Energy &Fuels 1994,8,1513-1521
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Density Gradient Centrifugation: Application to the Separation of Macerals of Type I, 11, and I11 Sedimentary Organic Matter B. Artur Stankiewicz,*Michael A. Kruge, and John C. Crelling Department of Geology, Southern Illinois University, Carbondale, Illinois 62901
Gary L. Salmon Illinois State Geological Survey, University of Illinois, Champaign, Illinois 61820 Received May 5, 1994@
Samples of organic matter from nine well-known geological units (Green River Fm., Tasmanian Tasmanite, Lower Toarcian Sh. of the Paris Basin, Duwi Fm., New Albany Sh., Monterey Fm., Herrin No. 6 coal, Eocene coal, and Miocene lignite from Kalimantan) were processed by density gradient centrifugation (DGC) to isolate the constituent macerals. Optimal separation, as well as the liberation of microcrystalline pyrite from the organic matter, was obtained by particle size minimization prior to DGC by treatment with liquid N2 and micronization in a fluid energy mill. The resulting small particle size limits the use of optical microscopy, thus microfluorimetry and analytical pyrolysis were also employed t o assess the quality and purity of the fractions. Each of the samples exhibits one dominant DGC peak (corresponding to alginite in the Green River Fm., amorphinite in the Lower Toarcian Sh., vitrinite in the Herrin No. 6, etc.) which shifts from 1.05g mL-' for the Type I kerogens t o between 1.18and 1.23 g mL-l for Type I1 and II-S. The characteristic densities for Type I11 organic matter are greater still, being 1.27g mL-l for the hydrogen-rich Eocene coal, 1.29 g mL-l for the Carboniferous coal and 1.43 g mL-l for the oxygen-rich Miocene lignite. Among Type I1 kerogens, the DGC profile represents a compositional continuum from undegraded alginite through (bacterial) degraded amorphinite; therefore chemical and optical properties change gradually with increasing density. The separation of useful quantities of macerals that occur in only minor amounts is difficult. Such separations require large amounts of starting material and require multiple processing steps. Complete maceral separation for some samples using present methods seems remote. Samples containing macerals with significant density differences due to heteroatom diversity (e.g., preferential sulfur or oxygen concentration in the one maceral), on the other hand, may be successfully separated (e.g., coals and Monterey kerogen).
Introduction
However, the heterogeneities of the more widespread, economically-important kerogens have, to date, largely resisted investigation. The application of the biological separation technique, density gradient centrifugation (DGC),t o coals6-10was a milestone in the quest to chemically characterize single macerals and produced several studies on coal macerals and their geochemical nature.l1-l6 Prior attempts to study separated kerogen macerals were more
The last two decades have witnessed continued progress in the investigation and understanding of organic matter in coals and kerogens. It has been demonstrated that bulk organic matter in sedimentary rocks is composed of the entities termed macerals. The concept of macerals,l first introduced in 1935,is based on the premise that organic matter is not a homogeneous material but is composed of a mixture of organic (3) Douglas, A. G.; Sinninghe Damst-6, J. S.; Fowler, M. G.; Eglinton, matter from various precursors (lignin, cutin, algae, T. I.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1991,55,275-291. etc.). The original concept remains intact, although (4) Hartgers, W. A.; Sinninghe Damst6, J. S.; de Leeuw, J. W. J. Chromatogr. 1992, 606, 211-220. continuing t o evolve. Reasoning suggests that macerals (5)Landais, P.; Rochdi, A.; Largeau, C.; Derenne, S. Geochim. derived from different precursors and having different Cosmochim. Acta 1993, 57, 2529-2539. (6) Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982, 61, 3-12. optical properties should be chemically distinct also. The (7) Dyrkacz, G. R.; Bloomquist, C. A. A,; Ruscic, L. Fuel 1984, 63, application of analytical methods such as pyrolysis-gas 1367-1374. chromatography and Fourier transform infrared spec(8) Crelling, J. C. Zronmaking Conf. Proc. 1988, 43, 351-356. (S)Crelling, J. C. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. troscopy (FTIR) has demonstrated that coals or kero1989; 34 (l), 249-255. gens dominated by particular macerals (i.e., natural (10)Taulbee, D.; Poe, S. H.; Robl, T.; Keogh, B. Energy Fuels 1989, maceral concentrates) are chemically d i ~ t i n c t i v e . ~ - ~ 3, 3662-670. (11)Senftle, J. T.; Larter, S. R. Org. Geochem. 1987,11, 407-409. (12) Nip, M.; de Leeuw, J. W.; Schenck, P. A. Geochim. Cosmochim.
@Abstractpublished in Advance ACS Abstracts, September 1,1994. (1) Stopes, M. Fuel 1935, 14, 4. (2) Larter, S. R. In Analytical Pyrolysis-Methods and Applications; Voorhees, K., Ed.; Butterworths: London, 1984, pp 212-275.
0887-0624l94l2508-1513$04.50/0
Acta 1988, 52, 637-648. (13) Nip, M.; de Leeuw, J. W.; Schenck, P. A.; Windig, W.; Meuzelam, H. L. C.; Crelling, J. C. Geochim. Cosmochim. Acta 1989,53,671683.
0 1994 American Chemical Society
1514 Energy & Fuels, Vol. 8, No. 6, 1994
limited.2,17-19Recent DGC work on k e r o g e n ~ ~ Ohas -~~ shown that the success of the separation is strongly dependent on organic matter type. The DGC method has also been successfully applied to coal maceral separation for study of coking and combustion properties.24 DGC has been utilized to separate and preconcentrate allochthonous dispersed organic matter (fusinite and vitrinite particles) from tempestite sediments in Mexico.25 In situ studies of single macerals utilizing FTIR,5 electron micro-probe,26and laser pyr~lysis-GC/MS~~ methods have recently improved our knowledge of maceral properties. However, the technical problems and limitations inherent in these techniques emphasize that clean, mechanically separated macerals, when available, are preferable. This paper presents recent advances in maceral separation by density, including innovations in the methods of sample pretreatment. Results from the analysis of the macerals by optical microscopy, fluorescence spectroscopy, and analytical pyrolysis-gas chromatography/mass spectrometry are presented. These techniques are usefid in characterization and verification of the quality of the maceral concentrates.
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Figure 1. Density gradient profile of New Albany Sh. kerogen showing the peak "tailing" resulting from the failure to remove the microcrystalline pyrite present in the sample.
Table 1. Vitrtinite Reflectance Values for All Samples sample vitrinite reflectance (R,ma) Green River Fm. Tasmanite Toarcian from Paris Basin Duwi Fm. Monterey Fm. Herrin No. 6 coal Eocene Kalimantan coal Miocene Kalimantan lignite
0.31 -
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USA, (2) the liptinite-rich Eocene high-volatile C bituminous
Experimental Section Samples. Samples used in this study were chosen because of their diverse petrographic and chemical character. These samples represent typical kerogens from petroleum source rocks, including Types I, 11, 11-S, and 111,and are well described in the literature. The Type I sample from the Mahogany Zone of the Eocene Green River Fm.was collected at the Cathedral Bluffs Oil Shale Mine in the Piceance Basin, western Colorado. The Type I Permian Tasmanite was collected from the Quamby Mudstone at Latrobe, Tasmania. Samples of Type I1 kerogen include one from the Lower Toarcian shale of the Paris Basin, France and one from the Upper Devonian New Albany Shale in Benton County, Indiana. The Campanian-Maastrichtian Duwi Fm. sample, a siliceous shale moderately rich in sulfur, was collected at the Dabaa phosphorite mine located near Quseir, Egypt. The sulfur-rich Type 11-S sample was collected from an outcrop of the Miocene Monterey Fm. at Naples Beach, near Santa Barbara, California. Type I11 organic matter is represented by (1)the Upper Carboniferous, high-volatile B bituminous coal from the Herrin No. 6 seam, Illinois Basin, (14)Nip, M.;de Leeuw, J. W.; Crelling, J. C. Energy Fuels 1992,6 , 125-136. (15)Kruge, M.A.;Crelling, J. C.; Hippo, E. J.; Palmer, S. R. Org. Geochem. 1991,17,193-204. (16)Kruge, M.A.;Landais, P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1992,37(41,1595-1600. (17) Senftle. J. T.: Yordv. K. L.: Barron. L. S.: Crelline. J. C. 1987 Eastern Oil Shale Sympoiium; Kentucky'Enerb Cabinet, 1987;pp 18-20. (18)Kruge, M.A.; Crelling, J. C.; Rimmer, S. M. In 1988 Eastern Oil Shale Symposium; Lazar, D. J., Ed.; Institute for Mining and Minerals Research; Lexington, KY, 1988;pp 411-417. (19)Pradier, B.;Landais, P.; Rochdi, A.; Davis, A. Org. Geochem. 1992,18, 241-248. (20)Kruge, M. A,; Stankiewicz, B. A,; Crelling, J. C. In Poster Sessions from the 16th International Meeting of Organic Geochemists; Oygard, K., Ed.; Stavanger 1993;pp 140-144. (21)Stankiewicz, B. A.;Kruge, M. A.; Crelling, J. C. Bull. Centres Rech. Exp1or.-Prod. Elf-aquitaine, in press. (22)Han, Z.; Kruge, M. A.; Crelling, J. C.; Stankiewicz, B. A. O g . Geochem., in press. (23)Stankiewicz, B. A,; Kruge, M. A. In Book ofAbstracts, Geochem. Division; 207th ACS National Meeting, San Diego, American Chemical Society: (Washington, DC, 1994;p 31. (24)Crelling, J. C.; Thomas, K. M.; Marsh, H. Fuel 1993,72,349357. (25)Kruge, M. A.;Stankiewicz, B. A.; Montanari, A.; Crelling, J. C.; Bensley, D. F. Geochim. Cosmochim. Acta 1994,58,1393-1397. (26)Mastalerz, M.; Bustin, R. M. J.Microsc. 1993,171, 153-166. (27) Stout, S. A.; Lin, R. Org. Geochem. 1992,18, 229-239.
coal from the Tanjung Fm. of southeastern Kalimantan, Indonesia, and (3) the Miocene lignite sample collected from the Warukin Fm., also of southeastern Kalimantan. All samples are immature or of low maturity (pre- or very early oil window) (Table 1). The low degree of thermal alteration assures that the macerals retain much of their original distinctive petrographic and chemical characteristics. Sample Preparation. Samples were crushed to -150 pm and extracted with excess CHzClz in an ultrasonic bath (10 min x 4). The extraction residues were demineralized by standard techniques using 20% HC1 and 48% HF for 24 h. Before micronization, samples were treated with liquid nitrogen t o induce fractures between macerals and t o promote brittle behavior in the normally ductile liptinitic macerals. Pretreatment with liquid nitrogen also liberates pyrite enclosed within algal bodies, which enhances their removal during floatation. Samples were then reduced to 0.5-5 pm in a Garlock FMT mill at 20 "C in a nitrogen atmosphere. This method further enhances the separability of the macerals from each other and from the non-acid-digestible minerals, especially authigenic microcrystalline pyrite. The removal of the pyrite from the sample is critical, because the DGC profile can be seriously disrupted and "tailing" in the upper density range can be observed (Figure 1). In the Garlock FMT mill, a partial segregation by density occurs during micronization. The mill has a glass container which collects the micronized sample, and a porous bag, which is an escape system for excess nitrogen gas, but also traps -20% of the sample. Less dense particles are preferentially concentrated in the bag by factor of 2. The particles in the bag are also more uniformly crushed, with -95% of the particle population reduced to -1 pm. This effect was perhaps unintended by the manufacturer, but it can be exploited to further enhance the efficacy of the maceral separation process. After micronization, the organic matter was suspended in water by means of an ultrasonicator and then layered upon a CsCl density gradient, following published The gradient was centrifuged in a Beckman J2-21M centrifuge using a zonal rotor at 17 500 rpm (g at rmax = 30 500) for 1 h. The gradient was fractionated and density of the fractions were measured using a Mettler density meter. The kerogen fractions were recovered by filtration for further chemical and petrologic analysis. X-ray diffraction analyses of the sink-floated kerogen were performed t o assess the effectiveness of the pyrite removal. The main peaks on the DGC profiles correspond to the dominant macerals in the
Density Gradient Centrifugation Table 2. Density Values and Descriptions of the Dominant Macerals density value (g mL-') sample maceral 1.04 Green River Fm. lamalginite 1.10 Tasmanite telalginite 1.18 Lower Toarcian Sh., yellow-brownish Paris Basin amorphinite 1.18 DuwiFm. yellow-brownish amorphinite 1.23 Monterey Fm. red-brownish amorphinite 1.29 Herrin No. 6 coal vitrinite 1.27 Eocene Kalimantan coal perhydrous vitrinite 1.43 Miocene Kalimantan lignite huminite samples. Descriptions of the major maceral concentrates for each density fraction are given in Table 2. "High-resolution" separations (HRDGC) were performed using narrower density range^,^^^^^^ in addition t o the normal DGC processing which yielded 40-50 fractions over a density range of 1.00-1.60 g mL-l. Petrographic Observations. Reflected white light and fluorescence microscopy were performed on aliquots of both whole rock and density fraction samples. Reflectance measurements were taken to determine maturity using rotational reflectance properties of the vitrinite maceral (Table 1).28 Microfluorimetric methods employing a diode array detection system were used t o match and assess the quality of the DGC fractions.29 The high sensitivity of the diode array system permits the recording of spectra from even weakly fluorescing macerals. Fluorescence spectra of DGC fractions were correlated with spectra taken from individual macerals in standard petrographic pellets made from the whole kerogen or coal. Analytical Pyrolysis-GC/MS. Analyses of the isolated kerogens and density fractions were performed utilizing CDS 120 pyroprobe, coupled to an HP 5890 gas chromatograph with an HP 5970 mass selective detector. A 25 m HP-1 column (0.2 mm i.d., film thickness 0.33 pm) was used for separation in the gas chromatograph. Up to 2 mg of each sample was pyrolyzed in a flow of helium for 20 s in a platinum coil at 610 "C, as measured by a thermocouple in the sample holder. The GC oven was operated under the following program: isothermal for 5 min at 0 "C: temperature ramped at 5 "C/min t o 300 "C and then held isothermal for 15 min. The mass spectrometer was operated in full scan mode (50-550 Da, 0.86 scansls, 70 eV ionization voltage). Peaks were identified by their mass spectra and GC retention times, with reference t o the U.S. National Bureau of Standards mass spectral library and the l i t e r a t ~ r e . ~ J ~ ~ ~ ~ - ~ ~
Results and Discussion Petrography and Microfluorimetry of the Concentrated Macerals. Determination of the initial maceral composition of the raw samples is the essential (28) Houseknecht, D. W.; Bensley, D. F.; Hathon, L. A.;Kastens, P. H. Org. Geochem. 1993,20,187-196. (29)Bensley, D. F.; Crelling, J. C. Org. Geochem. 1992,18,365-
372. (30)Radke, M.;Garrigues, P.; Willsch H. Org. Geochem. 1990,15, 17-34. (31)Douglas, A. G.; Sinninghe DamstB, J. S.; Fowler, M. G.; Eglinton, T. I.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1991,55, 275-291. (32)Eglinton, T.I.; Sinninghe Damste, J. S.; Pool, W.; de Leeuw, J. W.; Eijkel, G.; Boon, J. J. Geochim. Cosmochim. Acta 1992,56,15451560. (33)Sinninghe Damste, J. S.; Eglinton, T. I.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1992,56,1743-1751. (34)SinningheDamst6, J. S.; de Leeuw, J. W. Fuel Process. Technol. 1992,30,109-178. (35)Sinninghe Damste, J. S.; de las Heras, F. X. C.; de Leeuw, J. W. J. Chromatogr. 1992,607,361-376. (36)Sinninghe Damste, J. S.; de las Heras, F. X. C.; van Bergen, P. F.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1993,57,389-415.
Energy & Fuels, Vol. 8, No. 6, 1994 1515 Table 3. Maceral Composition by Percentage in the Investigated Kerogens and Coals" other sample Vit Am Al Res Lip Inert. Green River Fm. -5 90 -5 2 Tasmanite 98 Toarcian from < 1 85 10 4 Paris Basin Duwi Fm. -4 68 25 -3 Monterey Fm. -1 95 3 -1 Herrin No.6 coal 89 1.2 5 5.2 88 -5 -5 -2 Eocene Kalimantan coal Miocene Kalimantan lignite 80 -5 -9 -4 aVit = vitrinite, Am = amorphinite, Al = alginite, Res = resinite, Lip = liptinite, Inert = inertinite)
step in the experimental process. Based on these determinations, prediction and control of the quality of the DGC concentrates can be obtained. Table 3 describes the important maceral content of the investigated samples. Cursory inspection of these results allows for the prediction of separation efficiency. Predictions can be made that the separation of the minor resinite (1.2%)in the Herrin No. 6 coal sample will be extremely difficult; however, concentration of the resinite (-5%) in the Kalimantan coals may be successful. Predictions may also be made with respect to the amount of the sample which is needed for processing in order to get sufficient amount of the maceral concentrate after DGC. These predictions are based on the percentages of the minor macerals present in the sample. The quality of the concentrates was verified using spectral analysis. The spectra of the interesting macerals in the raw samples were compared with those of the DGC concentrates as well as spectra of the different density fractions (macerals) within the sample. Figure 2 shows fluorescence spectra of the maceral concentrates from the Duwi Fm., Monterey Fm., and Eocene Kalimantan coal samples. Major macerals show similar maximal as those of the minor ones; however, we do observe major difference in intensities, in each case. Alginite concentrates of the kerogens exhibit more intense spectra than the amorphinite concentrates. This suggests that these particular amorphinites (Duwi and Monterey Fms.) are products of bacterial degradation of the algal material. Similarities in the spectra of resinite (admixed with exsudatinite) and perhydrous vitrinite concentrates from the Eocene Kalimantan coal may result from the geochemical interaction between macerals during the coalification process (impregnation of the vitrinite by resinite is most probable). Density Profiles and Characterization of the Maceral Concentrates, The investigated Type I kerogens (Green River Fm. and Tasmanite) are nearly monomaceralic, which allowed for a clean separation of their dominant macerals. The density peak at 1.10 g mL-l on the Tasmanite profile represents practically pure Tasmanites algae (Figure 3A). The Green River Fm. kerogen's main DGC peak at 1.04 g mL-l is essentially pure lamalginite (Figure 3B). This kerogen also has 5% of what appears under the microscope to be vitrinite-like "gray bodies"; therefore HRDGC was performed over a density range of 1.10-1.50 g mL-l to preconcentrate this minor maceral (Figure 3C). No distinct peaks were observed, but investigation of an arbitrarily chosen fraction (e = 1.20 g mL-l) shows enrichment in the vitrinite-like material to -20% of the
1516 Energy &Fuels, Vol. 8, No. 6, 1994 90
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Figure 3. Density gradient profiles of A) Tasmanian tasmanite kerogen; (B) Green River Fm. kerogen; (C) Green River Fm. kerogen preconcentrate with e > 1.1g mI.-l. Fractions marked with arrows used in further analyses.
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Figure 4. Density gradient profiles of all samples used in the study showing changes in the density of the main macerals from Type I t o Type I11 organic matter.
The Type I1 kerogens are dominated by amorphous matter and minor amounts of marine alginite. The standard DGC profile of the Lower Toarcian Sh. kerogen from Paris Basin exhibits a maximum at 1.18 g mL-l, which denotes a mixture of amorphinite and alginite (Figure 4). A high-resolution DGC trace (after treatment with liquid nitrogen) shows small additional peaks at e = 1.15 and 1.23 g mL-l, which represent fractions enriched in alginite (-70% by volume) and yellowishbrown fluorescing amorphinite (-70% by volume), respectively.21 Our results suggest that there are no sharp differences between amorphinite and alginite, but rather a continuum of mixed macerals (alginite and amorphinite) representing different degrees of degradation which cannot be physically separated by the present method.21 However, another sample of the Lower Toarcian Sh. kerogen from Paris Basin has yielded an alginite fraction of high purity, which is currently being investigated. The Duwi Fm. sample has a maximum on the DGC profile at e = 1.18 g mL-l, shown petrographically to be 85% yellowish-brown fluorescing amorphinite and 15% alginite (Figure 5A). The cut points for HRDGC of the Duwi Fm. sample were set a t e = 1.18 g mL-l to concentrate the alginite and at 1.26 g mL-l t o isolate the traces of vitrinite and inertinite present (Figure 5B). The maceral concentrate at 1.05 g mL-l exhibited enrichment in the alginite (-85% dinoflagellates) with the balance being amorphous matter. The characteristic density peak at 1.43 g mL-l corresponds to various macerals of terrestrial origin (Figure 5B). Although not
total volume. The maceral was still unresolved from the dominant lamalginite. This and other s t ~ d i e s ~ ~ , ~ ~ (37) Han, Z.; Crelling, J. C.; Kruge, M. A. In Book of Abstracts, suggest that densities of 1.05-1.12 g mL-l are characGeochemistry Division, 207th ACS National Meeting, Sun Diego, teristic for freshwater and marine algae. American Chemical Society: Washington, DC, 1994, p 21.
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all the density fractions represent clean maceral concentrates, it was the first kerogen sample for which distinctive fractions were obtained. The DGC trace of the Monterey Fm. kerogen is dominated by reddish-brown fluorescing amorphinite at Q = 1.23 g mL-l. Multistep centrifugation allowed for a nearly perfect separation (95% pure) of the minor alginite (e = 1.11 g mL-l) (Figure 6A). Minor vitrinite could not be resolved completely and is concentrated (-30%) in a fraction mixed with highly degraded amorphinite (e = 1.35 g mL-l) (Figure 6B). The Type I1 kerogens demonstrate the most serious problem of DGC separation, arising from the heterogeneous nature of the amorphinite. The amorphinite has a wide density range (1.05-1.40 g mL-l), thus easily mixing with other macerals. However, one can general-
Figure 7. Density gradient profiles of (A) Eocene Kalimantan coal; (B) Eocene Kalimantan coal preconcentrates with e < 1.20 g mL-l and e > 1.28 g mL-l. Fractions marked with arrows used in further analyses. ize by saying that the most characteristic density for yellowish-brown fluorescing amorphinite is -1.18 g mL-' (e.g., Lower Toarcian Sh., Duwi Fm., Posidonia Sh.), while the higher density of -1.23 g mL-l may be characteristic of the reddish-brown (e.g., Monterey Fm.). Vitrinite and its precursor huminite are the dominant macerals in the three coal samples and comprise the major peak on their DGC traces. The Herrin No. 6 coal is composed of three main maceral groups (liptinite, vitrinite, ine~%inite).~* The standard DGC separation shows a main peak at 1.29 g mL-l which was determined to be vitrinite (97%)with only minor contamination by other macerals (Figure 4).21 The density range of 1.09-1.15 g mL-l represents a clean liptinite concentrate containing mixed cutinite (40%),resinite (35%), and sporinite (2O%hz1 Attempts to separate these individual liptinite components from each other failed due to their low concentration in the whole sample and their relatively large particle size (5-30 pm). The Herrin No. 6 sample was the first processed and was finished before the introduction of the liquid N2 pretreatment technique. Improvement of the liptinite separation may be observed in the Herrin coal sample with the Nz pretreatment. The density fraction in the 1.42-1.50 g mL-l range contained inertinite with only -5% vitrinite contamination; however, the individual semifusinite and fusinite macerals were not resolved.21 Specific studies targeting individual macerals have separated sporinite, vitrinite, semifusinite, and fusinite.14 The separation of the Eocene Kalimantan coal provided two clean (-95% purity) density fractions. Perhydrous (hydrogen-rich) vitrinite is represented on the DGC trace as the major peak a t 1.27 g mL-l (Figure 7A) and resinite mixed with exsudatinite (8 = 1.10 g mL-l) produces a small but distinct peak on the HRDGC profile (Figure 7B). The peak a t 1.18 g mL-l consists of mixed macerals (60% sporinite and 40% vitrinite). (38)Stach, E.; Taylor, G. H.; Mackowsky, M. T.; Chandra, D.; Teichler, M.; Teichmiiller, R. Stach's Textbook of Coal Petrology, 3rd ed.; Gebriider Borntraeger: Berlin, 1982.
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125
I 3
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1.45
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155
16
I
I05
11
115
I 2
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I ?
135 I 4
145
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16
0
4
Density [g ml I ]
Figure 8. Density gradient profiles of (A) Kalimantan lignite before and after cryogenic treatment; (B) Kalimantan lignite preconcentrate with e < 1.30 g mL-’. Fractions marked with arrows used in further analyses.
The Miocene Kalimantan lignite gives a broad density peak with the maximum a t 1.43 g mL-l, corresponding to macerals of the huminite group (Figure 8A). The fractions from 1.30 to 1.55 g mL-l do not display significant petrographic differences; thus separation of the huminite macerals observed in the whole coal (textinite and eu-ulminite) was not achieved. Similar results were obtained from earlier DGC separations of Utah lignites in this laboratory. Careful high-resolution DGC work successfully isolated the liptinites (resinite and exsudatinite) at a density of 1.10 g mL-l (Figure 8B). For the samples discussed in this paper and others processed by this laboratory, good separation of the dominant maceral has proven to be relatively easy. Those minor macerals which comprise less than 10%of the total sample volume, on the other hand, are extremely difficult to isolate, especially when several such macerals are present. However, separation can be affected when large quantities (up to kilograms) of starting material are available and a procedure utilizing semicontinuousfractionation preconcentrates the target material. A minor maceral accounting for at least 10% of the organic matter in coal can often be successfully isolated, even without extensive mi~ronization.~~ For Type I1 kerogens, where the minor and major macerals are intimately bound, particle size becomes crucial in DGC separations. Treatment with liquid nitrogen and true micronization are necessary and ultimately permit a more thorough separation. Segregation of the material during centrifugation can also occur because of particle size differences, with some larger particles (often mixed macerals) unable to attain their true density level due to hydrodynamic effects. Reduction of micronized particles t o a uniform size and shape is a critical step therefore, and eliminates the problem. Such careful preparations are extremely important if the macerals are being prepared for purposes of chemical analysis. Even a small percentage of vitrinite mixed with liptinite, for example, will introduce alkylphenol contamination upon pyrolysis.
It is important to mention that not every small peak o r bump on the DGC trace represents a pure maceral concentrate and petrographic or spectral observations are helpful and necessary. Also, an incomplete separation may actually reflect geological reality. Diagenetic interactions between nearby macerals as well as bacterial degradation of the primary organic matter can produce a range of intermediates between ideal endmember macerals. This appears to occur frequently in Type I1 kerogens where a gradual transition from alginite to amorphinite is found. Finally, an increase in density of the major maceral concentrates from the Type I, through Type I1 and 11-S toward Type I11 organic matter, with lignite being the densest (Figure 41, is observed. Fundamentally, the density differences are related to elemental composition, as Type I organic matter is the richest in hydrogen (the lightest element), while Type I1 and 11-Shave less hydrogen and more of the heavier element, sulfur. The Type I11 samples have even lower WC atomic ratios, in addition to being enriched in oxygen when low in rank with the lignite density trends being an extreme ~ a s e . 3Systematic ~ among the constituent macerals can now also be recognized (Table 4). Geochemical Characteristics of the DGC Fractions. Two to three DGC fractions each from seven samples were analyzed by pyrolysis-GCMS. Lamalginite, the dominant maceral in the Type I Green River Fm. kerogen, shows a strongly aliphatic character, with c6-c32 n-alk-1-enes and n-alkanes as the major components. These observations are in accord with those previously reported for alginite.35240,41The fraction containing the vitrinite-like “gray bodies” (e = 1.20 g mL-l) exhibits a similar geochemical signature, although alkylbenzenes and alkylphenols are slightly more in evidence (Figure 9). The Lower Toarcian Sh. Type I1 kerogen from Paris Basin, petrographically determined to be mostly amorphous matter with minor telalginite, did not yield clean fractions of the two macerals, even when thoroughly micronized and repeatedly processed by DGC. Results from py-GCNS of the fractions at densities of 1.15 and 1.23 g mL-l are similar and show n-alk-1-enes and n-alkanes from c6-c29 to be major constituents, together with alkylbenzenes and alkylthiophenes.21 The fraction at 1.11g mL-l was similar to the one at 1.15 g mL-l, and there was insufficient material of lower density for analysis. The only chemical distinction recognized is a slight increase in the relative abundance of the heterocompounds in the fraction at 1.23 g mL-’. There are also differences in the microstructure of the fractions revealed by electron micro~copy.~~ Lack of satisfactory separation of the macerals is most probably due to the continuum of intermediate products between alginite and amorphous matter.21 The Duwi Fm. Type 11-S kerogen DGC fractions show relatively good separation of the two main macerals, amorphinite, and alginite, and slight preconcentration of minor vitrinite and inertinite at g = 1.42 g mL-l. The chemical signatures of the three concentrates exhibit (39) Tissot, B.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1984; 699 p. (40) Largeau, C . ; Derrenne, S.; Casadevall, E.; Kadouri, A.; Sellier, N. In Advances in Organic Geochemistry 1985; Leythaeuser, D.; Rullkotter, J., Eds.; Org. Geochemistry; 1986; Vol. 10, pp 1023-1032. (41)Tegelaar, E. W.; de Leeuw, J. W.; Derenne, S.; Largeau, C. Geochim. Cosmochim. Acta 1989,53,3103-3106. (42) Largeau, C. personal communication.
Energy & Fuels, Vol. 8, No. 6, 1994 1619
Density Gradient Centrifugation BI
A
A
li.'
Green River Lamalginite @ 1.04 g ml-1
Duwi Alginite '3 1.05 g ml-1
Duwi Amomhinite
B Green River alginite with "graybodies" @ 1.20 e m1-1
1' C Duwi Mixed Amorphinite-Vitrinitelnertinite @ 1.42 g ml-1
30
Retention lime
50
70
Figure 9. Py-GCNS total ion current chromatograms of the maceral fractions separated by DGC from the Green River Sh. kerogen (A, alginite; B, alginite plus vitrinite-like "gray bodies". Key for the peaks: Bn, (alky1)benzenes;a n , (alkyl)phenols, where n indicates the extent of alkyl substitution (0, none; 1, methyl; 2, dimethyl or ethyl etc.); A, pairs of n-alk1-enes and n-alkanes; Pr, prist-1-ene; is, unidentified isoprenoids; H, hopeneshopanes.Number below chromatograms indicate the carbon numbers of the n-alkene + alkane series.
significant differences, with alginite being the most aliphatic (strong c6-c30 n-alk-1-enes and n-alkanes with minor prist-1-ene) with significant CO-c3 alkylbenzenes, CO-C~alkylindenes, and c1-c4 alkylthiophenes (Figure 1OA). The amorphinite shows a highly thiophenic character (note the predominance of alkylthiophenes over alkylbenzenes) and is less aliphatic than the alginite (Figure 10B). The total ion current chromatogram of the mixed vitrinite, inertinite, and amorphinite fraction is dominated by CO-C~alkylbenzenes, phenol, and cresols (Figure 1OC). These results indicate that chemical differences between Duwi Fm. alginite and amorphous matter are very distinct, especially the preferential concentration of thiophenes in amorphinite. The monoaromatics and alkylphenols in the densest fraction reflect in large measure the contribution of vitrinite to the mix.11J3J4vz1 The Monterey Fm. Type 11-S kerogen is composed mostly of amorphous matter, which was concentrated in high purity (-97% by volume) in the fraction a t e = 1.23 g mL-l. The total ion current chromatogram of this fraction shows the dominance of alkylpyrroles and alkylthiophenes (Figure 11B). The minor alginite component has been successfully separated and exhibits the classic aliphatic nature, with c6-c27 n-alk-l-enelnalkane pairs predominating on its py-GUMS trace (Figure 11A). The fraction composed of mixed vitrinite and highly degraded amorphinite shows a drop in the alkylpyrrole signal and an increase in phenols (Figure 1lC). The excellent separation of the macerals in Monterey Fm. kerogen probably reflects their distinctive chemistries. It is especially interesting that sulfur and
4
30
Retention time
io
io
Figure 10. Py-GCMS total ion current chromatograms of the maceral fractions separated by DGC from the Duwi Fm. kerogen (A, alginite; B, amorphinite; C, mixed vitrinite, inertinite and amorphinite).Key for the peaks: Bn, (alkyl)benzenes; @n,(alky1)phenols;On, (alky1)thiophenes;In, (alkyl) indenes; A, pairs of n-alk-1-enesand n-alkanes; Pr, prist-lene; is, isoprenoids; *, contaminant. Number below chromatograms indicate the carbon number in the n-alkendalkane series.
nitrogen compounds are preferentially concentrated in the amorphous matter. The three Type I1 kerogens examined in this study show improved separation efficiency in order of increasing heteroatom content (from Lower Toarcian t o Duwi t o Monterey). In the past, coals have been more extensively studied using DGC technique than have Type I and I1 kerogens and the results of successful separations have been p ~ b l i s h e d . ~ J ~ - ' Experiments ~J~J~ on the Herrin No.6 coal failed to achieve separation of the single liptinite or inertinite macerals, as discussed above. However, concentration of liptinite and inertinite group macerals was achieved, producing characteristic py-GC/MS signatures2J1J4 upon analysis. The liptinite exhibits a long-chain aliphatic structure, with by c6-c31 n-alk-lenes and n-alkanes as the dominant compounds.21 In contrast, vitrinite produces strong alkylphenols and monoaromatics, while the inertinite pyrolysate is dominated by di- and triaromatics.21 Phenols and alkylbenzenes are observed in the liptinite pyrolysate, largely attributable to minor admixed vitrinite. The pyrolysate of the high purity vitrinite (e = 1.27 g mL-l) separated from the Eocene Kalimantan coal contains abundant alkylphenols, alkylcatechols, and alkylbenzenes which are typical for low-rank vitrinite (Figure 12B).11J4This maceral is also highly aliphatic
1520 Energy & Fuels, Vol. 8, No. 6, 1994
Stankiewicz et al.
A Resinite with exsudatinite @ 1.11 g ml.1 A
N2
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34
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Figure 11. Py-GCIMStotal ion current chromatograms of the maceral fractions separated by DGC from the Monterey Fm. kerogen (A, alginite; B, amorphinite; C, mixed vitrinite and amorphinite). Key for the peaks: Bn, (a1kyl)benzenes; Qn, (alky1)phenols;On,(alky1)thiophenes;IIn, (alky1)pyrroles;In, (alky1)indenes;A, pairs of n-alk-1-enesand n-alkanes;Pr,prist-
1-ene. Number below chromatograms indicate the carbon number in the n-alkenelalkaneseries. (note the abundant long-chain alkenes and alkanes), due to its perhydrous nature, typical of Tertiary southeast Asian coals. Attempts to separate liptinite macerals met with partial success. The fraction at e = 1.11 g mL-l produced strong resin markers upon pyrolysis (1,6-dimethylnaphthalene and cadalene) (Figure 12A). However, the co-occurrence of major aliphatic hydrocarbons stands in contrast with geochemical studies on dammar resins from the region, which show that main pyrolysis products are derivatives of pentacyclic triterpenoids, such as cadalene.43,u Therefore the c6-c34 n-alk-1-enes and n-alkanes can be attributed to the presence of admixed exsudatinite (difficult to differentiate in micronized particles). Alternatively, the resulting signature could be due to the chemical interaction between liptinite macerals during diagenesis. Such intermaceral reactions would explain the similar fluorescence spectra of Kalimantan vitrinite and resinite (Figure 2), as well as the similar distribution of aliphatics in both concentrates (Figure 12). Similar findings were reported for Spanish coals.45 (43) Puttman, W.; Villar, H. Geochim. Cosmochim. Acta 1987,51,
__
302.1-3029. - --- - - .
(44) van Aarssen, B. G. K.; de Leeuw, J. W. J. Anal. App. Pyrol. 1991,20,125-139. (45) Jimenez, A.; Iglesias, M. J.; Laggoun-Defarge, J. G.; Prado, J . G.; Suarez-Ruiz, I. In Book of Abstracts, Geochemistry Division, 207th ACS National Meeting, San Diego, American Chemical Society: Washington, DC, 1994, p 33.
Figure 12. Py-GC/MStotal ion current chromatograms of the maceral fractions separated by DGC from the Eocene Kalimantan coal (A, resinite; B, vitrinite).Key for the peaks: Bn, (alky1)benzenes;an,(alky1)phenols;rn, (alky1)catechols;Nn, (alky1)naphthalenes;A, pairs of n-alk-1-enesand n-alkanes; H, hopeneshopanes; is, isoprenoids; C, cadalene. Number
below chromatograms indicate the carbon number in the n-alkenelalkane series. Several pyrolysis experiments were performed on the fractions along the broad peak on DGC profile (e = 1.35, 1.43, and 1.55 g mL-') of the Miocene lignite from Kalimantan. There are no significant differences in the resulting total ion chromatograms, except for a slight increase of the aliphatics in the less dense fraction. All these pyrolysates have lignin derivatives (alkylphenols, catechols, and g u a i a ~ o l s )as ~ ~the major compounds, most pronounced in the densest fraction (Figure 13B). Seven strongly pronounced peaks in the densest fraction have been tentatively identified as a homologous series of carboxylic acids. Resinite is the main liptinite maceral (-5% of total volume) and its concentrate produces abundant C15 compounds with the cadinane carbon skeleton,44 but also n-alkenes and alkanes (Figure 13A). Interestingly, the resin markers found to be typical for pyrolysates of dammar r e ~ i nare ~ ~ , ~ more pronounced in the coal of lower rank (compare with bituminous coal, Figure 12A). As in the case of the Eocene sample, the aliphatic hydrocarbons can be attributed to admixed exsudatinite or alternatively t o diagenetic modifications of the resinite macerals through interactions with other liptinites. These latest experiments on coals of different rank emphasize that good separation of minor macerals, as was previously reported,12-16 is difficult. For such separations, it appears that samples are required in which (1)minor macerals each comprise at least 10% each of the total volume and (2) the major maceral (Le., vitrinite) does not overwhelm the sample (
