4120
J. Phys. Chem. 1996, 100, 4120-4126
Dehydration of Cu-Hectorite: Water Isotherm, XRD, and EPR Studies Faı1za Annabi-Bergaya,* Henriette Estrade-Szwarckopf, and Henri Van Damme CNRS-UniVersite´ d’Orle´ ans, Centre de Recherche sur la Matie` re DiVise´ e, 45071 Orle´ ans Cedex 2, France ReceiVed: January 27, 1995X
Electron paramagnetic resonance (EPR) associated with adsorption gravimetry and X-ray diffraction (XRD) has been used simultaneously to study the dehydration process in a Cu2+-hectorite. The combination of these three techniques has provided information concerning the nature of copper hydrates, the total water content and the clay layers’ stacking order. Three copper hydrates are detected depending on the relative water vapor pressure: bare unhydrated ions in an isotropic environment, square planar tetrahydrates, and octahedral hexahydrates. “Pure” hydration states are observed at P/P0 ) 0.40 (tetrahydrates) and P/P0 ) 0.99 (hexahydrates), concomitantly with regular stacking order. Interstratification is observed in all other hydration states. A significant amount of water is found to be located outside the interlayer space, even after extensive evacuation at room temperature. A model of connected clay platelet stackings is proposed, which accounts quantitatively for the amount of water both inside and outside the interlayer space.
Introduction The interaction of water with interlamellar cations and the concomitant variation of the basal spacings in swelling clays have been the subject of numerous studies over several years,1-12 among which are several important reviews, published in 1980,2 later in 1982,3 1987,5 and 1989,8 and more recently in 1992.12 Due to their natural or industrial importance, Na- and Caexchanged smectites were the subject of most of these investigations. Other M2+-water-smectite systems may deserve interest for one reason or another such as catalytic or environmental applications or fundamental interest. It is well-known that the hydration process depends not only on factors concerning the cation, such as its size, its charge, and its hydration energy, but also on factors related to the clay itself, namely, the location of the layer charge, the particle size, and its micro- or mesoporosity. At relative pressure below 1, when less than three molecular layers of water are adsorbed in the interlamellar space, the organization of the adsorbed molecules is predominantly influenced by the nature of the exchangeable cations.11 The H-bond between molecules is also taken into account for the highly associated liquids like water. In fact, in the case of methanol, which is simpler than that of water (due to the presence of only one OH group per molecule), a similar situation has been observed13,14 when less than two layers are adsorbed. It was shown that these molecules can occupy three distinct environments: (i) molecules directly coordinated to exchangeable cations; (ii) molecules in the interlamellar space, but not directly coordinated to the cations; and (iii) molecules out of the interlamellar space in various kinds of porosities or adsorbed on the so-called external surfaces.15 To better understand such a complicated process and to tentatively build a textural model of the clay mineral, we have studied the dehydration phenomenon of Cu-hectorite. Cu2+ is a particularly interesting ion thanks to its beautiful spectroscopic signature, and hectorite is a trioctahedral clay well adapted to spectroscopic studies due to the absence of iron and to high local symmetry. An especially interesting EPR study had been performed through electron spin echo modulation (ESEM)7 on Cu2+-montmorillonite to determine the number of water molecules included in the hydrates interacting with * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4120$12.00/0
the cations. Despite the poor precision on their hydration states, they confirm that in the fully hydrated and in the one-waterlayer state the coordination numbers were 6 and 4, respectively, but for the “dehydrated state” they found one water molecule interacting with the Cu2+. However, as shown by their XRD pattern and by their ESR spectrum, the “dehydrated state” was not pure, and probably their ESEM spectrum results from an addition of several different types of Cu environment. Thus, the question of a dehydrated state in Cu2+-hectorite remained unsolved. Moreover, to our knowledge the water desorption isotherm of Cu-hectorite has not been published. Contrarily to the adsorption process, the desorption process is more reproducible.11 In addition, most of the published papers usually deal with only one or two (EPR and XRD, for instance) of the physical characteristics of the clay. In this paper three different techniques are simultaneously used: adsorption gravimetry, XRD, and EPR. Indeed, as we will show, the results are very sensitive to small variations of water content. The gravimetric isotherm allows us to know the total amount of water adsorbed in the clay at a given relative water vapor pressure. XRD on oriented films gives selective information about the organized domains of the interlamellar space of the clay, while EPR determines the local environment of the balancing cations. Thus, we will present here the dehydration properties of a trioctahedral smectite (hectorite), saturated by copper ions, studied in a vacuum apparatus, which allows one to follow the amount of water, the 001 basal distances, and the EPR data, all taken at the same relative vapor pressure in the range 0 e P/P0 e 0.99. Finally, taking into account the obtained data, we propose a model for the stacking of the clay platelets and for the kinds of porosity evidenced during the desorption. Experimental Section The smectite used in this study was a hectorite from Hector (California) containing about 50% of carbonated impurities. To remove these impurities, a 2% clay suspension was first treated with 0.1 N HCl, as in previous works.16,17 We took care that the pH was always higher than 4 to avoid the acid attack of the clay structure. Then a solution of 0.1 N NaOH was added to neutralize the excess of remaining acid, until reaching pH 7. After centrifugation, classical treatments with 1 N NaCl © 1996 American Chemical Society
Dehydration of Cu-Hectorite solutions allowed us to obtain a purified Na-hectorite. Copper-hectorite was obtained by shaking a 2% purified Nahectorite suspension with CuCl2 (1 N) solution and then centrifugating. This operation was repeated several times. After washing with distilled water to remove the salt excess (the pH was about 7, which prevents hydrolysis of Cu18) and drying at room temperature, the copper-saturated clay was used for the desorption isotherms in powder form, whereas for XRD and EPR studies, self-supporting films were used. The nitrogen BET surface area was measured to be 73 m2/g. The exchange capacity of the Cu-clay was 76 mequiv/100 g (on a calcined clay basis), which corresponds to 0.38 mM/g. Desorption isotherms of water were studied in a conventional apparatus19 which allowed us to prepare, simultaneously, the three samples for the three techniques: outlet 1 was attached to a XRD camera, outlet 2 to an EPR cell, and outlet 3 to the cell used for the desorption isotherm gravimetric measurements. All three cells could be isolated by means of a tap to maintain, during the experiments, the powder or film samples, in equilibrium with a given water vapor pressure. After the experiments, the cells were connected again to the gas-handling line and exposed to a new water vapor pressure without any contact with air. X-ray basal spacings (d00l) were determined on a selfsupported film in an XRD camera fitted with beryllium windows, using a Philips diffractometer with Ni-filtered Cu radiation. The spacings were calculated taking into account all the 00l reflections when the higher order reflections are rational or only the 001 reflection when the successive orders are irrational in the case of interstratification. The EPR experiments have been achieved with a Thomson spectrometer working at 9 GHz. The intensity of the lines could be followed by comparison with a reference sample located in half of the double cavity. The Lande´ g-factors and the hyperfine constants were measured with a NMR magnetometer together with a hyperfrequency meter measuring the irradiation frequency. The self-supporting hectorite film was located inside a quartz tube of rectangular cross section (inner dimensions 3 × 1 mm2), and the planarness and orientation of the film were maintained due to thin Teflon slabs. We took the EPR spectra in the two characteristic orientations of the film, i.e. with the mean c-axis of the film parallel or perpendicular to the main magnetic field H0, respectively called g| and g⊥ orientation. The temperature of the sample could be varied down to 80 K, due to cooled nitrogen flowing inside a double-walled vessel around the rectangular quartz tube. Experimental Results Desorption Isotherm. The water vapor desorption gravimetric isotherm obtained at room temperature (about 22 °C) is shown in Figure 1. The total adsorbed amount after equilibrium during 24 h at saturated pressure near P/P0 ) 0.99 is 19.65 mM/g. The desorption curve can be divided into three parts: (i) In the high relative pressure region (P/P0 g 0.82), after a plateau, a sharp decrease is observed at P/P0 ) 0.85. The shape of this part of the desorption curve lets us think that it is due to a capillary decondensation and that, probably, an important hysteresis loop should exist in the adsorption curve (not shown in Figure 1). This abrupt loss of water corresponds to 2.2 mM/ g, and by means of the Kelvin equation applied to it, we determined an average diameter of 13 nm for the concerned mesopores. (ii) In the P/P0 region between 0.78 and 0.44, there is a linear decrease of the adsorbed water amount until the approximate value of 9 mM/g. (iii) The last region, 0.43 g PP0 g 0, is very similar in shape to the usual Langmuir-type
J. Phys. Chem., Vol. 100, No. 10, 1996 4121
Figure 1. Water vapor desorption gravimetric isotherm at T ) 22 °C.
Figure 2. X Ray basal d001 spacings for different relative water pressures P/P0 at T ) 22 °C. Only for three narrow domains indicated by arrows (P/P0 ) 0, 0.40, and 0.98) do the higher reflections show rational d00l distances. Everywhere else, there is an interstratification.
isotherm. However, we can observe that a lot of water (2.94 mM/g) remains adsorbed after several hours at room temperature under vacuum (P/P0 about 0). This strongly bounded water could be partially desorbed when pumping at 300 °C. This desorption was reversible, contrarily to the complete desorption obtained after calcination. XRD Patterns. To investigate the evolution of the amount of water molecules in the interlamellar space at different stages of hydration, the previous desorption isotherm was followed by XRD experiments in the XRD camera. In Figure 2 the only 001 basal spacings are plotted versus relative vapor pressure. This curve can be divided into two well-defined domains, separated by clear frontiers: (i) At the beginning of the desorption (almost saturating relative pressure) we observe d00l reflections corresponding to an interlamellar distance of 1.63 nm, with higher order reflections corresponding to rational distances. In this narrow domain, the clay stacking order is homogeneous, with an interlayer distance of 0.67 nm, corresponding to copper cations surrounded by hexahydrates [Cu2+, 6H2O] called, in many previous papers, the two-water-layer state.3,5 When decreasing the relative pressure until P/P0 ) 0.40, the d001 distance decreases, and the appearance of irrational high-order reflections shows an interstratification between bilayer and monolayer hydrates. (ii) In a narrow domain around P/P0 ) 0.40, the d001 reflection corresponds to a distance of 1.26 nm with rational high-order reflections. Once again, the clay is in a pure water monolayer state, the copper cations being involved in flat tetrahydrates [Cu2+, 4H2O] with an interlayer distance of 0.3 nm. At smaller relative pressure (0.32 g P/P0 g 0) the d001 distance decreases, and again, irrational highorder reflections show that there is an interstratification between
4122 J. Phys. Chem., Vol. 100, No. 10, 1996 mono- and zero-layer hydrates. After several days under vacuum at room temperature, the 001 distance is about 1.1 nm, with still irrational higher order reflections. However, after heating at 100 °C an almost rational pattern is obtained with a characteristic distance of 1.03 nm. Such a distance could correspond to a dry state where the copper cation is partially embedded in the hexagonal cavities. EPR Spectra. To obtain further information about the local environment of the copper ions, we followed the EPR response of the self-supporting film for decreasing relative pressures. For high water content (P/P0 ) 0.99) the EPR room temperature spectra show what has usually been obtained in previous papers dealing with copper in clay minerals:18,20-22 the spectrum shows a single, symmetrical line without any hyperfine structure in any of the g| or g⊥ orientations relative to the magnetic field (Figure 3a). However, as soon as the water content is decreased, this hyperfine structure appears in the g| orientation and superimposes to the previous symmetrical single line (Figure 3b). For decreasing P/P0 values, the evolution is regular: decrease of the single line and increase of the hyperfinestructured line until P/P0 ) 0.40. This relative pressure corresponds to the water monolayer, and the spectrum is in fact unique, composed of the four lines of the hyperfine structure in the g| orientation and a single line in the other one (Figure 3c). Shapes observed in both directions remain unchanged when P/P0 continues to decrease, but as soon as the samples are evacuated, even at room temperature, a new line appears which gives the spectra a curious shape of five components instead of four in the g| orientation (Figure 3d). This “fifth line” is completely isotropic (g| ) g⊥), and if the pumping is pursued for several hours and even more when pumping at higher temperature, its intensity increases to the detriment of the hyperfine-structured one (such a line has already been published,18 but the authors did not seem to attach any importance to the new feature). After a few hours evacuation at 100 °C, this “fifth line” is largely the main one, the hyperfine-structured one having disappeared in the g| orientation. On the contrary, it seems that a very weak structured line is present, together with the main single one, in the perpendicular orientation. When opening the sample tube in air, the “fifth line” disappears very quickly, together with the appearance of the structured line. Corresponding g-values and hyperfine A constants are given in Table 1 for the different situations. In agreement with previous papers,18,20-22 we assign the single symmetrical lines observed for the highest water content (Figure 3a) to copper cations involved in a water octahedron [Cu2+, 6H2O], probably elongated by the Jahn-Teller effect and rapidly tumbling around its center of gravity. Similarly, we assign the axially symmetrical line, hyperfine structured in the g| orientation, and a single line in the g⊥ orientation to the copper cations involved in a square planar tetrahydrate [Cu2+, 4H2O], the axis of which is perpendicular to the film (Figure 3c). Both lines are present for 0.78 g P/P0 g 0.44. Taking into account the evolution of the “fifth component” of the spectra observed in vacuo, we assign this single isotropic line to copper ions, not bound to any water molecules, which we will call “bare copper” cations [Cu2+, 0H2O]. This line is present together with the monolayer one, when pumping on the sample at relatively low temperatures (
