Investigation of Structural Characteristics of Thermally

Sep 5, 2013 - The structural parameters of nine coal samples from a contact metamorphic zone were studied by Fourier transform infrared spectroscopy (...
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Investigation of Structural Characteristics of Thermally Metamorphosed Coal by FTIR Spectroscopy and X‑ray Diffraction Wu Dun,†,‡ Liu Guijian,†,‡,* Sun Ruoyu,† and Fan Xiang†,‡ †

CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, The Chinese Academy of Sciences, Xi’an, Shaanxi 710075, China ABSTRACT: The structural parameters of nine coal samples from a contact metamorphic zone were studied by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The FTIR spectra parameters composed of CH2/CH3, fa, Hal/Har, and (R/C)u indicate distinguishing features, including increase of aromaticity and the loss of aliphatic and oxygencontaining groups with thermally metamorphic evolution of coal. The XRD analysis shows that magmatic intrusion potentially caused rapid changes of structures in coal. With the decrease of distances from the intrusion rocks, the average lateral sizes (La), stacking heights (Lc), and interlayer spacing (d002) of the crystallite structures of coal range from 22.90 to 37.70 Å, 12.90 to 23.30 Å, and 3.80 to 3.50 Å, respectively. Exponential correlations are observed between structural parameters (fa, d002, La, Lc, Hal/H, (R/C)u, and Aar/Aal) and C/H of the coal, suggesting that the structures of coal are controlled by the degree of contact metamorphism.

1. INTRODUCTION The influence of magmatic intrusions on the petrology and geochemical characteristics of coal and coal bed methane development has been extensively discussed.1,2 However, its effect on the chemical structural evolution of coal has not been well investigated. The understanding of structural properties of coal would aid in the prediction of the behavior of coal during combustion, pyrolysis, liquefaction, and gasification processes.3−5 Meyer6 proposed that coal is a heterogeneous aggregate formed from a cross-linked molecular network of organic components. Subsequently, Solomon and Carangelo7,8 demonstrated that coal mainly consists of a three-dimensional macromolecular network made up of fused aromatic clusters, and the aromatic rings are linked by aliphatic or heteroaliphatic bridges. Nevertheless, fully characterizing coal structures is still a challenging issue due to the heterogeneity, noncrystalline structure, and insolubility of coal. Physical detection techniques such as X-ray diffraction techniques (XRD) and Fourier transform infrared spectroscopy (FTIR) have the advantage of minimal destruction of chemical structures in coal. Thus, they are widely applied in characterizing the chemical structures of coal. The crystal structure of a carbonaceous material is commonly studied under XRD spectroscopy, with its diffraction direction associated with the shape and size of crystalline cells and with its diffraction intensity connected with the arrangement of atoms in the unit cell. However, for the noncrystalline part of coal, XRD is still capable of revealing the order of arrangement of coal carbon atoms. The statistical interpretation of XRD profiles of carbonaceous materials with low-crystallinity has been applied by Hirsch9 and Diamond.10 Fujimoto and Shiraishi11 modified the Diamond’s method for estimating carbon-layer sizes, and Takagi and Morooka12 used a standard analysis of carbon stacking structure in coal by XRD to © 2013 American Chemical Society

understand the stacking structure of coal and other lesscrystalline carbonaceous materials. The degree of aromaticity, the interlayer spacing, and the crystallite size have been established as the parameters for evaluating the stacking structure of carbonaceous materials.13,14 The coal structural parameters from XRD can be evaluated by the Scherrer equation and an advanced approach using Gaussian curve fitting. FTIR can provide additional insight into the chemical structures of coal. Kister15 and Christy16 obtained the structural information of coal using FTIR. FTIR may further be used to know the chemical compositions of coal. In order to obtain the infrared spectral parameters, curve-fitting methods are often used to separate the overlapping bands in composite profiles. Different patterns of magmatic intrusions have been documented in the Lower Palaeozoic coal-bearing strata, Huainan coalfield, China, leading to the formation of coals of different ranks.17,18 However, the chemical and structural characteristics of magmatic-intruded coal have rarely been investigated.19 In the present study, FTIR and XRD techniques are applied to quantitatively study the chemically structural evolution of intruded coal using its derived structural parameters.

2. EXPERIMENTAL SECTION 2.1. Samples and Sample Preparation. Coal samples were collected from the Zhuji coal mine, northeast of the Huainan coalfield. This mine has abundant metamorphosed coal, occurring primarily in the Shanxi Formation, Lower Permian.18 The No.3 coal seam, with an average thickness of 3 m, was cross-cut by an Early Triassic dike which enhanced the adjacent low-rank bituminous coal into high-rank Received: July 5, 2013 Revised: September 5, 2013 Published: September 5, 2013 5823

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from 0.061 to 0.077, and high-rank bituminous coal (ZJ-2, ZJ-6, and ZJ-7), with H/C values ranging from 0.048 to 0.058. The metamorphosed degrees of coal samples studied here increase with decrease distance from the intrusive dike. In addition, the negatively significant correlation of volatile matter and coal ranks can also be seen from Table 1. 2.3. FTIR Spectroscopy Measurement. Quantitative FTIR transmission spectra analysis was conducted on coal KBr pellets. The pellets were prepared by mixing 1 mg of demineralized coal with 200 mg of KBr for 5 min and then pressing the mixture into pellets in an evacuated die under 10 MPa of pressure for 2 min. The pellets was dried in a vacuum oven for 24 h to exclude the interference of water on the spectrum.20 The infrared spectra were generated by collecting 128 scans at a resolution of 8 cm−1 using a Nicolet model 8700 Fourier transform infrared spectrometer. The measured region extended from 4000 to 400 cm−1. The peak separation and quantitative calculation were performed using a curve-fitting program of Origin 8.0 software. Selected regions of the FTIR spectra were linearized for baseline by connecting the left and right points of the interval with a straight line. An absorption normalization before curve-fitting for each spectrum was selected in the EZ OMNIC software. The positions and number of bands were established initially from the second derivative of the spectrum. The number of peaks in a given region was determined, and the frequency and intensity of each peak was estimated. All the band shapes, heights, and widths were obtained using a Lorentz combination and were adjustable.21 It is assumed that all carbon atoms are either aliphatic or aromatic, and the apparent aromaticity (fa) of the samples was estimated according to the method of Mielczarski et al.:22

Figure 1. Stratigraphic profile of the No.3 coal seam, Zhuji coal mine, Huainan coalfield showing the locations of dike and coal samples. bituminous coal and anthracite. Nine coal samples were collected from the #13−4 borehole along both sides of the dike (Figure 1). Bulk coal samples of approximate 200 g were pulverized and sieved to obtain particles of 4]

W S M M M M S S S W W M M S S M M M M

S M S S S S M M M S M W W M M S S S W

S W W W W W W W W M S W W W W W W M W

L, low-rank bituminous coal; H, high-rank bituminous coal; A, anthracite; S, strong; M, medium; W, weak.

by deconvoluting the spectral region of 2800−3100 cm−1. The 2922 and 2854 cm−1 bands are attributed respectively to the asymmetric stretch of the CH3 and CH2 groups. A higher CH2/CH3 implies longer aliphatic chains binding to aromatic rings, in contrast to a relative compact structure of less aromatic clusters space for a lower CH2/ CH3.25 The parameter Aar/Aal is an indication of aromaticity and ranks of coal, and is estimated by the following: A ar /A al = A 900 − 700cm−1/A3000 − 2815cm−1

representing respectively the γ-band and ∏-band (002). The ∏-band represents the spacing of aromatic ring layers, while the γband represents the packing distance of saturated structures.26 The position (φ), intensity (I), full width at half-maximum (B), and integrated area (A) of these fitting peaks were determined. Theoretically, the areas under the γ-band and ∏-band are attributed to the number of aromatic carbon atoms (Car) and aliphatic carbon atoms (Cal), respectively.27,28 The aromaticity ( fa′) of coal in aliphatic chains relative to aromatic rings can be defined as follows:

(5)

fa′ = Car /(Car + Cal) = A∏ /(A∏ + A γ )

2.4. X-ray Diffraction Measurement. A Philips X’Pert PRO Xray diffractometer was used to record X-ray intensities scattered from the examined coals. Cu Kα radiation (60 kV, 55 mA) was used as the X-ray source. Powdered coal samples were packed into a rectangular cavity in an aluminum holder and scanned from 10 to 70° in the 2θ range with a 0.1° step interval and a 2 s/step counter time. Origin 8.0 software was used for the deconvolution of the diffractograms in the 2θ region of 15−35°. The broad hump in this region was fitted to two Gaussian peaks around 20 and 26°,

(6)

Coal rank was calculated from the peak intensities at positions of the γband and the ∏-band using the following: coal rank = I∏/Iγ

(7)

The structural parameters of coal crystallite, including the lateral size (La) and the stacking height (Lc), were determined using the conventional Scherrer equations: 5825

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La = 1.84λ /Ba cos(φa)

(8)

Lc = 0.89λ /Bc cos(φc )

(9)

where λ is the wavelength of the radiation used, Ba and Bc are the full width at half-maximum of the (100) and (002) peaks, and φa and φc are the corresponding scattering angles or peak positions.

3. RESULTS AND DISCUSSION 3.1. FTIR Spectral Characteristics of Coal Samples. The absorption peaks of functional groups in organic matter of all Table 3. Structural Parameters Derived from the FTIR Spectruma sample

coal rank

fa

Hal/H

(R/C)u

CH2/CH3

Aar /Aal

ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6 ZJ-7 ZJ-8 ZJ-9

lb hb A A A hb hb lb lb

0.758 0.836 0.932 0.942 0.893 0.888 0.781 0.752 0.750

0.631 0.421 0.219 0.157 0.284 0.328 0.561 0.648 0.696

0.543 0.584 0.624 0.636 0.611 0.603 0.557 0.540 0.530

1.57 4.69 2.41 2.63 3.47 3.37 4.45 1.83 1.48

0.349 1.305 3.761 4.818 2.810 2.246 0.488 0.343 0.385

Figure 4. Curve-fitted FTIR spectrum of the aromatic bending modes (900−700 cm−1) for illustrated anthracite ZJ-4.

a

lb, low-rank bituminous coal; hb, high-rank bituminous coal; A, anthracite.

coal samples are recorded by infrared spectra (Figure 2). The corresponding functional groups29−31 of every band are tabulated in Table 2. According to the FTIR curves (Figure 2), coals of different ranks have similar characteristic absorption bands, consisting of primarily of aromatic nuclei, aliphatic sidechains, and oxygen-containing groups. However, their intensities of absorption bands vary considerably, indicating significantly structural difference among coals of different ranks. The broad band at 3400−3320 cm−1 represents −OH groups of water. We suspected that a small fraction of water was probably still retained into coal pellets even after dryness. In order to obtain quantitative information on structural changes in function of coal ranks, a series of structural

Figure 5. Curve-fitted FTIR spectrum of aliphatic and aromatic stretching modes (3000−2815 cm−1) for illustrated anthracite ZJ-4.

parameters are used. The parameters ( fa, Hal/H, and (R/C)u) for FTIR are calculated according to eqs 1−3 and are included in Table 3. We used anthracite ZJ-4 to illustrate the calculation

Figure 3. Curve-fitted FTIR spectrum of the aliphatic C−H stretching bands (3200−2600 cm−1) in illustrated anthracite ZJ-4. 5826

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Figure 6. Relationship between FTIR spectrum parameters and coal ranks (represented by H/C ratio).

Figure 7. X-ray diffraction profiles of the demineralized coal samples.

of other parameters. The 3200−2600 cm−1 spectral region (Figure 3) was first curve-fitted to derive A2854cm−1 and A2922cm−1 for calculating CH2/CH3 by eq 4. Then, the 900−700 cm−1 spectral region, representing aromatic bending modes (Figure 4), and the 3000−2815 cm−1 spectral region (Figure 5), representing aliphatic and aromatic stretching modes, were respectively curve-fitted to derive A 9 00−7 00cm − 1 and A3000−2815cm−1for calculating Aar/Aal by eq 5. In detail, the 900−700 cm−1 bands are assigned to aromatic structures with isolated aromatic H (873 cm−1), two adjacent aromatic H in each ring (849 cm−1), and four adjacent aromatic H (776 cm−1). The aliphatic C−H stretching bands reveal the presence of four peaks at 2919, 2848, 2955, and 2863 cm−1, all attributed to methyl groups, while the aromatic rings at 3080−3035 cm−1 correspond to the aromatic stretching band (Figure 5). 3.1.1. Evolution of Aliphatic Structures. CH2/CH3 values in Table 3 indicate that methylene groups increased from lowrank to high-rank bituminous coal. This is probably due to a loss of alkyl chains and the conversion of hydroaromatic methyl structures to aromatic rings or the branched aliphatic structures.25 However, for anthracite, the contributions of methylene and methyl groups were very low due to a compact structure with less space between aromatic clusters in

Figure 8. Curve-fitting of two Gaussian peaks for three ranks of coal in the 2θ range 8−36°.

anthracite. However, the methylene groups were more preferably lost than methyl groups, resulting in a relatively lower CH2/CH3 value in anthracite than high-rank bituminous coal.32 The peak at 730−720 cm−1 arises from long-chain alkanes [(CH2)n, n ≥ 4] in aliphatic structures of coals. Longchain alkanes are obvious in the low-rank bituminous coal but invisible in high-rank bituminous coal and anthracite. 5827

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Table 4. Structural Parameters Derived from the Curve-Fitting of XRD Spectraa

a

sample

coal rank

d002 (Å)

d100 (Å)

fa′

I26/I20

Lc (Å)

La (Å)

ZJ-1 ZJ-2 ZJ-3 ZJ-4 ZJ-5 ZJ-6 ZJ-7 ZJ-8 ZJ-9

lb hb A A A hb hb lb lb

3.7615 3.6692 3.5039 3.4892 3.4988 3.5343 3.6258 3.7630 3.7680

2.2485 2.2186 2.1178 2.0996 2.0860 2.1499 2.2014 2.2460 2.3427

0.7367 0.7683 0.8945 0.9233 0.8782 0.8668 0.7869 0.7367 0.7265

2.9962 3.2172 4.1706 4.4676 4.2457 3.9749 3.5515 3.2529 2.8486

13.30 13.67 20.50 23.30 21.40 18.87 14.93 13.20 12.90

23.74 24.18 28.64 37.70 29.83 26.18 24.51 23.50 22.93

lb, low-rank bituminous coal; hb, high-rank bituminous coal; A, anthracite.

(indicated by increased fa, (R/C)u, and Aar/Aal values in Table 3) and the decreased H/C ratio (Table 1) in high-rank coals as compared to low-rank coals. As shown in Figure 6, fa, (R/C)u, and Aar/Aal are all inversely correlated with H/C. Low-rank bituminous coal has an obvious aromatic band in the 849 cm−1 band (Figure 2). The relatively low aromaticity ( fa) in low-rank bituminous coal demonstrates that such a band represents highly substituted aromatic rings rather than large stacked aromatic rings.27 An important contributed band at 870 cm−1 is observed in high-rank bituminous coal to anthracite. However, the peak intensity of anthracite is weaker than that of high-rank bituminous coal, suggesting the degree of aromatic substitution decreased and the aromaticity increased in anthracite. On the basis of values of fa, (R/C)u, and Aar/Aal and reported spectral band assignations,33 the 870 cm−1 band in the high-rank bituminous coal and anthracite could be attributed to aromatic structures with 3−4 rings or more.31 These observations are in good agreement with previous studies that the increase of aromaticity in thermally metamorphosed coal is mainly due to its higher content of polynuclear aromatic or graphite-like structures.34−36 3.2. X-ray Diffraction Analysis of Coal Samples. X-ray diffractograms for coal samples of different ranks are shown in Figure 7. The high background intensity of the diffractograms indicates that coal contains a significant proportion of highly disordered materials composed of amorphous carbon. In addition, anthracite contains graphite-like structures (crystalline carbon) indicated by a clear (002) band at ∼26° and a characteristic (10) band at the neighborhood of the (100) band at ∼42°. The presence of the asymmetric (002) suggests the existence of another band (γ) at ∼20°. This band is attributed to the saturated structures such as aliphatic side-chains attaching to the edge of coal crystallites.37 The (002) peak indicates the spacing of aromatic ring layers, while (γ) reflects the packing distance of saturated structures.38 With the increase of coal ranks, the position of the (002) peak shifts to a high diffraction angle accompanied with increased sharpness and uplift (Figure 7), and the peak (γ) sharp band gradually disappears. Two Gaussian peak fittings for the ∼20 and 26° bands in coal of different ranks are shown in Figure 8. The position (φ), intensity (I), full width at half-maximum (B), and integrated area (A) are derived according to eqs 6−9, and the structural parameters aromaticity (fa′), coal rank (I26/I20), crystallite diameter (La), and crystallite height (Lc) are calculated and listed in Table 4. The interlayer spacing (d002) of the crystallite structure in coal ranges from 3.489 to 3.768 Å. The d002 values are higher than that of pure graphite (3.354 Å), suggesting a lower degree of crystalline order in the studied coal samples.

Figure 9. Relationships between coal structural parameters and coal ranks (represented by H/C ratio).

3.1.2. Evolution of Oxygen-Containing Structures. The 1750−1000 cm−1 spectral zone represents oxygen-containing functional groups, which are different for coals of different ranks. The band of carbonxyl group shifts from 1721 cm−1 in low-rank bituminous coal to 1695 cm −1 in high-rank bituminous coal and anthracite, indicating a conversion of aliphatic carbonxyls into aromatic carbonxyls (Figure 2).31 A remarkable peak of highly conjugated CO structures was detected in the high-rank bituminous coal at 1650 cm−1. Carbonyl groups are more stable than carboxyl in high-rank bituminous coal than low-rank bituminous coal and anthracite.31 Low-rank bituminous coal has a higher content of C− O−R groups due to the presence of lignin-like structures inferred from the bands at 1450, 1330, and 1220 cm−1. However, −OH groups are enriched in high-rank bituminous coal and anthracite due to the presence of phenols, ethers, and alcohols inferred from the stretching vibration at 3419−3359 cm−1. 3.1.3. Evolution of Aromatic Structures. Aromatic nucleus CC stretching vibration absorption peaks are primarily located in the 1615−1585 cm−1 spectral region (Figure 2). A peak shift from 1615 cm−1 of the aromatic CC stretching band in low-rank bituminous coal to 1585 cm−1 in high-rank bituminous coal and anthracite is evident. This observation agrees well with the increased condensation of aromatic rings 5828

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (No. 41173032 and No. 41373110), the National Basic Research Program of China (973 Program, 2014CB238903), the National Science and Technology Support Program (1012BAC10B02), the Key Program for Science and Technology Development of Anhui Province (No. 12010402111 and 11010401015), and the Creative Project of the Huainan Mining Industry (Group) Co. Ltd. The authors thank Research Associates Zuo Jian and Huang Jianliu of Instruments Center for Physical Science, USTC, China, for assistance with FTIR spectroscopy and XRD measurement. The comments and corrections from two anonymous reviewers are highly appreciated.

The d002 value of high-rank coal approaches the value for graphite (3.354 Å) (Table 4). Significant exponential correlations between the structural parameters of coal and its rank (represented by H/C) are shown in Figure 9. With the increase in coal ranks (i.e., decrease in H/C), La, Lc, and fa′ increase, whereas the d002 decreases. The short interlayer spacing (d002) between organic groups in anthracite is a result of orientation due to heating under a confined pressure. On the basis of the above observations, low-rank bituminous coal possesses pronounced polymeric character and certain degrees of open structure.39 With the increase of colification due to magmatic intrusion, the bridge structures of coal became unstable because of increased interaction forces between the aromatic nuclei, forming a “liquid” structure.40 Subsequently, this structure stiffened again (anthracitization) under a magmatic effect and rendered anthracite with a graphite-like structure.

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Table 5. Correlations between Structural Parameters and Ranks of Coal

fa Hal/H Aar/Aal (R/C)u fa′ La Lc d002

volatile matter (R2)

H/C (R2)

fa = 0.96 exp(−VM/95.4) + 0.07 (R2 = 0.98) Hal/H = 7.83 exp(−VM/0.53) + 0.02 (R2 = 0.97) Aar/Aal = −11.39 exp(−VM/ 2869.11) + 12.07 (R2 = 0.99) (R/C)u = 12.04 exp(−VM/ 11.60) − 0.31 (R2 = 0.99) fa′ = 0.49 exp(−VM/33.52) + 0.56 (R2 = 0.92)

fa = 1.04 exp[−(H/C)/0.19] + 0.01 (R2 = 0.83) Hal/H = 0.05 exp[−(H/C)/ 0.03] + 0.05 (R2 = 0.87) Aar/Aal = 18.7 exp[(H/C)/ 0.02] + 0.021 (R2 = 0.75)

La = 237.5 exp(−VM/11.14) + 0.03 (R2 = 0.81) Lc = −24.2 exp(−VM/17.0) + 9.87 (R2 = 0.85) d002 = −0.00005 exp(−VM/ 0.28) + 0.12 (R2 = 0.85)

(R/C)u = −0.009 exp[−(H/ C)/0.03] + 0.66 (R2 = 0.87) fa′ = 44881.7 exp[−(H/C)/ 10544.21] − 44880.7 (R2 = 0.92) La = 42.4 exp[−(H/C)/0.11] + 0.02 (R2 = 0.99) Lc = 31.7 exp[−(H/C)/0.08] + 0.01 (R2 = 0.87) d002 = 0.01 exp[−(H/C)/0.02] + 0.11 (R2 = 0.85)

by FTIR and XRD. The exponential function model is suitable to explain the variation of coal chemical structural parameters. These two techniques complement each other. The mathematical equations presented in Table 5 show significant correlations between FTIR parameters (Hal/H, Aar/Aal, (R/ C)u, fa) and XRD parameters (fa′, La, Lc, and d002) and coal rank parameters (H/C or VM). With the increase of coal ranks, the aromaticity increases, whereas aliphatic and oxygen-containing groups decrease. Highly substituted aromatic rings seem to be the main aromatic structures in low-rank bituminous coal, whereas condensed aromatic nuclei are present in the high-rank bituminous coal and anthracite.

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REFERENCES

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4. CONCLUDING REMARKS Metamorphism of coal due to magmatic intrusion could significantly modify the chemical structures of coal, as indicated

structural parameters

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Corresponding Author

*Tel: +86-551-3603714. Fax: +86-551-3621485. E-mail address: [email protected]. Notes

The authors declare no competing financial interest. 5829

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dx.doi.org/10.1021/ef401276h | Energy Fuels 2013, 27, 5823−5830