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Adsorption Study of Acetylacetone on Cation-Exchanged Montmorillonite by Infrared Spectroscopy Jong Rack Sohn* and Sang Il Lee Department of Industrial Chemistry, Kyungpook National University, Taegu 702-701, Korea Received June 1, 1998. In Final Form: February 22, 2000 The adsorption of acetylacetone on layer silicates containing various cations has been studied by means of infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. An analysis of several split bands in the region of 1740-1520 cm-1 was made. The bands were attributed to the interaction of carbonyl with structural hydroxyl and cationic hydroxyl groups and were responsible for the formation of acetylacetonate complexes between enolic or anionic acetylacetone and metal cations in the silicate layers.
1. Introduction Montmorillonite (Mont) is a naturally occurring layer aluminosilicate containing exchangeable cations between the layers. The Mont minerals have characteristically high cation-exchange capacities and high surface areas and consequently great capacities for adsorption due to Coulombic and van der Waals forces. Various polar organic molecules penetrate into the interlayer spaces to form clay-organic complexes. The adsorption of organic molecules on the clay surface is highly dependent on the type of exchangeable cation, saturating at the negatively charged surface of the clay. The nature of clay-organic complexes has long been a subject for research and studied by many workers.1-5 Recent investigations have shown that transition metal cations can form complexes with organic molecules at the interlamellar surface of layer silicate minerals.6,7 Serratosa studied the orientation of pyridine molecules in clay complexes by infrared analysis and the X-ray diffraction method.8 Kanamaru and Vand determined the crystal structure of a 6-aminohexanoic acid-vermiculite complex using the X-ray single-crystal diffraction method.9 Recently, Koizumi and co-workers studied the formation of acrylonitrile-montmorillonite complexes and observed that the stability of the complex was directly related to the polarizing power of the interlayer cation.4 However, due to the complicated states of cations exchanged at the clay mineral surface as well as the existence of various types of adsorption sites, the bonding characteristics of adsorbed species are not as unique as a homogeneous solution. The versatility of acetylacetone as a coordinating ligand is well-known. It is the purpose of this paper to examine the effect of pretreatment of acetylacetone adsorbed on cation-exchanged Mont samples * To whom correspondence should be addressed. (1) Choudary, B. M.; Kumar, K. R.; Kantam, M. L. J. Catal. 1991, 13, 41. (2) Jamis, J.; Drljaca, A.; Spiccia, L.; Smith, T. D. Chem. Mater. 1995, 7, 2086. (3) Logan, M. B.; Howe, R. F.; Cooney, R. P. J. Mol. Catal. 1992, 74, 285. (4) Yamanaka, S.; Kanamaru, F.; Koizumi, M. J. Phys. Chem. 1975, 79, 1285. (5) Sackett, D. D.; Fox, M. A. Langmuir 1990, 6, 1237. (6) Rao, Y. V. S.; Rani, S. S.; Choudary, B. M. J. Mol. Catal. 1992, 75, 141. (7) Kijima, T.; Nakazawa, H.; Takenouchi, S. Bull. Chem. Soc. Jpn. 1991, 64, 1686. (8) Serratosa, J. M. Clays Clay Miner. 1969, 14, 385. (9) Kanamaru, F.; Vand, V. Am. Mineral. 1970, 55, 1550.
at different temperatures to clarify the involvement of adsorbed water and hydroxyl groups in the bonding mechanism and to understand the nature of interaction of acetylacetone at the surface of the adsorbent. 2. Experimental Section The principal Mont used was Wyo-gel, a commercial Wyoming bentonite obtained from Archer Daniels Midland Co., Cleveland, OH. Acetylacetone was obtained from Wako Pure Chemical Industry Ltd, Japan. The fraction of less than 1 µm size Mont whose cation-exchange capacity was known to be 69 mequiv/100 g were exchanged by the cations Li+, Na+, Ca2+, Ni2+, Co2+, Cu2+, Fe3+, and Al3+. The self-supported film specimens of those cationexchanged Mont, approximately 0.003 g in weight, were prepared by air drying fresh suspensions within a circular aluminum dish 2 cm in diameter.10 Since copper ions reacted with the aluminum dish, clay suspensions had to be dried on a flat polyethylene film. The IR spectra of adsorbents and adsorbed compounds were obtained in the range of 4000-1200 cm-1 by a Mattson GL 6030E FTIR spectrometer equipped with a heatable gas cell with potassium bromide windows. The thin film air-dried at room temperature was put in the heatable gas cell connected to the oil-diffused vacuum system and was evacuated at 10-5 Torr for 3 h prior to each adsorption process. Then it was allowed to contact with the vapor of acetylacetone (about 6 Torr) for 3 h and the free vapor was degassed to 10-5 Torr for 0.5 h. The degassing for at least 1 h and spectroscopic measurement were repeated at different temperatures to examine how the shape and size of absorption bands changed due to the degree of dehydration and to ascertain the adsorption mechanism. X-ray diffraction patterns of the film specimen used for IR measurement were taken with a JEOL model JDX-8030 diffractometer, employing Ni-filtered Cu KR radiation, a 1.0° beam divergence slit, and 0.1° detecter slit. The scan rate was 2° 2θ/ min. The interlamellar spacings of the clay-acetylacetone complexes were calculated by assuming a nonexpanded interlayer d(001) spacing of 9.6 Å. TGA-DSC measurements were performed with a PL-STA model 1500H apparatus in air, and the heating rate was 5 °C/min.
Results and Discussion The Mont film air-dried at room temperature holds much adsorbed water, whose absorption bands occur broadly at 3600-3200 cm-1 for the stretching vibration and 1630 cm-1 for the deformation vibration, respectively, as shown in Figures 1 and 3. When this air-dried film is exposed to acetylacetone vapor, a number of shifted bands are observed along with those of adsorbed water, as illustrated in Figures 1 and 3. The intensity of the (10) Kim, J. T.; Sohn, J. R. J. Korean Chem. Soc. 1973, 17, 247.
10.1021/la980637k CCC: $19.00 © 2000 American Chemical Society Published on Web 04/22/2000
Adsorption of Acetylacetone on Montmorillonite
Figure 1. Variation of OH stretching bands for the acetylacetone adsorbed on Co2+-Mont followed by degassing at different temperatures.
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Figure 3. Infrared spectra of acetylacetone adsorbed on Li+Mont and Co2+-Mont followed by degassing at different temperatures.
The versatility of acetylacetone as a coordinating ligand is well recognized. Acetylacetone consists of keto and enol tautomers as follows:
In general, the CdO stretching vibration band of the keto form11,12 appears at 1730 cm-1, while that of the enol form appears at 1625 cm-1. As shown in Figure 2, in the IR spectrum of acetylacetone vapor (6 Torr) only the carbonyl band at 1625 cm-1 due to the enol form was observed, indicating that the enol form is mainly present in the vapor state. However, the IR spectra of acetylacetone adsorbed on cation-Mont are quite different from that of the vapor. The shift of carbonyl stretching frequency due to acetylacetone adsorption showed that the adsorption took place through the carbonyl oxygen instead of other sites of the molecule. Especially, several carbonyl bands appeared in the region of 1740-1520 cm-1, showing that there are several adsorption mechanisms involving the carbonyl group of acetylacetone. Adsorbed acetylacetone also exhibited bands at 1415, 1389, and 1364 cm-1, those being attributed to the CH3 and CH2 deformation vibrations.13 The band at 1306 cm-1 is assignable to the C-C-O Figure 2. Infrared spectra of acetylacetone adsorbed on cation-Mont followed by degassing at 25 °C.
water band in the range of 3600-3200 cm-1 dimished gradually according to the elevation of the dehydration temperature.
(11) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; John Wiley & Sons: New York, 1978; pp 249-258. (12) Nakamura, Y.; Isobe, K.; Morita, H.; Yamazaki, S.; Kawaguchi, S. Inorg. Chem. 1972, 11, 1573. (13) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1964; p 322.
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Sohn and Lee Table 1. Infrared Frequency Shift of the Carbonyl Group of Acetylacetone Complexes at Dehydration Temperature of 190 °C and the Polarizing Power of the Interlayer Cation
interlayer cation
ionic radius (R), Å
charge (Z)
polarizing power (Z/R)
Na+ Li+ Ca2+ Co2+ Cu2+ Ni2+ Fe3+ Al3+
0.95 0.60 0.99 0.72 0.72 0.69 0.64 0.50
+1 +1 +2 +2 +2 +2 +3 +3
1.05 1.67 2.02 2.78 2.78 2.90 4.68 6.00
a
Figure 4. Infrared spectra of acetylacetone adsorbed on Cu2+Mont and Fe3+-Mont followed by degassing at different temperatures.
stretching vibration. However, absorption bands appearing below 1200 cm-1 have been omitted since they were obscured by the silicate skeletal vibrations. Our primary interest was to identify active sites for acetylacetone adsorption. An analysis of several split bands in the region of 1740-1520 cm-1 was made. For representative examples, the variations of IR spectra of acetylacetone adsorbed on Li+-Mont, Co2+-Mont, Cu2+Mont, and Fe3+-Mont at different dehydration temperatures are illustrated in Figures 3 and 4. The A band in Figures 3 and 4 always appears as a sharp independent band at 1740 cm-1 with (2 cm-1 variation. Since the independent nature of the band exhibits no correlation with the species of cation, we have assigned this band to the carbonyl band adsorbed on the structural hydroxyl group14,15 as >CdO‚‚‚HO-Struct (structural hydroxyl). The B band around 1700 cm-1 shows appreciable variation in its frequency and shape according to the species of cation as well as dehydration temperature. The B bands exhibit shifts toward the higher frequencies on the basis of the enolic carbonyl band (1625 cm-1) for acetylacetone vaper as listed in Table 1, where the + and - signs mean higher and lower shifts, respectively. After the adsorbate vapor is pumped off from the gas cell at an elevated temperature, the B band shifts to lower frequencies and simultaneously the intensity of the band reduces. The frequency of the B band for Cu2+-Mont was plotted as a function of temperature. As illustrated in Figure 4, the higher the temperature, the lower the frequency of the B band. Similar spectra were recorded with other cation-Mont samples. This result supports both the resonance theory reported previously10 and the fact that the B band is related to the interlayer cation. It seems (14) Farmer, V. C.; Russell, J. D. Spectrochim. Acta 1964, 20, 1149. (15) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966; p 334.
carbonyl band shift (cm-1) B C E banda band band +75 +75 +75 +77 +82 +77 +95 +95
-6 -8 -18 -19 -21 -23 -35 -41
-83 -83 -83 -83 -83 -83 -85 -85
The dehydration temperature is 25 °C.
likely that there is also a correlation between the band shift and the polarization power of the interlayer cation as defined later. At low dehydration temperature, it involves the linking of acetylacetone to an exchangeable metal cation through a water bridge bonding as > CdO‚‚‚H-O(H)-Mn+. This kind of bond has been demonstrated for Mont complexes with pyridine, benzoic acid, nitrobenzene, and amides by other authors.16,17 An attempt to represent a schematic diagram of the hydroxylation and dehydration process of cationic water was made in view of results obtained by Mortland and his co-worker.18 They investigated the stereochemistry of the hydrated cation in the interlamellar surface of layer silicate by ESR and proposed a inclined octahedral arrangement of water for the double layer and a planar arrangement for the single layer of water. There is also general agreement that the formation of hydroxyl groups in the cationexchanged zeolites is due to hydrolysis of the cation and dissociation of the water molecule by the electrostatic field created by the cation.19,20 Similarly, in the case of cationexchanged Mont under a reasonably dehydrated condition, the equilibrium between the adsorbed water and cationic hydroxyl formation is established, which involves the formation of a complex with a cationic hydroxyl group as >CdO‚‚‚H-O-M(n-1)+. The hydroxylation of cationic water is promoted by the hydrogen interaction with adjacent surface oxygen.17 The size of the cation and orientation of water around the cation, controlled by the coordinating power of the cation and the space around it, should also be an important factor. Koda et al. reported that the IR spectra of the molecular complex of acetylacetone with manganese(II) bromide exhibits CdO and CdC stretching bands at 1627 and 1564 cm-1 bearing a close resemblance to those of the enolic form of acetylacetone.21 A molecular adduct of acetylacetone with dioxobis(acetylacetonato)uranium(VI) was also suggested to contain the enolic molecule as a unidentate ligand.22 Consequently, the C and D bands around 1620 and 1570 cm-1 in Figures 3 and 4 are assigned to CdO and CdC stretching vibrations of enolic acetylacetone linked to cations as unidentate ligands. (16) Tahoun, S.; Mortland, M. M. Soil Sci. 1966, 102, 314. (17) Mortland, M. M. Adv. Agron. 1970, 22, 75. (18) Clementz, D. M.; Pinnavaia, T. J.; Montland, M. M. J. Phys. Chem. 1973, 77, 196. (19) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974; p 462. (20) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases; Kodansha: Tokyo, 1989; p 147. (21) Koda, S.; Ooi, S.; Kuroya, H.; Nakamura, Y.; Kawaguchi, S. Chem. Commum. 1971, 280. (22) Haigh, J. M.; Thornton, D. A. Inorg. Nuclear Chem. Lett. 1970, 6, 231.
Adsorption of Acetylacetone on Montmorillonite
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Table 2. Interlamellar Spacing of Cation-Mont and Acetylacetone-Mont Complexes Evacuated at 25 °C spacing for cation-Mont (Å) spacing for acetylacetone-Mont complex (Å)
Na+
Li+
Ca2+
Co2+
Cu2+
Ni2+
Fe3+
Al3+
2.8 3.1
3.7 3.2
5.1 3.2
5.2 3.3
5.9 3.2
5.2 3.2
5.8 3.2
4.9 3.2
The C band exhibits a systematic shift toward the lower frequencies, and simultaneously the intensity of the band reduces as the dehydration temperature increases. The extent of the C band shift is dependent on the cation and dehydration temperature. It is of great interest to relate the band shift with the properties of cations. There is a good correlation between the band shift and the polarizing power of the interlayer cation as listed in Table 1, where the band shift was calculated on the basis of the enolic carbonyl band (1625 cm-1) for acetylacetone vapor. Polarizing power (Z/R) is defined as a ratio of the charge of the cation (Z) to its ionic radius (R) and is directly related to the strength of the electrostatic field around the cation. Since no information is available on the size of these exchangeable cations in Mont, Pauling’s crystal radii have been used.23 Carbonyl frequency shifts (C band) at 190 °C are listed in Table 1 together with the polarizing power of the interlayer cation. These results suggest that cations of higher polarizing power are able to interact more powerfully with the carbonyl oxygen of acetylacetone. In general, the CdO stretching band of the chelated acetylacetonate group24 appears at 1540-1560 cm-1. Therefore, E bands in Figures 2-4 are assigned to CdO stretching vibration of anionic acetylacetone complex, where acetylacetone forms the complex as an anion ligand as shown in following scheme:
The G band at 1547 cm-1 in Figures 3 and 4 is attributed to CdC stretching of the anionic acetylacetone complex. As seen in Figure 2, the E band around 1530 cm-1 is predominant in transition-metal and aluminum cationexchanged Mont samples even at the dehydration temperature at 25 °C, showing remarkably enhanced intensities for Cu2+, Fe3+, and Al3+. As illustrated in Figures 3 and 4, the intensity of the carbonyl E band increases gradually with increasing dehydration temperature. It is of interest that the intensity of E band increases simultaneously at the expense of the carbonyl C band. To make this point clearer, we plotted the intensity ratio of E band to C band for Cu2+-Mont and Co2+-Mont as a function of dehydration temperature, where the curves were analyzed by appropriate Gaussion curve fitting to calculate the band intensities. As shown in Figure 6, the intensity ratios of the E band to C band increase as the dehydration temperature increases. Thus, we propose that this type of complex (E band) is originated from an enolic acetylacetone complex (C band). As listed in Table 1, most E bands appeared in the region of 1542-1540 cm-1, showing the nearly constant band shift of -83 to -85 cm-1. So it seems likely that there is no correlation between the shift of E band and the polarizing power of the interlayer cation. (23) Sohn, J. R.; Lee, S. I. Stud. Surf. Sci. Catal. 1997, 105, 1763. (24) Fackler, J. P.; Mittleman, M. L.; Weigold, H.; Barrow, G. M. J. Phys. Chem. 1968, 72, 4631.
Figure 5. Frequency of the B band for Cu2+-Mont as a function of dehydration temperature.
Figure 6. Intensity ratio of the E band to C band for Cu2+Mont (b) and Co2+-Mont (O) as a function of dehydration temperature.
The preparation of the clay-acetylacetone complexes in the form of films resulted in a well-ordered state in which the clay plates lie parallel to the supporting surface. The thickness of the covering around cations can be measured by X-ray diffraction analysis. Interlamellar spacings of the samples prepared under various conditions are listed in Table 2, where the spacings were calculated assuming a nonexpanded interlayer d(001) spacing of 9.6 Å.25 Values of these spacings of cation-Mont samples can be classified into two main groups. Na+- and Li+-Mont belong to the first group whose interlamellar spacings are from 2.8 to 3.7 Å, while the other samples belong to a second group whose spacings are from 4.9 to 5.9 Å. This means that adsorbents of the first group hold a monolayer of water and those of the second group hold a double layer of water in the air-dried state. As listed in Table 2, the interlamellar spacing of most acetylacetone complexes decreases to the extent of 0.5(25) Kim, J. T.; Sohn, J. R. J. Korean Chem. Soc. 1977, 21, 246.
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on the interlamellar cation. The stability of acetylacetone complex on Mont was also correlated with the polarizing power of the interlayer cations. That is, the temperature for complete desorption of acetylacetone was 240 °C for Na+, 300 °C for Li+, 320 °C for Ca2+, 360 °C for Co2+, 350 °C for Cu2+, 360 °C for Ni2+, 420 °C for Fe3+, and 430 °C for Al3+. Conclusion
Figure 7. Proposed orientation of the acetylacetone complex in the interlamellar region of Mont.
2.7 Å under dehydration at 25 °C. The decrease of the interlamellar spacing is attributed to the removal of retained water instead of acetylacetone, because the IR study shows that the intensities of adsorbed water bands decrease after adsorption of acetylacetone. On the basis of interlamellar spacings (3.1-3.3 Å), it is suggested that the intercalated acetylacetone molecules coordinate to cation in such a way that the molecular plane is parallel to silicate layers as shown in Figure 7. To examine the desorption tendency of acetylacetone adsorbed on cation-Mont, thermogravimetric analyses have been performed. The temperature of complete desorption of the acetylacetone was different depending
An analysis of characteristic split bands in the region of 1740-1520 cm-1 was made. A sharp band (A) which appeared at 1740 cm-1 was attributed to the interaction of carbonyl with structural hydroxyl. The band (B) around 1700 cm-1 was responsible for the linking of acetylacetone to an exchangeable metal cation through a water bridge or cationic hydroxyl group. The bands (C and D) which appeared around 1620 and 1570 cm-1 were assigned to CdO and CdC stretching vibrations of enolic acetylacetone linked to the cation as a unidentate ligand, respectively. The band at 1540-1560 cm-1 (E) was attributed to the formation of anionic acetylacetone complex, while the band (G) at 1547 cm-1 was assigned to CdC stretching of the anionic acetylacetone complex. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation through the Research Center for Catalytic Technology at Pohang University of Science and Technology. LA980637K
