ARTICLE pubs.acs.org/EF
FTIR and 13C NMR Investigation of Coal Component of Late Permian Coals from Southern China Shaoqing Wang,†,‡ Yuegang Tang,*,†,‡ Harold H. Schobert,*,§ Ya’nan Guo,† and Yufei Su† †
College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), D11, Xueyuan Road, Beijing 100083, P.R. China ‡ State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology (Beijing), D11, Xueyuan Road, Beijing 100083, P.R. China § The EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Two separated components, vitrinite and barkinite, and one associated coal sample obtained from South China were studied by Fourier transform infrared spectroscopy (FTIR) and carbon-13 nuclear magnetic resonance (13C NMR) with curvefitting analysis to obtain information on the concentrations of several functionalities in the samples. These two components were also studied in situ in coal by micro-FTIR. Some parameters derived from FTIR, micro-FTIR, and 13C NMR were selected to obtain a better understanding of the chemical structure of barkinite. All the FTIR and 13C NMR spectra and the values of I1 and I2 show that the most distinct structural feature of barkinite is a rich concentration of aliphatic structures. Furthermore, the aliphatic structures are longer and less branched. Oxygen is mainly bound to aliphatic carbon. For barkinite, the ratio of integrated areas of 30002700 cm1 and 31003000 cm1 regions of FTIR spectra suggest that its aliphatic hydrogen concentration is larger than its aromatic hydrogen concentration. In the aromatic structures, barkinite has a higher ratio of integrated areas of 900700 cm1 to 1600 cm1 than vitrinite, and the aromatic ring number per cluster is mainly 2 or 3. In addition, the intensity of the aliphatic peaks relative to the aromatic peaks (the A factor) of barkinite shows that barkinite can be inferred to be a type I kerogen and that barkinite has good hydrocarbon-generating potential.
1. INTRODUCTION The Late Permian coals, from the Leping coal basin in Jiangxi province, have relatively high values of hydrogen and volatile matter in comparison with other humic coals of similar rank.13 To explore the source of hydrogen, Wang and Tang et al.4 analyzed some coals and found that the high hydrogen content was related to the barkinite concentration in the coal. Barkinite, classified as a liptinite maceral in Chinese nomenclature, has been widely studied over the past 80 years.1,2,514 In more recent years, research on barkinite has been published especially in international journals.4,812,1417 However, “barkinite” has not yet been recognized as a maceral by organizations such as the International Committee for Coal and Organic Petrology (ICCP) or The Society for Organic Petrology (TSOP). Hower et al.6 pointed out that it was more appropriate to use the term “component” instead of “maceral” for barkinite. Some questions on barkinite have not been satisfactorily solved, such as the morphological differences between barkinite and other liptinitic macerals (especially suberinite or cutinite) and the chemical differences between barkinite and other liptinitic macerals of the same rank and age. Some analytical methods, specifically Fourier transform infrared spectroscopy (FTIR) and solid-state carbon-13 nuclear magnetic resonance (13C NMR), have been used extensively to characterize the chemical structure of coals and individual macerals. FTIR is capable of providing information on functional groups on coal structure.1824 The solid-state 13C NMR technique can provide the information related to the type and distribution of aromatic and aliphatic carbons in coal and r 2011 American Chemical Society
macerals.2533 The techniques of cross-polarization (CP)34 and magic angle spinning (MAS)35 can provide high-resolution 13 C NMR spectra in solids when they were used separately or combined (CP/MAS). In addition, the chemical structural characteristics of coal macerals can be studied using the combination of transmission and reflectance micro-FTIR.3639 Although these techniques have been applied in many investigations, very few applications have been made for the characterization of barkinite. Wu et al.40 examined some coals from the southern China using FTIR. Qin et al.41 analyzed the chemical structure of two barkinite-rich coals (of purity: 80% and 86%, separately) from the Chuangguang coalfield using 13C NMR. Guo et al.1 and Sun11 characterized the chemical structure of lopinite from the Leping, Changguang, and Shuicheng coalfields using mirco-FTIR and made comparisons with other macerals. Wang and Tang et al.42 characterized several different coals using 13 C NMR and attempted to correlate their liquefaction reactivity with structural characteristics. However, little information on detailed chemical structural characteristics of barkinite has been obtained to date from studies that bring several methods to bear on the structural analysis. The present work concentrates mainly on studying the chemical structural characteristics of barkinite, and not on its petrology. The objectives of the present work were (1) to characterize the structural features of barkinite, (2) to make a Received: August 9, 2011 Revised: October 24, 2011 Published: November 01, 2011 5672
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comparison of data obtained with FTIR and 13C NMR between barkinite and other samples.
2. SAMPLES AND EXPERIMENTAL A column sample of coal was collected from the Mingshan (MS) coal mine in the south of China for both separating macerals and examination by micro-FTIR. The individual macerals (barkinite and vitrinite) were concentrated by combining hand picking and the density gradient centrifugation (DGC) method. The purities of both barkinite (BaS) and vitrinite (VS) are 96%. Vitrinite (V) and barkinite (Ba) in the MS sample were also determined in situ by micro-FTIR, separately. The reflectance reported was the mean value of 100 measurements. Maceral compositions were determined in the State Key Laboratory of Coal Resources and Safe Mining (Beijing, China). Proximate analysis and ultimate analysis were carried out following methods GB/T 212-2008,43 GB/T 476-2008,44 GB/T 214-2007,45 and GB/T 215-2003,46 separately. The elemental composition of MS was 80.75% carbon, 5.82% hydrogen, 9.64% oxygen, 1.99% total sulfur, 1.53% organic sulfur, on a dry ash-free basis (daf), and 0.86 H/C atomic ratio. The maceral composition is vitrinite, 39.7 vol %; liptinite, 39.9 vol %; barkinite, 31.7 vol %; and inertinite, 20.4 vol %. The value of mean maximum vitrinite reflectance of MS was 0.69%, and the value of barkinite (Ba) reflectance was 0.21%. FTIR Measurement. Measurements were made with a Nicolet model 6700 Fourier transform-infrared spectrometer. Quantitative FTIR transmission spectra of coals were obtained using finely ground samples pressed in KBr pellets. A small sample (typically 13.0 mg) of this finely ground dry coal was weighed in a drybox and added to a weighed amount, about 300 mg, of KBr. Both the KBr and the coal were then mixed by grinding for 120 s and pressed into a pellet in an evacuated die under 10 MPa pressure for 2 min. The pellets were dried in a vacuum oven for 48 h to minimize the contribution of water to the spectrum. Spectra were recorded by co-adding 300 scans at a resolution of 2 cm1. FTIR with Microscopy Measurement. The infrared spectra were generated by collecting 400 scans at a resolution of 6 cm1, using a Bruker IFS 66/s FTIR instrument equipped with a Bruker Hyperion 3000 microscope. The infrared spectra of individual macerals were measured with cross sectional areas of 60 60 μm. Spectra were referenced to a bare gold mirror and then KramersKronig transformed to obtain an absorbance spectrum. Solid State CP/MAS 13C NMR Measurement. The NMR spectra of MS and the two separated macerals were acquired on a Varian Unity Inova 300 M NMR spectrometer with a double resonance probe by using cross-polarization and magic-angle-spinning (CP/MAS) techniques. The 13C frequency was set to 75.5 MHz. The spectral width was 3 kHz, and the recycle delay was 4 s. The contact time was 0.005 s, and the spin rate was 12 kHz. To obtain further data, peak separation and semiquantitative calculation of FTIR and micro-FTIR were done using the curve-fitting program of PeakFit software, and those of 13C NMR were carried out by Nuts 2000 software.
3. RESULTS AND DISCUSSION FTIR Characteristics of Coals. The peaks were assigned in this paper on the basis of the work of Painter et al.23,47 Figure 1 shows the FTIR spectra of the samples used. It can be clearly seen that the spectra are characterized by strong aliphatic CH stretching bands near 2920 cm1 and 2850 cm1, which were predominantly assigned to methylene groups, strong intensities of aromatic (CdC) stretching vibration at 16011610 cm1, and aliphatic CHx deformation vibration at 14491455 cm1. The spectra are also characterized by the clear aromatic CHx
Figure 1. FTIR spectra of individual macerals and MS sample.
stretching band at 30523049 cm1, the symmetric methyl bending at 13701375 cm1, aromatic CO stretching band at 11001300 cm1, and various aromatic CH out-of-plane bending modes in the 700900 cm1 spectral region. Absorptions near 865 cm1, 812 cm1, and 753 cm1 observed in the spectra of most coals are assigned to the vibrations of substituted aromatic rings having one, two, three, or four adjacent hydrogens, respectively. The weak peak at 16901720 cm1 was assigned to aromatic CdO group stretching vibration, which appears as a shoulder on the 1600 cm1 peak in coals. The intensity and shape of the band near 3400 cm1 was assigned to OH groups present in samples, with water absorbed by KBr during sample preparation. The pellet was dried overnight at 105 °C under vacuum, but this serves only to minimize water content rather than eliminate it.48 Compared with the absorption IR spectra of the MS sample (Figure 1), the intensities of CHx stretching vibration peaks of individual macerals both from FTIR and micro-FTIR spectra are stronger. BaS has the highest intensity at 2920 cm1 and 2850 cm1, compared to other macerals, and was assigned to aliphatic methylene group stretching bands. The highest intensity of aromatic nucleus stretching vibration at 1601 cm1 is in VS, in comparison with other samples. The peak at 1735 cm1, due to CdO groups in aliphatic esters, is of strong intensity in barkinite in situ in MS coal. However, the results of Guo et al.1 and Sun,11 when studying barkinite structure by micro-FTIR, show that there is not a strong peak, or even no peak at all, at 1735 cm1. Moreover, the peak at 1735 cm1 of separated barkinite was not found in this study. It seems that the existence of a peak at 1735 cm1 in barkinite should be further investigated. CP/MAS 13C NMR Characteristics of Coals. Figure 2 presents the CP/MAS 13C NMR spectra for the three samples (MS, BaS, and VS). The chemical shift ranges for the different carbon types are based on the work of Yoshida et al.33 The spectra were clearly divided into two regions, an aromatic band from 90 to 170 ppm and an aliphatic band from 0 to 90 ppm. The unsubstituted aromatic carbon atom was found at approximately 129 ppm. The bands around 150 ppm were assigned to oxygenated aromatic carbon atoms (ArO) bound to hydroxyl, methoxy, and aromatic ether oxygen atoms. The bands around 126 ppm were 5673
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the degree of aromatic ring condensation.11 The A factor (30002700 cm1/30002700 cm1 + 1600 cm1) and C factor (1705 cm1/1705 cm1 + 1600 cm1), which represents the intensity of aliphatic relative to aromatic peaks, used for describing kerogen type and maturation levels.53 In addition, the A factor can also be used to define the hydrocarbon-generating potential of source rocks. The integrated areas of HAL (30002700 cm1) and HAR (31003000 cm1) might be considered to estimate the concentration of aliphatic and aromatic hydrogen, respectively.19 The semiquantitative data from the useful regions of the FTIR spectrum are given in Table 1. The ratio of HAR/HAL is also listed in the same table. For coals, the bands between 3100 and 3000 cm1 are characterized by aromatic CH stretching vibrations, whereas 30002700 cm1 is aliphatic CH stretching vibrations. So, the concentration of aromatic and aliphatic hydrogen can be estimated by calculating the integrated areas of these two regions, respectively. The values of HAL, HAR, and HAR/HAL obtained from FTIR spectra are listed in Table 1. The ratios of HAR/HAL of three samples used (BaS, VS, and MS) are less than 0.1, showing that the intensities of the aliphatic CH stretching absorption of these samples are significantly greater than that of the aromatic CH stretching absorption. BaS has the highest HAL and the lowest HAR/HAL values, which suggests that the aliphatic hydrogen concentration of barkinite is largest, relative to the other two samples. The intensity between 3000 and 2800 cm1 is attributed to the aliphatic CH groups (CH3, CH2, and CH). The integrated area between 3000 and 2800 cm1 can be used to estimate the total aliphatic CH content. The results of the ratios of integrated areas of 30002800 cm1 to 1600 cm1 (I1) and to 900700 cm1 (I2) are given in Table 1. Both BaS and Ba have higher values of I1 and I2, especially I1, than other samples, suggesting that the relative concentration of aliphatic groups in barkinite is the highest among these samples. Both the I1 and I2 values of BaS and VS indicate that the aliphatic concentration of barkinite is greater than that of vitrinite. The higher values of I1 and I2 in barkinite also show that barkinite has a relatively high concentration of aliphatic groups compared to that of aromatic groups. To study the aliphatic structure of barkinite, curve-fitting analysis between 3000 and 2800 cm1 was conducted. An example of band-fitting for the aliphatic CH stretching band for BaS is shown in Figure 3. Five bands were separated in this absorption range. Of these bands, the CH3 asymmetrical stretching vibration at 2950 cm1 and the CH2 asymmetrical stretching vibration at 2920 cm1 were clearly separated. The CH2/CH3 (2920 cm1/ 2950 cm1) ratio can be used to estimate the length and degree of branching of aliphatic side chains.49,51,52 From Table 1, it can be seen that the CH2/CH3 (2920 cm1/2950 cm1) ratios of both BaS and Ba are higher than those of VS and V. So, the content of CH2 of the aliphatic side chains in barkinite is larger
assigned to aromatic carbon atoms bound to hydrogen. The bridgehead and substituted aromatic carbon atoms bound to hydroaromatic CH2 carbon and alkyl side chains (ArC) were found around 138 ppm. In the aliphatic band (090 ppm), the most prominent resonance is found at approximately 30 ppm and is assigned to methylene group in saturated alkyl chains and in bridges. The signal at 15 ppm is caused by the methyl group. From Figure 2, it can be seen that BaS has the highest peak intensity in the aliphatic band, especially at 30 ppm, relative to the other two samples. The highest intensity of the aromatic band is concentrated in VS, among these three samples. The same result is also observed by FTIR analysis in this study. The two analytical methods results to show that the chemical structure of barkinite is characterized by more aliphatic carbons than those of vitrinite and MS samples. A similar result has been reported by Guo et al.1 and Sun.11 Structural Parameters of FTIR Analysis. Indexes derived from FTIR spectra have been applied to evaluate the chemical characteristics of coal and/or macerals.1,36,38,4951 To examine the structural characteristics of barkinite, the following structural parameters obtained from FTIR by curve fitting analysis were utilized in this study. The peaks selected are the relatively more intense and stable absorption peaks. The band area (not peak intensity) was used in this work. The CH2/CH3 (2920 cm1/2950 cm1) ratio is used to estimate the length and degree of branching aliphatic side chains.49,51,52 The ratio of integrated areas of 30002800 cm1 to 1600 cm1 (I1) and to 900700 cm1 (I2) can be used to compare the relative abundance of aliphatic and aromatic functional groups.1,36,37 The ratio of integrated areas of 900700 cm1 to 1600 cm1 can be used as a suitable index for assessing
Figure 2.
13
C NMR spectra of whole coal and individual macerals.
Table 1. Some Structural Parameters Derived from FTIR of Samples Used sample
a
HAL
HAR
HAR / HAL
CH2/CH3
I1
I2
A
C
δa
BaS
0.14
0.005
0.03
6.9
2.27
3.11
0.69
0.23
0.73
VS
0.11
0.009
0.08
5.75
0.7
2.78
0.41
0.06
0.25
MS
0.09
0.006
0.06
4.61
0.88
3.15
0.47
0.15
0.28
Ba V
0.12 0.14
0.002 0.008
0.02 0.06
8.35 1.8
3.59 0.96
4.94 3.38
0.78 0.49
0.67 0.16
0.71 0.28
Area (900700 cm1)/area (1600 cm1). 5674
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than that of vitrinite. Furthermore, with the aliphatic chains becoming larger or less branched, the CH2/CH3 ratio in the aliphatic CH stretching region should increase.50 Therefore, the higher CH2/CH3 ratios of BaS and Ba suggest that barkinite contains aliphatic structures that are longer and less branched. A similar result was also obtained by Guo et al.1 The region between 1800 and 1500 cm1 is attributed to aromatic rings and carbonyl and carboxylic groups, and it is used in the classification of kerogen types and their degree of maturation.53 In the samples studied here, only BaS, VS, and MS have clear absorption peaks of aromatic carbonyl/carboxyl CdO at 1700 cm1. In the Ba sample, the peak at 1735 cm1 due to aliphatic ester groups is clearly shown.39 For the V sample, the peak nominally expected at 1700 cm1 is shifted to 1685 cm1. The A factor and C factor are calculated and listed in Table 1. Both BaS and Ba have higher values of A and C factors than VS and V. According to the results of Ganz and Kalkreuth,53 barkinite should be type I kerogen. The high A factor value obtained for BaS agrees with its high hydrogen content (6.91%,
Figure 3. Curve-fitted FTIR spectrum of the aliphatic CH stretching band for BaS.
Figure 4. Curve-fitted FTIR spectrum of the aromatic CH out-ofplane deformation bands for BaS.
daf) and high volatile matter (62.16%, daf). Meanwhile, the high value of the A factor of barkinite indicates that barkinite has good hydrocarbon-generating potential, which is in good agreement with earlier works.8,9,40 The spectral region of 900700 cm1 is used to study the substitution of the aromatic structure. Three principal out-ofplane CH deformation bands were observed in this region for the samples used. They are centered at 870, 815, and 750 cm1 and assigned to aromatic structures with isolated aromatic hydrogens, two adjacent hydrogens per ring and four adjacent aromatic hydrogens, respectively.54 The curve-fitting analysis of BaS, for example, for the region of 900700 cm1 is shown in Figure 4. The ratio of integrated areas of 900700 cm1 to 1600 cm1 (δ value, Table 1) is a suitable index for assessing the degree of aromatic ring condensation.11 From Table 1, it can be seen that both BaS and Ba have higher ratio values than VS and V, which suggests that the degree of aromatic ring condensation of barkinite is higher than that of vitrinite. However, the result should be further discussed because fewer samples were used in the paper. Carbon and Oxygen Distributions. Structural parameters are useful for the representation of characteristics of coals and macerals. To determine some structural parameters of the samples used in this study, the peaks of three selected samples were fitted using the curve-fitting program in Nuts 2000 software. Table 2 gives the results of semiquantitative calculation for the MS sample and separated macerals. Barkinite has the lowest aromaticity (0.36) of these samples and the highest aliphatic carbon (0.64), with a higher H/C atomic ratio, which shows that barkinite has an aliphatic-rich structure. Furthermore, in the aliphatic carbon range, the falH value of barkinite is much larger than that of fal*, showing that the aliphatic carbon in barkinite is primarily in long-chain structures. The result is the same with obtained from the FTIR analysis in this study. The oxygen-containing peak groups are divided into oxygenbound aromatic CO and aliphatic CO. For VS and BaS samples, the numbers of oxygen-bound carbons per 100 carbon atoms are 10 and 9, respectively, which is much more than that of MS coal (4). Furthermore, an interesting result is that most of oxygen is bound to aliphatic carbon, although the difference of numbers of oxygen bound to aliphatic and to aromatic carbon is not great. For barkinite, about 60% of all the oxygens are bound to aliphatic carbon. The mole fraction of aromatic bridgehead carbon (χb) is used to estimate the aromatic cluster size.27 χb is calculated as χb = faB/ fa 0 . The results calculated are also listed in Table 2. BaS has the highest χb value (0.25). According to Solum et al.,27 when χb equals 0.25, the number of carbon atoms per cluster is between 10 and 14. The χb value of naphthalene is 0.25. So, it can be inferred that the aromatic ring number per cluster of barkinite is predominately 2 or 3. For the VS sample, χb = 0.17. The number
Table 2. Some Carbon Structural Distribution and H/C Atomic Ratio of the Samples Used samples
fa a
fa0
b
faP c
faS d
faB e
fal f
fal* g
falH h
falO i
χb
H/C
MS
0.52
0.50
0.01
0.34
0.12
0.48
0.02
0.46
0.03
0.24
0.86
VS
0.41
0.41
0.04
0.30
0.07
0.59
0.03
0.56
0.06
0.17
0.81
BaS
0.36
0.36
0.04
0.22
0.09
0.64
0.04
0.60
0.05
0.25
1.01
fa, total aromatic carbon. b fa0 , in an aromatic ring. c faP, phenolic, δ = 150165 ppm. d faS, alkylated aromatic, δ = 135150 ppm. e faB, aromatic bridgehead. f fal, total aliphatic carbon. g fal*, CH3 or nonprotonated. h falH, CH or CH2. i falO, bonded to oxygen. a
5675
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Energy & Fuels of carbon atoms per cluster is between 6 and 10, depending on the result of Solum et al.,27 which indicates that the aromatic ring number per cluster of vitrinite is mainly 1 or 2.
4. CONCLUSIONS The resolution of overlapping bands in the FTIR and 13C NMR spectras of the studied samples by curve-fitting methods can provide detailed chemical information on the samples used. All the FTIR and 13C NMR spectras and the values of I1 and I2 show that barkinite is characterized by an aliphatic-rich structure. The higher CH2/CH3 ratio in the aliphatic CH stretching region suggests that barkinite contains aliphatic structures that are longer and less branched. The hydrogen distribution, calculated according to HAL and HAR/HAL values, shows that the aliphatic hydrogen concentration of barkinite is larger than that of the aromatic hydrogen concentration. The ratio of integrated areas of 900700 cm1 to 1600 cm1 derived from FTIR spectra shows that barkinite has higher ratio values than vitrinite. Furthermore, the χb value calculated from 13 C NMR implies that the aromatic ring number per cluster of barkinite structure is mainly 2 or 3. The oxygen-containing functional groups are often divided into oxygen-bound aromatic CO and aliphatic CO. For barkinite, oxygen-containing functional groups can be seen in both aromatic and aliphatic structures. However, most of the oxygen atoms are bound to aliphatic carbon. According to the A factor and the C factor calculated from FTIR spectra, barkinite can be inferred to belong to type I kerogen. Depending on the value of the A factor, barkinite has good hydrocarbon-generating potential. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: +86-10-6233-9302. *E-mail:
[email protected]. Tel: +1-814-863-1337.
’ ACKNOWLEDGMENT The authors gratefully thank the National Natural Science Foundation of China for financial support (Research Project No. 40772101). The authors also acknowledge the support by State Key Laboratory for Coal Resources and Safe Mining, China University of Mining and Technology (Beijing) (Project No. SKLCRSM10KFB08) and The Fundamental Research Funds for the Central Universities (Project No. 2010QD02). ’ REFERENCES (1) Guo, Y. T.; Renton, J. J.; Penn, J. H. Int. J. Coal. Geol 1996, 29, 187–197. (2) Hsieh, C. Y. Bull. Geol. Soc. China 1933, 12, 469–490. (3) Han, D. X.; Ren, D. Y.; Wang, Y. B.; Jin, K. L.; Mao, H. L.; Qin, Y. Coal Petrology of China; China University of Mining and Technology Press: Xuzhou, 1996 (in Chinese). (4) Wang, S. Q.; Tang, Y. G.; Schobert, H. H.; Mitchell, G. D.; Liao, F. R.; Liu, Z. Z. Int. J. Coal. Geol 2010a, 81, 37–44. (5) Han, D. X.; Ren, D. Y.; Guo, M. T. J. Sediment 1983, 1 (4), 1–14(in Chinese). (6) Hower, J. C.; Suarez-Ruiz, I.; Mastalerz, M.; Cook, A. C. Spectrochim. Acta, Part A 2007, 67, 1433–1437. (7) Ren, D. Y.; Gao, Q. C.; Liu, X. S.; Wang, M. Y. The 32rd Annual Academic Symposium 1963, 124–128(in Chinese). (8) Sun, X. G. Int. J. Coal. Geol 2002, 51 (4), 251–261.
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dx.doi.org/10.1021/ef201196v |Energy Fuels 2011, 25, 5672–5677
