Theoretical Study of the Intercalation Behavior of Ethylene Glycol on

Article pubs.acs.org/JPCC

Theoretical Study of the Intercalation Behavior of Ethylene Glycol on Kaolinite Xin-Juan Hou,* Huiquan Li,* Shaopeng Li, and Peng He Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Different kinds of models of ethylene glycol (EG) intercalated on kaolinite are investigated by means of density functional theory (DFT). It is found that the most possible mode of EG intercalated on kaolinite is a mixing of physical intercalation and covalent grafting of EG. If the existence of water was considered, the possibility for EG intercalating on kaolinite is as follows: mixing of physical intercalation and covalent grafting > physical intercalation > covalent grafting. The distance between the carbon atom of the bonded EG and the Al atom of the kaolinite lamella are calculated to be about 3 Å, which is in good agreement with the experimental value. The methylene and hydroxyl groups of EG or grafting EG can form hydrogen bonds with both adjacent layers of kaolinite with d(001) of 9.40 Å. With the expanding of the d(001) value from 9.40 to 10.85 Å, the van der Waals forces become more important in the stabilization of EG molecule in the interlayer space.

1. INTRODUCTION Traditional materials, such as clay minerals, can be combined with substances of organic molecules or biological molecules to form novel functional materials. Organically modified clays exhibit adsorption capacities for cations, anions, and organic compounds, which make them valuable for various technical applications. Intermolecular interactions between organic molecules and clay minerals are important in a wide range of chemical applications.1−3 Research into organic molecule modified clay minerals should place importance on studying the mechanism of the intercalation process. The theoretical investigations for the adsorption of organic compounds, water and metal ions on kaolinite have been performed by various methods.4−14 The related theoretical calculation also extended to other aluminosilicate layers, such as pyrophyllite.15−17 These theoretical works provide an microscopic insight about the physical adsorption mechanism of organic compounds, water, and metal ions on clay minerals. Kaolinite is the most ubiquitous clay and an important industrial raw material. It has a wide variety of applications in industry, as a glossy surface agent in coated papers, diluting agent of titanium dioxide, white pigment, paint extender, or rubber filler. It is a hydrated aluminosilicate with the chemical composition Al2Si2O5(OH)4, which has two kinds of interlayer surface. An individual layer consists of two connected sheets: a tetrahedral sheet formed from SiO4 tetrahedral sharing corners, and an octahedral sheet consisting of AlO6 octahedral sharing edges. The weak hydrogen interactions and van der Waals (vdW) forces in consecutive layers offer the possibility of intercalation.18−20 When interacting with organic molecules, the original hydrogen bonds of the kaolinite break, and there are two possible interaction forms: one is new hydrogen bonds forming between molecules and surface hydroxyl groups on the octahedral side as well as oxygen atoms on the tetrahedral side, © 2014 American Chemical Society

the other is the interlayer aluminol groups are susceptible to grafting with organic molecules.21,22 Layered materials are promising precursors in the synthesis of inorganic−organic compounds. Some researchers reported intercalation of ethylene glycol and/or glycerol into various layered materials such as kaolinite.23,24 Covalent grafting diols would offer an attractive approach to surface modification. The surface modification of such materials could be realized by covalent grafting by etherification, enabling intercalates of these molecules to be used in the guest-displacement reaction to form new supramolecular hybrid systems with substances which are not able intercalate directly between the single kaolinite layers. Ethylene glycol intercalated kaolinite has beyond a purely physical incorporation of EG into inter lamellar space and the reaction of the EG molecules and the μ-hydroxyl might take place. Tunney et al.25 first reported a covalent bond between the intercalated EG molecules and kaolinite with d(001) value of 9.40 Å based on IR and thermogravimetric analysis (TGA) data, which provide indirect evidence. Hirsemannet al.26 analyzed the mode of bonding of EG, intercalated into kaolinite by solid-state NMR techniques. All NMR data7 clearly showed that kaolinite was grafted covalently by EG and the distance between the bonded 13C nucleus of EG molecules and the 27Al nuclei of the kaolinite layer was about 3.1 Å.7 Qiao et al.27 characterized the intercalated complex of kaolinite with EG with d(001) value of 10.85 Å, they also performed theoretical optimization for the complex with EG weakly interacting with kaolinite with hydrogen bonding. The thermogram (TG) curve of intercalated compound of kaolinite with EG showed that the continuous weight loss occurred before the dehydroxylation Received: July 3, 2014 Revised: October 15, 2014 Published: October 20, 2014 26017

dx.doi.org/10.1021/jp506628n | J. Phys. Chem. C 2014, 118, 26017−26026

The Journal of Physical Chemistry C

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and decomposition of kaolinite, which was obviously different from that of physical intercalated kaolinite-dimethyl sulfoxide complex (K-DMSO).27 Until now, there is no comprehensive theoretical study about the mode of diols intercalation on kaolinite including both physical intercalation and chemical grafting of diols. An explicit description for the intercalation behavior of EG on kaolinite is very necessary to obtain a consistent picture of the intercalated kaolinite complex and to validate experimental results. In this work, a further theoretical study is performed to judge the intercalation mode of EG on kaolinite, namely, intercalation or grafting compound, or combining of physical and grafted incorporation. Theoretical calculations of quantum chemistry are used to study six kinds of possible intercalation forms of EG in kaolinite with d(001) value of 9.40 and 10.85 Å, respectively. The role of hydrogen bond and vdW forces in the interaction between two sheets of kaolinite layer and the physical intercalated EG or covalent grafting EG are discussed.

Figure 1. Atomic partial charges of K-940 (Al, magenta; O, red; C, gray; Si, orange; H, white).

Table 1. Potential Parameters for Intercalated Kaolinite Complexes

2. MODEL AND CALCULATIONS a. Computational Method. The primitive unit cell of kaolinite was optimized using the CASTEP program package.28 The generalized gradient approximation (GGA) for the exchange-correlation potential (PW91),29 which is appropriate for the relatively weak interactions.14 The threshold values of the convergence criteria were 10−5 eV/atom for energy, 0.03 eV/Å for maximum force, 0.001 Å for maximum displacement, and 10−6 eV/atom for self-consistent field tolerance. In the geometry optimization for pure kaolinite, all the atoms and unit cell parameters were relaxed. The optimized primitive unit cell is characterized by the parameters a = 5.192 Å, b = 9.007 Å, c = 7.434 Å, and α = 91.7°, β = 105.3°, γ = 89.8°, which is consistent with the experimental values30 (a = 5.149 Å, b = 8.934 Å, c = 7.384 Å, and α = 91.9°, β = 105.0°, γ =89.8°). Based on the primitive unit cell, a series of (2 × 2 × 1) supercell were build with d(001) value of 9.40 and 10.85 Å, respectively. These (2 × 2 × 1) supercell with d(001) value of 9.40 and 10.85 Å are denoted as K-720, K-940, and K-1085, respectively. The EG and water molecules are also put into the same supercells with d(001) value of 7.20, 9.40, and 10.85 Å for optimization by using the GGA/PW91 method. In order to investigate the interaction mechanism of EG and water in the layered structure of kaolinite, a simulated annealing algorithm was used to perform canonical Monte Carlo (MC) simulation with EG and H2O simulated as adsorbate on the layer of pure kaolinite or the kaolinite complex grafting by EG. For calculating the interactions between kaolinite and adsorbates, the atomic partial charges obtained by GGA/ PW91 method were adopted. Part of the atomic partial charges of K-940 is shown in Figure 1. In the MC simulation, the cell parameter were fixed, and the atoms of kaolinite were frozen except for the hydroxyl groups of octahedral sheet and the basal oxygen atoms of tetrahedral sheet, while EG (or grafting EG) and H2O molecules full relaxed. The adsorption behavior was modeled using the universal force field (UFF).31 UFF has been successfully used in the study for aluminosilicate minerals.32−36 The Lorenz−Berthelot mixing rules37 were applied to obtain the LJ cross potentials. The atomic partial charges of adsorbates and LJ parameters of UFF are shown in Table 1. The cycle is 5, and the step of one cycle is 106, a representative part of the interface devoid of any arbitrary boundary effects. The threshold values of the convergence criteria were 10−4 kcal/ mol for energy, 0.005 kcal/mol/Å for force, and 5 × 10−5 Å for

species kaolinite Al Si O H H2O O H EG C H−CH2 O H−OH

σ[Å]

ε/kB[k]

q[e]

4.01 3.83 3.12 2.57

254.28 202.41 30.21 22.16

-

3.01 2.96

82.58 34.20

−0.708 0.354

3.43 2.57 3.01 2.57

52.87 22.16 82.96 22.16

−0.300 0.280 −0.790 0.530

displacement. The columbic interactions were calculated by using the Ewald summation method,38 while the van der Waals interactions were evaluated within a cutoff radius of 15.5 Å. Based on the preferential adsorption models of CH3OH in the layer of kaolinite predicted by the MC calculation, the covalent grafting kaolinites were constructed. GGA-PW91 was used to further optimize the structures of kaolinite, adsorbates, and intercalated or grafted kaolinite complexes in order to predict more accurate formation energies of different kinds of kaolinite complexes. In all GGA/PW91 calculations for intercalated or grafting kaolinite complexes, DFT-D correction was adopted, and all the cell parameters are fixed. b. Computational models. According to previous theoretical studies, the hydrogen bonds between organic molecules and kaolinite layer dominated the status of the physical intercalated compounds. For the covalent grafted kaolinite, the hydroxyl of ethylene glycol has an etherification with the hydroxyl of the Al surface of kaolinite to produce the grafted complex and water molecule. In order to judge the intercalation mode of EG on kaolinite, we need to consider intercalated and grafted complexes, and the complexes combining both physical and grafted incorporation of EG molecules into the lamellar space. The role of water molecule in these complexes is also investigated. Ten kinds of structural models reflecting possible EG intercalated or grafted kaolinite complexes are constructed for the computational study. The first kinds of models (K-INT) are kaolinite complexes intercalated by EG. K-INT-940 and K-INT-1085 represent 26018



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the d(001) value of the intercalated kaolinite complex and are 9.40 and 10.85 Å, respectively. The second kinds of models represent kaolinite intercalated by both EG and H2O are denoted as K-INT-W. The earlier DFT study suggested that the Al surface of kaolinite exhibits two types of hydroxyl groups: one is an −OH group oriented perpendicularly to the surface (−OHa), the other is an −OH group oriented parallel to the surface (OHb).11,14,39,40 Our GGA optimization for kaolinite also confirmed it. Thus, two positions with EG grafting on −OHa and −OHb were considered, respectively, and labeled as K-GRA-A and K-GRA-B. The models represent kaolinite grafting with EG and physical incorporating with water are denoted as K-GRA-A(B)-W. The models grafting and physically incorporating EG molecules are denoted as K-GRA-A(B)INT. The kaolinite complex grafting EG and physically incorporating EG and water molecules at the same time is denoted as K-GRA-A(B)-INT-W. The starting crystal structures of 10 kinds of models for GGA optimization were obtained with a different approach. For KINT and K-INT-W, a simulated annealing algorithm was used to perform MC simulation for EG or both EG and H2O simulated as adsorbate on the layer of GGA optimized pure kaolinite, then the lowest energy configuration was chosen as the starting structure for further GGA optimization. After obtaining the GGA optimized K-INT, the starting structure of K-GRA-A(B) can be obtained by connecting EG with the oxygen atom of the octahedral sheet that has the nearest OHa(b)···O distance. The starting structure of K-GRA-A(B)-W was the lowest energy configuration obtained with MC simulation with water simulated as adsorbate on the layer of GGA-optimized K-GRA-A(B). For K-GRA-A(B)-INT or KGRA-A(B)-INT-W, The starting structure was the lowest energy configuration obtained with MC simulation with EG or both EG and water simulated as adsorbate on the layer of GGA-optimized K-GRA. In the GGA optimization for these models, the oxygen atoms in the SiO4 tetrahedral sheet, the hydroxyl groups in the AlO6 octahedral sheet, the grafted EG, the intercalated EG, and water were relaxed; other atoms were frozen. c. Formation Energy. In order to confirm the preferred intercalation behavior of EG on kaolinite layer, the formation energies of intercalated and grafted kaolinite complexes need to be calculated. Taking the kaolinite complexes with d(001) of 9.40 Å as an example, the formation energy can be defined as

EK‑INT‑940 and EK‑INT‑W‑940 are the energies of the optimized kaolinite complexes intercalated by EG and by both EG and water, respectively; EK‑GRA‑A(B)‑940 is the energy of the optimized kaolinite complexes with EG grafting on an octahedral sheet; EK‑GRA‑A(B)‑W‑940 is the energy of the optimized kaolinite complexes for K-GRA-A(B)-940 including water in a layer; EK‑GRA‑A(B)‑INT‑940 is the energy of optimized kaolinite complex with extra EG physical intercalated on K-GRA-A(B)-940, EK‑GRA‑A‑EG‑W‑940 is the energy of optimized K-GRA-A(B)-INT940 with water physical intercalated on it. d. Noncovalent Interaction Analysis Method. In addition to examining the configuration of kaolinite complex, we performed noncovalent interaction analysis (NCI) to further investigate the interaction between the kaolinite layer and intercalated or grafted molecules by examining the reduced electron density gradient that comes from the electron density and its first derivative.42 The reduced electron density gradient (RDG) is defined as RDG(r ) =

|∇ρ(r )| 1 2 1/3 2(3π ) ρ(r )4/3

Weak interactions exist in the regions with low electron density and low RDG value. By multiplying the density by the sign of the second density Hessian eigenvalue (λ2), different types of interactions (attraction and repulsion) can be distinguished. This noncovalent interaction analysis has been developed to visualize the noncovalent interaction by plotting the electron density versus the reduced density gradient. It is a powerful tool to study noncovalent attractive interaction. For NCI surfaces, red color is a sign of steric effect; blue color indicates strong attraction such as hydrogen bonding (between two bonded atoms, the blue region means the bonding); and green color means weak van de Waal interaction. The related interaction analysis and plotting of figure are performed by Multiwfn program.42

3. RESULTS AND DISCUSSION 3.1. Structural Relaxations of EG. The optimized geometry parameters of EG molecules on free status, K-INT940 and K-INT-1085 are listed in Table 2. The free EG are in symmetry. This equivalency is lost for the intercalated EG molecules. As one can see, the EG molecules on K-INT-940 has longer O−H and shorter C−H bond lengths than on free status, which is mainly caused by the hydrogen bonding between hydroxyl groups and octahedral or tetrahedral sheets, and the steric effect between methylene group and the nearby atoms in two sheets, respectively. These interactions between EG and kaolinite also leads the large difference of the dihedral angle of EG skeleton comparing with free EG. The changes for geometry parameters of K-INT-1085 are smaller than those of K-INT-940. It means that, in K-INT-1085, the effects of octahedral and tetrahedral sheet to EG molecule are weaker than those in K-INT-940. This phenomena can be observed in Figure 4. 3.2. Physical Intercalation. The structure of EG and/or water molecules intercalated into kaolinite and the corresponding gradient isosurfaces of the interactions are depicted in Figures 2 and 3, and Figures 4−6, respectively. In the hydrogen bond studies, a distance cutoff limit of 3.2 Å for H···Y and an angular cutoff of 90° for the X−H···Y angle is usually proposed as hydrogen bond classification.43−47 As shown in Figures 2 and 3, some H···Y distances are shorter than 3.2 Å, but the related

ΔE K ‐ INT ‐ 940 = E K ‐ INT ‐ 940 − (E K + E EG) ΔE K ‐ INT ‐ W ‐ 940 = E K ‐ INT ‐ W ‐ 940 − (E K + E EG + E W )

ΔE K ‐ GRA ‐ A ‐ 940 = E K ‐ GRA ‐ A ‐ 940 − (E K + E EG − E W )

ΔE K ‐ GRA ‐ A ‐ W ‐ 940 = E K ‐ GRA ‐ A ‐ W ‐ 940 − (E K + E EG) ΔE K ‐ GRA ‐ B ‐ 940 = E K ‐ GRA ‐ B ‐ 940 − (E K + E EG − E W )

ΔE K ‐ GRA ‐ B ‐ W ‐ 940 = E K ‐ GRA ‐ B ‐ 940 − (E K + E EG) ΔE K ‐ GRA ‐ A ‐ INT ‐ 940 = E K ‐ GRA ‐ A ‐ INT ‐ 940 − (E K + 2E EG − E W )

ΔE K ‐ GRA ‐ A ‐ INT ‐ W ‐ 940 = E K ‐ GRA ‐ A ‐ INT ‐ W ‐ 940 − (E K + 2E EG) ΔE K ‐ GRA ‐ B ‐ INT ‐ 940 = E K ‐ GRA ‐ B ‐ INT ‐ 940 − (E K + 2E EG − E W ) ΔE K ‐ GRA ‐ B ‐ INT ‐ W ‐ 940 = E K ‐ GRA ‐ B ‐ INT ‐ W ‐ 940 − (E K + 2E EG) 26019



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Table 2. Structural Parameters of the EG Molecule Obtained as a Free Molecule and in the Models of K-INT-940 and KINT-1085 parametersa

free EGb

K-INT-940

K-INT-1085

O1−C1 C1−C2 O1−H1c O4−C2 O2−H2c C1−H1a C1−H1b C2−H2a C2−H2b H1c−O1−C1 H2c−O2−C2 C1−C2−O2 C2−C1−O1 O1−C1−C2−O2

1.434 1.511 0.974 1.434 0.974 1.100 1.101 1.100 1.101 107.1 106.9 107.3 107.3 −179.6

1.459 1.500 0.986 1.455 0.982 1.085 1.095 1.085 1.086 104.8 105.1 112.1 109.2 −135.969

1.447 1.523 0.978 1.424 0.981 1.096 1.099 1.100 1.103 107.7 106.9 113.0 109.9 170.2

a

Bond lengths in angstroms; angles in degrees. bThe free molecule is located in the cell in which the parameters are the same as that of KINT-940. Figure 4. Gradient isosurfaces (s = 0.32 au) for intercalated kaolinite complex with basal spacing of 9.40 and 10.85 Å. (Al, coral; O, red; C, cyan; Si, yellow; H, white).

angle is smaller than 90°, which leads to the weakening or disappearance of the hydrogen bond. In K-INT-940 with d(001) values of 9.40 Å, strong hydrogen bonds are formed between the oxygen atoms of two hydroxyls of EG and the hydrogen atoms of surface hydroxyls of Al octahedral sheet; in the meantime, the hydrogen atoms of two hydroxyls of EG form weak hydrogen bonds with the basal oxygen atoms of the Si tetrahedral sheet and the vdW force exist between the methylene group and the Si tetrahedral sheet. This means that the hydroxyl groups of EG play the most dominate role in

Figure 2. Structure of K-INT-940 (Al, magenta; O, red; C, gray; Si, orange; H, white).

Figure 3. Possible kaolinite complexes intercalated by EG and/or water molecules with basal spacing of 9.40 and 10.85 Å. The distances are in angstrom. 26020



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Figure 5. Possible kaolinite complexes grafted by EG and/or water molecules with basal spacing of 9.40 and 10.85 Å. The distances are in angstrom.

stabilizing the molecule on kaolinite with the formation of hydrogen bonds with both octahedral and tetrahedral sheets at a distance between 1.6−2.2 Å. For K-INT-W-940, the addition of a water molecule causes the slight changing of structure of adsorbed K-INT compared with the nonhydrated complexes. In K-INT-W-940, one of the hydroxyls of the EG molecule forms a hydrogen bond with the water molecule. The participation of the hydroxyl H atom in the formation of the hydrogen bond leads the −OH bond of EG to elongate about 0.01 Å. In KINT-940-W, the vdW force dominates the interaction between the water molecule and two sheets, and a hydrogen bond exists between water and the octahedral sheet. In K-INT-1085 with a d(001) value of 10.85 Å, the O atom of one hydroxyl group of EG forms a hydrogen bond with the surface “up” hydrogen atom of the aluminol group, and the other hydroxyl group exhibits vdW interaction with the tetrahedral sheet. Two methylene groups of K-INT-1085 have vdW interaction with

both adjacent sheets. For K-INT-W-1085, the oxygen atom of the water molecule forms hydrogen bonds with the “up” hydrogen atoms of the octahedral sheet. Different from K-INTW-940, vdW force is more important for the interaction between water and nearby EG molecules for K-INT-W-1085. 3.3. Covalent Grafting. There are two possible covalent grafting modes considered for EG molecule. The kaolinite complex of EG molecule interaction with −OHa and −OHb groups is labeled as K-GRA-A and K-GRA-B, respectively. In KGRA-A(B)-940, the free C−O bond of grafting EG is almost parallel with the plane of the basal oxygen atoms. For K-GRAA-940, the residual −OH group of the grafting EG molecule is “up” and forms two hydrogen bonds with nearby H atoms. The methylene group of the grafting EG adjacent with the residual −OH group also forms a hydrogen bond with the basal O atom of tetrahedral sheet. In K-GRA-A-W-94, the water formed hydrogen bonds with the basal O atom of the tetrahedral sheet 26021



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bond formed between the grafting EG and the two sheets, while the hydroxyl group of EG has vdW interaction with the tetrahedral sheet. In K-GRA-B-1085, the vdW force is mainly cause by the methylene group of EG and the tetrahedral sheet. In K-GRA-A(B)-W-1085, the interaction between water and the grafting kaolinite is mainly contributed by the hydrogen bonding between water and the octahedral sheet. The distance between the bonded C atom of the grafting EG molecules and the Al atom of the kaolinite layer (Figure 7) was

Figure 7. Structures of grafting EG in K-GRA-940 and K-GRA-1085.

determined by REAPDOR measurements to be 3.1 Å.26 As shown in Table 3, our calculations predict that in all kinds of Table 3. Partial Structural Parameters of the EG Grafting Kaolinite Complexes parametera

K-GRA-A940

K-GRA-B940

K-GRA-A1085

K-GRA-B1085

Al1−C1 Al2−C2 C1−O1−Al1 C1−O1−Al1

2.977 2.928 124.9 119.9

2.883 3.031 120.9 128.8

2.970 2.959 125.6 125.7

3.006 3.013 128.9 128.9

a

Bond lengths in Angstroms; angles in degrees.

EG covalent grafting kaolinite, the C−Al distances are about 3.0 Å. The distance for covalent grafting EG is much shorter than those in physically intercalated kaolinite. For instance, in KINT-940, the shortest distance between the carbon atom of EG and the Al atom of kaolinite is about 4.132 Å. 3.4. Mixing of Physical Intercalation and Covalent Grafting. As shown in Figures 8 and 9, when free EG is physically intercalated in EG grafting kaolinite complex as KGRA-A(B)-EG-940 and K-GRA-A(B)-W-EG-940, the free EG molecules form strong hydrogen bonds with kaolinite layers. Namely, the hydroxyl groups interact with the “up” hydrogen atoms of the octahedral sheet, and methylene groups interact with the basal oxygen atom of the tetrahedral sheet. At the same time, water molecules form hydrogen bonds with both octahedral and tetrahedral sheets, while the interaction between water and grafting EG are mainly contributed by vdW force. With the enlarging of layer spacing, in K-GRA-A-EG-1085 and K-GRA-A-EG-W-1085, the grafting EG is in “trans” configuration. The interaction of grafting EG and free EG with kaolinite layers are mainly contributed by vdW force, partially by the hydroxyl group of free EG. For K-GRA-B-EG1085 and K-GRA-B-EG-W-1085 with grafting EG of “cis” configuration, similar to the complex grafting “trans” EG, the interaction of EG with kaolinite is mainly caused by vdW force. In K-GRA-A(B)-EG-W-1085, the localization and stabilization are mainly contributed by the hydrogen bond formed between the water molecule and the octahedral sheet. 3.5. Intercalation Energy. Calculated intercalation energy for different kinds of models of EG in kaolinite with d(001)

Figure 6. Gradient isosurfaces (s = 0.32au) for covalent grafting kaolinite complex with basal spacing of 9.40 and 10.85 Å.

and “up” hydrogen atom of octahedral sheet, but the interactions between water and grafting EG are mainly contributed by vdW force. In K-GRA-B-94, the residual −OH group of the grafting EG molecule is “down” and the hydrogen atom of the residual −OH group forms a hydrogen bond with the oxygen atom of −OHb in the octahedral sheet. In K-GRAA(B)-940, the interaction between the grafting EG and the tetrahedral sheet is mainly caused by the hydrogen bond forming between the methylene group and the basal oxygen atoms. In K-GRA-A(B)-W-940, the water interacts with both adjacent sheets, and the interactions between water and grafting EG are all contributed by vdW force. As mentioned above, in all EG grafting kaolinite complexes with spacing of 9.40 Å, the hydroxyl and methylene groups of EG form hydrogen bonds with octahedral and tetrahedral sheets, respectively. This phenomenon is different when the d(001) values enlarge to 10.85 Å. The grafting EG molecules in K-GRA-A-1085 and K-GRA-B-1085 are in “trans” and “cis” configurations, which means that two C−O bonds of the EG are located at a different side and the same side of the C−C bond, respectively. In K-GRA-A-1085, there is no hydrogen 26022



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Figure 8. Possible kaolinite complexes with mixing of physical intercalation and covalent grafting by EG and/or water molecules with basal spacing of 9.40 and 10.85 Å. The distances are in angstrom.

value of 9.40 and 10.85 Å are presented in Tables 4 and 5, respectively. The energies of kaolinite and EG in this way are obtained after the expansion of the kaolinite layer and extraction of EG molecules from the liquid phase. During the intercalation process, the expansion of the kaolinite structure and extraction of EG molecules from the liquid phase are endothermic, while the EG inserting into the expanded interlayer space is an exothermic process. The calculated energy difference between K-720 and K-940 and between K720 and K-1085 are 462.1 and 590.1 kJ/mol, respectively, which represent the energy necessary for the expansion of the kaolinite layer from 7.20 to 9.40 Å and 10.85 Å. The liquid density of EG at 298 K are 1.11 × 10−24 g/Å3,43 which means there are about 19 EG molecules in the space, which is the same as K-940 (space A), and 22 EG molecules in the space, which is same as K-1085 (space B). The energy difference for space A including 19 EG molecules and 18 EG molecules, and for space B including 22 EG molecules and 21 EG molecules are 252.3 and 290.1 kJ/mol, respectively, which can be considered as a very simple estimation of the energy necessary

for extracting the EG molecules from liquid phase during the real intercalation process. As shown in Table 4 and Table 5, the formation energy of KINT-940 is slightly smaller than that of K-GRA-A(B)-W-940, which demonstrates the coexistence of the physical intercalation and covalent grafting of the kaolinite complex in the experimental process. The production of kaolinite complex as K-GRA-A(B)-EG-940 includes two steps. In the first step, EG is intercalated or grafted on kaolinite to produce K-INT-940 or KGRA-A(B)-W-940. In the second step, extra EG is intercalated or grafted on K-INT-940 or K-GRA-A-W-940 to produce KGRA-A(B)-EG-W-940. So the formation energy of K-GRAA(B)-EG-W-940 includes the energies of two-step exothermic reactions. If the existence of water was considered, the possibility of EG intercalated kaolinite is as follows: mixing of physical intercalation and covalent grafting > physical intercalation > covalent grafting. Note that the appearance of a water molecule in the kaolinite layer is possibly intercalated from the external environment or produced during the process of covalent grafting of EG molecule. The data in Tables 4 and 5 also indicate that with the increasing of the d(001) value, the 26023



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Table 5. Formation Energies (ΔEform, in kJ mol−1) of Different Kinds of Intercalated and Grafted Kaolinite Complex with d(001) Value of 10.85 Åa ΔEform (kJ mol−1)

reaction Physical Intercalation (1) K-1085+EG → K-INT-1085 (2) K-1085+EG+H2O → K-INT-W-1085 Covalent Grafting (3) K-1085+EG-H2O → K-GRA-A-1085 (4) K-1085+EG → K-GRA-A-W-1085 (5) K-1085+EG-H2O → K-GRA-B-1085 (6) K-1085+EG → K-GRA−B-W-1085 Mixing of Physical Intercalation and Covalent Grafting (7) K-1085 + 2EG-H2O → K-GRA-A-EG-1085 (8) K-1085 + 2EG → K-GRA-A-EG-W-1085 (9) K-1085 + 2EG-H2O → K-GRA-B-EG-1085 (10) K-1085 + 2EG → K-GRA-B-EG-W-1085

−190.7 −289.5 −85.3 −213.0 −120.0 −217.4 −306.0 −388.8 −297.1 −396.3

The label “A” and “B” indicate that the corresponding grafted kaolinite was produced by the methanol reacting with −OHa and −OHb, respectively.

a

electrostatic environment of the 27Al nuclei. Yet the experiment cannot rule out the existence of physical intercalated EG molecules because the physical intercalation stabilized via hydrogen bonding does not significantly change the electrostatic environment of the 27Al nuclei. Qiao et al.27 presented the thermograms of kaolinite, K-DMSO and K-EG. The thermal behavior of K-EG is obviously different from that in K-DMSO. The intercalation of DMSO in kaolinite has been verified as physical interaction,23,24,49,50 which means that the intercalation of EG is not a simply physical process. The weight-loss percentage corresponding to EG removal in K-EG was 6.9% between 109 and 200 °C, and the continuous weight loss was 7.3% between 200 and 440 °C before the dehydroxylation and decomposition of kaolinite at about 400 °C.27 According to the calculated interaction energy, the most possible complexes after the intercalation of EG should include the covalent EG, the physical intercalation EG, and water molecules, which are the byproduct of EG with kaolinite or come from the external environment. The energy difference between K-GRA-A(B)-EGW and K-GRA-A(B)-EG is about 80−100 kJ/mol, while the corresponding values for K-GRA-A(B)-EG-W and K-GRAA(B)-W are about 180−258 kJ/mol. This means water molecules are more easily removed than EG molecules in KGRA-A(B)-EG-W. The weight-loss process between 109 and 400 °C are in the following order: (a) the elimination of water; (b) the physically intercalated EG removal; (c) decomposition reactions of grafted molecules on the kaolinite surface.

Figure 9. Gradient isosurfaces (s = 0.32au) for mixing kaolinite complex with basal spacing of 9.40 and 10.85 Å.

Table 4. Formation Energies of Different Kinds of Intercalated and Grafted Kaolinite Complex with d(001) Value of 9.40 Åa reaction Physical Intercalation (1) K-940+EG → K-INT-940 (2) K-940+EG+H2O → K-INT-W-940 Covalent Grafting (3) K-940+EG-H2O → K-GRA-A-940 (4) K-940+EG → K-GRA-A-W-940 (5) K-940+EG-H2O → K-GRA-B-940 (6) K-940+EG → K-GRA−B-W-940 Mixing of Physical Intercalation and Covalent Grafting (7) K-940 + 2EG-H2O → K-GRA-A-EG-940 (8) K-940 + 2EG → K-GRA-A-EG-W-940 (9) K-940 + 2EG-H2O → K-GRA-B-EG-940 (10) K-940 + 2EG → K-GRA-B-EG-W-940

ΔEform (kJ mol−1) −203.1 −313.3 −106.6 −223.0 −127.9 −226.9 −334.6 −455.4 −361.1 −484.7

4. CONCLUSIONS The above reported results show the structural details and interaction modes of different kinds of models of ethylene glycol intercalated on kaolinite, including the kaolinite complexes with physical intercalation of EG, covalent grafting of EG and mixing of physical intercalation and covalent grafting of EG. The calculated formation energy of these kaolinite complexes indicated that EG preferred to exist with a mixing of physical intercalation and covalent grafting. The calculation results also confirmed the result of 27Al NMR7 that indicated two different kinds of 27Al chemical surroundings. The distance between the carbon atom of the bonded EG and the Al atom of the kaolinite lamella are calculated to be about 3 Å, which is in

The labels “A” and “B” indicate that the corresponding grafted kaolinite was produced by the methanol reacting with −OHa and −OHb, respectively.

a

formation energies of different kinds of intercalation models of EG and water on kaolinite decrease compared with the corresponding values with d(001) of 9.40 Å. In the work of Hirsemann,26 the 27Al MQMAS spectrum of EG kaolinite clearly showed two different signals, which indicated that the covalent grafting significantly changed the 26024



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good agreement with the experimental value.26 Our work also explored the effect of expanding basal spacing for the interaction between intercalated molecules and both adjacent layers. The methylene and hydroxyl groups of EG or grafting EG can form hydrogen bonds with both adjacent layers in kaolinite with d(001) of 9.40 Å. With the expanding of d(001) value from 9.40 to 10.85 Å, the vdW forces become more important in the stabilization of EG molecule in the interlayer space. These studies provide new insight for the intercalation of the diols in layered aluminosilicate at the molecular level.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the NSFC (Grant No. 51304184) for its financial support. The results described in this paper were obtained on the Deepcomp7000 of the Supercomputing Center in the Computer Network Information Center of the Chinese Academy of Sciences.

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Theoretical Study of the Intercalation Behavior of Ethylene Glycol on

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