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Cheng, Hongfei, Hou, Xinjuan, Liu, Qinfu, Li, Xiaoguang, & Frost, Ray(2015)New insights into the molecular structure of kaolinite - methanol intercala-tion complexes.Applied Clay Science, 109 - 110, pp. 55-63.

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https://doi.org/10.1016/j.clay.2015.03.010

1

New insights into the molecular structure of kaolinite-methanol intercalation complexes

Hongfei Cheng a,b, Xinjuan Hou c, Qinfu Liu b•, Xiaoguang Li b, Ray L. Frost d,* a State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 100083, P.R.

China b School of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, P.R. China c Institute of Process Engineering , Chinese Academy of Sciences, Beijing 100080,P.R.China

d School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of

Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia

Corresponding author: Qinfu Liu

Fax: +86 10 62331248,

E-mail addresses: lqf@cumtb.edu.cn

Corresponding author: Ray L. Frost

Fax: +61 7 3138 2407,

E-mail addresses: r.frost@qut.edu.au

The emails of the other authors

Hongfei Cheng h.cheng@cumtb.edu.cn Xinjuan Hou xjhou@home.ipe.ac.cn Qinfu Liu lqf@cumtb.edu.cn Xiaoguang lixiaoguang12345@126.com

• Corresponding authors. Fax:+86 10 62331825 (Q. Liu) , +61 7 3138 2407(R. L. Frost) E-mail addresses: lqf@cumtb.edu.cn (Q. Liu), r.frost@qut.edu.au (R. L. Frost)

2

New insights into the molecular structure of kaolinite-methanol intercalation complexes

Hongfei Cheng a,c, Xinjuan Hou b, Qinfu Liu a•, Xiaoguang Li a, Ray L. Frost d,* a School of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, P.R. China b Institute of Process Engineering , Chinese Academy of Sciences, Beijing 100080,P.R.China c State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Beijing 100083, P.R. China d School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, 2 George Street,

GPO Box 2434, Brisbane, Queensland 4001, Australia

Abstract A series of kaolinite-methanol complexes with different basal spacings were synthesized using guest

displacement reactions of the intercalation precursors kaolinite-N-methyformamide (Kaol-NMF), kaolinite-urea (Kaol-U),

or kaolinite-dimethylsulfoxide (Kaol-DMSO), with methanol (Me). The interaction of methanol with kaolinite was

examined using X-ray diffraction (XRD), infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). Kaolinite

(Kaol) initially intercalated with N-methyformamide (NMF), urea (U), or dimethylsulfoxide (DMSO) before subsequent

reaction with Me formed final kaolinite-methanol (Kaol-Me) complexes characterized by basal spacing ranging between

8.6 Å and 9.6 Å, depending on the pre-intercalated reagent. Based on a comparative analysis of the three Kaol-Me

displacement intercalation complexes, three types of Me intercalation products were suggested to have been present in

the interlayer space of Kaol: (1) molecules grafted onto a kaolinite octahedral sheet in the form of a methoxy group

(Al-O-C bond); (2) mobile Me and/or water molecules kept in the interlayer space via hydrogen bonds that could be

partially removed during drying; and (3) a mixture of types 1 and 2, with the methoxy group (Al-O-C bond) grafted onto

the Kaol sheet and mobile Me and/or water molecules coexisted in the system after the displacement reaction by Me.

Various structural models that reflected four possible complexes of Kaol-Me were constructed for use in a

complimentary computational study. Results from the calculation of the methanol kaolinite interaction indicate that the

hydroxyl oxygen atom of methanol plays the dominant role in the stabilization and localization of the molecule

intercalated in the interlayer space, and that water existing in the intercalated Kaol layer is inevitable.

Keywords kaolinite, methanol, intercalation complex, structural model

• Corresponding authors. Fax:+86 10 62331825 (Q. Liu) , +61 7 3138 2407(R. L. Frost) E-mail addresses: lqf@cumtb.edu.cn (Q. Liu), r.frost@qut.edu.au (R. L. Frost)

3

1. Introduction

Intercalation is a well-known means of forming inorganic-organic nanocomposites under mild conditions, with clay

minerals commonly utilized as host materials for this process (Li et al., 2009). Kaolinite (Kaol) intercalation and its

applications for polymer-based functional composites have attracted great interest, both in industry and in academia,

because these materials frequently exhibit remarkable improvements in desired properties as compared to the virgin

polymer or conventional micro and macro-composites (Frost et al., 2003, 2004; Gardolinski and Lagaly, 2005; Jia et al.,

2008; Detellier and Letaief, 2013). A variety of inorganic and organic species can be used for the intercalation of the

interlayer spaces of Kaol, including formamide (Churchman et al., 1984; Joussein et al., 2007), dimethylsulfoxide

(DMSO) (Costanzo and R. F. Giese, 1986), methanol (Me) (Komori et al., 1999; Kuroda et al., 2011; Caglar et al., 2013),

urea (U) (Nicolini et al., 2009), potassium acetate (Frost et al., 2000a; Cheng et al., 2010, 2011, 2012a), aniline (Luca and

Thomson, 2000), and hydrazine (Horváth et al., 2003). The intercalation of the small molecules into the interlamellar

spaces of Kaol causes an increase in the basal spacing from 7.2 Å to 14.2 Å, and can be used as a preliminary expansion

step permitting insertion of large-sized, non-reactive molecules by displacement intercalation methods. For example,

based on previous reports, a Kaol-Me complex can be an effective intermediate for further intercalation reactions.

Through utilizing a Kaol-Me complex as the second intermediate, further intercalation reactions of ethylene glycol

(Hirsemann et al., 2011), hexylamine (Matusik et al., 2012a), quaternary ammonium salts (Kuroda et al., 2011; Yuan et

al., 2013), and n-alkylamines (Gardolinski and Lagaly, 2005) have also been reported.

More recently, several studies were conducted using molecular simulation studies to assess the intercalation of

simple molecules in Kaol (Fang et al., 2005; Vasconcelos et al., 2007; Rutkai et al., 2009; Benazzouz and Zaoui, 2012).

Molecular simulations are useful because they allow a more detailed interpretation of experimental results, as well as

provide findings that cannot easily be derived from strictly experimental data. In most cases, experimental observations

alone cannot provide a clear picture of the mechanism of the molecular intercalation process, or the influences of

4

intercalation on the clay structure. On the other hand, molecular simulations can contribute information about

intercalation phenomena at the microscopic level. Despite previous research, the exact structure of Kaol-Me complex

remains largely unknown. However, detailed and accurate molecular simulations can be used to furnish evidence about

the structure of Kaol-Me complex. Therefore, it is necessary that research be completed to study the exact structure of

Kaol-Me complex.

In the present study, the desired intercalation product was made by displacing pre-intercalation guest molecules

from the intermediate intercalate complex. Data obtained using X-ray diffraction (XRD), infrared spectroscopy (IR), and

nuclear magnetic resonance(NMR) were combined in order to elucidate the molecular arrangement of Kaol-Me

complexes from different pre-intercalated guest molecules. In addition, molecular simulation studies were completed to

provide valuable insights into the structural arrangement of the complexes. The purpose of the present study is to make

clear the intercalation interaction mechanism for Kaol-Me complex, as well as provide novel structural insight regarding

the structural model of the intercalation complex.

2. Experimental

2.1.Materials

Natural, pure Kaol with an average size of 20 μm was obtained from Hebei Zhangjiakou in China for use in this

study. The chemical composition of the obtained Kaol by percent mass was SiO2 44.64%, Al2O3 38.05%, Fe2O3 0.22%,

MgO 0.06%, CaO 0.11%, Na2O 0.27%, K2O 0.08%, TiO2 1.13%, P2O5 0.13%, and MnO 0.002%, with a loss on ignition

of 15.06%. The major mineral constituent is well ordered Kaol (95 % by mass) with a Hinckley index of 1.31. The

DMSO, N-methyformamide (NMF), U and Me used in this study were purchased from Beijing Chemical Reagents

Company, China.

2.2.Intercalation reaction

5

Intercalation complexes of Kaol with DMSO and NMF were prepared using the method described by Tunney and

Detellier (1993, 1996). Kaol-DMSO intercalation complex as the precursor was prepared by adding 50 g Kaol into a

closed container that contained 90 ml DMSO and 10 ml deionized water, and Kaol-NMF intercalation complex was

prepared by adding 50 g Kaol into a closed container that contained 90 ml NMF and 10 ml deionized water. The mixture

was stirred with the magnetic stirrer at 60 °C for 12 h, and then the suspension was separated by centrifugation with ethyl

alcohol. The intercalation complex of Kaol with urea was prepared using the method by Frost et al.(2000). Kaol-U

intercalation complex as the intermediate was prepared by adding 50 g Kaol into a closed container that 100 ml urea

saturated solution. The mixture was stirred with the magnetic stirrer at 60 °C for 12 h, and then the suspension was

separated by centrifugation with ethyl alcohol. The Kaol-DMSO, Kaol-NMF and Kaol-U intercalates were used as

precursors for further intercalation with Me. Me was added to the pre-intercalation complexes (Kaol-DMSO, Kaol-NMF

and Kaol-U) and the reaction mixtures were stirred for ten days replacing the Me with fresh Me each day. The resulting

materials are noted Kaol-DMSO-Me, Kaol-NMF-Me, and Kaol-U-Me derived respectively from the Kaol-DMSO,

Kaol-NMF and Kaol-U pre-intercalation complexes reacted with Me. Following these steps, the reaction products were

centrifuged and then allowed to dry at room temperature before subsequent XRD, FTIR, and NMR analysis.

2.3.Characterization

X-ray diffraction (XRD). The XRD patterns of the prepared samples were collected using a Rigaku D/max 2500 PC

powder X-ray diffraction meter with Cu Ka radiation of 1.540596 Å and a scanning rate of 2°/min in the 2θ range of

1.0-45°, while operating at 40 kV and 150 mA.

Infrared spectroscopy. Fourier-transform infrared spectroscopy (FTIR) was performed using a Thermofisher Nicolet

6700 spectrometer. The samples were prepared as potassium bromide (KBr) pellets (ca. 2% by mass in KBr). The FTIR

spectra of prepared samples were recorded between 400 and 4000 cm-1.

6

Solid state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurement. Solid-state 1H NMR

spectra were recorded using a Bruker MSL400 spectrometer with a Larmor frequency of 61.42 MHz. A quadrupole echo

pulse sequence was used, with a 90° pulse width of 2.5 µs and a 15 µs pulse interval between the two pulses. Solid-state

13C CP (cross-polarization)/MAS (magic-angle-spinning) NMR measurements were performed using Bruker ASX400

(magnetic field of 9.4 T) and ASX200 (4.7 T) spectrometers with Larmor frequencies of 100.63 and 50.33 MHz,

respectively. The sample spin rate was 5 kHz. 2000 data points were recorded over a spectral width of 500 ppm.

Transients were acquired using a recycle delay of 1 s until a sufficient signal-to-noise ratio had been achieved (ranging

from -250 to 250 ppm).

2.4.Computational Details

The primitive unit cell of Kaol was optimized using the generalized gradient approximation (GGA) for the

exchange-correlation potential (PW91) (Perdew et al., 1996). This approach is appropriate for the relatively weak

interactions present in the models studied (Martorell et al., 2010). The resulting primitive unit cell is characterized by the

parameters a = 10.384Å, b = 18.013 Å, c=7.434 Å, and α=91.7°, β=105.3°, γ = 89.8°. Based on this primitive unit cell, a

series of (2×2×1) supercells were built with the basal spacing of layers set to 8.6 Å, 8.8 Å, or 9.6 Å.

Four structural models that reflect four possible models of Kaol-Me complexes were constructed for use in the

computational study. The first model represents Kaol intercalated by one Me molecule (Kaol-Me). The second model

represents Kaol intercalated by one Me and one water molecule simultaneously (Kaol-Me-W). The third model

represents Kaol with one grafted methoxy group (Kaol-gMeO). The forth model represents Kaol with one grafted

methoxy group as well as one intercalated water (Kaol-gMeO-W). In order to investigate the intercalation mechanism of

Me into a pure Kaol layer, a simulated annealing algorithm was used to perform canonical Monte Carlo (MC)

simulations with one Me and one water molecule as adsorbates on the layer of pure Kaol. The d(001) values were 8.6 Å,

7

8.8 Å, and 9.6 Å. For Kaol-gMeO and Kaol-Me-W, MC simulation was firstly performed with one Me or both one Me

and one water molecule as adsorbate. The adsorption behavior was modeled using the Universal force field (Rappe et al.,

1992). The number of cycles was 3 and the step of one cycle was 106, a representative piece of the interface devoid of

any arbitrary boundary effects. Based on the preferential adsorption models of Me and water in the layer of Kaol

predicted by the MC calculation, GGA-PW91 was used to further optimize the structure and predict the interaction

energy between the Me and the Kaol complex with greater accuracy. Results from an earlier DFT study suggested that

the Al surface of Kaol exhibits two types of hydroxyl groups: one with the –OH group oriented perpendicular to the

surface (-OHv), and the other with the –OH group oriented parallel to the surface (OHl). In the model of Kaol-gMeO and

Kaol-gMeO-W, these two positions of Me grafting, –OHv and –OHl respectively, were considered. The GGA-PW91

calculations were performed using a double numerical plus polarization function (DNP) as the basis set and a DFT-D

correction. In all of the calculations, the heavy atoms of Kaol were frozen, while the hydrogen of Kaol, and the Me and

water molecules were fully relaxed.

3. Results and discussion

3.1.XRD results

The XRD patterns collected for both original Kaol and the intercalation complexes are shown in Fig. 1. The pattern

measured for the original Kaol displays a typical and well-ordered layer structure with a basal spacing (d001) of 7.2 Å.

This value matches well with the standard ICDD reference pattern 14-0164 (kaolinite, Al2Si2O5(OH)4). It is expected that

during the filling of capillaries used for X-ray diffraction studies, both Kaol and intercalated phases will align along the

c-axis due to their plate-like particle shapes (White et al., 2011). The intercalation of Kaol with DMSO, NMF, or U

expands the structure along the c-axis, resulting in the dominant (001) reflection appearing at 11.2, 10.7, and 10.9 Å

respectively, in agreement with previous XRD investigations on Kaol intercalation complexes (Lapides and Yariv, 2009;

Li et al., 2009; Mako et al., 2009; Rutkai et al., 2009; Tonle et al., 2009; Hirsemann et al., 2011; Cheng et al., 2012b;

8

Matusik et al., 2012a;Caglar et al., 2013). When Kaol-DMSO was used as an intermediate, a reflection at 11.2 Å was

observed, indicative of the DMSO intercalation into the interlamellar spaces of Kaol (Elbokl and Detellier, 2009). The

intercalation of Me into the interlamellar spaces of Kaol-DMSO was clearly observed by a shift of the (001) reflection to

the lower angles 2θ=9.14° (d(001)=9.6 Å) upon displacement. A recent investigation by Matusik et al. (Matusik et al.,

2012a) reported that Kaol-DMSO-Me sample was dried at 110 °C the reflection with d=11.2 Å disappeared and a broad

reflection with maximum at 9.5 Å was observed. These authors (Matusik et al., 2012a) also reported that the chemical

formula of Kaol-DMSO intercalation complex was calculated to be Al2Si2O5(OH)3.20(OCH3)0.80 on the basis of CHNS

analysis. Based on this formula, results showed that about 1/3 of the inner surface OH groups were replaced by methoxy

groups, in agreement with previous reports (Tunney and Detellier, 1996). This observation suggests the formation of a

Kaol-Me intercalation complex. Upon direct intercalation of NMF into the interlamellar spacing of Kaol, a very strong

peak with the distance of 10.7 Å is observed at 8.23(2θ), and corresponds to the newly formed diffraction plane. The

other d(001) and d(002) diffraction peaks of Kaol are positioned at 12.37(2θ) and 24.87(2θ) with the distances of 7.2 and 3.6

Å, respectively, in agreement with previous data reported in the literature (Komori et al., 1999; Caglar, 2012). After the

treatment of Kaol-NMF with Me, the basal spacing decreased to 8.6 Å. This is in opposition to research completed by

Komori and Sugahara (1998), which reported that when Kaol-NMF was treated with Me, the d-spacing slightly increased

to 11.1 Å, with this value obtained only when the sample was stored under wet conditions with Me. These opposing

results indicate that the resulting Kaol-Me made from Kaol-NMF as the starting material is inherently unstable. In the

case of Kaol initially intercalated by U, the observed 10.9 Å d-value corresponds to a 3.7 Å expansion as compared to

original Kaol. This value is indicative of a flattened monolayer arrangement of U molecules intercalated into the

interlamellar space (Letaief et al., 2006). Upon the intercalation of Me, the 10.9 Å d (001) peak characteristic of Kaol-U

was fully replaced by a new peak at 8.8 Å. This decrease in d-value indicates a different organization of Me in the

interlamellar space of Kaol as compared to final complexes made using DMSO or NMF as intermediate intercalates.

9

After treatment of the pre-intercalate Kaol complexes with Me, the pre-intercalate reagents (DMSO, NMF, and U) were

removed from the interlayer space of Kaol through a concerted replacement by Me. In summary, the d(001) value were

9.6 Å for Kaol-DMSO-Me, 8.6 Å for Kaol-NMF-Me, and 8.8 Å for Kaol-U-Me, compared with recorded values in this

case of 11.2 Å for the starting material (Kaol-DMSO), 10.7 Å for Kaol-NMF, and 10.9 Å for Kaol-U. These correspond

to interlamellar distances of 2.4 Å, 1.4 Å, and 1.6 Å, respectively, with these values obtained by subtracting a clay layer

thickness of 7.2 Å from the observed d(001) values for Kaol. These values are indicative of the various arrangements of

Me molecules between Kaol layers. During the process of intercalation, a fraction of the organic cations is intercalated

while another fraction is covalently linked to the aluminol surfaces. The relationship between Kaol-Me complexes

(Kaol-DMSO-Me 9.6 Å phase, Kaol-NMF-Me 8.6 Å phase, and Kaol-U-Me 8.8 Å phase) and pre-intercalation

complexes (Kaol-DMSO, Kaol-NMF, and Kaol-U) is thought to be related to Me molecules having different angles

between Kaol layers.

It has been reported that an increase of structural disorder led to an obvious weakening of reflections 11l and 02l (2θ

between 17 and 27°), which were replaced by a broad peak indicative of scattering with weak modulations (Hinckley,

1963; Frost et al., 1998a, 1998b). This disorder is due to the intercalation molecules breaking the hydrogen bonding

between adjacent Kaol layers. The intercalation of molecules into Kaol causes the expansion of the Kaol layers in the c

direction, and results in significant changes to the Kaol surface properties. For example, intercalation can cause an

increase in surface areas and provide more surface hydroxyl, which are now more readily available for chemical

reactions. When the Kaol was first intercalated with DMSO, NMF, or U and then reacted with Me, a new product was

formed which is characterized by a basal spacing ranging between 8.6 Å and 9.6 Å, depending on the pre-intercalated

reagent. Tunney and Detellier (1996) reported that water is formed as a byproduct during the reaction and the intercalated

guest species (DMSO or NMF) are expelled during the course of reaction. However, the work of Matusik et al (2012b)

was the only investigation to propose that two types of Me molecule arrangements existed in the interlayer space in the

10

wet state: (1) molecules grafted to an octahedral sheet in the form of a methoxy group (Al-O-C bond); and (2) mobile Me

and/or water molecules kept in the interlayer space via hydrogen bonds that can be partially removed during drying.

3.2.Infrared spectroscopy

IR analysis of Kaol intercalation complexes is useful for understanding the mechanism of the intercalation reaction

and the structural arrangement of intercalated molecules in the interlayer of Kaol (Ledoux and White, 1964; Frost et al.,

1998b, 2000a, 2000b). The IR spectra for the original Kaol, Kaol-DMSO, Kaol-NMF, Kaol-U, Kaol-DMSO-Me,

Kaol-NMF-Me, and Kaol-U-Me was present in Fig.2. Ledoux and White (1964) reported that the unit cell of Kaol shows

the existence of four hydroxyl groups. One hydroxyl group, termed the inner hydroxyl, lies in the ab plane and points

toward the apical oxygen, the bridging atom between the siloxane and gibbsite-like surface. These inner hydroxyls refer

to the OH groups in the plane common to the octahedral and tetrahedral sheets and to OH groups at 4.4 Å, all having

their dipoles directed towards an empty octahedral site. The other three hydroxyl groups, termed the inner surface

hydroxyl groups, lie at angles between 65° and 73° to the ab plane, point away from the surface, and hydrogen bond to

the oxygens of the next adjacent siloxane layer. Inner surface hydroxyls refer to OH groups located at the surface of the

octahedral sheets opposite the tetrahedral oxygen molecules of the adjacent Kaol layer and having their OH dipole

normal or nearly normal to the (001) plane. In terms of spectroscopy, these hydroxyls function as in-phase and

out-of-phase vibrations. Four distinct bands are observed in the infrared spectrum of the original Kaol at 3621, 3650,

3668, and 3696 cm-1. The three higher frequency bands (3696, 3668, and 3650 cm-1) are assigned to OH stretching modes

of the three inner surface hydroxyl groups. The band at 3621 cm-1 is attributed to the stretching frequency of the inner

hydroxyl groups of Kaol that are aligned parallel to the direction of the (001) layers and towards the unoccupied

octahedral hole. After DMSO intercalation, the bands observed at 3538 and 3503 cm-1 are assigned to the stretching

frequency of coupled inner-surface hydroxyl groups that are perpendicular to the (001) plane and hydrogen-bonded to

11

DMSO molecules in the interlamellar space. Duer et al. (1992) reported that the bands at 3662, 3538, and 3503 cm-1 can

be attributed to the formation of moderately strong hydrogen bonds between some of the inner surface hydroxyls facing

the interlayer space of Kaol and the sulfonyl oxygen. After displacement of DMSO by Me, the disappearance of the two

bands at 3538 and 3503 cm-1 and the appearance of two bands at 3600 and 3550 cm-1 were observed. However, it is

worth noting that the band 3550 cm-1 is due to the formation of hydrogen bonds between some of the inner surface

hydroxyls and intercalated Me molecules. Upon intercalation of NMF molecules, bands were observed at 3678 and 3419

cm-1, and are assigned to hydrogen-bonded OH stretching and NH stretching, respectively. The band at 3419 cm-1

originates from the formation of hydrogen bonds between NMF and the inner surface hydroxyl groups of Kaol (Caglar et

al. 2013). After displacement intercalation of Kaol-NMF with Me, the above two bands are no longer observed, and a

band at 3550 cm-1 is detected. These changes indicate that the hydrogen bonding pattern varied based on Me treatment. In

the case of Kaol-U, new bands at 3410 and 3380 cm-1 are assigned to the asymmetric and symmetric NH2 stretching

frequencies, and the band observed at 3503 cm-1 are correlated with the H bonding between the NH2 groups and the O in

the tetrahedral sheet. As a result of displacement intercalation by Me, the characteristic bands at 3410 and 3380 cm-1 for

Kaol-U disappeared and the band assigned to hydrogen bonded OH stretching was detected at 3550 cm-1. The appearance

of the new band indicates the formation of characteristic hydrogen bonding between Me and Kaol. Based on a

comparative analysis of the three Kaol-Me displacement intercalation complexes Kaol-DMSO-Me, Kaol-NMF-Me, and

Kaol-U-Me, three types of Me molecule arrangements are suggested to be present in the interlayer space of Kaol: (1)

molecules grafted to an octahedral sheet in the form of methoxy groups (Al-O-C bond); (2) mobile Me and/or water

molecules kept in the interlayer space via hydrogen bonds that can be removed partially during drying; and (3) a

combination of cases 1 and 2, with both methoxy groups (Al-O-C bond) grafted in and mobile Me and/or water

molecules coexisting in the system after the displacement reaction by Me. The intensity of the inner surface hydroxyl

bands decrease significantly upon intercalation, indicating the insertion of intercalating molecules into the layers. At the

12

same time, the inner hydroxyl bands are broader after the displacement reaction, indicating a disturbance of the inner

hydroxyl group.

The presence of a band at 1658 cm-1 for Kaol and its complexes suggests that co-intercalated water molecules are

present to some extent (Fig.2). In case of Kaol-DMSO-Me, the observed band at 1631 cm-1 indicates that water molecules

were also adsorbed on the surface in addition to being co-intercalated. Furthermore, the bands in this region provide

information regarding any perturbations to Kaol lattice vibrations (1150-1000 cm-1) and any changes in the Al-O-H

bending modes of Kaol (900-980 cm-1). The two most intense vibrations of Kaol at 1039 and 1009 cm-1 remain relatively

unaffected, and are assigned to the in-plane Si-O-Si stretching vibrations of Kaol. By comparing these bands of Kaol and

its intercalation complexes, appearances and/or shifts of several bands are observed. The variations in wavenumber in

this region are complex because the lattice modes and the intercalated molecule also have some influence in this region

of the spectra (Suarez and Garcia-Romero, 2006). The intercalation reaction destroys the inherent hydrogen bonds in

Kaol and presents new bonds (Mellouk et al., 2009). Upon intercalation of NMF, a band is observed at 1682 cm-1 and is

attributed to the carbonyl stretching vibration. After reaction with Me, the intensity of this band almost disappears and a

broad band around 1658 cm-1 appears. Because it is known that most NMF molecules had been deintercalated, the band

at 1658 cm-1 was assigned to water bending vibrations, indicating that some amount of water is present in the Kaol-Me

(Komori et al., 2000). A band is also observed at 1537 cm-1, and is attributed to the NH2 bending mode. For Kaol-NMF

intercalation complex, the N-H in-plane bending vibration is observed at 1422 cm-1, and the symmetric deformation band

of the CH3 group is observed at 1381 cm-1. It has been proposed that this shift of the CH3 group bands for NMF is due to

the effects on the interlayer environment (Horváth et al., 2010). The bending vibrational band of the C-H unit in NMF

was seen to shift to 1381 cm-1 as a result of hydrogen-bonding with inner surface OH groups in the intercalation complex.

Upon intercalation of U molecules, the bands observed at 1631 and 1583 cm-1 were assigned to stretching modes of the

free and hydrogen bonded C=O group, respectively. Following the reaction with Me, a band is observed at 1658 cm-1,

13

and is assigned to water bending modes within the interlayers of Kaol (Liu et al., 2009). The fact that this band is

observed suggests that there are water molecules in the structure of intercalated Kaol, and that this complex is different

than the interlayer structure of Kaol alone. It is important to note that the spectra in this region not only provides

information about the nature of the inserted molecule, but also displays the characteristic bands of OH deformation. The

bands in the region of 1150-980 cm-1 provide information regarding any perturbations to Kaol lattice vibrations as well as

any changes in the Al-O-H bending modes of Kaol. The Kaol intercalation caused significant changes to the relative

intensities of the bands at 742 and 784 cm-1, which are typical of the OH translational vibrations. It was observed that the

intensity of the OH translational vibrational band (784 cm-1) decreased when the Kaol reacted with methanol. The bands

at 912 and 943 cm-1 have been attributed to OH bending vibrations. By comparing the original Kaol spectra to that of its

intercalation complexes, it was found that an increase in the intensity of the band at 646 cm-1 coincided with a decrease in

intensity for the band at 692 cm-1. It was reported that two strong bands at 998 and 1031 cm-1 were assigned to the

Si-O-Si in-plane vibrations, and the band occurring at 1124 cm-1 is the Si-O stretching mode (Frost, 1995).

Understanding these variations in wavenumber is complex because the lattice modes and the intercalated molecules also

have some influence on this region of the spectra (Suarez and Garcia-Romero, 2006). Based on these results, it is

proposed that the hydrogen atoms of the methyl group form hydrogen bonds with the oxygen atoms of the Si sheet for

the grafted Kaol with a spacing layer of 8.6 Å and 8.8 Å.

3.3.Solid-state 13C NMR analysis

The 13C MAS NMR spectra of Kaol and its intercalation complexes are shown in Fig. 3. No 13C signal was observed

in the spectrum of the native Kaol (not show). Two signals observed for Kaol-DMSO at 44.1 and 44.3 ppm were

attributed to the methyl group located parallel to the sheet and the methyl group keyed into the ditrigonal hole of the

silicate sheet. According to the report by Thompson (1985), the two equally intense methyl carbon resonances at 43.7 and

14

42.5 were due to nonequivalent chemical environments. This conclusion was reached because in the liquid state the two

methyl carbons are chemically equivalent. When Kaol is intercalated by DMSO the two methyl groups are held in

chemically nonequivalent position, paralleling the explanation for the two Si resonances observed in untreated Kaol. The

13C NMR spectrum of Kaol-DMSO contained two resonances at 43.1 and 44.2 ppm (Duer et al. 1992). These were

assigned to two nonequivalent 13C atoms in the same DMSO molecule: one methyl group is thought to be keyed into the

ditrigonal holes in the silicate sheet, and the other being approximately parallel to the sheet (Duer et al., 1992). An

explanation given by Thompson (1985) proposed that the two methylene carbons could be nonequivalent due to a small

difference in the C-S bond lengths, as is found in solid DMSO. However, the static 13C study of solid DMSO detected

only a single resonance at 40 ppm. Therefore, the doublet observed in the spectrum of Kaol-DMSO complex probably

does not arise from small differences in the C-S bond lengths, but rather can be assigned to two 13C resonances from two

different DMSO sites within the unit cell. For the intercalation complex Kaol-NMF, the signal at 27.2 pm can be

unambiguously attributed to the methylene carbons of the intercalated NMF molecule, while the signal at 164.5 ppm is

assigned to carbons of the carbonyl groups for the NMF molecule in the interlayer of Kaol. Xie and Hayashi (1999a,

1999b) reported that the peaks observed at 164.2 and 26.9 ppm were due to the carbonyl and methyl groups, respectively,

of the interlayer guest molecules. The author of this paper also reported that the residual NMF in the sample has chemical

shift values of 165.2 and 27.3 ppm, which are different than the values of the intercalated NMF. In the case of Kaol-U,

one signal at 162.7 ppm is observed for the carbonyl carbon of the intercalated urea molecule (Cai et al., 2006).

Upon displacement intercalation by Me, the spectrum of Kaol-DMSO-Me no longer shows the resonances

corresponding to Kaol-DMSO. A signal at 51.3 ppm, attributed to the methyl carbon of the intercalated Me molecule

(Letaief and Detellier, 2009), is the only signal observed. This result indicates that DMSO was completely replaced by

Me during the reaction process. Similarly, the disappearance of all signals for Kaol-NMF after displacement by Me

indicates that the intercalated Me is rigidly fixed in the interlamellar space of Kaol.

15

3.4.Solid-state 1H NMR analysis

1H MAS NMR studies have proven very useful for chemical structural studies of intercalated Kaol. 1H MAS NMR

spectra of Kaol and its intercalation complexes was shown in Fig. 4. Kaol has an intense peak at 4.9 ppm which is due to

the host hydroxyl protons (Xie and Hayashi, 1999a). The 1H MAS NMR spectrum of Kaol-DMSO shows a peak at 1.3

ppm with a shoulder at 3.0 ppm ascribed to the methyl 1H and the hydroxyl group for Kaol. The spectrum of Kaol-NMF

sample has a sharp central peak at 2.7 ppm caused by the methyl 1H, the peak at 4.5 ppm is attributed to the hydroxyl

group of Kaol and the signals observed at 8.1 ppm due to the HCO protons respectively of the intercalated NMF

molecules. It was previously reported that the chemical shifts observed for the amide proton in neat NMF liquid are 7.87

and 7.65 ppm, whereas the corresponding value in Kaol-NMF is observed at 8.1 ppm (Xie and Hayashi, 1999a). The 1H

isotropic chemical shift values may serve as a practical tool for analyzing hydrogen bonds, where hydrogen bonds cause

a high-frequency shift in the 1H resonance (Xie and Hayashi, 1999a). The obvious increase in the chemical shift upon

intercalation can be seen as evidence of the formation of a hydrogen bond between the amide proton and the adjacent

silica sheet of Kaol host at room temperature, as was also suggested by the crystal structure data. The spectra collected

for Kaol-U has a similar central peak at 1.4 ppm attributed to the NH protons, and a second peak at 5.8 ppm showing

evidence of the hydroxyl group of Kaol. After displacement intercalation by Me, the sample Kaol-DMSO-Me has two

peaks located at 1.4 and 3.1 ppm, which are ascribed to the methyl protons and the hydroxyl group for Kaol, respectively.

The spectrum collected for Kaol-NMF-Me sample has a peak at 1.3 ppm due to the methyl protons and a shoulder at 3.5

ppm due to the hydroxyl group for Kaol. The Kaol-U-Me sample spectrum shows signals at 1.5 due to the methyl protons,

and 5.0 ppm due to the hydroxyl group of the intercalated Kaol.

The 1H MAS NMR data for Kaol proton signals in the literature occur at a rather broad range of chemical shifts.

Wang et al. (2002) assigned signals at 2.4-3.0 ppm to the inner surface hydroxyls for Kaol. Similarly, Fitzgerald et al.

16

(1996, 1997) reported that the signal due to the pyrophyllite mineral showed a peak at 2.4 ppm, and is assigned to OH

groups in the gibbsite like AlOH layer. On the contrary, Hunger et al. (1991) reported that the 1H NMR data on Kaol

proton signals were observed at higher chemical shifts, e.g. 4.0±0.2 ppm, 5.0±0.2 ppm and 5-6 ppm. These results would

indicate that the sharper peak observed in the Kaol spectrum at 4.9 ppm could stem from the inner surface hydroxyl

proton Al-OH-Al on the exposed aluminum hydroxide basal surface of Kaol. It can be observed a broad peak at ~1.4 ppm

with a shoulder at 1.0 ppm, which might be due to the signal of mobile methanol molecules in the interlayer of Kaol.

Therefore, the signal at 4.9 ppm for the 1H NMR spectrum of Kaol should be attributed to the chemical shifts of the inner

surface hydroxyls for Kaol.

4. Computational simulations

4.1.Interaction Energies

The interaction energy (IE) of the first reaction is defined as the energy difference between the reactants Kaol and

Me and the product Kaol-Me (as shown in interaction (1) of Table 1), while for the second reaction it is defined as the

energy difference between the reactants pure Kaol, Me, and water, and the product Kaol-Me-W (as shown in interaction

(2) of Table 1). For the process of Me grafting in Kaol and the movement of formed H2O out of Kaol layer (as shown in

interaction (3) of Table 1), the IE is defined as the energy difference between the reactant Kaol and Me, and the products

grafted Kaol and isolated H2O. For the process of Me grafting in Kaol and the formed H2O remaining in Kaol layer (as

shown in interaction (4) of Table 1), the IE is defined as the energy difference between the reactants Kaol and Me, and

the products grafted Kaol and H2O within the layer. The four intercalation modes calculated based on the above criteria

with layer spacing of 8.6 Å are depicted in Figs. 5-6, while the corresponding information for Kaol complexes with layer

spacing of 8.86 and 9.6 Å are shown in supporting information.

As shown in Table 1, the IEs of the four possible reactions are ordered interaction (4) > interaction (2) > interaction

17

(3) > interaction (1), specifically; the likelihoods of the models for intercalating Me into Kaol layers are in the order

Kaol-gMeO-W > Kaol-Me-W > Kaol-gMeO > Kaol-Me. These results mean that the water existing in Kaol layer is

inevitably producing the reaction between Me and Kaol. The IR band observed at 1658 cm-1 is evidence of water bending

modes within the interlayers of Kaol. Comparing the IE of the same intercalate model with different basal spacing, it was

found that with an increase of the d(001) values, the corresponding IEs of the same reaction decreased. After comparing

these four interactions, it was noticed that the existence of H2O can dramatically increase the IE of intercalated molecules

with Kaol, specifically; water molecules can promote Me intercalation into Kaol layers. For Kaol with a spacing layer of

9.650 Å, it is almost impossible for intercalation to occur without the presence of H2O. In interactions (3) and (4), the

reaction of Me with Kaol has two possible modes: Me reacting with the –OH group oriented perpendicularly to the

surface (-OHv), or Me reacting with –OH group oriented parallel to the surface (OHl). The formation energy of the Kaol

complex in which –OCH3 substituted –OHv was seen to be much stronger than the corresponding complex where –OCH3

substituted –OHl. The –OCH3 perpendicular to the surface leads the methoxy group to make more easily formed

hydrogen bonds with the nearby surrounding atoms.

4.2.Structure

The hydrogen bonds formed between Me molecule and the surface of Kaol layers with layer spacing at 8.6 Å, 8.8 Å, and

9.6 Å are shown in Fig. 5. Me intercalated into Kaol layer through two types of interactions: (a) the interaction of the

hydroxyl oxygen atom with the surface hydroxyl groups of the octahedral Al sheet, and (b) the interaction of the methyl

group and the hydroxyl H atom of the tetrahedral Si sheet with the opposite layer. To provide a better illustration, three

figures with different views are presented for each Kaol layer in Figs. 5-6. As the distance of hydrogen bonding indicates,

the hydroxyl oxygen atom of Me molecule plays a dominant role in the stabilization and the localization of the molecule

in the interlayer space. As our infrared spectra of intercalated Kaol indicated, the band at 3550 cm-1 is due to the

formation of hydrogen bonds between some of the inner surface hydroxyls and the intercalated Me molecules. In the case

18

of Me intercalation in Kaol with a d(001) value of 8.6 Å, the hydroxyl oxygen atom forms hydrogen bonds with two

nearby upright –OH groups in the octahedral Al sheet, with the O-H⋅⋅⋅OMe distances measured at 1.632 Å and 1.688 Å,

with angles of 99.0° and 110.2°, respectively. The hydroxyl oxygen atom also forms a hydrogen bond with one lying –

OH group with an O-H⋅⋅⋅OMe distance of 2.372 Å and angle of 101.7°. As for the interaction of Me with the silicon-oxide

layer, the H atom of hydroxyl forms two hydrogen bonds with the nearby oxygens of the tetrahedral silicate with

OMe-H⋅⋅⋅O distances measured at 1.624 Å and 2.493 Å, and the angles 105.6° and 161.4°. One C-H bond in the methyl of

Me is almost vertical to the silicon-oxide layer, the hydrogen atom in this position forms hydrogen bonds with nearby

oxygens of the tetrahedral silicate, with the CMe-H⋅⋅⋅O distances of 2.524 Å, 2.642 Å, and 2.372 Å, and angles of 111.2°,

106.5°, and 101.7°, respectively. Meanwhile, the other two H atoms of the methyl groups also form hydrogen bonds with

nearby oxygens of the tetrahedral silicate with the CMe-H⋅⋅⋅O distances of 2.709 Å, 2.532 Å, and 2.383 Å, and angles of

102.2°, 92.5°, and 114.0°, respectively. These measurements make it obvious that the oxygen atom in the silicate layer is

involved in weak hydrogen bonding with Me molecule.

The interactions between intercalated Me and Kaol layer, as well as water molecules and Kaol layer, are shown in

Fig. 6. For all types of intercalated Kaol, hydrogen bonds formed between the hydrogen atoms of the methyl and

hydroxyl groups and the oxygen atoms of the Si sheet. Hydrogen bonds also formed between the hydrogen atoms of

water molecules and the oxygen atoms of the Si sheet. Meanwhile, oxygen atoms in the water and Me molecules formed

hydrogen bonds with the hydrogen atoms in the Al sheet. However, there is no observed hydrogen bonding between the

water and Me molecules. For the grafted Kaol with layers spaced at 8.6 Å and 8.8 Å, the hydrogen atoms of the methyl

group form hydrogen bonds with the oxygen atom of the Si sheet. For the grafted Kaol with a spacing layer of 9.6 Å, no

hydrogen bond was observed to form between the methyl group and the opposite Si sheet. It was noticed that throughout

the variety of d-values in Kaol, the oxygen of water forms hydrogen bonds with H atoms of the surface hydroxyl groups

of the octahedral Al sheet, while the hydrogen atoms of water form hydrogen bonds with the oxygen atom of the

19

tetrahedral Si sheet. The grafted methoxy group forms weak hydrogen bonds with the oxygen atom of the tetrahedral Si

sheet in Kaol with the d-values at 8.6 and 8.8 Å. In Kaol with layer spacing of 9.6 Å, there are no hydrogen bonds

formed between the grafted methoxy group and the opposite Si sheet.

5. Conclusions

The intercalation of Kaol with DMSO, NMF, and U expands the structure along the c-axis resulting in the dominant

(001) reflection appearing at 11.2, 10.7, and 10.9 Å, respectively. After treatment of the pre-intercalate Kaol complexes

with Me, the pre-intercalate reagents (DMSO, NMF, and U) were removed from the interlayer space of Kaol through a

concerted replacement by Me. The (001) reflection is 9.6 Å for Kaol-DMSO-Me, 8.6 Å for Kaol-NMF-Me, and 8.8 Å for

Kaol-U-Me, compared with a recorded value dependent on the starting material, 11.2 Å in the case of Kaol-DMSO, 10.7

Å for Kaol-NMF, and 10.9 Å for Kaol-U. These values are indicative of the various arrangements of Me molecules

between Kaol layers. The relationship between Kaol-Me complexes (Kaol-DMSO-Me 9.6 Å phase, Kaol-NMF-Me 8.6 Å

phase, and Kaol-U-Me 8.8 Å phase) and pre-intercalation complexes (Kaol-DMSO, Kaol-NMF, and Kaol-U) is thought

to be related to the differing angles of Me molecules intercalated between the Kaol layers.

The Me intercalated into Kaol layers through two types of interactions – (a) the interaction of the hydroxyl oxygen

atom with the surface hydroxyl groups of the octahedral Al sheet, and (b) the interaction of the methyl group and

hydroxyl H atoms of the tetrahedral Si sheet with the opposite layer. The existence of water can dramatically increase the

IE of intercalated molecules with Kaol, specifically; water molecules can promote methanol intercalation into the Kaol

layers. It was found that with the increase of space between Kaol layers, the IE is seen to decrease. For all types of

intercalated Kaol, hydrogen bonds form between the hydrogen atoms of the methyl and hydroxyl groups with the oxygen

atoms of the Si sheet. Hydrogen bonds also form between the hydrogen atoms of water and the oxygen atoms of the Si

sheet. The Kaol-Me complexes described offer interesting possibilities for the synthesis of Kaol/polymer nanocomposites

with optimized interactions between the clay mineral and the polymer phase.

20

Acknowledgments

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No.

51034006), Beijing Nova Program (xxjh2015B***) and the Open Research Project of State Key Laboratory for Coal Resources and

Safe Mining, China University of Mining &Technology (SKLCRSM14KFB02). The calculation 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.

21

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LIST OF TABLES

Table 1. Interaction energies (IE, in kJ mol-1) of four models of Me and water intercalated in Kaol.a

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Table 1. Interaction energies (IE, in kJ mol-1) of four models of Me and water intercalated in Kaol.a

Interaction Model IE (1) Kaol+Me → Kaol-Me (a-1) -299.9 (b-1) -118.5 (c-1) 68.6 (2) Kaol+Me +water → Kaol-Me-W (a-1) -421.4 (b-1) -215.0 (c-1) -36.3 (3) Kaol+Me → Kaol-gMeO + H2O (a-1-l) -247.2 (a-1-v) -356.0 (b-1-l) -32.2 (b-1-v) -151.0 (c-1-l) 164.9 (c-1-v) 61.2 (4) Kaol+Me → Kaol-gMeO -W (a-1-l) -365.3 (a-1-v) -594.3 (b-1-l) -142.4 (b-1-v) -361.7 (c-1-l) 54.3 (c-1-v) -144.7

a The label “l” and “v” indicate that the corresponding grafted Kaol was produced by Me reacting with –OHl and –OHv, respectively.

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LIST OF FIGURES

Fig.1 The XRD patterns of Kaol, Kaol-DMSO, Kaol-NMF, Kaol-U, Kaol-DMSO-Me, Kaol-NMF-Me, and Kaol-U-Me

Fig.2 The infrared spectra of Kaol, Kaol-DMSO, Kaol-NMF, Kaol-U, Kaol-DMSO-Me, Kaol-NMF-Me, and Kaol-U-Me

Fig.3 13C MAS NMR spectra for Kaol, Kaol-DMSO, Kaol-NMF, Kaol-U, Kaol-DMSO-Me, Kaol-NMF-Me, and Kaol-U-Me

Fig.4 1H MAS NMR spectra for Kaol, Kaol-DMSO, Kaol-NMF, Kaol-U, Kaol-DMSO-Me, Kaol-NMF-Me, and Kaol-U-Me

Fig.5 Hydrogen bonds formed between adsorbates the surface of the Kaol layers with d-values of 8.6Å.

Fig.6 Hydrogen bonds formed between the grafted methoxy group, water and the surface of Kaol layers with d-values of 8.6Å. The label “l” and “v” indicate that the corresponding grafted Kaol was produced by the Me reacting with –OHl and –OHv, respectively.

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Fig. 1

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Fig. 2

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Fig. 3

31

Fig. 4

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(a-1) (a-2) (a-3)

(b-1) (b-2) (b-3)

Fig. 5

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(a-1-l) (a-2-l)

(a-1-v) (a-2-v)

(b-1-l) (b-2-l)

(b-1-v) (b-2-v)

Fig. 6