4722
J. Phys. Chem. C 2007, 111, 4722-4726
Cation Dynamics upon Adsorption of Methanol in Na-Y Faujasite Type Zeolites: A Dielectric Relaxation Spectroscopy Investigation A. Nicolas, S. Devautour-Vinot,* G. Maurin, J. C. Giuntini, and F. Henn Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Physicochimie des Mate´ riaux De´ sordonne´ s et Poreux, CC003, 34095 Montpellier Cedex 05, France ReceiVed: September 28, 2006; In Final Form: January 25, 2007
Dielectric relaxation experiments are carried out to address the question of cation location upon adsorption of methanol in a Na-Y faujasite system. They allow us to probe both the evolutions of the number of extraframework sodium ions in the different crystallographic sites and of their de-trapping energy as a function of the adsorbate loading. Redistributions of the cation positions are explained in light of previous molecular dynamic results. It is shown that SI cations remain mainly trapped in their initial sites whatever the methanol loading. At low and intermediate loadings, SII cations migrate within the supercage, due to strong interaction with the adsorbate molecules. They are followed by hopping of SI′ cations from the sodalite cage toward the supercage to fill vacant SII sites. At higher loading, the cation motions are hindered due to steric effects induced by absorbates within the supercage. These cation motions are shown to be consistent with the evolution of the de-trapping energy of Na+.
1. Introduction Zeolites are used in a variety of important industrial applications such as drying agent, molecular sieves, or heterogeneous catalyst.1 Knowledge of the location and distribution of the extraframework cations and of the interaction between these cations and both the zeolite framework and/or the adsorbed reactants is a crucial point for understanding the solid-state, adsorption, and catalytic properties of zeolites.2,3 Methanol, the subject of this study, is involved in several industrially important reactions such as Mobil’s methanol to gasoline (MTG) and methanol to olefin (MTO) processes.4 It is also used as an alkylating agent for aromatic compounds in zeolite-catalyzed reactions.5 These reactions take place in a number of zeolites, including alkali metal-exchanged X and Y zeolites, where the extraframework cations play a crucial role by activating the methanol molecules.6 A deeper understanding of the interactions between the cations and methanol can thus provide insights into the initial stages of the alkylation reaction. Some experimental and theoretical studies were devoted to the determination of the cation position in the dehydrated faujasite.7-9 The sodium distribution among the different crystallographic sites is usually described according to Figure 1. The SI cations are located in the hexagonal prisms, which connect the sodalite cages (β-cages). The SII cations are in the six-ring windows of the supercage. SI′ and SII′ cations are inside the sodalite cage facing positions I and II, respectively. Some additional cations, named SIII and SIII′, were also found (in the case of Na-X) in the supercage.10,11 In the case of the dehydrated Na-Y type faujasite with a corresponding standard Si/Al ) 2.4, it was reported that sodium ions are distributed as follows: 6, 18, and 32 Na+ in SI, SI′, and SII sites, respectively.7,12-14 However, a migration of these extraframework cations can occur when adsorbing or desorbing molecules such as water.15-17 In a previous work, we investigated the * Corresponding author. E-mail:
[email protected].
Figure 1. Representation of the extraframework cation sites in the faujasite system.
redistribution of the cations in a Na-mordenite zeolite system upon water adsorption by combining dielectric relaxation spectroscopy (DRS) and computational simulation based on energy minimization techniques.18 More recently, it was established that the extraframework cations of faujasite can be substantially perturbed upon adsorption of halocarbon molecules, leading to possible changes in the adsorption properties due to the increasing accessibility of these specific adsorption sites.19-22 In a recent paper,23 molecular dynamics (MD) simulations were performed to investigate the cation migration upon methanol adsorption in Na-Y. It was shown that at low and intermediate loadings, from 16 to 64 methanol molecules per unit cell, the SII cations can migrate within the supercage due to strong interactions with the adsorbates, and these motions can be followed by a hopping of SI′ from the sodalite cage into the supercage to fill the vacant SII site, emphasizing that both single and concerted cation rearrangements can take place in this material. Furthermore, the SI cations were revealed to be mainly trapped in their initial sites whatever the loading. At high loading, only limited motions were observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage.
10.1021/jp066383q CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
Methanol Adsorption in Na-Y Faujasite Zeolites In the present work, the Na-Ysmethanol system is studied by means of DRS and analyzed in light of data obtained from MD simulations.23 It is clearly established that several parameters, such as the nature of the cations, their size and charge, the lattice structure, and the zeolite composition as well as the hydration degree have a strong influence on the dielectric properties of zeolites.24-27 The complex interplay of all these parameters is a key point to better understand adsorption and/ or catalytic properties of such solids. As an example, DRS was used to investigate in detail both the mobility of sorbed molecules and the influence of adsorbed phases on the relaxation mechanisms due to extraframework cation hops.15,28-31 It was then revealed that the coupling of experimental DRS and molecular simulations is a powerful tool in order to deeper understand the cation motion in zeolite in both dehydrated32 and hydrated18 states. It was shown33 that an appropriate analysis of the DRS signal, in terms of a distribution of the relaxation times, allows us to access the multiplicity of relaxation hopping mechanisms in iono-covalent solids such as zeolites. Applied to a Namordenite type zeolite, this approach provided quantitative information about the location of sodium ions in the different crystallographic sites and the distribution energy related to the de-trapping motions of the different exchangeable cations, as a function of the hydration state.27 Here, we propose to use the same approach, in order to investigate the influence of methanol adsorption on the dielectric relaxation of sodium ions in a Na-Y faujasite system. The remainder of the paper is organized as follows. In section 2, a brief description of the investigated sample as well as of the techniques used to characterize the solids (thermogravimetry and DRS) is given. Then, the data analysis, which yields the de-trapping energy and the location of the sodium ions in Na-Y faujasite as a function of the methanol loading, is discussed in section 3. 2. Materials and Methods Materials. The zeolite Na-Y (Si/Al ) 2.4) used for the present investigation has the following composition Na56Si136Al56O384‚yH2O. The high crystallinity of the material is confirmed by X-ray powder diffraction, and the absence of extraframework aluminum species is examined by 27Al MAS NMR spectroscopy. First, the zeolite powder, compacted into a pellet form, is dried at 400 °C, under vacuum, for at least 24 h in order to dehydrate the sample. Adsorption is then carried out by keeping the dried pellet in a saturated methanol vapor atmosphere, at ambient temperature, for 24 h. Finally, the pristine saturated sample is introduced in the DRS cell. To have a range of different methanol loadings, the zeolite pellet is heated in situ at different treatment temperatures TT (TT ) 30, 60, 80, 100, 120, 150, 170, 190, and 220 °C), during 2 h, prior to dielectric measurements. The methanol loading corresponding to each TT is controlled by thermogravimetry analysis (TGA). Thermogravimetry Analysis. TG experiments are performed with Setaram Labsystem TG-DTA/DSC equipment, under an argon flow, in the 25-400 °C temperature range. The mass loss, due to the methanol departure, is recorded after treating during 2 h the pristine saturated powder sample at the same TT as the ones used in DRS. After 2 h, the TG signal shows that there is no more mass loss, so that we can assume that the equilibrium conditions are reached. Weight loss values as a function of TT are reported in Table 1. Dielectric Relaxation Spectroscopy. The complex permittivity * ) ′ - i′′ is measured in the frequency domain of 10-2-106 Hz with a Novocontrol Alpha dielectric analyzer. The
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4723 TABLE 1: Number of Methanol Molecules Remaining Per Unit Cell, as a Function of the Temperature Treatment, Determined from Thermogravimetric Analysis treatment temp (°C)
nMeOH remaining/ (unit cell)
treatment temp (°C)
nMeOH remaining/ (unit cell)
25 30 60 80 100 120
132 110 88 71 51 34
150 170 190 220 400
17 9 5 3 0
spectrometer cell is placed in a cryostat and maintained under a dry nitrogen flow. Experiments are carried at temperatures ranging from TT ) -50 to -100 °C, in order to avoid the desorption of methanol during measurements. Blocking electrodes (PTFE, 10 µm thin films) are inserted between the sample and the parallel plates of the spectrometer in order to transform the direct current (dc) conductivity contribution of the sample into a dielectric loss peak. In zeolites, it is well-established that the dielectric loss ′′(ω) measured using DRS can be related to the localized hops of extraframework cations between neighboring vacant sites34 and that each hop is characterized by a thermally activated relaxation time τ which follows a simple Maxwell-Boltzmann statistic:
(∆E kT )
τ ) τo exp
(1)
where ∆E is the activation energy, k the Boltzmann constant, and τo the inverse of the so-called natural frequency. Due to the multiplicity of cationic hops and hence of relaxation times, the dielectric loss ′′(ω) spectrum can then be expressed by33
′′(ω) ∝
∫0∞G(τ)1 +ωτω2τ2 dτ
(2)
where ω is the electric field pulsation (ω ) 2πf ) and G(τ)the distribution of relaxation times. If τo is assumed to be constant, then G(τ) can be transformed (eq 1) to a distribution of detrapping energies G(∆E). The determination of G(∆E), which characterizes the studied system, can thus be obtained by fitting the experimental ′′(ω) spectrum with eq 2. 3. Results and Discussion Case of a Dry Na-Y. The presence of PTFE thin films placed between the zeolite pellet and the metallic electrodes of the spectrometer cell precludes the use of temperatures higher than 200-250 °C. The zeolite sample must therefore be preheated ex situ at 400 °C, prior to being mounted into the spectrometer cell. However, because the sample can take up water during its transfer to the dielectric spectrometer cell, it is in situ reheated at 220 °C, during 2 h, under a dry nitrogen flow in order to remove the adsorbed water molecules as much as possible. In such experimental conditions, TGA measurements reveal that a maximum of three water molecules per faujasite unit cell can be adsorbed on the most hydrophilic sites of Na-Y. The sample, the so-called “dry” sample in this study, is therefore not fully dehydrated. However, this number of residual water molecules can be neglected since it only represents, in the less favorable case, 1.5% of the total number of water molecules that Na-Y can adsorb. Consequently, it can thus be reasonably assumed that this negligible amount of water does not significantly influence the mechanism and thermodynamics of MeOH adsorption.
4724 J. Phys. Chem. C, Vol. 111, No. 12, 2007
Figure 2. Imaginary part of the permittivity losses ′′(ω) as a function of log(f ), in the case of the dry Na-Y, for temperatures ranging from 170 to 0 °C.
Nicolas et al.
Figure 5. Evolution of the global energy distribution function (solid line) and of its decomposition into three Gaussian functions (dotted line), assigned to each Na+ site in the dry Na-Y.
TABLE 2: Interatomic Distances and Angles Involving Sodium Cations among Crystallographic Sites in the Case of the Dry Na-Y7
site II site I site I′
Figure 3. Arrhenius plot of the dielectric losses, in the case of the dry Na-Y.
Figure 4. Comparison between the calculated ′′ signal with eq 2 (solid line) and the experimental data (open square), in the case of the dry Na-Y at T ) 60 °C.
Figure 2 illustrates the isothermal spectra of ′′ versus frequency for different temperatures (from 170 to 0 °C). The dielectric relaxation peak maximum is shifted toward higher frequencies upon increasing temperature as expected from eq 1. Measuring τmax at the peak maximum, assuming ωτmax ) 1and then plotting ln(τmax) vs T-1, we determined the value of ∆E for Na+ ion hopping (Figure 3). However, this procedure which yields a unique value of ∆E, i.e. the most probable activation energy, does not account for the distribution of energy barriers G(∆E). This can be achieved, by fitting any isothermal ′′(ω) spectrum with eq 2 (Figure 4). Due to the time/ temperature superposition principle (eq 1) which is applicable in the present case, all the isothermal ′′(ω) spectra correspond to the same distribution function. In the case studied here, the so-obtained G(∆E) function (Figure 5) appears to be a contribution of three elementary Gaussian functions, each of them being related to the reorientational motion of Na+ trapped in given crystallographic sites of the faujasite framework, i.e., SI, SI′, and SII. A more detailed analysis of G(∆E) and of the elementary Gaussian functions can thus provide relevant information on each cation site energy and occupancy.
distances (Å) Na-O
angles (deg) O-Na-O
2.38 (3d1) 2.86 (3d2) 2.71 (3d1) 3.52 (3d2) 2.24 (3d1) 2.93 (3d2)
149.53 128.47 156.15
First of all, it is possible to determine the number of Na+ ions in each crystallographic site26 and to compare our data with those obtained from other experimental techniques or computational simulations reported in literature. The site occupancy can be calculated by considering that it is proportional to the surface of each elementary Gaussian function. In the dry NaY, 32, 18, and 6 Na+ cations are obtained for the high-, intermediate-, and low-energy elementary Gaussian, respectively (see Figure 5). The comparison of this result with data obtained from neutron scattering techniques,7 molecular simulations,19 X-ray diffraction,20 or 23Na solid-state NMR measurements35,36, which give a distribution of 32, 16, and 8 Na+ in the SII, SI′, and SI sites, respectively, allows us to assign each elementary Gaussian to the Na+ hops embedded in these sites. Following this assignment, it becomes possible to clarify the energy sequence: ∆EsiteI (0.64 eV) < ∆EsiteI′ (0.71 eV) < ∆EsiteII (0.74 eV) from structural arguments. Noteworthy, this explanation is based on the assumption that the deeper the cationic site energy level the higher the barrier for de-trapping. The energy sequence given above can be explained by considering both the geometry of the sites in which Na+ ions are embedded and the coordination of the cation to the oxygen atoms. In this considered structure,7 the site geometry is summarized as follows: the SI site belongs to the center of the double hexagonal prism, the SI′ site in the sodalite cavity faces the six oxygen ions of one hexagonal plane of the prism mentioned above, and the SII site located in the supercage shares the six oxygens of the six ring. The corresponding interatomic distances and bond angles are given in Table 2, in which d1 and d2 are the smallest and the largest distances between O and Na for each site, respectively. It is interesting to note that the SI site is shared between two hexagonal plans, whereas the SI′ and SII sites face only one hexagonal plan. As a consequence, the distances between cations and oxygen atoms are larger for the SI site than for both SI′ and SII sites. This implies that the interaction between the zeolite framework and the exchangeable cations is lower for the SI sites than for the other sites, making the SI cations less trapped, as shown from DRS data. The interatomic distances and bond angles do not allow us to discriminate the energy difference
Methanol Adsorption in Na-Y Faujasite Zeolites
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4725
Figure 6. Imaginary part of the permittivity losses ′′(ω) as a function of log(f ), in the case of methanol adsorbed in Na-Y for four loadings (88 (O), 51 (4), 34 (0), and 5 (3) methanol molecules/(unit cell)).
Figure 8. Evolution of the number of cations per site in Na-Y as a function of the methanol loading measured by DRS. Dotted lines are guides to eyes.
Figure 7. Evolution of the global energy distribution function (solid line) and of its decomposition into three Gaussian functions (dotted line) in the Na-Ysmethanol system, for a loading of 88 adsorbed molecules.
Figure 9. Evolution of the activation energy per site in Na-Y as a function of the methanol loading measured by DRS.
observed for cations embedded in SI′ and SII sites, because their difference in energy is probably too small to be explained with so simple considerations. A more complex analysis of the interactions between Na+ and these sites would be needed. Nevertheless, it is shown that the so-obtained energy sequence is in good agreement with the population of each type of sites: the deeper the site energy the larger the number of Na+ ions trapped in it. This explains why SII is always saturated with 32 Na+ whatever the Si/Al ratio in faujasites.17 Case of Methanol Adsorbed in Na-Y. Following the procedure described in Materials and Methods, the ′′(ω) spectra are recorded under isothermal conditions as the function of the methanol loading. Typical spectra recorded at 10 °C are reported in Figure 6, where it can be seen that both the ′′ shape and frequency position are modified by the presence of methanol molecules into the zeolite framework: the higher the loading, the higher frequency and the broader the ′′(ω) peak. Each spectrum is fitted with eq 2, and the so-extracted distribution functions G(∆E) are analyzed in terms of site population and energy as for the dry Na-Y sample. As an example, a typical energy distribution function is given in Figure 7 for 88 methanol molecules adsorbed in Na-Y. Comparing with G(∆E) of the dry Na-Y (Figure 5), we observe that the G(∆E) position and shape are strongly influenced by the adsorption of methanol. Figures 8 and 9 report the evolution of the cation sites occupancy and of the de-trapping energy, respectively, upon methanol adsorption. The site occupancies (Figure 8) are determined by assuming that, for all loadings, the SI cations remain located in their initial site. This assumption results from MD simulations23 which showed a flat profile of the mean square displacement plots for this cation. It is thus observed that the evolutions of the cation site occupancy and of the detrapping energy depend on the methanol loading. First of all, for the lowest loadings (0 < nMeOH/(unit cell) < 16), the SII Na+
occupancy remains almost constant, whereas that of SI′ sharply decreases. In this domain, ∆E significantly decreases with the methanol content for all the considered sites. At intermediate loading (16 < nMeOH/(unit cell) < 50), the cation populations of SII and SI′ decrease, the decrease being sharper for Na+ initially embedded in site SII than in site SI′. Concomitantly, ∆E continuously decreases with increasing methanol content. Finally, for higher loading (nMeOH/(unit cell) > 50), the occupancy degree and de-trapping energy of both sites are roughly constant. These evolutions are consistent with those previously observed by MD simulations23 and can be explained as follows: in the first domain, the cations initially located in the SII sites are extracted, due to a strong interaction with the first adsorbed molecules in the supercage creating vacant SII sites. This displacement consequently induces the migration of cations from SI′ toward free SII sites which are more stable (see discussion on the dry state). In the second domain, the same cation displacements occur, but the departure of Na+ from SII is not anymore compensated by the migration of cations from SI′ which is now much less occupied. As previously reported in the literature in different cation exchanged zeolite,23,37 it is known that, in these domains, the adsorbed methanol molecules can directly interact with the aluminosilicate framework via hydrogen bonding. This interaction lowers the cation/site interaction, so that ∆E continuously decreases with the methanol content (Figure 9), even though there is no direct interaction between the methanol molecules and the considered cation. Finally, in the third domain, SI′ cations remain embedded in their initial sites, while SII Na+ cations are only slightly extracted from their initial site toward the supercage. Again, this observation is in agreement with MD calculations,23 which showed that the SII cation exhibits only short-range displacement. Our experimental data, thus confirming the theoretical finding, emphasize that the de-trapping of SII Na+ is hindered by steric effect induced by the methanol molecules adsorbed within the supercage.23,38 In this range of adsorption, ∆E is more or less constant (Figure 9). This can be explained by considering
4726 J. Phys. Chem. C, Vol. 111, No. 12, 2007 that the adsorbed molecules mainly interact with each other via hydrogen-bonding rather than with the zeolite framework. This assumption is supported by a previous paper,23 showing the existence of dimers of methanol molecules surrounding the cations at high loading. Whatever the methanol loading, the site classification in terms of de-trapping energy remains unchanged: ∆EsiteI < ∆EsiteI′ < ∆EsiteII. This energetic sequence is, again, in good agreement with MD simulations,23 which showed that the cations initially located in SI′ can migrate from one SI′ site to another one within the same sodalite cage, with a residence time very limited in the intermediate SI site. This result confirms that the activation barrier corresponding to the de-trapping of the SI cation is quite low. Furthermore, it was reported that the cations preferentially hop from SI′ to SII sites, suggesting a deeper potential well in this later case. To summarize, the adsorption of methanol molecules tends to decrease the cation-framework interaction energy for every cation, even if they are not directly in interaction with adsorbate molecules. This can be clearly observed in the case of the sodium embedded in the SI site and can be interpreted as resulting from an inductive effect, due to the strong hydrogen bonding of the methanol molecules with the lattice oxygen.37 Indeed, this interaction is likely to pump up the negative charge from the zeolitic network, such as the sodium cation embedded in the neighboring SI site “feels” a lower electrostatic interaction and, hence, can pass over lower energy barrier. This energy barrier decrease can be even more pronounced than that observed for the cations in direct interaction with the methanol molecules since the later is likely to hinder, and, hence, to slow down, the cation hopping mechanism. 4. Conclusion In this work, we demonstrate that DRS can be used to investigate the location of the extraframework cations and their de-trapping energy as a function of the methanol loading in a Na-Y type zeolite. It is then observed that these structural and energetic characteristics are strongly dependent on the methanol content. Noteworthy, it is clearly emphasized that the coupling of DRS and MD simulation is necessary to get a coherent analysis of both the experimental and computational data so that relevant information about the cation motions upon adsorption process in microporous materials as zeolites can be gained. It is thus shown that SI cations remain mainly trapped in their initial sites whatever the methanol loading. However, at low and intermediate loadings, SII cations migrate within the supercage, due to strong interaction with the adsorbate molecules. This cationic displacement is then followed by the SI′ cations hopping from the sodalite cage toward the supercage to fill vacant SII sites. At higher loading, the cation motions are limited due to steric effects induced by absorbates within the supercage. Acknowledgment. Many thanks to Dominique Granier for the thermogravimetry measurements.
Nicolas et al. References and Notes (1) VanBekkum, H., Flanigen, E. M., Jacobs, P. A., Jansens, J., Eds. Stud. Surf. Sci. Catal. 2001, 137. (2) Bosch, E.; Huber, S.; Weikamp, J.; Knozinger, H. Phys. Chem. Chem. Phys. 1999, 1, 575. (3) Maurin, G.; Llewellyn, P.; Poyet, T.; Kuchta, B. J. Phys. Chem. B 2005, 109, 125. (4) Patca, F. C. J. Catal. 2005, 231, 194. (5) Borgna, A.; Sepulveda, J.; Magni, S. I.; Apesteguia, C. R. Appl. Catal., A 2004, 276, 207. (6) Philippou, A.; Anderson, M. W. J. Am. Chem. Soc. 1994, 116, 5774. (7) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (8) Lignieres, J.; Newsam, J. M. Microporous Mesoporous Mater. 1999, 28, 305. (9) Mellot, C. F.; Cheetham, A. K. Comptes Rendus de l’Acade´mie des Sciences Se´rie II, Fascicule C-Chimie 1 1998, 11, 737. (10) Zhu, L.; Seff, K. J. Phys. Chem. B 1999, 103, 9512. (11) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. J. Phys. Chem. B 1997, 101, 4559. (12) Boddenberg, B.; Rakhmatkariev, G. U.; Hufnagel, S.; Salimov, Z. Phys. Chem. Chem. Phys. 2002, 4, 4172. (13) Eulenberger, G. R.; Shoemaker, D. P.; Keil, J. G. J. Phys. Chem. 1967, 71, 1812. (14) Lievens, J. L.; Mortier, W. J.; Chao, K. J. J. Phys. Chem. Solids 1992, 53, 1163. (15) Pissis, P.; Daoukaki-Diamanti, D. J. Phys. Chem. Solids 1993, 54, 701. (16) Mortier, W. J.; Bossche, E. V. d.;Uytterhoeven, J. B. Zeolites 1984, 4, 41. (17) Beauvais, C.; Boutin, A.; Fuchs, A. H. ChemPhysChem 2004, 5, 1791. (18) Maurin, G.; Bell, R. G.; Devautour, S.; Henn, F.; Giuntini, J. C. J. Phys. Chem. B 2004, 108, 3739. (19) Ramsahye, N. A.; Bell, R. G. J. Phys. Chem. B 2005, 109, 4738. (20) Grey, C. P.; Poshni, F. I.; Gualtieri, F.; Norby, P.; Hanson, J. C.; Corbin, D. R. J. Am. Chem. Soc. 1997, 119, 1981. (21) Sanchez-Sanchez, M.; Blasco, T.; Rey, F. Phys. Chem. Phys. Chem. 1999, 1, 4529. (22) Mellot-Draznieks, C.; Rodriguez-Carvajal, J.; Cox, D. E.; Cheetham, A. K. Phys. Chem. Chem. Phys. 2003, 5, 1882. (23) Maurin, G.; Plant, D.; Henn, F.; Bell, R. G. J. Phys. Chem. B 2006, 110, 18447. (24) Pamba, M.; Maurin, G.; Devautour, S.; Vanderschueren, J.; Giuntini, J. C.; Renzo, F. D.; Hamidi, F. Phys. Chem. Chem. Phys. 2000, 2, 2027. (25) Giuntini, J. C.; Maurin, G.; Devautour, S.; Henn, F.; Zanchetta, J. V. J. Chem. Phys. 2000, 113, 4498. (26) Devautour, S.; Vanderschueren, J.; Giuntini, J. C.; Henn, F.; Zanchetta, J. V.; Ginoux, J. L. J. Phys. Chem. B 1998, 102, 3749. (27) Devautour, S.; Abdoulaye, A.; Giuntini, J. C.; Henn, F. J. Phys. Chem. B 2001, 105, 9297. (28) Kalogeras, J. M.; Vassilikou-Dova, A. Cryst. Res. Technol. 1996, 31, 693. (29) Ohgushi, T.; Kataoka, S. J. Colloid Surf. Sci. 1992, 148, 148. (30) Simon, U.; Franke, M. E. Microporous Mesoporous Mater. 2000, 41, 1. (31) Frunza, L.; Kosslick, H.; Frunza, S.; Schonhals, A. J. Phys. Chem. B 2002, 106, 9191. (32) Maurin, G.; Senet, P.; Devautour, S.; Gaveau, P.; Henn, F.; Doren, V. E. V.; Giuntini, J. C. J. Phys. Chem. B 2001, 105, 9157. (33) Devautour, S.; Vanderschueren, J.; Giuntini, J. C.; Henn, F.; Zanchetta, J. V. J. Appl. Phys. 1997, 82, 5057. (34) Franke, M. E.; Simon, U. Solid State Ionics 1999, 118, 311. (35) Lim, K. H.; Grey, C. P. J. Am. Chem. Soc. 2000, 122, 9768. (36) Sanchez-Sanchez, M.; Blasco, T. J. Am. Chem. Soc. 2002, 124, 3443. (37) Rep, M.; Palomares, A. E.; Eder-Mirth, G.; Ommen, J. G. V.; Rosch, N.; Lercher, J. A. J. Phys. Chem. B 2000, 104, 8624. (38) Devautour-Vinot, S.; Giuntini, J. C.; Henn, F. IEEE Trans. Dielectr. Electr. Insul. 2004, 11, 320.
