Exfoliation Properties of Acid-Activated Montmorillonites and Their

Dec 18, 2008 - Ca-rich montmorillonite (STx-1, with a CEC of 92 mequiv/100 g, Mt) was ... The 29Si and 13C chemical shifts were reported with respect ...
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Langmuir 2009, 25, 724-730

Exfoliation Properties of Acid-Activated Montmorillonites and Their Resulting Organoclays Fethi Kooli† Institute of Chemical and Engineering Sciences, Pesek Road, Jurong Island, Singapore 627833 ReceiVed August 5, 2008. ReVised Manuscript ReceiVed October 12, 2008 The intercalation process of acid-treated montmorillonite clays by a cationinc surfactant (decyltrimethylammonuium) from a hydroxide solution was affected by the temperature of acid activation. Although the cation exchange capacity of the treated clay at 90 °C (0.74 mequiv g-1) was lower compared to that treated at room temperature (0.84 mequiv g-1), the uptaken amount of the surfactant (1.24 mmol g-1) and thus the basal spacing (3.83 nm) were higher. These values depended on the initial loading concentrations. However, when the clay was treated at room temperature, the uptaken amounts of surfactant (0.81 mmol g-1) and the basal spacing (2.20 nm) were lower. These values were independent of the initial loading concentration. The higher basal spacing (3.83 nm) was also affected by the type of the exchange medium and the washing solution by a mixture of ethanolic solutions. The intercalation of the surfactants occurred in two different ways, and was related to exfoliation properties of the acid activated clays. The intercalated surfactant exhibited different conformations in the interlayer space and different thermal stability.

Introduction The expression “activation of clay minerals” covers a wide range of chemical treatments, most of them aimed at producing modifications on the surface of the clay mineral crystals. Besides acid-activated smectites or bentonites, the following are some treatments used: quaternary ammonium exchanged smectites (organophilic bentonites),1 surface modified kaolinites,2 pillared smectites,3 and thermally activated kaolinites.4 During the acid activation, the edges of the crystals are opened, and the Al3+ and Mg2+ cations of the octahedral sheet (from the 2:1 layers) are exposed to the acid and become soluble; the surface pore diameter expands; the degree of crystallinity of the clay mineral, as tentatively evaluated by the peak intensity of the (001) peak is reduced; and the specific surface area increases to a maximum and then is reduced by additional treatment.5 Usually 80% or more of the available surface area correspond to pores with diameters ranging from 2.0 to 6.0 nm, and the surface area can be increased with acid treatment to 200-400 m2 g-1.5 The treatment with mineral acid is also known to impart surface acidity of the clay, which improves its catalytic properties.6 The product of acid dissolution of these minerals is a hydrous, partly protonated amorphous silica phase, but highly disordered tetrahedral layers still persist, which are no longer lying parallel to each other.7 The effect of acid concentration and the time and temperature of acid treatment on the acidity and catalytic activity of the obtained clays have been studied by many researchers.8-10 For catalytic properties, the complete dissolution of parent clay † Present address: Taibah University, Department of Chemistry, P.O. Box 30002, Al-Madinah Al-Munawwarah, Saudi Arabia. E-mail: fkooli@ taibahu.edu.sa. (1) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986, 34, 581–585. (2) Frost, R. L.; Mako´, E´.; Kristo´f, J.; Horvath, E.; Kloprogge, J. T. Langmuir 2001, 17, 4731–4738. (3) Pinnavaia, J. T. Science 1983, 220, 365–371. (4) Kristo´f, J.; Frost, R. L.; Kloprogge, J. T.; Horva´th, E.; Mako´, E´. J. Therm. Anal. Calorim. 2002, 69, 77–83. (5) Komadel, P.; Madejova´, J. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006; Vol. 1, p 263. (6) Breen, C.; Madejova´, J.; Komadel, P. Appl. Clay. Sci. 1995, 10, 219–230. (7) Vicente, M. A.; Suarez, M.; Lopez-Gonzalez, J. D.; Banares-Munoz, M. A. Langmuir 1996, 12, 566–572.

is not desirable. Komadel et al. found mild acid-treated Mg-rich montmorillonite and hectorite to be catalytically active; treatment for a prolonged time or with higher acid concentration resulted in the decrease of activity or complete catalytic inactivity.8 Few studies were reported on further modification of acidactivated clays by inorganic cations or organic ones. For instance, intercalation of inorganic polyhydroxications led to stable pillared acid-activated clays with more mesoporosity and acidity compared to the pillared nontreated clays, which affected their cracking catalytic activities.11-13 The acid-activated clays were also modified with an organic exchange reaction, e.g., the short alkyl ammonium cations leading to a basal spacing of 1.50 nm, and the catalytic properties of these systems for the isomerization of R-pinene to camphene and limonene were reported.14,15 The modification with long alkyl ammonium (cetyltrimethylammonium, C16TMA) cations resulted in organo acid-activated clays with a basal spacing about 2.0 nm, and they were used as intermediate precursors for the preparation of porous clay heterostructures.16 The structural modification of the clay sheets during the acid treatment at higher acid\clay ratios and at 90 °C was the origin of obtaining higher basal spacing of 3.90 nm from C16TMAOH solution. while, at mild acid treatment with lower acid\clay ratios at 90 °C, a basal spacing of 2.0 nm was obtained.17 The high uptaken amount of the C16TMA surfactants above the cation exchange capacity (CEC) is not fully understood. In this study, we focus on understanding the origin of the high uptaken amount of C16TMA cations by acid-activated clays treated under specific conditions. The extent the structural modification of the clay sheets plays a key role on the exfoliation properties during the reaction with quaternary alkylammonium ions from C16TMAOH solution, and a schematic mechanism is proposed. For this purpose, the acid activation of the clay was performed (8) Komadel, P.; Janek, M.; Madejova´, J.; Weekes, A.; Breen, C. J. Chem. Soc. Faraday Trans. 1997, 93, 4207–4210. (9) Bovey, J.; Jones, W. J. Mater. Chem. 1996, 4, 2027–2035. (10) Kooli, F.; Jones, W. Clay Miner. 1997, 32, 633–643. (11) Mokaya, R.; Jones, W. J. Chem. Soc. Commun. 1994, 929–930. (12) Bovey, J.; Kooli, F.; Jones, W. Clay Miner. 1996, 31, 501–506. (13) Kooli, F.; Jones, W. J. Chem. Mater. 1998, 8, 2119–2124. (14) Breen, C.; Watson, R.; Madejova, J.; Komadel, P.; Klapyta, Z. Langmuir 1997, 13, 6473–6479.

10.1021/la802533y CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008

Acid ActiVation of Montmorillonite Clays

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at room temperature (RT) or at 90 °C with a higher acid\clay ratio of 0.3 (w/w) before the reaction with C16TMAOH solution. Detailed characterization of the obtained acid-treated clays and their intercalated counterparts were performed by different techniques. The conformer configuration of the intercalated cations were examined by Fourier transform infrared (FTIR) and 13C cross-polarization (CP)/NMR techniques. The chemical stability of the organo-acid activated clays was modified by the nature of the washing solution, and the thermal behavior of the organo acid-activated clays is examined by mean of thermogravimetric analysis (TGA).

Experiment Sample Preparation. Ca-rich montmorillonite (STx-1, with a CEC of 92 mequiv/100 g, Mt) was obtained from the Source Clays repository, Purdue University (USA) and used as received. The method of acid activation has been described in detail elsewhere.10 Briefly, the montmorillonite was slurred in a solution of water and sulfuric acid (maintaining a solution/clay ratio of 20 mL/g) at RT or at 90 °C for 16 h. An acid/clay (w/w) ratio of 0.3 was calculated using the dry weight of clay and 98% H2SO4. The resulting acidactivated clay was repeatedly washed with deionized water and dried at ambient temperature. The sample is assigned as follows: ACMtRT indicates that Mt was acid treated at an acid\clay ratio of 0.3 at RT. When the acid activation was carried out at 90 °C (instead of RT), the product is designated ACMt-90. The C16TMA-clays were prepared at different amounts of surfactant cations. One gram of each clay was mixed with different amounts (in milliliters) of C16TMAOH (25% solution) and 22 g of deionized water. The mixture was stirred overnight at RT. The resulting products were separated by filtration and washed with deionized water, then air-dried at RT. The intercalated acid-activated clays C16TMA-ACMt-RT and C16TMA-AMCt-90 were prepared from a specific amount of 3 mL of C16TMAOH dissolved into 22 mL of deionized water, and corresponded to an initial loading concentration of 2.47 mmol. To study the effect of ethanol solution on the washing process. The same procedure described above was followed; however, the wet filtrate was washed using a mixture of ethanol and deionized water at different ratios in volume. The effect of the ethanolic solution on the intercalation process was also examined. Three milliliters of C16TMAOH solution was added to 22 mL of a mixture of ethanol and deionized water at 50% (in volume) instead of pure deionized water. The obtained product was washed only with deionized water. Characterization Techniques. The powder X-ray diffraction (XRD) patterns were collected on a Bruker Advance 8 diffractometer (Ni-filtered Cu KR radiation). The elemental analysis of the raw clay before and after acid activation was carried out by an X-ray fluorescence (XRF) Bruker S4 explorer. The content of carbon, nitrogen, and hydrogen of the pure C16TMABr and in the intercalated clays were determined by a EURO EA elemental analyzer. The FTIR spectra were collected using a Digilab Excalibur FTS 3000 series and KBr technique. TGA features were recorded on a TA Instruments, SDT2960. The measurements were carried out in air or nitrogen flow at 100 mL min-1heated from 25 to 800 °C, at a heating rate of 5 °C min-1. Solid-state NMR experiments were performed on a Bruker 400 spectrometer operating at a 29Si NMR frequency of 78 MHz. A 4-mm magic-angle spinning (MAS) probehead was used with sample-rotation rates of 4.0 kHz for 29Si NMR experiments. A total of 80-100 scans were accumulated with the recycle delay of 200s. 1H-13C cross-polarization solid-state (13C CP MAS) NMR were acquired with a Bruker Advance DSX400 spectrometer operating at 400.16 MHz for 1H and 100.56 MHz for (15) Moranta, A.; Ferrer, V.; Quero, J.; Ateaga, G.; Choren, E. Appl. Catal. A: Gen. 2002, 230, 127–135. (16) Kooli, F.; Hian, P. C.; Weirong, Q.; Alshahateet, S. F.; Chen, F. J. Porous Mater. 2006, 13, 319–324. (17) Kooli, F.; Khimyak, Y. Z.; Alshahateet, S. F.; Chen, F. Langmuir 2005, 21, 8717–8723.

Figure 1. Relationship between the initial C16TMAOH/CEC molar ratios and the amounts uptaken by (a) ACMt-Rt and (b) ACMt-90 clays. 13

C with a MAS triple resonance probehead using zirconia rotors 4 mm in diameter. The spinning rate was 4.0 kHz, the 1H π/2 pulse length was 4.40 µs, the contact time was 1.0 ms, and the pulse delay was 10.0 s. The two pulse phase modulated (TPPM) decoupling was used during the acquisition. The Hartmann-Hahn condition was set with hexamethylbenzene. The 29Si and 13C chemical shifts were reported with respect to tetramethylsilane (TMS). The surface area and pore volume of the different clays were measured by nitrogen sorption using a quantachrome Autosorb 6 instrument. Prior to analysis, the samples were degassed under vacuum at 120 °C, overnight.

Results and Discussion Elemental Analysis. During the acid treatment, variable amounts of the structural cations of the clay are removed, depending on the temperature of acid treatment. Indeed, the XRF data (not shown) indicates that the dissolution amount of CaO, MgO, Al2O3 increased along with the treatment temperature. The SiO2 content and the SiO2/Al2O3 ratio increased as a result of partial dissolving of aluminum ions and the formation of amorphous silica. Few traces of calcium cations were present. The leaching of these cations led to the decrease of the CEC value,9 thus the ACMt-RT exhibited higher value (0.84 mequiv g-1) compared to ACMt-90 clay (0.75 mequiv g-1). The acid treatment of the clay mineral at RT has an effect, however, to a lesser extent compared to that treated at 90 °C. Figure 1 depicts the relationship between the amount of intercalated surfactant with the C16TMAOH/CEC molar ratios. As the amount of the added surfactant increases, the uptaken amount of surfactants increases, and then a plateau appears. The saturated amount was 0.84 mmol g-1 for ACMt-RT and 1.24 mmol g-1 for ACMt-90 (Figure 1a,b). These data indicate that ACMt-90 adsorbed more C16TMA cations than ACMt-RT clay. Table 1 summarizes the C and N elemental analysis of C16TMAACMt-RT and C16TMA-ACMt-90 clays at the plateau; solid C16TMABr was analyzed as a reference, and the molar ratio of C/N was about 19, which is closed to that of pure C16TMABr. The surprising high content for the former was not related to the cation exchange reaction, because the CEC of ACMt-90 was lower than that of ACMt-RT. In addition, it could not be attributed to the adsorption on the amorphous silica formed during the

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Figure 2. Proposed models for the different stages during the uptake of C16TMA surfactants with initial C16TMAOH/CEC molar ratios of 1 and above 2. Table 1. C and N Contents of C16TMABr Salt and Different C16TMA Intercalated Clays a samples

C%

C16TMABr 61.52 C16TMA-ACMt-RT 18.9 C16TMA-ACMt-90 28.37 b

I. A. mass N % C/N+ (mmol g-1) lossb (%) 3.74 1.18 1.74

19.19 18.97 19.02

0.84 1.24

99 23 31

a C/N+ corresponds to C/N molar ratios; I. A. is Intercalated amount. Deduced from TGA data.

acid-activation,18 because this phase was dissolved during the exchange reaction (see below). One can anticipate that, when the silica phase is dissolved, the remaining solid may be composed of nonaltered layers, and its CEC may not be lower than that of the original clay; however, the high content still could not explained, because the nontreated clay with a higher CEC value of 0.92 mequiv g-1 adsorbed only an amount of 0.79 mmol g-1.17 The high content could be explained in the case of ACMt90 at C16TMAOH/CEC above 2, by the high pH values of the C16TMA-clay suspensions (above 10). At this value, the surface of ACMt-90 layers became more negatively charged, and thus favored the exfoliation of the clay sheets, and the adsorption of C16TMA cations occurred via electrostatic interactions between the charged surface and the cationic surfactants on both surfaces of the exfoliated sheets, followed by restacking (Figure 2). The proposed scheme is similar to that reported for the pillaring of different layered material types with exfoliation properties.19-21 The intercalation mechanism was modified when ethanolic solution (50% in volume) was used during the exchange reaction of ACMt-90 at a C16TMAOH/CEC molar ratio of 3. Only an amount of 0.78 mmol/g was obtained. This fact could indicate that the easy exfoliation process was modified in the presence of ethanol; as a consequence, the up taken amount could be related only to a cation exchange reaction, with possible physiadsorbed molecules adsorbed on the external surfaces. In the case of ACMt-RT, the adsorption of C16TMA cations occurred via cation exchange reaction, and no exfoliation was achieved at similar or higher C16TMAOH/ CEC molar ratios. This fact could be related to less modification (18) Favoriti, P.; Mannebach, M. H.; Treiner, C. Langmuir 1996, 12, 4691– 4696. (19) Kooli, F.; Sasaki, T.; Rives, V.; Watanabe, M. J. Mater. Chem. 2000, 10, 497–501.

Figure 3. Variation of basal spacing of (a) ACMt-Rt and (b) ACMt-90 with the initial C16TMAOH/CEC molar ratios.

of the clay layers during the acid treatment at RT. The uptaken amount of C16TMA by ACMt-RT was higher compared to that of ACMt-90 reacted in ethanolic solution or after washing with etahnolic solution. This fact is due the higher CEC of ACMt-RT (0.84 mequiv g-1) compared to ACMt-90 (0.74 mequiv g-1). XRD. The powder XRD showed that a partial destruction of ordered structure occurred. Its extent depends on the temperature of acid treatment.10 ACMt-90 exhibited a higher partial destruction compared to ACMt-RT, with decrease in intensity of 001 reflection was observed with a relative increase of the amorphous silica phase in the range between 25 and 30° (see the Supporting Information, Figure 1). After reaction with C16TMAOH solutions at different initial concentrations, the basal spacing of ACMt-RT increased from 1.54 nm to a maximum value of 2.20 nm, independently of C16TMAOH/CEC molar ratios above 2 (Figure 3a). Similar observation were reported for organoclays prepared from C16TMABr solution.22 However, ACMt-90 exhibited different behavior, and two kinds of structures with different basal spacing were obtained (Figure 3b). When the molar ratio of C16TMAOH/ CEC was below 2, the basal spacing was 2.18 nm. With increasing molar ratios above 2, an abrupt increase of the basal spacing occurred from 2.18 to 3.83 nm. Further increasing the C16TMAOH/CEC molar ratios did not change the basal spacing of 3.83 nm. The significant expansion of the basal spacing from 2.18 to 3.83 nm was due to different orientations of the surfactant ions in the host clay sheets from a monolayer paraffin complex (2.18 nm) to a bilayer paraffin structure (see the Supporting Information, Figure 1). At an initial molar ratio of 2, the clay sheets of ACMt90 were exfoliated, and interaction between the negative charge of the clay sheets and surfactant cations occurred in both sides, thus it formed a double layer during the restacking as discussed above. This exfoliation seemed to be difficult to occur in the case of ACMt-RT clay, independently of the molar ratios (see the Supporting Information, Figure 1) This fact could be related to a less modification of the layered structure during the acid activation process.8 Then the organoclay prepared from ACMt-90 (with a C16TMAOH/CEC molar ratio of 3) was washed with ethanolic solution at different compositions (water/ethanol). The resulting powder XRD patterns are presented in Figure 4. The initial

Acid ActiVation of Montmorillonite Clays

Figure 4. Powder XRD patterns of C16TMA-ACMT-90 washed with different mixtures of ethanolic solution (% in volume): (a) 0%, (b) 25%, (c) 50%, and (d) 75%.

expansion of 3.83 nm was observed when the wet intercalate was washed with a mixture solution of 25% of ethanol. However, it decreased to 2.20 nm when solutions of 50% and above were used. For the C16TMA-ACMt-RT, the ethanolic mixture used during the washing process did not affect the basal spacing; it remained constant at about 2.20 nm. Similar observations were reported in the case of vermiculite intercalated with other alkylammonium cations.23 He et al. studied the stability of surfactant within the organoclay in water\ethanol media, however, after drying the organoclays. The decrease of the interlayer spacing was due to the dissolution of the physically adsorbed surfactants and rearrangement of the intercalated surfactants.24 The decrease of the basal spacing of ACMt-90 was related to a partial dissolution of surfactants located between the clay sheets due to the easy accessibility of the ethanol molecules to these surfactants, which was difficult in the case of ACMt-RT. When the intercalation was performed in a mixture of ethanol and water (50%), only a basal spacing of 2.20 nm was obtained, independently of the initial loading concentrations and the used clays. This fact could indicate that the mechanism of intercalation was modified to the cation exchange process, because the full exfoliation process was not achieved in this case for ACMt-90. The alkylammonium ions in the interlayer space of smectite may adopt different orientations from monolayer, double-layer, pseudotrimolecular layer, or paraffin-type structures depending on the magnitude of the layer charges.25 The montmorillonite sheet thickness is about 0.96 nm. The length of fully stretched C16TMA cations was reported to be in the range of 2.20-2.53 nm.26-29 The difference between the different lengths was related (20) Maireles-Torres, P.; Olivera-Pastor, P.; Redriguez-castellon, E.; JimenezLopez, A.; Tomlinson, A. A. G. J. Mater. Chem. 1991, 1, 739–746. (21) Wu, J.; Lerner, M. M. Chem. Mater. 1994, 6, 207–210. (22) Xi, Y.; Ding, Z.; He, H.; Frost, L. R. J. Colloid Interface Sci. 2004, 277, 116–120. (23) Williams-Daryn, S.; Thomas, R. K. J. Colloid Interface Sci. 2002, 255, 303–311. (24) He, H.; Duchet, J.; Galy, J.; Gerard, J. F. J. Colloid Interface Sci. 2006, 295, 202–208. (25) Lagaly, G.; Ogawa, M.; De´ka´ny, I. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006; Vol. 1, p 309. (26) Venkataraman, N. V.; Vasudevan, S. J. Phys. Chem. B 2001, 105, 1805– 1812. (27) Williams-Daryn, S.; Thomas, R. K. J. Colloid Interface Sci. 2002, 255, 303–311. (28) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 891–896.

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Figure 5. FTIR spectra of (a) ACMt-RT and (b) ACMt-90 and their organoclay counterparts (a′) C16TMA-ACMt-RT and (b′) C16TMAACMt-90, respectively. The FTIR spectrum of the solid C16TMABr (c) is presented for comparison.

to the type of calculation based on different models. The C16TMA-ACMt-90 material exhibited an interlayer gallery of 2.84 nm, which exceeded the length of the C16TMA cation (about 2.53 nm).29 The intercalated surfactant cations adopted a tilted bilayer paraffin arrangement. For a simple case of a bilayer structure where methylene chains are exclusively alltrans and interdigitation is absent, the tilt angle is about 35° with respect to the layers. When this intercalate is washed with a mixture of water/ethanol (50%), the interlayer gallery is about of 1.22 nm, similar to that obtained for the ACMt-RT. In this case, the intercalated molecules adopted different rearrangement, with a monolayer paraffin-type arrangement of C16TMA cations tilted 29°. This value was close to that reported for a monolayer paraffin structure in the case of non-acid-activated montmorillonite.29 Fourier Transform Infrared Spectroscopy. The ACMt-RT and ACMt-90 exhibited bands at 3440 and 1642 cm-1 due the stretching and bending vibrations for the hydroxyl groups of water molecules present in the clay. No significant changes were observed for the bands at 3440 and 1642 cm-1 as the acid activation temperature increased (Figure 5a,b). However, the shape of the Si-O stretching band at 1040 cm-1 was affected by changes in the Si environment from RT to 90 °C, indicating textural changes in the ACMt-90.30,31 The band at 1040 cm-1 decreased in intensity, while the band at 1225 and 1090 cm-1 increase in intensity (Figure 5a,b). The shape changes are a characteristic of amorphous silica present in ACMt-90. Clear diminution in the intensity of the OH bending vibration at 913 cm-1 (AlAlOH) and 840 cm-1 (AlMgOH) along with that of the Al-O-Si bending band at 520 cm-1for ACMt-90 compared to ACMt-RT (Figure 5a,b). This data indicated that a reduction of the octahedral cations occurred at 90 °C compared to that at RT.32,33 After reaction with C16TMAOH solution at a similar C16TMAOH/CEC molar ratio of 3, the shape of the Si-O (29) He, H.; Frost, R. L.; Bostrom, T.; Yang, D.; Xi, Y.; Kloprogge, T. Appl. Clay Sci. 2006, 31, 262–271. (30) Tka`c, I.; Komadel, P.; Muller, D. Clay Miner. 1994, 29, 11–19. (31) Komadel, P.; Madejova´, J.; Janek, M.; Gates, W. P.; Kirkpatrick, R. J.; Stucki, J. W. Clays Clay Miner. 1996, 44, 228–236. (32) Madejova´, J. Vib. Spectrosc. 2003, 31, 1–10. (33) Tyagi, B.; Chudasama, C. D.; Jasra, R. V. Spectrochim. Acta, Part A 2006, 64, 273–278.

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stretching bands was modified due to the dissolution of the amorphous silica phase; in the case of C16TMA-ACMt-90, the intensity of the band at 793 cm-1 decreased after reaction with C16TMAOH (Figure 5b′). There were no significant changes in the characteristic bands of the C16TMA-ACMt-Rt (Figure 3a′). A decrease of the stretching and bending vibration for the water molecules decreased in intensity asa result of the hydrophobic character of organoclays. Similar observations were reported for organoclays prepared from raw montmorillonite and C16TMABr solution.34 FTIR has been found to be a very powerful method for studying the surfactant molecule orientation and configuration of surfactant molecules, or the conformation of the methylene chains, where the vibration bands shift accordingly.34,35 For example, C16TMA monomer and micelles have vibrations at 2860 and 2930 cm-1 and 2853 and 2923 cm-1, respectively.36 For C16TMABr solid, the vibration decreased to 2849 and 2917 cm-1 37 (Figure 5c). The FTIR spectra of organoclays showed similar absorption bands in the 3020-2800 cm-1 region, with two strong bands near 2850 cm-1 and 2920 cm-1 assigned to symmetric (νs(CH2)) and antisymmetric (νas(CH2)) stretching bands of the methylene groups, respectively (Figure 5a′,b′). These wavenumbers are almost identical to those for the corresponding bands in solid C16TMABr. It was difficult to see any changes in the position of these bands since the shift was very small (about 2-4 cm-1), and some errors could be made during the reading. He et al. reported both symmetric (νs(CH2)) and antisymmetric (νas(CH2)) absorption bands shifted to lower wavenumbers for organoclays prepared from C16TMABr.34 However, in this study, only a change in intensity was observed, depending on the density of the alkyl chains within the interlayer space.38 Indeed, the organaoclay prepared from ACMt-90 exhibited qualitatively higher bands intensities compared to that prepared from ACMtRT (Figure 3a′,b′). These results agree with those of the elemental analysis. The position of these two bands are quite similar, with small variation of the vibrational bands, indicating that the intercalated cations have similar configuration. The wavenumber of these two bands are slightly higher than those for the corresponding modes in the all-trans crystalline n-alkanes, but are still considerably lower than the observed values in the disordered liquid phase of n-alkanes. This suggests that the methylene chains of the intercalated surfactant adopt an essentially all-trans conformation with additional gauche conformers.34 Figure 5a′,b′ shows bands at 1440-1480 cm-1 and 700-750 cm-1, corresponding to the CH2 scissoring and rocking modes (encircled ones). These modes depended on the amine concentration, chain packing, and conformation ordering. The positions of these bands were quite similar to those of pure C16TMABr. The splitting of these bands was related to the increase of alltrans conformers as the uptake of C16TMA cations increased.34,38 MAS NMR Studies. During the acid activation, the intensity of the resonance peak at -99 ppm (Q3 Si species) decreased, and a peak at -110 ppm appeared, related to the amorphous silica (Q4 Si species) species.17,30 The intensity of the latter depended on the temperature of acid activation, with a higher intensity at 90 °C (see the Supporting Information, Figure 2a,b). (34) He, H.; Frost, L. R.; Zhu, J. Spectrochim. Acta, Part A 2004, 60, 2853– 285. (35) Vaia, R. A.; Teukolsky, R. K.; Giannnelis, E. P. Chem. Mater. 1994, 6, 1017–1022. (36) Kung, K.; Hayes, F. Langmuir 1993, 9, 263–267. (37) Li, Z.; Gallus, L. Colloids Surf., A: Physicochem. Eng. Sci. Aspects 2005, 264, 61–67. (38) Peker, S.; Yapar, S.; Besun, N. Colloids Surf. 1995, 104, 249–257.

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Figure 6. 13 C CP NMR of pure C16TMABr (a) and different C16TMA intercalated clays, (b) C16TMA-ACMt-RT, and (c) C16TMA-ACMt90. Table 2. 13C CP NMR of C16TMABr and C16TMA Intercalated Clays samples

CN

C1

C4-13

C2

C3,15

C16

C16TMABr C16TMABra C16TMA-ACMt-RT C16TMA-ACMt-90

55.0 56.8 54.7 54.1

63.5 64.4 68.1 67.1

35.0 34.5 33.2 33.0

30.0 31.6 31.3 31.1

24.9-24.1 26.5-25.7 24.7 23.7

17.0 18.6 15.6 15.0

a

Taken from ref 41.

After exchange with C16TMAOH solution, a decrease of the peak at -110 ppm related to amorphous silica was observed as a result of the dissolution of this phase at high pH medium for ACMt-90,17 in good agreement with FTIR data (see the Supporting Information, Figure 2a′,b′). The solid 13C CP NMR is considered as a powerful tool for probing the structure, conformation, and dynamics of molecules at an interface.39 The 13 C resonance for long carbon-chain surfactant molecules is sensitive to the difference in conformation and packing in addition to the chemical structure. The chemical shift difference has been used to characterize the chain conformations of surfactantexchanged clays.40,41 The 13C CP/MAS spectrum of the solid C16TMABr is characterized by a main signal at 32.9 ppm, indicating that the trans conformation of the CH2 groups is dominant (Figure 6a).17 The position of the other signals agreed well with the data reported in the literature,41 and summarized in Table 2. In the crystalline phase, the all trans conformation of the hydrocarbon chains was dominant with the inner methylens resonance occurring in the range 34.2-32.8 ppm,42 while a gauche conformation is adopted in solution state. The 13C CP\MAS spectrum of the intercalated C16TMA cations into ACMt-RT showed two resolved resonance peaks for the main methylene signals in the range 30-34 ppm, resulting from a mixture of all trans and gauche conformations at 33.2 and 31.3 ppm, respectively (Figure 6b). However, the C16TMA-0.3ACMt-90 exhibited mainly one peak at 31.1 ppm, indicating that the C16TMA surfactants have a gauche conformation with a shoulder at 33.0 ppm, related to minor contribution of the all-trans conformation (39) Grandjean, J. Encyclopedia of Surfaces and Colloid Science; Habbard, A. T., Ed.; Marcel Dekker: New York, 2002; pp 3700-3712. (40) Wang, L. Q.; Liu, J.; Exarhos, G. J.; Flanigan, K. Y.; Bordia, R. J. Phys. Chem. B 2000, 104, 2810–2816. (41) Kubies, D.; Je´roˆme, R.; Grandjean, J. Langmuir 2002, 18, 6159–6163. (42) Zhu, J.; He, H.; Zhu, L.; Wen, L.; Deng, F. J. Colloid Interface Sci. 2005, 286, 239–244.

Acid ActiVation of Montmorillonite Clays

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Table 3. Textural Properties of the Different Clays before and after Reaction with C16TMAOH Solutiona samples

SBET (m2g-1)

TPV (mLg-1)

APS (nm)

C16TMA-ACMT-RT C16TMA-ACMt-90

17 (54) 20 (154)

0.05 (0.21) 0.05 (0.30)

8.6 (7.3) 9.5 (7.4)

a TPV: total pore volume at relative pressure p/po ) 0.95; APS: average pore size. The values in brackets correspond to starting materials.

(Figure 6c). While the FTIR spectra indicated that the intercalated surfactants had a configuration similar to that of the C16TMABr solid, however, the 13C CP/NMR showed that the intercalated surfactants had different conformation than the solid C16TMABr. The CN (54.7 ppm) and C1 (68.1 ppm) resonance peaks of the C16TMA intercalates are more intense compared to those of the crystalline C16TMABr, whereas the sharp C16 resonance peak (17.0 ppm) has a higher intensity in crystalline C16TMABr. This fact reflected the difference in the mobility of these carbons in the different materials, and the broadness of the peaks indicated that the C16TMA cation mobility was reduced between the clay sheets. Such reduction of mobility and heterogeneity has also been shown for intercalated surfactants into montmroillonite clay and laponites.40,41,43 In contrast, good homogeneity of the alltrans conformation was observed for C16TMA intercalated magadiites.44 Thus, the mobility of the intercalated aliphatic chains depended of the structure of the inorganic host materials.45,46 Textural Properties. Table 3 summarizes the total BrunauerEmmett-Teller (BET) surface areas and pore volumes for acidactivated clays before and after exchange reaction with C16TMAOH solution. The ACMt-RT exhibited a surface area of 54 m2g-1 and a pore volume of 0.11 mL g-1. However, these values increased when the activation was performed at 90 °C with a surface area of 164 m2 g-1 and a pore volume of 0.30 mL/g, as a result of the contribution of the resulting amorphous silica phase from the acid activation process. These results are in good agreement with those reported for the acid-activated clays.5 After intercalation of C16TMA cations, the interlayer spacing is increased, and these cations may act as pillars. However, a decrease of the surface area was observed. This decrease could be related to the nonaccessibility of the nitrogen molecules to the adsorption sites that were blocked by the surfactants or to the dissolution of the amorphous silica phase during the exchange process.17,46 An average of 20 m2 g-1 was obtained for the samples. A low pore volume (0.05 mL g-1) was also obtained (Table 3) resulting from the presence of some physiadsorbed surfactant molecules, which blocked the interparticle space and leading to the dissolution of the amorphous phase. The variation of the textural properties indicated a rearrangement of the clay sheets during the exchange reaction and the stacking of exfoliated sheets. TGA. The derivative of thermogravimetry (DTG) of ACMtRT clay exhibited two mass losses, due the release of physisorbed water molecules and interlayer water in the range of 50-200 °C. A second one, at a lesser extent, is due to the dehydroxylation process in the temperature range of 500-800 °C (Figure 7a). Meanwhile, ACMt-90 exhibited only one mass loss in the range of 50-200 °C (Figure 7b). The second mass loss at higher temperatures was almost absent, indicating a destruction of the clay sheets during the acid activation.10,30 (43) Ishikawa, S.; Kurosu, H.; Ando, I. J. Mol. Struct. 1991, 248, 361. (44) Kooli, F.; Mianhui, L.; Alshahateet, S. F.; Chen, F.; Yinghuai, Z. J. Phys. Chem. Solids 2006, 67, 926–931. (45) Muller, R.; Hrobarikova, J.; Calberg, C.; Je´roˆme, R.; Grandjean, J. Langmuir 2004, 20, 2982–2985. (46) Wang, C. C.; Juang, L. C.; Lee, C. K.; Hsu, T. C.; Lee, J. F.; Chao, H. P. J. Colloid Interface Sci. 2004, 280, 27–35.

Figure 7. DTG features of (a) ACMt-RT and (b) ACMt-90 and their organoclay counterparts (a′) C16TMA-ACMt-RT and (b′) C16TMAACMt-90, respectively. The DTG feature of the solid C16TMABr (c) is presented for comparison.

Figure 7 presents the DTG curves of the pure C16TMABr and the different intercalated clays. The DTG curve of the pure C16TMABr in nitrogen atmosphere exhibited only one peak at the maximum mass loss of 266 °C because of one mass loss in the range of 180-380 °C (Figure 7c). However, the DTG of the intercalated C16TMA-ACMt-RT exhibited three main mass loss steps, and the surfactant began to decompose at 200 °C (Figure 7a′). The DTG of C16TMA-ACMt-90 presents different features with a shift of temperatures at the maximum rate of mass loss (peak in DTG) to lower values (see Figure 7b′). This shift could be related to the acidity of the host clays, which made the release of C16TMA cations at lower temperatures easy, or to the structure of the bilayer paraffin structure with higher basal spacing, which makes the release of organic surfactants easy. The higher content of the surfactants could be also the reason for the shift of temperatures at the maximum rate of mass loss, as reported for octadecyltrimethylammonium organoclays.47 The TGA curves showed losses of 23% and 31%, which roughly correspond to the loss of all the organic cations, and is in good agreement with the elemental analysis of C and N (see Table 1). For C16TMA-ACMt-90, the first mass loss at relatively lower temperature (at 140 °C) could be related to physiadsorbed surfactants,24 or to the degradation of the excess surfactant above the CEC that resides exterior to the layers.47 The sample was submitted to a purge of N2 gas between 140 and 160 °C, then cooled to RT. Prior the TGA measurements, the peaks in DTG at low temperatures between RT and 150 °C vanished (see the Supporting Information, Figure 3c,d′) and indicated that the mass loss was attributed to possible physiadsorbed molecules of water and some intercalated surfactants. No variation of DTG features for the C16TMA-ACMt-Rt was observed (Figure 3a,b). Overall, the intercalated C16TMA cations had lower thermal stability compared to the pure C16TMABr, as indicated by the shift of the DTG peaks to lower temperatures compared to those of pure C16TMABr in the case of organo acid-activated clays.

Conclusions Acid-activated montmorillonite clays exhibited different intercalation properties. The uptaken amounts of C16TMA cations (47) Xi, Y.; Ding, H.; He, H.; Frost, L. R. J. Colloid Interface Sci. 2004, 277, 116–120.

730 Langmuir, Vol. 25, No. 2, 2009

depended on the initial C16TMAOH/H+ molar ratios for ACMt90. The excess of uptaken amounts above the CEC value indicated that the intercalation of C16TMA cations did not occur via simple cation exchange reaction. Exfoliation of the clay sheets was achieved from an initial loading concentration above 1.60 mmol, where the surfactant cations were adsorbed on both sides via interaction between the negatively charged sheets and the positive surfactant cations, and formed a double layer paraffin structure during the restacking of the clay sheets. When an ethalonic solution was used, this exfoliation did not occur. Partly intercalated surfactants were washed out with ethanol, and only one monolayer paraffin structure was then obtained. FTIR spectroscopy indicated that the intercalated surfactants exhibited the same configuration as the solid C16TMABr. However, the solid 13C CP/NMR indicated that the C16TMA cations have different conformations compared to those of pure C16TMABr, depending on the paraffin structure, due to the restrictions on conformational freedom and mobility imposed by the acid0activated clays. The decomposition of surfactants shifted to low temperatures as the amount of

Kooli

intercalated surfactant increased, but was still lower than the solid C16TMABr, as deduced from TGA. Acknowledgment. The author would like to thank the Institute of Chemical and Engineering Sciences, Agency of Science, Technology and Research (A-STAR), Singapore, and Taibah University (Grant 429/220) for the financial support. Supporting Information Available: Figure 1 presents the variation of the basal spacing of the intercalated organo acid- activated clays, deduced from powder XRD. Powder XRD of acid-activated montmorillonite clay at RT (a) followed by a reaction of C16TMAOH solution at C16TMAOH/CEC molar ratio of 3 (a′). (b) ACMt-90 reacted with C16TMAOH solution at C16TMAOH/CEC molar ratios of (a′) 1 and (a′′) 3. Figure 2 illustrates the 29Si MAS NMR spectra of (a) ACMtRT and (b) ACMt-90 followed by a reaction of C16TMAOH solution at a C16TMAOH/CEC molar ratio of 3 with (a′) C16TMA-ACMt-RT and (b′) C16TMA-ACMt-90. Figure 3 depicts the DTG curves of (a, a′) C16TMA-ACMt-RT and (b, b′) C16TMA-ACMt-90 before and after the pretreatment at 140 °C prior to the TGA measurement. This information is available free of charge via the Internet at http://pubs.acs.org. LA802533Y