Theoretical Investigation of Noncovalent Interactions between Low

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Theoretical Investigation of Noncovalent Interactions between LowRank Coal and Water Junhong Wu, Jianzhong Liu,* Shao Yuan, Zhihua Wang, Junhu Zhou, and Kefa Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China ABSTRACT: The high moisture content of low-rank coals (LRCs) is associated with the noncovalent interactions between coal and water, such as hydrogen bonding and van der Waals attraction. In this study, the molecular model of lignite was constructed and its electrostatic potential (ESP) on a van der Waals surface was analyzed. The mechanism of water absorption on the hydrophilic or hydrophobic sites of the lignite surface was investigated based on the atoms in molecules analysis and reduced density gradient analysis of seven typical lignite···water complexes. The regions with the most negative and positive ESP values were associated with oxygen in oxygen functional groups, and hydrogen was associated with smaller electronegativity. The typical hydrogen bonds O−H···O were formed between water and oxygen functional groups, whereas the weaker hydrogen bonds C− H···O were formed between water and the skeleton components of lignite, such as the benzene ring and methyl and methylene groups. Oxygen functional groups were revealed to exhibit more hydrophilic than skeleton structures of lignite, including benzene rings and aliphatic chains. The hydrophilicity sequences for the different oxygen functional groups in the lignite model were determined as carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether. Furthermore, the carbon bond C···O was also observed in the lignite···water complex via van der Waals interactions, exhibiting a hydrophobic effect. The color-mapped reduced density gradient isosurface accurately demonstrated the noncovalent interactions of lignite···water complexes.

1. INTRODUCTION Lignite and sub-bituminous coals are frequently referred to as low-rank coals (LRCs), which account for nearly half of the world’s coal reserves.1 LRCs have several advantages over bituminous coals, such as low mining cost, high reactivity, high volatile contents, and low environmental pollutants, such as sulfur, nitrogen, and heavy metals.2 However, the high inherent moisture in LRC significantly restricts its large-scale utilization. The significant amount of inherent moisture degrades the energy quality of the coal and increases the long-distance transportation cost. The high moisture further leads to high fuel consumption, high stack flue gas flow rate, high power consumption, low efficiency, and high risk of spontaneous combustion during the utilization processes of LRCs.3 For the preparation of coal water slurry (CWS), the vast amount of inherent water bounded on the coal surface cannot flow freely between coal particles to reduce the viscosity, thereby leading to inferior slurryability.4,5 Consequently, developing an efficient and feasible upgrading technique for LRCs is imperative. Given the structural complexity and component diversity of LRCs, a great number of noncovalent intermolecular and intramolecular interactions have been observed for LRCs, including hydrogen bonding, van der Waals interaction, and charge transfer interaction.6−9 The surfaces of LRCs with oxygen functional groups, such as carboxyl and hydroxyl, generally exhibit strong hydrophilicity. The hydrogen bonding between oxygen functional groups and water is the dominant reason for their moisture content of as much as 30−60% on a wet basis.10 Arisoy and Akgün classified the types of coal···water linkage as chemically bonded water, water adsorbed by physiochemical forces, and free water held by physicomechanical forces.11 Gutierrez-Rodriguez et al.12 stated that coal structure was composed of three sites with regard to hydrophilicity, namely, hydrophilic, weak hydrophobic, and © 2016 American Chemical Society

strongly hydrophobic sites. The hydrophilicity of coal decreased when increasing the coal rank and carbon content, and when decreasing oxygen and hydroxyl content until it turns into bituminous coals. Nishino13 observed that the carboxyl groups with high acidity were considered the preferential sites of adsorption compared with the other groups, and the water adsorption on the coal surface depended on the carboxyl group concentration. Allardice and Evans estimated the heat of water adsorption during the process of water desorption from a raw brown coal and found that the oxygen functional groups had different hydrogen bonding abilities.14 Although a substantial number of techniques, including 1H NMR, DSC, and FTIR, have been used to investigate coal···water interactions in the past,10,15−17 less is known about the theoretical description of the hydrophilicity of oxygen functional groups and their noncovalent interactions with water at the molecular level. This paper aims to provide a molecular-level description of the noncovalent interactions between water and sites with both hydrophilicity and hydrophobicity by quantum-chemistry calculations, and to compare the hydrophilicity of different functional groups in LRCs.

2. METHODS 2.1. Model Construction of Lignite Molecule. The molecular model of lignite was constructed based on Wender’s model18 in combination with Kumagai’s model19 and Tang’s model,20 as shown in Figure 1. The ubiquitous oxygen functional groups (carboxyl, carbonyl, hydroxyl) and molecular skeleton of lignite composed of cyclopentene and benzene rings can be observed in this lignite model. Received: June 6, 2016 Revised: July 29, 2016 Published: August 2, 2016 7118

DOI: 10.1021/acs.energyfuels.6b01377 Energy Fuels 2016, 30, 7118−7124

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Energy & Fuels

critical points have only one negative eigenvalue and generally occur at the center of the ring system, displaying steric repulsions. The bond path is defined as the maximal gradient path that links a BCP and two attractive atom pairs correspondingly, thereby revealing an atomic interaction path for all types of bonding.28 AIM analysis was conducted using the Multiwfn 3.3.8 program. The wave functions for AIM analysis of lignite···water complexes were produced under the same level as the one employed for geometry optimization, namely, M062X-D3/6-311+G(d, p). 2.3.3. RDG Analysis. The reduced density gradient (RDG) function is a fundamental dimensionless quantity used to identify noncovalent interactions, which is defined as29 Figure 1. Lignite model constructed in this study.

RDG(r) = 21

∑ A

ZA − |r − RA|

∫ |rρ−(r′r)′| d r′

(2)

where ρ(r) and ∇ρ(r) are the electron density and its first derivative, respectively. Gradient isosurface plots of RDG versus the electron density ρ(r) multiplied by the sign of the second Hessian eigenvalues sign [λ2(r)] were also rendered by the VMD1.9.2 program based on the outputs of Multiwfn. For clarity, only the grids of the intermolecular interaction regions between lignite and water are displayed.

2.2. Geometry Optimization. The Gaussian 09 program was used to perform all geometry optimizations. The lignite molecule was optimized using the DFT method B3LYP in conjunction with the 6311G(d, p) basis set. All geometry optimizations of lignite···water complexes in this study were performed at the M062X-D3/6311+G(d, p) level. M062X with Grimme’s DFT-D3 dispersion correction using the Becke−Johnson damping function is highly suitable and reliable for noncovalent interaction calculations.22,23 2.3. Wave Function Analysis. 2.3.1. ESP. The electrostatic potential V(r) created by both the nuclei and electrons of a molecule in the surrounding space is given by24

Vtot(r) = Vnuc(r) + Vele(r) =

|∇ρ(r)| 1 2(3π 2)1/3 ρ(r)4/3

3. RESULTS AND DISCUSSION 3.1. Electrostatic Potential of Lignite Molecule. ESP is a useful guide to predict the sites of electrophilic and nucleophilic attack on the molecular vdW surface.30 The positive ESP regions with several maxima are associated with the hydrogen and occasionally with σ-holes of Group IV−VII atoms,31 whereas the negative ESP value is constantly associated with lone pairs of electronegative atoms (such as oxygen, fluorine, and chlorine), and with π electrons of unsaturated molecules.32 Each negative region has one or more local minima, where V(r) reaches its most negative value in that region. Electrophiles are positively charged and have vacant orbitals attracted to an electron rich center with negative ESP. Consequently, the ESP minima on the vdW surface tend to be the favorable sites for electrophilic attack. The ESP and surface extrema of lignite on the vdW surface are plotted in Figure 2. A lone pair of each oxygen atom leads to one or more ESP minima with a more negative ESP value.

(1)

where ZA is the charge on nucleus A, which is located at RA, and ρ(r) is the molecular electron density. The ESP of a lignite molecule on the van der Waals (vdW) surface (electron density = 0.001 au) was analyzed by Multiwfn 3.3.8.25 The wave functions for ESP analysis were obtained at the density functional B3LYP/6-311G(d, p) level. The isosurface map of ESP was rendered by the VMD1.9.2 program26 based on the outputs of Multiwfn. 2.3.2. AIM Analysis. Topology analysis is widely used to analyze real space functions, such as electron density in atom in molecules (AIM) theory. In AIM theory,27 critical points (CPs) are the positions where gradients of electron density vanish. CPs can be classified into four types according to the sign of eigenvalues of the Hessian matrix of electron density. Bond critical points (BCPs) have two negative eigenvalues and mostly appear between attractive atom pairs. Ring

Figure 2. Electrostatic potential of lignite molecule calculated at the B3LYP/6-311G(d, p) level. The local minima and maxima of ESP are plotted as cyan and orange spheres, respectively. 7119

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Figure 3. BCPs and their concomitant bond paths in six optimized lignite···water complexes at the hydrophilic sites. Orange and yellow spheres correspond to bond critical points and ring critical points. Brown lines denote bond paths.

The global minimum of the ESP on the surface is −40.2 kcal/ mol, which is assigned to the overlap region of the carbonyl and benzene ring. Other local minima of the ESP on the surface are −36.98, −35.49, −30.06, and −28.79 kcal/mol, which correspond to the oxygen in the ether, alcoholic hydroxyl, carboxyl, and phenolic hydroxyl groups, respectively. The ESP value over the benzene rings is moderately negative because of the rich π-cloud of benzene rings.33 The main positive ESP regions are associated with hydrogen atoms, given that the carbon (or oxygen) atoms that bond with hydrogen have a relatively larger electronegativity than hydrogen.28 The global maximum of the ESP on the surface is +49.12 kcal/mol, and this is attributed to the hydrogen in carboxyl groups. The other two major local maxima of ESP with the values of +42.29 and +42.68 kcal/mol are located around the hydrogen atoms in the phenolic and alcoholic hydroxyl groups, respectively. The ESP on the vdW surface of lignite is crucial to the understanding and prediction of intermolecular or intramolecular interactions, especially noncovalent interactions. A prominent noncovalent interaction in lignite is hydrogen bonding. The local maxima with most positive ESP values, such as hydrogen atoms in carboxyl and hydroxyl groups, indicates possible hydrogen bond donors, whereas local minima with most negative ESP values, such as lone pairs in carbonyl and ether groups, indicate potential hydrogen bond acceptors. Hydrogen (as local maxima) and oxygen (as local minima) have well-documented correlations with empirical measures of

hydrogen bond-donating and -accepting tendencies, respectively.34 Consequently, ESP on the vdW surface of lignite is a critical guide to establish a stable structure of lignite···water complexes. When the water approaches the lignite molecule, the regions of positive and negative ESP on lignite and water are the the driving force to form the stable complex and are then partially neutralized by the interaction.32 3.2. AIM Characteristics of Lignite···Water Complexes. On the basis of the ESP results, we optimized the lignite···water complexes with the initial geometry in which the hydrogen (or oxygen) of water faces the local minima (or maxima) of lignite to obtain a more stable structure with lower electron energy. Figure 3 shows all the AIM molecular graphs of optimized lignite···water complexes that show critical points and their concomitant bond paths. The BCPs between hydrogen and oxygen atoms and the corresponding bond paths that connect the BCP to the two interacting atoms (hydrogen and oxygen) are observed, thereby suggesting the existence of hydrogen bonding in all these complexes. One water molecule generally interacts with two or more adjacent functional groups in lignite via hydrogen bonding. Taking complex A for example, two hydrogen bonds are formed. One is O55···H59−O57 with O55 in alcoholic hydroxyl as the hydrogen bond acceptor, and the other one is O57···H42−C39 with H42−C39 in the benzene ring as the hydrogen bond donor. The hydrogen bonds in Figure 3 are classified into two types according to the different hydrogen bond donors, namely, O− 7120

DOI: 10.1021/acs.energyfuels.6b01377 Energy Fuels 2016, 30, 7118−7124

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Energy & Fuels Table 1. Properties of Typical O−H···O Hydrogen Bonds in Lignite···Water Complexes Complexes

BCP

Hydrogen bonds

H···O distance (Å)

H-bond angle (deg)

ρ(rbcp) (a.u.)

V(rbcp) (a.u.)

EHB (kJ·mol−1)

A B B C C D D E E F

a1 b1 b2 c1 c2 d1 d2 e1 e2 f1

O55···H59−O57 O34···H58−O57 O57···H21−O20 O57···H56−O55 O34···H59−O57 O57···H27−O26 O25···H59−O57 O11···H58−O57 O34···H59−O57 O20···H58−O57

1.87 1.83 1.78 1.90 2.04 1.72 1.90 2.02 2.46 1.99

168.0 177.4 160.1 164.3 151.3 157.9 140.8 173.0 143.3 157.3

0.0291 0.0291 0.0367 0.0279 0.0176 0.0434 0.0288 0.0191 0.0079 0.0211

−0.0244 −0.0251 −0.0335 −0.0229 −0.0132 −0.0416 −0.0243 −0.0143 −0.0049 −0.0167

−32.01 −32.95 −43.96 −30.05 −17.32 −54.59 −31.91 −18.73 −6.37 −21.91

Table 2. Properties of Weak C−H···O Hydrogen Bonds in Lignite···Water Complexes Complexes

BCP

Hydrogen bonds

H···O distance (Å)

H-bond angle (deg)

ρ(rbcp) (a.u.)

V(rbcp) (a.u.)

EHB (kJ·mol−1)

A B E E E F

a2 b2 e3 e4 e5 f2

O57···H42−C39 O57···H30−C28 O57···H17−C16 O57···H42−C39 O57···H36−C35 O57···H7−O6

2.39 2.41 2.63 2.59 2.45 2.41

135.1 132.7 124.9 132.2 128.2 137.3

0.0120 0.0110 0.0078 0.0072 0.0103 0.0114

−0.0074 −0.0741 −0.0049 −0.0045 −0.0065 −0.0070

−9.71 −9.73 −6.40 −5.87 −8.52 −9.19

bond. Given that all the lignite···water complexes comprise two or more hydrogen bonds, thus the EHB obtained from formula 3 is inaccurate. To provide a quantitative insight into hydrogen bonding on the lignite surface, electron density ρ(r) and potential energy density V(r) at BCPs of hydrogen bonds were calculated, as listed in Tables 1 and 2. The ρ(rbcp) range from 0.0079 to 0.0434 au and from 0.0072 to 0.012 au for typical (O−H···O) and weak (C−H···O) hydrogen bonding interactions, respectively. Nearly all of them fulfilled Lipkowski’s topology criteria for the existence of hydrogen bonds.38 The electron density at the BCP is a good measure of the interaction energies in complexes bound by hydrogen bonding and other noncovalent interactions.39 A high electron density corresponds to a strong hydrogen bonding interaction. Espinosa et al.40 stated that for the hydrogen bond [X−H···O (X = C, N, O)], the relationship between bond energy EHB and potential energy density V(r) at the corresponding BCP can be described as

H···O and C−H···O. The geometries of O−H···O differ from those of C−H···O hydrogen bonds in lignite···water complexes, as shown in Tables 1 and 2. The distances between H and O range from 1.72 to 2.46 Å for O−H···O interactions, and from 2.39 to 2.63 Å for C−H···O interactions. The angles change from 140.8 to 177.4° and from 124.9 to 137.3° for O−H···O and C−H···O hydrogen bonds, respectively. Furthermore, both O−H···O and C−H···O hydrogen bonds apparently have smaller H···O distances in comparison with the corresponding sum of vdW radii of oxygen and hydrogen (1.52 + 1.2 = 2.72 Å),35 thereby indicating that these two attractive atoms are mutually penetrated. The positive mutual penetration between oxygen and hydrogen is taken as an infallible indicator of hydrogen bonding between the water and oxygen functional groups in lignite.36 O−H···O hydrogen bonds have larger angles and shorter H···O lengths compared with C−H···O hydrogen bonds. A well-documented finding is that the hydrogen bond is strong when an angle is closer to being linear (180°) and the bond length is short. Thus, the geometry results indicate that the O−H···O hydrogen bonds are commonly stronger than the C−H···O hydrogen bonds. However, the geometry is occasionally incapable of discriminating the strengths of the hydrogen bonds even for the same type. For example, complex B has two typical hydrogen bonds, namely, O34···H58−O57 and O57···H21−O20. The former has a larger angle (177.4°) than the latter (160.1°), whereas the H···O length of the former (1.83 Å) is slightly larger than that of the latter (1.78 Å). According to IUPAC,37 the hydrogen bond energy is defined as the energy required to separate the two hydrogen bonding species. In the supermolecular approach, complex A···B including a hydrogen bond is handled as one molecule and the total energy E(A···B) is determined. If the energy of monomers A and B is taken as reference, the hydrogen bond energy can be described as E HB = E(A···B) − E(A) − E(B)

E HB = V (rbcp)/2

(4)

EHB values were calculated for all hydrogen bonds in lignite··· water complexes, as listed in Tables 1 and 2. O57···H27−O26 in complex D clearly possesses the largest EHB, thereby indicating that the electrostatic interaction between water and the carboxyl group is dominant, whereas the EHB of O34··· H59−O57 in complex E is the smallest, predominantly because of its geometry with a large bond length (2.46 Å) and a relatively small bond angle (143.3°). A large value of EHB implies a strong bonding interaction between the functional groups and water. The hydrogen bonds associated with the carboxyl are generally stronger than those with the other functional groups, such as the hydroxyl, which is in better agreement with Takanohashi’s experimental results.41 The polar solvents were found to have the ability to release the hydrogen bonds related with the phenolic hydroxyl, whereas neither the nonpolar solvent nor the polar solvent released the strong hydrogen bonds related with the carboxyl. Accordingly, the value of EHB of the hydrogen bond can be used to predict the hydrophilicity of the corresponding oxygen functional groups.

(3)

However, the hydrogen bond energy determined by formula 3 only applies to the complex A···B with one single hydrogen 7121

DOI: 10.1021/acs.energyfuels.6b01377 Energy Fuels 2016, 30, 7118−7124

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Energy & Fuels On the basis of the ρ(rbcp) and EHB results, the hydrophilicity sequences for the different oxygen functional groups in our lignite model are basically the same, namely, carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether. Note that C−H···O hydrogen bonds (Table 2) are 3−5 times weaker than the typical O−H···O hydrogen bonds (Table 1). The bond energies of these C−H···O hydrogen bonds are lower than 10 kJ·mol−1, and they are the same order of magnitude as van der Waals attractions. The attractive contribution of C−H···O hydrogen bonds from dispersion energy is comparable to that from electrostatic interaction. For these weak hydrogen bonds, the enthalpy-reducing contribution to the free energy of these weak hydrogen bonds is not insufficient to compensate for the entropy increase contribution because of the hydrophobic effects of these regions, which would account for the fact that these sites still possess hydrophobicity.42 As shown in Figure 3, C−H donors in the C−H···O weak hydrogen bonds generally appear in benzene rings and methyl and methylene groups, which represent the hydrophobic sites on the lignite surface. For comparison, we further optimized the lignite···water complex with the initial geometry in which the oxygen of the water faces the tetrahedral methyl face (a local maxima) of lignite. The electron deficient face is attributed to the bond between C12 of methyl and O11 of methoxyl, which is an electron withdrawing group. Figure 4 shows a BCP and its

indicative of attractive interactions (such as hydrogen bonding), whereas if sign (λ2)ρ is large and positive, the interaction is nonbonding (usually steric effect). Values near zero show weak vdW interactions.29 The interactions between lignite and water are relatively different for complexes C, D, and G. For complex C, an obvious hydrogen bond is formed between alcoholic hydroxyl and water according to the blue isosurface (also see the spike that ranges from −0.03 to −0.02 au). A weaker hydrogen bond is also observed between the adjacent carbonyl and water corresponding to the light blue isosurface (also see the spike that ranges from −0.02 to −0.01 au). The large green or brown-green isosurface between water and the benzene ring in lignite suggests that the weak vdW interaction is rather evident in complex C. The complex further exhibits a small but nonignorable steric repulsion. For complex D, most of the isosurface is blue, whereas only a small part is red. A strong hydrogen bonding between the lignite and water appears to dominate the interactions, although a moderate steric repulsion between oxygen (in water) and carbon (in carboxyl) can be observed as well. Two spikes are found at the range from −0.045 to −0.02 au, representing two typical hydrogen bonds. The left spike corresponds to O57···H27−O26 with larger bond strength (as shown in Table 1), and the right one is O25···H59−O57. The results reconfirm the remarkable hydrophilicity of carboxyl groups in lignite. Complex G is a typical van der Waals complex where only the weak vdW attraction is dominant. The overall interaction between lignite and water is a weak vdW interaction plotted by the green isosurface, whose spikes locate only in the low electron density and low gradient regions.

4. CONCLUSION The high moisture content of LRCs, which significantly restricts its large-scale utilization, is related to the noncovalent interactions between coal and water as described in this study. The molecular model of lignite was constructed, and its ESP on the van der Waals surface was analyzed. The regions with the most negative and positive ESP values were associated with oxygen in oxygen functional groups, and hydrogen was associated with smaller electronegativity. The mechanism of water absorption on the hydrophilic or hydrophobic sites of the lignite surface was investigated based on the AIM analysis and RDG analysis of seven typical lignite···water complexes. Typical hydrogen bonds O−H···O were formed between water and oxygen functional groups, whereas weaker hydrogen bonds C− H···O were formed between water and the skeleton components of lignite, such as the benzene ring and methyl and methylene groups. Oxygen functional groups were revealed to exhibit more hydrophilic behavior than the skeleton structures of lignite, including benzene rings and aliphatic chains. The hydrophilicity sequences for the different oxygen functional groups in the lignite model were determined as carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether. Furthermore, the carbon bond C···O was also observed in the lignite···water complex via van der Waals interactions, thereby exhibiting a hydrophobic effect. The color-mapped RDG isosurface can be used to accurately demonstrate the noncovalent interactions of lignite···water complexes.

Figure 4. BCP and bond path in the optimized lignite···water complex at the hydrophobic site.

concomitant bond path linking C12 and O57. The C12···O57 distance, 3.10 Å, is slightly smaller than the sum of the vdW radii of oxygen and carbon (1.70 + 1.52 = 3.22 Å),35 thereby indicating that this interaction is extremely weak. The C12··· O57 interaction is actually well documented as a carbon bond,30 which displays a small hydrophobic effect. Consequently, complex G is a typical van der Waals complex, whose interaction is nearly solely dominated by the dispersion effect. 3.3. RDG Analysis of Lignite···Water Complexes. Three typical lignite···water complexes (i.e., complex C, D, and G) with different types of intermolecular interactions were chosen for RDG analysis. The color mapped isosurfaces and corresponding scatter diagrams of RDG versus sign (λ2)ρ for complexes C, D, and G are shown in Figure 5. The results could be used to characterize the type of multiple weak interactions between lignite and water via colored isosurfaces according to the values of sign (λ2)ρ. The sign of λ2 can be used to distinguish bonded (λ2 < 0) and nonbonded (λ2 > 0) interactions, whereas the electron density is an indicator of the bonding strength. Large negative values of sign (λ2)ρ are 7122

DOI: 10.1021/acs.energyfuels.6b01377 Energy Fuels 2016, 30, 7118−7124

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Figure 5. Color mapped RDG isosurface graphs and scatter diagrams of complexes C, D, and G. The isovalue is set to 0.5. The surfaces are colored on a blue-green-red scale according to the values of sign (λ2)ρ ranging from −0.03 to 0.02 au. Blue represents strong attractive interactions, green represents vdW interactions, and red indicates strong steric effects.

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AUTHOR INFORMATION

(1) Osman, H.; Jangam, S. V.; Lease, J. D.; Mujumdar, A. S. Drying of Low-Rank Coal (LRC)-A Review of Recent Patents and Innovations. Drying Technol. 2011, 29 (15), 1763−1783. (2) Willson, W. G.; Walsh, D.; Irwin, W. Overview of low-rank coal (LRC) drying. Coal Prep. 1997, 18 (1−2), 1−15. (3) Yu, J.; Tahmasebi, A.; Han, Y.; Yin, F.; Li, X. A review on water in low rank coals: The existence, interaction with coal structure and effects on coal utilization. Fuel Process. Technol. 2013, 106 (0), 9−20. (4) Atesok, G.; Boylu, F.; Sirkeci, A. A.; Dincer, H. The effect of coal properties on the viscosity of coal-water slurries. Fuel 2002, 81 (14), 1855−1858. (5) Yu, Y.; Liu, J.; Hu, Y.; Gao, F.; Zhou, J.; Cen, K. The properties of Chinese typical brown coal water slurries. Energy Sources, Part A 2016, 38 (9), 1176−1182. (6) Larsen, J. W.; Baskar, A. J. Hydrogen bonds from a subbituminous coal to sorbed solvents. An infrared study. Energy Fuels 1987, 1 (2), 230−232.

Corresponding Author

*Tel.: +86 571 87952443 5302, fax: +86 571 87952884, e-mail address: [email protected] (Jianzhong Liu). Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors acknowledge the financial support provided by the National Key Research and Development Plan of China (Grant No. 2016YFB0600505) and the National Basic Research Program of China (Grant No. 2012CB214906). The authors also wish to thank Dr. Sobereva for his enlightening advice on DFT calculations. 7123

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DOI: 10.1021/acs.energyfuels.6b01377 Energy Fuels 2016, 30, 7118−7124