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Thermal and Spectroscopic Characterization of N-Methylformamide/Ca-, Mg-, and Na-Exchanged Montmorillonite Intercalates 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 January 28, 2000. In Final Form: May 18, 2000 The progressive adsorption of N-methylformamide (NMF) onto Ca-, Mg-, and Na-exchanged Wyoming bentonite (Mn+-SWy-2) together with its subsequent thermal desorption has been studied using a variety of complementary techniques. The derivative thermograms (DTG) for the desorption of NMF from Mn+SWy-2 exhibit three maxima at temperatures which depend on the exchange cation. In Mg-SWy-2 these maxima occur at 130, 200, and 400 °C, whereas in Na-SWy-2 they occur at 100, 150, and 190 °C. Each of these maxima has been assigned to different sites and/or environments for sorbed NMF using variable temperature-X-ray diffraction (VT-XRD) and variable temperature-diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS). Each fully loaded Mn+-SWy-2/NMF complex has two layers of NMF in the gallery, which decreases to a single layer prior to the complete removal of NMF from the complex. The temperatures at which major weight losses occur coincide with decreases in the interlayer spacing. VT-DRIFTS has shown that at low temperatures NMF was removed from NMF clusters, similar to those in liquid NMF, while at high temperatures the NMF molecules are firmly bound and directly coordinated to the exchangeable cations. Unusual shifts in the C-H and N-H absorption bands were observed, indicating a unique orientation of these groups, which probably reflects their keying into the hexagonal cavities of the tetrahedral sheet of the aluminosilicate layer.
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. The smectite group, which includes montmorillonite, has enjoyed the most attention due to its useful adsorptive/intercalation and cation-exchange properties, which may be modified so that, for example, their catalytic behavior can be tuned/ manipulated.4,5 The relatively high exchange capacity and the ready exchange of the naturally occurring cations by others can be utilized to alter the properties of the mineral significantly. For example, replacement of the naturally occurring inorganic cations by tetramethylammonium6 or long-chain alkytrimethylammonium 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 list of organic molecules that interact with smectites is extensive and the interaction of amides with these * To whom correspondence should be addressed. † Sheffield Hallam University. ‡ Schlumberger Cambridge Research. (1) Essington, M. E. Soil Sci. 1994, 158, 181. (2) Vaccari, A. Catal. Today 1998, 41, 53. (3) Sastry, N. V.; Sequaris, J. M.; Schwuger, 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: Dordrecht, 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) Sawhney, B. L., Ed.; CMS Workshop Lectures Vol. 8, Organic Pollutants in the Environment; The Clay Minerals Society: Boulder, CO, 1996.
minerals have been investigated in a large number of publications. For example, polyamides have been incorporated in drilling muds used for the exploration of oil9 and a number of studies have investigated the interaction of smaller amides such as formamide,10 N-methylformamide11 and N,N-dimethylacetamide12 with kaolinite. The interaction of NMF with kaolins has been extensively studied since NMF is one of only a limited number of molecules that intercalate directly into kaolin. In contrast, the adsorption of NMF into smectites has received scant attention. However, NMF and dimethylformamide (DMF) have recently proven to be effective vapor-phase probes to determine swelling clays in the presence of nonexpanding minerals.13 The thermal characterization of organic-clay complexes is best achieved using a variety of complementary techniques, which include thermogravimetric analysis (TGA),14 X-ray diffraction (XRD),15 infrared spectroscopy,16-18 and evolved gas analysis.19 Infrared spectroscopy is particularly useful since it enables the identification of bonds (9) Peker, S.; Yapar S.; Besun, N. Colloids Surf. 1995, 104, 249. (10) Frost, R. L.; Lack, D. A.; Paroz, G. N.; Tran, T. H. T. Clays Clay Miner. 1999, 47, 297. (11) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clays Clay Miner. 1971, 19, 83. (12) Reis, A. S., Jr.; Simoni, J. de A.; Chagas, A. P. J. Colloid Interface Sci. 1996, 177, 1. (13) Clegg, F. Ph.D. Thesis, Sheffield Hallam University, Sheffield, UK, 1998. (14) Stuki, J. W., Bish, D. L., Mumpton, F. A., Eds.; CMS Workshop Lectures Vol. 3, Thermal Analysis in Clay Science; The Clay Minerals Society: Boulder, CO, 1990. (15) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978. (16) Rochester, C. H. Chem. Ind. 1981, March 21, 175. (17) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (18) Yariv, S. Thermochim. Acta 1996, 274, 1. (19) Breen, C.; Thompson, G.; Webb, M. J. Mater. Chem. 1999, 9, 3159.
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that are formed between functional organic groups and active sites at the clay surface. In addition, both the formation and strength of bonds formed can be estimated from the perturbation of characteristic infrared absorption bands. VT-DRIFTS is a particularly useful infrared technique for this purpose, as illustrated by Parker and Frost20 who successfully used it in their studies on the adsorption and desorption of volatile organics from montmorillonites as a means of controlling odor release agents for attracting pest animals, in particular, wild dogs. Preliminary studies of the interaction of NMF with Ca-, Mg-, and Na-exchanged Wyoming bentonite revealed that the different exchange cations could be readily distinguished simply by the temperature at which the most strongly held NMF molecules were desorbed during thermogravimetric analysis. This is not known for any other absorbate on clay surfaces; hence, a concerted study utilizing TGA, VT-XRD, and VT-DRIFTS was initiated to identify the nature and strength of the interactions occurring between NMF and Wyoming bentonite containing three different exchange cations (Ca, Mg, and Na). Particular emphasis is given to complexes formed during progressive exposure to NMF vapor and their subsequent decomposition when the fully loaded complexes are heated. Experimental Section NMF (CH3NHCHO) was obtained from Aldrich (99%) and used without further purification. Montmorillonite (SWy-2) was obtained from the Source Clay Repository and naturally contains a high proportion of sodium exchange cations. Homoionic cation exchanged SWy-2 was prepared by dispersing SWy-2 in deionized water and treating with 3.3 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 the exchanged clays (≈50 mg) to NMF vapor was achieved at atmospheric pressure and room temperature using a sealed glass saturator (volume ) 200 cm3), which contained 5 mL of NMF. 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 NMF 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 PW1830 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 NMF 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 an 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 (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 Spectrum 1995, 24, 205. (22) Brown, G.; Edwards, B.; Ormerod, E. C.; Weir, A. H. Clay Miner. 1972, 9, 407.
Figure 1. DTG traces of Ca-SWy-2 after exposure to NMF vapor for 0 h, 30 h, 3 days, 8 days, 15 days, and 30 days. 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
20.16 68.47 1.70 2.77 1.77 0.69 0.16 4.13 0.15
20.77 68.43 0.27 2.48 3.05 0.50 0.13 4.22 0.15
21.12. 69.09 0.12 4.29 0.3 0.43 0.14 4.38 0.13
20.84 68.75 3.06 2.46 0.05 0.36 0.13 4.23 0.12
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, and a DRIFTS spectrum collected prior to any heating or purging treatment at 25 °C. The sample was purged with nitrogen gas (20 cm3/min) for 15 min before another spectrum was collected. The sample was then heated to 50 °C and allowed to equilibrate in the nitrogen flow for 15 min before the spectrum was collected. This process was repeated at 75 and 100 °C and then at 50 °C increments until the NMF 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 procedures were effective in ensuring that the majority of the exchange sites were occupied by the chosen cation. Thermogravimetric Analysis. The DTG traces of Ca-SWy-2 prior to exposure to NMF vapor and after exposure for specific periods of time are shown in Figure 1. The samples lose weight at a constant rate during the 15 min isothermal (not shown in diagram) under nitrogen and this will be considered later when the VT-DRIFTS
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Figure 3. Gallery heights of N-Methylformamide/Mn+-SWy-2 complexes obtained from VT-XRD patterns. Diagrams within correlate the gallery height to the expected number of intercalated NMF layers.
Figure 2. DTG traces for untreated Na- and Mg-SWy-2 and after exposure to N-methylformamide vapor for 8 and 30 days. Table 2. Temperatures and Percentage Weight Losses of the Maxima Observed in the DTG Traces of Untreated Mn+-SWy-2 and N-Methylformamide/Ca-, Mg-, and Na-Exchanged SWy-2 Complexes Mn+-SWy-2 Ca-SWy-2 Na-SWy-2 Mg-SWy-2
temp. (°C) and (% weight losses) Nontreated 90 and 130 (10.3), 700 (4.6) 90 (2.2), 700 (4.0) 90 and 130 (10.8), 700 (4.5)
NMF Exposed Mn+-SWy-2 Ca-SWy-2/NMF (30 h) 90 (4.0), 160-260 (0.9), 320 (4.0), 700 (4.2) Ca-SWy-2/NMF (3 days) 90 (2.7), 160-260 (1.2), 320 (6.4), 700 (4.6) Ca-SWy-2/NMF (8 days) 80-160 (3.2), 190 (6.0), 320 (7.3), 700 (4.8) Ca-SWy-2/NMF (15 days) 80-160 (4.1), 190 (10.1), 320 (7.8), 700 (5.4) Ca-SWy-2/NMF (31 days) 140 (7.7), 200 (13.0), 320 (8.3), 700 (4.5) Mg-SWy-2/NMF (30 days) 130 (12.4), 200 (8.9), 260-340 (5), 400 (5.6), 670 (5.4) Na-SWy-2/NMF (30 days) 80-150 (26.2), 190 (7.6), 690 (7.3)
spectra are discussed. The numbers in brackets in Table 2 denote the percentage weight loss associated with each maximum in all DTG traces discussed herein. The maxima denoted -H2O and -OH are due to the loss of water and dehydroxylation of the clay structure, respectively.13,23,24 The loss of water from Ca-SWy-2 below 200 °C occurs in two stages, giving two maxima in the DTG trace. This is also the case for Mg-SWy-2 but not Na-SWy-2 (Figure 2) and reflects the different water binding sites. The weight loss below 200 °C depends on the hydration state of the (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.
sample but the weight loss due to dehydroxylation should remain essentially constant and may be used as a visual guide to the amount desorbed under the other maxima present. There is also a continuous loss of water between 200 and 500 °C that does not register in the DTG trace because the slope is unchanging. After exposure of Mn+-SWy-2 to NMF, additional maxima were observed in the DTG traces. Weight loss data alone cannot determine whether water and/or NMF were being desorbed in a particular temperature region, but it was possible to observe trends in the magnitude and temperatures of the maxima as the clay was exposed to NMF for longer periods of time. When Ca-SWy-2 was fully loaded (i.e., after 30 days), three maxima were observed in the DTG trace (Figure 1). The maximum at highest temperature (320 °C) was clearly apparent after 30 h, and when this maximized (≈8.0% weight loss) after 3 days a second feature began to grow in at 200 °C. This was followed by a third feature at 140 °C. Note that as the maximum at 320 °C grew in, there was a corresponding decrease in the maximum attributed to water desorption (90 °C). Similar DTG traces were obtained for the Mg-SWy-2/NMF complexes, but the maxima were at different temperatures, particularly the highest temperature maximum, which occurred at 400 °C (Figure 2). Moreover, the trace did not return to baseline between the maxima at 200 and 400 °C, indicating that desorption was also occurring in this region. The DTG traces of the Na-SWy-2/NMF complexes also exhibited three features but the highest temperature maximum was at 190 °C, and the lowest temperature maximum creates the asymmetry on the low-temperature side of the feature at 150 °C (Figure 2). A similar progressive population of sorption sites was observed for the adsorption of cyclohexylamine on Al3+ montmorillonites.25 The speed at which the maxima accumulated (in particular, the highest temperature maximium in each DTG) as the exposure time increased was much faster for Na-SWy-2 than for Ca-SWy-2 and in turn for Mg-SWy2. These differences are probably due to the polarizing power of each cation, which increases in the order Na < Ca < Mg and reflects their increasing ability to retain sorbed water. A slower accumulation of NMF on MgSWy-2 was thus anticipated since it is more difficult to displace water from this more polarizing cation. The characteristic desorption maxima observed in the DTG traces of the Mn+-SWy-2/NMF complexes are probably associated with distinctive sites or environments (25) Ballantine, J. A.; Graham, P.; Patel, I.; Purnell, J. H.; Williams, K.; Thomas, J. M. Proc. Int. Clay Conf. 1987, 311-318.
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Figure 4. VT-XRD traces of the Ca-SWy-2/NMF complex (30 days in vapor).
Figure 5. VT-XRD traces of the Mg-SWy-2/NMF complex (30 days in vapor).
for the sorbed NMF molecules. FTIR can provide useful information regarding the nature/strength of interaction between sorbed organics and the exchange cations in smectites. However, it is important to know how much gallery space is available to the sorbed species prior to detailed FTIR investigation. Variable Temperature-X-ray Diffraction. XRD results have shown that the interlayer space of the Mn+SWy-2 increased upon exposure to NMF, thus indicating that NMF resides within the galleries. Moreover, VT-XRD confirmed that as the complexes were heated, the interlayer spacing decreased in a stepwise manner at temperatures commensurate with the corresponding DTG traces (Figure 3). Water-saturated Mn+-SWy-2 form a collapsed clay (d001 ≈ 10 Å) at temperatures above 200 °C. The VT-XRD traces of Ca-SWy-2 after exposure to NMF vapor for 30 days (Figure 4) show that the d spacing of the complex decreased in two distinct stages. The first step, a decrease of 3.9 Å, occurred when the sample was heated to 170 °C and coincided with the end of the first major weight loss in the DTG trace (Figure 1), which maximized at 90 °C. The second step, a decrease of 3 Å, occurred upon heating to 300 °C and correlated with the loss of the high-temperature maximum at 320 °C. The minor discrepancy between the DTG results and the VTXRD data can be attributed to the more protracted heating regime in VT-XRD compared with the controlled linear heating rate in TGA.19,26 This indicates that the complex contained two layers of NMF molecules in the gallery at 25 °C, which decreased to one layer upon heating to 170
°C. At 350 °C the d001 spacing was 10.2 Å and remained constant at higher temperatures, indicating that the clay layers had collapsed and all the interlayer material had been removed. Some NMF molecules may have been trapped in the collapsed structure since the peak at 10.2 Å was more asymmetric to low angles compared to a clay heated in the absence of NMF. The presence of two layers is commensurate with the size of the NMF molecules, the interlayer spacing, and the general trend observed for other organo-clay complexes.13 Similar behavior was observed for the Na-SWy-2/NMF complexes (not illustrated), but the correlation between the reductions in interlayer spacing and the maxima in the DTG traces was not as clear due to the proximity of the maxima and the heating parameters used. Three distinct spacings were observed when the fully loaded Mg-SWy-2/NMF complex was heated (Figure 5). The first step (-4.9 Å) occurred when the complex was heated from room temperature to 170 °C, thus coinciding with the two low-temperature maxima in the DTG trace. This was followed by a gradual contraction (-1.3 Å) between 220 and 350 °C, which coincided with the desorption between 260 and 340 °C. The third step (-1.7 Å) occurred when the complex was heated from 350 to 400 °C and relates to the high-temperature maximum. The Mg-SWy-2/NMF complex prepared at 25 °C was also expected to contain two layers of NMF molecules, one (26) Breen, C.; Rawson, J. O.; Mann, B. E. J. Mater. Chem. 1996, 6, 849.
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Scheme 1. Possible Adsorption Mechanisms between NMF and Mn+-SWy-2
layer between 170 and 300 °C and above 400 °C the clay will have collapsed. In each of the Mn+-SWy-2/NMF complexes the NMF molecules desorbed during the high-temperature maximum were in the interlayer region and infrared studies suggest that these were most likely coordinated to the cations (vide infra). If all the NMF molecules contributing to the highest temperature maximum in each Mn+-SWy2/NMF complex were indeed associated with exchangeable cations, then the TGA data suggest that there were approximately 4 NMF molecules per Mg cation, 3 NMF molecules per Ca cation, and 2 NMF molecules per Na cation. The number of NMF molecules associated with each exchange cation were calculated from the weight loss associated with the highest temperature desorption maximum for NMF relative to the weight of the Mn+SWy-2 prior to the onset of dehydroxylation at 550 °C. VT-DRIFTS of Ca-SWy-2/NMF (after 30 Days of Exposure). Assuming that NMF was associated with clay sites of different affinities, the vibrational frequencies of the NMF bands associated with these different sites should cause shifts in the associated infrared bands and thus provide a means of identifying the nature of the sites present. NMF is a small, yet complex molecule when considering its possible adsorption sites on montmorillonites because of its carbonyl and N-H groups. Prior to the infrared studies herein, it was not known whether direct coordination to exchange cations would occur via the lone pair of electrons of the nitrogen or oxygen atom. Self-intermolecular hydrogen bonding is also possible and can be present in clusters or associated with NMF directly coordinated to exchange cations (Scheme 1). Residual water can also hydrogen bond to NMF and in some organo-clay complexes there is spectral evidence to support water molecules acting as a bridge between the organic and exchange cation.18 Furthermore, protonation of NMF may occur, which could result in the displacement of exchangeable cations. This is known to happen with alklyamines and alkyldiamines27 but is considered much less likely herein. The desorption of residual water from montmorillonites has been discussed extensively in the literature28-30 and is not emphasized here. However, it is necessary to point out that the infrared spectra of heated Mn+-SWy-2 did show a decrease in intensity of the broad O-H stretching bands, νs(OH), (≈3420 and 3225 cm-1) and the O-H (27) Ruiz-Conde, A.; Ruiz-Amil, A.; Perez-Rodriguez, J. L.; SanchezSoto, P. J.; de la Cruz, F. A. Clays Clay Miner. 1997, 45, 311. (28) Farmer, V. C.; Russel, J. D. Spectrochim. Acta 1964, 20, 1149. (29) Bishop, J. L.; Pieters, C. M.; Edwards, J. O. Clays Clay Miner. 1994, 42, 702. (30) Shewring, N. I. E.; Jones, T. G. J.; Maitland, G.; Yarwood, J. J. Colloid Interface Sci. 1995, 176, 308.
Figure 6. VT-DRIFTS spectra of Ca-SWy-2 after exposure to NMF vapor for 30 and 3 days (3800-2500 cm-1) together with the transmission spectrum of liquid NMF. Table 3. Assignment of the Bands Observed in the Infrared Spectrum of Pure Liquid NMF band position (cm-1)
assignment
3300 3066 2944 2879 2807 (shoulder) 2756 (shoulder) 1667 1543 1453 1412 1384 1321 (shoulder) 1242 1148
νs(NH) Fermi resonance band νas(CH3) νs(C-H) νs(CH3) amide I, (mostly νCdO) amide II (δNH + νCN) δas(CH3) δs(CH3) δ(CH) amide III, (νCN + δNH) CH3 rocking
bending band, δOH, (1627 cm-1). These water molecules are either firmly bound (directly coordinated) to exchange cations or weakly bound (physisorbed) occupying interlamellar spaces between saturated exchangeable cations and/or on polar sites on external surfaces. The VT-DRIFTS spectra (3800-2500 cm-1) of the CaSWy-2/NMF complex formed after exposure to NMF vapor for 30 days (Figure 6, middle spectra) showed a decrease in the intensity of the bands as the complex was heated. The assignment of the NMF bands was extracted from a number of references11,31,32 and are collected in Table 3. The first prominent bands to decrease were those at 3300 and 3075 cm-1. These bands were assigned to the N-H stretching (νs(NH)) and Fermi resonance bands (of νs(NH) and the first overtone of the δNH band of NMF), respectively, and are similar to those of NMF when it is in its liquid state (Figure 6, top spectrum). The loss of these (31) De Graaf, D. E.; Sutherland, G. B. B. M. J. Chem. Phys. 1957, 26, 716. (32) Forarasi G.; Balaza, A. J. Mol. Struct. (THEOCHEM) 1985, 133, 105.
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Figure 7. Schematic representation of the correlation between NMF sites/environments and the maxima observed in the DTGs.
bands correlated with the low-temperature maximum (140 °C) in the corresponding DTG trace, which suggests that the NMF present up to 150 °C in the complex was in a form similar to that of the liquid NMF (i.e., self-associated with a strong hydrogen-bonding network). Given the concomitant decrease in layer spacing (Figure 4), the majority of this H-bonded network of NMF molecules was probably in the clay gallery. The next prominent band to decrease in intensity was that at 3420 cm-1 between 250 and 300 °C. This band was also assigned to the νs(NH) of NMF, but occurred at a higher wavenumber because it was not as strongly involved in hydrogen bonding. This band is unlikely to be due to water since TG-FTIR data13 showed the majority of water had been replaced. The position of this band was at a lower wavenumber than that observed for NMF molecules diluted in CCl4, 3466 cm-1 (i.e., effectively nonhydrogen-bonded), and therefore suggested that the NH bonds were not totally “free” but were still experiencing some form of interaction, possibly with other NMF molecules or with the clay structure. The intensity of this band was substantially reduced between 200 and 350 °C, which relates to the high-temperature maximum (320 °C) observed in the DTG trace. These NMF molecules, which were retained at such a high temperature, must be coordinated with a “strong” bond and since the N-H bonds were not undergoing a strong interaction, it indicates that the strong bond must be between the carbonyl group and exchangeable cations. When the sample was heated from 150 to 250 °C, the asymmetry on the low-wavenumber side of the band at 3420 cm-1 was reduced, presumably due to the removal of NMF molecules involved in interactions of varying strength via the N-H bond. This loss in asymmetry correlates with the maximum observed at 200 °C in the DTG trace. The NMF removed was probably present as a species coordinated to external exchangeable cations found at broken edges and surfaces (which account for 20% of the CEC) or to directly coordinated NMF molecules. These latter type of NMF molecules form the second coordination sphere, as depicted in Figure 7. At 500 °C the νs(NH) and Fermi resonance bands had been completely removed and only the broad stretching bands of the structural OH groups remained. It is possible that the H atom of the N-H bond keys into the hexagonal cavity of the silicon tetrahedral sheets, particularly at higher temperatures as the gallery space decreases. Similar keying behavior involving the H atoms of hydrazine and
Figure 8. VT-DRIFTS spectra of Ca-SWy-2 after exposure to NMF vapor for 30 and 3 days (2000-1200 cm-1) together with the transmission spectrum of liquid NMF.
the hexagonal cavities of kaolinite has been shown via infrared spectroscopy by Johnstone and Stone.33 They noted frequency shifts for the inner hydroxyl bands, which were located between the silicate tetrahedral and alumina octahedral layer that is not usually accessible to guest species. In the C-H stretching region (3000-2600 cm-1) of liquid NMF the 2879 cm-1 band (νs(C-H)) of the single C-H group was more intense than the band at 2944 cm-1 (νas(CH3)). However, at 25 °C the νs(C-H) band was relatively reduced in intensity and shifted slightly to higher wavenumber, which indicates that the environment of the C-H bond in the complex had changed compared to that in the liquid. When the liquidlike NMF had been removed from the complex at 150 °C (absence of the bands at 3300 and 3076 cm-1), the νs(C-H) band of the remaining NMF molecules appeared more distinct and was at an even higher wavenumber (2923 cm-1). Such a large change (44 cm-1) is very uncommon for a C-H stretching band and indicates that the C-H bond is in a unique environment. The minor changes in νas(CH3) (2944 cm-1) and δs(CH3) (1413 cm-1) indicated that the CH3 group was not significantly involved in the clay/NMF interaction. The first observation to note in the VT-DRIFTS spectra (Figure 8, middle spectra) of the Ca-SWy-2/NMF complex formed after 30 days in vapor in the 2000-1000 cm-1 region was the intensity of the 1627 cm-1 band had decreased considerably in comparison with a watersaturated clay and was only a weak shoulder, indicating that much of the water had been replaced by NMF. This water was weakly held and removed by 100 °C. The band at 1664 cm-1 is the amide I band, which is due mostly to the stretching vibration of the carbonyl bond (νs(CdO)).11 The intensity of this band decreased less between 25 and 250 °C than between 250 and 350 °C, which was not expected given that 70% of the total weight (33) Johnston, C. T.; Stone, D. A. Clays Clay Miner. 1990, 38, 121.
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loss occurred between 25 and 250 °C. This was attributed to the NMF molecules retained at higher temperatures having a higher extinction coefficient than those removed at lower temperatures. This interpretation was supported by the observation that the decrease in intensity of the bands at 1542 and 1383 cm-1 (also due to NMF) was in proportion to the loss in weight of the complex. The anticipated large decrease in intensity was therefore compensated by the increase in the extinction coefficient of the remaining NMF molecules that were coordinated directly to the exchange cation. Incidentally, it has been shown that the molar absorptivity of the O-H bending mode of water increases upon removal of water from montmorillonite and that the effect was more pronounced for more highly polarizing exchange cations.34 The fact that the νs(CdO) was at 1664 cm-1 and not at a higher wavenumber than that in liquid NMF means that the NMF molecules were interacting very strongly with the clay via their carbonyl group. This is because NMF molecules in the liquid phase form very strong intermolecular bonds, which is reflected in NMF’s high boiling point (180-185 °C). Coordination to the clay therefore occurs through the carbonyl group rather than the N-H group, which was supported by the shift of the νs(N-H) from 3300 to 3420 cm-1 (Figure 6). The band at 1542 cm-1 was assigned to the amide II band and was due to almost pure N-H deformation (δNH).11 This band increased slightly in frequency (≈5 cm-1) as the temperature was increased from 25 to 250 °C and indicated that the environment of the N-H bond was progressively changing. It is possible that this band shifted to higher frequency and led to the low intensity band at 1595 cm-1 in the complex heated to 300 °C. If this is the case, then the N-H bond must be experiencing a higher degree of interaction because more energy is required to bend the constrained N-H bond. This may be true at 300 °C given that the clay layers have collapsed and the N-H bond of any trapped NMF molecules would be in close proximity to the clay layers. Since such a large shift was considered unlikely for a δNH, the band at 1595 cm-1 may be due to the νs(CdO) of NMF molecules retained at high temperatures that were very strongly coordinated (via the exchangeable cations) to the clay. However, although a shift to lower frequency was anticipated, a massive shift of ≈70 cm-1 was considered unlikely and thus leaves the exact assignment of the 1595 cm-1 band uncertain. The appearance of the band at 1334 cm-1 when the complex was heated between 100 and 250 °C was attributed to a shift of the amide III band11 (almost pure C-N stretching (νs(CN))) originally positioned at 1383 cm-1. The presence of this band relates to the maximum at 200 °C in the DTG trace and suggests that some of the NMF molecules lose their CN double-bond character as they interact more strongly with the clay. The band at 1453 cm-1, δas(CH3), observed in the spectrum of liquid NMF and the Ca-SWy-2/NMF complexes collected below 150 °C was very broad. However, above 150 °C the complex exhibited a more distinct band at lower frequency (1444 cm-1). This suggests the number of environments (and range of interactions) of CH3 groups decreases greatly as the liquidlike NMF in the clay gallery is desorbed. VT-DRIFTS of Ca-SWy-2/NMF (after 3 Days of Exposure). The VT-DRIFTS spectra of the Ca-SWy-2/ NMF complex formed after 3 days of exposure to NMF (Figures 6 and 8, bottom spectra) have also been studied (34) Johnston, C. T.; Sposito, G.; Erickson, C. Clays Clay Miner. 1992, 40, 722.
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to support the foregoing interpretations and determine whether the presence of extra NMF molecules (i.e., those contributing to the two low-temperature maxima in the DTG trace of complex exposed for 30 days) affected the infrared bands of the NMF molecules associated with the desorption maximum at 320 °C (Figures 1 and 7). The spectra therefore represent the sites relating to the maximum at 320 °C in the DTG trace. The first difference was that at 25 °C the prominent band previously observed at 3300 cm-1 has been replaced by a broad band at 3392 cm-1. This broad band was due to the νs(OH) of water and the underlying NMF bands. This is supported by the relatively more intense band at 1627 cm-1 (δOH (water)) in Figure 8, lower spectra. These bands were removed when the complex was heated to 100 °C, indicating that the 90 °C maximum in the corresponding DTG trace (Figure 1) was due to the loss of water. The absence of the bands at 3300 and 3075 cm-1 shows that there were no intermolecular hydrogen-bonded NMF molecules present in the gallery. The N-H stretching bands were the same in both complexes above 200 °C, confirming that the N-H bonds were interacting less strongly. The C-H stretching region was not masked by bands due to liquidlike NMF molecules. This reinforced the view that the C-H bonds in the NMF molecules associated with the high-temperature maximum were uniquely positioned. The gallery heights of Ca-SWy-2 after exposure to NMF vapor for 3 days (Figure 3) were almost identical to those exposed for 30 days. VT-DRIFTS spectra confirmed that all the water had been removed from the complex by 100 °C and the only NMF present was that associated with the maximum at 320 °C in the DTG trace. This suggests that the NMF molecules directly associated with the interlayer cation controlled the gallery height and not those present as NMF clusters (i.e., those directly coordinated to the cation as depicted by the inner coordination sphere in Figure 7). The νs(CdO) bands at 1664 cm-1 in both complexes were almost the same relative intensity whereas the bands at 1542 and 1383 cm-1 were relatively less intense in the spectra of the complex formed after 3 days. This supports the interpretation above regarding the extinction coefficient of the νs(CdO) of the NMF molecules, which increased as the NMF loading decreased and the weakly bound NMF molecules were desorbed. Note also that the 1442 cm-1 band was clear and distinct at all temperatures. VT-DRIFTS of Mg- and Na-SWy-2/NMF (Each after 30 Days of Exposure). The VT-DRIFTS spectra of the Mg- and Na-SWy-2/NMF complexes formed after 30 days of exposure (Figures 9 and 10) exhibited similar trends, although there were differences in the detail, particularly, the temperatures at which the bands characteristic of both non- and intermolecular hydrogenbonded NMF molecules were removed. First, the 3420cm-1 band in the Mg-SWy-2/NMF complex shifted to 3405 cm-1 as it was heated from 150 to 300 °C, which coincided with the decrease in the gallery height and thus indicates the N-H bond was interacting more strongly, probably with the clay. The second difference occurred in the C-H stretching region. When the νs(C-H) band due to intermolecular hydrogen-bonded NMF (2887 cm-1) was removed in the Mg-SWy-2/NMF complex, the νs(C-H) band due to non-intermolecular hydrogen-bonded NMF was closer to the νas(CH3) band (2944 cm-1) and contributed to the low-wavenumber asymmetry of this band. As a result, the intensity of the band at 2944 cm-1 relative to the band at 2809 cm-1 was larger than the corresponding bands in the Ca-SWy-2/NMF complex (i.e., bands at 2944
NMF/Ca-, Mg-, and Na-Exchanged Montmorillonite
Figure 9. VT-DRIFTS spectra of Mg- and Na-SWy-2 after exposure to NMF vapor for 30 days (3800-2500 cm-1).
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It is interesting to note that there were several distinct bands at lower frequency in the carbonyl region of the spectra (1642-1597 cm-1) of Mg-SWy-2/NMF at 400 °C. This could indicate a range of strong interactions between the carbonyl bond and the clay, which is realistic given that these NMF molecules were retained to a much higher temperature than those in the Ca-SWy-2/NMF complex and would therefore be strongly bound. The position of the carbonyl band in the Na-SWy-2/NMF complex at 25 °C was at a higher frequency (1678 cm-1) than the respective spectra in the Mg- and Ca-SWy-2/NMF complexes (both at 1664 cm-1) and indicates the NMF molecules were interacting less strongly with the Na+ cation. It was anticipated that the νs(CdO) band of NMF molecules would exhibit more pronounced shifts when they were complexed in different environments, as reported for other amide-montmorillonite complexes. Tahoun and Mortland35 observed shifts in the carbonyl band of acetamide and N-ethylacetamide when they were complexed with Ca-, Na-, and Cu-exchanged montmorillonites. The carbonyl band of the acetamide shifted from 1681 to 1659, 1667, and 1661 cm-1, respectively, while the carbonyl band of the N-ethylacetamide shifted from 1650 cm-1 in the liquid to 1637, 1635, and 1621 cm-1, respectively. Affrossman et al.36 studied the adsorption of N-ethylacetamide and 2-pyrolidone on alumina by DRIFTS and reported carbonyl band shifts from 1715 cm-1 in the vapor to 1634 cm-1 for the former and from 1681 to 1668 cm-1 for the latter. It was also anticipated that the shifts in the νs(CdO) would relate to the polarizing power of the exchange cations, as was observed in the infrared studies completed by Onikata et al.37 for propylene carbonate on various cation-exchanged montmorillonites. The νs(CdO) in the spectra collected below 50 °C from the 30-day-exposed Ca-SWy-2/NMF complex was at 1664 cm-1 and shifted to 1671 cm-1 after the liquidlike NMF had been removed (i.e., at 150 °C). This indicated that the NMF molecules were interacting less strongly through their carbonyl group and infers that the formation of a hydrogen bond between NMF molecules was stronger than the bond formed between the NMF carbonyl group and a cation. This was unexpected but may be the case because the strength of the hydrogen-bonding network is reflected in the relatively high boiling point of NMF liquid (180185 °C). The effect on the νs(CdO) band of adding cations to pure liquid NMF has received little attention, but Bonner and Jordan38 studied this using both infrared and Raman spectroscopy and found that the addition of some salts caused the band to shift to higher wavenumbers. Some examples are NaI (1671 cm-1), LiI (1667 cm-1), LiCl (1667 cm-1), and NaPF6 (1675 cm-1). This increase was attributed to the disruption of the strong hydrogenbonding network between the NMF molecules, but herein the effect of the large planar clay anion may also exert an influence on the νs(CdO).38 Summary and Conclusions
Figure 10. VT-DRIFTS spectra of Mg- and Na-SWy-2 after exposure to NMF vapor for 30 days (2000-1200 cm-1).
and 2814 cm-1). This more pronounced increase in band frequency indicated that the C-H bond in the NMF molecules retained at higher temperature occupied a more perturbed environment in the Mg than in the Ca complex. It is possible that the C-H group was positioned within the hexagonal cavity, perturbing its movement and thus shifting the frequency of the band.
The DTGs of the Mn+-SWy-2/NMF complexes showed that the temperature of desorption maxima for NMF can be used to clearly distinguish between the type of interlayer cations present in the clay. The thermal (35) Tahoun, S. A.; Mortland, M. M. Soil Sci. 1965, 102, 314. (36) Affrossman, S.; Armstrong, D. R.; Robb, D.; Treverton, J. A. Langmuir 1995, 11, 2060. (37) Onikata, M.; Kondo, M.; Hayashi, N.; Yamanaka, S. Clays Clay Miner. 1999, 47, 672. (38) Bonner, O. D.; Jordan, C. F. Physiol. Chem. Phys. 1976, 8, 293.
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techniques used herein showed the most thermally stable complex to be Mg-SWy-2/NMF, whereas the least thermally stable was Na-SWy-2/NMF. The gallery heights of the complexes at room temperature indicated two NMF layers were present, which subsequently reduced to one layer when heated to 175 °C. These single-layer complexes contained approximately 4, 3, and 2 NMF molecules per Mg-, Ca-, and Na-exchanges cations, respectively. Excellent agreement was observed for the major changes in intensity of the infrared bands with the TG and VT-XRD
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data. The VT-DRIFTS data showed the NMF was directly coordinated to the exchange cations through the CdO bond. In addition, NMF was present as clusters within the gallery and hydrogen bonded to directly coordinated species. Unusual shifts in the C-H and N-H adsorption bands indicated a unique orientation of these groups, which are believed to be interacting strongly with the clay layers. LA000124P
