Applied Clay Science 43 (2009) 357–363

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Applied Clay Science

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Reaction of acid activated montmorillonites with hexadecyl trimethylammoniumbromide solution

Fethi Kooli a,⁎, Yan Liu b, Solhe F. Alshahateet c, Mouslim Messali a, Faiza Bergaya d

a Department of Chemistry, Taibah University, P.O.BOX 30002, Al-Madinnah Al-Munawwarah, Saudi Arabiab Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833 Singaporec Department of Chemistry, Mutah University, POBOX 7, Mutah 61710, Karak, Jordand CNRS-CRMD, 1b Rue de la Férollerie, Orléans 45071 Cedex 02, France

⁎ Corresponding author. Tel.: +966 8460008x1405; faE-mail address: fkooli@taibahu.edu.sa (F. Kooli).

0169-1317/$ – see front matter © 2008 Elsevier B.V. Alldoi:10.1016/j.clay.2008.10.006

a b s t r a c t

a r t i c l e i n f o

Article history:

Acid-activated montmorillo Received 14 July 2008Received in revised form 11 October 2008Accepted 16 October 2008Available online 28 October 2008

Keywords:Hexadecyl trimethylammonium bromideOrganoclaysAcid activationCP/ MAS NMR

nites (AMt) prepared at different acid\montmorillonite ratios were reacted withhexadecyl trimethylammonium (C16TMA) bromide solution. The acid treated Mt incorporated smalleramounts of the surfactants than the parent Mt, due to the reduction of the cation exchange capacity afteracid-treatment. The powder X-ray diffraction patterns exhibited a similar basal spacing of 3.80 nm but withless ordered structure at higher acid\montmorillonite ratios. A certain degree of conformationalheterogeneity was observed by 13C CP\NMR spectroscopy due to the different local environment ofC16TMA+ ions in the interlayer space. The in-situ PXRD patterns showed an increase of the basal spacing oforgano acid-activated montmorillonites when heated at intermediate temperatures (100–200 °C), while thebasal spacing was almost constant in this range of temperature for the organomontmorillonite. Generally, thestability of the surfactant decreased when intercalated into the montmorillonites compared to the pureC16TMABr. This fact implies that the interlayer space influences the decomposition steps.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The treatment of clayminerals, mainly swelling ones, with inorganicacids of high concentration and at elevated temperatures, referred to as“acid activation” or “acid dissolution” (Komadel andMadejová, 2006) iscommonly used for production of relatively inexpensive adsorbents orcatalysts used in industry or environmental protection (Mokaya et al.,1994; Ravichandran and Sivasankar, 1997; Bergaya and Lagaly, 2001;Komadel and Madejová, 2006). During the acid treatment, manychanges occurred in the aluminosilicate structure due to dissolution ofstructural ions and rearrangement of the structure (Komadel andMadejová, 2006). The acid treatment firstly renders the surface of clayacidic. It leaches metal ions from the clay mineral lattice (Komadel andMadejová, 2006). It also increases the external specific surface area ofthe clay and introduces permanent mesoporosity (Hart and Brown,2004; Kooli and Jones, 1997). The proton exchanged clay minerals arenot stable. Autotransformation results in gradual dissolution of metalions, mainly Al3+ (Janek et al., 1997).

When the exchangeable inorganic cations are replaced by organiccations, the clay mineral surface properties are greatly modified (Lagalyet al., 2006). As a result, the organoclay becomes an excellent adsorbentfor poorly water-soluble organic species. The commonly used types oforganic cations for the exchange reaction are tetramethylammonium

x: +966 48470235.

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(TMA+) and its long-chain alkyl trimethylammonium homologues suchas trimethyl phenylammonium (TMPA+), and benzyl trimethylammo-nium cation (BTMA+) (Brixie and Boyd,1994; Kwolek et al., 2003; Lagalyet al., 2006). The quaternary ammonium is adsorbed on the claymineralsurface and the attached long carbon chain renders the interlayer spaceorganophilic, swelling in water is minimized but swelling in organicsolvents or organic fluids is greatly enhanced.

The combination of acid-activation with alkylammonium intercala-tion provides a potential enhancement of the clay mineral properties.The clay mineral presents a proton rich environment when mixed toorganic molecules. On the other hand, the catalytic abilities can beimproved by providing organic cations in the interlamellar space, whichenable the access to/and interact effectivelywith organicmolecules. Thecatalytic properties in these systems are enhanced, the effect of differentacid-treatments on organoclays formed from clay minerals of differentinitial composition was studied by evaluating the resistance of theseorganocations to acid leaching (Breen et al., 1997). These catalysts werealso tested in the isomerization ofα-pinene to camphene and limonene(Breen et al., 1997).

The ammonium salt used for the most organoclay preparations hasbeen hexadecyl trimethylammonium (C16TMA) from its bromide orchloride forms (Jaynes and Boyd, 1991; Dentel et al., 1995; Guangyaoet al., 1996; Zhao et al., 1996; Boyd and Mortland, 1998). Recently, theeffect of the counteranionwas studied on the intercalation properties ofC16TMA into clayminerals and layered silicates such asmagadiite (Kooliet al., 2006a,b). The effect of acid activation on the intercalation of

Fig. 1. Cation exchange capacity (CEC, ▲) and amount of C16TMA intercalated (●) byacid activated montmorillonites.

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C16TMA+with different anionswas also investigated using bromide andhydroxide solutions (Kooli and Magussin, 2005; Kooli et al., 2005). Theacid-activation process affected the amount of intercalated C16TMA+

from its hydroxide form on acid-activated clay minerals and magadiites(Kooli et al., 2005, 2006b). However, there was not a systematic ordetailed study on the intercalation of these cations from their bromideform using acid activated clay minerals treated at different ratios. Fewstudies were reported on the intercalation of C16TMA+ cations into acidactivated clay minerals at different given acid\clay mineral ratios,although they were often used as an intermediate stage to preparemesoporous or porous clay heterostructures (Pichowicz and Mokaya,2001; Linssen et al., 2002; Kooli et al., 2006c). In this study, we report astudy on the acid treatment effect of a selected montmorillonite atdifferent acid\montmorillonite ratios on the intercalation properties ofC16TMA+ from its bromide solution. The thermal properties will be alsoreported using in-situ X-ray diffraction at increasing temperatures.

2. Experimental

2.1. Materials

A Ca2+-montmorillonite (STx-1, with CEC of 0.92 meq/g) wasobtained from the Source Clays repository, Purdue University (USA)and used as received. The method of acid-activation has beendescribed in detail elsewhere (Kooli and Jones, 1998). Acid activationwas accomplished by mixing the Mt with aqueous sulfuric acid(maintaining an acid/Mt ratio of 20mL g−1) at 90 °C for overnight. Thisw\w ratio calculated by using the dry mass of montmorillonite andH2SO4 (98%) was varied between 0.1 and 0.5. These acid-activatedmontmorillonites (AMt) were repeatedly washed with distilled wateruntil free SO4

2− ions, as tested by BaCl2. The samples (noted 0.1, 0.2, 0.3,0.4 and 0.5AMt) were dried at room temperature.

2.2. Preparation of organo acid-activated clays

Amounts of 1 g of AMt was dispersed in a solution containing 0.3 gof C16TMABr in 25 g of deionized water. The suspension was stirredfor overnight at room temperature. The material was collected byfiltration, washed with deionized water until free of Br- ions, as testedby AgNO3, and dried at room temperature. The sample will beassigned as C16TMA-XAMt (where X is equal to acid to montmor-illonite ratios (w/w) 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5). In some cases,C16TMA-AMt was prepared at different quantity of the organic cation.The resulting organoclay was identified by the amount in mmol per gof organic cations adsorbed.

2.3. Characterization

The powder X-ray diffraction (PXRD) patterns were recorded on aBruker Advance 8 diffractometer (Ni-filtered Cu-Kα radiation). In-situhigh temperature PXRD patterns between room temperature and425 °C, were recorded using an Anton Parr heating stage KT450, undernitrogen atmosphere. The cation exchange capacity (CECs) ofmontmor-illonite and AMt were measured by the micro-Kjedahl method(Mackenzie, 1951). The contents of carbon, nitrogen, and hydrogen inthe organoclayswere determinedbyEUROEAelemental analyzer. Thesedata were used to estimate the intercalated amounts of C16TMA+.Thermogravimetric analysis (TGA)was performed on the TA instrumentcalorimeter, SDT2960, in airflow100mlmin−1 heated from25 to 900 °C,at a heating rate of 5 °C min−1. Solid-state nuclear magnetic resonance(NMR) experiments were performed on a Bruker 400 spectrometeroperating at 29Si NMR frequency 78MHz. A 4-mmmagic-angle spinning(MAS)probeheadwasusedwith sample-rotation rates of 4.0 kHz for 29SiNMR experiments. 80–100 scans were accumulated with the recycledelay of 200 s. The 29Si chemical shift is reportedwith tetramethylsilane(TMS) taken as reference. 1H–13C cross-polarisation solid-state NMR

(1H–13C CP/MAS) were acquired with a Bruker Advance DSX400spectrometer operating at 400.16 MHz for 1H and 100.56 MHz for 13Cwith a MAS triple resonance probehead using zirconia rotors 4 mm indiameter. The spinning ratewas 4.0 kHz, 1H π/2 pulse lengthwas 4.40 μsand the pulse delay 10.0 s. 13C chemical shifts were quoted in ppmwithrespect to TMS. The specific surface area and pore volume of the acidactivated clays and organoclays were measured by nitrogen sorptionusing a quantachrome Autosorb 6 instrument. Prior to analysis, thesamples were degassed under vacuum at 120 °C, overnight. The BETequation was used to calculate the surface areas, while the total porevolume have been calculated from the adsorbed amount at relativepressure p/po=0.95.

3. Results and discussion

3.1. C, H, N contents

Gradual increase of adsorbed C16TMA+ cationswas observed as theinitial loading concentrations arise, then the amount of organiccations adsorbed reached a plateau. The value of this plateau dependson the extent of acid activation extent (Fig. 1). The parent Mt containsthe largest amount of intercalated C16TMA cations. With increasingextent of acid-activation, the surfactant value decreases (Table 1).Similar results were obtained for the alumina and zirconia pillaredacid-activated saponites (Kooli and Jones, 1998). The surfactantamount decreases with increasing the acidic treatment, and couldbe related to the decrease in CEC during acid activation, as alreadyshown by Bovey and Jones (1995). Previously, different results wereobtained by using C16TMAOH solution. The content of C16TMA+

increased with the increase of the extent of acid-activation process(Kooli et al., 2005). The amount of adsorbed surfactant was distinctlyhigher than that predicted by the CEC of the starting montmorillonite,and could be attributed to a partial adsorption of C16TMA+ cations onamorphous silica formed during the acid-activation (Pradubmooket al., 2003). However in this study, the surfactant content decreasesas the silica phase increases with acid\montmorillonite ratio. Theadsorption of long chain quaternary ammonium ions on clay mineralsinvolves at least three reactions i) cation exchange, ii) adsorption ofion-pairs and iii) chain–chain interactions (Zhang et al., 2003). Thearrangement and the orientation of the intercalated surfactant

Table 1Elemental analysis of different organo-montmorillonites

Samples CEC meq g−1 C% H% N% Mass lossa (%) Intercalated (mmol/g)

C16TMA-0.0Mt 92 28.14 5.40 1.87 33.51 1.22C16TMA-0.1Amt 89 24.84 4.84 1.64 28.59 1.08C16TMA-0.2AMt 81 21.40 4.33 0.88 25.42 0.93C16TMA-0.3 AMt 75 20.52 4.03 0.78 21.79 0.90C16TMA-0.4 AMt 69 19.61 3.88 0.71 22.01 0.85C16TMA-0.5AMt 64 18.26 3.55 0.68 21.73 0.80

a from mass loss in TG run in the range 200 – 400 °C.

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strongly dependent on the Van der Waals interactions and lead to abilayer of alkyl chains (called paraffin-type arrangements) (Lagalyet al., 2006). Thus, the C16TMA+ cations are initially adsorbed bycation exchange in the interlayer space and on the external surface. Asloading increases, C16TMABr is adsorbed via van der Waals interac-tions (Xu, and Boyd, 1995; Lagaly et al., 2006).

Fig. 3. Basal spacing of Mt after exchange with loading concentrations of C16TMABr(mmol).

3.2. X-ray diffraction

In the PXRD patterns of the C16TMA-AMt the degradation of thelayered structure is indicated by the progressive loss in intensity andthe broadening of the reflections as the severity of acid treatmentincreased (Bovey and Jones, 1995; Kooli and Jones, 1998; Komadel andMadejová, 2006). The parent Mt (with a basal spacing of 1.54 nm,Fig. 4) shows a good stability toward the acid treatment with a certainloss in intensity of the (001) reflection, but without undergoing linebroadening due to the partial decomposition of the clay minerallayers. After reaction with C16TMABr solution, a basal spacing of3.80 nm was observed (Fig. 2). This expansion of the basal spacingindicates that Ca2+ or protons are replaced by C16TMA+ cations. TillX=0.3, the PXRD patterns exhibited three 00l orders showing wellordered phases of the organoclays. However, for X=0.4 and 0.5, thePXRD patterns exhibited a main reflection at 1.92 nm with a broadreflection at 3.90 nm. The broadening of the reflection at lower angleis related to the degradation of the clay mineral during the acidactivation process. The reflection at 1.92 nm could be either a secondorder of the phase at about 3.90 nm or a new phase as confirmed bythe in-situ XRD studies (described in Section 3.6).

Fig. 2. PXRD patterns of obtained organo-montmorillonites after exchange withC16TMABr solution (2.47 mmol).

A gradual increase of the basal spacing from 1.80 nm to 2.15 nmwas observed with increasing surfactant concentration up to1.23 mmol C16TMABr. At 1.64 mmol, an abrupt expansion to3.70 nm was observed, and for higher initial concentrations nosubstantial further change in the basal spacing was noted (Fig. 3).Similar trends were observed for 0.3AMt compared to Mt, but thestructure was better ordered at higher initial loading (Fig. 4). It isinteresting to notice that another studied Mt (Los trances) and its acidactivated counterpart behave differently with a basal spacingincreasing from 1.54 nm to only 1.84 nm (Kooli and Magussin,2005). This difference may be related to the charge distribution of theclay layers (Lee and Kim, 2002).

After subtraction of the layer thickness (0.96 nm), the basal spacingof 3.80 nm corresponds to an interlayer space of 2.84 nm. This valueexceeds the length of the full extension of C16TMA+ cations about

Fig. 4. PXRD of 0.3AMt exchanged with different initial loading concentrations ofC16TMABr (mmol).

Table 2Textural properties of the organo-montmorillonites

Samples SBET (m2 g−1) Pore volume (mL g−1)

C16TMA-Mt 25 (80)a 0.080 (0.13)C16TMA-0.1AMt 22 (98) 0.084 (0.16)C16TMA-0.2AMt 27 (150) 0.091 (0.26)C16TMA-0.3 AMt 29 (164) 0.081 (0.30)C16TMA-0.4 AMt 38 (176) 0.096 (0.32)C16TMA-0.5AMt 49 (183) 0.090 (0.31)

a corresponds to the starting montmorilloinites.

Fig. 5. 13C CP/NMR spectra of different organo-montmorillonites. The pure solidC16TMABr is presented for comparison.

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2.20 nm (Slade and Gates, 2004). Similar situation was previouslyreported for the C16TMA surfactant from its hydroxide form and usingacid activated clays at acid/montmorillonite ratios N0.2 (Kooli et al.,2005). The interlayer spacing of 1.24 nm derived from the basal spacingof 2.20 nm reflects a pseudotrimolecular layer arrangement of C16TMA+

cations between the clay mineral layers (Janeck and Lagaly, 2003).

3.3. Microtextural studies

It iswell known thatMt is capable of interlamellar expansion and it isoften found that larger organic cations may act as pillars, increasing thespacing between the clay layers. This behavior could lead to specific BETsurface area increase or decrease, depending on the arrangement, andthe packingof organic cations in the interlayer spacing (Boydet al.,1988;Lee et al., 1999). The specific BET surface area of the raw clay increasedduring acid-activation process, due to the formation of amorphous silicaphase (Kooli et al., 2005). After exchange with C16TMABr solution, thespecific BETsurface areadecreases significantly. The organic cationsmayblock the access of nitrogen molecules to the adsorption sites and thepore network (Table 2). The values of specific BET surface area areindependent of the density of C16TMA+ cations in the interlayer space.The specific BET surface area values of the prepared organoclays agreewell with the values reported in the literature (Wang et al., 2004).

3.4. 29Si and 13C NMR data

The changes in 29Si MAS NMR spectra of the starting Mt (STx 2) andits acid-activated counterparts have been reported in detail recently(Kooli et al., 2005). The contribution from the Q3 type Si species at−93 ppm decreased, and that of Q4 type Si species in the silica phase at−110 ppm increased as the acid\montmorillonite ratio increased.Additional weak resonances in the range of −100 to −105 ppm wereattributed to Si bound in amorphous silica with a three-dimensionalcross-linked framework (Komadel et al., 1996). After exchange withC16TMBr solution, the features of the spectra did not change andindicated that the startingmaterials were intact and the structure of claymineral layers and amorphous silica were preserved. The pH ofC16TMABr solution was not strong enough to affect the host montmor-illonite. However, using C16TMAOH solution, the 29Si spectra weredifferent due to the dissolution of the amorphous silica phase.

NMR spectroscopy has been proven to be one of the most powerfultechnique for probing structure, conformation and dynamics ofsurfactants molecules at interfaces (Grandjean, 1988; Gao and Reven,

Table 313C CP MAS NMR chemical shift of solid C16TMABr and organo-montmorillonites

Carbon atom C16TMABr (solid) C16TMA-Mt C16TMA-0.3AMt

C1 63.1 (64.4) 66.3 66.7CN(N–CH3) 56.2 (56.8) 54.3 54.3C14 – (36.6) – –

C4–13 32.4 (34.5) 33.6 33.6C2 – (31.6) – 31.6C3,15 24.5 (26,5/ 25,7) 25.3 25.2C16 16.5 (18.6) 16.1 16.0

– not detected. Values in brackets are taken from Zhu et al., 2005.

1995; Badia et al., 1996). Solid state 13C NMR was used to investigatethe conformation heterogeneity and mobility of surfactant moleculesin intercalated smectites and layered materials (Khimyak andKlinowski, 2001; Simonutti et al., 2001; Kubies et al., 2002; Kooliet al., 2006b). The C16TMA cations exhibited two main resonancepeaks associated with backbonemethylene groups, i) the resonance at33 ppm corresponds to the ordered (all-trans) conformation and ii) theresonance at 30 ppm to the disordered gauche conformation (Table 3,Zhu et al., 2005). In the 13C CP/NMR spectrum of pure C16TMABr oneresolved resonance peak at 33.6 ppm indicated that the surfactantsexhibited mainly all-trans conformation, while a certain amount ofgauche conformation (resonance peak at 31.6 ppm) in addition to all-trans conformation was detected for C16TMA-0.3AMt (Fig. 5). Similarbehavior was observed for the other AMt with higher acid\clay ratios.The area ratios of ordered conformation at 33.6 ppm to the disorderedconformation at about 31.6 ppm are 100:1 and 20:6, respectively.

Fig. 6. DTG curves of Mt treated at different acid\montmorillonite ratios.

Fig. 8. In-situ PXRD patterns of some selected montmorillonites calcined at 400 °C.

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Although the PXRD patterns showed that all the intercalatedmaterialsexhibited similar interlayer spacing, the presence of trans and gaucheconformations detected by NMR could be related to the difference inthe local environment of C16TMA+ cations in the interlayer space,which was affected by the acid treatment. The heterogeneity of theC16TMA+ conformation was also related to the counteranion usedduring the exchange reaction. Indeed, using C16TMAOH solution,mainly gauche conformation was obtained with a minor contributionof the trans conformation, using the same AMt (Kooli et al., 2005). Thisdifference could be related to a better ordered structure due to therearrangement of the clay layers during the intercalation process. TheCN and C1 resonance peaks of the C16TMA intercalates are moreintense compared to those of C16TMABr whereas the sharp C16resonance peak has a higher intensity in C16TMABr. This fact reflectedthe difference in the mobility of these carbons in the differentmaterials, and the broadness of the peaks indicated that the C16mobility was reduced between the claymineral layers (Ishikawa et al.,1991; Kubies et al., 2002).

3.5. Thermogravimetric data

The DTG curve of Mt shows two mass losses at 71 and 137 °C(Fig. 6), due the release of physisorbedwatermolecules, and interlayerwater directly attached to the hydrated Ca2+ cations via H bonds, in therange of 50 to 200 °C. The only thermal effect between 500 and 800 °Cis connected with the dehydroxylation process of Mt structure. As theacid\montmorillonite ratio increases, the first mass loss remainsrelatively constant. The secondmass loss at 137 °C disappeared, due tothe exchange of Ca2+ ions by protons. The mass loss due thedehydroxylation of the silicate layers decrease, indicating that a partof the hydroxyl groups are removed during the acid treatment (Breenet al., 1995; Kooli and Jones, 1997). The temperature of dehydroxyla-tion (obtained from the derivative curve) decreased from 653 to572 °C as the level of acid activation increases, reflecting the easier lossof the remaining hydroxyl groups after acid treatment.

In air atmosphere, the DTG of C16TMABr exhibits threemass lossesbetween 173 and 320 °C (Fig. 7), due to different fragmentation steps,in addition to another one at higher temperatures related to thecomplete combustion of the residual carbonaceous materials. How-ever, for the intercalated C16TMA+ cations, the loss occurs mainly atone step between 150 and 290 °C followed bya shoulder at 307–317 °C.

Fig. 7. DTG curves of raw and acid treated Mts after exchange with C16TMABr solution.

A continuous mass loss was also observed above 320 °C and could berelated to the complete combustion of the residual carbon materialsand dehydroxylation of the claymineral layers. The DTG curves showaweak peak at low temperatures related to physi-adsorbed water(about 3%), and confirmed the organophilic character of the obtainedmaterials compared to the starting materials. The mass loss due to thesurfactants in the range of 200–400 °C, decreases with the increase ofthe acid\clay ratios, in good agreement with the decrease of the CECand the C, H, N elemental analysis (Table 1). However, the mass lossincreases with the increase of the initial loading concentration of thesurfactant. The intensity of the peak at 235 °C is enhanced and reachesa maximum at loading concentration of 2.47 mmol. The intensities ofthe two peaks at 580 and 680 °C are independent of the loadingconcentrations, and could be related respectively to the completedehydroxylation of the silicate layers and the total combustion of theresidual materials.

The fact that the stability of the surfactant decreases whenintercalated into raw Mt and AMt (Fig. 7) implies that the interlayerspace influences the decomposition steps, the reaction kinetics, theproduct transfer and the volatilization of the evolved species (Xieet al., 2001).

3.6. Thermal stability

The structural changes of the intercalated surfactants between theclay mineral layers are followed by in-situ PXRD. The basal spacing ofmontmorillonite decreased from 1.54 nm to 1.13 nm at 100 °C and to0.98 nm at temperatures above 150 °C. The acid activated montmor-illonited (AMt) heated at 400 °C, showed a higher basal spacing of1.31 nm (Fig. 8). This difference in basal spacing could indicate thepresence of some silica species between the clay mineral layers whichacts as pillars and prevents the collapse of the clay mineral layers. Theordered phase of the calcined products decreases with the increase ofheating temperatures (Fig. 8).

The solid C16TMABr has a lamellar ionic structure composed ofalternating layers of paraffinic alkyl chains and of ionic groups andassociated hydration water. In Fig. 9, in-situ PXRD patterns in nitrogenatmosphere, indicate a continuous increase of the basal spacing from2.6 nm to 3.02 nm between room temperature and 200 °C. At 215 °C,the basal spacing reaches 3.11 nmwith a significant loss of intensity. At250 °C, the crystal structure was completely destroyed due to melting

Fig. 9. In-situ PXRD patterns of pure C16TMABr heated at different temperatures (°C) innitrogen atmosphere.

Fig. 11. In-situ PXRD patterns of C16TMA-0.3Mt heated at different temperatures (°C) inair atmosphere.

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at of 237–243 °C. Similar observations were recorded when the in-situstudies were performed in air atmosphere (Vaia et al., 1994; Kooliet al., 2005).

The in-situ PXRD patterns of C16TMA-Mt in air atmosphere arepresented in Fig. 10. The basal spacing at 3.80 nmwith the presence ofa higher rational orders, remained stable till 150 °C, then decreasedgradually from 3.61 nm at 200 °C to 1.91 nm at 250 °C. A collapse to1.42–1.32 nm was observed at 300–400 °C, due to the decompositionof the surfactants. This value was larger compared to that of startingMt (0.96 nm), due to the residual organic material between the claymineral layers.

The C16TMA-0.3AMt behave differently from the organo-mon-tmorillonite. C16TMA-0.3AMt had a basal spacing at 4.2 nm till 215 °C,which decreased above this temperature to 1.42 nm and reached1.31 nm at 400 °C (Fig. 11). For C16TMA-0.5AMt (Fig. 12) the basal

Fig. 10. In-situ PXRD patterns of C16TMA-Mt heated at different temperatures (°C) in airatmosphere.

spacing also increased from4.10 nm at RT to 4.20 nm at 150 °C, with animprovement of reflection's intensity at lower angle. The position ofthe second reflection about 1.92 nm varies in the same manner as thereflection at 4.10 nm to 2.05 nm suggesting that this reflection wasindeed a second order reflection of the 4.1–4.2 nmphase. Between 200and 250 °C, a continuous decrease was observed from 4.10 to 1.61 nm.The basal spacing decreased at higher temperatures and a basalspacing of 1.41 nm was obtained at 300 °C, due to the collapse of theorganic surfactants.

A difference in the thermal behavior of the pure surfactant andC16TMA intercalated materials is noted by in-situ PXRD as well as byTGA technique. An increase of the basal spacing between 50 and 100 °Chas been observed for other alkylammonium smectites and attributedto themelting of the intercalated surfactants, which creates a fluid-likeenvironment between the layers, enabling local relaxation of residual

Fig. 12. In-situ PXRD patterns of C16TMA-0.5Mt heated at different temperatures (°C) inair atmosphere.

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stress and packing imperfections (Vaia et al., 1994; Wang et al., 2000).In this study, an increase is only observed from 100 °C to 200 °C for theintercalated C16TMA-AMt, and was attributed to the thermal expan-sion of the structure perpendicular to the layers, due to the productionof volatiles and the associated increase of internal pressure within theinterlayer region. This leads to the expansion of the structure (Xie et al.,2001; Kooli et al., 2005) with a better ordered solid phase of the heatedmaterials. Lee and Kim (2003) have associated the continuous increasein the basal spacing of the organoclays up to 200 °C to a dehydrationprocess. However, in our TG experiments, no significant weight lossrelated to water molecules was detected in the range of 50–120 °C.

4. Conclusions

During acid activation, the removal of some charge sites along withthe dissolution of the clay mineral decreased the CEC. Therefore byusing C16TMABr solution, the amount of intercalated C16TMA cationswas lower in the acid-activated Mt than in the raw Mt. The basalspacing of the intercalated C16TMA-Mt which increased significantlyto 3.8 nm was dependent on the initial concentrations of thesurfactant. At higher acid activation level, it was difficult to confirmthe presence of 3.80 nm phase or another phase at 1.92 nm frompowder XRD. However, the in-situ XRD technique at high tempera-tures proved to be an interesting tool to confirm doubtful conclusions,the broad reflection at lower angle detected by PXRD at roomtemperature was indeed the first order reflection of 3.80 nm, and thereflection 1.92 nm was the second order reflection of this phase. The13C CP/NMR studies indicated a certain degree of conformationheterogeneity of the intercalated surfactant in acid-activated mon-tmorillonite, mainly homogeneous conformation was obtained forC16TMA-Mt without previous acid activation, although both materialsexhibit similar basal spacing.

Acknowledgments

The Institute of Chemical and Engineering Sciences, Agency ofScience Technology and Research (A-star) Singapore and TaibahUniversity (Grant 429/220) are gratefully thanked for the financialsupport.

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