- Email: [email protected]
Electron spin resonance study of copper acetylacetonate adsorbed on clays
MICROPOROUS MATERIALS E L S EV I E R
Microporous Materials 4 ( 1995) 187-193
I
Electron spin resonance study of copper acetylacetonate adsorbed on clays J.V. Zanchetta *, B. Deroide, B. Asri, J.C. Giuntini Laboratoire de Physicochimie des Matkriaux Solides, Equipe de Chimie Physique (URA D0407 CNRS), Universitb Montpellier II, Place E. Bataillon. F-34095 Montpellier Cedex 5, France
Received 16 July 1994; accepted 14 December 1994
Abstract
Two clays have been studied by means of a copper acetylacetonate probe. Swelling clays (montmorillotaites) and non-swelling clays (kaolinites) have been investigated. Kaolinite-Na (K1-Na) shows a clear interaction with the copper complex. At room temperature, the electron spin resonance (ESR) spectra of K1-Ca do not show a real axial symmetry. In spite of the large basal spacing of the montmorillonites (Na, Ca, and Zr) the spectra are less well resolved than those of kaolinite. The tensor components gll, g±, All and A± are obtained by numerical simulation of the spectra. Adapted parameters are calculated in order to determine the magnitude and nature of the interaction of the copper complex with the clays. Keywords." Clay; Kaolinite; Montmorillonite; Electron spin resonance; Copper complex
1. Introduction
The introduction of a paramagnetic species into a solid is a well known method used in the study of the local structure of compounds, crystalline or amorphous. This can be performed in porous substances in several ways. Organic free radicals, such as tanol, can be introduced into the medium [1,2]. Paramagnetic ions can also be used. Another possibility is the creation of radicals by using molecules such as perylene and tetracyanoethylene [3,4]. In previous work [4] we have shown that charge transfer can exist between clays and organic molecules, showing the acido-basic properties of these aluminosilicates. A correlation with the catalytic activities of a montmorillonite pillared
* Corresponding author. 0927-6513/95/$9.50© 1995ElsevierScienceB.V. All rights reserved SSDI 0927-6513(95)00002-X
by Zr-hydroxyl cations has also been demonstrated [5]. Among the paramagnetic ions, copper has been widely used with success. Several methods were based on cationic exchange with the paramagnetic metal in zeolites [6-10] or clays I l l 13]. In some favorable cases, the paramagnetic ion was already present within the structure and was an integral part of the starting clay [ 14,15]. An interesting way to introduce the paramagnetic metallic probe consists of using a copper complex, such as acetylacetonate, leading to an interaction essentially connected with an adsorption phenomenon [16]. The salt has been largely studied itself [17-19] and after adsorption on different substrates [20-23]. The aim of the different experiments was to understand the porosity of the solids and to study the activity of biological systems.
188
J.V. Zanchetta et al./Microporous Materials 4 (1995) 187-193
In order to obtain further information on the interaction between copper and clays we investigated the adsorption of copper acetylacetonate by ESR experiments. For that purpose we used two different clays: kaolinite (a non-swelling clay) and montmorillonite (a swelling-clay).
solid was crystallized several times from the solvent (mainly chloroform) and then dried. Solutions of copper acetylacetonate in chloroform were prepared for sorption on clays.
2.2. Introduction of the copper salt
2. Experimental 2.1. Sample preparation Two clays have been chosen, a kaolinite and a montmorillonite, which are, respectively, a nonswelling and a swelling clay. The crude kaolinite (of known low cation exchange capacity) was exchanged with Na + and Ca 2+. The surface area measured with N2 is near 30 m 2 g- 1 From the Wyoming montmorillonite we obtained three materials, characterized by their basal spacing, surface area and microporosity. The crude clay was exchanged with Na + and Ca 2+. The Na + form had a basal spacing of 1.28 nm and a surface area of 44 m 2 g-1 while for the Ca 2+ form values of 1.54nm and 70m z g-1 were observed. The exchange capacity was near 1 meq./g of clay. The third material used was a pillared clay obtained by treating the sodium form with a solution of ZrOCI2'8H20 [4,5]. This process which consisted of an exchange with Zr-hydroxycations, led to a permanent swelling of the clay layers and, therefore, to a Zr-pillared clay. The interlayer distance was 2.0 nm and the surface area near 300 m 2 g-1 for 11.5 wt.-% ZrO2. The exchange led to hydroxy cations. In the case of Zr-clay, the cation was very likely hydrolyzed to a large extent. The hydroxyl environment was not really well known and depended, of course, on the calcining temperature. The pore distribution was determined by the BET method and application of the Dubinin equation [4]. There was a distribution of pores between 2 and 6 nm for montmorillonite-Ca corresponding to mesopores at the external surface of the grains. For montmorillonite-Zr, 60% of the pore volume had a diameter less than 2 nm perfectly consistent with the interlayer spacing [4]. Copper acetylacetonate was prepared according to a well known procedure [24]. The obtained
The clays were degassed at 250°C for 48 h under vacuum (0.1 Pa) in order to remove the adsorbed water without dehydroxylation. Thermogravimetric analysis showed that water was released near 130°C whatever the cation. A second peak near 420°C showed a beginning of dehydroxylation. Under these circumstances the solid was not structurally altered. After heat treatment and cooling of the sample (typically 500 mg), the solid was left in contact with 5 ml of the copper acetylacetonate solution [15]. The suspension was stirred for 24 h (a longer time does not lead to a modification of the ESR signal, showing that equilibrium is reached). The mixture was filtered, dried in air, introduced into an ESR tube (20 rag) and sealed. The observed signal remained unchanged after several weeks. The concentration of the copper salt in the solution before adsorption was an important parameter. Several solvents could be used, such as dioxane or toluene. Attempts performed with apolar solvents were disappointing, the solubility of the complex being very low. The most convincing results were obtained with chloroform, in the concentration range 5.10-3 to 5.10-2 M, depending on the nature of the clay studied. The amount of the adsorbed complex was not evaluated. The theoretical value (maximum) would be ca. 1 mg (to be compared with 500 mg of clay). It is unlikely that the introduction of the probe modifies the interlayer spacing.
2.3. ESR experiments The ESR spectra were recorded with a Bruker spectrometer operating in the X band (v= 9.6 GHz). Measurements of g factors were carried out with maximum accuracy by means of a double cavity.
J. V. Zanchetta et aL/Microporous Materials 4 (1995) 187-193
189
3. Results
3.1. Adsorption of the copper salt
At room temperature, the complex gave the well known hyperfine line pattern with a line width depending on the spin number. When the temperature was decreased, typical spectra were obtained, and near 153 K all the parallel components characteristic of the solid-state spectrum appeared. The spectrum obtained at 77 K was the basis of the analysis of the adsorption phenomenon of the complex on the different solids. Table 1 shows the values of ( g ) and ( A ) for the spectrum at room temperature and of g±, gI], A± and Aii obtained by numerical simulation (see Section 4) of the spectrum at 77 K (Fig. 1). The values are in good agreement with those reported in the literature [16,21,23,25].
After contact with the clay, a hyperfine structure with partially or totally resolved low-field components should be observed if an adsorption process took place.
Table 1 Tensor values calculated for 5.10 -3 M copper acetylacetonate in chloroform at 298 and 77 K (g>
T
(A) (m 1)
(K) 298
2.125
gbl
-0.78
g±
All (m-l)
--
--
A± (m-l) m
(8O G) 77
2.135
-0.64 (65 G)
2 . 2 8 5 2.060
-1.80 (170 G)
-0.12 13 G)
Kaolinite-Na (KI-Na) and kaolinite-Ca (KI-Ca) The surface area, as already mentioned, was low. The exchange rate of this clay was very low. However, we tried to introduce Ca 2÷ ions in the solid. Before any contact with the probe a very weak and narrow signal (0.4-0.5 mT, g=2.0026) could be observed, the intensity of which was reduced by action of hydrogen peroxide. No signal' connected with iron was observed on this sample [5]. The signal observed was a rather complex solid-like signal with partially resolved low-field components at room temperature (Fig. 2). The concentration in chloroform chosen was 5-10 - 3 M. The different axial magnetic parameters, obtained after simulation of the experimental spectra, are reported in Table 2. Kaolinite exchanged with Ca 2 + (KI-Ca) did not show an axial symmetry at room temperature. Table 2 shows the different tensor values at 77 K. The axial symmetry signal appears near 200 K, temperature to be compared with that of the pure complex.
Montmorillonites All the montmorillonite samples showed a very weak and narrow signal (g ~ 2.0026). At very low
ij'.~
H(fi.) l
J
I,
2500
~,
, [ t 3000
I
i
J
l
3500
l
~
i
HIG.) I
,
/,000
Fig. 1. Experimental and computed line shapes of copper acetylacetonate (5.10 -3 M in chloroform) at 77 K. The dashed curve represents the calculated spectrum with gaussian functions, full line widths of 1.5 mT being used in the calculation of the parallel and the perpendicular components.
2500
3000
3500
WOO
Fig. 2. Experimental (room temperature) and calculated absorption curves for K1-Na, after contact with the copper complex (5.10 3 M in chloroform). The dashed aurve represents the theoretical plot. Lorentzian functions of line widths (3Hxx=5.0 mT, A/~vy= 1.4 mT, A H ~ = 1.0 mT) are used.
190
J. V. Zanchetta et aL/Microporous Materials 4 (1995) 187 193
Table 2 ESR parameters determined for two kaolinites after contact with the copper complex T (K)
(g)
(A) (m -1)
glt
g±
All (m -1)
Cu(acac)2 on KI-Na (5.10-3M in chloroform) 298 2.145 -0.82 2.280 2.070 -1.85 (85 G) (175 G) 77 2.145 -0.81 2.285 2.070 - 2 . 0 (84 G) (189 G)
/
A± (m 1)
Y
-0.31
(32 G) -0.25 (25 G)
Cu(acac)2 on KI-Ca (5.10-3M in chloroform) 77 2,123 0.77 2.270 2.050 - 1 . 9 6 -0.25 (78 G) (185 G) (25 G)
magnetic field an absorption due to Fe 3+ ( g - 4 ) was observed. This well known signal has been described elsewhere [13,26 ] and did not hinder the observation of the different components of the spectra. For the montmorillonite-Na (M-Na), the lowfrequency components were partially resolved. At 233 K the spectrum was totally resolved, and the solid-like signal was well defined. The different parameters obtained after simulation of the spectrum are listed in Table 3. The signal related to the M-Ca was partially resolved at room temperature. As an example, the corresponding simulated spectrum at 77 K is given in Fig. 3. The different parameters are listed in Table 3. The typical hyperfine spectrum was never observed in the case of M-Zr, whatever the temperature was (Fig. 4). The structure of the signal did
I
H(fi.) i
2500
1
i
I
]
I
I
3000
I
I
3500
,
,
,
I
J
t,000
Fig. 3. ESR spectra of M-Ca at 77 K after contact with the copper complex (5.10 3M in chloroform). The dashed curve represents the computed spectrum. Lorentzian functions of line widths (,ffH~x= 5.0 mT, zfHrr =,d//~ z = 1.0 roT) are used.
9;2.av
•:2.006 1006.
Fig. 4. Typical spectrum of M-Zr after interaction with the copper complex. There is no modification of the observed lines when the temperature decreases.
not change with decreasing temperature. The position of gt corresponded to a value very close to that of the free electron and the other components due to Cu 2+.
4. Analysis of the ESR spectra The tensor components gll, g±, All and A± obtained from the ESR measurements can be used
Table 3 ESR parameters of montmorillonites after contact with the copper complex T (K)
(g)
(A) (m ')
gll
g±
AiI (m 1)
Ax (m 1)
Cu(acac)2 on M-Na (10 -2 M in chloroform)
77
2.123
-0.74 (75 G)
2.250
2.060
- 1.98 (190 G)
-0.17 (18 G)
Cu(acac)2 on M-Ca (5.10 -3 M in chloroform)
77
2.123
-0.74 (75 G)
2.250
2.060
1.98 (190 G)
-0.17 (18 G)
-
J. lC Zanchetta et al./Microporous Materials 4 (1995) 187 193
to determine the nature of the chemical bonding of copper in clay. For this purpose the well known axially symmetrical spin Hamiltonian was considered [ 17,18]. Therefore the spectroscopic splitting factors gll and g± and the hyperfine spectrum constants A, and AI could be evaluated. In order to obtain the best values of the tensor g and A we used a simulation method, which need not be described here, based on the spin Hamiltonian, taking into account the second-order hyperfine interaction (perturbation theory). Indications on the line widths are given in the figure captions. The uncertainties in the values of g and A are +0.015 for
5. Discussion and conclusion The complexes formed by contact of copper acetylacetonate with clays are not readily removed
191
Table 4 Antibonding molecular orbitals coefficients for the two clays
Cu(acac)2 C+KI-Na C+K1-Ca C+M-Na C+M-Ca
T(K)
~2
32
:(
fi,
77 298 77 77 77 77
0.84 0.86 0.90 0.87 0.86 0.86
0.67
0.48
0.90
0.78 0.55 0.67 0.67
0.43 0.44 0.45 0.45
0.87 0.86 0.84 0.84
Values of fl, ~' and fll are not significantly different in the case of C + K I - N a at 298 and 77K.
from the surface by washing with the corresponding solvent, indicating that they are bonded to the surface. The appearance of hyperfine structure in the solid with resolved axial symmetry (at room temperature or at lower temperatures)clearly shows an interaction with the clay.
5.1. Kaolinite complexes At room temperature, there is a clear adsorption, with K1-Na as shown by the axial character of the spectrum. Surprisingly, this axial symmetry appears only at 200 K for K1-Ca, despite the low exchange capacity. It is not easy to draw conclusions from the observed values of the g tensor. The ESR spectra of the surface-adsorbed complexes are always broader than those of the pure salt in a glassy solvent (values of All ~nd A±). What seems most convincing is the temperature of appearance of an axial symmetry. The fact that the spectrum of K1-Ca has always been found very different from that of KI-Na leads to the following considerations. The adsorption of the complex is probably located on the clay layer edges: In view of this, the calcium would be able to block the "active sites" of the grain boundaries and of the basal surfaces [28]. Indeed, the hexagonal sites of the basal planes can accept Ca 2+ and Na + ions [28,29]. Otherwise, the solvent can interfere, as already suggested [16]. This model leads to octahedral sites connected with an A1 atom in a hydroxyl and oxygen environment. This representation is consistent with practically constant N and 2 parameters, whatever the nature of the solvent. There is a significant increase of parameter ~,
192
J. K Zanchetta et al./Microporous Materials 4 (1995) 187-193
after interaction of the copper complex with the kaolinite, showing a decrease of the o- character of the Cu...O bond. The evolution of parameter ~' confirms this tendency. Parameter /~1 decreases showing that the covalency of the in-plane zc bonding increases. In summary these results imply that there is (1) a decrease of the covalent character of the Cu...O bond, (2) an increase of the cr character of the in-plane 7r bonding and (3) an increase of the zc bonding nature in the out-of-plane bonding. The decrease of the ~ character of Cu...O with increasing covalency of the zc bonding shows a clear interaction of the copper salt with the clay. This fact corresponds to larger values of All with decreasing gJl values. This implies a participation of the solid ligands in the process of immobilization of the complex. The differences between the two kaolinites are weak. This is partly explained by the low ion exchange capacity of these clays.
5.2. Montmorillonite complexes The ESR spectra are less well resolved than those of kaolinite. The parameters reported in Table 4 reflect an increase of All and a decrease of gll compared with the values of the isolated complex. Particularly, the spectrum of M-Ca is not totally resolved at 298 K. However, the axial character of the signal appears at 173 K, namely at a higher temperature than that of the isolated complex, but at a lower temperature than that of M-Na showing a weaker interaction although the characteristic parameters are exactly the same (Table 4). The values of parameter a are close to those of the various kaolinite complexes. Their increase reflects the decrease of the covalent character of the Cu...O bond. In the same way, a' and fll indicate that the rc character of the in-plane bonding decreases. This indicates necessarily an interaction of the clay with Cu 2+ ions. As already mentioned, the clay-complex interaction is weaker for M-Ca than for M-Na, in spite of the smaller values of the basal spacing and of the specific area of the latter. Changing the solvent (non-polar like dioxane) does not alter the conclusions. The case of Zr-pillared montmorillonite is quite different.
As shown in Fig. 4 an axial spectrum is never observed. The part of the signal characterized by gl = 2.006 is certainly due to paramagnetic impurities partly eliminated by treatment with peroxide. As the intensities of the lines are weak (especially compared with the other clay spectra), a very weak signal remains. The part of the spectrum at g3-~2.17 is generally ascribed to the existence of an isotropic environment [27,30,31]. The species involved in these studies was identified as [Cu(H20)6] 2+, since the experiments were performed in aqueous medium. This is not the case in the present work. However, the water related to solvation of the zirconium remains which is not affected by the heat treatment limited to 250°C. This mobile species seems to exist and is not connected with water here. The line centered at g2 = 2.05 is found to be characteristic of a chemisorbed species [30] generally ascribed to hexahydrated copper. Several hypotheses can be proposed. The existence of the hydrated species cannot be supported, even if there are water molecules present in the interlamellar spacing. This leads to the possibility of an axial symmetry due to the complex with two molecules of adsorbed solvent in order to create an octahedral environment, giving parameters g close to those of the hydrated species. This hypothesis seems more realistic than that of a hexa-coordinated complex with oxygen atoms of the layers [32]. In conclusion, this study shows that the copper complex interacts mainly with layer edges of the kaolinite and the sodium montmorillonite. The fact that the spectra are comparable for the two clays leads to the conclusion that the complex does not penetrate into the interlayer spacing of the montmorillonite. The cases of M-Ca and M-Zr are different. Indeed, the basal spacing becomes larger than that of the parent sodium product. The spectra are totally different and do not show an axial-like symmetry. The interlayer distance seems adequate for an interaction of the complex with the zirconium-pillared clay. What seems rather peculiar is the analogy between the spectra of M-Zr in interaction with copper in aqueous and non-aqueous media. The present method can give information on complex solids like clays and seems suitable for
J. V. Zanchetta et al./Microporous Materials 4 (1995) 187-193
distinguishing between non-swelling and swelling aluminosilicates. The ESR study shows a difference in the behaviour of the different clays. The interaction of such compounds with a copper complex appears possible, even in a non-aqueous medium.
Acknowledgements We are grateful to Dr. F. Fajula for helpful discussions on clays and to Dr. M. Chachaty for his assistance in the simulation of the ESR spectra.
References [1] A. Jeunet, B. Nickel and A. Rassat, Nouv. J. Chim., 10 (1986) 123. [2] A. Jeunet, B. Nickel and A. Rassat, J. Chim. Phys., 85 (1988) 97. [3] L. Bonneau and H. Pezerat, J. Chim. Phys., 80 (1983) 275. [4] F. Figueras, A. Matrod-Bashi, G. Fetter, A. Thrierr and J.V. Zanchetta, J. Catal., 199 (1989) 91. [5] J.V. Zanchetta, A. Matrod-Bashi and F. Figueras, Clay Miner., 27 (1992) 343. [6] E.G. Derouane, M. Mestdagh and L. Vielvoye, J. Catal., 3 (1974) 169. [7] G. Martini, M.F, Ottaviani and G.L. Seravalli, J. Phys. Chem., 79 (1975) 1716. [8] C. Naccache and Y. Ben Taarit, J. Catal., 22 (1971) 171. [9] J.C. Conesa and J. Soria, J. Phys. Chem., 82 (1978) 1575. [10] J.C. Vedrine, E.G. Derouane and Y. Ben Taarit, J. Chem. Phys., 98 (1974) 531.
193
[11] M. Che, J. Fraissard and J.C. Verdrine, Bul. Groupe Fr. Argiles, 25 (1974) 1. [12] R.C. Kemp, J~ Phys. C: Solid State Phys., 5 (1972) 3566. [13] D. Olivier, J.C. Vedrine and H. Pezerat, Bul. Groupe Fr. Argiles, 27 (1975) 153. [14] C. Mosser, M. Mestdagh, A. Decarreau and A. Herbillon, 9th International Clay Conferences, Strasbaurg, Aug. 28-Sept. 2, 1989. [15] T. Delineau, T. Allard, J.P. Muller, J. Yvon, O. Barres and J.M. Cases, Clays Clay Miner., in press. [16] G. Martini and M.F. OttavianL J. Phys. Chem., 85 (1981) 1922. [17] D. Kivelson and R. Neiman, J. Chem. Phys., 35 ( 1961 ) 149. [18] H.R. Gersmann and J.D. Swalen, J. ('hem. Phys., 36 (1962) 3221. [19] A. Nicula, D. Stamires and J. Turkevich, J. Chem. Phys., 42 (1965) 3684. [20] P.A. Berger and J.F. Roth, J. Phys. Chem., 71 (1967) 4037. [21] H.A. Kuska, M.T. Rogers and R.E. Drullinger, Z Phys. Chem., 71 (1967) 109. [22] T. Ogata, F. Fujisawa, N. Tanaka and H. Yokoi, Bull. Chem. Soc. Jpn., 49 (1976) 2759. [23] I. Adato and 1. Eliezer, J. Chem. Phys., 54 (197l) 1472. [24] M.M. Jones, J. Am. Chem. Soc., 8t (1959) 3188. [25] C.C. Chao and J,H. Lunsford, J. (Twin. Phys., 57 (1972) 2890. [26] K. Komusinski, L. Stoch and S.M. Dubiel, Clays Clay Miner., 29 ( 1981 ) 123. [27] O. Bradel, R.C. Barklie, D.H. Doff, N.H.J. Gangas and A. McKimm, Z. Phys. Chem. Neue Folge, 151 (1987) 157. [28] A.T. Pilipenko, B.I. Kornilovich, V.I. Lysenko and V.V. Maliarenko, Dokl. Akad. Nauk. S.S.S.R., 305 (1989) 1408. [29] R. Calvet, Thesis, Paris VI, 1972. [30] K.R. Kukkadapu and L. Kevan, J. Phys. (Twin., 92 ( 1988 ) 6073. [31] D.R. Brown and L. Kevan, Z Am. Chem. Sot., 110 (1988) 2743. [32] S. Gonzales Garcia, J.M. Suarez Cardeso, Si Rodriguez Alonso and G. Dios Cancela, Ann. Quim., 82(1986) 13.
Microporous Materials 4 ( 1995) 187-193
I
Electron spin resonance study of copper acetylacetonate adsorbed on clays J.V. Zanchetta *, B. Deroide, B. Asri, J.C. Giuntini Laboratoire de Physicochimie des Matkriaux Solides, Equipe de Chimie Physique (URA D0407 CNRS), Universitb Montpellier II, Place E. Bataillon. F-34095 Montpellier Cedex 5, France
Received 16 July 1994; accepted 14 December 1994
Abstract
Two clays have been studied by means of a copper acetylacetonate probe. Swelling clays (montmorillotaites) and non-swelling clays (kaolinites) have been investigated. Kaolinite-Na (K1-Na) shows a clear interaction with the copper complex. At room temperature, the electron spin resonance (ESR) spectra of K1-Ca do not show a real axial symmetry. In spite of the large basal spacing of the montmorillonites (Na, Ca, and Zr) the spectra are less well resolved than those of kaolinite. The tensor components gll, g±, All and A± are obtained by numerical simulation of the spectra. Adapted parameters are calculated in order to determine the magnitude and nature of the interaction of the copper complex with the clays. Keywords." Clay; Kaolinite; Montmorillonite; Electron spin resonance; Copper complex
1. Introduction
The introduction of a paramagnetic species into a solid is a well known method used in the study of the local structure of compounds, crystalline or amorphous. This can be performed in porous substances in several ways. Organic free radicals, such as tanol, can be introduced into the medium [1,2]. Paramagnetic ions can also be used. Another possibility is the creation of radicals by using molecules such as perylene and tetracyanoethylene [3,4]. In previous work [4] we have shown that charge transfer can exist between clays and organic molecules, showing the acido-basic properties of these aluminosilicates. A correlation with the catalytic activities of a montmorillonite pillared
* Corresponding author. 0927-6513/95/$9.50© 1995ElsevierScienceB.V. All rights reserved SSDI 0927-6513(95)00002-X
by Zr-hydroxyl cations has also been demonstrated [5]. Among the paramagnetic ions, copper has been widely used with success. Several methods were based on cationic exchange with the paramagnetic metal in zeolites [6-10] or clays I l l 13]. In some favorable cases, the paramagnetic ion was already present within the structure and was an integral part of the starting clay [ 14,15]. An interesting way to introduce the paramagnetic metallic probe consists of using a copper complex, such as acetylacetonate, leading to an interaction essentially connected with an adsorption phenomenon [16]. The salt has been largely studied itself [17-19] and after adsorption on different substrates [20-23]. The aim of the different experiments was to understand the porosity of the solids and to study the activity of biological systems.
188
J.V. Zanchetta et al./Microporous Materials 4 (1995) 187-193
In order to obtain further information on the interaction between copper and clays we investigated the adsorption of copper acetylacetonate by ESR experiments. For that purpose we used two different clays: kaolinite (a non-swelling clay) and montmorillonite (a swelling-clay).
solid was crystallized several times from the solvent (mainly chloroform) and then dried. Solutions of copper acetylacetonate in chloroform were prepared for sorption on clays.
2.2. Introduction of the copper salt
2. Experimental 2.1. Sample preparation Two clays have been chosen, a kaolinite and a montmorillonite, which are, respectively, a nonswelling and a swelling clay. The crude kaolinite (of known low cation exchange capacity) was exchanged with Na + and Ca 2+. The surface area measured with N2 is near 30 m 2 g- 1 From the Wyoming montmorillonite we obtained three materials, characterized by their basal spacing, surface area and microporosity. The crude clay was exchanged with Na + and Ca 2+. The Na + form had a basal spacing of 1.28 nm and a surface area of 44 m 2 g-1 while for the Ca 2+ form values of 1.54nm and 70m z g-1 were observed. The exchange capacity was near 1 meq./g of clay. The third material used was a pillared clay obtained by treating the sodium form with a solution of ZrOCI2'8H20 [4,5]. This process which consisted of an exchange with Zr-hydroxycations, led to a permanent swelling of the clay layers and, therefore, to a Zr-pillared clay. The interlayer distance was 2.0 nm and the surface area near 300 m 2 g-1 for 11.5 wt.-% ZrO2. The exchange led to hydroxy cations. In the case of Zr-clay, the cation was very likely hydrolyzed to a large extent. The hydroxyl environment was not really well known and depended, of course, on the calcining temperature. The pore distribution was determined by the BET method and application of the Dubinin equation [4]. There was a distribution of pores between 2 and 6 nm for montmorillonite-Ca corresponding to mesopores at the external surface of the grains. For montmorillonite-Zr, 60% of the pore volume had a diameter less than 2 nm perfectly consistent with the interlayer spacing [4]. Copper acetylacetonate was prepared according to a well known procedure [24]. The obtained
The clays were degassed at 250°C for 48 h under vacuum (0.1 Pa) in order to remove the adsorbed water without dehydroxylation. Thermogravimetric analysis showed that water was released near 130°C whatever the cation. A second peak near 420°C showed a beginning of dehydroxylation. Under these circumstances the solid was not structurally altered. After heat treatment and cooling of the sample (typically 500 mg), the solid was left in contact with 5 ml of the copper acetylacetonate solution [15]. The suspension was stirred for 24 h (a longer time does not lead to a modification of the ESR signal, showing that equilibrium is reached). The mixture was filtered, dried in air, introduced into an ESR tube (20 rag) and sealed. The observed signal remained unchanged after several weeks. The concentration of the copper salt in the solution before adsorption was an important parameter. Several solvents could be used, such as dioxane or toluene. Attempts performed with apolar solvents were disappointing, the solubility of the complex being very low. The most convincing results were obtained with chloroform, in the concentration range 5.10-3 to 5.10-2 M, depending on the nature of the clay studied. The amount of the adsorbed complex was not evaluated. The theoretical value (maximum) would be ca. 1 mg (to be compared with 500 mg of clay). It is unlikely that the introduction of the probe modifies the interlayer spacing.
2.3. ESR experiments The ESR spectra were recorded with a Bruker spectrometer operating in the X band (v= 9.6 GHz). Measurements of g factors were carried out with maximum accuracy by means of a double cavity.
J. V. Zanchetta et aL/Microporous Materials 4 (1995) 187-193
189
3. Results
3.1. Adsorption of the copper salt
At room temperature, the complex gave the well known hyperfine line pattern with a line width depending on the spin number. When the temperature was decreased, typical spectra were obtained, and near 153 K all the parallel components characteristic of the solid-state spectrum appeared. The spectrum obtained at 77 K was the basis of the analysis of the adsorption phenomenon of the complex on the different solids. Table 1 shows the values of ( g ) and ( A ) for the spectrum at room temperature and of g±, gI], A± and Aii obtained by numerical simulation (see Section 4) of the spectrum at 77 K (Fig. 1). The values are in good agreement with those reported in the literature [16,21,23,25].
After contact with the clay, a hyperfine structure with partially or totally resolved low-field components should be observed if an adsorption process took place.
Table 1 Tensor values calculated for 5.10 -3 M copper acetylacetonate in chloroform at 298 and 77 K (g>
T
(A) (m 1)
(K) 298
2.125
gbl
-0.78
g±
All (m-l)
--
--
A± (m-l) m
(8O G) 77
2.135
-0.64 (65 G)
2 . 2 8 5 2.060
-1.80 (170 G)
-0.12 13 G)
Kaolinite-Na (KI-Na) and kaolinite-Ca (KI-Ca) The surface area, as already mentioned, was low. The exchange rate of this clay was very low. However, we tried to introduce Ca 2÷ ions in the solid. Before any contact with the probe a very weak and narrow signal (0.4-0.5 mT, g=2.0026) could be observed, the intensity of which was reduced by action of hydrogen peroxide. No signal' connected with iron was observed on this sample [5]. The signal observed was a rather complex solid-like signal with partially resolved low-field components at room temperature (Fig. 2). The concentration in chloroform chosen was 5-10 - 3 M. The different axial magnetic parameters, obtained after simulation of the experimental spectra, are reported in Table 2. Kaolinite exchanged with Ca 2 + (KI-Ca) did not show an axial symmetry at room temperature. Table 2 shows the different tensor values at 77 K. The axial symmetry signal appears near 200 K, temperature to be compared with that of the pure complex.
Montmorillonites All the montmorillonite samples showed a very weak and narrow signal (g ~ 2.0026). At very low
ij'.~
H(fi.) l
J
I,
2500
~,
, [ t 3000
I
i
J
l
3500
l
~
i
HIG.) I
,
/,000
Fig. 1. Experimental and computed line shapes of copper acetylacetonate (5.10 -3 M in chloroform) at 77 K. The dashed curve represents the calculated spectrum with gaussian functions, full line widths of 1.5 mT being used in the calculation of the parallel and the perpendicular components.
2500
3000
3500
WOO
Fig. 2. Experimental (room temperature) and calculated absorption curves for K1-Na, after contact with the copper complex (5.10 3 M in chloroform). The dashed aurve represents the theoretical plot. Lorentzian functions of line widths (3Hxx=5.0 mT, A/~vy= 1.4 mT, A H ~ = 1.0 mT) are used.
190
J. V. Zanchetta et aL/Microporous Materials 4 (1995) 187 193
Table 2 ESR parameters determined for two kaolinites after contact with the copper complex T (K)
(g)
(A) (m -1)
glt
g±
All (m -1)
Cu(acac)2 on KI-Na (5.10-3M in chloroform) 298 2.145 -0.82 2.280 2.070 -1.85 (85 G) (175 G) 77 2.145 -0.81 2.285 2.070 - 2 . 0 (84 G) (189 G)
/
A± (m 1)
Y
-0.31
(32 G) -0.25 (25 G)
Cu(acac)2 on KI-Ca (5.10-3M in chloroform) 77 2,123 0.77 2.270 2.050 - 1 . 9 6 -0.25 (78 G) (185 G) (25 G)
magnetic field an absorption due to Fe 3+ ( g - 4 ) was observed. This well known signal has been described elsewhere [13,26 ] and did not hinder the observation of the different components of the spectra. For the montmorillonite-Na (M-Na), the lowfrequency components were partially resolved. At 233 K the spectrum was totally resolved, and the solid-like signal was well defined. The different parameters obtained after simulation of the spectrum are listed in Table 3. The signal related to the M-Ca was partially resolved at room temperature. As an example, the corresponding simulated spectrum at 77 K is given in Fig. 3. The different parameters are listed in Table 3. The typical hyperfine spectrum was never observed in the case of M-Zr, whatever the temperature was (Fig. 4). The structure of the signal did
I
H(fi.) i
2500
1
i
I
]
I
I
3000
I
I
3500
,
,
,
I
J
t,000
Fig. 3. ESR spectra of M-Ca at 77 K after contact with the copper complex (5.10 3M in chloroform). The dashed curve represents the computed spectrum. Lorentzian functions of line widths (,ffH~x= 5.0 mT, zfHrr =,d//~ z = 1.0 roT) are used.
9;2.av
•:2.006 1006.
Fig. 4. Typical spectrum of M-Zr after interaction with the copper complex. There is no modification of the observed lines when the temperature decreases.
not change with decreasing temperature. The position of gt corresponded to a value very close to that of the free electron and the other components due to Cu 2+.
4. Analysis of the ESR spectra The tensor components gll, g±, All and A± obtained from the ESR measurements can be used
Table 3 ESR parameters of montmorillonites after contact with the copper complex T (K)
(g)
(A) (m ')
gll
g±
AiI (m 1)
Ax (m 1)
Cu(acac)2 on M-Na (10 -2 M in chloroform)
77
2.123
-0.74 (75 G)
2.250
2.060
- 1.98 (190 G)
-0.17 (18 G)
Cu(acac)2 on M-Ca (5.10 -3 M in chloroform)
77
2.123
-0.74 (75 G)
2.250
2.060
1.98 (190 G)
-0.17 (18 G)
-
J. lC Zanchetta et al./Microporous Materials 4 (1995) 187 193
to determine the nature of the chemical bonding of copper in clay. For this purpose the well known axially symmetrical spin Hamiltonian was considered [ 17,18]. Therefore the spectroscopic splitting factors gll and g± and the hyperfine spectrum constants A, and AI could be evaluated. In order to obtain the best values of the tensor g and A we used a simulation method, which need not be described here, based on the spin Hamiltonian, taking into account the second-order hyperfine interaction (perturbation theory). Indications on the line widths are given in the figure captions. The uncertainties in the values of g and A are +0.015 for
5. Discussion and conclusion The complexes formed by contact of copper acetylacetonate with clays are not readily removed
191
Table 4 Antibonding molecular orbitals coefficients for the two clays
Cu(acac)2 C+KI-Na C+K1-Ca C+M-Na C+M-Ca
T(K)
~2
32
:(
fi,
77 298 77 77 77 77
0.84 0.86 0.90 0.87 0.86 0.86
0.67
0.48
0.90
0.78 0.55 0.67 0.67
0.43 0.44 0.45 0.45
0.87 0.86 0.84 0.84
Values of fl, ~' and fll are not significantly different in the case of C + K I - N a at 298 and 77K.
from the surface by washing with the corresponding solvent, indicating that they are bonded to the surface. The appearance of hyperfine structure in the solid with resolved axial symmetry (at room temperature or at lower temperatures)clearly shows an interaction with the clay.
5.1. Kaolinite complexes At room temperature, there is a clear adsorption, with K1-Na as shown by the axial character of the spectrum. Surprisingly, this axial symmetry appears only at 200 K for K1-Ca, despite the low exchange capacity. It is not easy to draw conclusions from the observed values of the g tensor. The ESR spectra of the surface-adsorbed complexes are always broader than those of the pure salt in a glassy solvent (values of All ~nd A±). What seems most convincing is the temperature of appearance of an axial symmetry. The fact that the spectrum of K1-Ca has always been found very different from that of KI-Na leads to the following considerations. The adsorption of the complex is probably located on the clay layer edges: In view of this, the calcium would be able to block the "active sites" of the grain boundaries and of the basal surfaces [28]. Indeed, the hexagonal sites of the basal planes can accept Ca 2+ and Na + ions [28,29]. Otherwise, the solvent can interfere, as already suggested [16]. This model leads to octahedral sites connected with an A1 atom in a hydroxyl and oxygen environment. This representation is consistent with practically constant N and 2 parameters, whatever the nature of the solvent. There is a significant increase of parameter ~,
192
J. K Zanchetta et al./Microporous Materials 4 (1995) 187-193
after interaction of the copper complex with the kaolinite, showing a decrease of the o- character of the Cu...O bond. The evolution of parameter ~' confirms this tendency. Parameter /~1 decreases showing that the covalency of the in-plane zc bonding increases. In summary these results imply that there is (1) a decrease of the covalent character of the Cu...O bond, (2) an increase of the cr character of the in-plane 7r bonding and (3) an increase of the zc bonding nature in the out-of-plane bonding. The decrease of the ~ character of Cu...O with increasing covalency of the zc bonding shows a clear interaction of the copper salt with the clay. This fact corresponds to larger values of All with decreasing gJl values. This implies a participation of the solid ligands in the process of immobilization of the complex. The differences between the two kaolinites are weak. This is partly explained by the low ion exchange capacity of these clays.
5.2. Montmorillonite complexes The ESR spectra are less well resolved than those of kaolinite. The parameters reported in Table 4 reflect an increase of All and a decrease of gll compared with the values of the isolated complex. Particularly, the spectrum of M-Ca is not totally resolved at 298 K. However, the axial character of the signal appears at 173 K, namely at a higher temperature than that of the isolated complex, but at a lower temperature than that of M-Na showing a weaker interaction although the characteristic parameters are exactly the same (Table 4). The values of parameter a are close to those of the various kaolinite complexes. Their increase reflects the decrease of the covalent character of the Cu...O bond. In the same way, a' and fll indicate that the rc character of the in-plane bonding decreases. This indicates necessarily an interaction of the clay with Cu 2+ ions. As already mentioned, the clay-complex interaction is weaker for M-Ca than for M-Na, in spite of the smaller values of the basal spacing and of the specific area of the latter. Changing the solvent (non-polar like dioxane) does not alter the conclusions. The case of Zr-pillared montmorillonite is quite different.
As shown in Fig. 4 an axial spectrum is never observed. The part of the signal characterized by gl = 2.006 is certainly due to paramagnetic impurities partly eliminated by treatment with peroxide. As the intensities of the lines are weak (especially compared with the other clay spectra), a very weak signal remains. The part of the spectrum at g3-~2.17 is generally ascribed to the existence of an isotropic environment [27,30,31]. The species involved in these studies was identified as [Cu(H20)6] 2+, since the experiments were performed in aqueous medium. This is not the case in the present work. However, the water related to solvation of the zirconium remains which is not affected by the heat treatment limited to 250°C. This mobile species seems to exist and is not connected with water here. The line centered at g2 = 2.05 is found to be characteristic of a chemisorbed species [30] generally ascribed to hexahydrated copper. Several hypotheses can be proposed. The existence of the hydrated species cannot be supported, even if there are water molecules present in the interlamellar spacing. This leads to the possibility of an axial symmetry due to the complex with two molecules of adsorbed solvent in order to create an octahedral environment, giving parameters g close to those of the hydrated species. This hypothesis seems more realistic than that of a hexa-coordinated complex with oxygen atoms of the layers [32]. In conclusion, this study shows that the copper complex interacts mainly with layer edges of the kaolinite and the sodium montmorillonite. The fact that the spectra are comparable for the two clays leads to the conclusion that the complex does not penetrate into the interlayer spacing of the montmorillonite. The cases of M-Ca and M-Zr are different. Indeed, the basal spacing becomes larger than that of the parent sodium product. The spectra are totally different and do not show an axial-like symmetry. The interlayer distance seems adequate for an interaction of the complex with the zirconium-pillared clay. What seems rather peculiar is the analogy between the spectra of M-Zr in interaction with copper in aqueous and non-aqueous media. The present method can give information on complex solids like clays and seems suitable for
J. V. Zanchetta et al./Microporous Materials 4 (1995) 187-193
distinguishing between non-swelling and swelling aluminosilicates. The ESR study shows a difference in the behaviour of the different clays. The interaction of such compounds with a copper complex appears possible, even in a non-aqueous medium.
Acknowledgements We are grateful to Dr. F. Fajula for helpful discussions on clays and to Dr. M. Chachaty for his assistance in the simulation of the ESR spectra.
References [1] A. Jeunet, B. Nickel and A. Rassat, Nouv. J. Chim., 10 (1986) 123. [2] A. Jeunet, B. Nickel and A. Rassat, J. Chim. Phys., 85 (1988) 97. [3] L. Bonneau and H. Pezerat, J. Chim. Phys., 80 (1983) 275. [4] F. Figueras, A. Matrod-Bashi, G. Fetter, A. Thrierr and J.V. Zanchetta, J. Catal., 199 (1989) 91. [5] J.V. Zanchetta, A. Matrod-Bashi and F. Figueras, Clay Miner., 27 (1992) 343. [6] E.G. Derouane, M. Mestdagh and L. Vielvoye, J. Catal., 3 (1974) 169. [7] G. Martini, M.F, Ottaviani and G.L. Seravalli, J. Phys. Chem., 79 (1975) 1716. [8] C. Naccache and Y. Ben Taarit, J. Catal., 22 (1971) 171. [9] J.C. Conesa and J. Soria, J. Phys. Chem., 82 (1978) 1575. [10] J.C. Vedrine, E.G. Derouane and Y. Ben Taarit, J. Chem. Phys., 98 (1974) 531.
193
[11] M. Che, J. Fraissard and J.C. Verdrine, Bul. Groupe Fr. Argiles, 25 (1974) 1. [12] R.C. Kemp, J~ Phys. C: Solid State Phys., 5 (1972) 3566. [13] D. Olivier, J.C. Vedrine and H. Pezerat, Bul. Groupe Fr. Argiles, 27 (1975) 153. [14] C. Mosser, M. Mestdagh, A. Decarreau and A. Herbillon, 9th International Clay Conferences, Strasbaurg, Aug. 28-Sept. 2, 1989. [15] T. Delineau, T. Allard, J.P. Muller, J. Yvon, O. Barres and J.M. Cases, Clays Clay Miner., in press. [16] G. Martini and M.F. OttavianL J. Phys. Chem., 85 (1981) 1922. [17] D. Kivelson and R. Neiman, J. Chem. Phys., 35 ( 1961 ) 149. [18] H.R. Gersmann and J.D. Swalen, J. ('hem. Phys., 36 (1962) 3221. [19] A. Nicula, D. Stamires and J. Turkevich, J. Chem. Phys., 42 (1965) 3684. [20] P.A. Berger and J.F. Roth, J. Phys. Chem., 71 (1967) 4037. [21] H.A. Kuska, M.T. Rogers and R.E. Drullinger, Z Phys. Chem., 71 (1967) 109. [22] T. Ogata, F. Fujisawa, N. Tanaka and H. Yokoi, Bull. Chem. Soc. Jpn., 49 (1976) 2759. [23] I. Adato and 1. Eliezer, J. Chem. Phys., 54 (197l) 1472. [24] M.M. Jones, J. Am. Chem. Soc., 8t (1959) 3188. [25] C.C. Chao and J,H. Lunsford, J. (Twin. Phys., 57 (1972) 2890. [26] K. Komusinski, L. Stoch and S.M. Dubiel, Clays Clay Miner., 29 ( 1981 ) 123. [27] O. Bradel, R.C. Barklie, D.H. Doff, N.H.J. Gangas and A. McKimm, Z. Phys. Chem. Neue Folge, 151 (1987) 157. [28] A.T. Pilipenko, B.I. Kornilovich, V.I. Lysenko and V.V. Maliarenko, Dokl. Akad. Nauk. S.S.S.R., 305 (1989) 1408. [29] R. Calvet, Thesis, Paris VI, 1972. [30] K.R. Kukkadapu and L. Kevan, J. Phys. (Twin., 92 ( 1988 ) 6073. [31] D.R. Brown and L. Kevan, Z Am. Chem. Sot., 110 (1988) 2743. [32] S. Gonzales Garcia, J.M. Suarez Cardeso, Si Rodriguez Alonso and G. Dios Cancela, Ann. Quim., 82(1986) 13.