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Sequential Extraction and Thermal Dissolution of Baiyinhua Lignite in Isometric CS2/Acetone and Toluene/Methanol Binary Solvents Yun-Peng Zhao,*,†,‡ Jian Xiao,† Man Ding,† Eric G. Eddings,‡ Xian-Yong Wei,*,† Xing Fan,† and Zhi-Min Zong† †

Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ‡ Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: Baiyinhua lignite (BL) was sequentially extracted and thermally dissolved in isometric CS2/acetone and toluene/ methanol binary solvents to obtain an extract in isometric CS2/acetone (EICA) and a soluble portion (SP) in isometric toluene/ methanol (SPITM). The yields of EICA and SPITM are notably higher than the total extract yield from sequential extraction with CS2 and acetone (or acetone and CS2) and the total SP yield from sequential thermal dissolution in toluene and methanol (or methanol and toluene), indicating that there exists an obvious synergic effect between CS2 and acetone during the extraction and between toluene and methanol during the thermal dissolution. EICA and SPITM mainly consist of hydrocarbons and oxygencontaining organic species, respectively. Little difference in Fourier transform infrared spectroscopy spectra of BL and its extraction residue was observed, while the intensities of absorbances assigned to the phenolic OH, CO, and C−O/C−O−C groups of the thermal dissolution residue are obviously lower than those of BL and its extraction residue. X-ray photoelectron spectroscopy analysis shows that C−O/C−O−C groups in BL remarkably decreased after thermal dissolution, corresponding to the abundant phenols dissolved in SPITM. The difference in weight loss between BL and its extraction residue is close to the yield of EICA, while the difference in weight loss between extraction and thermal dissolution residues is significantly lower than the yield of SPITM.

1. INTRODUCTION With the depletion of petroleum and high-quality coal resources, the utilization of lignites has attracted the attention of researchers recently.1 Unfortunately, high inherent moisture and oxygen contents and low calorific values greatly restrict the large-scale application of lignites in traditional industry.2 Therefore, it is necessary to develop alternative processing technologies for effective utilization of lignites. Extraction and thermal dissolution (TD) have attracted more and more attention as effective approaches for identifying the structural characteristics of coals and to using coals in a clean and ecofriendly manner.3−5 Takanohashi et al.6,7 pointed out that some strongly polar solvents and binary solvents, such as pyridine, tetrahydrofuran (THF), and carbon disulfide/Nmethyl-2-pyrrolidinone (CS2/NMP), are excellent solvents to dissolve the organic species of bituminous coals. Additionally, a small amount of additives, such as tetracyanoethylene and halogenide salt, are beneficial for increasing the extract yield of bituminous coals in CS2/NMP.8,9 Ashida et al.10,11 fractionated two bituminous coals and a lignite into six fractions by tetralin and 1-methylnaphthalene (1-MN) using a sequential TD method and examined the difference in the chemical structure and thermal properties of the fractions. Shui et al.12 investigated the TD behavior of Shenfu subbituminous coal in different solvents. They found that 1-MN/NMP and 1-MN/methanol binary solvents greatly increased the soluble portion (SP) yield. Nevertheless, it is difficult to separate the soluble organic species from the above involved high-boiling-point solvents, resulting in the difficulty to identify the composition and © 2015 American Chemical Society

structural characteristics of the extracts or SPs and to further upgrade them as fuels or fine chemicals. Yan et al.13 found that high-boiling-point solvents can tightly couple with extraction or TD residues and cannot be removed completely, which may cause some problems when using these residues as a feedstock for combustion or gasification. Moreover, the poor recoverability of high-boiling-point solvents also influences the economics of the extraction process. Therefore, it is necessary to develop an extraction technology using solvents with low boiling points that can be easily separated with both extract and residue. Coal is a sedimentary rock with a large three-dimensional cross-linked macromolecular network of polynuclear aromatic clusters connected by relatively strong chemical bonds, along with the soluble part embedded in the macromolecular network.14,15 Because the soluble part can be extracted out with suitable organic solvents at room temperature, it can be defined as free organic species in coals, whereas a high extraction temperature is necessary to dissolve more organic species from coals through the destruction of the weak bridged bonds in the macromolecular network, such as the ether bond, thioether bond, methylene bond, and methylene ether bond. Therefore, a sequence of extraction at room temperature followed by TD with recyclable solvents is a potential method to isolate the free organic species embedded in the macroReceived: August 3, 2015 Revised: November 17, 2015 Published: November 18, 2015 47

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Energy & Fuels molecular network and the organic species originated from the destruction of the weak bridge bonds in the macromolecular network and to provide the possibility for identifying the composition and structural characteristics of the organic species in coals in detail.2 Although several investigations have been carried out previously on the extraction or TD of coals in binary solvents with high boiling points, to the best of our knowledge, sequential extraction and TD of lignite in binary solvents with low boiling points, along with delineating the differences in the composition and structural characteristics between the free organic species and the organic species originated from the destruction of the weak bridged bonds in the macromolecule network of lignites, have not been reported. China possesses nearly 130 billion tons of lignite proven reserves, accounting for about 13% of the total coal reserves of the country.16 In this work, Baiyinhua lignite (BL) was sequentially extracted and thermally dissolved in an isometric CS2/acetone binary solvent and an isometric toluene/methanol binary solvent. To investigate the synergic effect of the binary solvents during extraction and TD, BL was also sequentially extracted with CS2 and then acetone (or acetone and then CS2) and the extraction residue in the isometric CS2/acetone binary solvent was also sequentially thermally dissolved in toluene and then methanol (or methanol and then toluene). The extract and SP from the binary solvents were characterized with Fourier transform infrared (FTIR) spectroscopy and gas chromatography/mass spectrometry (GC/MS), and the BL and its extraction and TD residues from the binary solvents were analyzed with FTIR, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA).

Figure 1. Sequential extraction of BL in CS2 and acetone. basis), respectively. ERICA was dried in a vacuum oven at 105 °C for 24 h. As exhibited in Figure 2, 2 g of ERICA and 40 mL of toluene were placed into a 100 mL stainless-steel, magnetically stirred autoclave.

Figure 2. Sequential TD of ERICA in toluene and methanol.

2. EXPERIMENTAL SECTION

Then, the autoclave was sealed, and the air inside the reactor was replaced with high-purity nitrogen (99.99%) at an initial pressure of 1 MPa. Afterward, the autoclave was heated to 300 °C by an external electric furnace and held for 1 h, after which the autoclave was cooled to room temperature in a water bath. The reaction mixture was taken out as cleanly as possible with the appropriate TD solvents from the autoclave, filtered through a Teflon membrane filter with a 0.45 μm pore size, and repeatedly washed with the appropriate solvents to produce filtrates and filter cakes. The filtrate was concentrated with a rotary evaporator to remove solvent under reduced pressure to produce SP1. The filter cake was dried at 105 °C for 24 h in a vacuum oven to produce a TD residue in toluene, TDR1. Then, the TDR1 was thermally dissolved in 40 mL of methanol at 300 °C to produce SP2 and TDR2. Similarly, ERICA was sequentially thermally dissolved in methanol and toluene to produce SP1′ and SP2′, respectively. ERICA was thermally dissolved in an isometric toluene/methanol binary solvent to produce SPITM and TDRITM. The SP yield was calculated according to the formula: mSP/mBL,daf, where mSP and mBL,daf denote the weights of SP and BL used (dry and ash-free basis), respectively. 2.3. Characterization. FTIR spectra of BL, EICA, SPITM, ERICA, and TDRITM were recorded from 4000 to 400 cm−1 at a resolution of 2 cm−1 by an EQUINOX55 spectrometer using the KBr pellet technique. EICA and SPITM were analyzed with Hewlett-Packard 7890/5975 GC/MS equipped with a capillary column coated with HP5 (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, and 0.25 μm film thickness) and a quadrupole analyzer and operated in electron impact (70 eV) mode. The GC column temperature was raised from 60 to 300 °C at a rate of 5 °C/min and held at 300 °C for 10 min. The mass range scanned was from 30 to 500 amu, and compounds were identified by comparing mass spectra to NIST11 library data. BL, ERICA, and TDRITM were analyzed by use of an X-ray photoelectron spectrometer ESCALAB 250Xi (Thermo Fisher, Waltham, MA) equipped with an Al Kα source and 180° hemisphere energy analyzer. The spectra were recorded in the fixed analyser transmission (FAT) mode (ΔE = constant) with a pass energy of 20 eV. The calibration was carried out with the main C 1s

2.1. Materials. The BL sample was collected from the Baiyinhua 1 basin in the Inner Mongolia Autonomous Region of China. It was pulverized to pass through a 200-mesh sieve (22.19

1.40

a

Mad, moisture (air-dried basis); Ad, ash (dry basis, i.e., moisture-free basis); and Vdaf, volatile matter (dry and ash-free basis). bBy difference.

lists the results of the proximate and ultimate analyses of BL. All solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. and are analytical reagents (>99.8%) that were also distilled with a Büchi R-134 rotary evaporator prior to use. 2.2. Extraction and TD Procedure. As depicted in Figure 1, About 30 g of BL was first extracted with CS2 in an ultrasonic bath at room temperature to produce the extract (E1) and extraction residue (ER1). Each run of the extraction was conducted for 2 h, followed by filtration and distillation. Such operations were repeated with fresh CS2 until few GC/MS-detectable compounds were detected in the solvent, indicating that the exhaustive extraction was achieved. Then, the ER1 was extracted exhaustively with acetone to produce the extract (E2) and extraction residue (ER2). All of the concentrated filtrates from the extraction with the same solvent were incorporated. Similarly, BL was sequentially extracted with acetone and CS2 to produce the extracts, E1′ and E2′, respectively. BL was extracted in the isometric CS2/ acetone binary solvent to produce EICA and ERICA. The extract yield Y was calculated according to the formula: Y = mE/mBL,daf, where mE and mBL,daf denote the masses of the extract and BL used (dry and ash-free 48

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Energy & Fuels Table 2. Yields (wt %, daf) of the Extracts and SPs extract

SP

E1

E2

E1′

E2′

EICA

SP1

SP2

SP1′

SP2′

SPITM

0.33

2.69

2.69

0.1

6.10

4.08

13.83

17.60

1.55

30.32

peak at 284.8 eV. TGA of BL, ERICA, and TDRITM were performed on a Mettler Toledo TGA/SDTA851e analyzer. During the experiment, about 15 mg of sample was placed in a ceramic crucible and heated from 25 to 900 °C at 10 °C/min using N2 as the carrier gas at a constant flow rate of 60 mL/min.

resulting in more organic species release from the macromolecular network.20,21 These organic species released from the breakage of oxygen bridged bonds possible easily dissolve in toluene rather than polymerize each other into residues. 3.2. Analyses of EICA and SPITM. As presented in Figure 3, the FTIR spectra of both EICA and SPITM have absorption peaks

3. RESULTS AND DISCUSSION 3.1. Yields of Extracts and SPs. As listed in Table 2, the yields of E1, E2, E1′, and E2′ from BL are 0.33, 2.69, 2.69, and 0.10%, respectively, whereas the yield of EICA is 6.10%, which is clearly higher than the total extract yield in sequential individual solvents, regardless of the solvent sequence. The yields of SP1, SP2, SP1′, and SP2′ are 4.08, 13.83, 17.60, and 1.55%, respectively, whereas the yield of SPITM is 30.32%, which is also substantially higher than the total SP yield in sequential individual solvents, regardless of the solvent sequence. Previous studies demonstrated that low liquid yields were obtained during coal TD in toluene, even with the temperature higher than 400 °C.17,18 During extraction and TD, a synergic effect arising between two or more solvent possibly produces a higher yield of extract or SP than the total yield of the extracts or SPs from individual solvents. Therefore, the results from this work suggest that there exists obvious synergic effects between both CS2 and acetone during the extraction of BL and toluene and methanol during the TD of ERICA. Additionally, it is observed that the total yields of EICA and SPITM are higher than the difference in weight loss between BL and TDRITM at 900 °C (Figure 6), indicating that the organic species in EICA and SPITM not only originate from the release of the volatile matter but also from the breakage of weak bridged bonds in the macromolecular network of BL. The extraction of coal is mainly controlled by two factors: one is the solubility of the solvent to coal, and another is the penetration ability of the solvent into the coal cross-link structure.19 A number of studies have been carried out to examine the existence of synergic effects between two solvents.7−9,12,15,19 The viscosities of high-boiling-point solvents, such as NMP, are too high to enter the cross-link structure of coals. Shui et al.19 reviewed the speculations about the reasons for extraction enhancement in the CS2/NMP mixed solvent, and they pointed out that CS2 may disrupt the dipolebased association of NMP and, thus, decrease the viscosity of the CS2/NMP mixed solvent, resulting in NMP penetrating more quickly into the macromolecular network structure of coal and breaking the stronger coal−coal interaction. Nevertheless, the solvents used in this work are low-boiling-point solvents with low viscosity; therefore, the reasons for the extraction and TD enhancement in isometric CS2/acetone and toluene/methanol binary solvents are different from those in the CS2/NMP mixed solvent. CS2 and acetone tend to extract different types of organic species in BL for their different polarities and solubilities. The dissolution of extractable organic species in CS2 and the swelling effect of CS2 on the macromolecular network possibly accelerate the dissolution of extractable organic species in acetone and vice versa. As a nucleophilic and H-donor solvent, methanol can attack and break the oxygen bridged bonds in coals and stabilize radicals,

Figure 3. FTIR spectra of EICA and SPITM..

attributed to phenolic OH (3700−3000 cm−1), aliphatic moieties (2929, 2865, 1450, and 1376 m−1), CO (1702 cm−1), aromatic CC (1600 cm−1), and C−O (1330−1000 cm−1) groups.22,23 Nevertheless, the intensity of the absorption peak assigned to phenolic OH in the FTIR spectra of SPITM is stronger, and the peak center shifts to a higher wavenumber region compared to those of EICA, indicating that more phenols exist in SPITM than those in EICA. Additionally, the absorption peak ascribed to ether bond vibration (1090 cm−1) is only detected in the FTIR spectrum of SPITM, implying that there are abundant compounds containing ether bonds in SPITM.24 Moreover, there are more absorption peaks assigned to the C− H bending vibrations of aromatics at the band of 650−920 cm−1 in the FTIR spectrum of SPITM than those in the FTIR spectrum of EICA, indicating more types of alkyl substituents in the arenes of SPITM than those of EICA. To further investigate the component difference between EICA and SPITM, they were characterized with GC/MS analysis. As presented in Figure S1 and Table 3, there are significant differences in the components between EICA and SPITM. It should be noted that the compounds presented in Table 3 only represent the detected compounds with a relative content over 1% by GC/MS analysis. The main compounds identified in EICA are hydrocarbons, including condensed arenes (54.11%) and alkanes (9.41%), which suggests that the majority of free organic species embedded in the macromolecular network of BL are hydrocarbons. Among the detected condensed arenes, the relative contents of naphthalene and 7-isopropyl-1-methylphenanthrene are greater than 10%. Condensed arenes have the properties of teratogenicity, carcinogenicity, and mutagenicity; therefore, the generation of condensed arenes during coal utilization processes, including combustion, gasification, and carbonization, has received considerable attention.25 It was proposed that the condensed arenes in the “mobile” phase of 49

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XPS is an effective non-destructive method to determine the occurrence forms of elements in coals.30,31 As seen in Figure 5,

coals are of particular environmental concern, because they can be more easily released into the environment than those interlinked with the macromolecular network.26,27 Additionally, two organonitrogen compounds, i.e., 7-ethyl-2,4dimethylbenzo[b][1,8]naphthyridin-5(10H)-one (9.06%) and 4′-cyanobiphenyl-4-yl 4-(4-pentylcyclohexyl)benzoate (1.29%), were extracted from BL. Nitrogen atoms are considered as precursors for pollutant formation when coals are used as fuels; therefore, the isolation and identification of organonitrogen compounds in coals before use are beneficial for the clean utilization of coals.28 With a difference from those of EICA, the dominant compounds identified in SPITM are oxygen-containing organic compounds, including phenols (37.04%), alcohols (9.05%), 3,4,5-trimethylcyclopent-2-enone (3.64%), and methyl heptacosanoate (1.39%). In accordance with the strong phenolic OH absorption peak in the FTIR spectrum of SPITM, the phenols are the most abundant compounds in SPITM. Most of the phenols in SPITM are alkylphenols, suggesting that there possibly exists abundant (CH3)nAr−OAr structural units in the macromolecular network of BL.29 Among these oxygencontaining organic compounds in SPITM, both 2-methoxypropan-1-ol and 3-methoxy-2,4,6-trimethylphenol contain a methoxyl group. Actually, many compounds with a relative content less than 1% in SPITM also contain a methoxyl group, such as 2-methoxy-4-methylphenol (0.57%), 2-methoxy-3methylphenol (0.21%), dimethoxybenzene (0.20%), 2-methoxy-1,3,5-trimethylbenzene (0.38%), etc. These results are in agreement with the obvious peak at 1090 cm−1 assigned to ether bond vibration in the FTIR spectrum of SPITM. 3.3. Analyses of BL, ERICA, and TDRITM. As shown in Figure 4, the FTIR spectra of BL and ERICA exhibit similar

Figure 5. O 1s and N 1s XPS spectra of BL, ERICA, and TDRITM..

the O 1s spectra were curve-resolved into three peaks at 531.4, 532.8, and 534.0 eV, which were assigned to CO, C−O/C− O−C, and O−CO groups, respectively. The N 1s spectra were curve-resolved into three peaks at 398.8, 400.2, and 401.4 eV, which were ascribed to pyridinic N (Np), pyrrolic N (Np′), and quaternary N (Nq), respectively. As shown in Table 4. The relative content of the O−CO group decreases in the order of BL > ERICA > TDRITM as a result of the gradual dissolution of esters or decomposition of carboxyl groups in BL. It is proposed that carboxylic groups in coals can be decomposed to aldehydes and then to alcohols during TD, which may be the reason why some alcohols exist in SPITM.32 The amount of the CO group in ERICA is clearly less than that in BL, corresponding to the high content of compounds with the CO group in EICA, e.g., 7-ethyl-2,4dimethylbenzo[b][1,8]naphthyridin-5(10H)-one (Table 3). The TD treatment results in the dramatic decrease of C−O/ C−O−C groups in TDRITM compared to ERICA, which coincides with the high content of phenols in SPITM and the low intensity of the phenolic OH absorption peak in the FTIR spectrum of TDRITM. The relative contents of Np, Np′, and Nq in BL are 38.19, 38.59, and 23.22%, respectively. Corresponding to the high relative content of 7-ethyl-2,4-dimethylbenzo[b][1,8]naphthyridin-5(10H)-one with Np′ in EICA, the relative content of Np′ in ERICA is less than that in BL. The relative content of Nq decreases in the order of BL > ERICA > TDRITM. It was reported that the decomposition of acidic oxygenic functional groups, such as the carboxyl group, possibly leads to the deprotonation of Nq to produce Np′ during TD; therefore, the

Figure 4. FTIR spectra of BL, ERICA, and TDRITM..

functionalities, reflecting that the extraction process has little influence on the macromolecular network of BL, which also indicates that the extract mainly comes from the free organic species in BL. Nevertheless, the FTIR spectrum of TDRITM is clearly different from the FTIR spectra of BL and ERICA. In particular, the intensities of the absorption peaks assigned to the oxygen-containing groups, including phenolic OH (3600− 3000 cm−1), CO (1700 cm−1), and C−O (1100 cm−1), in the FTIR spectrum of TDRITM are weaker than those in the FTIR spectra of BL and ERICA. These results indicate that abundant organic species with oxygen-containing groups are dissolved out and that the macromolecular network was partly destroyed during TD of ERICA in the isometric toluene/ methanol binary solvent. 50

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Energy & Fuels Table 3. Compounds Detected in EICA and SPITM relative content (%) compound

EICA

2-methoxypropan-1-ol (2E,4E)-hexa-2,4-diene 3,4,5-trimethylcyclopent-2-enone naphthalene dimethylphenol azulene ethylmethylphenol trimethylphenol 2-ethyl-4,5-dimethylphenol 4-isopropyl-3-methylphenol methylnaphthalene 2,6,10,14-tetramethylhexadecane 3-methoxy-2,4,6-trimethylphenol tetramethylphenol tetradecane (4-tert-butylphenyl)methanol pentadecane acenaphthene fluorene 4-isopropyl-1,6-dimethylnaphthalene N1,N1,N4,N4-tetramethylbenzene-1,4-diamine trimethylnaphthalene phenanthrene 1,2-diethyl-3,4,5,6-tetramethylbenzene 7-isopropyl-1,1,4a-trimethyl-1,2,3,4,4a,9,10,10a-octahydrophenanthrene 7-butyl-1-hexylnaphthalene 7-ethyl-2,4-dimethylbenzo[b][1,8]naphthyridin-5(10H)-one 7-isopropyl-1-methylphenanthrene tricosane tetracosane methyl heptacosanoate heptacosane (5α)-ergost-14-ene 4′-cyanobiphenyl-4-yl 4-(4-pentylcyclohexyl)benzoate total a

SPITM

a

n n n 13.67 n 1.17 n n n n 1.71 1.40 n n 1.95 n 2.32 3.84 1.46 1.34 n n 1.35 n 5.83 1.18 9.06 22.56 1.24 1.35 n 1.15 1.07 1.29 74.94

7.62 3.27 3.64 n 6.11 n 1.21 14.49 1.26 4.99 n n 1.80 7.15 n 1.43 n n n n 6.11 1.25 n 1.41 n n n 1.01 n n 1.39 n n n 64.14

n = not including the small peaks with an area less than 1% of the total area.

Table 4. Distribution of Oxygen and Nitrogen Forms (Molar Content, %) in BL, ERICA, and TDRITM from XPS Analysis nitrogen forma

oxygen form

a

sample

CO

C−O/C−O−C

O−CO

Np

Np′

Nq

BL ERICDA TDRITM

54.35 22.28 67.74

20.75 66.68 22.75

25.02 11.04 9.51

38.19 48.35 48.07

38.59 30.48 32.20

23.22 21.17 19.73

Np, pyrrolic N; Np′, pyridinic N; and Nq, quaternary N.

solid fuels to some extent.33 The thermogravimetry (TG)/ differential thermogravimetry (DTG) curves of BL, ERICA, and TDRITM are shown in Figure 6. For these three samples, the weight loss is in the order of BL > ERICA > TDRITM, as expected. The difference in weight loss between BL and ERICA at 900 °C is 7.11%, which is slightly higher than the yield of EICA, demonstrating that the extraction process mainly extracts the free organic species embedded in the macromolecular network of BL, which are easily released during pyrolysis. However, the difference in weight loss at 900 °C between ERICA and TDRITM is only 6.15%, which is clearly less than the SPITM yield (30.32%). This result indicates that most organic species in the SPITM originate from the breakage of the

relative content of Np′ in TDRITM is slightly higher than that of ERICA.32 The relative amount of Np in ERICA is more than that in BL as a result of the decrease of Np′ and Nq, while there is almost no difference in the relative content of Np between ERICA and TDRITM. Geng et al.32 found that the relative content of Np in the residues of lignite from hydrothermal treatment did not change with the temperature, and they proposed that Np has not been involved in acid−base interactions. TGA is a useful technique to obtain the weight loss and rate of weight loss of solid fuels with increasing temperature, which can be used to determine the pyrolysis reactively and the information on composition and physicochemical structure of 51

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in EICA and SPITM mainly consist of hydrocarbons and oxygencontaining organic species, respectively. FTIR and XPS analysis results show that the extraction in the isometric CS2/acetone binary solvent has little influence on the macromolecular network structure of BL, while the TD in the isometric toluene/methanol binary solvent results in dramatic breakage of oxygen bridge bonds or decomposition of the oxygencontaining functional groups. TG/DTG analysis results demonstrate that almost all of the components in EICA come from the free organic species embedded in the macromolecular network of BL, while most of the components in SPITM originate from the breakage of the macromolecular network rather than the release of volatile matter. These findings indicate that sequential extraction and TD in the two binary solvents are beneficial for isolating and identifying the organic species of lignites and facilitating the development of alternative utilization technologies for lignites.



ASSOCIATED CONTENT

S Supporting Information *

Figure 6. TG and DTG curves of BL, ERICA, and TDRITM..

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01775. Total ion chromatogram of EICA and SPITM (Figure S1) (PDF)

macromolecular network rather than the release of volatile matter in BL. Because of the removal of free organic species in BL during extraction, the temperature corresponding to the maximum weight loss rate (Tp) of ERICA is slightly higher than that of BL, whereas the maximum weight loss rate of ERICA is slightly higher than that of BL, which may possibly be ascribed to the swelling effect of isometric CS2/acetone binary solvent on the macromolecular network of BL. Swelling of coal could break weaker non-covalent bonds and increase macropores in coals, resulting in the relaxation of the coal macromolecular network and the decrease of diffusive limitation.34,35 Shui et al.19 defined the height ratio of the peak originated from OH self-associated hydrogen bond vibration to the peak assigned to aromatic vibration in FTIR spectra as 1.0 for raw coal. In comparison to the ratio for raw coal, the ratios for the tetrahydronaphthalene swollen coal and the NMP swollen coal are 0.97 and 0.74. According to Figure 4, the height ratio of the peak originated from OH self-associated hydrogen bond vibration (3170 cm−1) to the peak assigned to aromatic vibration (1600 cm−1) for ERICA is 0.78 compared to the ratio for BL (1.0), indicating that the extraction process of BL in the isometric CS2/acetone binary solvent obviously decreased OH self-associated hydrogen bonds in BL, resulting in structural relaxation of ERICA compared to BL. Moreover, a small weight loss peak appears at around 700 °C in the DTG curve of ERICA, which may also be due to the swelling effect of the isometric CS2/acetone binary solvent on the macromolecular network of BL. The Tp of TDRITM is notably higher and the maximum weight loss rate is considerably lower than those of ERICA, indicating that TD process enhances the cross-linking strength of the macromolecular network structure of BL.36,37



AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-516-83995916. E-mail: yunpengzhao2009@ 163.com. *Telephone: +86-516-83995916. E-mail: wei_xianyong@163. com. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was subsidized by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology; Grant 2015QNA25) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.



4. CONCLUSION The yields of extract and SP in isometric CS2/acetone and isometric toluene/methanol binary solvents were found to be substantially higher than the total yields of extracts and SPs obtained from extraction and TD using the same solvents individually but in sequence, indicating that synergic effects exist between CS2 and acetone during the extraction and between toluene and methanol during the TD. The compounds 52

NOMENCLATURE BL = Baiyinhua lignite EICA = extract in the isometric CS2/acetone binary solvent ERICA = extraction residue in the isometric CS2/acetone binary solvent FTIR = Fourier transform infrared spectroscopy GC/MS = gas chromatography/mass spectrometry NMP = N-methyl-2-pyrrolidinone SP = soluble portion SPITM = soluble portion in the isometric toluene/methanol binary solvent TD = thermal dissolution TDRITM = thermal dissolution residue in the isometric toluene/methanol binary solvent TGA = thermogravimetric analysis THF = tetrahydrofuran XPS = X-ray photoelectron spectroscopy 1-MN = 1-methylnaphthalene DOI: 10.1021/acs.energyfuels.5b01775 Energy Fuels 2016, 30, 47−53

Article

Energy & Fuels



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DOI: 10.1021/acs.energyfuels.5b01775 Energy Fuels 2016, 30, 47−53