Influence of Calcination and Acidification on Structural

Oct 8, 2013 - Anthracite represents the highest metamorphic rank in coal. ... PeakFit V4.0 software was applied to fit the diffractograms using Gaussi...
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Influence of Calcination and Acidification on Structural Characterization of Anyang Anthracites Yude Zhang,*,†,‡ Xiaojuan Kang,† Jinlong Tan,† and Ray L. Frost*,‡ †

Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, P.R. China ‡ School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia S Supporting Information *

ABSTRACT: The structural parameters of six coals were determined by X-ray diffraction (XRD), scanning electron microscope (SEM), and Raman and FTIR spectroscopy. The results reveal that the derivative coals prepared by calcination and HF acidification contain much crystalline carbon like graphite structure and are improved significantly in aromaticity, coal rank, and hydroxyl concentration. The increase of hydroxyl (OH) bonds is very beneficial to the surface modification of coal crystalline layers. In addition, the derivative coals have an obvious decrease of the aliphatic C−H content and a significant increase of the aromatic C−H content in FTIR spectra compared to that for the raw coal (AY). They are consistent with the changes of aromaticity and crystalline carbon measured by XRD and Raman spectra. With the increase of coal rank, the content of defect crystalline carbon and amorphous carbon decreases gradually from AY to the coal prepared by calcination and HF acidification (AY-C-HF). The particles of AY-C-HF with about 20 nm in thickness have better dispersibility with simultaneously a lot of pore space. The process of first calcination and then acidification can help us delaminate the aromatic layers in the coals and obtain the superfine crystalline carbon materials like graphite structure.

1. INTRODUCTION Coal is an organic, combustible, sedimentary rock of infinite variations, formed from layered plant remains consolidated under superimposed strata.1 Depending on the maturation stage reached in the coalification process, coal is mainly composed of carbon, oxygen, nitrogen, and sulfur in variable proportions. Graphite, essentially the end point of coalification, only contains carbon. Anthracite represents the highest metamorphic rank in coal.2 The aromatic structure is mainly composed of anthracite, and the condensation degree is further improved.3 Anthracite coals are known to be highly ordered and are thus similar to graphite in terms of structure and thermodynamics, yet the substitution of heteroatoms and certain cross-linkages within anthracite make anthracite slightly less stable than graphite.4 Unlike graphite, coal crystallite is extremely small and contains significant amounts of aliphatic side chains on its edges. These small crystallites can be linked via their side chains to form the so-called macromolecule, with the amorphous carbon being trapped in it.5 The anthracite is more complicated than the graphite in structure. Therefore, the structural properties of anthracite have received much attention among coal chemists. Different instrumental techniques, including Xray diffraction (XRD),1,5−7 infrared spectroscopy (IR),3,8,9 Raman spectroscopy, 2,6 nuclear magnetic resonance (NMR)10−12 spectroscopy, Mössbauer spectroscopy,11 scanning electron microscopy (SEM),13,14 and transmission electron microscopy (TEM)15 have been applied to the investigation of the chemical structure of coal. By means of XRD quantitative analysis,16 structural parameters, such as amount of amorphous carbon (Cam), aromaticity (fa), interlayer © 2013 American Chemical Society

spacing of the crystalline structure (d002), and crystallite sizes (La and Lc) can be determined for evaluating the carbon stacking structure in carbon materials.16 Raman spectral characteristics, mainly those of the G (graphite) and D (defect) bands at 1580 and 1350 cm−1, respectively, can provide information for evaluating the degree of ordering and crystallinity defect in carbonaceous materials.17−20 FTIR is very useful in probing the functional groups in coal and carbonaceous materials and thus provides additional insight into the structure of coal.6,9 Graphite after delaminated can be blended with the polymers to make the functional composites with special performances. Coal crystallites in anthracite are similar to the graphite in structure. The delaminated structure of coal crystallite can be obtained through destroying the cross-linkages of aliphatic side chains on its edges. In this work, the combined technologies of calcination and HF acidification were adapted to reduce the linkages within coal crystallites, increase the reactive point on its surface, and prepare the superfine layer coal-based filler. XRD, SEM, and Raman and FTIR spectroscopy were used to investigate the structure changes of an anthracite collected from Anyang in China and its five derivative samples prepared by calcination and acidification. In addition, the selected spectra were deconvoluted in order to gain further structural information of the different coal samples. The aim of the present work is to establish the correlation among the structural Received: August 20, 2013 Revised: October 8, 2013 Published: October 8, 2013 7191

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peaks.2,19,22 The content of amorphous carbon (Cam) in the coals was the peaks D3 and D4 intensity relative to the total intensity (ITotal), i.e., Cam = I(D3+D4)/ITotal. 2.4. FTIR Spectroscopy Analysis. The FTIR spectra of the six anthracite samples were recorded on a Bruker Tensor27-spectrometer over the range of 4000−400 cm−1 after using potassium bromide/coal pellet technique at the ratio of 200 to 1. The total numbers of scans were 50 with spectral resolution of 4 cm−1. For all spectra, a linear baseline correction was used and the peak fitting was performed by PeakFit v4.0 software in the regions of 2600−4000 and 600−1300 cm−1. The band positions, intensities, widths, and areas were determined. The fitted spectral regions can provide the most valuable data of the aliphatic carbon (Cal), aromatic carbon (Car), carbonyl, and hydroxyl groups. 2.5. SEM Analysis. The coal powders were adhered to Cu stubs using conductive adhesive, and scanning electron microscopy (SEM) micrographs were obtained with a S4800 LV electron microscope under 50 kV accelerating voltage.

parameters and obtain an impactful method to delaminate the aromatic carbon layer of the coals.

2. EXPERIMENTAL SECTION 2.1. Samples and Sample Preparation. The anthracite was collected from Anyang in China. The Anyang raw coal was ground into powder (200 mesh) and analyzed for moisture, ash, and elemental composition. The content of moisture, air-dried basis ash, air-dried basis volatile, and fixed carbon was determined according to GB/T 212-200 standard. The Ultimate analysis (C, H, N, O, and S) was performed using a Vario MACRO Cube elemental analyzer. The corresponding results are as following (%): moisture 1.79, ash 12.98, volatiles 5.13, fixed carbon 80.09; C 81.93, H, 5.18, O 10.72, N 2.17, S 0.51. Five derivative samples were obtained after the Anyang raw coal (AY) was treated by calcination and HF acidification. First, 50 g of the Anyang raw coal (AY) was loaded into a corundum crucible, then heated for 1 h at 800 °C in a high temperature furnace with nitrogen atmosphere, and cooled to ambient temperature, named as AY-C. Second, 30 g of the AY-C was dispersed in 150 mL of concentrated HF solution (40.0 wt.%), and the mixture was stirred for 72 h at ambient temperature. The coal was filtered and washed with distilled water, dried at 80 °C, and named as AY-C-HF. In addition, 50 g of AY was dispersed in 250 mL of concentrated HF solution (40.0 wt.%), and the mixture was stirred for 72h at ambient temperature. The coal was filtered and washed with distilled water, dried at 80 °C, and named as AY-HF. Then 30 g of the AY-HF was heated according to the conditions of AY-C and named as AY-HF-C. Finally, 15 g of the AYHF-C was acidized again according to the conditions of AY-HF and named as AY-HF-C-HF. 2.2. X-ray Diffraction Analysis. The XRD data collection was acquired with a RagKu D/max-2000 18 kW X-ray diffractometer with Cu Kα radiation, 40 kV, 150 mA, and a scanning rate of 2°/min over the range 2.5−75° (2θ). PeakFit V4.0 software was applied to fit the diffractograms using Gaussian functions in the 2θ regions of 16−33° and 36−53°. The broad hump in the region of 16−33° (2θ) was fitted to two Gaussian peaks around 20° and 26°, representing γ-band and Π-band (d002), respectively. The peak positions, intensities, widths, and area were determined. Theoretically, the areas under the γ- and Π -peaks are believed to be equal to the number of aliphatic carbon atoms (Cal) and aromatic carbon atoms (Car), respectively.5 Therefore, the aromaticity (fa) of coal, i.e., the ratio of carbon atoms in aliphatic chains to aromatic rings, can be defined as fa = Car/(Car+ Cal) = A002/(A002+Aγ). Coal rank was determined from the peak intensities at position 20° and 26°,7 defined as A26/A20 in this work. The lateral size (La) and the stacking height (Lc) of the crystallite were determined using the conventional Scherrer equations:21 La = 1.84 λ/(βa cos φa) and Lc = 1.84 λ/(βc cos φc), where λ is the wavelength of the radiation used, βa and βc are the widths of the (100) and (002) peaks, respectively, at 50% height, and φa and φc are the corresponding scattering angles or peak positions. The aspect ratio of stacking aromatic layer is equal to the ratio of La vs Lc. The stacking layer number (n) of crystalline carbon can be defined as the ratio of Lc to interlayer spacing (d002) within aromatic layers. 2.3. Raman Spectroscopy Analysis. The Raman spectra of the raw coal and five derivative coal samples were recorded at a resolution of 4 cm−1 using an inVia Laser confocal Raman spectroscopy system, at the conditions of 514.5 nm laser wavelength, 65 um slit-width, 20 s data acquisition time, and three scanning times. The laser power of the incident beam on the sample was kept below 2mW to prevent irreversible thermal damage to the sample surface. For all spectra, a linear baseline correction was used and the band fitting was performed by PeakFit V4.0 software in the region of 800−2000 cm−1.The band positions, intensities, widths, and areas were determined. The defect degree (AD1/AG) of microcrystalline carbon was equal to the integrated area ratio of the D1 (1350 cm−1) and G bands (1580 cm−1). The concentration of crystalline carbon (Ccr) was calculated from the ratio of the intensity of D1 and G peaks (I(D1+G)) to the total intensity (ITotal) of D1, D3 (1530 cm−1), D4 (1200 cm−1), and G

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis of the Coals. The AY contains 12.98% ash content according to the proximate analysis of coal. It can be seen that some minerals exist in this coal. XRD profiles for the AY and its derivatives prepared by acidification-calcination are shown in Figure 1. The mineral

Figure 1. X-ray diffraction profiles of the anthracites prepared by acidification and calcination.

compositions of the AY and the corresponding position (2θ) are summarized in Table 1. There are some kaolinite, calcite, calcium sulfate, dolomite, boehmite, and quartz and a little elpidite. All samples exhibit high background intensity, indicating that the coals contain a proportion of disordered materials in the form of amorphous carbon. In addition, the coals also contain some graphite-like structures (crystalline carbon) indicated by the presence of a clear (002) band at ∼26° and (100) band in the neighborhood of the graphite (100) at ∼42.3°. These observations suggest that the crystallites in all of the coals have intermediate structures between graphite and the amorphous state, so-called turbostratic structure or random layer lattice structure. In the diffractograms, the diffraction profiles show the presence of a clear asymmetric (002) band around 26° which suggests the existence of another band (γ) on its left-hand side. The (γ) band around 20° has 7192

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Table 1. Minerals and the Corresponding Position of Diffraction Peak of the Raw Coal (AY) 2 θ (deg)

minerals calcite (CaCO3) calcium sulfate (CaSO4) dolomite (CaMg(CO3)2) boehmite (AlOOH) quartz (SiO2) elpidite (NaZrSiO15·3H2O) kaolinite (Al2(Si2O5)(OH)4)

29.38 25.36 24.06 14.33 26.64 12.46 12.38

35.99 28.67 30.80 22.218 36.58 17.12 21.30

39.42 29.20 37.18 34.90

43.18 40.97 43.85 42.34

25.90 25.92

54.18

48.52 43.44 44.78 54.59

47.42 50.17 62.67

56.29 59.53

sample has much more crystalline carbon like graphite and indicates the highest ordered degree. The La and Lc values range from 19.19 to 26.85 Å and 10.04 to 16.00 Å, respectively (Table 2). The d002 of the crystallite structure ranges from 3.43 to 3.57 Å. The aspect ratio and n values range from 1.34 to 2.51 and 2.82 to 4.66. The stacking layer number of crystalline carbon for the derivative coals was decreased compared to AY, and the crystalline layers become thinner. The d002 values of the derivative coals prepared by HF acidification and calcination are higher than that of pure graphite (3.36−3.37 Å), suggesting a low degree of crystalline order. 3.2. Raman Characterization of the Coals. The Raman spectra of the six coal samples in the range 4000−400 cm−1 are presented in Figure 3. Figure 4 shows the fitted spectra of AY in

been reported by many other authors and was attributed to the presence of saturated structure such as aliphatic side chains, which is attached to the edge of coal crystallites.16 The (002) band indicates the spacing of aromatic ring layers, whereas (γ) band reflects the packing distance of saturated structures. Representative fits of three Gaussian peaks for the bands around 26° and 42.3° for AY are shown in Figure 2. The XRD

Figure 2. Curve-fitting of the peaks for the raw coal (AY) in 2θ range 16−33° and 36−53°.

pattern fits of the derivatives prepared by calcination and HF acidification are presented in Figure S1 and S2 in the Supporting Information. Deconvoluted structure parameters extracted from the curve-fitting of XRD profile are listed in Table S1. The derived structure parameters obtained after curve-fitting, including peak positions, intensity, area and fwhm, are summarized in Table 2. The fa and A26/A20 values for the six samples range from 0.721 to 0.843 and 2.58 to 5.38, respectively. All of the derivative coals are improved in the aromaticity and endowed with much crystalline carbon. The AY-C-HF has the highest aromaticity up to 0.83 and simultaneously with the largest A26/A20 value. The AY-C-HF

Figure 3. Raman spectra of the anthracite prepared by acidification and calcination.

the range 2000−800 cm−1, and Figure S3 in the Supporting Information displays the fitted spectra of all the coals. All of the

Table 2. Derived Structure Parameters Extracted from the Curve-Fitting of XRD Profile samples

La (Å)

Lc (Å)

d002 (Å)

A26/A20

La/Lc

n

fa

AY-C-HF AY-HF-C-HF AY-HF AY-HF-C AY-C AY

23.91 25.18 19.19 22.36 21.10 26.85

10.28 10.04 14.33 10.18 11.41 16.00

3.56 3.57 3.47 3.55 3.54 3.43

5.38 5.27 4.70 4.10 2.81 2.58

2.33 2.51 1.34 2.20 1.85 1.68

2.89 2.82 4.12 2.87 3.22 4.66

0.843 0.840 0.825 0.804 0.737 0.721

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derivative coals via calcination and acidification are reduced remarkably (Table 3). The value of Ccr in six coals varies from 51.0% (AY) to 76.5% (AY-C-HF) and shows the same change tendency as the fa measured by XRD (Table 2). The process of calcination first and HF acidification next is much available to improve the aromaticity and crystalline carbon for this coal. 3.3. FTIR Spectra Analysis of the Coals. The infrared spectra of coals have been studied by many researchers with the emission and transmission spectra. The major bands evident in both spectra are assigned as follows:6,9 ∼3700 and 3650 cm−1, OH stretching in kaolinite mineral, and organic compounds having oxygen functional groups found in coal including phenols, alcohols, and carboxylic acid; 3100−3000 cm−1, aromatic C−H stretch; 3000−2800 cm−1, aliphatic C−H stretch; ∼1600 cm−1, aromatic C−C stretch, probably as well as CO stretching vibration of carbonyl groups; 1450 cm−1, aliphatic C−H bend, probably as well as CO vibration of carbonyl groups; 1200−900 cm−1, Si−O−Al vibrations of clay minerals; 900−700 cm−1, aromatic out-of-plane C−H bend; and 600−480 cm−1, AlO6 and SiO4 vibrations of clay minerals. The FTIR spectra of all coal samples and the fitting spectra of AY at the selected regions (4000−2600 and 1300−600 cm−1) are shown in Figures 5 and 6, respectively. The fitting

Figure 4. Raman spectra of the raw coal (AY) with the corresponding curve fitted bands in range 2000−800 cm−1.

curves in Figure 3 exhibit two narrow and overlapping peaks with intensity maxima at ∼1340 and ∼1590 cm−1 which correspond to the D1 and G bands of disordered and ordered graphite, respectively.23 The G peak at 1580 cm−1 is assigned to the stretching vibrations of sp2 bonds in hexagonal aromatic molecules of graphitic carbon.19 The D1 peak between 1330 and 1400 cm−1 results from the broadening of the G peak due to the introduced disorder carbon. The high signal intensity between the two peak maxima can be attributed to a band between 1500 and 1550 cm−1, designated D3. The D3 band has been associated with amorphous sp2-bonded forms of carbon.24 In addition, the broad D band conceals another band of amorphous carbon, namely as D4, and the main part of D band attributed to D1. Then the region of 1800−1000 cm−1 is composed of four bands including D1, D3, D4, and G. The Raman spectra of all coal samples were analyzed by curve-fitting the four peaks (D1, D3, D4, and G). The deconvoluted parameters such as peak positions, integral area of intensity and fwhm are listed in Table S2. The derived parameters including AD1/AG, Ccr, and Cam are summarized in Table 3. The fwhm of the G band ranges from 54 to 76 cm−1 which is far larger than that recorded for highly oriented pyrolytic graphite of about 15−23 cm−1.24 This result suggests a low crystalline order degree in the studied coals. The defect degree of crystallite graphite in the coal samples ranges from 1.725 to 2.777 (Table 3). AY-C-HF has the lowest defect degree, and AY has the highest defect degree in graphite crystalline layers. It shows that the AY sample may supply many active points which can react with the molecules of the modifier, intercalator, and sweller. The Cam values of the

Figure 5. FT-IR spectra of the anthracite prepared by acidification and calcination.

spectra of the derivative coals can be seen in Figures S4 and S5 in the Supporting Information. The FTIR spectra of all of the

Table 3. Derived Parameters Obtained from Fitting Raman Spectra of the Anthracite Prepared by Acidification and Calcination peak area/%

AY-C-HF

AY-HF-C-HF

AY-HF

AY-HF-C

AY-C

AY

AD1 AD3 AD4 AG Ccr Cam AD1/AG

48.41 14.06 9.47 28.06 76.47 23.53 1.725

46.59 16.89 10.76 25.76 72.35 27.65 1.809

44.64 16.10 14.81 24.45 69.09 30.91 1.826

43.81 17.95 16.83 21.41 65.22 34.78 2.046

42.46 24.43 13.47 19.64 62.10 37.90 2.162

35.41 26.71 22.34 15.55 50.96 49.05 2.777

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The broad absorption band around 1620 cm−1 is assigned to the stretching vibration of CC bonds in aromatic structure and probably as well as the CO vibration of carbonyl groups.6 The band becomes stronger in five derivative coals compared to AY due to the increase of aromaticity (Figure 5). The bands of the derivative coals around 1450 cm−1 assigned to carbonyl vibration in this work are reduced compared with that of AY. The coals exhibit some bands in the 1250−1000 cm−1 region due to Si−O−Al bending vibrations.6,9 The region of 900−700 cm−1 is observed to contain some low intensity outof-plane vibration bands of aromatic −CH in six coal samples (Figures 6 and S5). The total aromatic carbon at 800 and 880 cm−1 is about 21.38% for AY-C-HF and about 7.24% for AY, which shows similar changes with that in the region of 4000− 2600 cm−1. 3.4. Relationship between Structural Parameters and Coal Rank. The coal samples analyzed belong to the anthracitic groups, with an elemental carbon content of 81.09 wt %. The A26/A20 values are plotted against fa, AD1/AG, and Cam ratios in Figure 7. It shows that a good positive correlation

Figure 6. Infrared spectra of the raw coal (AY) with the corresponding curve fitted bands in the ranges 4000−2600 and 1300−600 cm−1.

samples exhibit a broad absorption band between 4000 and 2600 cm−1 due to −OH and NH groups.25 Oxygen functional groups contained in phenols, alcohols, and carboxylic acid are found in the coals. The fitting band of the −OH group at 3420 cm−1 is broad and very weak and absent at 3470 cm−1 in AY (Figure 6); The bands at 3115 and 3180 cm−1 are assigned to the tightly bound cyclic OH tetramer and OH−N hydrogen bond vibration.26 The deconvoluted parameters of all the samples are summarized in Tables S3 and S4. The derived parameters are shown in Table 4. The total integral area of OH bands is 80.92% for AY. These hydroxyls are much stronger comparatively for the five derivative coals prepared by calcination and HF acidification. It suggests that the absorption bands of the most hydroxyls should be attributed to the organic compounds including phenols, alcohols, and carboxylic acid in coal. A little of them is associated with either clay minerals or other minerals containing water of crystallization.9 The increase of OH bonds is beneficial to modify the surface chemical performance of coal crystalline layers. The coal samples exhibit two fitted bands at about 3006 cm−1 attributed to the aromatic C−H stretch and 2850 cm−1 due to aliphatic −CH, −CH2, and −CH3 stretching vibration (Figure 6). The derivative coals show an obvious decrease in the integral intensity of the aliphatic C−H (Cal−H) range from 2.90% to 8.01% compared to that for AY with 16.75% and display a significant increase in the content of the aromatic C−H (Ccr−H) (Table 4). There is a good agreement with the changes of fa (Table 2) and Ccr (Table 3) measured by XRD and Raman spectra for the six coals.

Figure 7. Relationship between coal rank (A26/A20) and aromaticity ( fa), defect crystalline carbon (AD1/AG), and amorphous carbon (Cam).

exists between the coal rank and the aromaticity (R2 = 0.979). Both parameters reflect the increase of maturity degree for the five derivative coals. There is a negative correlation between the coal rank and defect crystalline carbon, amorphous carbon, R2 is 0.731 and 0.836, respectively. With the increase of coal rank, the concentration of defect crystalline carbon and amorphous carbon decreases gradually from AY to AY-C-HF. 3.5. SEM Analysis. Anthracite is a kind of coal with high aromaticity. The stacking of aromatic layers is similar to the

Table 4. Derived Parameters Obtained from Fitting FT-IR Spectra of the Anthracite Prepared by Acidification and Calcination peak area/% range

group

4000−2600

Cal−H Car−H −OH Car−H Al−O Si−O

1300−600

position/cm−1 2850 3006 ∼3110 900 937 990−1200

AY-C-HF

AY-HF-C-HF

AY-HF

AY-HF-C

AY-C

AY

2.90 6.04 91.06 21.38 6.84 71.78

5.24 5.88 88.88 19.48 26.99 52.56

9.41 5.39 85.2 16.89 7.12 76

8.24 2.85 88.91 13.49 12.64 73.41

8.01 2.68 89.31 11.10 12.14 76.75

16.75 2.33 80.92 7.24 12.33 79.71

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comb-like structure. This structure may be attributed to the curling and defect of the small aromatic layers due to the serious acidification. There are some obvious changes in microstructure for the coals prepared by calcination and acidification. The particles in AY-C-HF have better dispersibility compared to other coal samples. The particles exist in the form of separated flakes of about 20 nm and display the edge-face (E-F) contact with simultaneously a lot of pore space. Furthermore, some small aromatic flakes emerge that are about 100 nm in width. Therefore, the process of first calcination and then acidification is not only beneficial to delaminate the aromatic crystalline carbon layers of the coals but also can increase the content of crystal graphite carbon and the surface activation.

graphite structure. In this work, AY belongs to the anthracite with the 0.721 aromaticity. The layer structure composed of aromatic rings is very developed, as shown in Figure 8a. Most

4. CONCLUSION The structural parameter investigation of six coals reveals that the derivative coals prepared by calcination and HF acidification contain many crystalline carbon like graphite structures and are improved significantly in aromaticity, coal rank, and hydroxyl concentration. The increase of hydroxyl bonds is very beneficial to the surface modification of coal crystalline layers. In addition, the derivative coals have an obvious decrease of the aliphatic C−H content and a significant increase of the aromatic C−H content in FTIR spectra compared to that for AY. There is a good agreement with the changes of aromaticity and crystalline carbon content measured by XRD and Raman spectra. A good positive correlation exists between the coal rank and the aromaticity. A negative correlation is shown between the coal rank and defect crystalline carbon and amorphous carbon. With the increase of coal rank, the content of defect crystalline carbon and amorphous carbon decreases gradually from AY to AY-C-HF. The particles of AY-C-HF with about 20 nm in thickness have better dispersibility with simultaneously a lot of pore space. In a word, The combined technologies of calcination and HF acidification remarkably weakened the cross-linkages of aliphatic side chains on coal crystallite edges. The derivative coals present obvious changes in microstructure. The process of first calcination and then acidification effectively helps us carry out the delaminating for the aromatic layers in the coals and obtain the superfine crystalline carbon materials like graphite.

Figure 8. SEM micrographs of the coals: AY (a), AY-C (b), AY-C-HF (c), AY-HF (d), AY-HF-C (e), and AY-HF-C-HF (f).

of them are stacked tightly in the form of face to face (F−F) contact and made up of many crystalline aromatic flakes. The lateral size is about 1 μm, and the thickness of the aggregates is close to 0.2 μm. There are some individual layer particles about 20 nm in thickness; it is highly consistent with the values of Lc measured by XRD. In addition, the layer surface is not very clean due to the coating of clay and other minerals. Figure 8b is the SEM image of AY-C. The micrograph shows that the stacking phenomenon becomes weak in this derivative coal, and the thickness of the aggregates decreases due to the break of bridge bonds (aliphatic carbon chains) among the aromatic layers. There are many small flakes. The layer surface becomes very clean because of the change of clay and other minerals after calcination. Figure 8c displays the structure change of AYC-HF prepared by calcination first and then HF acidification. The particles exist in the form of separated flake about 20 nm in thickness and have a much higher aspect ratio. Most of the aliphatic carbon chains are broken due to calcination and acidification. The association is weakened significantly among the layers. The particles are in contact via edge-face (E-F) with simultaneously a lot of pore space. In addition, some large aromatic layers are broken due to the HF acidification, and its size becomes much smaller, about 100 nm in width. AY-HF acidized by HF displays some changes in structure, as shown in Figure 8d. The layers are aligned in parallel and stacked tightly compared to that of AY-C-HF. After calcination, AY-HF-C becomes much tighter in the form of face to face (F− F) contact. Some small flakes are observed on the surface of the aggregates. When AY-HF-C acidized by HF again, the particles of AY-HF-C-HF are associated together and form a honey-

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ASSOCIATED CONTENT

S Supporting Information *

Additional material as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the technical support of Science and Engineering Faculty in Queensland University of Technology and the financial support provided by the National Natural Science Foundation Project of China (51104060), the Opening Project of Henan Key Discipline Open Laboratory of Mining Engineering Materials (MEM11-2), Cultivating Base 7196

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for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, and the Young Teacher Programs Foundation of Henan Polytechnic University in China.

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REFERENCES

(1) Saikia, B.; Boruah, R. K.; Gogoi, P. K. Bull. Mater. Sci. 2007, 30 (4), 421−426. (2) Potgieter-Vermaak, S.; Maledi, N.; Wagner, N.; Van Heerden, J. H. P.; Van Grieken, R.; Potgieter, J. H. J. Raman Spectrosc. 2011, 42 (2), 123−129. (3) Li, X.; Ju, Y.; Hou, Q.; Li, Z.; Fan, J. J. Geol. Res. 2012, 2012, 1−8. (4) Lueking, A. D.; Gutierrez, H. R.; Jain, P.; Van Essandelft, D. T.; Burgess-Clifford, C. E. Carbon 2007, 45 (11), 2297−2306. (5) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39 (12), 1821−1833. (6) Sonibare, O. O.; Haeger, T.; Foley, S. F. Energy 2010, 35 (12), 5347−5353. (7) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martinez-Alonso, A.; M. D. Tascon, J. J. Mater. Chem. 1998, 8 (12), 2875−2879. (8) Rodrigues, S.; Marques, M.; Suárez-Ruiz, I.; Camean, I.; Flores, D.; Kwiecinska, B. Int. J. Coal Geol. 2013, 111 (1), 67−79. (9) Cole-Clarke, P. A.; Vassallo, A. M. Fuel 1992, 71 (4), 469−470. (10) Dieckman, S. L.; Gopalsami, N.; Botto, R. E. Energy Fuels 1990, 4 (4), 417−418. (11) Wang, S.; Tang, Y.; Schobert, H. H.; Guo, Y. n.; Su, Y. Energy Fuels 2011, 25 (12), 5672−5677. (12) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3 (2), 187−193. (13) Valimbe, P. S. Fuel Energy Abstr. 1996, 37 (3), 209. (14) Waanders, F. B.; Dyk, J. C.; Prinsloo, C. J. Hyperfine Interact. 2009, 190 (1−3), 109−114. (15) Painter, P. C.; Coleman, M. M. Fuel 1979, 58 (4), 301−308. (16) Ludwig Schoening, F. R. Fuel 1983, 62 (11), 1315−1320. (17) Li, X.; Hayashi, J.-i.; Li, C.-Z. Fuel 2006, 85 (12−13), 1700− 1707. (18) Kostova, I.; Tormo, L.; Crespo-Feo, E.; Garcia-Guinea, J. Spectrochim. Acta Part A 2012, 91 (6), 67−74. (19) Reich, S.; Thomsen, C. Philos. Trans.: Math., Phys. Eng. Sci. 2004, 362 (1824), 2271−2288. (20) Cançado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Speziali, N. L.; Jorio, A.; Pimenta, M. A. Carbon 2008, 46 (2), 272−275. (21) Johnson, C. A.; Patrick, J. W.; Mark Thomas, K. Fuel 1986, 65 (9), 1284−1290. (22) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61 (20), 14095− 14107. (23) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Pöschl, U. Carbon 2005, 43 (8), 1731−1742. (24) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martínez-Alonso, A.; Tascón, J. M. D. Carbon 1994, 32 (8), 1523−1532. (25) Geng, W.; Nakajima, T.; Takanashi, H.; Ohki, A. Fuel 2009, 88 (1), 139−144. (26) Wang, G.; Zhou, A. Int. J. Min. Sci. Technol. 2012, 22 (4), 517− 521.

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