The Study of Australian Coal Maturity: Relationship between Solid

School of Chemistry, University of Sydney, NSW 2006, Australia ... Samples of a range of Australian bituminous coals from Pelican-5, which is a petrol...
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Energy & Fuels 2005, 19, 954-959

The Study of Australian Coal Maturity: Relationship between Solid-State NMR Aromaticities and Organic Free-Radical Count Thilanga S. Bandara, G. S. Kamali Kannangara,* and Michael A. Wilson* College of Science, Technology and Environment, University of Western Sydney, Locked Bag 1797, Penrith South DC 1797, Australia

Christopher J. Boreham Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia

Keith Fisher School of Chemistry, University of Sydney, NSW 2006, Australia Received September 10, 2004. Revised Manuscript Received December 20, 2004

Samples of a range of Australian bituminous coals from Pelican-5, which is a petroleum drill site in the Bass Basin, located in Australia offshore between Victoria and Tasmania, were studied by 13C solid-state nuclear magnetic resonance (13C NMR) spectroscopy and electron spin resonance (ESR) spectroscopy. As expected, the fraction of carbon that is aromatic in these coals, measured using a single-pulse (Bloch decay) method, was higher than values obtained by the crosspolarization (CP) method, because some carbon observed using the single-pulse method is not observed by the CP method. Loss of signal through rapid spin-spin relaxation due to inorganic paramagnetics has been excluded as a source of differences in the aromaticity measurements; however, the organic free-electron content correlates well with observed spin-lattice relaxation times in the rotating frame and the difference between aromaticities measured by the Bloch decay and CP methods decreases as the electron count and rank increase. Some aromatic carbon is observed in Bloch decay experiments in low-rank coals but not by CP, because organic free electrons alter quantitation in CH/T1FH dynamics more than they do in spin-spin relaxation. Because the aliphatic chains are longer at lower rank, they are more remote from the free electrons on aromatic rings and less influenced by free radicals on the aromatic rings.

Introduction The fraction of carbon that is aromatic in coals is important in regard to determining rank, utilization, and oil generation capacity. It has been measured since the pioneering experiments of VanderHart and Retcofsky1,2 and Maciel et al.3 The issue of just how quantitative NMR measurements are has been a matter of debate for more than twenty years.4-12 Nevertheless, this has not stopped a range of studies that have * Authors to whom correspondence should be addressed. E-mail addresses: [email protected], [email protected]. (1) VanderHart, D. L.; Retcofsky, H. L. Fuel 1976, 55, 202-204. (2) Retcofsky, H. L.; VanderHart, D. L. Fuel 1978, 57, 421-423. (3) Maciel, G. E.; Bartuska, V. J.; Miknis, F. P. Fuel 1979, 58, 391394. (4) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J. Am. Chem. Soc. 1981b, 105, 2142. (5) Wilson, M. A.; Vassallo, A. M.; Russell, N. J. Org. Geochem. 1983, 5, 35-42. (6) Wilson, M. A. NMR Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: Oxford, U.K., 1987; pp 248277. (7) Suggate, R. P.; Dickinson, W. W. Int. J. Coal Geol. 2004, 57, 1-22. (8) 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, 547-560.

characterized coals and source rocks from around the world, because of the inadequacies of other rank techniques. Although vitrinite reflectance (VR) has been the reference point for most geological studies, it bears only a complex and unknown relationship with chemical structure and, hence, measurements such as aromaticity by NMR techniques, which, with all their limitations, offer a better and more reliable alternative than petrographic methods, which are still subject to operator interpretation. In this work, we study a range of coaly source rocks which, in themselves, have significant geological interest, because they appear in a region known to provide oil from land-based plants.6 Here, we study the aromaticity changes during coalification with depth using NMR to determine differences in measurements by cross-polarization (CP) and Bloch decay techniques. (9) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 1, 173-181. (10) Maroto-Valer, M. M.; Andresen, J. M.; Rocha, J. D.; Snape, C. E. Fuel 1996, 75, 1721-1726. (11) Jurkiewicz, A.; Maciel, G. E. Anal. Chem. 1995, 67, 2188-2194. (12) Franco, D. V.; Gelan, J. M.; Martens, H. J.; Vanderzande, D. J. M. Fuel 1991, 70, 811-817.

10.1021/ef040085f CCC: $30.25 © 2005 American Chemical Society Published on Web 03/01/2005

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Table 1. Elemental Analysis Data for Ten Coal Samples Elemental Analysis (wt %) H N S

GA sample reference numbera

depth from KB (m)

our sample number

C

20010049 20010052 20010377 20010378 20010379 20010381 20010382 20010385 20010387 20010391

2034.0 2791.9 2304.0 2409.0 2502.0 2721.0 2799.0 3162.0 3414.0 3960.0

049 052 377 378 379 381 382 385 387 391

69.5 71.5 80.7 70.2 81.6 76.8 77.2 80.1 81.6 73.9

5.3 4.9 6.1 5.0 6.1 5.6 5.8 5.2 5.0 4.0

1.5 0.7 1.7 1.0 1.8 1.9 1.8 1.8 2.1 1.6

1.9 0.6 0.5 0.7 1.5 0.4 0.6 0.5 0.5 0.5

Ob

H/C

21.8 22.3 11.2 23.13 9.0 15.2 14.6 12.5 10.9 20.1

0.92 0.82 0.91 0.85 0.90 0.87 0.90 0.78 0.74 0.65

a Sample reference numbers were sent by Geoscience Australia. b Oxygen content calculated by difference before rounding to two or three decimal figures.

Table 2. Vitrinite Reflectance and Hydrogen Index of Coal Samples our sample number

depth from KB (m)

vitrinite reflectance, VR

hydrogen index, HI (mg/g TOC)

049 052 377 378 379 381 382 385 387 391

2034.0 2791.9 2304.0 2409.0 2502.0 2721.0 2799.0 3162.0 3414.0 3960.0

0.53 0.76 0.60 0.63 0.65 0.73 0.77 0.95 1.09 1.50

220 407 265 299 340 377 441 257 206 121

Results have been correlated with the VR value, hydrogen index (HI), and organic free-radical content. Although these results are important in their own right in describing the deposit, here, we also investigate quantitation of carbons, to put the results on a more secure footing. Experimental Section Sample Coals. Ten Australian bituminous coals used in this study were taken from Pelican-5, which is a petroleum exploration well in the Bass Basin, located in Australia offshore between Victoria and Tasmania, which lies over an area of ∼42 000 km2.13 All the samples were from different depths from the sea bed, ranging from 2034 m to 3960 m below the platform of the drill rig (KB). Table 1 lists the elemental and proximate analyses, as well as the depth level for each of the coal sample. Of the 12 coal samples, 10 have been demineralized using a hydrofluoric acid (HF) treatment, as outlined elsewhere.14 The elemental analysis data for the 10 coal samples are shown in Table 1. All samples were stored in the cool room at a temperature of 2 °C, to ensure that oxidation and decomposition did not occur. VR and HI values were measured by standard methods13,15 and are recorded in Table 2. Nuclear Magnetic Resonance (NMR) Spectroscopy. The 10 demineralized coals, together with two untreated “as received” coals, were analyzed by solid-state nuclear magnetic resonance (NMR) spectroscopy. Approximately 75-85 mg of powdered sample was directly packed into zirconia rotors with Kel-F caps. Spectra were recorded using a Bruker model DPX200W Avance spectrometer that was operating at 50 MHz (13) Boreham, C. J.; Blevin, J. E.; Radlinski, A. P.; Trigg, K. R. APPEA J. 2003, 43, 117-148. (14) Saxby, J. D. Chemical Separation and Characterization of Kerogen from Oil Shale; In Oil Shale; Yen, T. F., Chilingarian, G. V., Eds.; Developments in Petroleum Science 5; Elsevier: New York, 1976; pp 103-128. (15) van Krevelen, D. W. Coal. Typology-Physics-ChemistryConstitution, 3rd Edition; Elsevier: Amsterdam, The Netherlands, 1993; pp 365-367, 687.

for 13C and 200 MHz for 1H at ambient temperature with magic angle spinning (MAS). A potassium bromide (KBr) sample spinning at 5 kHz was used to set the magic angle. Chemical shifts were measured, relative to the adamantane that was used as an external reference, but recorded with respect to tetramethylsilane (TMS). Standard 13C CP spectra for the 12 coal samples were recorded using a CP contact time of 1.0 ms and a recycle delay time of 2.0 s with a 90° pulse of 3.2 µs. Approximately 3072 scans were collected at a spinning speed of 8 kHz. Variablecontact-time experiments were performed, using CP contact times ranging from 0.005 ms up to 15.0 ms. Integrated signal intensities for aromatic and aliphatic carbons of each of the 10 coal samples were recorded as a function of contact time. 13 C Bloch decay experiments, using high-power decoupling and MAS, required recycling times up to 20 s and 3072 scans. The fraction of aromatic carbons (the aromaticity, denoted as fa) was measured using standard CP and Bloch decay spectra for each coal sample. The fraction of aromatic carbons was determined by directly integrating the spectra, assigning the aliphatic and the aromatic carbon regions to vary from 0 to 90 ppm and 100-200 ppm, respectively, and calibrating the total integrated area as 100.

fa )

aromatic carbon aromatic carbon + aliphatic carbon

The relaxation time constants, which describe the CP dynamics (TCH represents the CP relaxation time and T1FH represents the proton spin lattice relaxation time in the rotating frame) for aliphatic and aromatic carbon species, were measured by variable-contact-time experiments from the approximation

I)

C{exp[- (tcp/T1FH)] - exp[- (tcp/TCH)]} 1 - (TCH/T1FH)

(1)

where I is the signal intensity, C a proportionality constant, T1FH the spin lattice relaxation time in the proton rotating frame, TCH the CP relaxation time, and tcp the CP contact time.16 A typical plot for change in carbon magnetization with contact time is given in Figure 1. Aromaticities of the coals studied here by the Bloch decay and CP methods, corrected for T1FH effects, are listed in Table 3. Electron Spin Resonance (ESR) Spectroscopy. ESR spectra were recorded on a sample of coal and finely ground silicon dioxide (SiO2), mixed thoroughly to obtain a homogeneous mixture with a coal:SiO2 weight ratio of 1:5. The coalSiO2 mixtures, with known constant weights, were then packed into the sample tubes for analysis. ESR spectra were recorded using Bruker EMX EPR spectrometer, applying a modulation frequency of 100 Hz. A sweep width of 50 G was used, and the center fields of the spectra were set at 3490 G. (16) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J. Am. Chem. Soc. 1981a, 105, 2133.

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Table 3. NMR Parameters for Coals from the Pelican-5 Well Aromaticity, fa our sample number

CP/MAS, fa(CP)

Bloch decay, fa(BD)

difference, fa(BD) - fa(CP)

049 052 377 378 379 381 382 385 387 391 052A 049A

0.53 0.47 0.52 0.47 0.52 0.58 0.57 0.63 0.68 0.77 0.51 0.51

0.77 0.70 0.72 0.69 0.72 0.82 0.73 0.78 0.84 0.94 0.61 0.67

0.24 0.23 0.20 0.22 0.20 0.24 0.16 0.15 0.16 0.17 0.10 0.15

TCH (µs)a

T1FH (ms)a

22.0 (ali), 27.0 (arom) 28.0 (ali), 45.0 (arom) ndb 22.0 (ali), 28.0 (arom) 28.1 (ali), 36.0 (arom) 28.0 (ali), 40.0 (arom) 42.0 (ali), 59.7 (arom) 23.7 (ali), 29.5 (arom) 21.4 (ali), 29.0 (arom) 18.1 (ali), 24.0 (arom)

12.1 (ali), 24.9 (arom) 15.2 (ali), 15.0 (arom) ndb 11.7 (ali), 9.6 (arom) 9.5 (ali), 24.6 (arom) 8.3 (ali), 19.1 (arom) 7.9 (ali), 12.0 (arom) 8.1 (ali), 12.6 (arom) 5.3 (ali), 6.9 (arom) 5.7 (ali), 5.7 (arom)

a Values listed are typical for samples that were not demineralized. Time constants were determined for samples with mineral matter; no time constants for samples 049A and 052A were obtained, because of time limitations. b Not determined; we were unable to obtain a good fit to eq 1.

Figure 1. Behavior of carbon magnetization, relative to changes in the contact time in a typical cross-polarization (CP) experiment (for coal sample 052). Given that a constant amount of sample mixture was analyzed, integrated ESR spectral intensities were considered as relative measurements of the organic free radical count. Electron g values were also recorded from the ESR spectra of the 10 coal samples.

Results and Discussion Similar to other coals, solid-state 13C NMR spectra of all coal samples used for this study had two dominant peaks that appeared at ∼125 and ∼30 ppm, corresponding to aromatic and aliphatic carbons, respectively, with some additional features that have been described extensively elsewhere17-20 and, hence, will not be discussed further, except that it is worth noting that phenoxy carbon functional groups showed a significant decrease with increasing aromaticity (Figure 2). Because of the difficulties in obtaining good signalto-noise ratios, the aromaticity of coals have been measured using CP techniques, despite the fact that carbons with short spin-lattice relaxation times in the rotating frame, very long CP times, or very short spin(17) Axelson, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels: An Experimental Approach; Multiscience: Montreal, Canada, 1985; pp 198-204, 219. (18) Hatcher, P. G.; Breger, I. A.; Szeverenyi, N.; Maciel, G. E. Org. Geochem. 1982, 4, 9-18. (19) Hatcher, P. G.; Clifford, D. J. Org. Geochem. 1997, 27, 251274. (20) Newman, R. H.; Davenport, S. J. Fuel 1983, 62, 601-605.

Figure 2. Change in the intensity of carbon signal belonging to the phenoxy signal (∼150 ppm) with aromaticity (fa) obtained from 13C NMR CP experiment. (Correlation coefficient of R2 ) 0.566.)

spin relaxation times will not be observed. Elsewhere reasonable correlations have been obtained between the CP aromaticity and H/C ratio, vitrinite reflectance (VR), and hydrogen index (HI).6,21,22 The reasons for any scatter in these plots are, in part, due to errors and assumptions in the CP method, and also because of other factors that alter the H/C ratios, VR values, and HI values. For example, the cross-linking of aromatic rings or changes in aliphatic structural groups will alter H/C ratios; VR, being a physical property, relates more directly to surface structure than chemical composition23 and yields of oil and gas are affected by the mineral matter content. Moderate correlations (correlation coefficient of R2 ) 0.648) were obtained between CP aromaticity and H/C ratio (Figure 3), aromaticity and VR (Figure 4) and aromaticity and HI (Figure 5) for the coals studied here as well. It is interesting to note that the H/C ratios and geological parameters such as the VR value and the HI index also gave moderate correlations with fa values determined by the Bloch decay technique (see Figures 6-8). The fa values determined using the Bloch decay method for these coal samples are greater than those (21) Carr, A. D.; Williamson, J. E. Org. Geochem. 1990, 16, 313323. (22) Hayatsu, R.; McBeth, R. L.; Scott, R. G.; Botto, R. E.; Winans, R. E. Org. Geochem. 1984, 6, 463-471. (23) Stach, E.; Mackowsky, M. T.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, R. Stach’s Textbook of Coal Petrology, 3rd Edition; Gebru¨der Borntraeger: Berlin, 1975; pp 34-53.

Study of Australian Coal Maturity

Figure 3. Plot of the elemental ratio of hydrogen to carbon (H/C) versus the aromaticity (fa) obtained using the CP method. (Note: The correlation coefficient is R2 ) 0.648.)

Figure 4. Plot of vitrinite reflectance (VR) versus the aromaticity (fa) obtained in CP NMR experiments. (Note: The correlation coefficient is R2 ) 0.8323.)

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Figure 6. Plot of the H/C elemental ratio versus the aromaticity (fa) obtained using the Bloch decay method. (Note: The correlation coefficient is R2 ) 0.5787.)

Figure 7. Plot of vitrinite reflectance (VR) versus the aromaticity (fa) obtained using the Bloch decay method. (Note: The correlation coefficient is R2 ) 0.7149.)

Figure 5. Plot of the hydrogen index (HI) versus the aromaticity (fa) obtained using the CP method. (Note: The correlation coefficient is R2 ) 0.419.)

of CP experiments (see Table 3), because some aromatic carbon must have very short spin-lattice relaxation times in the rotating frame or very long CP times. Even though such differences between the fa values determined using the CP and Bloch decay pulse sequences have been previously discussed in the literature,6,24 the current set of data showed a considerably larger difference between the two fa values than most of the literature-reported values and, hence, was worthy of further investigation. (24) Wilson, M. A.; La Farque, E.; Gizachew, D. APPEA J. 1994, 34, 210-215.

Figure 8. Plot of the hydrogen index (HI) versus the aromaticity (fa) determined using the Bloch decay method. (Note: The correlation coefficient is R2 ) 0.4542.)

To test the impact of paramagnetics on the magnitude of the fa values, some demineralized and undemineralized samples were examined (see Table 3 for examples). The methodology for demineralization removes both aluminum- and silicon-based minerals, as well as ironcontaining compounds and others with free electrons.14 Samples were also checked by X-ray diffraction (XRD) to determine if inorganic materials with oxidation states that contain free electrons were present. However, no

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Figure 9. Relationship between electron spin count and CP aromaticity (fa). (Note: The correlation coefficient is R2 ) 0.7558.)

Figure 10. Effect of free electrons on the spin-lattice relaxation time in the proton rotating frame (T1FH) of aliphatic and aromatic carbons.

Table 4. Electron g Values of the Ten Coal Samples Calculated from ESR Experiments our sample number

g value

our sample number

g value

049 052 377 378 379

2.00364 2.00333 2.00352 2.00351 2.00357

381 382 385 387 391

2.00338 2.00334 2.00331 2.00317 2.00308

significant difference was observed in the fa values between demineralized and undemineralized samples. In the absence of a substantial influence of mineral matter on the carbon signal intensities, the organic free electrons in the coal samples are probable causes of a loss of signal. As one would expect, carbons adjacent to unpaired electrons in coals can have extremely short spin spin relaxation (T2) times, so that the NMR signal decays before the acquisition of the NMR signal is complete.6 The presence of these electrons in close proximity to the aromatic rings will create a possibility of delocalization and thereby increase the stability of the free electrons. Therefore, it is highly probable that these free electrons are selectively associated with aromatic carbons, thereby giving lower aromaticity estimates for aromatic carbons. These would include coals with high inertinite maceral contents; it has been reported that samples with high inertinite contents may exhibit lower fa values in CP versus Bloch decay experiments.24 As mentioned previously, the amount of organic free electrons present in a sample can have a considerable effect on relaxation processes. Table 4 lists the electron g values for the coals. As expected, the g values of these coals, ranging from 2.00308 to 2.00364, indicated that the free electrons in these coal samples are primarily centered on aromatic rings.25,26 Interestingly, when examining the plot drawn against the electron spin counts and the fa values of CP, the free electrons were observed to increase fairly consistently with increasing CP fa values of the samples (Figure 9). The fact that these free electrons are also significant in spin-lattice relaxation in the rotating frame relaxation is demonstrated by a plot of electron spin counts versus T1FH of (25) Retcofsky, H. L.; Stark, J. M.; Friedel, R. A. Anal. Chem. 1968, 40, 1699-1704. (26) Petrakis, L.; Grandy, D. W. Anal. Chem. 1978, 50, 303-308.

Figure 11. Relationship between the difference in aromaticities (fa) and electron spin count. (Note: The correlation coefficient is R2 ) 0.4009.)

observed carbons (Figure 10), which clearly shows that there is a good correlation. Thus, electrons are hiding carbons both in Bloch decay and CP experiments. Significant variation exists for different samples between aromatic and aliphatic carbons in T1FH, which shows the selective discrimination of carbon. However, the difference between fa values measured by the Bloch decay and CP methods decreases as the electron count increases (Figure 11). Thus, at higher fa values, electrons hide both aliphatic and aromatic carbons, but they are more selective in hiding carbons at lower fa values. This is almost certainly true, because the aliphatic chains are shorter at higher fa values (higher rank) and longer at lower rank. In lower ranks, many of these aliphatic chain carbons are not located R to aromatic rings and are more remote from the free electrons, and, thus, they are observed. A significant point is that free electrons affect CP fa values in a manner that is different from that for Bloch decay fa values. That is, some carbon is observed in Bloch decay experiments but is not detectable using CP methods because electrons alter the CH/T1FH dynamics as well as T2 relaxation. Finally, in passing, note that there is an excellent relationship between the VR value and the g values for these samples (Figure 12). VR is a surface phenomenon, whereas g values characterize the location of the electrons. As the coals increase in rank, the surface properties and g values will be dependent directly on how delocalized the π electrons in the system are, more so than on the aromaticity, which also reflects aromatic

Study of Australian Coal Maturity

Figure 12. Change in g values with increasing coal rank with vitrinite reflectance (VR). (Note: The correlation coefficient is R2 ) 0.847.)

ring size and aliphatic heterogeneity in structure. This paper adds evidence to the fact that rank, as defined as VR, is better related to rank that has been defined chemically as the degree of delocalization of π electrons, rather than H/C ratio, the percentage carbon, or the aromaticity. Conclusions (1) Coal samples from the Pelican-5 exploration well, which is located in Australia between Victoria and

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Tasmania, show a good correlation between depth and vitrinite reflectance (VR) as rank increases. Similar correlations are observed between Bloch decay and cross-polarization (CP) aromaticities determined by nuclear magnetic resonance. (2) For coal samples from Pelican-5, aromaticity measurements by Bloch decay produced higher aromaticities, compared to those from the CP method, because some carbon is selectively not observed by the CP method. Inorganic free radicals were excluded as a factor causing the differences; however, electron spin resonance (ESR) spectroscopy showed that the organic free-electron content is important and correlates well with observed spin-lattice relaxation times in the rotating frame. (3) The difference between aromaticities measured by the Bloch decay and CP methods decreases as the electron count increases. Thus, at higher aromaticities, electrons mask both aliphatic and aromatic carbons but are more selective at lower aromaticities. Some carbon is observed in Bloch decay experiments but is not observed by CP methods, because the electrons alter the CH/T1FH dynamics. Acknowledgment. We thank Dr. Alan McCutcheon for technical assistance in NMR spectroscopy. EF040085F