Molecular-Level Investigation of the Adsorption Mechanisms of

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Molecular-Level Investigation of the Adsorption Mechanisms of Toluene and Aniline on Natural and Organically Modified Montmorillonite Xin-Juan Hou,† Huiquan Li,*,† Peng He,† Shaopeng Li,† and Qinfu Liu‡ †

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 ‡ School of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China S Supporting Information *

ABSTRACT: The present work reports the adsorption mechanisms of aniline and toluene in dry and hydrated montmorillonite (MMT−Na and MMT−Na−W) and tetramethylammonium-cation-modified MMT (MMT−TMA) as determined through density functional theory. These theoretical investigations explicitly demonstrate that cation−π interactions between Na+/TMA+ cations and aromatics play the key role in adsorption of organics over MMT−Na and MMT−TMA. Weak hydrogen bonds between the H atoms of organics and basal O atoms of tetrahedral silicate also stabilize the location of organics. The combination of interactions between water and basal O atoms and between organics and water molecules in hydrated MMT complexes strengthens the adsorption of organics on MMT, resulting in higher formation energies in hydrated organically intercalated MMTs than in the corresponding dry complexes. The adsorption of organics also changes frontier orbital distributions and consequently promotes the preferential occurrence of reactions on the organics rather than on the MMT layers. These adsorption mechanisms predicted by theoretical investigation can be used to explicate the adsorption of aromatic organics on aluminosilicates with different external environment. Berghout et al.8 calculated the effects of divalent cations on the structural and mechanical elasticity of montmorillonite under different degrees of hydration by using the DFT method. Mignon et al.9 using the AIMD method to study the effect of three types of isomorphic substitutions in the montmorillonite layer. The potential use of organoclays as effective sorbents, particularly for the removal of toxic organic compounds, has been extensively investigated. One of the most intensively studied intercalated clays is prepared by intercalation of tetramethylammonium cation (TMA+) into MMT.10−12Although adsorption of organic compounds is important for understanding the mechanism of water and soil pollution caused by harmful organics and improving the design of new polymer nanocomposites, evaluating the reactions occurring on the inner surfaces of the clays is challenging. The adsorption behavior of a series of organics on MMT has been studied by means of empirical force field methods. For example, molecular dynamics (MD) and Monte Carlo (MC) simulations have been used to study the interactions of 2,4-dichlorophenoxyacetic acid, benzene, and hexadecyltrimethylammonium with clays.13−15 Shi et al.16 investigated the interlayer expansion

1. INTRODUCTION Montmorillonite (MMT) is a representative 2:1 layered phyllosilicate featuring an octahedral sheet of AlO6 sandwiched between two tetrahedral sheets of SiO4. Tetrahedral cations can be replaced by Al3+, and octahedral cations can be replaced by Fe2+ or Mg2+. The resultant layer charge is compensated by exchangeable metal cations (Na+, K+, Ca2+, and Mg2+) in the interlayer space. These metal cations can be exchanged with various organic cations to form organically modified clay mineral. Intercalation of guest organic molecules into the interlayer space of clay minerals is an important process to obtain new nanostructure composites with desirable properties. Organoclays have gained increased research attention because of their potential use in adsorbents, catalysts, molecular sieves, ion conductors, fireproofing coats, and varnishes.1−5 Recently, density functional theory (DFT) and ab initio molecular dynamics (AIMD) have been used to study the structure and swelling adsorption of layered aluminosilicate, such as kaolinite and montmorillonite. For example, Lee et al.6 performed first-principles molecular dynamics simulations to explore the structural changes in variably hydrated Camontmorillonite induced by supercritical CO2 and CO2−SO2 mixtures under geologic storage conditions. Schaef et al.7 employed atomistic density functional simulations to explain CO2 sorption observations with kaolinite surface models. © 2015 American Chemical Society

Received: September 29, 2015 Revised: October 19, 2015 Published: October 20, 2015 11199

DOI: 10.1021/acs.jpca.5b09475 J. Phys. Chem. A 2015, 119, 11199−11207

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The Journal of Physical Chemistry A

a Ca−MMT unit cell.24 A model was constructed based on a supercell consisting of 2 × 2 × 1 cells of the former model. One Al3+ and two Mg2+ isomorphic substitutions in the tetrahedral sheet of SiO4 and octahedral sheet of AlO6 was constructed, and charge compensation was achieved by adding three Na+ atoms. The MMT (MMT−Na) cell was optimized by using the CASTEP program package.25 Generalized gradient approximation (GGA) for the exchange−correlation potential (PW91)26 is appropriate for relatively weak interactions.27The threshold values of the convergence criteria were 2.0 × 10−5eV/ atom for energy, 0.05 eV/Å for maximum force, 0.002 Å for maximum displacement, and 10−6 Ha for self-consistent field tolerance. All of the atoms and unit cell parameters were relaxed in the geometry optimization for MMT−Na. The optimized cell was characterized by the following parameters: a = 10.55 Å, b = 18.08 Å, and c = 9.84 Å; and α = 88.9°, β = 93.7°, and γ = 89.9°. A simulated annealing algorithm was then used to perform canonical MC simulations, with Na+, water, TMA+, aniline, and toluene simulated as adsorbates on the layer of MMT. The atomic partial charges obtained by the GGA/PW91 method were adopted. Some of the atomic partial charges of MMT are shown in Figure S1 (Supporting Information (SI)). The adsorption behavior was modeled using a universal force field (UFF).28 UFF has been successfully used in the study of aluminosilicate minerals.29−33The atomic partial charges of adsorbates and LJ parameters of UFF are shown in Table S1 (SI). Coulombic interactions were calculated by using the Ewald summation method,34 and vdW interactions were evaluated within a cutoff radius of 15.5 Å. The cycle number was 5, and the step of one cycle was 106, which is large enough to guarantee system equilibrium. 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 displacement. Based on MC simulation structures, a series of models was constructed. These models include hydrated MMT−Na (MMT−Na−W), MMT modified with TMA + cations (MMT−TMA), hydrated MMT−TMA (MMT−TMA−W), MMT−Na intercalated with neutral aniline or toluene (MMT− Na−Ani or MMT−Na−Tol) and the corresponding hydrated complexes (MMT−Na−Ani−W or MMT−Na−Tol−W), MMT−TMA intercalated with aniline or toluene (MMT− TMA−Ani or MMT−TMA−Tol), and the corresponding hydrated complexes (MMT−TMA−Ani−W or MMT− TMA−Tol−W). The MMT−Na model was constructed on the basis of a supercell consisting of 2 × 2 × 1 cells of a unit-cell formula of Na0.75[Si7.75Al0.25][Al3.5Mg0.5]O20(OH)4, which means the simulated MMT−Na cell possesses three Na+. For MMT−TMA and its hydrate, three TMA+ cations are included to exchange with three Na+ cations. In this work, we only considered one-layer hydrate of MMT−Na−W. Based on an interlayer water density of 1.10 mg/m3 for one-layer MMT− Na,35,36 this MMT−Na−W included 12 water molecules in the 2 × 2 × 1 simulated cell. Limited by the computational cost, only one toluene or aniline molecule adsorption on MMT−Na or its hydrate is considered. A sequence of MD simulations was performed to obtain the original structures of MMT complexes for DFT optimization. For instance, the first round of MD simulations for MMT−TMA−W was carried out in the NVT ensemble for 200 ps at 298 K. The lowest-energy configuration obtained from the process was then used to start a further 5000 ps simulation in the NVT ensemble. Based on the lowest energy configuration of the second round of MD simulations,

behaviors of Na+ and NH4+, and formamide-intercalated MMT systems by DFT methods and found that interactions between adsorbates and MMT include ionic bonding, hydrogen bonding, electrostatic interactions, and van der Waals forces. Stevens et al.17 studied competitive water−arene sorption on TMA and trimethylphenylammonium (TMPA); their experimental results revealed that, in the absence of water, benzene, and ethylbenzene interact with adsorbed TMA and TMPA ions and subsequently adsorb on the siloxane surface.17 When water and arene vapors are present, water sorption inhibits arene− cation interactions but shows little or no effect on arene sorption by siloxane surfaces.17Lee et al.10 found that, in the presence of water, the adsorption capacities of MMT for aromatic compounds are significantly reduced. Experimental and molecular modeling investigations demonstrate that water molecules enable adsorption of aniline on MMT and repel phenol molecules from MMT.18 However, the presence of water in TMA−MMT increases aniline and phenol adsorption.18 Researchers have suggested that noncovalent electron donor−acceptor interactions, including cation−π, n−π, and hydrogen−π interactions are responsible for the adsorption between aromatic compounds and mineral surfaces. Zhu et al.19,20 suggested that cation−π bonding forms between polycyclic aromatic hydrocarbons (PAHs) and exchangeable metal cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces. Keiluweit et al.21 pointed out that aromatic π-systems within organic compounds can adsorb to minerals, organic oil, and sediment compounds. Vasudevan et al.22demonstrated that cooperative cation−π, π−π, and van der Waals interactions can increase aromatic cationic amine sorption to Na/Ca−MMT well beyond the extent expected in cation interactions alone. Qu et al.23 suggested that cation−π interactions between ammonium cations and PAHs are incapable of the interactions of chlorobenzene with tetra-alkyl ammonium modified smectites. The distribution and orientation of molecules in the interlayer space can be predicted by the MC and MD methods, but comprehensive information on bonding relations in organoclays, interaction energies, and the types of interactions between clay layers and guest cations can only be predicted by quantum chemistry calculations. While the swelling and hydration of montmorillonite minerals has been studied extensively by means of empirical force field methods, quantum chemistry simulations of the intercalation of organics on MMT are less common because of the high computational cost of their application to complex clay systems. This cost is generally attributed to the large size of the simulation cell. The influences of sorption behavior of aromatics on MMT and organic modified MMT are not clearly. The present theoretical investigation presents fundamental details on the intercalation of organics with MMT and may help in the future design of materials with predetermined properties based on MMT. The adsorption mechanism of aromatics on different kinds of MMT complexes is valuable to characterize molecular sorptive interactions of aromatic hydrocarbons on clay mineral or corresponding organically modified clays.

2. THEORETICAL DETAILS a. Computational Methods. The model clay mineral studied in this work is a Wyoming-type MMT with a unit-cell formula of Na0.75[Si7.75Al0.25][Al3.5Mg0.5]O20(OH)4. In the present work, the model system was constructed according to 11200

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The Journal of Physical Chemistry A Table 1. DFT Optimized Cell Volume (V0) and Corresponding Cell Parameters of Natural and Organically Modified Montmorillonite (MMT) Models with Toluene and Aniline model

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

V0 (Å3)

MMT−Na MMT−Na−Tol MMT−Na−Ani MMT−Na−Tol−W MMT−Na−Ani−W MMT−Na−W MMT−TMA MMT−TMA−Tol MMT−TMA−Ani MMT−TMA−W

10.55 10.57 10.55 10.54 10.55 10.55 10.55 10.53 10.55 10.53

18.08 18.11 18.08 18.12 18.12 18.11 18.08 18.09 18.08 18.03

9.84 16.02 16.09 16.03 16.06 13.56 15.89 15.98 15.86 15.79

88.9 90.9 90.0 89.8 89.8 90.2 90.1 90.3 91.1 90.0

93.7 91.7 91.5 91.5 91.3 91.5 91.5 91.3 91.6 91.1

89.9 89.8 89.8 89.8 89.8 89.8 90.0 89.9 89.9 89.8

1873.32 3064.0 3069.4 3062.1 3069.2 2589.0 3031.2 3042.7 3022.8 2995.6

the sign of the second density Hessian eigenvalue (λ2). The sign λ2 distinguishes the bonded (λ2 < 0) from nonbonded (λ2 > 0) interactions. NCI visualized noncovalent interactions by plotting electron density against reduced density gradient. In the NCI surfaces, red signifies steric effects, blue indicates strong attraction, and green denotes weak vdW interactions. The related interaction analysis and figure plotting were performed using the Multiwfn program.39

GGA/PW91 further full optimized the structures of a series of MMT complexes to predict more accurate formation energies. In all GGA/PW91 calculations, the method proposed by Ortmann, Bechstedt, and Schmidt was used for DFT-D corrections.37 During geometry optimization of all MMT complexes, all of the atoms of the MMT layers were relaxed, and adsorbates, such as Na+, TMA+, aromatics, and water molecules, as well as the unit cell parameters were also full relaxed. b. Formation Energy. The formation energies of a series of the MMT complexes were calculated to predict the adsorption ability of different types of adsorbates on the clay layer. The formation energies can be defined as follows:

3. RESULTS AND DISCUSSION 3.1. Structural Relaxation. Table 1 shows the structural parameters of the relaxed computational cells of all of the models. The validity of computational structures can be verified by comparing the predicted structures of dry and hydrated MMT−Na with the structures found in other theoretical and experimental studies.40,41 Our computed lattice parameters (a = 10.55 Å, b = 18.08 Å, c = 9.84 Å, α = 88.9°, β = 93.7°, γ = 89.9° for MMT−Na; a = 10.55 Å, b = 18.06 Å, c = 13.51 Å, α = 90.0°, β = 91.2°, γ = 89.9° for MMT−Na−W) closely coincide with these measurements.40,41 The lattice parameters obtained also reveal that the d(001) spacing of the MMT−Na−W, MMT− TMA, MMT−TMA−W and the corresponding complexes adsorbing aniline and toluene predicted by PW91 functional are slightly larger than the data of PBE functional.37 In MMT−Na, the Na+ cations are “sandwiched” between two layers and located near the upper or lower layers (Figure 1a). These cations are also positioned approximately above/ below the center of a “cavity” in adjacent layers. This cavity corresponds to a six-oxygen ring (SOR) formed by six basal O atoms of the tetrahedral sheets. In MMT−Na−W, the Na+ cations move near the upper or lower inner surfaces and to the corner of the SOR (Figure 1b); some Na+ cations are surrounded by two water molecules while others are surrounded by four water molecules. In MMT−TMA (Figure 1c), the TMA+ cations are located near the upper or lower sheets with hydrogen bonds formed between methyl groups and O atoms of the siloxane surface. In MMT−TMA−W (Figure 1d), water molecules prefer to adsorb on the siloxane surface and repel TMA cations to the center of layer space. The FTIR7 indicated that for TMA−montmorillonite there were almost four water molecules per cation at the lowest partial pressure, this can be verified by Figure 1d. On the other hand, TMA+ cations in dry MMT−TMA preferred to adsorb near the tetrahedral silica surface. In MMT−Na−Tol and MMT−Na− Ani (Figure 1e and f), the Na+ cations are located near one tetrahedral sheet and deviate from the center of the SOR, while the phenyl rings are obliquely intercalated into the layers and located near Na+ atoms with the distances between Na+ and six

ΔEMMT − Na − W = EMMT − Na − W − (EMMT − Na + 12Ew ) ΔEMMT − Na − Tol = EMMT − Na − Tol − (EMMT − Na + E Tol )

ΔEMMT − Na − Ani = EMMT − Na − Ani − (EMMT − Na + EAni)

ΔEMMT − Na − Tol − W = EMMT − Na − Tol − W − (EMMT − Na − W + E Tol ) ΔEMMT − Na − Ani − W = EMMT − Na − Ani − W − (EMMT − Na − W + EAni) ΔEMMT − TMA = EMMT − Na + 3E TMA − 3E Na +

ΔEMMT − TMA − W = EMMT − TMA − W − (EMMT − TMA + E12W ) ΔE′MMT − TMA − W = EMMT − Na − W + 3E TMA − 3E Na +

ΔEMMT − TMA − Ani = EMMT − TMA − Ani − (EMMT − TMA + EAni) ΔEMMT − TMA − Tol = EMMT − TMA − Tol − (EMMT − TMA + E Tol)

Taking MMT−Na−W as an example, EMMT−Na−W is the energy of optimized MMT−Na−W, EMMT−Na is the energy of optimized MMT−Na, and Ew is the optimized water molecules located in a cell with the same cell parameters of optimized MMT−Na. c. Noncovalent Interaction Analysis Method. Noncovalent interaction analysis (NCI) was performed to further identify the interaction between clay layers and adsorbates. This method was applied by examining the reduced electron density gradient from the electron density and its first derivative.38 The reduced electron density gradient (RDG) was defined as follows: RDG(r) =

|∇ρ(r)| 1 2 1/3 2(3π ) ρ(r)4 / 3

Weak interactions exist in regions with low electron density and low RDG. Different interactions (e.g., attraction and repulsion) were distinguished by multiplying the density with 11201

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Figure 1. Structures of MMT−Na, MMT−Na−W, MMT−TMA, MMT−TMA−W, and a series of MMT complexes intercalated with aniline and toluene(Al, magenta; O, red; Mg, green; Si, yellow; H, white; C, cyan; N and Na, blue).

intercalated parallel to the layers and located near the Na+ cation inside the SOR cavity and aniline and toluene molecules and are surrounded by water molecules; the other Na+ cations are surrounded by water molecules. 3.2. Type of Interactions. In hydrogen-bonding studies, a distance cutoff limit of 3.2 Å for H···Y and an angular cutoff of

C atoms of the phenyl ring ranging from 2.7 to 3.2 Å. In MMT−TMA−Tol and MMT−TMA−Ani (Figure 1g and 1h), the phenyl rings are almost vertically intercalated into the layers and one methyl group of TMA+ is located near the center of the phenyl ring (Figure 1 h). In MMT−Na−Tol−W and MMT−Na−Ani−W (Figure 1i and 1j), the aromatics are 11202

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The Journal of Physical Chemistry A 90° for the X−H···Y angle are usually proposed for hydrogen bond classification.42−46 In hydrated MMT and organically modified MMT, the hydrogen bonds among the water molecules, organics, and tetrahedral sheets play a key role in maintaining the stability of the resultant MMT complexes. Cation−π interactions refer to noncovalent interactions between a cation and the planar surface of an aromatic π− donor system.47 Aromatic π systems in organic compounds are known to adsorb to minerals, organic soils, and sediments via π intermolecular interactions.17 Neutral polycyclic aromatic hydrocarbon sorption onto MMT may also be explained as a cation−π interaction resulting from attraction of the π system to the polarizable inorganic cation associated with the surface of the MMT.15−17 However, no theoretical studies have yet investigated cation−π interactions in MMT. Different MMT complexes vary in adsorption ability for aromatic compounds because of differences in their adsorption mechanisms. The effects of hydrogen bonds and cation−π interactions on the adsorption of metal cations, water molecules, and aromatic compounds in MMT−Na−W, MMT−TMA−W, MMT−Na− Ani/Tol, MMT−TMA−Ani/Tol, and MMT−TMA−W−Ani/ Tol are discussed in the following section. As illustrated in the structure of dry MMT−Na in Figure 2 a, the location of Na+ is mainly attributed to the balance between the electrostatic attraction of Na+ and oxygen atoms and the electronic repulsion of Na+ and Si atoms. In MMT−Na−W (Figure 2b), some Na+ cations are surrounded by four water molecules, and the dominant interactions between Na+ and two water molecules are contributed by electrostatic forces between Na+ and O atoms of water. The locations of two other water molecules are mainly determined by the presence of nearby water molecules with hydrogen bonds. Some of the charges in MMT−Na−Tol−W, MMT−Na−Tol, and MMT−TMA−Tol are presented in Figure 3. In MMT−Na−Tol−W, the charges of the O atoms of the SOR and the nearby Na+ cation are about −1.15 ∼ −1.18e and 1.68e, respectively. The charge of the C atom of the phenyl rings connecting to the methyl group is −0.08e, while the charges of the remaining five C atoms are about −0.32 ∼ −0.35e, and (Figure 3). This charge distribution indicates that the interaction between the metal and the SOR is dominated by electrostatic forces between the Na+ and O atoms of the SOR, while the interactions between Na+ and the aromatic π system correspond to cation−π interactions. These interaction modes can also be found in MMT−Na−Tol, where in phenyl rings are obliquely intercalated into the layers to obtain a balance between cation−π interactions and the hydrogen bonds formed between organics and O atoms of siloxane surface. More positive charges are notably found in the Na+ cations of MMT−Na−Tol−W than in those of MMT− Na−Tol. This observation suggests that the cation−π interactions in MMT−Na−Tol−W are stronger than those in MMT−Na−Tol. As the gradient isosurfaces show in Figure 2, the existence of water molecules form a hydrogen-bonding network that stabilizes the aniline/toluene more extensively in hydrated MMT than in dry MMT. Adsorption of TMA+ cations on dry MMT (Figure2c) is mainly attributed to hydrogen-bond formation between methyl groups of TMA+ and the basal oxygen atoms of the tetrahedral sheets. As shown in Figure 2d, water molecules located near TMA+ with the hydrogen bond between oxygen atoms of water molecules and hydrogen atoms of TMA+. In the meanwhile, hydrogen bonds also formed between hydrogen atoms of water molecules and basal O atoms. This finding indicates that water molecules tend

Figure 2. Gradient isosurfaces (s = 0.32 au) of a series of MMT complexes(Al, magenta; O, red; Mg, green; Si, yellow; H, white; C, cyan; N and Na, blue).

to hydrate adsorbed TMA cations.7 In Figure 3c, the electrons of two methyl groups near the benzene ring of toluene possess charges of 0.27e and form cation−π interactions with the phenyl ring of toluene. These cation−π interactions and the hydrogen bonds formed between the H and basal O atoms promote the vertical positioning of toluene in the layers. One phenyl ring in MMT−TMA−Tol appears to form cation−π interactions with two TMA+ cations simultaneously, which means aniline and toluene adsorption in the TMA−modified MMT is stronger compared with that in the MMT−Na layer. The FTIR for benzene and ethylbenzene adsorbed on montmorillonites and MMT−TMA indicated that in the absence of water, benzene and ethylbenzene adsorbed on the siloxane surface as well as interacted directly with TMA ions.13 This experimental conclusion can be compliant with the optimized structures and gradient isosurface of MMT−TMA− Tol shown in Figure 1g and Figure 2h. Cation−π complexation between PAHs and tetra-alkyl ammonium cations in chloroform was also verified by ring-current-induced upfield chemical shifts of the alkyl groups of cations in the 1H NMR spectrum.19 In general, the adsorption of water molecules and other organic cations such as TMA+ in MMT may be mainly attributed to hydrogen bonds, and the adsorption of aromatic organics is mainly caused by cation−π interactions between 11203

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Figure 3. Atomic partial charges of aromatics and nearby basal oxygen atoms in the optimized structure of MMT−Na−Tol−W (a), MMT− Na−Tol (b), and MMT−TMA−Tol (c) (Al, magenta; O, red; Mg, green; Si, yellow; H, white; C, cyan; N and Na, blue).

Na+ or organic cations and phenyl rings. The simultaneous hydrogen bonds’ interaction of the planar aromatic rings with siloxane surfaces also contribute to the stability of the aromatic organics in the clay layer. 3.3. Orbital Analysis. The highest occupied molecular orbital (HOMO) and lowest unoccupied orbital (LUMO) provide information about the potential reactivity of a system, as a chemical reaction requires excitation of electrons from their Fermi levels to the lowest conduction band. The HOMOs and LUMOs of dry and hydrated MMT−Na and other organically intercalated MMT complexes (Figure 4 and Figure S2 in SI) were examined to determine changes in their chemical reactivity. The HOMO of MMT−Na (Figure 4a1) is mainly located at the O atoms of hydroxyl groups connected to Mg atoms and nearby bridging O atoms connected to Al atoms. Its LUMO (Figure 4a2) is mainly located at the O atoms of hydroxyl groups and one Na+ cation, partially involving bridging O atoms. The presence of water molecules partially contributes to HOMO and LUMO (Figure S2a1′ and a2′ in SI) of MMT−Na−W. When the Na+ cations are exchanged by TMA+ cations, the LUMO orbital moves toward TMA+ cations (Figure 4b2 in SI). In hydrated MMT−TMA, the frontier orbitals mainly located at some water molecules. The HOMO of MMT−Na−Ani (Figure S2d1′ in SI) is located at aniline, whereas the HOMO of MMT−Na−Tol (Figure 4c1) is similar to those of MMT−Na and MMT−TMA. The frontier orbitals of MMT−Na−Tol−W and MMT−Na−Ani−W differ completely. The HOMOs of MMT−Na−Tol−W and MMT−Na− Ani−W (Figure S2c1′ and e1′ in SI) are mainly located at some water molecules and aniline, respectively. The LUMO of MMT−Na−Tol−W (Figure S2c2′ in SI) is located at toluene and that of MMT−Na−Ani−W (Figure S2e2′ in SI) is located at most of the oxygen atoms of the MMT layers and water molecules. Although the HOMO and LUMO of MMT− TMA−Ani (Figure S2f1′ and f2′ in SI) are both located at

Figure 4. Frontier orbital distributions of a series of MMT complexes.

aniline, those of MMT−TMA−Tol (Figure 4d1 and d2) are mainly located at the MMT layers and toluene, respectively. In dry MMT−Na and MMT−TMA, toluene intercalation does not change the distribution of the HOMO, but aniline intercalation relocates the HOMO. In dry MMT−TMA and MMT−Na−Tol, water molecules obviously change the frontier orbital distribution. The HOMO and LUMO occupations observed indicate that aniline intercalated on dry and hydrated MMT−Na or MMT−TMA possesses strong chemical reactivity as an electron donor. In general, the adsorption of organics changes the HOMO and LUMO distributions in the MMT complexes, thereby promoting further reaction of the organics instead of MMT layers. 3.4. Formation Energy. The formation energies calculated for the different MMT complexes are presented in Table 2. It is apparent that adsorption ability involves many factors, including not only interaction energy but also availability of adsorption sites, among other factors. Therefore, the formation energy of these MMT complexes only reflects one important aspect of the adsorption ability. During intercalation, expansion of the MMT−Na layers is an endothermic processes. For reactions (1), (2), (3), and (6), the basal spacing of MMT complexes shows evident expansion. For reactions (4), (5), and (7), the basal spacing of MMT 11204

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The Journal of Physical Chemistry A Table 2. Formation Energies (ΔEform, in kJ mol−1) of the Different Intercalation Reactions of the MMT Complexes ΔEform

reactions (1) MMT−Na + 12W (2) MMT−Na + aniline (3) MMT−Na + toluene (4) MMT−Na−W + aniline (5) MMT−Na−W + toluene (6) MMT−Na + 3TMA+ (7) MMT−Na−W + 3TMA+ (8) MMT−TMA +12W (9) MMT−TMA + aniline (10) MMT−TMA + toluene

→ → → → → → → → → →

MMT−Na−W MMT−Na−Ani MMT−Na−Tol MMT−Na−Ani−W MMT−Na−Tol−W MMT−TMA + 3Na+ MMT−TMA−W + 3Na+ MMT−TMA−W MMT−TMA−Ani MMT−TMA−Tol

136.7 55.9 58.4 −74.3 −69.5 197.6 100.5 −216.2 −55.9 −39.0

Figure 5. Total densities of state of the MMT complexes.

the hydrogen-bond formation of organics with water, which strengthen the stability of organics on layers. By comparing reactions (9) and (10) with reactions (2) and (3), TMAmodified MMT is observed to adsorb organics containing benzene rings more easily than dry MMT−Na, likely because one phenyl ring can simultaneously form cation−π interactions with two TMA+ cations, thereby strengthening the interaction between aromatics and TMA+ cations. 3.5. Density of States. The calculated total densities of states (DOS) of the MMT−Na and different MMT complexes such as MMT−Na−Tol, MMT−TMA, MMT−TMA−Tol are shown in Figure 5 and Figure S3−S6 in SI. The low-valence band (VB) DOS of all of the MMTs is predominantly contributed by Al 2s/2p, Si 2s/2p, and O 2p. The DOS of Al 2s/2p, Si 2s/2p, and O 2p also demonstrate a covalent interplay between Al/Si and O. In all of the MMT complexes, the DOS of O 2p is higher than those of Al 2p and Si 2p. This result shows that some electrons from Al 2p and Si 2p transform into the VB and take part in the Al/Si and O interaction. This

complexes shows slightly expansion. During the adsorption process in reactions (9) and (10), the basal spacing exhibits almost no change. These discrepancies in basal spacing for the different reactions indicate that reactions (1), (2), (3), and (6) are endothermic. The formation energy of reaction (7) is obviously positive, which is partially caused by expansion of the MMT layers and extraction of the Na+ cations. Comparison of the formation energies of reaction (6) and (7) reveals that hydrated MMT−Na more readily exchanges Na+ with TMA+ than dry MMT−Na. The adsorption energies of toluene and aniline on dry MMT−Na or hydrated MMT and MMT−TMA are nearly identical because the interaction of these organics with different MMT complexes is mainly contributed by cation−π effects, and the interaction of methyl and amino groups with basal O atoms of silicate layers is fairly weak. Thus, substituted groups, such as methyl and amine groups, show no obvious effects on the adsorption of aromatic organics. Hydrated MMT−Na more easily adsorbs toluene or aniline than dry MMT−Na because the presence of water promotes 11205

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The Journal of Physical Chemistry A observation also implies the occurrence of hybridization between Al/Si 2s and O 2p. Conversely, in the conduction band (CB), the DOS of Al/Si 2s is higher than that of O 2p, which suggests that few O 2p electrons transfer to the CB and hybridize with Al 2s and Si 2s electrons. The VB of MMT−Na is separated in approximately 4.5 eV of the CB. The gap decreases to 2.5 eV with respect to MMT−TMA and MMT− Tol. As shown in Figures S3−S6, the DOS of C 2p in the VB for MMT−TMA is higher than those of N 2p, which indicates that some electrons in the VB are transformed from C 2p to N 2p. The DOS of phenyl C 2p in MMT−Na−Tol is higher than those of methyl C 2p. These findings reveal that some electrons from phenyl C 2p transform into the VB and become part of the phenyl ring and methyl group interaction.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b09475. Potential parameters of the montmorillonite complexes, full description of the material, atomic partial charges of MMT−Na, frontier orbital distributions of a series of MMT complexes, the partial density of states of different elements in MMT−Na, MMT−Na−Tol, MMT−TMA, and MMT−TMA−Tol (PDF)

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We would like to thank the National Program on Key Basic Research Project (973 Program, No. 2013CB632605) and the National Natural Science Foundation of China (NSFC, No. 51034006) for their financial supports. The results described in this paper were obtained on Deepcomp7000 of the Supercomputing Center at the Computer Network Information Center of the Chinese Academy of Sciences.

4. CONCLUSIONS The adsorption mechanisms of aromatics such as aniline and toluene in dry and hydrated MMT−Na and MMT−TMA were investigated by using density functional theory. We explicitly demonstrated that cation−π interactions between Na+/TMA+ and aromatics play a key role in the adsorption of aromatic organics by MMT and TMA modified MMT. Weak hydrogen bonds between the H atoms of organics and the basal O atoms of siloxane surface further stabilize these adsorptions. Water molecules forming strong hydrogen bonds with basal O atoms push organics away from the silicate surface. The combination of interactions between water and basal O atoms and between organics and water molecules in the hydrated MMT complexes strengthens the adsorption of organics on MMT, resulting in higher formation energies in hydrated MMT−Na−Tol/Ani than in the corresponding dry complexes. The adsorption of organics changes the HOMO and LUMO distributions in the MMT complexes, thereby promoting further reaction of the organics instead of MMT layers. In MMT complexes containing organics, the gap between VB and CB decreases to 2.5 eV from 4.5 eV in MMT−Na. This theoretical investigation presents fundamental details on the intercalation of organics with MMT and may help in the future design of materials with predetermined properties based on MMT. The adsorption mechanism of aromatics on different kinds of MMT complexes is valuable to characterize molecular sorptive interactions of aromatic hydrocarbons on clay mineral or organic modified clay minerals.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 11206

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