Distinguishing Interlayer Cations in Montmorillonite by Thermal

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Distinguishing Interlayer Cations in Montmorillonite by Thermal Analysis of Dimethylformamide-Saturated Samples C. Breen,*,† F. Clegg,† T. L. Hughes,‡ and J. Yarwood† Materials Research Institute, Sheffield Hallam UniVersity, Howard Street, Sheffield, S1 1WB, United Kingdom, and Schlumberger Cambridge Research, Madingley Road, Cambridge, CB3 0EL, United Kingdom ReceiVed: June 5, 2000; In Final Form: March 12, 2001

Derivative thermograms (DTG) for the desorption of dimethylformamide (DMF) from Mn+-SWy-2/DMF complexes can be used to distinguish between different exchange cations simply by the temperature at which the most strongly held DMF molecules are desorbed. The maxima for the desorption of this strongly bound DMF in the DTG traces for Mg2+-, Ca2+- and Na+-SWy were at 420, 330, and 220 °C, respectively. Variable-temperature X-ray diffraction confirms that the DMF is present as a single layer of molecules in the clay gallery at these temperatures. Variable-temperature diffuse reflectance Fourier transform spectroscopy (VT-DRIFTS) has shown that the strongly held DMF molecules are directly coordinated to the exchangeable cations via the carbonyl group. The method is simple and robust, requires no specialized preparation methods and the diagnostic capability is not adversely influenced by the presence of moisture. VT-DRIFTS has shown that coadsorbed water is desorbed at temperatures below 100 °C

Introduction The interactions between clay minerals and organic molecules have received widespread attention within a number of industrial sectors including; waste treatment,1 catalysis,2 laundry powders,3 and many more. This interest arises because the useful adsorptive/intercalation and cation exchange properties of these minerals may be modified so that, for example, their catalytic behavior can be tuned/manipulated.2,4-5 The smectite group receives the most attention because its members have a relatively high cation exchange capacity and the resident cations can be readily substituted by other cations thus significantly changing the properties. For example, the replacement of the naturally occurring inorganic cations by tetramethylammonium6 or long chain alkyltrimethylammonium cations7 can change the surface character from hydrophilic to organophillic, thus making these organoclays very efficient scavengers of benzene, toluene, and xylene from aqueous waste streams.8 The interactions of small amide molecules with kaolinites have been investigated extensively in the literature, with particular emphasis on the interactions of N-methylformamide (NMF, CH3HNCHO) and N,N-dimethylacetamide because these are two of only a limited number of molecules that intercalate directly into kaolinite.9,10 The interaction of dimethylformamide (DMF, (CH3)2NCHO) with kaolinite has received less attention because the intercalation rate is very slow and only a minimal portion of the kaolinite expands.11 Recently, DMF and NMF have proven to be effective vapor phase probes to determine swelling clays in the presence of nonexpanding minerals.12 Moreover, these studies have revealed that the different exchange cations within montmorillonites can be readily distinguished simply by the temperature at which the most strongly held NMF12,13 or DMF molecules12 were desorbed during * Author to whom correspondence should be addressed. E-mail: c.breen@ shu.ac.uk. Fax: +44 (0) 114 253 3501. † Sheffield Hallam University. ‡ Schlumberger Cambridge Research.

thermogravimetric analysis. A detailed thermal and spectroscopic study of the interactions between NMF and Ca-, Mg-, and Na-exchanged Wyoming has shown that a variety of NMF environments were present, in addition to those that were diagnostic of the resident exchange cation. The same complementary thermoanalytical and spectroscopic techniques have been used herein to further investigate the interactions occurring between DMF and the same cationexchanged clays. These include; thermogravimetric analysis (TGA),14 variable-temperature X-ray diffraction,15 infrared spectroscopy,18-19 and evolved gas analysis.19 Infrared spectroscopy is particularly useful since it enables the interactions between functional organic groups and active sites at the clay surface to be identified. In addition, the bonds formed and their relative strengths can be estimated from the perturbation of characteristic infrared absorption bands. Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) is a particularly useful technique in this regard as illustrated by Parker and Frost20 who successfully used it in their studies on the adsorption and desorption of volatile organics from montmorillonite as a means of controlling odor release agents for attracting pest animals, in particular wild dogs. Particular emphasis herein is given to the complexes formed during progressive exposure to DMF vapor and their subsequent decomposition when the fully loaded complexes are heated. Experimental Section DMF ((CH3)2NCHO) was obtained from Aldrich (99%) and used without further purification. Montmorillonite (SWy-2) was obtained from the Source Clays Repository of The Clay Minerals Society and naturally contains a high proportion of exchanged sodium cations. Homoionic cation exchanged SWy-2 was prepared by dispersing SWy-2 in deionized water and treating with 3.0 times the cation exchange capacity (CEC) of the chosen cation. This process was repeated 3 times before the clay was repeatedly washed and centrifuged. The source of all the cations was the chloride salt (Aldrich-Analytical grade). Exposure of

10.1021/jp002027a CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001

Distinguishing Interlayer Cations in Montmorillonite

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4873

TABLE 1: XRF Results of the Original and Ca-, Mg-, and Na-exchanged SWy-2 % oxide

original

Ca-SWy-2

Mg-SWy-2

Na-SWy-2

Al2O3 SiO2 Na2O MgO CaO K2O TiO2 Fe2O3 others

20.2 68.5 1.7 2.8 1.8 0.7 0.2 4.1 0.2

20.8 68.4 0.3 2.5 3.1 0.5 0.1 4.2 0.2

21.1 69.1 0.1 4.3 0.3 0.4 0.1 4.4 0.2

20.8 68.8 3.1 2.5 0.1 0.4 0.1 4.2 0.1

the exchanged clays (≈50 mg) to DMF vapor was achieved at atmospheric pressure and room temperature using a sealed glass saturator (volume ) 200 cm3) which contained 5 mL of DMF. Samples for X-ray fluorescence analysis were prepared using the Li2B4O7 fusion method and the beads analyzed on a Philips PW2400 XRF spectrometer, using calibration software prepared from standard reference materials.21 Thermogravimetric analysis was performed using a Mettler TA3000 thermogravimetric analyzer. Each sample (6-12 mg) was transferred from DMF vapor and placed directly in the thermobalance. The sample was preconditioned in a nitrogen gas flow (20 cm3/min) for 15 min at 35 °C. The sample was then heated to 800 °C at a rate of 20 °C/min. The measurement was recorded as weight loss but is presented as the negative of the first derivative (i.e., -dw/dT). X-ray diffraction patterns were obtained using a Philips PW3710 diffractometer operating at 35 kV and 45 mA with a copper target (λ ) 1.5418 Å). Slurried samples were coated on glass slides, dried in air, and exposed to DMF vapor. Diffraction patterns were collected from 5 to 65 [°2θ] at 2 [°2θ]/min. A simple heating stage was utilized for VT-XRD.22 Diffractograms were collected at a specific temperature (accurate to (8 °C) after a 15 min equilibration period. The maximum temperature of the heating stage was 300 °C. Higher temperature treatment was achieved using a solvent-free oven and the sample was immediately replaced on the heating stage (at 300 °C to prevent hydration in air) and the XRD pattern collected. VT-DRIFTS was performed using a Mattson Polaris FTIR spectrometer, a Graseby Selector DRIFTS accessory, and its associated environmental chamber controlled by an automatic temperature controller (20-500 °C) in which the compartments were purged continuously with nitrogen. All samples were prepared by mixing with finely ground KBr as a 10% clay mixture. The sample (≈0.25 mg) was transferred to the diffuse reflectance cup positioned in the heating chamber, the surface leveled with a microspatula (in order to ensure a reproducible sample height), and a DRIFTS spectrum collected at 25 °C prior to any heating or purging treatment. The sample was purged with nitrogen gas (20 cm3/min) for 15 min before collecting another spectrum. The sample was then heated to 50 °C and allowed to equilibrate in the nitrogen flow for 15 min before collecting the spectrum. This process was repeated after raising the temperature by 50 °C increments until the DMF was completely desorbed. Background spectra were collected using KBr powder alone using the same procedure and used to ratio against the respective sample spectra. Results and Discussion X-ray Fluorescence. The oxides of the major elements in SWy-2 and its cation-exchanged forms are listed in Table 1. SWy-2 is a mixed Ca/Na/Mg bentonite which also contains a small quantity of K+ on the exchange sites. The exchange

Figure 1. DTG traces of Ca-SWy-2 after exposure to DMF vapor for 0, 5, 24, 41 h, and 16 days.

TABLE 2: Temperatures and Percentage Weight Losses (in parentheses) of the Maxima Observed in the DTG Traces of Untreated Mn+-SWy-2 and Dimethylformamide/Ca-, Mg-, and Na-Exchanged SWy-2 Complexes Mn+-SWy-2 Ca-SWy-2 Na-SWy-2 Mg-SWy-2

temperatures (°C) and (% weight losses) Non-Treated 90 & 130 (10.3), 700 (4.6) 90 (2.2), 700 (4.0) 90 & 130 (10.8), 700 (4.5)

DMF-Exposed Mn+-SWy-2 Ca-SWy-2/DMF (5 h) 90 & 130 (5.2), 330 (5.1), 700 (4.7) Ca-SWy-2/DMF (24 h) 60-100 (3.8), 130 (14.0), 330 (9.8), 700 (4.8) Ca-SWy-2/DMF (41 h) 60-100 (3.9), 130 (15.5), 330 (11.1), 700 (4.2) Ca-SWy-2/DMF (16 days) 80 (6.7), 140 (17.2), 330 (10.0), 700 (4.4) Mg-SWy-2/DMF (5 h) 80 & 160 (5.5), 410 (6.1), 690 (5.3) Mg-SWy-2/DMF (41 h) 60-100 (3.3), 140 (13.4), 420 (7.5), 670 (4.2) Mg-SWy-2/DMF (16 days) 80 (7.2), 150 (19.7), 420 (6.4), 680 (4.6) Na-SWy-2/DMF (24 h) 100 (6.0), 220 (11.5), 690 (5.3) Na-SWy-2/DMF (16 days) 105 (22.7), 205 (10.2), 690 (5.0)

procedures were effective in ensuring that the majority of the exchange sites were occupied by the chosen cation. Thermogravimetric Analysis. The numbers in parentheses in Table 2 denote the percentage weight loss associated with each maximum in all the DTG traces discussed herein. Figure 1 shows the DTG traces of Ca-SWy-2 prior to exposure to DMF vapor and after exposure for specific periods of time, whereas Figure 2 shows those for Mg-SWy-2 and Na-SWy2. The maxima denoted H2O and -OH are due to the loss of water and dehydroxylation of the clay structure, respectively.12,23,24 Before exposure to DMF, the loss of water from Ca- and Mg-SWy-2 exhibited a strong maximum at 90 °C with a weaker, unresolved shoulder near 130 °C whereas NaSWy-2 had a single, strong maximum at 90 °C. These maxima reflect the desorption of water from different binding sites. The DTG trace did not detect the observed continuous loss of water

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Figure 3. VT-XRD traces of the Ca-SWy-2/DMF complex (16 days in vapor).

TABLE 3: Interlayer Spacing of the Mn+-SWy-2 prior to and after Exposure to Dimethylformamide Vapor for 16 Days Mn+-SWy-2 Ca-SWy-2 Mg-SWy-2 Na-SWy-2

Figure 2. DTG traces for untreated Mg- and Na-SWy-2 and after exposure to dimethylformamide vapor.

between 200 and 500 °C because the rate of weight loss in this temperature range was constant. The weight loss due to dehydroxylation should remain essentially constant and may be used as a visual guide to the amount of DMF desorbed under the other maxima present. During the 15 min equilibration period under flowing nitrogen at 35 °C (not shown in diagram) all the samples lost weight at a constant rate and this will be considered below with the VT-DRIFTS spectra. After exposure to DMF the DTG profiles were changed considerably in that the intensities and positions of the maxima changed and additional maxima in the 200-450 °C region arose. Clearly, the gases evolved from the complexes could not be determined from gravimetric analysis alone, but systematic trends in the magnitude and temperatures of the maxima as the Mn+-SWy-2 was exposed to DMF for longer periods of time were evident. Excluding the dehydroxylation maximum (≈700 °C), three maxima were observed in the DTG trace of the fully loaded Ca-SWy-2/DMF complex (i.e., after 16 days). The maximum at highest temperature (330 °C) was clearly apparent after 5 h and when this maximized (≈10.0% weight loss) after 24 h a second sharp, intense maximum developed at 135 °C. This was followed by a third feature at 80 °C, which was clearly present as a shoulder in the sample exposed for 16 days. Note that as the maximum at 330 °C increased in intensity it was accompanied by a corresponding decrease in the maximum attributed to water desorption (90 °C) in the Ca-SWy-2 sample not exposed to DMF. Similar DTG traces were obtained for the Mg-SWy-2/DMF complexes but the maxima were at different temperatures, particularly the highest temperature maximum which occurred at 420 °C (Figure 2). A small weight loss (≈2.0%) occurred between 300 and 350 °C as indicated

Interlayer spacing Interlayer spacing Interlayer spacing prior to exposure after 16 days according to (hydrated state) exposure to DMF Olejnik et al.9 12.2 Å 15.1 Å 12.5 Å

19.3 Å 19.2 Å 19.8 Å

19.1 Å 19.2 Å 19.8 Å

by the low-temperature asymmetry on the maximum at 420 °C. The three maxima observed in the Ca and Mg-SWy-2/DMF complexes are considered to represent three types of adsorption sites or environments of DMF and/or water molecules within the clay complex. The presence of only two maxima, at 100 °C and 220 °C, in the DTG traces of the Na-SWy-2/DMF complexes suggest that there were only two types of DMF and/ or water sites present (Figure 2). A comparison of the DTG traces of the Mn+-SWy-2/DMF complexes with those formed when Mn+-SWy-2 was exposed to NMF vapor12,13 showed that DMF adsorbed at a much faster rate than NMF. This difference was attributed to the higher vapor pressure of DMF compared to that of NMF because DMF molecules do not form intermolecular hydrogen bonds since the N-H bond has been replaced by a N-CH3 bond.25,26 The increased speed at which the buildup of DMF occurs on Mn+SWy-2 means a more rapid analysis should this methodology be used to identify the type of cation present in a montmorillonite. Note that the highest temperature desorption maximum in comparable Mn+-SWy-2 clays was always at a higher temperature for the DMF complexes than for the NMF complexes (Table 3). This may imply that DMF binds more strongly to Mn+-SWy-2 than NMF but the possibility of different diffusional processes through the sample bed may play an important role.27,28 Nonetheless, the diagnostic DMF desorption maxima from clays of different locus and density of charge mirror those reported here which identifies the exchange cation as the discriminating factor for Na-, Ca-, and Mg-exchanged smectites. Variable-Temperature X-ray Diffraction. The interlayer spacings for the fully loaded Mn+-SWy-2/DMF complexes (Table 3) are in excellent agreement with the work of Olejnik et al.29 and confirm that the DMF molecules have penetrated the interlayer space and are likely to be interacting directly with the cations. Water-saturated M2+-SWy-2 formed a collapsed clay (d001 ≈ 10 Å) at temperatures above 200 °C. The VT-XRD traces of Ca-SWy-2 after exposure to DMF vapor for 16 days (Figure 3) show that the basal spacing of the complex decreased in three distinct stages. The first of these, a decrease from 19.3 to 14.5 Å, occurred when the sample was heated to 100 °C and coincided with the end of the first major weight loss in the DTG trace (Figure 1), which appeared as the


Figure 4. VT-XRD traces of the Mg-SWy-2/DMF complex (16 days in vapor).

shoulder at 80 °C. The second stage, an additional decrease of 1.4 Å to 13.1 Å, occurred when the sample was heated from 100 to 170 °C and coincided with the desorption maximum observed in the same DTG trace at 130 °C. By 220 °C the intensity of the 001 peak associated with the 13.1 Å spacing was significantly reduced in intensity but there was no obvious increase in the number of collapsed layers which suggests that the number of DMF molecules in the interlayer space had been greatly reduced. The third reduction in d spacing at 300 °C resulted in a collapsed clay structure. This decrease in gallery height was expected to correlate with the high-temperature maximum (330 °C) in the DTG trace but the discrepancy can be attributed to the more protracted heating regime in VT-XRD compared with the controlled linear heating rate in TG.19,30 These observations suggest that two molecular layers existed in the gallery at 25 °C, which decreased to one layer upon heating to 100 °C. Thermogravimetry-FTIR (TG-FTIR)12 and VT-DRIFTS (vide infra) confirmed that the dominant species in the two-layer species was DMF although a relatively small amount of water was present in the complex below 100 °C. Above 100 °C water was no longer present and thus the single layer was due to only DMF. As the complex was heated from 100 to 220 °C the gallery height contracted by 1.4 Å confirming that the layer of DMF molecules remained in the interlayer but had altered their spatial arrangement around the cation and/or their packing density in the interlayer. At 300 °C the single layer of DMF molecules had been removed and the clay layers had collapsed. Layered aluminosilicates commonly display a range of different d spacings over small temperature intervals as a sample is heated and the interlayer guest is removed. For example, Reichenbach and Beyer have recently reported distinct states of hydrated Sr2+-vermiculite characterized by basal spacings of 15.4, 15.1, 14.9, 12.3, 12.1, and 9.6 Å.31 It is important to note that the highest temperature maximum, which distinguishes between the different cations, was associated with interlamellar DMF molecules and that the presence of water below 100 °C did not influence the temperature at which it occurred. Similar behavior was observed for the Na-SWy-2/DMF complex (not illustrated), in that an initial decrease of 5.3 Å (19.8 to 14.5 Å) occurred upon heating to 80 °C, after which the spacing reduced to 12.6 Å at 170 °C and was at 10.9 Å by 220 °C. A water expanded Na-SWy-2 sample collapsed from 12.5 to 9.8 Å after 30 min at 50 °C. The VT-XRD traces of the Mg-SWy-2/DMF complex (Figure 4) showed two distinct decreases in the gallery height as it was heated. The first major decrease of 4.5 Å to 14.7 Å, occurred when the complex was heated to 80 °C and coincided with the loss of the lowtemperature maximum observed in the DTG trace at 80 °C (Figure 2). The second major decrease from 12.7 Å to 10.2 Å

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4875 occurred when the complex was heated from 300 to 420 °C. The gallery height contracted gradually from 14.7 to 12.7 Å as the complex was heated from 80 to 220 °C, as did that for CaSWy-2/DMF. During the last temperature increment (220300 °C) the DMF population in the expanded layers was reduced and the proportion of smaller gallery heights increased as indicated by the broadening of the peak toward higher angles. VT-XRD studies have shown that the DMF molecules desorbed during the high-temperature maximum (i.e., at a d spacing ≈ 13.1 Å) were in the interlayer region and infrared studies (vide infra) strongly suggest that these were coordinated to the cations. If all the DMF molecules contributing to the highest temperature maximum in each Mn+-SWy-2/DMF complex were associated with exchangeable cations then the thermogravimetric data suggests that there were approximately 3.8, 2.4, and 1.9 DMF molecules per Ca, Mg, and Na cation, respectively. VT-DRIFTS of Ca-SWy-2/DMF. Infrared spectroscopy has been used to investigate whether the mode of interaction between DMF and the different cation exchanged clays varied during each characteristic weight loss in the TGA traces. In particular, changes in position, intensity, and breadth of the infrared bands are used to identify the similarities and differences between the interaction of DMF with the different exchange cations. In general, significant reductions in band intensities were observed over temperature intervals which coincided with major weight losses in the DTG profiles. DMF is likely to interact most strongly with the clay via its carbonyl group since the oxygen atom is a strong electron pair donor and can strongly coordinate to metal ions.32 Thus, the most significant interaction is likely to be between the carbonyl group and the exchange cations either directly or via a bridging water molecule, both of which have been reported to occur in other organoclay complexes.18 It is also possible for DMF to interact with the clay via the lone pair of electrons on the N atom or for the DMF to become protonated. However, given that the lone pair on the nitrogen is less available in DMF than in NMF and that recent work has shown that NMF coordinates to the interlayer cations via the carbonyl group,13 it is considered that DMF will interact via the carbonyl oxygen. The infrared spectrum (transmission) of liquid DMF is shown at the top of Figure 5 (3800-2500 cm-1) and Figure 6 (19001200 cm-1). The assignments of the DMF bands are given in Table 4.9,25,26,32 The broad bands at 3552 and 3334 cm-1 are due to water present in liquid DMF. Figures 5 and 6 show how the VT-DRIFTS spectra (in the regions 3800-2500 cm-1 and 1900-1200 cm-1, respectively) of the Ca-SWy-2/DMF complex, formed after exposure to DMF vapor for 16 days, varied as the sample temperature was increased. There was a significant decrease in the intensity of the broad bands between 3500 and 3100 cm-1. The shape of these bands, assigned to νs(OH), is characteristic of sorbed water and therefore indicates its presence in the complex. These bands were significantly reduced by 100 °C, which infers that the water was weakly bound and contributed to the maximum at 80 °C observed in the corresponding DTG trace (Figure 1). TG-FTIR also confirmed the desorption of water from the Mn+-SWy-2/DMF complexes as they were heated from 35 to 100 °C.12 The spectrum of a water-saturated Mn+-SWy-2 shows two broad bands centered at approximately 3419 and 3225 cm-1, which are also removed when the clay is heated to 100 °C (spectra illustrated in ref 12) in agreement with work of others.34-36 The dominant νs(OH) band observed in water-saturated Mn+-SWy-2 at 3419 cm-1 was shifted to lower frequency (3350 cm-1, Figure

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TABLE 4: Assignment of the Bands Observed in the Infrared Spectrum of Liquid DMF (transmission) and Position of the Respective Bands (at room temperature) in the Mn+-SWy-2/DMF Complexes band position (cm-1) assignment

liquid (transmission)

Ca-SWy-2 /DMF

Mg-SWy-2 /DMF

Na-SWy-2 /DMF

νs(OH) water νas(CH3) νs(CH3) νs(CH) νs(CH3) amide I ν(CdO) ν(CN) δas(CH3) δs(CH3) δs(CH3) & δ(NCH) νas(CH3)2N

3552/3334 2993 2929 2859 2809/2779 1723 (shoulder) 1675 1503 1457 (shoulder) 1439 1406 1388 1253

3350 3002 2936 2896 2815/2787 1719 (shoulder) 1658 1499 1457 (shoulder) 1437 1419 1390 1253



Figure 5. VT-DRIFTS spectra of Ca-SWy-2 after exposure to DMF vapor for 16 days (3800-2500 cm-1). Top: transmission spectrum of liquid DMF.

Figure 6. VT-DRIFTS spectra of Ca-SWy-2 after exposure to DMF vapor for 16 days (1900-1200 cm-1). Top: transmission spectrum of liquid DMF.

5) in the presence of DMF. This implies that the water molecules were involved in stronger hydrogen-bonding interactions in the presence of DMF than in the presence of water and suggested that the small amount of water present at low temperatures was interacting with some of the DMF molecules. The shoulder (1627 cm-1) on the intense νs(CdO) band at 1658 cm-1 is due to δ(O-H) of water and indicates its presence in the complex (Figure 6). This shoulder was removed when heated at 100 °C and coincided with the loss of the water bands between 3500 and 3100 cm-1. Similar observations were made for all the Mn+-Swy-2/DMF complexes and are in accord with the DTG results (Figure 1) and the reduction in d spacing (Figure 2). A comparison of the C-H stretching bands (3000-2750 cm-1) of DMF liquid to those of DMF in the Ca-SWy-2/ DMF complex showed that the bands had shifted to higher frequency and had become sharper. The bands at 2929, 2809, and 3002 cm-1 shifted approximately 8 cm-1, whereas the band originally positioned at 2859 cm-1 underwent a marked shift to 2896 cm-1. The bands at 2929 and 2809 cm-1 were both assigned to the stretching vibration of the C-H bond in the CH3 groups (νs(CH3)). The shift of these bands to higher frequency indicated that the CH3 groups were in a repulsive environment, possibly in the interlayer region. The 37 cm-1 shift

of the νs(C-H) band, originally positioned at 2859 cm-1, indicated that the environment of this bond had changed considerably and it may be in a unique position. The decrease in bandwidth as the complex was heated was attributed to a decrease in the width of the distribution of DMF environments. The infrared spectrum of liquid DMF exhibited a very strong band at 1675 cm-1 and a small shoulder at 1723 cm-1 (Figure 6, top spectrum). The spectrum of DMF vapor9 had a single band at 1714 cm-1 which indicates that the high-frequency bands in liquid DMF can be attributed to DMF molecules that are undergoing weaker interactions via the carbonyl group than those molecules associated with the 1675 cm-1 band. The carbonyl band in the clay complex was 17 cm-1 lower (1658 cm-1) than the carbonyl band in liquid DMF. This together with the thermal stability the DMF molecules associated with the 1658 cm-1 band, confirmed that the carbonyl bond was interacting strongly with the exchangeable cations in the clay as does NMF.13 Thus, the DMF molecules adsorbed to the clay via their carbonyl group and were not simply present as DMF clusters. The 1658 cm-1 band was asymmetric to the high wavenumber side which indicated that a proportion of the DMF molecules in the clay were more liquidlike (i.e., interacting less strongly through their carbonyl group).


J. Phys. Chem. B, Vol. 105, No. 21, 2001 4877

Figure 9. Deconvoluted spectra of the Ca-SWy-2/DMF complex formed after 16 days in DMF vapor (1800-1500 cm-1).

Figure 7. VT-DRIFTS spectra of Mg and Na-SWy-2 after exposure to DMF vapor for 16 days (3800-2500 cm-1).

Figure 8. VT-DRIFTS spectra of Mg and Na-SWy-2 after exposure to DMF vapor for 16 days (1900-1200 cm-1).

VT-DRIFTS of Mg- and Na-SWy-2/DMF. Figures 7 and 8 illustrate that the Mg- and Na-SWy-2/DMF complexes displayed the same general trends as those described for CaSWy-2/DMF except that the significant changes were in accord with the corresponding maxima in the respective DTG profiles. The νs(CdO) band of the Mg-SWy-2/DMF complex was at 1655 cm-1 which is 20 cm-1 less than that in liquid DMF and 3 cm-1 less than the corresponding band in the Ca-SWy-2/ DMF complex. The reduction in intensity of the broad bands between 3500 and 3100 cm-1, together with the shoulder near

1630 cm-1, confirm that the small amount of water present in the DMF complexes was removed by 100 °C (Figures 7 and 8). Carbonyl Stretching Region. When examined in detail it was apparent that the νs(CdO) bands in the three Mn+-SWy2/DMF complexes were not symmetric and encompassed at least 4 major components as shown in the deconvoluted spectra (Figure 9). The first component of the νs(CdO) bands was the shoulder near 1720 cm-1, which related to vapor-like DMF. This indicated that there were very weakly adsorbed DMF molecules associated with the clay, which were removed by 100 °C, suggesting that they were located on external clay surfaces. The second component dominated and governed the frequency of the band (e.g., 1658 cm-1 in the Ca and MgSWy-2/DMF complex). This component arose from DMF molecules that were directly coordinated to exchangeable cations and thus the band was still present at higher temperatures (e.g., >250 °C in the Mg-SWy-2/DMF complex). These bands progressively shifted to lower frequency as the temperature was increased, indicating a progressively stronger interaction between DMF and exchangeable cations. Incidentally, the infrared evidence supports the TGA data in that DMF molecules bind more strongly to the exchange cations than NMF molecules do.13 The evidence is that the νs(CdO) band shifts to a significantly lower frequency in the spectra of the Ca-, Mg-, and NaSWy-2/DMF complexes (1658, 1655, and 1658 cm-1, respectively) than those in the spectra of the comparable Ca-, Mg-, and Na-SWy-2/NMF complexes (1664, 1664, and 1678 cm-1, respectively). Shifts in the νs(CdO) of similar amide molecules have been used by others to estimate the strength with which the amides interact.37-39 The third component was the source of the asymmetry on the high wavenumber side of the 1658 cm-1 band. This occurred at 1683, 1692, and 1680 cm-1 in the Ca-, Mg-, and NaSWy-2/DMF complexes, respectively, and indicated that liquidlike DMF molecules were present in the complex. Compared with the corresponding bands attributed to the cationcoordinated DMF molecules these bands decreased more in intensity between 25 and 250 °C and are thus assigned to the low-temperature maxima (≈125 °C) in the DTG traces. The fourth component was a shoulder (≈1627 cm-1) to the lowfrequency side of the ν(CdO) band and was attributed to weakly bound water molecules. This component decreased in parallel with the νs(O-H) band at 3419 cm-1 and confirmed its assignment. It is proposed that the DMF molecules (1683, 1692, and 1680 cm-1) and the small amount of water (≈1627 cm-1) reside in small clusters on clay surfaces, between clay stacks or in the interlayer region. Although the VT-DRIFTS spectra

4878 J. Phys. Chem. B, Vol. 105, No. 21, 2001 show no definitive evidence for the formation of these mixed DMF-water clusters they were likely to occur because the hydrogen bonds formed between DMF and water molecules are stronger than the attractive forces between DMF molecules.40 This could explain why very little water was present in the Mn+-SWy-2/NMF complexes,13 since the strong intermolecular hydrogen bonding between NMF molecules would dominate and dictate against NMF-water interactions. Summary and Conclusions The DTG traces of the Mn+-SWy-2/DMF complexes showed that the temperature of desorption maxima for the most strongly bound DMF can be used to clearly distinguish between the type of interlayer cation present in the clay. These desorption maxima occur, respectively, at 420, 330, and 220 °C for the Mg-, Ca-, and Na-SWy-2/DMF complexes, indicating that the thermal stability decreased as Mg > Ca > Na. VT-XRD confirmed that DMF was still in the gallery at these temperatures and VT-DRIFTS analysis showed that this DMF was directly coordinated to the exchangeable cations via its carbonyl group. In addition, DMF was also present as DMF-DMF clusters and DMF-water clusters within the interlayer space and on external surfaces. The presence of small amounts of water in the complexes at low-temperature did not interfere with the diagnostic desorption maxima thus removing the need to dry samples before exposure to DMF. Shifts in the infrared band assigned to the νs(CH3) vibration indicates that the DMF molecules are in a more perturbed environment caused by the reduction in interlayer spacing as the Mn+-SWy-2/DMF complexes are heated. References and Notes (1) Essington, M. E. Soil Sci. 1994, 158, 181. (2) Vaccari, A. Catal. Today 1998, 41, 53. (3) Sastry, N. V.; Sequaris, J. M.; Suhwuger, M. J. J. Colloid Interface Sci. 1995, 171, 224. (4) Adams, J. M. Appl. Clay Sci. 1987, 2, 309. (5) Ballantine, J. A. In Chemical Reactions in Organic and Inorganic Constrained Systems; Burton, R., Ed.; Reidel: Dordecht, 1986; pp 197212. (6) Lee, J. F.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Clays Clay Miner. 1990, 38, 113. (7) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986, 34, 581. (8) CMS Workshop Lectures Vol. 8, Organic Pollutants in the enVironment; Sawhney, B. L., Ed.; The Clay Minerals Society: Boulder, CO, 1996.

Breen et al. (9) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clays Clay Miner. 1971, 19, 83. (10) Reis, A. S., Jr.; Simoni, J. de A.; Chagas, A. P. J. Colloid Interface Sci. 1996, 177, 1. (11) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clay Miner. 1970, 8, 421. (12) Clegg, F. Thesis, Sheffield Hallam University, Sheffield, U.K., 1998. (13) Breen, C.; Clegg, F.; Hughes, T. L.; Yarwood, J. Langmuir 2000, 16, 6648. (14) CMS Workshop Lectures Vol. 3, Thermal Analysis in Clay Science; Stucki, J. W., Bish, D. L., Mumpton, F. A., Eds.; The Clay Minerals Society: Boulder: CO, 1990. (15) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, 1978. (16) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1996. (17) Rochester, C. H. The characterisation of surfaces by infrared spectroscopy. Chem. Ind. March, 175-179, 1981. (18) Yariv, S. Thermochim. Acta 1996, 274, 1. (19) Breen, C.; Thompson, G.; Webb, M. J. Mater. Chem. 1999, 9, 3159. (20) Parker, R. W.; Frost, R. L. Clays Clay Miner. 1996, 44, 32. (21) Giles, H. L.; Hurley, P. W.; Webster, H. W. M. X-ray Spectrom. 1995, 24, 205. (22) Brown, G.; Edwards, B.; Ormerod, E. C.; Weir, A. H. Clay Miner. 1972, 9, 407. (23) El-Shabiny, A. M.; Hammad, S. M.; Ibrahim, I. A.; Ismail, A. K. J. Therm. Anal. 1996, 46, 1421. (24) Worrall, W. E. Clays and Ceramic Raw Materials, 2nd ed.; Elsevier Applied Science Publishers: London, 1986. (25) Zhou, X.; Krauser, J. A.; Tate, D. R.; VanBuren, A. S.; Clark, J. A.; Moody, P. R.; Liu, R. J. J. Phys. Chem. 1996, 100, 16822. (26) Stålhandke, C. M. V.; Mink, J.; Sandstro¨m, M.; Pa´pai, I.; Johansson, P. Vib. Spectrosc. 1997, 14, 207. (27) Tronconi, E.; Forzatti, P. J. Catal. 1985, 93, 197. (28) Breen, C. Clay Miner. 1991, 26, 487. (29) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clays Clay Miner. 1974, 22, 361. (30) Breen, C.; Rawson, J. O.; Mann, B. E. J. Mater. Chem. 1996, 6, 253. (31) Reichenbach, H. G. v.; Beyer, J. Clay Miner. 1997, 32, 573. (32) Yokoyama, H.; Suzuki, S.; Goto, M.; Shinozaki, K.; Abe, Y.; Ishiguro, S. Z. Naturforsch. 1995, 50a, 301. (33) Jao, T. C.; Scott, I.; Steele, D. J. Mol. Spectrosc. 1982, 92, 1. (34) Farmer, V. C.; Russell, J. D. Spectrochim. Acta 1964, 20, 1149. (35) Bishop, J. L.; Pieters, C. M.; Edwards, J. O. Clays Clay Miner. 1994, 42, 702. (36) Shewring, N. I. E.; Jones, T. G. J.; Maitland, G.; Yarwood, J. J. Colloid Interface Sci. 1995, 176, 308. (37) Tahoun, S. A.; Mortland, M. M. Soil Sci. 1965, 102, 314. (38) Affrosman, S.; Armstrong, D. R.; Robb, D.; Treverton, J. A. Langmuir 1995, 11, 2060. (39) Onikata, M.; Kondo, M.; Hayashi, N.; Yamanaka, S. Clays Clay Miner. 1999, 47, 672. (40) Lee, Y. T. J. Raman Spectrosc. 1997, 28, 45.

Distinguishing Interlayer Cations in Montmorillonite by Thermal

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