical and structural parameters.45,47-51 II. Experimental Section (A) Samples. Fresh starting coal samples were obtained in sealed ampules from the Argonne Premium (AP) Coal Sample Program.61 Other coal samples, DECS (Department of Energy Coal Samples) and PSOC (Penn. State Coals), were obtained from the Pennsylvania State University coal sample bank62 and are identified by their DECS or PSOC numbers. The elemental data for these coals are listed in Table 1. The peat samples were obtained from the peat sample bank of the University of South Carolina.63 A detailed description of the peat samples used in the present study appear elsewhere.63 The elemental data for the peat samples are shown in Table 2. These peats differ significantly in composition due to the variety of different plant inputs and depositional settings. For example, the “Sh. Riv. Rhiz” (SRR) and “Coot Bay” (CB) peats are both from intertidal mangrove swamps of South Florida and thus are much higher in inorganic content and total sulfur than all other samples.63 However, they also differ in that the SRR sample has a dominance of roots and the CB sample contains significantly more leaves and bark, making it poten(61) Vorres, K. S., Ed. The Users Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, IL, 1989; Report ANL-PCSP-89-1. Vorres, K. S. Energy Fuels 1990, 4, 420. (62) Glick, D. C.; Davis, A. Org. Geochem. 1991, 17, 421. (63) Cohen, A. D.; Rollins, M. S.; Durig, J. R.; Raymond, R. J. Coal Qual. 1991, 10, 145.

Organically Bound Oxygen Forms tially more aliphatic-rich. The “Oke. Tax.” (OT), “Minn. Hem.” (MH), and “NC 1st Col” (NCC) peat samples, on the other hand, are derived primarily from freshwater, swamp-forest settings (i.e., woody, lignin-rich precursors), but the NCC sample is also noticeably more coalified than any of the other peat samples (i.e., its previtrinitic macerals are more orange) and its humotelinitic macerals have the highest initial reflectance of any samples tested.63 Thus, the NCC sample might be inherently lower in aliphatics and reactivity. On the other hand, although the MH, OT, and NCC samples are all derived from woody plants, MH and OT are derived from gymnosperms and might be more resin-rich than the NCC sample, which is derived primarily from hardwood trees. The “Me Sph.” (MS), “Lox. Nym.” (LN), and “Oke. Nym.” (ON) peat samples are the least decomposed of any samples tested and, therefore, would be expected to be somewhat enriched in aliphatics. The MS sample is, in fact, the better preserved of the three. These samples would be expected to differ somewhat in chemistry, however, as MS is derived primarily from a moss (nonvascular plant), whereas both LN and ON are derived primarily from a vascular plant (water lily). Because ON and LN are derived primarily from the same plant, one would expect them to have similar chemical properties, even though the former comes from a temperate swamp (Okefenokee Swamp in Georgia) and the latter from a tropical setting. The “Lox. Saw.” (LS) and LN samples were collected in the same location (Loxahatchee Wildlife Refuge of Florida), but one comes from a setting where sedge debris (saw grass) is accumulating and the other where water lilies dominate. Laboratory pyrolysis of peat samples was done in a quartz lined furnace in 1 atm of helium. The outlet of the reactor was connected to a bellows pump and a recirculation loop. The total volume of the reactor and recirculation loop was 100 cm3. The sample size for these experiments was 20 to 40 mg. Prior to pyrolysis the sample was introduced into the quartz reactor vessel which was evacuated to 0.1 Pa (1 × 10-3 Torr). The reactor was then pressurized to 200 kPa with helium and reevacuated. This purging was repeated several times to ensure the removal of air and water from the system before heat up. The conversion of the thermolysis time and temperature parameters into a laboratory vitrinite reflectance (R0) scale was done based on a vitrinite maturation model.64 Pyrolysis for five minutes at 350, 375, and 400 °C corresponds to R0 values of 0.5, 0.6 and 0.7, respectively, on this laboratory scale. Notice that these R0 values correspond to subbituminous and higher ranking coal. It is recognized that vitrinite maturation kinetic model64 is not necessarily optimized for the earliest stages of vitrinite maturation. Indeed, the results from the present study show that the response of many of the carbon and oxygen structural features in pyrolyzed peats resemble the features found in lignites (R0 ≈ 0.25-0.4). Therefore the pyrolysis time and temperature conditions used in the present work likely cover earlier stages of coalification than those implied using the specific vitrinite maturation model.64 (B) X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were obtained with a Vacuum Generators (VG) ESCA Lab system using Al K alpha nonmonochromatic radiation and a five-channel detection arrangement. The samples were evacuated to remove moisture and ground into fine powders. The powders were mounted on a metallic nub via nonconducting double-sided tape. 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 correcting for atomic sensitivity. The amount of organic oxygen was derived from the total oxygen (1s) signal by taking into account inorganic contributions.40 For coal, the amount of each inor(64) Sweeney, J. J.; Burnham, A. K, Bull. Am. Assoc. Pet. Geol. 1990, 74, 1559.


Figure 1. Example XPS carbon (1s) spectrum and curve resolution into different components for peat, pyrolyzed peat, lignite, and higher ranking coals. ganic element determined by XPS may differ from the bulk inorganic elemental composition due to particle size effects and organic encapsulation.40 The relative amount of aromatic carbon was determined by the method of Π to Π* signal intensity.60 Organic oxygen forms in the peats, lignites, and higher ranking coals were determined by analyzing oxygen’s effect on the XPS carbon (1s) signal of adjacent carbon atoms. Five peaks were used to curve-resolve the XPS carbon (1s) signal of peat. These occur at 284.8, 285.3. 286.3, 287.5, and 289.0 ((0.1) eV. Figure 1 shows examples of XPS carbon (1s) spectra and the results of curve resolution into different components for a peat, a pyrolyzed peat, a lignite, and a coal. The 284.8 eV peak represents contributions from both aromatic and aliphatic carbon. The 286.3 eV peak represents carbon bound to one oxygen by a single bond (e.g., C-O, C-OH, etc.). The 287.5 eV peak corresponds to carbon bound to oxygen by two oxygen bonds (CdO and O-C-O). The 289.0 eV peak corresponds mainly to carbon bound to oxygen by three bonds (Od C-O). The 285.3 peak will have contributions mainly from carbon adjacent from carboxyl carbon (beta peak) and carbon bound to nitrogen (i.e., pyrrole and pyridinic). The 285.3 eV peak is therefore fixed to the sum of the intensity of the 289.0 eV peak and the intensity of carbon adjacent to nitrogen (i.e., twice the nitrogen level). For pyrolyzed peats and coals, the amount of carboxyl (Od C-O) oxygen is associated with twice the intensity of the 289 eV curve-resolved peak. The amount of carbonyl oxygen intensity corresponds to the 287.5 eV peak intensity and the amount of oxygen associated with carbon-oxygen single-bond species is determined by subtracting the carbonyl and carboxyl amounts from the total amount of organic oxygen. The interpretation of the curve-resolved carbon (1s) spectrum for peats is different in one aspect. The 287.5 eV peak is associated with O-C-O species. Therefore, the total amount of oxygen associated with carbon-oxygen single-bond species is determined by subtracting the carboxyl oxygen level (twice the intensity of the 289 eV curve-resolved peak) from the total amount of organic oxygen. (C) Solid-State 13C NMR Spectroscopy. High-resolution solid-state 13C NMR measurements of peat samples were performed using a Chemagnetics CMXII-200 spectrometer operating at a static magnetic field of 4.7 T (50.2 MHz 13C). Peat samples were packed into a 5-mm diameter zirconia rotor and spun at 8-kHz. Cross-polarization magic-angle spinning (CPMAS) NMR was used to characterize the peat samples. The

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Figure 2. (A) Example solid state of peats.

Kelemen et al.

13

C NMR spectrum with different chemical shift ranges and (B) solid state

Table 3. Definition of structural parameter

NMR Structural and Lattice Parameters carbon type

90-240 90-165 165-240 150-165 135-150 90-135 0-90 22-50 0-22 & 50-60 50-90 50-60

aromatic/carboxyl/carbonyl/amide aromatic carboxyl/carbonyl/amide phenoxy/phenolic alkyl-substituted aromatic bridgehead aromatic aliphatic methylene/methine methyl/methoxy alcohol/ether methoxy

definition (faP +

FAA Cn′

C NMR spectra

chemical shift range (ppm)

fa fa′ faC faP faS faB fal falH fal* falO Fa-OCH3 lattice parameter

13C

13

faS)/fa′

fal/faS

CPMAS experiments were performed at a 1H-13C CP contact time of 3 ms and a pulse repetition delay of 2 s. The 1H and 13 C radio frequency fields for Hartmann-Hahn match during cross-polarization and the proton high power decoupling during data acquisition were at 62.5 kHz. The definition of 13 C NMR structural parameters and chemical shift ranges are the same as those reported previously45,50,51 and are shown in Table 3. An example solid state 13C NMR spectrum of the division into different chemical shift ranges are shown in Figure 2(A). Figure 2(B) shows the solid state 13C NMR spectra of peats. A complete list of structural parameters, obtained by integration of the solid-state 13C NMR spectrum, is found in Tables 6 and 7. For peats, the methoxy peak is fairly wellresolved and could be deconvolved. In fact, integration of the NMR signal over the selected chemical shift range for the methoxy group yielded results that were comparable to those obtained by deconvolution.

III. Results (A) Elemental and Structural Analyses. A simple plot of O/C versus H/C (called a Van Krevelan Diagram) is recognized as a way to understand the fundamental chemical evolution pathways of different types of sedimentary organic matter.1-3 However, total organic oxygen is usually determined by difference. XPS offers one way to quantify directly the level of organic oxygen. The H/C and O/C atomic ratios are plotted for peat,

description fraction of aromatic carbons with attachments average aliphatic carbon chain length Table 4. XPS Results for Lignites and Higher Ranking Coals Per 100 Carbon Atoms sample

aromatic carbon

organic oxygen

North Dakota-B (lignite) Montana (lignite) Alaska (lignite) Texas (lignite) North Dakota-H (lignite) Beulah Zap (lignite) Wyodak (coal) Illinois No. 6 (coal) Blind Canyon (coal) Pitt. No. 8 (coal) Lewiston (coal) U. Freeport (coal) Pocahontas (coal) Buck Mountain (coal)

55 52 38 61 56 53 52 64 59 65 73 71 80 91

20.3 18.0 23.5 23.3 22.4 18.8 16.9 10.9 10.0 7.8 8.0 4.5 3.2 1.0

lignite, and higher ranking coal in Figure 3. The H/C values come from bulk elemental analysis data. The line in Figure 3 is the fit to the coal data. The O/C values were obtained from XPS data (Tables 4 and 5). All peat samples have higher H/C and O/C values than lignites an other coals. These results are in good general agreement with generally established ordering for these kinds of materials.1 The range of organic oxygen values determined by XPS for peat is roughly between 20 and 30 oxygen atoms per 100 carbons (Table 5). These results are significantly lower than the amount of



Table 5. XPS Results for Peats and Laboratory Pyrolyzed Peats Per 100 Carbon Atoms sample

hydrogen

aromatic carbon

organic oxygen

Oke. Nym. (peat) Me. Sph. (peat) Minn. Hem. (peat) Minn. Hem. (R0 ) 0.5) Minn. Hem. (R0 ) 0.6) Lox. Nym. (peat) Lox. Nym. (R0 ) 0.5) Lox. Nym. (R0 ) 0.6) Lox. Nym. (R0 ) 0.7) Lox. Saw. (peat) Lox. Saw. (R0 ) 0.5) Lox. Saw. (R0 ) 0.6) Lox. Saw. (R0 ) 0.7) Sh. Rv. Rhiz. (peat) Sh. Rv. Rhiz. (R0 ) 0.5) Sh. Rv. Rhiz. (R0 ) 0.6) Sh. Rv. Rhiz. (R0 ) 0.7) Oke. Tax. (peat) Oke. Tax. (R0 ) 0.5) Oke. Tax. (R0 ) 0.6) Oke. Tax. (R0 ) 0.7) Coot Bay (peat) Coot Bay (R0 ) 0.5) Coot Bay (R0 ) 0.6) Coot Bay (R0 ) 0.7) N. C. 1st Col. (peat) N. C. 1st Col. (R0 ) 0.5) N. C. 1st Col. (R0 ) 0.6) N. C. 1st Col. (R0 ) 0.7)

122 131 117 98 91 130 105 98 97 111 95 91 96 118 102 95 88 121 106 91 84 131 109 109 100 107 92 80 75

23 24 26 43 43 23 48 50 53 28 52 58 52 35 49 53 61 25 46 55 59 33 37 45 49 29 48 53 60

27.2 27.3 29.6 17.2 16.1 27.1 18.2 18.7 15.3 32.9 21.7 18.5 20.0 29.6 19.6 17.0 14.6 31.2 15.0 13.9 13.4 28.4 22.8 16.7 19.3 25.9 16.1 14.8 14.3

oxygen determined by difference (Table 2). The values of organic oxygen determined by-difference are believed to be inaccurate. Selected peats were pyrolyzed in the laboratory to R0 equal to 0.5, 0.6, and 0.7 calculated from experimental time and temperature conditions using the EasyRo method.64 Figure 4 shows that there are significant changes in the 13C NMR spectrum of Shark River Rhiz. peat after laboratory pyrolysis. These changes are similar among all peats studied in this work. There is a loss of intensity between 50 and 90 ppm and above 165 ppm due to the loss of organic oxygen species. For fresh peat, the methoxy signal at 56 ppm is fairly wellresolved and this signal disappears upon pyrolysis. The relative intensity of the signals between 90 and 165 ppm increases due to a relative increase in the amount of aromatic carbon. On the bases of these results, the oxygen to carbon atom ratio and the hydrogen to carbon atom ratio are expected to decline after laboratory pyrolysis. The H/C and O/C data for pyrolyzed peats are plotted in Figure 3 along with peat, lignites, and higher ranking coals. The H/C and O/C values for pyrolyzed peats are much lower than the original peats. Pyrolyzed peats have higher H/C levels than lignites at comparable organic oxygen level. Some of the H/C data for the pyrolyzed peats fall close to coals at comparable organic oxygen level. The fact that H/C level of pyrolyzed peats fail to H/C levels of lignites of comparable organic oxygen level, is one indication that the laboratory pyrolysis conditions do not completely simulate natural coalification. Natural coal maturation is a complex process involving diagenetic and catagenetic changes to the sedimentary organic matter. Microbial activity as well as thermal chemistry and other chemical and biological processes participate in the overall coalifica-

tion process. A more detailed examination, using 13C NMR and XPS methods, into the chemical structural transformations that occur during pyrolysis of peat, discussed below, show that mildly pyrolyzed peats share some common chemical structural features with lignites and other coals. Both solid-state 13C NMR and XPS were used to quantify the amount of aromatic carbon in peat, pyrolyzed peat, lignite, and higher ranking coals. Figure 5 shows a comparison between the percent aromatic carbon determined by XPS and NMR. Published results for coal45,59,60 are included for comparison. There is good to excellent agreement between the XPS and the 13C NMR data. Not surprisingly, peat has significantly lower levels of aromatic carbon than lignites and higher ranking coals.6-9,34 The level of aromatic carbon in the laboratory-pyrolyzed peats is much greater than in the starting materials. The level of aromatic carbon in the pyrolyzed peats is comparable to lignites and lower rank coals. While there is a tendency for the 13C NMR aromaticity values to be slightly higher than the XPS values, a tendency which has been noted before with other carbonaceous samples,50 the close correspondence between the XPS and NMR values indicate that the aromatic carbon level at the surface of these finely ground peat, pyrolyzed peat, and coal are comparable to those in the bulk. The reflectance of vitrinite (Ro) is an internationally recognized rank parameter of coals.65 It is wellestablished that the cellulose-related components are lost very early in the coalification process.66 The huminite/vitrinite reflectance from peat to the anthracite stage is controlled primarily by the chemical changes in the lignin derived components.65 For coal, it is wellestablished that the amount of aromatic carbon increases with sample maturity. Figure 6A shows that the percent of aromatic carbon increases with increasing vitrinite reflectance for the present set of lignite and higher ranking coal samples. The reflectance data for peat is based on the mean random reflectance of humocollinites.63 Further details about the reflectance measurements for peats appear elsewhere.63 Vitrinites make up a larger proportion of the sample for coals than do the humocollinites in peats. The sudden drop in the aromatic carbon level for peat at low reflectance is associated with the presence of a large amount of cellulose-related material in peats. For coal, it is well-known that the amount of organic oxygen decreases with increasing coal metamorphism (sample maturity). The measured levels of organic oxygen determined by XPS are plotted versus vitrinite reflectance in Figure 6B. Peats, lignites, and other coals follow the pattern of decreasing amounts of organic oxygen with increasing reflectance. For this series of samples, the organic oxygen level drops rapidly up to a reflectance of 0.5. The higher level of oxygen in peats is partially due to the presence of cellulose-derived material. (65) Mukhopadhyay, P. K. Vitrinite Reflectance as a Maturity Parameter. In Vitrinite Reflectance as a Maturity Parameter; ACS Symposium Series 570; American Chemical Society: Washington, DC, 1994; p 1. (66) Hatcher, P. G.; Wenzel, K. A.; Cody, G. D. Vitrinite Reflectance as a Maturity Parameter. In Vitrinite Reflectance as a Maturity Parameter; ACS Symposium Series 570; American Chemical Society: Washington, DC, 1994; p 112.

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Table 6. Solid State N. Dakota-B Montana Alaska

13C

NMR Results for Lignites

fa

fa′

faC

faP

faS

fal

falH

faOCH3

fal*

falO

0.66 0.58 0.48

0.62 0.54 0.41

0.04 0.04 0.07

0.07 0.06 0.05

0.15 0.14 0.09

0.34 0.43 0.52

0.22 0.28 0.34

0.04 0.03 0.04

0.09 0.09 0.12

0.07 0.07 0.10

Table 7. Solid State sample

fa

fa′

faC

N. C. 1st Col. Oke. Nym. Lox. Saw. Me. Sph. Oke. Tax. Minn. Hem. Coot Bay Sh. Rv. Rhiz. Lox. Nym.

0.45 0.40 0.43 0.40 0.38 0.41 0.36 0.40 0.35

0.36 0.30 0.33 0.33 0.29 0.35 0.28 0.33 0.24

0.09 0.10 0.10 0.06 0.09 0.07 0.09 0.06 0.11

13C

NMR Results for Fresh Peats

faP

faS

fal

falH

faOCH3

fal*

falO

0.05 0.04 0.05 0.04 0.04 0.04 0.04 0.04 0.03

0.07 0.05 0.06 0.04 0.05 0.06 0.05 0.06 0.04

0.56 0.60 0.57 0.60 0.62 0.59 0.63 0.60 0.65

0.33 0.18 0.20 0.18 0.30 0.19 0.22 0.18 0.30

0.04 0.07 0.07 0.04 0.06 0.07 0.08 0.08 0.07

0.09 0.11 0.11 0.07 0.10 0.11 0.14 0.13 0.12

0.18 0.38 0.33 0.39 0.28 0.37 0.35 0.38 0.30

Figure 3. Van Krevelen plot for lignites, higher ranking coals, peats, and laboratory pyrolyzed peats.

Figure 5. Comparison of the XPS and 13C NMR results for aromatic carbon from lignites, higher ranking coals, peats, and laboratory-pyrolyzed peats.

Figure 6. Relationship between reflectance and the aromatic carbon and organic oxygen content of peats lignites, and higher ranking coals.

Figure 4. Changes in the solid-state peat during laboratory pyrolysis.

13

C NMR spectrum of

The relationship between the amount of aromatic carbon and hydrogen is shown in Figure 7. For bituminous coals, detailed solid-state 13C NMR67-69 studies have demonstrated that the hydrogen to carbon atom ratio is linearly and inversely related to the percent (67) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1998, 77, 783. (68) Franz, J. A.; Garcia, R.; Lineham, J. C.; Love, G. D.; Snape, C. E. Energy Fuels 1992, 6, 598. (69) Maroto-Valer, M. M.; Love, G. D.; Snape, C. E. Fuel 1994, 73, 1926.

aromatic carbon. This correlation is independent of the inertinite content and geological province.67 This rank range extends from 75 to 90% aromatic carbon and the correlation line is included in Figure 7A for reference. The previously determined values shown here for coal are in agreement with those of this work. Figure 7B shows the plot of H/C vs aromatic carbon using data obtained by XPS. Peats are expected to be less aromatic and have more hydrogen than lignites and higher coals. The pyrolyzed peats cover the range of aromatic carbon levels found for lignites. However, Figure 7A,B show that most of these pyrolyzed peats have more hydrogen than the lignites at equivalent levels of aromatic carbon. The average aliphatic carbon chain length is determined by dividing the percentage of aliphatic carbon by the percentage of alkyl substituted aromatics (fal/faS). The results for peats, lignites, and pyrolyzed peats are plotted in Figure 8 along with published results for


Figure 7. Relationship between aromatic carbon and hydrogen content in peats, pyrolyzed peats, lignites, and higher ranking coals. (A) 13C NMR results for aromatic carbon and (B) XPS results for aromatic carbon.

Figure 8. Variation in the average aliphatic carbon chain length with the percent aromatic carbon in peats, pyrolyzed peats, lignites, and higher ranking coals.

coal.45,61 Although the spectra of peats in Figure 2 clearly show presence of long-chain aliphatics including crystalline (CH2)n peaks, similar to those observed in humic substances,70 the term “average aliphatic chain length” does not necessarily imply the presence of straight or branched aliphatic chains of specific length. Naphthenic and other aliphatic structures contribute to the NMR parameter “fal” and thus influence the NMR parameter for the average aliphatic chain length (fal/ faS). With this qualification, the average aliphatic carbon chain length decreases with increasing aromatic carbon for peats, lignites, and coals. The data for laboratory pyrolyzed peats fall on the same trend line found for peats, lignites, and coals. The fraction of aromatic carbons with attachments (FAA) is determined by dividing the percentage of phenoxy/phenolic groups plus alkyl-substituted aromatics by the percentage of aromatic carbon, i.e., (faP+faS)/ fa′. This is a measure of the degree of aromatic carbon substitution and is independent of aromatic carbon cluster size.50,51 In Figure 10, FAA is plotted versus the percent aromatic carbon in peats, lignites, and pyrolyzed peats along with published results for coals.45,59 For coals, FAA tends to decrease with increasing level of aromatic carbon and a line has been drawn in Figure 10 to highlight this tendency. Peats appear to have slightly lower FAA than pyrolyzed peats or lignites. For the same amount of aromatic carbon, pyrolyzed peats have similar values for FAA as those of lignites and other low ranking coals. (B) Organic Oxygen Forms. XPS was used to quantify the total amount of organic oxygen species in (70) Hu, W.-G.; Mao, J.; Xing, B.; Schmidt-Rohr, K. Environ. Sci. Technol. 2000, 34, 530.


Figure 9. Variation of the fraction of aromatic carbons with attachments for peats, pyrolyzed peats, lignites, and higher ranking coals.

Figure 10. Variation in the amount of organic oxygen with aromatic carbon content for peats, pyrolyzed peats, lignites, and higher ranking coals.

Figure 11. Comparison of the XPS and 13C NMR derived parameters for total organic oxygen in peats, pyrolyzed peats, lignites, and higher ranking coals using (A) 13C NMR lower limit estimate and (B) 13C NMR upper limit estimate.

peats, pyrolyzed peats, lignites, and higher ranking coals. Figure 11 shows that there is a general trend of decreasing amount of organic oxygen with increasing amount of aromatic carbon, however, considerable variation is seen among the different types of samples. Almost all of the pyrolyzed peats have less organic oxygen than lignites and other low ranking coals at comparable level of aromatic carbon. The solid-state 13C NMR parameters faC (carboxyl, carbonyl, and amide), faP (phenolic and phenoxy), and falO (alcohol and ether) have oxygen associated with them. It is not possible to determine the total amount of oxygen relative to carbon from the sum of oxygen related 13C NMR signals (faC, faP, and falO) because it is uncertain what oxygen stoichiometry to assign to each oxygen related 13C NMR signal. Nevertheless, a lower limit estimate for the amount of oxygen associated with

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Table 8. Solid State

13C

NMR Results for Pyrolyzed Peats

sample

laboratory R0

fa

fa′

faC

faP

faS

fal

falH

faOCH3

fal*

falO

N. C. 1st Col. N. C. 1st Col. N. C. 1st Col. Oke. Tax Oke. Tax Oke. Tax Minn. Hem. Minn. Hem. Coot Bay Coot Bay Coot Bay Sh. Rv. Rhiz. Sh. Rv. Rhiz. Lox. Nym.

0.5 0.6 0.7 0.5 0.6 0.7 0.5 0.6 0.5 0.6 0.7 0.5 0.6 0.7

0.60 0.59 0.74 0.58 0.63 0.66 0.58 0.58 0.47 0.59 0.53 0.68 0.72 0.69

0.58 0.54 0.70 0.52 0.57 0.64 0.52 0.54 0.40 0.49 0.48 0.57 0.65 0.67

0.03 0.04 0.04 0.05 0.06 0.02 0.06 0.04 0.08 0.10 0.05 0.11 0.08 0.02

0.06 0.07 0.09 0.07 0.07 0.06 0.06 0.06 0.05 0.08 0.05 0.09 0.09 0.07

0.13 0.13 0.16 0.12 0.13 0.13 0.12 0.12 0.08 0.11 0.10 0.15 0.16 0.15

0.40 0.41 0.26 0.43 0.38 0.35 0.42 0.42 0.53 0.41 0.47 0.32 0.28 0.31

0.30 0.31 0.19 0.32 0.27 0.25 0.28 0.29 0.29 0.25 0.29 0.22 0.19 0.20

0.03 0.03 0.02 0.03 0.03 0.02 0.04 0.03 0.06 0.04 0.04 0.03 0.02 0.01

0.8 0.08 0.06 0.08 0.07 0.08 0.10 0.10 0.15 0.12 0.15 0.07 0.08 0.10

0.04 0.06 0.03 0.07 0.07 0.04 0.08 0.06 0.14 0.09 0.07 0.06 0.04 0.03

Table 9. XPS Results for the Amount of Oxygen Associated with Different Peaks in the Curve-Resolved XPS Carbon (1s) Spectrum of Lignites and higher ranking Coals

sample

total organic oxygen

286.3 eV amount by difference (C-O)

287.5 eV 1× peak amount (CdO)

289.0 eV 2× amount O-CdO

North Dakota-B (lignite) Montana (lignite) Alaska (lignite) Texas (lignite) North Dakota-H (lignite) Beulah Zap (lignite) Wyodak (coal) Illinois No. 6 (coal) Blind Canyon (coal) Pitt. No. 8 (coal) Lewiston (coal) U. Freeport (coal) Pocahontas (coal) Buck Mountain (coal)

20.3 18.0 23.5 23.3 22.4 18.8 16.9 10.9 10.0 7.8 8.0 4.5 3.2 1.0

12.6 10.9 15.1 14.5 13.5 11.2 10.4 9.9 9.6 6.9 6.8 3.9 3.2 1.0

3.1 3.8 5.9 3.6 3.6 1.4 1.3 0.4 0.4 0.9 1.2 0.6 0 0

4.6 4.2 5.0 5.2 5.4 6.2 5.2 0.6 0 0 0 0 0 0

faC is 1.0 (i.e., ketone, amide, etc.) and 0.5 for faP and falO (i.e., aliphatic ether, methoxy, etc.). Figure 11A shows the comparison of the XPS and the 13C NMR derived lower limit estimate for total organic oxygen (faC + 0.5(faP + falO) in peats, lignites, and pyrolyzed peats as well as published results for coal.45,59 It is clear that all pyrolyzed peats have lower organic oxygen levels than the starting peats. Most of the data fall below the parity line, in agreement with the view that the NMRderived equation used to estimate the amount of organic oxygen is a lower limit estimate. This would be expected if hydroxyl species make up a portion of the organic oxygen single-bond species and if carboxyl groups contribute to faC. The maximum amount of oxygen associated with faC is 2.0 (i.e., carboxyl or ester) and 1.0 for faP and falO(i.e., hydroxyl, phenolic, etc.). Figure 11B shows the comparison of the XPS results for total organic oxygen and the 13C NMR derived upper limit estimate for total oxygen (2.0(faC) + faP + falO). Most of the 13C NMR data for peats lie far above the parity line. A discussion concerning the kinds of organic oxygen functionalities in peats and pyrolyzed peats appears below. For lignites and other coals, XPS and 13C NMR derived results for total organic oxygen are in excellent agreement. Parts A and B Figure 11 show that the XPS results for total organic oxygen fall between the 13C NMR-derived upper and lower limit estimates for total oxygen. A combination of XPS and solid-state 13C NMR approaches has been used to quantify the organic oxygen species. Table 9 shows the XPS values for total organic oxygen and the XPS results from the curve resolution of the carbon (1s) spectra for lignites and

higher ranking coals. The intensity of each curveresolved peak is related to the amounts of different kinds of organic oxygen species for lignites and other coals. These results can be compared to the NMR results. Twice the amount the 289 eV peak is associated with the amount of oxygen in carboxyl groups. The 287.5 eV peak is associated with the amount of oxygen in carbonyl groups. The amount of oxygen associated with carbon-oxygen single-bond species is determined by difference. Carboxyl and carbonyl is subtracted from the total amount of organic oxygen. XPS and solid-state 13C NMR methods together were also used to distinguish the amounts and kinds of organic oxygen species in peats. Table 10 shows the XPS results based on the methodology described in the Experimental Section for peat. Species such as O-C-O (present in cellulose) are very likely to contribute to the 287.5 eV carbon (1s) signal. The presence of a significant amount of O-C-O species in peat would complicate the usual interpretation for the amount of oxygen associated with 287.5 eV carbon (1s) signal. For peat, it is assumed that the 287.5 eV peak is associated with carbon bonded to two oxygen atoms each by a single bond. The 289.0 eV peak is associated with carboxyl oxygen. The sum of the 286.3 and 287.5 eV peaks are associated with carbon-oxygen single-bond species. The results shown in Table 10 and Figure 12 Aassume that two oxygen atoms are associated with the curve-resolved 289.0 eV carbon (1s) peak and that the remainder of the oxygen is associated with the 286.3 and 287.5 eV peaks as carbon-oxygen singly bonded species. With these assumptions, there is reasonable agreement between the XPS and 13C NMR derived parameters for organic



Table 10. XPS Results for the Amount of Oxygen Associated with Different Peaks in the Curve-Resolved XPS Carbon (1s) Spectrum of Peats

Minn. Hem. Me. Sph. Lox. Nym. Lox. Saw. Sh. Rv. Rhiz. Oke. Tax. Oke. Nym Coot Bay N. C. first Col.



287.5 eV 0.5× peak amount (O-C-O)

289.0 eV 2× peak amount O-CdO

29.6 27.3 27.1 32.9 29.6 31.2 36.8 31.7 25.9

20.1 18.7 16.9 19.4 17.3 18.4 25.1 21.8 17.7

3.4 3.1 3.3 4.2 3.9 3.5 4.0 3.9 2.6

6.2 5.6 7.0 9.4 8.4 9.4 7.8 6.0 5.6

peats, lignites, and other coals. Figure 13B shows the NMR results for carbon-oxygen multiply bonded species (faC) plotted versus the fraction of aromatic carbon (fa′) for peats, pyrolyzed peats, lignites, and other coals. The solid line in each figure is drawn only through the data points for lignite and coal. The plots of the XPS and 13C NMR data have the same appearance, namely an abrupt drop in the amount of carboxyls and carbonyls near 60% aromatic carbon. Figures 14A shows the XPS results for carbon-oxygen single-bond species plotted against the percent aromatic carbon for peats, lignites, coals and pyrolyzed peats. Figure 14B shows the 13C NMR results for carbon-oxygen single-bond species (faP + falO) plotted against the fraction of aromatic carbon (fa′) for peats, pyrolyzed peats, lignites and other coals. The solid line in each figure is drawn through the data points for coal. The XPS and NMR data plots also have the same appearance, namely a progressive decline in the amount of carbon-oxygen single-bond species with increasing amount of aromatic carbon. XPS and 13C NMR results are sensitive to the basic difference in the kinds of organic oxygen species found in peats and those found in coal. There is a general correspondence between the NMR parameter faC and the XPS peaks found at 287.5, and 289.0 eV in the XPS carbon (1s) curve-resolved spectrum for lignites and other coals. These peaks are interpreted to be primarily due to carbonyl and carboxyl forms of oxygen, respectively. For lignites and other coals, the 13C NMR value of 0.5(faP + falO) agrees well with the amount of oxygen associated with the 286.3 eV peak in the XPS carbon (1s) curve-resolved spectrum, which is associated with carbon-oxygen singly bonded species. For peats, a distinction between carbonyl carbon and carbon-oxygen single-bonded species cannot be made based on the carbon (1s) line shape because of the presence of anomeric carbon (O-C-O) known to be present in cellulose-related material. For peats, different relationships exist between these oxygen related XPS and 13C NMR signals. Here there is a general correspondence between the NMR parameter faC and the 289.0 eV peak in the XPS carbon (1s) curve-resolved spectrum. The 13C NMR value of 0.5(faP + falO) agrees well with the amount of oxygen associated from the sum of 286.3 and 287.5 eV peak in the XPS carbon (1s) curveresolved spectrum. The XPS 287.5 eV peak is usually interpreted as being due to carbonyl oxygen species. However, it can also arise from carbon that is bound to two oxygen each by a single bond. For peats, the general agreement between the value of 0.5(faP + falO) and the amount of oxygen associated with the XPS 286.3 and 13C

Figure 12. Comparison of the 13C NMR results for organic oxygen forms in peat and XPS results. (A) O-C-O associated with the 287.5 eV peak in the XPS-curve-resolved XPS spectrum and (B) CdO associated with the 287.5 eV peak in the XPS-curve-resolved spectrum.

oxygen bearing in mind the value of 0.5(faP + falO)100 is a lower limit estimate for the amount of oxygen associated with single-bond species. These data are expected to fall below the parity line in Figure 12A. The situation is different for lignites and higher ranking coals, where it is assumed that carbonyl is the predominant species associated with the 287.5 eV peak in the XPS curve-resolved carbon (1s) spectrum (Table 9). Figure 12B shows the results when this assumption is used to interpret the data for peat. Clearly, there is much poorer agreement between the XPS and 13C NMR data for oxygen forms using this assumption. Figure 12B shows that this interpretation of the 287.5 eV XPS peak as CdO rather than O-C-O results in an undercount of carbon-oxygen single-bonded species and an over-count of carbonyl species, bearing in mind the value of 0.5(faP + falO)100 is a lower limit estimate for the amount of oxygen associated with single-bond species. Table 11 shows the amount of oxygen associated with different peaks in the curve-resolved XPS carbon (1s) spectrum for pyrolyzed peats using the same assumptions used for lignites and coals. Again, the results shown in Table 11 assume that two oxygen atoms are associated with the curve-resolved 289.0 eV peak and that one oxygen is associated with the 287.5 eV peak in the curve-resolved carbon (1s) spectrum of pyrolyzed peats. The remainder of the oxygen is associated with the 286.3 eV. With these assumptions, there is relatively good agreement between the XPS and 13C NMR derived parameters for individual groups of organic oxygen forms. Figure 13A shows the XPS results for carbon-oxygen multiply bonded species (carbonyl and carboxyl) plotted against the percent aromatic carbon for peats, pyrolyzed

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Table 11. XPS Results for the Amount of Oxygen Associated with Different Peaks in the Curve-Resolved XPS Carbon (1s) Spectrum of Pyrolyzed Peats sample



287.5 eV 1× peak amount (CdO)

289.0 eV 2× peak amount O-CdO

Minn. Hem. (R0 ) 0.5) Minn. Hem. (R0 ) 0.6) Lox. Nym. (R0 ) 0.5) Lox. Nym. (R0 ) 0.6) Lox. Nym. (R0 ) 0.7) Lox. Saw. (R0 ) 0.5) Lox. Saw. (R0 ) 0.6) Lox. Saw. (R0 ) 0.7) Sh. Rv. Rhiz. (R0 ) 0.5) Sh. Rv. Rhiz. (R0 ) 0.6) Sh. Rv. Rhiz. (R0 ) 0.7) Oke. Tax. (R0 ) 0.5) Oke. Tax. (R0 ) 0.6) Oke. Tax. (R0 ) 0.7) Coot Bay (R0 ) 0.5) Coot Bay (R0 ) 0.6) Coot Bay (R0 ) 0.7) N. C. 1st Col. (R0 ) 0.5) N. C. 1st Col. (R0 ) 0.6) N. C. 1st Col. (R0 ) 0.7)

17.2 16.1 18.2 18.7 15.3 21.7 18.5 20.0 19.6 17.0 14.6 15.0 13.9 13.4 22.8 20.4 19.3 16.1 14.8 14.3

11.7 11.2 9.7 9.7 10.7 11.7 10.5 11.5 10.1 11.1 9.8 8.6 9.6 10.4 13.8 12.4 11.3 12.3 11.7 11.7

3.2 2.5 4.5 5.0 2.6 3.0 4.0 3.0 3.4 3.5 2.0 2.6 2.1 2.2 5.0 4.0 4.0 2.0 2.1 1.6

2.3 2.4 4.0 4.0 2.0 7.0 4.0 5.5 3.2 2.4 2.8 3.8 2.2 0.8 4.0 4.0 4.0 1.8 1.0 1.0

Figure 13. (A) XPS and (B) 13C NMR results for carbonoxygen multiply bonded species plotted as a function of the amount of aromatic carbon.

Figure 14. (A) XPS and (B) 13C NMR results for carbonoxygen single-bond species plotted as a function of the amount of aromatic carbon.

287.5 eV peaks indicates that in peats, the XPS 287.5 eV peak is primarily due to carbon that is bound to two oxygen each by a single bond. Anomeric carbon is an example of species such as this and would be expected to be present if a significant amount of cellulose derived material were also present in peats. The fresh peats studied do indeed exhibit a significant amount of anomeric carbon as shown by a peak 105 ppm in the 13C NMR spectra. For pyrolyzed peats, the 13C NMR and XPS relationships for oxygen species follow that of lignites and higher ranking coals. These results are consistent with the absence of significant amounts of cellulose-derived materials in lignites and higher ranking coals, and the concomitant absence of the anomeric carbon peak in the 13C NMR spectra.

Figure 15. 13C NMR results for individual carbon-oxygen single-bond species plotted as a function of the amount of aromatic carbon.

Figure 16. 13C NMR results for methoxy species plotted as a function of the amount of aromatic carbon.

A distinction among carbon-oxygen single-bond species (faP and falO) is possible using 13C NMR but not using XPS. The data for falO and faP are plotted versus the percent aromatic carbon in parts A and B of Figure 15, respectively, for peats, lignites, coals, and pyrolyzed peats. Different patterns emerges for falO and faP. There is a general decrease in falO with increasing amount of aromatic carbon. On the other hand, faP appears to increases up to about 60% aromatic carbon and then declines. 13C NMR can distinguish methoxy species (faOCH3) from other carbon-oxygen single-bond species. The data for faOCH3 are plotted as a function of aromatic carbon in Figure 16 for peats, lignites, and pyrolyzed peats. The level of methoxy species declines as the amount of aromatic carbon increases. Peats have the highest levels of methoxy species. The level of methoxy species for lignites and pyrolyzed peats are comparable.


IV. Discussion A combination of 13C NMR and XPS has been used to reveal different aspects of the organic oxygen functional groups in a range of complex carbonaceous solids. Quantitative characterization results have been presented based on XPS and solid-state 13C NMR data for peats, pyrolyzed peats, lignites, and higher ranking coals. Different underlying assumptions are used in the interpretation of 13C NMR and XPS spectra for oxygen’s effect on the carbon signal. The kinds of materials used in our study have been extensively studied in the past by a variety of different techniques and valuable qualitative data exists concerning oxygen functionalities. Therefore, these are ideal well-defined complex materials to further develop 13C NMR and XPS methods for quantitative analysis of organic oxygen species. There is good general agreement between XPS, 13C NMR and previous findings for lignites and other coals. The results for peats and pyrolyzed peats highlight potential thermal chemical transformations occurring during coalification. A van Krevelen plot for coals and lignites based on elemental analysis of hydrogen and carbon for H/C and organic oxygen derived for the XPS oxygen (1s) intensity and carbon (1s) for O/C follow the established pattern for these materials. All peats have higher H/C and O/C values than lignites and other coals. The H/C and O/C value of every pyrolyzed peat is less than those of the parent peat; however, the H/C and O/C data for pyrolyzed peats do not coincide with the values of coals and lignites. At comparable O/C, most of the H/C data for pyrolyzed peats are higher than those of lignites and other coals. There is good agreement between the 13C NMR and XPS derived values for aromatic carbon in peats and pyrolyzed peats, lignites and other coals. The percent aromatic carbon increases in a regular way with increasing reflectance for peats, lignites, higher ranking coals. In general, H/C decreases as the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher than lignites and other coals of comparable aromaticity. This is likely due to differences in the carbon structural framework between pyrolyzed peats and coals. Although the percent of aromatic carbon increases for pyrolyzed peats relative to the parent peat, the differences in H/C relative to lignites and coals imply that the carbon structural framework for pyrolyzed peats is quantifiably different. This difference indicates that (laboratory) pyrolysis does not necessarily mimic all of the chemical transformations encountered during the natural maturation process of coal. Some carbon structural features of pyrolyzed peats are similar to lignites and other coals. The 13C NMR parameter for the average aliphatic carbon chain length (Cn′) decreases in a regular way with increasing percent of aromatic carbon for peats, pyrolyzed peats, lignites and other coals. Pyrolyzed peats have FAA values similar to lignites and other low ranking coals. These correspond to aromatic carbon levels roughly between 40 and 70%. The fraction of aromatic carbons with attachments is only slightly lower in peats than in lignites and other low ranking coals. The XPS derived value for total organic oxygen decreases with increasing reflectance values for peats and coals. There is an initial steep drop in the organic


oxygen level at low reflectance values. In general, O/C decreases as the percent of aromatic carbon increases. The expected ordering of magnitude of O/C levels is found for these materials, peats > lignites > higher ranking coals. A range of O/C levels (0.23 to 0.13) were produced following pyrolysis of peat. These data, when plotted against the percent of aromatic carbon, fall below the values for lignites. This result implies that the level and kinds of organic oxygen species developed during the pyrolysis of peat may be different than those developed during the natural maturation process of coal. Nevertheless, the subsequent discussion on the kinds of organic oxygen species found in mildly pyrolyzed peats by 13C NMR and XPS techniques show that they share some common features with lignites and higher ranking coals and can be overlaid with those that appear as a result of natural coalification. The fact that some of these chemical and structural features associated with coalification can be produced by simple pyrolysis indicates that thermal chemistry is the primary driver of natural maturation of peat. For lignites and other coals, the percent of aromatic carbon increases with increasing vitrinite reflectance. The relative amount of aromatic carbon increases after pyrolysis of peat. Previous Fourier transform infrared studies of British coals indicated that the percentage of hydroxyl groups increases through the bituminous stage of coalification while overall oxygen is lost by the decline in carbonyls and/or ethers.26 Current 13C NMR results show a maximum in phenolic/phenoxy species near 60% aromatic carbon while both 13C NMR and XPS results show the loss of carbonyl/carboxyl and the decline in carbonoxygen single-bond species with coalification. The increase in the number of phenolic/phenoxy species is found for pyrolyzed peats along with the decline in carbonyl/carboxyl and carbon-oxygen single-bond species. These 13C NMR data for oxygen overlap the data for lignites and other coals. The loss of aromatic methoxyl functionalities from lignin-derived material progresses through the lignite stage and appears completed by the sub-bituminous stage of coalification.16 The present work shows that 13C NMR can differentiate among carbon and oxygen singly bonded species and that the amount of methoxy species (fOCH3) decreases as the percent aromatic carbon increases. The methoxy levels in pyrolyzed peats are similar to the levels in lignites at comparable levels of aromatic carbon. Pyrolysis of peat to an equivalent R0 ) 0.7 (ref 64) results in loss of most methoy species. Decarboxylation reactions are associated with the loss of carboxyl and carbonyl groups following the lignite stage of maturation.16 Our XPS and 13C NMR results show that carboxyl and carbonyl groups are present in peats and are abundant in lignites but are lost with increasing coalification. Pyrolyzed peats show the increasing loss of carboxyl and carbonyl groups with increasing amount of aromatic carbon similar to lignites and other coals at comparable levels of aromatic carbon. The chemical structural changes in peat produced during pyrolysis are undoubtedly accompanied by alterations to the physical structure of peat. These physical alterations to peat have been examined petrographi-

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cally in other work and some features, reminiscent of those that appear in coals, were identified.10-14 V. Conclusions There is good agreement between the 13C NMR and XPS derived values for aromatic carbon in peats, pyrolyzed peats, lignites, and higher ranking coals. The percent aromatic carbon increases as vitrinite reflectance increases. XPS and 13C NMR data for oxygen and carbon chemical/structural features show regular patterns when plotted against the aromatic carbon content. A van Krevelen plot for lignites and other coals, based on elemental analysis for H/C and XPS for O/C, follow the established pattern for these materials. Most of the H/C data for pyrolyzed peats are higher than lignites of comparable O/C level and these differences reflect carbon structural differences between these pyrolyzed peats and lignites. However, some carbon structural features of pyrolyzed peats are similar to lignites and higher ranking coals, The average aliphatic carbon chain length (Cn′) and the fraction of aromatic carbons with attachments (FAA) are similar to lignites and higher ranking coals. In general, O/C decreases as the percent of aromatic carbon increases for peats, lignites and other coals. The expected ordering of the magnitudes of O/C levels for these materials, peats > lignites > higher ranking coals, holds. The data for pyrolyzed peats fall below the values for lignites and coals at comparable levels of aromatic carbon. These results indicate that simple laboratory pyrolysis experiments of the type described in the Experimental Section do not mimic all of the chemical transformations of oxygen encountered during the natural transformation from peats to coals. There is good agreement between the total amount of organic oxygen derived from the XPS oxygen (1s) signal relative to carbon and 13C NMR-derived estimates for total oxygen for peats, pyrolyzed peats, lignites and other coals. A lower limit estimate for total oxygen is based on the 13C NMR value (faC + 0.5(faP + falO) and an upper limit estimate is based on the value (2.0(faC) + faP + falO). XPS and 13C NMR results are sensitive to the basic difference in the kinds of organic oxygen species found in peats and those found in coals. For pyrolyzed peats, lignites, and higher ranking coals, there is agreement between the amount of oxygen associated with the NMR parameter faC and the XPS peaks found at 287.5, and 289.0 eV in the XPS carbon (1s) curve-resolved spec-

Kelemen et al.

trum. These NMR and XPS measures are tied to carbonyl and carboxyl forms of oxygen. For pyrolyzed peats, lignites, and higher ranking coals, there is agreement between the amount of oxygen associated with the NMR value of 0.5(faP + falO) and the 286.3 eV peak in the XPS carbon (1s) curve-resolved spectrum. These NMR and XPS measures are tied to carbonoxygen singly bonded species. For peats, a distinction between carbonyl carbon and carbon-oxygen single-bonded species cannot be made based on the XPS carbon (1s) line shape because of the presence of anomeric carbon. For peats, there is agreement between the amount of oxygen associated with the 13C NMR value of 0.5(faP + falO) and the sum of 286.3 and 287.5 eV peak in the XPS carbon (1s) curve-resolved spectrum. The agreement is expected if the XPS 287.5 eV peaks are primarily due to carbon that is bound to two oxygen each by a single-bond (e.g., anomeric carbon). Anomeric carbon would be expected to be present if a significant amount of cellulose derived material were also present. 13C NMR can differentiate among carbon and oxygen singly bonded species and the amounts for pyrolyzed peats are similar to those in lignites and higher ranking coals at comparable level of aromatic carbon. The amount of methoxy species (fOCH3) decreases as the percent aromatic carbon increases. The total amount of aliphatic carbon singly bonded to oxygen (falO) decreases with increasing level of aromatic carbon. Phenoxy species (faP) reach a maximum near 60% aromatic carbon. Both 13C NMR and XPS results show that there is a sharp drop in the level of CdO and O-CdO groups near 60% aromatic carbon. Taken together, all these results indicate that thermal decarboxylation/decarbonylation and demethoxylation pathways exist for peat and suggest that similar pathways are available during natural coalification processes. These thermal mechanisms should be considered along with biological and catalytic explanations to account for methane formation via demethoxylation. Further work aimed at characterizing other heteroatom behaviors during pyrolysis of peats is in progress. Acknowledgment. We thank Mr. P. J. Kwiatek for conducting the XPS experiments and Mr. B. Liang for conducting the NMR experiments. We acknowledge Drs. R. J. Pugmire, M. S. Solum, Y. Xiao, D. J. Curry, A. E. Bence, and H. Freund for valuable discussions. EF020050K

Characterization of Organically Bound Oxygen Forms in Lignites

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