Chemical Composition and Structural Characteristics of Oil Shales

Article pubs.acs.org/EF

Chemical Composition and Structural Characteristics of Oil Shales and Their Kerogens Using Fourier Transform Infrared (FTIR) Spectroscopy and Solid-State 13C Nuclear Magnetic Resonance (NMR) Qing Wang,* Jiang-bin Ye, Hong-yang Yang, and Qi Liu Engineering Research Centre of Oil shale Comprehensive Utilization, Ministry of Education, Northeast Dianli University, Jilin 132012, Jilin, People’s Republic of China ABSTRACT: The chemical composition and chemical structure of selected oil shales and the kerogens isolated from them were studied by solid-state 13C nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy with curvefitting analysis, and the changes during the removal of the inorganic matrix were also investigated. The 13C NMR results indicate that the oil shales and kerogens are mainly composed of aliphatic carbon (≥55%). FTIR analysis indicates that aliphatic hydrocarbons mainly contain the methylene group, mostly forming long straight chains and a small amount of branched chains, and the hydroxy groups mainly contain OH−OH and OH−O bonds (≥65%). Both the 13C NMR and FTIR analyses show that the acid treatment improved the hydrocarbon-generating ability in kerogen. Furthermore, the curve-fitting analysis indicates that the HCl and HF treatments slightly affected the aliphatic and hydroxyl structures, but significantly affected the oxygen-containing and aromatic structures in the oil shales. The oxygen-containing groups in the oil shales are mainly the C−OH group, followed by the C−O, OC−O, and CO groups, in descending order. After the acid treatment, the main oxygen-containing groups were the C−OH and C−O groups, because of the hydrolysis, substitution, and ion exchange. The 13C NMR results indicate that the acid treatment not only decreased the extent of aromatic ring condensation (fused aromatic rings), but also decreased the number of condensed aromatic rings.

1. INTRODUCTION The increasing consumption and cost of petroleum have prompted extensive search for an alternative source of energy. Oil shale, which is an important auxiliary energy source with huge reserves and diverse utilization, has attracted significant global attention.1 Previous studies indicate that carbonates and silicates constitute 66% and 20% of the entire oil shale reserves, respectively.2 Moreover, the mineral composition affects the physical and chemical properties of oil shale. For example, Patterson et al.3 reported that smectite, kaolinite, calcite, pyrite, and siderite significantly affected the processing of Australian oil shale. Similarly, Karabakan et al.4 reported the inhibitory effects of silicate and catalytic effects of carbonate minerals on the pyrolysis of U.S. Green River oil shales. The separation of kerogen from its mineral matrix has been studied extensively.5 Several isolation or concentration procedures have been proposed, and these procedures can be divided into physical and chemical methods. Because of efficiency and high removal rate, the chemical methods based on the dissolution of carbonates with hydrochloric acid (HCl) and dissolution of silicates with hydrofluoric acid (HF) in oil shale have been widely used.6,7 However, it has been generally concluded that better methods should be developed for the isolation of kerogen, because the isolated kerogen obtained via traditional chemical methods did not always represent the original organic structure. Solomon et al.8 reported that the methods for the characterization of oil shales, particularly those applied to raw shales, are the most useful, because they are not affected by possible changes in the organic structure caused by © XXXX American Chemical Society

the extraction procedure. Therefore, the fundamental understanding of the inorganic and organic structures of oil shales is essential to finding better ways to utilize this resource. Fourier transform infrared (FTIR) spectroscopy is a nondestructive, highly sensitive technique for studying the structures and reactions of fossil fuels. Particularly, some of the problems associated with organic and mineralogical analysis can be solved by FTIR spectroscopy, because the characteristic FTIR signals of most of the molecular vibrations of organic matter and minerals appear in the mid-IR spectral region (wavenumber range of 4000−400 cm−1).9 13C nuclear magnetic resonance (NMR) is an effective and direct technique to characterize the carbon skeleton structure of solid fossil fuels.10 The use of both FTIR and 13C NMR spectroscopies can provide more information about the chemical structure of materials. In recent years, the chemical characteristics of oil shale and kerogen, particularly in pyrolysis, have been extensively studied. In this study, the kerogen of oil shales was isolated, and the changes during the removal of the inorganic matrix were investigated. Opaprakasit et al.11 reported the FTIR spectroscopic studies of coal after acid treatments. The results show that coal may undergo ionic cross-linking during pyrolysis, and this cross-linking is unstable and easily destroyed by HCl and HF treatments. Larsen et al.12 also Received: April 1, 2016 Revised: June 7, 2016

A

DOI: 10.1021/acs.energyfuels.6b00770 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Ultimate and Proximate Analyses of the Studied Samples Proximate Analysisa (%)

Ultimate Analysis wdaf (%) sample

Ctb

c

Co

YJ HD YJK HDK

32.63 32.88 71.02 67.15

30.05 30.89 70.98 67.12

Ced

e

H

O*

2.58 1.98 0.06 0.02

3.44 4.63 6.97 7.85

3.15 6.51 13.64 16.47

N

S

Mad

Vad

Aad

FCad

0.96 0.74 4.85 5.15

2.56 2.3 1.68 1.82

1.43 3.84

25.94 42.36

55.74 49.11

16.89 4.69

LOIdaff (%)

98.16 98.02

a Mad = moisture content; Vad = volatiles content; Aad = ash content; and FCad = fixed carbon. bCt = total carbon. cCo = organic carbon. dCe = inorganic carbon. e[O*] = LOIdaf − Ctdaf − Hdaf − Ndaf − Sdaf. fLOI = loss on ignition.

reported that acid treatments mainly remove the inorganic substances from coal and hinder ionic cross-linking. The objective of this study was to investigate the chemical changes in selected oil shales and their kerogens during the HCl and HF treatments using FTIR and 13C NMR spectroscopies with curve-fitting analysis. Moreover, the chemical composition and chemical structure of oil shales and kerogens were studied. This study would provide a theoretical basis for the comprehensive utilization of oil shales.

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, two types of oil shale samples have been obtained from the Huadian mine in Jilin Province and the Yaojie mine in Gansu Province and are labeled as HD and YJ, respectively. Kerogen is isolated according to the standard procedure reported by Durand and Nicaise.5 Dried oil shale (50 g) was treated with chloroform (CHCl3), to extract the bitumens until the solvent in the arm of the Soxhlet was colorless. The bitumen-free shale was dried and weighed. The shale then was stirred at 343 K with HCl (using concentrated HCl (20%) and 10 mL/g bitumen-free shale) for 4 h until no carbon dioxide evolved. The residue was washed with distilled water until the silver nitrate test for chloride was negative. The HCl treatment was repeated twice to eliminate all of the calcium products. The carbonate-free shale was first stirred with 40% HF (10 mL/g) at 343 K for 4 h, and then washed with 20% HCl. Finally, the products were washed thrice with distilled water to remove chloride and dried overnight at 333 K. The obtained kerogens were labeled as HDK (with the HD oil shale) and YJK (with the YJ oil shale). Ultimate and proximate analyses of the oil shales and their kerogens have been conducted, according to China National Standards GB/T 476-2001 and GB/T 212-2008. The C, H, N, and S contents have been determined in triplicate. The results are listed in Table 1. 2.2. FTIR Measurements. FTIR measurement was carried out using an FTIR absorption spectrometer (Model TENSOR27, Bruker, Germany). The samples (1 ± 0.005 mg) were ground with KBr (500 mg) for 2 min and pressed into pellets in an evacuated die under a pressure of 10 MPa for 2 min. The FTIR spectra were recorded with 120 scans at a resolution of 4 cm−1. The pellets were dried under vacuum with P2O5 for 48 h to minimize water in the spectrum. The Omnic Quantpad software was used for the baseline correction of the spectra, and scaled to 1 mgsample cm−2. 2.3. Solid-State 13C NMR Measurements. The solid-state 13C NMR spectroscopic measurements were carried out at room temperature using a spectrometer (Bruker, Model Avance III) at 100.62 MHz. The sample was packed in a 4-mm-diameter zirconia rotor and spun at 5 kHz. A contact time of 2 ms and recycle delay time of 6 s were used in the cross-polarization (CP) experiments. A total of 9000 data points were collected for each sample.

Figure 1. FTIR spectra of (a) HD, (b) HDK, (c) YJ, and (d) YJK.

similar band shape of the two oil shales indicates their similar mineral composition. The peaks in the range of 1170−1060 cm−1 are the characteristic of Si−O stretching vibration, arising from the overlap of strong absorption bands of kaolinite, Illite, smectite, quartz, and clay minerals present in shales. The double bands at 780 and 804 cm−1 were assigned to quartz. The bands near 424, 466, and 540 cm−1 were assigned to silicate minerals including quartz and clay minerals. The peaks at 1430 and 877 cm−1 are the characteristic peaks of calcite.11 After the acid treatment, a significant decrease in the intensity of these peaks was observed in the isolated kerogens, indicating that all the minerals except pyrite were removed. Treatment with HCl and HF does not remove pyrite.14 After the HCl and HF treatments, a broad band appeared in the range 1380−1100 cm−1. This is probably because minerals were removed, and the organic functional groups in oil shales appeared. The broad band in the range of 3600−3000 cm−1 can be attributed to hydroxyl groups, which is the main functional group responsible for the hydrogen bonding in oil shales and kerogens. The sharp bands in the region of 3700−3600 cm−1, in addition to the normal silicate absorption bands, are characteristic of kaolinite minerals.13 Two sharp bands appeared at 3614 and 3703 cm−1 in the spectra of raw shales, because of the presence of kaolinite minerals. Aliphatic and aromatic hydrogens constitute the main macromolecular framework of oil shales and kerogens. A significant amount of important information such as hydrocarbon generation and reactivity are embodied in them.15 In this study, the absorption intensity of the band in the range of 3000−2800 cm−1 was used to determine the content of aliphatic hydrogens, and the absorption intensity of the band in the range of 900−700 cm−1 was used to determine the content

3. RESULTS AND DISCUSSION 3.1. FTIR Characteristics of the Studied Samples. Figure 1 shows the FTIR spectra of the samples. The minerals in this study were assigned according to the work of Adams et al.13 Clearly, the peaks for minerals are prominent in the spectra of oil shales, compared to the peaks of their kerogens. A B


Article

Energy & Fuels

Gaussian function has been applied. The positions and number of bands were established initially from the second derivative of spectrum. The analysis of spectrum is based on the studies of Machnikowska et al.16 and Painter et al.17 3.2.1. Effect of Acid Dissolution on Aromatic Structures. Three out-of-plane C−H deformation bands were observed in the 900−700 cm−1 region for the studied samples. Good fits were obtained with a relatively less number of bands (between 6−9 bands). The band-fitting analysis for the aromatic zone is shown in Figure 3. Tables 2 and 3 show the results of the curvefitting analysis. The curve-fitting analysis shows that the aromatic structure in the oil shales was dominated by three adjacent aromatic hydrogens per ring (>50%). After the acid treatment, the two and three adjacent aromatic hydrogens per ring became dominant in their kerogens. Previous studies show that the hydrogens on the aromatic rings were easily substituted by other groups.18 Aromatic substitution reactions occurred via acid attack. 3.2.2. Effect of Acid Dissolution on Oxygen-Containing Structures. The 1800−1000 cm−1 zone shows the most marked differences for the oil shales and kerogens. Good fits were obtained with 16−19 bands, based on the second derivative and the type of oxygen groups. Figure 4 shows the curve-fitted FTIR spectra for YJ and HD and their kerogens in this range. Tables 4 and 5 show the results of the curve-fitting analysis for the oxygen-containing functional groups. The curve-fitting analysis allows the specification of C−O, OC−O (carboxyl), C−OH (alcohol, phenol, and ether), and CO (carbonyl) or O−C−O groups.10 The peak centered at 1040 cm−1 can be attributed to alkyl ether. Its contents in YJ and HD were 11.1% and 13.0%, respectively, and 3.09% and 2.77% in their kerogens, respectively. That is because the

of aromatic hydrogens. The results are shown in Figure 2. The ratio of aliphatic hydrogen to aromatic hydrogen (Hal/Har)

Figure 2. Ratio of aliphatic hydrogen to aromatic hydrogen (Hal/Har) for the oil shale and kerogen samples.

dramatically increased after the acid treatments, indicating that the acid treatment improved the hydrocarbon-generating ability in kerogens. 3.2. Curve-Fitting Analysis for Oil Shales and Their Kerogens. Selected zones of the FTIR spectra were studied by curve-fitting analysis using a commercially available dataprocessing program (OMNIC Quantpad software). Selected regions of the FTIR spectra were baseline-linearized by connecting the left and right points of interval with a straight line. This baseline adjustment resulted in zero intensities at both the ends of the region, thus eliminating a possible source of artifacts in the deconvoluted spectra. The band shapes with

Figure 3. Curve-fitted FTIR spectrum of aromatic C−H out-of-plane deformation bands for oil shales and their kerogens. C


Article

Energy & Fuels Table 2. Curve-Fitting Analysis for the Aromatic C−H Out-of-Plane Deformation Bands for YJ and YJK Sample YJ

Sample YJK

peak No.

position σ (cm−1)

assignment

area percentage (%)

peak No.


assignment

area percentage (%)

1 2 3 4 5 6

733.024 744.759 759.35 778.866 798.082 817.5

4H 4H 3H 3H 3H 2H

0.90 14.9 17.0 34.7 32.4 0.07

1 2 3 4 5 6 7 8

719.869 737.316 748.981 765.659 783.348 801.771 822.392 840.518

(CH3)n, where n ≥ 4 4H 4H 3H 3H 3H 2H 2H

17.4 7.67 20.7 12.7 5.72 8.93 20.3 6.45

Table 3. Curve-Fitting Analysis for the Aromatic C−H Out-of-Plane Deformation Bands for HD and HDK Sample HD

Sample HDK

peak No.


assignment

area percentage (%)

peak No.


assignment

area percentage (%)

1 2 3 4 5 6 7 8

716.317 763.005 777.057 786.861 799.500 837.731 870.108 873.397


15.8 5.32 17.79 18.89 16.4 2.21 10.9 12.5

1 2 3 4 5 6 7 8

720.887 736.448 750.981 759.717 777.834 819.935 849.357 880.117


19.2 36.0 25.30 12.03 0.45 0.37 5.52 0.46

Figure 4. Curve-fitted FTIR spectra of the oxygen-containing functional groups for oil shales and their kerogens.

treatment. The peaks at 1100 cm−1 and 1170 cm−1 can be attributed to the C−OH (alcohol) and C−OH (phenol) groups, respectively. The curve-fitting analysis show that the content of C−OH (alcohol) groups in YJK and HDK significantly decreased by 87% and 84%, respectively, mainly because of the substitution of C−OH (alcohol) groups with HF during the acid treatment.18 Moreover, because the metals ion

hydrolytic cleavage of alkyl ether bonds occurs easily under acidic or heating conditions and produces free radicals.19 Feng et al.20 reported that alkyl ether bonds easily form phenol after their cleavage, indicating that, under acidic conditions, alkyl ether bonds are hydrolyzed and form phenols after the cleavage. The band from 1338 cm−1 to 1260 cm−1 can be assigned to aryl ether, and its content increased drastically after the acid D


Article

Energy & Fuels Table 4. Curve-Fitting for the Oxygen-Containing Functional Groups for YJ and YJK Sample YJ

a

peak No.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

1029.529 1049.281 1081.171 1107.045 1128.435 1147.034 1173.665 1213.427 1260.552 1316.9 1374.875 1427.867 1460.51 1500.241 1542.497 1585.684 1620.482 1657.835 1706.505

Sample YJK

assignmenta

area percentage (%)

peak No.


assignmenta

area percentage (%)

alkyl ethers alkyl ethers £ C−O sec. alcohols £ C−O sec. alcohols £ C−O sec. alcohols £ C−O phenols, ethers £ C−O phenols, ethers £ C−O phenols, ethers £ C−O in aryl ethers £ C−O in aryl ethers £ s CH3−Ar, R £ s CH3−Ar, R μ as. CH3−, CH2− aromatic CC aromatic CC aromatic CC aromatic CC conjugated CO carboxyl acids

3.97 7.13 11.1 11.7 7.53 2.77 5.79 2.90 2.55 2.59 3.33 4.69 3.60 1.61 1.14 2.45 3.46 0.75 1.59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

1049.108 1098.119 1148.206 1193.373 1237.079 1279.845 1327.026 1366.1 1400.029 1436.055 1462.068 1486.495 1523.936 1549.649 1596.765 1648.744 1711.658 1764.66

alkyl ethers £ C−O sec. alcohols £ C−O phenols, ethers £ C−O phenols, ethers £ C−O phenols, ethers £ C−O in aryl ethers £ C−O in aryl ethers £ s CH3−Ar, R £ s CH3−Ar, R μ as. CH3−, CH2− μ as. CH3−, CH2− μ as. CH3−, CH2− aromatic CC aromatic CC aromatic CC conjugated CO carboxyl acids aryl esters

3.09 3.92 4.92 5.92 6.37 7.95 9.02 5.66 4.13 5.84 4.15 5.60 3.91 6.54 1.02 2.28 7.95 1.56

μ denotes stretch, £ denotes deformation vibration, and as denotes asymmetric.

Table 5. Curve-Fitting for the Oxygen-Containing Functional Groups for HD and HDK Sample HD

a

peak No.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

1020.432 1041.142 1079.476 1104.473 1132.607 1175.052 1225.241 1282.136 1327.371 1369.525 1406.222 1428.77 1464.484 1501.98 1541.227 1582.61 1630.471 1679.989 1714.783

Sample HDK

assignmenta

area percentage (%)

peak No.


assignmenta

area percentage (%)

alkyl ethers alkyl ethers £ C−O sec. alcohols £ C−O sec. alcohols £ C−O sec. alcohols £ C−O phenols, ethers £ C−O phenols, ethers £ C−O in aryl ethers £ C−O in aryl ethers £ s CH3−Ar, R £ s CH3−Ar, R £ s CH3−Ar, R μ as. CH3−, CH2− aromatic CC aromatic CC aromatic CC aromatic CC conjugated CO carboxyl acids

4.92 8.08 10.2 8.36 5.94 5.01 1.74 0.98 0.96 2.36 4.27 1.11 8.79 8.33 5.19 4.09 5.60 1.66 1.88

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1038.518 1104.827 1179.743 1245.135 1298.617 1351.156 1382.147 1414.805 1449.25 1466.271 1486.579 1532.369 1556.17 1583.32 1633.307 1667.858 1708.522 1716.569

alkyl ethers £ C−O sec. alcohols £ C−O phenols, ethers £ C−O phenols, ethers £ C−O in aryl ethers £ C−O in aryl ethers £ s CH3−Ar, R £ s CH3−Ar, R μ as. CH3−, CH2− μ as. CH3−, CH2− μ as. CH3−, CH2− aromatic CC aromatic CC aromatic CC aromatic CC conjugated CO carboxyl acids carboxyl acids

2.77 4.58 10.1 7.70 7.41 6.84 3.49 3.52 5.65 1.71 1.22 2.67 3.47 5.93 9.66 0.06 1.43 2.18

μ denotes stretch, £ denotes deformation vibration, and as denotes asymmetric.

carboxylic acid (1700 cm−1) increased, compared to that of the original shales. According to the curve-fitting analysis, the content of oxygen functional groups in the oil shales was dominated by the C−OH group, followed by the C−O, OC−O, and CO groups, in descending order. After the acid treatment, the content of C−OH and C−O groups increased because of hydrolysis, substitution, and ion exchange. The details of the content of oxygen-containing functional groups in oil shales and their kerogens are shown in Figure 5. In the FTIR spectra of kerogen and coal, the absorption peak at 1600 cm−1 is usually attributed to aromatic groups, whereas

bound to the oxygen functional groups were removed in large quantities,21 the content of C−OH (phenol) groups increased in their kerogens, which corresponds to the reduction in the content of alkyl ether. The peak near 1700 cm−1 can be attributed to the CO group of carboxylic acids, its contents were 0.59% and 1.88% in YJ and HD, respectively. After the acid treatment, they increased to 7.95% and 2.18%. Geng et al.22 reported that the content of carboxylic acid in lignite after the HCl treatment increased (not less than 10%), because of the protonation of OC−O groups in HCl, generating carboxylic acid. As shown in Tables 2 and 3, the content of E


Article

Energy & Fuels

used the same method as Painter et al. and normalized the fitting area (1700−1500 cm−1), relative to the total area. The calculations showed that the contents of aromatic CC groups in YJ and HD were 92.2% and 84.1%, respectively, and those in their kerogens were 99.7% and 90.1%, respectively. This indicates that the absorption peak at 1600 cm−1 in oil shales and kerogens are mainly attributed to the aromatic CC structures. Moreover, the contribution of conjugated CO groups to 1600 cm−1 decreased after the acid treatment. This is due to the fact that the acid treatment converted the OC−O groups in kerogens to carboxylic acid, and the center of the peak shifted to 1700 cm−1. 3.2.3. Effect of Acid Dissolution on Aliphatic Structures. The second derivative of the 3000−2800 cm−1 zone in the FTIR spectra of the studied samples showed the presence of 5− 8 bands, consistent with the results reported previously.24 This region includes diverse structural vibrations of CH2 (chain), CH2 (ring), CH3 (branch), CH3 (end), and CH groups with a high absorption intensity without miscellaneous interference. The band-fitting analysis for the aliphatic C−H stretching bands is shown in Figure 6. Tables 6 and 7 list the results of this study. Figure 6 shows that the oil shales and kerogens exhibit two prominent peaks, at 2860 and 2930 cm−1, respectively, which can be attributed to the symmetric and asymmetric −CH2− stretching, respectively.25,26 Tables 6 and 7 show that the aliphatic hydrogens in the oil shales and kerogens are dominated by methylene groups, with a lesser content of methyl and methine groups. This indicates that the aliphatic hydrogens in the oil shales and kerogens mainly exist as long straight chains or alicyclic structures, with a few branched chains. The curve-fitting analysis showed that the acid

Figure 5. Content of oxygen functional groups in oil shales and their kerogens.

the assignment of 1600 cm−1 band has been quite controversial. It is generally believed that this peak is mainly due to the presence of aromatic CC groups. However, some researchers reported that the contribution of π-conjugated CO groups such as those in quinones cannot be ignored. Painter et al.23 used the second derivative method to analyze the band in the range of 1500−1700 cm−1 in the bituminous coal IR spectrum, and the overlapping bands mainly contained three peaks at 1589, 1614, and 1656 cm−1. The former two peaks belong to the aromatic CC groups, whereas the peak at 1656 cm−1 can be attributed to the presence of conjugated CO groups. The curve-fitting results show that the conjugated CO groups contributed slightly to the overall band in bituminous coal. We

Figure 6. Curve-fitted FTIR spectra of the aliphatic C−H stretching bands for oil shales and their kerogens. F


Article

Energy & Fuels Table 6. Curve-Fitting Analysis for the Aliphatic C−H Stretching Bands of YJ and YJK Sample YJ

Sample YJK

peak No.


assignment

area percentage (%)

peak No.


assignment

area percentage (%)

1 2 3 4 5 6

2825.631 2850.164 2874.905 2896.218 2922.286 2952.619

sym. R2CH2 sym. R2CH2 sym. R2CH2 −R3CH asym. R2CH2 asym. RCH3

5.39 20.5 9.01 14.6 38.8 11.2

1 2 3 4 5 6

2824.548 2849.841 2876.437 2896.399 2921.813 2952.912

sym. R2CH2 sym. R2CH2 sym. R2CH2 −R3CH asym. R2CH2 asym. RCH3

5.41 21.7 10.5 12.5 38.7 11.0

Table 7. Curve-Fitting Analysis for the Aliphatic C−H Stretching Bands of HD and HDK Sample HD −1

Sample HDK −1

peak No.

position σ (cm )

assignment

area percentage (%)

peak No.

position σ (cm )

assignment

area percentage (%)

1 2 3 4 5 6

2828.481 2851.511 2902.679 2922.132 2939.647 2970.357

sym. R2CH2 sym. R2CH2 −R3CH asym. R2CH2 asym. R2CH2 asym. RCH3

3.40 19.3 36.0 15.3 21.2 4.62

1 2 3 4 5 6

2826.886 2851.695 2897.295 2923.256 2945.469 2971.817

sym. R2CH2 sym. R2CH2 −R3CH asym. R2CH2 asym. R2CH2 asym. RCH3

3.47 21.8 33.68 16.97 22.68 4.08

contents of OH−π, ring hydroxyl, and OH−N bonds decreased after the HCl and HF treatments, indicating that the acid treatment slightly destroyed these three hydrogen bonds.

treatment of the oil shales slightly affected the aliphatic structure. To further investigate the aliphatic structure, the ratio of antisymmetric −CH2− and −CH3 stretching was used to estimate the length and branching of the aliphatic chains in the oil shales and kerogens.27 The details are shown in Table 8.

4. SOLID-STATE 13C NMR ANALYSIS OF OIL SHALES AND THEIR KEROGENS 13 C NMR spectroscopy can provide additional details about the multistructure information on carbon, hydrogen, and oxygen functional groups present in the samples.29 Figure 8 shows the high-resolution 13C NMR spectra of the studied raw shales and their kerogens. To further investigate the differences between the spectra of oil shales and kerogens, the NTUTS NMR software was used to obtain the structural parameters of the oil shales and isolated kerogens, according to the methods proposed by Solum et al.30,31 The results are shown in Table 11. The small sharp peak at 15 ppm and the peak at 25 ppm are attributed to the terminal methylene carbon and aliphatic C(2) carbon, respectively. Moreover, some weak resonance absorption that arises from oxygen−aliphatic carbon also appeared in the region of 50−90 ppm. Table 11 shows that aliphatic carbon ( fal) structure is dominant in both oil shales and kerogens (≥55%). A strong positive correlation was observed between the total aliphatic carbon and Fischer assay oil yield.17,32 After the acid treatment, the content of aliphatic carbon in kerogens increased, and this result is consistent with the results shown in Figure 2. The methylene percentages of aliphatic carbon (Ai) in YJ and HD were 76% and 85%, respectively, indicating ∼76 and 85 methylene carbons per 100 carbon atoms in YJ and HD, respectively. After the acid treatment, the number of methylene carbons in YJK and HDK increased to 85 and 89 per 100 carbon atoms, respectively. The average carbon numbers of the methylene chain (Cn) in YJ and HD were 5 and 20, respectively, whereas those in YJK and HDK were 6 and 22, respectively. The number of aliphatic chains (N) in the YJ and HD samples increased from 5 and 3 to 7 and 3 in YJK and HDK, respectively. The combined results of Ai, Cn, and N indicates that the oil shales and kerogens were dominated by methylene carbon, and the majority of them existed as long straight chains. Moreover, a few weak branches were preferably lost from the aliphatic chains in the raw shales by the attack of

Table 8. Effect of the Acid Treatment on CH2/CH3 Ratio of Oil Shales and Their Kerogens sample

A(CH2)/A(CH3)

YJ YJK HD HDK

4.3 4.7 7.9 8.3

The CH2/CH3 ratio slightly increased after the acid treatment. This indicates that the oil shale contains more methyl groups, and the aliphatic structure has a higher branching, compared to their kerogens. This is probably because a few weak branches were removed from the main chains in oil shales by the attack of acids. 3.2.4. Effect of Acid Dissolution on Hydroxyl Structures. The broad band in the region of 3600−3000 cm−1 can be attributed to the hydroxyl groups that significantly affect the reactivity of oil shales; the determination of this band is easily influenced by water in the oil shale, or water absorbed in preparing the KBr pellet. However, Solomon et al.28 reported that, if the sample is carefully prepared, the FTIR method can provide a good estimate of the hydroxyl content. Figure 7 shows the curve-fitting analysis of the hydroxyl groups in YJ and HD and their kerogens. More details about the structural characteristics are shown in Tables 9 and 10. The hydrogen bonding plays an important role in stabilizing the macromolecular framework of oil shale and kerogen. Tables 8 and 9 show that both the oil shales and kerogens contain five types of hydrogen bonding: OH−π bonding, OH−OH bonding, OH−O bonding, ring hydroxyl bonding, and OH−N bonding. The curve-fitting analysis shows that the hydroxyl groups had mainly OH−OH and OH−O bonding, reaching more than 65% in oil shale and kerogen. After the acid treatment, these contents slightly increased. Moreover, the G


Article

Energy & Fuels

Figure 7. Curve-fitted FTIR spectra of 3000−3600 cm−1 zone for oil shales and their kerogens.

Table 9. Curve-Fitting Analysis of 3000−3600 cm−1 Zone for YJ and YJK Sample YJ

Sample YJK

peak No.


assignment

area percentage (%)

peak No.


assignment

area percentage (%)

1 2 3 4 5 6 7 8

3076.128 3234.37 3298.895 3342.895 3373.573 3431.909 3478.258 3535.637

OH−N ring hydroxyl OH−O OH−O OH−O OH−OH OH−OH OH−π

5.67 11.3 5.67 15.41 7.34 19.21 16.89 6.23

1 2 3 4 5 6 7 8

3077.631 3161.098 3234.021 3298.807 3367.924 3424.422 3477.449 3536.192

OH−N OH−N ring hydroxyl OH−O OH−O OH−OH OH−OH OH−π

2.05 3.50 10.5 13.9 22.5 18.3 17.9 10.1

Table 10. Curve-Fitting Analysis of 3000−3600 cm−1 Zone for HD and HDK Sample HD

Sample HDK

peak No.


assignment

area percentage (%)

peak No.


assignment

area percentage (%)

1 2 3 4 5 6 7 8

3176.128 3234.37 3288.895 3342.895 3393.573 3441.909 3488.258 3535.637


3.32 7.67 9.69 14.2 24.0 20.0 15.4 5.54

1 2 3 4 5 6 7 8

3070.655 3193.786 3252.934 3310.773 3369.009 3420.182 3470.829 3542.855


0.14 5.65 8.55 11.7 21.1 23.3 20.5 8.77

acids. The results are in good agreement with the FTIR analysis. The resonance absorption in the range of δ 100−165 ppm was assigned to aromatic carbons. Table 11 shows that the contents of alkylated aromatic carbons ( fSa ) and bridged aromatic carbons (fBa ) were almost the same in the HD sample, indicating that the contents of monocyclic and polycyclic

aromatic structures were almost the same in the HD sample, whereas in the HDK sample, the value of fSa was higher than that of fBa , indicating that the content of monocyclic aromatic structure increased, compared to that of the polycyclic aromatic structure after the acid treatment. Moreover, the contents of bridged aromatic carbons (fBa ) in oil shales were more than those of fBa in their kerogens, indicating that, after the acid H


Article

Energy & Fuels

Figure 8. Solid-state 13C CP/MAS NMR spectra for samples.

(iii) The curve-fitting analysis shows that the acid treatment slightly affected the aliphatic and hydroxyl structures. The CH2/CH3 ratio indicates that the aliphatic structure in raw shales has a higher branching, compared to their kerogens. The hydroxy groups in oil shales and kerogens mainly had OH−OH and OH−O bonding (≥65%). The acid treatment slightly destroyed the OH−N, OH−O, and ring hydrogen bonding structures. (iv) The FTIR results show that the acid treatment significantly affected the oxygen-containing and aromatic structures in the oil shales. The oxygen-containing functional groups in the oil shales were mainly C−OH groups, followed by C−O, OC−O, and CO groups, in descending order. After the acid treatment, the main oxygen functional groups were C−OH and C−O groups, because of the hydrolysis, substitution, and ion exchange. In the aromatic structure, the main content of three adjacent H deformation in the oil shales became the two and three adjacent H deformation in the kerogens. The 13C NMR results indicate that the HCl and HF treatments not only decreased the extent of aromatic ring condensation (fused aromatic rings), but also decreased the number of condensed aromatic rings.

Table 11. Structural Parameters of Samples parameter aromatic carbon rate, fa aliphatic carbon rate, fal oxy-aliphatic carbon rate, fOal carbonyl carbon rate, fCa alkylated-substituted carbon rate, fSa bridged aromatic carbon rate, fBa methylene rate, fHal methylene percentage of aliphatic carbon, Ai number of aliphatic chains, N aromatic cluster size, XBP average carbon number of the methylene chain, Cn substitution of aromatic ring, D

YJ

HD

YJK

HDK

30.1 56.8 1.55 1.01 7.67 9.75 48.6 76.12

18.17 80.65 1.20 2.01 3.03 4.11 76.05 85.43

39.98 60.58 2.56 0.00 8.78 7.75 46.07 78.32

11.35 85.61 1.67 2.97 3.43 1.31 76.79 89.70

5.6 76.12 5.98

2.75 29.06 20.33

7.74 24.05 6.25

3.14 13.05 22.39

35.33

56.82

46.77

60.44

treatment, the degree of condensation of the aromatic rings (aromatic fused rings) decreased. XBP is applied to characterize the size of aromatic clusters in oil shales. The XBP values of naphthalene with two condensed aromatic rings and phenanthrene with three condensed aromatic rings are 0.25 and 0.4, respectively.32 The XBP values of YJ and HD are 0.53 and 0.29, respectively. This indicates that the degree of aromatic condensation in YJ (at least four rings) is more than that of phenanthrene, and the HD sample contains at least three rings. After the acid treatment, the XBP values of YJK and HDK were 0.24 and 0.13, respectively, both within two rings, indicating that the degree of condensation also decreased.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study is supported by the National Natural Science Foundation of China (No. 51276034) and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13052).

5. CONCLUSIONS The following conclusions can be drawn from our study: (i) The resolution of overlapping bands in the FTIR spectra of the studied oil shales and their kerogens by curve-fitting methods provides more information on the main structural changes that occur during the acid treatments. The FTIR analyses show that HCl and HF treatments not only removed the particulate form of minerals in oil shale, but also the minerals bonded to the oil shale structure. Both the 13C NMR and FTIR analyses show that the acid treatments improved the hydrocarbon-generating ability and reactivity in kerogens. (ii) The 13C NMR results show that the carbon skeletal structure of oil shales and kerogens is mainly composed of aliphatic carbon (≥55%) and aromaticity (≥10%). Moreover, the FTIR analysis indicates that aliphatic hydrocarbons containing mainly the methylene group mostly formed long straight chains and a small amount of branched chains.

■

REFERENCES

(1) Yan, J.; Jiang, X.; Han, X.; Liu, J. A TG−FTIR investigation to the catalytic effect of mineral matrix in oil shale on the pyrolysis and combustion of kerogen. Fuel 2013, 104, 307−317. (2) Ginzburg, M.; Ginzburg, B. Z. Ion and glycerol concentrations in 12 isolates of Dunaliella. J. Exp. Bot. 1985, 36 (7), 1064−1074. (3) Patterson, J. H.; Henstridge, D. A. Comparison of the mineralogy and geochemistry of the Kerosene Creek Member. Chem. Geol. 1990, 82, 319−339. (4) Karabakan, A.; Yürüm, Y. Effect of the mineral matrix in the reactions of shales. Part 2. Oxidation reactions of Turkish Goynuk and US Western Reference oil shales. Fuel 2000, 79 (7), 785−792. I


Article

Energy & Fuels (5) Durand, B.; Nicaise, G. Procedures for Kerogen Isolation. In Kerogen: Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 35−53. (6) Aboulkas, A.; El Harfi, K. E. Effects of acid treatments on Moroccan Tarfaya oil shale and pyrolysis of oil shale and their kerogen. J. Fuel Chem. Technol. 2009, 37 (6), 659−667. (7) Sugano, M.; Mashimo, K.; Wainai, T. Structural changes of lower rank coals by cation exchange. Fuel 1999, 78 (8), 945−951. (8) Solomon, P. R.; Miknis, F. P. Use of Fourier transform infrared spectroscopy for determining oil shale properties. Fuel 1980, 59 (12), 893−896. (9) Chen, Y.; Furmann, A.; Mastalerz, M.; et al. Quantitative analysis of shales by KBr-FTIR and micro-FTIR. Fuel 2014, 116, 538−549. (10) Tong, J.; Han, X.; Wang, S.; et al. Evaluation of structural characteristics of Huadian oil shale kerogen using direct techniques (solid-state 13C NMR, XPS, FT-IR, and XRD). Energy Fuels 2011, 25 (9), 4006−4013. (11) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Ionomer-like structures and π-cation interactions in Argonne Premium coals. Energy Fuels 2002, 16 (3), 543−551. (12) Larsen, J. W.; Pan, C. S.; Shawver, S. Effect of demineralization on the macromolecular structure of coals. Energy Fuels 1989, 3 (5), 557−561. (13) Adams, M. J.; Awaja, F.; Bhargava, S.; et al. Prediction of oil yield from oil shale minerals using diffuse reflectance infrared Fourier transform spectroscopy. Fuel 2005, 84 (14), 1986−1991. (14) Yürüm, Y.; Dror, Y.; Levy, M. Effect of acid dissolution on the mineral matrix and organic matter of Zefa Efe oil shale. Fuel Process. Technol. 1985, 11 (1), 71−86. (15) Xue, X.; Liu, Y.; Li, Y.; et al. Study on Oil Producibility from Oil Shale and Kerogen by Infrared Spectra. J. Northeast. Univ. 2010, 31 (9), 1292−1295. (16) Machnikowska, H.; Krztoń, A.; Machnikowski, J. The characterization of coal macerals by diffuse reflectance infrared spectroscopy. Fuel 2002, 81 (2), 245−252. (17) Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. Concerning the Application of FTIR to the Study of Coal: A Critical Assessment of Band Assignments and the Application of Spectral Analysis Programs. Appl. Spectrosc. 1981, 35 (5), 475−485. (18) Liang, H.-Z.; Wang, C.-G.; Zeng, F.-G.; Xiang, J.-H.; et al. Effect of demineralization on lignite structure from Yinmin coalfield by FTIR investigation. J. Fuel Chem. Technol. 2014, 42 (2), 129−137. (19) Pang, Y. Mechanism and model of phenols formation during multistage rotary furnace pyrolysis. Fuel Energy Abstr. 1995, 18 (1), 75−81. (20) Feng, H.; Yanguo, Z.; Aihong, M. FTIR analysis of Yunnan lignite. J. China Coal Soc. 2014, 39 (11), 2293−2299. (21) Domazetis, G.; James, B. D. Molecular models of brown coal containing inorganic species. Org. Geochem. 2006, 37 (2), 244−259. (22) Geng, W.; Nakajima, T.; Takanashi, H.; et al. Analysis of carboxyl group in coal and coal aromaticity by Fourier transform infrared (FT-IR) spectrometry. Fuel 2009, 88 (1), 139−144. (23) Painter, P. C.; Coleman, M. M.; Jenkins, R. G.; et al. Fourier transform infrared study of mineral matter in coal. A novel method for quantitative mineralogical analysis. Fuel 1978, 57 (6), 337−344. (24) Michaelian, K. H.; Friesen, W. I. Photoacoustic FT-ir spectra of separated western Canadian coal macerals: Analysis of the CH stretching region by curve-fitting and deconvolution. Fuel 1990, 69 (10), 1271−1275. (25) Bhargava, S.; Awaja, F.; Subasinghe, N. D. Characterisation of some Australian oil shale using thermal, X-ray and IR techniques. Fuel 2005, 84 (6), 707−715. (26) Petersen, H. I.; Rosenberg, P.; Nytoft, H. P. Oxygen groups in coals and alginite-rich kerogen revisited. Int. J. Coal Geol. 2008, 74 (2), 93−113. (27) Ibarra, J. V.; Moliner, R.; Bonet, A. J. FTIR investigation on char formation during the early stages of coal pyrolysis. Fuel 1994, 73 (6), 918−924.

(28) Solomon, P. R.; Carangelo, R. M. FTIR analysis of coal. 1. Techniques and determination of hydroxyl concentrations. Fuel 1982, 61 (7), 663−669. (29) Ju, Y.; Jiang, B.; Hou, Q.; et al. 13 C NMR spectra of tectonic coals and the effects of stress on structural components. Sci. China, Ser. D: Earth Sci. 2005, 48 (9), 1418−1437. (30) Solum, M. S.; Pugmire, R. J.; Grant, D. M. 13C Solid-State NMR of Argonne Premium Coals. Energy Fuels 1989, 3 (2), 187−193. (31) Solum, M. S.; Sarofim, A. F.; Pugmire, R. J.; et al. 13C NMR analysis of soot produced from model compounds and a coal. Energy Fuels 2001, 15 (4), 961−971. (32) Wang, Q.; et al. Chemical structure analysis of oil shale kerogen. J. Chem. Eng. (Beijing, China) 2015, 5, 1861−1866.

J


Chemical Composition and Structural Characteristics of Oil Shales

Recommend Documents