Theoretical and Experimental Insight into Coal Structure: Establishing

Dec 1, 2016 - Because of their complex supermolecular structure, coal structures are challenging. We have tried to establish a novel method to constru...
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Theoretical and Experimental Insight into Coal Structure: Establishing a Chemical Model for Yuzhou Lignite Jie-Ping Wang, Guang-Yue Li,* Rui Guo, An-Qi Li, and Ying-Hua Liang* College of Chemical Engineering, North China University of Science and Technology, Tangshan, P. R. China, 063009 S Supporting Information *

ABSTRACT: Because of their complex supermolecular structure, coal structures are challenging. We have tried to establish a novel method to construct a three-dimensional chemical model with proper reactivity for Yuzhou lignite using experimental and theoretical methods. Structural units of the Yuzhou lignite were built using elemental analysis, infrared analysis, 13C nuclear magnetic resonance spectra, and density functional theory calculation. A three-dimensional chemical model was built in a periodic box in order to rationalize intra- and intermolecular interactions. To confirm the validity of the proposed model, we compared bond cracking pathways occurring during pyrolysis experiments using thermal gravimetric analysis and numerical dynamic reactive simulations employing the ReaxFF of thermal cracking of the model that we proposed. Experimental and numerical simulations results agreed, validating our building procedure. Therefore, this model could reflect both the basic structural characteristics and the thermal reactivity of Yuzhou lignite. samples can be described20 by quantum-chemical calculations, such as density functional theory (DFT).21,22 Because of the large numbers of intermediates and reactions in the coal thermal process, the quantum-chemical calculations of coal reactions is too expensive. Small molecular models (such as phenol23 and benzoic acid24) are used in the thermal reaction, but such simple structures cannot be employed to study interactions between structural moieties of coal. The reactive force field (ReaxFF)25−27 method can simulate bond cleavage and formation between atoms with lower computational cost than quantum-chemical calculations. ReaxFF has been widely used to investigate coal pyrolysis28,29 processes. In these studies, 3D coal structures are employed as structural units19 to provide proper density and intermolecular interactions. ReaxFF simulations describe the relationship between coal structure and reactivity, which enhances the reliability of the coal models. Coal models have always been used directly to simulate pyrolysis in these simulations, and only a few researches19 have verified the correctness of the models by integrating experimental and computational methods during the thermal conversion process. In order to surmount the limitations of the above-mentioned methods when used alone, we highlight the merits of these methods, and build a continuous 3D model for a Yuzhou lignite sample. The structural units in the model are built from elemental and proximate analysis, IR spectroscopy, 13C NMR spectroscopy, and DFT calculations. The model is then verified by ReaxFF simulations, to determine whether the sequence of bond-cleavage reactions in the model agrees with the experimental results of the lignite sample. This work explores the benefits of building a complex macromolecular model of coal by integrating experimental and computational methods, especially

1. INTRODUCTION The complexity of the coal structure results in a complexity of its reactions, which limits the development of coal thermal conversion processes. Coal structures have been studied using a variety of chemical methods, such as proximate analysis,1 elemental analysis,2 and solvent extraction.3 Coal samples and resultant fragments or products have been characterized by spectral or chromatographic methods,4,5 such as infrared (IR) spectroscopy,6 nuclear magnetic resonance (NMR),7 X-ray diffractometry,8 and gas chromatography−mass spectrometry (GC−MS).9,10 However, these methods provide only partial or empirical structures, or derive molecular structures from a fragment of the macromolecular structure of coal. Coal structural models are average molecular representations of coal. They are usually extremely large molecules, which indicate the complexity of the coal structure. More than 100 coal structural models have been built using the above-mentioned chemical and physical analytical methods.11 Shinn12 built a molecular model for Illinois 6 coal by analyzing its liquefaction products in terms of elemental distribution, aromaticity, functional groups, and reactivity. A structural model for coalified wood of high-volatile bituminous coal rank was constructed by Hatcher et al.13 from elemental analysis, solid-state 13C NMR, and pyrolysis/GC−MS. These models can characterize the basic structures and functional groups of rank coals,14,15 but most of them are two-dimensional structures. They do not describe the spatial structures and intermolecular interactions. Developments in numerical simulations and computer technologies allowed the development of three-dimensional (3D) models of coal. As early as 1981, Spiro developed four 3D coal models for well-known Wiser, Given, Solomon, and Herdy− Wender models.16 From then, more coal 3D models were generated by molecular modeling.17,18 Especially, Mathews et al. have constructed a large-scale model for Illinois No. 6 Argonne Premium coal based on structural data.19 By using these stereoscopic models, the bond energy or thermal reaction in coal © XXXX American Chemical Society

Received: July 28, 2016 Revised: October 20, 2016 Published: December 1, 2016 A

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Energy & Fuels Table 1. Proximate and Ultimate Analysis of Yuzhou Lignite and Elemental Proportion of the Model M proximate analysis (wt %)a Mad YZ M a

8.42

Ad 15.14

ultimate analysis (wt %, daf) Vdaf 37.63

C

H

65.76 65.74

4.72 4.82

O 27.78 27.76

b

N

S

0.95 0.96

0.78 0.73

ad: air-dried base; d: dry base; daf: dry and ash-free base. bBy difference.

Figure 1. Flow diagram for the construction of Yuzhou lignite structural macromodel. was used at 50 mL/min. The differential thermogravimetric (DTG) curve at 20 °C/min is fitted by six Gaussian-shaped standard curves with a coefficient of determination (R2) of 0.9984. According to the TG analysis method by Liu,30 each Gaussian-shaped curve represents a group of covalent bonds. Thus, the sequence of bond-cleavage reactions in Yuzhou lignite is obtained. 2.3. Model Construction Procedure. The construction procedure of the chemical model is shown in Figure 1. Two structural units C1 and C2 (see Figure 2a) of the model were constructed based on data from elemental analysis, FTIR and 13C NMR spectroscopy, DFT calculations, and a combined common knowledge of lignite structure. Two C1 and six C2 molecules were added to a 30.00 × 30.00 × 30.00 Å cell to construct a 3D chemical model (M for short; see Figure 3) by the Packmol program.31 Then, model M was optimized using the Dreiding force field32 in the LAMMPS program33,34 and was verified by comparing the bond-cleavage distributions from TG analysis and ReaxFF simulations. 2.4. DFT Simulations of IR and 13C NMR Spectra. All DFT calculations of C1 and C2, including geometry optimizations, IR, and 13C NMR spectra simulations, were carried out using the ORCA program.35 Becke’s three-parameter hybrid exchange functional with Lee−Yang−Parr gradient-corrected correlation (B3LYP functional)36−39 was used in the subsequent work, which provides reasonable agreement with experimental results and is not time-consuming.40,41 A triple-valence quality with one set of polarization functions (TZVP)42 was used as the basis set throughout, as it is an appropriate basis set for organic compounds. No constraints for symmetry, bonds, angles, or dihedral angles were applied in the geometry optimization calculations. Calculated IR spectra are obtained by vibrational analysis. NMR shielding tensors are computed with the Gauge-independent atomic orbital method. 2.5. ReaxFF Simulations of Pyrolysis Process. The model M included H, C, O, N, and S elements, so we selected the C/H/O/N/S parameters as reported by Castro-Marcano,43 Kamat, and van Duin27 in

verified the correctness of the complex coal structures in the thermal reaction process.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Sample Preparation. Yuzhou lignite was provided by the Kailuan Group. This is a young low-rank coal (originated form angiosperms in the Tertiary period) from Henan Province of China. Yuzhou lignite samples were dried at 50 °C for 2 h, and were then crushed and sieved to less than 0.074 mm. Tests at different granularities showed that the pyrolysis curve was reproducible at 0.074 mm. 2.2. Apparatus and Procedure. A proximate analysis was obtained according to the Chinese National Standard GB/T 212-2008 for moisture, ash, and volatile matter contents. An elemental analysis was obtained using a Vario EL III Elementar elemental analyzer. The proximate and elemental analysis results are listed in Table 1. IR spectra were obtained using a VERTEX70 FT-IR spectrometer. The ratio of dried samples to KBr was approximately 1:100. The wave range of the spectra was located from 4000 to 400 cm−1 with an interval of 2 cm−1. 13C NMR analysis was performed using a Bruker Avance III 400 MHz solid-state spectrometer. Coal samples were packed into a 4 mm diameter zirconia rotor. Cross-polarization magic angle spinning techniques were applied. The total sideband suppression technique was used to remove the sidebands. Cross-polarization/total sideband suppression sequences were recorded at a spinning speed of 5 kHz, a polarization contact time of 3 ms, and a recycle delay time of 2 s. Pyrolysis cracking patterns of Yuzhou lignite were studied using a Netzsch STA449F3 thermogravimetric (TG) analyzer. The sample mass was approximately 0.0300 g. Temperature conditions varied from room temperature to 110 °C at 10 °C/min and were maintained at 110 °C for half an hour to remove moisture. The heating rate from 110 to 900 °C was controlled at 1, 2, 5, 10, and 20 °C/min, and was kept at 900 °C for 30 min to remove volatiles. Inert high-purity Ar (99.999%) B

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Figure 2. Two-dimensional (a) structures of units C1 and C2; (b, c) two other possible structures of C1.

Figure 3. Three-dimensional structures of C1, C2, and M. subsequent ReaxFF molecular dynamics simulations. In the LAMMPS program, we performed heating ReaxFF simulations of M using an isothermal−isochoric (NVT) ensemble of 10 ps at 300 K, then heated the systems to 3000 K at 30, 20, 10, and 5 K/ps.27 The bond order cutoff was 0.3, and the nonbonded cutoff was 10 Å. Moreover, the number of trajectories needs to be adequate to obtain meaningful statistical data from ReaxFF simulations. For classification on the reactions, calculation results for the two trajectories agree well with those with three trajectories. That is, if there are more than two trajectories, the simulation

gives converging results for classification on the reactions. Herein, for the simulations of each heating rate, we run three times using model M with different initial velocities. All atoms in M were marked and traced in the simulations. The ReaxFF trajectories were analyzed using C++ programs to obtain the sequence of bond-cleavage reactions in these simulations. The program for searching for elementary reactions was written according to our previously reported work.44 Source codes are provided in Section 3 of the Supporting Information. C

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3. RESULTS AND DISCUSSION 3.1. Structure Analysis of Lignite. We used IR and 13C NMR spectra to define molecular fragments of Yuzhou lignite. We obtained information on specific functional groups of lignite by IR. Five characteristic peaks exist in the IR spectrum (see Figure 4c). (1) The broad peak at >3000 cm−1 was caused by

Figure 5. Experimental and calculated 13C NMR spectra. (a) C1 simulated spectrum, (b) C2 simulated spectrum, (c) experimental data.

Because of the expensive computational cost for spectral simulations of such a large molecule C240H211N3SO76 by DFT, we constructed two relatively small structural units C1 (C60H52SO19) and C2 (C60H53NO19) for M. When the ratio of C1 and C2 is 1:3 in the model, the elemental distribution of M is close to the elemental analysis results of the Yuzhou lignite. From the molecular formula, we can also deduce that the unsaturation degrees (Ωtotal) is equal to 35 for C1/C2. It means there are 35 carbon rings or double bonds in C1/C2. According to the saturated/unsaturated carbon ratio (0.31) from 13C NMR spectra, C1 and C2 each have 46 unsaturated carbon atoms. These unsaturated carbon atoms exist in the form of aromatic rings or double bonds. The 60 carbon atoms in each structural unit include 42 aromatic carbon atoms (it is the equal of 7 benzene rings, because the number of lignite aromatic ring structural units is less than 2), 14 aliphatic carbon atoms, and 4 carbonyl carbon atoms, which is determined by integrating the 13 C NMR curve. Because each benzene moiety has 1 carbon ring and 3 double bonds and each carbonyl group has 1 double bond, there must be 3 aliphatic rings in each structural unit. The types of functional groups and bridge bonds are mainly determined by IR as well as common knowledge of lignite structures. As mentioned above, there are 4 carboxyl or ketocarbonyl groups in C1/C2. C1/C2 should also contain −OH groups (broad peak at >3000 cm−1 in IR), C−O bridge bonds (−CH2−O− or −O−, 1116 cm−1 in IR), and −OCH3 groups (originated from lignin). Furthermore, sulfur atoms exist mainly in thiophene and nitrogen atoms exist mainly in the form of pyridine or pyrrole types.11,46 The similarity in C1 and C2 structures would lead to similar IR and 13C NMR spectra. Thus, the simulated spectra of the two structural units could be used to represent the spectra of M. We combined carbon skeletons and oxygen-containing groups in structural units, and constructed the primary structures of C1 and C2. The numbers and connections of these functional groups and bridge bonds cannot be determined just by elemental analysis, IR, and 13C NMR. Here, DFT calculations are needed to verify 3D molecular structures of C1 and C2, by identifying the bands in IR and 13C NMR. If a calculated band could not be observed in the experimental spectra, the number or position of corresponding functional groups or bridge bonds

Figure 4. Experimental and calculated IR spectra. (a) C1 simulated spectrum, (b) C2 simulated spectrum, (c) experimental data.

hydrogen-bonded hydroxy groups. It indicates the existence of intra- or intermolecular hydrogen-bonding interactions between oxygen-containing groups in Yuzhou lignite, such as phenolic hydroxyl, alcohol hydroxyl, or carboxyl. This peak may overlap the stretching vibration peak of the aromatic C−H bond at ∼3100 cm−1. (2) The peak at ∼2900 cm−1 was caused by the stretching vibration of the saturated C−H bond. It indicates the existence of −CH2−, −CH3 moieties. These moieties may be from alkyl side chain, bridge bond, or saturated carbon rings in Yuzhou lignite. (3) The peak at 1700 cm−1 corresponds to the CO group, such as carboxyl or carbonyl. They are both the main forms of oxygen-containing groups in lignite. (4) The peak at 1400 cm−1 corresponds to a saturated or unsaturated C−H bending vibration. (5) The peak at 1116 cm−1 contributes to the C−O stretching vibration or C−H bending vibration; it indicates the existence of C−O bridge bonds, such as −CH2−O− or −O−.45,46 The 13C NMR spectra (see Figure 5c) provide the saturated/ unsaturated carbon ratio of 0.31 (the integral curve is shown in Figure S1 in the Supporting Information). The percentage of unsaturated carbon is 76%, which is the major form of carbon atoms in Yuzhou lignite. Most unsaturated carbon atoms exist in aromatic moieties according to common coal knowledge. Peaks at ∼180 and ∼150 ppm indicate there may be different types of CO functional groups, such as carbonyl or carboxyl groups. The broad peak at ∼30 ppm indicates the existence of saturated carbon atoms in the lignite. 3.2. Coal Chemical Model Construction. The formula of M is determined first from the elemental content of Yuzhou lignite. Because of its lowest content30 and highest atomic mass, sulfur element is chosen as the reference standard. Thus, the most simplest formula of M can be expressed as (C240H211N3SO76)n. D

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Energy & Fuels should be modified. Then, IR and 13C NMR spectra are calculated again until they can well reproduce the experimental result. By geometrical optimization and spectrum simulation with DFT, we modified the proposed structures, relative numbers, and functional group positions iteratively and obtained the final 3D structures of C1 and C2. Their structural formulas are shown in Figure 2a, and their 3D structures are shown in Figure 3a,b. When searching for possible structures of the two structural units, we also found other similar 3D structures that conform to the elemental distribution, IR, and 13C NMR spectra of Yuzhou lignite (see Figure S2, Supporting Information). The structures in Figure 2a are not the only feasible structures for C1 and C2. However, a network structure, such as a macrocyclic rigid structure in the molecule in Figure 2b, is not suitable to describe the abundant intermolecular interactions. The chain structure (in Figure 2c) needs a bigger box and could lead to a partially ordered model in the following model construction process. It is not suitable for describing the amorphous coal structure. Herein, a structural unit with some side chains always has a small molecular length, which is beneficial to reducing the model volume and the computational cost. Two C1 and six C2 molecules were inserted into a periodic 30.00 × 30.00 × 30.00 Å box to construct the model M (C480H422O152S2N6, molecular weight: 8762 g/mol). The main functional groups and the mass fractions of each atom (see Table 2)

agree, the reliability of the coal structure model M will be confirmed. 3.3. Thermal Gravimetric/Differential Thermal Gravimetric Analysis. In coal study, pyrolysis47 is commonly employed to identify and define the structural composition of coal samples. TG analysis is a technique commonly employed that enables one to study thermochemical transformations pathways.48,49 We used the procedure provided by Liu30 to obtain the sequence of bond cleavage at different temperatures qualitatively. We analyzed the TG and DTG curves at a heating rate of 1, 2, 5, 10, and 20 °C/min (see Figures 6, S4, and S5) at 110−900 °C. Results confirm that temperatures at maximum mass loss rates (Tmax) shifted toward a higher temperature with increasing heating rate. Tmax obtained at 20 °C/min lagged approximately 50 °C compared to Tmax obtained at 1 °C/min because complete volatile release requires an extended period. However, the overall TG or DTG trend is consistent for different heating rates. Liu30 indicates that the interval of each peak is greater than 50 °C, so different heating rates in our work will not affect the fitting results. The DTG curve was fitted using six subcurves (R2 = 0.9984; see Figure 7). As shown in Table 3, five main peaks were detected at 195, 390, 449, 552, and 690 °C. Because Yuzhou lignite does not contain sufficient aromatic rings, the peak of condensation of aromatic rings above 740 °C is not detected. The Yuzhou lignite peak at 195 °C (Peak 1) can be attributed to the release of bound water and the decomposition of carboxylic acid. It is always accompanied by the release of CO2 and H2O. The main decomposition of volatiles appeared at Peaks 2, 3, and 4. Peak 2 at 390 °C occurs because of the breakage of bonds Cal−O, Cal−S, or Cal−N. Peak 3 at 449 °C is thought to be Cal−Cal, Cal−H, or Car−N. Peak 4 at 552 °C corresponds to Car−Cal, Car−O, or Car−S. Peak 5 at 690 °C is supposed to be carbonate decomposition leading to the generation of CO2 and the condensed aromatic ring and the release of H2. These five subcurve peaks and their respective relative heights are consistent with the findings of Liu.30 3.4. ReaxFF Simulation of Lignite Pyrolysis. We simulated employing ReaxFF reactive dynamics simulations pyrolysis of the model M and using the ReaxFF force field. Low heating rates were used to distinguish between different cleavage pathways. Processes were heated from 300 to 3000 K at 5, 10, 20,

Table 2. Atomic Ratio of Yuzhou Lignite and of the Model M, in Which Carbon Element Is Selected as Reference Standard sample

H/C

O/C

N/C

S/C

YZ M

0.861 0.879

0.317 0.317

0.012 0.012

0.004 0.004

agreed well with experimental data. The model can also describe intra- and intermolecular interactions, such as hydrogen bond or π−π stacking (see Figure S3, Supporting Information). Using our procedure, we have been able to propose a lignite structural model that is consistent with our analytical results. Validation of structural model M will be completed only if thermal decomposition reaction pathways obtained by numerical simulations and by experimental results agree. Thus, coal pyrolysis has been performed using TG analysis and molecular dynamic simulations employing the ReaxFF. If the theoretical and experimental results

Figure 6. Overlaps of DTG curves at 1 and 20 °C/min. E

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Figure 7. DTG curve and fitting results.

Table 3. Categories of Bond-Cleavage Reactions in TG Experiment and ReaxFF Simulations #

reactions

fitted peak temperature in TG (°C)

ReaxFF results

1 2 3 4 5 6

decarboxylation cleavage of Cal−O, Cal−S, and Cal−N cleavage of Cal−Cal, Cal−H, and Car−N cleavage of Car−Cal, Car−O, and Car−S carbonate decomposition condensation of aromatic rings

195 390 449 552 690

Peak 1 Peak 2 Peak 3 Peak 4

Figure 8. Bond-cleavage distribution obtained from ReaxFF simulations of M at 10 K/ps.

and 30 K/ps to determine the cleavage pathways patterns in the pyrolysis process. Results show that the heating rate of 10 K/ps is the most efficient simulation set up to distinguish

between different cleavage pathways. To guarantee the accuracy of the data, we conducted tests three times at a heating rate of 10 K/ps. Consequently, the trends in statistical results are similar. F

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theses bond cleavages could happen at relatively low temperatures. The bond-cleavage reactions have been listed (see Table 3) . Thus, the sequence of cleavage reactions in the reactive dynamics simulations of model M agrees well with that obtained by TG experiments. Moisture and inorganic minerals are not considered in the model M, so carbonate decomposition reactions (corresponding to specific bond cleavage) under Peak 5 are not observed in the reactive dynamic simulations. These results suggest that model M is representative of lignite structures such as Yuzhou lignite or of their thermochemical stability.

We compared our ReaxFF results with previous theoretical work50,51 and found that the dissociation pathways of main moieties, such as benzoic acid and thiophene, etc., showed good agreements with quantum chemical results. It could illustrate the correctness of our model. In the simulations, the cleavage reaction number of specific bonds is defined as the difference between the number of bonds cleavaged and generated of the same type. We count up four categories of bonds cleavage as reported.30 Figure 8 displays the cleavage reaction number for the four categories of bond cleavage. The decarboxylation reactions occur quickly at a relatively low temperature (Peak 1 in Figure 8). Carboxyl groups (R−COOH) first release H atoms to form a carboxyl radical (R−COO·) as shown in Figure 9a. The

4. CONCLUSIONS We constructed a 3D chemical model C480H422O152S2N6 of Yuzhou lignite using experimental and theoretical methods. The results of experimental methods of elemental analysis provided the element ratio of C, H, O, N, and S. IR spectra provided the specific functional groups, for example, the existence of the main bonds including aromatic CC, aliphatic C−H, C−O bonds, etc. 13C NMR spectra provided that the percentage of Cal/Car is 0.73 and showed that the unsaturated carbon is the major existing form of C atoms in Yuzhou lignite. Then, we constructed two similar structural units (C1 and C2) of lignite by experimental methods using elemental analysis and IR and 13C NMR spectra combined with common knowledge of the chemistry of coal. The model was modified iteratively until its DFT calculations could agree with the spectroscopic data. The calculated IR and 13C NMR spectra of the structural units agreed well with the experimental spectra by DFT calculation. The final chemical model M contains two C1 and six C2. To confirm the validity of the 3D chemical structure, we employed pyrolysis experiments by DTG and the ReaxFF reactive dynamics simulations. The DTG curves can be fitted by six subcurves that each represent a group of covalent bond cleavages in a certain temperature range. The ReaxFF simulation results demonstrate that the sequence of bond-cleavage reactions is significantly similar to the experimental results. The carboxyl group (R-COOH) first decomposes at a relatively low temperature, and the cleavage reactions are always the bridge bonds linking aliphatic carbon and aromatic moieties at a high temperature. The chemical structure model more accurately represents the intuitionistic structure of Yuzhou lignite and can describe intermolecular interactions during the thermal conversion process. The method developed in this work may provide a platform for further studies in coal structure and the mechanism of coal pyrolysis.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. Reaction pathways obtained in the ReaxFF molecular dynamics simulation of M. (a) Decarboxylation reaction, (b) Cal−O bond-cleavage reaction, (c) Cal−Cal bond-cleavage reaction, (d) Car−O bond-cleavage reaction.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01854. Source code of searching elementary reactions in ReaxFF simulations, Car/Cal ratio obtained from the integral curve of 13C NMR spectra, disadvantages of two other possible structures of C1, intermolecular interaction in M, and TG and DTG curves of Yuzhou lignite at different heating rates (PDF)

C1−C2 bond in the carboxyl radical moieties then breaks to release C radical moieties and a CO2 molecule. In the lignite structure, O-substituted aliphatic chains are connecting aromatic rings. These aliphatic chains are cleaved at higher temperature by rupture of Cal−O, Cal−S, or Cal−N bonds. An example is shown in Figure 9b. A Cal−O bond breaks down into a C and an O radical. As the temperature continues to increase, cleavage reactions at Peak 3 occur (such as the reaction shown in Figure 9c). Because Peak 3 corresponds to the highest peak in the TGA curve, these are the main reactions in the simulations. Finally, cleavage reactions of Peak 4 occur at a very high temperature. An example is shown in Figure 9d. In the thermal reactions of coal,

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-Y.L.). *E-mail: [email protected] (Y.-H.L.). Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS This research was financed by the National Natural Science Foundation of China (U1361212 and 21506047), the Natural Science Foundation of Hebei Province (B2014209261), and the Key Fund Project of Hebei Province Department of Education (ZD2016068). G.-Y.L. and J.-P.W. thank the State Key Laboratory of Molecular Reaction Dynamics of the Chinese Academy of Sciences for computing and financial support (SKLMRD-K201601). The authors also thank the Kailuan Group for providing the Yuzhou lignite sample.

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DOI: 10.1021/acs.energyfuels.6b01854 Energy Fuels XXXX, XXX, XXX−XXX