Mechanisms Responsible for Dielectric Properties of Various

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Mechanisms Responsible for Dielectric Properties of Various Faujasites and Linde Type A Zeolites in the Microwave Frequency Range Benoit Legras,† Isabelle Polaert,*,† Lionel Estel,† and Michel Thomas‡ † ‡

LSPC, INSA de Rouen, avenue de l’Universite, Saint Etienne du Rouvray, France IFP Energies nouvelles - Lyon, Direction Catalyse et Separation, Solaize, France ABSTRACT: Microwave energy is used in zeolite sorption process intensification and more specifically in the desorption step. To optimize a microwave process, the knowledge of dielectric properties of the used materials is required. The complex permittivities of several zeolites (NaA, NaK-LSX, NaX, NaY, and DAYs) were measured and interpreted using phenomenological models. Permittivities are linked to the various properties of the zeolites such as structures (LTA and FAU), number of exchangeable cations, silanols density, and hydration level. Three phenomena have been proven to occur: two relaxation processes and one ionic conductivity contribution. Rotational polarization of water molecules adsorbed is revealed, as well as interfacial polarization of charges in intercrystalline void and orientational polarization of water molecules adsorbed on hydroxyl sites. Water loading strongly affects the charge carriers mobility and improves the conductivity phenomenon observed for low and intermediate silica zeolites. Cation location is also of main importance in the involved mechanisms.

’ INTRODUCTION Zeolites are microporous crystalline aluminosilicates, composed of SiO4 and AlO4 tetrahedra with oxygen atoms bonding neighboring tetrahedra. Zeolites types differ by the way Si, O, and Al atoms are interconnected creating different and well-defined crystalline structures. Faujasite (FAU) and Linde Type A (LTA) are two of many different types of zeolites, widely used in industrial applications as adsorbents, catalysts, and ion exchange supports.1 LTA and FAU differ in sodalite unit layout. Their unique layout confers specific pore opening size and void in framework. Selectivity and adsorption capacities strongly depend on specific interactions (electrostatic) as well as on steric interactions. Upon incorporation of Al into a silica framework, the þ3 charge on the Al makes the framework negatively charged, and requires the presence of extraframework cations within the structure to keep the overall framework neutral.2 The general chemical formula of zeolites unit cell (u.c.), with eight supercages superposed, is Mx/n(AlO2)x(SiO2)192-x 3 mH2O, where m is the number of water molecules and n is the valence of the exchangeable cation M (with x = 96 for LTA), as indicated in the Atlas of Zeolite Framework Type.3 In adsorbent applications, extraframework cations are usually of the alkali and alkaline earth metal species (Naþ, Kþ, Liþ, Ca2þ, Mg2þ). Considering FAU and LTA zeolites, cations are distributed between different cationic sites (I/I0 , II/II0 , III/III0 ) of framework4 of the zeolites to maximize interactions with framework oxygens and minimize cationcation electrostatic repulsions. Sites I (SI) are localized in sodalite cages, sites III (SIII) in the supercages, and sites II (SII) at the center of hexagonal prism between the sodalite cage and the r 2011 American Chemical Society

supercage. Generally, cations preferentially locate in SII and SI, then finally in SIII. The size and valence of the cation have already been proved to be critically important in adsorption capacities and catalytic properties of zeolites.3 To improve industrial applications for zeolites, microwave energy is used, particularly in desorption processes.5-7 Microwaves are electromagnetic waves in a frequency range from 300 MHz to 30 GHz. Among the authorized frequencies (ISM frequency), 2.45 GHz is the most often used, even if other frequencies would sometimes be more convenient and favorable to electromagnetic energy conversion into heat. Because microwave heating occurs in a volumetric manner, dielectric substances can be quickly and internally heated8,9 without requiring heat transfer from the surrounding gas as in thermal swing adsorption processes.10 Desorption under microwave heating is a rapid and effective way to regenerate adsorbent beds leading to energy savings in some cases as compared to classical methods.10 To optimize a microwave process, the knowledge of dielectric properties (ε0 (F m-1), dielectric constant; ε00 (F m-1), dielectric loss factor) of the materials used is required. Microwave absorbed power by a material is expressed by eq 1 involving ε0 and ε00 . Pabs ðWÞ ¼ πf ε00 E2 V

ð1Þ

where f (s-1) is the wave frequency, E (V m-1) the electric field module, and V (m3) is the volume of the microwave irradiated Received: December 1, 2010 Revised: January 13, 2011 Published: February 3, 2011 3090

dx.doi.org/10.1021/jp111423z | J. Phys. Chem. C 2011, 115, 3090–3098

The Journal of Physical Chemistry C sample. ε0 is related specifically to the electric field distribution, and ε00 is related to the microwave energy conversion into heat in the medium. The electromagnetic field distribution is ruled by the Maxwell equations, and no microwave process simulation would be possible without these essential data. As already reported in the literature,9,11 dielectric losses are mainly due to two phenomena: dipole orientation and electrical conduction. Orientational polarization of polar molecules like water is the most common and is required for many microwave heating applications in the food industry,12 material drying,10 and the processing industry.13 In case of microwave irradiation of electrically conductive powders,14 the conduction phenomenon is the main heating mechanism. Sometimes both phenomena occur as in the case of zeolite microwave heating. Because of the complexity of structures and chemical compositions, it is difficult to understand the contribution of each phenomenon, yet this is essential to make the appropriate selection of zeolites for microwave irradiation applications. To our knowledge, there is a lack of data for dielectric properties of zeolites in the commonly used microwave frequency range. Most of the previous studies dealing with hydrated or nonhydrated zeolites focused on phenomena occurring below 10 MHz.15-29 Ionic conductivity was first the most studied phenomenon. Conductivity results from the migration of exchangeable cations along the channels and cavities of the zeolite framework according to an ion-hopping mechanism.16-18 Relationships between the dielectric properties and the zeolite’s structure, the number and size of extraframework cations, and the hydration level were clearly demonstrated,15,16,19,20 in 1961 with Freeman and Stamires.15 It has been established below 10 MHz that these zeolite properties affect cation’s mobility in zeolite framework by the steric effect (due to cation and channel sizes), electrostatic interactions, and intercationic repulsions (due to cation’s nature), and therefore influence the overall electrical conductivity. When water is present, water molecules coordinate the cations. Interactions between the cations and the negatively charged framework of zeolite are then weakened and increase their mobility. Multiple relaxation domains were identified and attributed to numerous cation jumps between cationic framework sites.20-23 For example, Ohgushi et al. (2001)23 studied Naþ jumps in NaA hydrated and dehydrated. In these studies, after calculations on dielectric spectra, the relaxations were considered as the consequence of specific Naþ jumps between SIII and SI, and between SIII and SII at different frequencies, around 1 MHz depending on temperature. Ionic conductivity also has an influence on the dielectric properties of materials at the commonly used frequency, 2.45 GHz, but with much lower values. Ohgushi and co-workers (2001, 2001, 2003, and 2009)23-27 specifically concluded on the importance of cations in site III in microwave heating. Nevertheless, this conclusion was issued from extrapolation to 2.45 GHz, and of permittivity data obtained between 1 kHz and 1 MHz.26,27 They also compared temperature profiles obtained by microwave heating of different LTA zeolites at 2.45 GHz and linked the results to the extrapolated dielectric loss factors. According to them, SIII cation relaxation, which is predominant in the megahertz frequency region, is the only one still strong enough at 2.45 GHz, to induce microwave heating. Only a few studies have focused particularly on the microwave frequency range. Or they are limited to 2.45 GHz30,31 (Roussy et al., 1984, Thiebaut et al., 1988), or to a particular kind of zeolites28,29 (Chapoton et al., 1975, Ravalitera et al., 1977). The last two authors worked in a large frequency band but deleted

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Table 1. Chemical Compositions of Zeolites maximum water content measured

zeolite

framework nSi/ extraframework structure nAl cations

H2O/u. % (mH2O/ c. mzeolite)

NaK-LSX

FAU

1

(Na0.7K0.3)þ

247

31.6

NaX (13X)

FAU

1.3

Naþ

238

32

NaY

FAU

2.4

Naþ

227

32

DAY (cbv712)

FAU

6

Hþ

142

22.1

DAY (cbv760)

FAU

30

Hþ

110

17.2

NaA (4A)

LTA

1

Naþ

201

26.5

conductivity of zeolites by using a silicon coating required by their method: they blocked the samples between two electrical polarized electrodes, but they ignored a part of the conductivity and interfacial phenomena. Furthermore, all of them used zeolites associated with binder. The nature and content of binder is variable from one zeolite to another and is not always wellknown. It is then difficult to extrapolate their results to other zeolites of the same type. The results of these studies are not sufficient to conclude on the mechanisms that govern the interactions between microwaves and zeolites in the whole microwave frequency range, and comparison with ideal models describing dielectric relaxations is not possible. In this study, complex permittivities at room temperature between 500 MHz and 20 GHz have been measured for several pure zeolite powders. Zeolites have been chosen with various properties such as structures (LTA and FAU), number of exchangeable cations (nSi/nAl ratio), and silanols density number. The effect of the hydration level has also been studied. More than simply collecting the dielectric properties of various zeolites in the whole microwave frequency range, the aim of the present work is to understand the way microwaves interact with microporous materials and more specifically with the investigated zeolites. For that, permittivity measurements have been interpreted using phenomenological models, such as the modified Debye formulation. The relationships between the phenomena involved during microwave irradiation and zeolite properties such as cation number and nature, zeolite structure, and water loading are also discussed.

’ EXPERIMENTAL METHODS 1. Materials. Zeolites NaA (4A), NaK-LSX, NaX (13X), and NaY were commercially supplied by Ceca, and DAYs (cbv712 and cbv760) was from Zeolyst. Materials were powder of micrometric size without any binder. The compositions of materials are listed in Table 1. The maximum water content obtained for each solid at atmospheric pressure is expressed as the number of water molecules per unit cell (u.c.) with eight supercages superposed (for both FAU and LTA types) and in weight percentage of the dry solid weight as well. Cbv712 was thermally treated at 450 C for 3 h to degrade exchangeable cations NH4þ to Hþ as in cbv760. 2. Dielectric Property Measurements. For dielectric measurements, material powders were manually compacted in a cylindrical Teflon PTFE block, 20 mm high and 20 mm in diameter. To obtain dielectric measurements of partially dry powders, the zeolites were first dehydrated in an oven for 12 h at 250 C. Materials were then slowly cooled at room temperature 3091

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The Journal of Physical Chemistry C in a desiccator. Compactions and dielectric measurements of nearly dry powder were conducted in a glovebox under dry nitrogen flow at room temperature (20 C ( 1). The external porosity of the compacted powder bed was evaluated for each sample by using the apparent bed density and zeolite microporosity measurements. Very comparable values for all the solids studied were obtained, varying from 0.5 to 0.6 with a most common value of 0.55 regularly reached. Just after the dielectric properties measurement, a sample was taken for hydration level evaluation by thermogravimetric analysis (TGA). The remaining material was placed in a large beaker at room atmosphere for progressive rehydration. Samples were regularly taken and compacted. Dielectric measurements and thermogravimetric analysis

Figure 1. Complex permittivity test apparatus: (a) vector network analyzer, (b) isolated box, (c) open-ended coaxial probe, and (d) PTFE cup with sample.

Figure 2. Schematic diagram of the open-ended coaxial probe. Streamlines represent electric field.

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were conducted on these partially hydrated powders. All steps were repeated until full hydration. Thermal analysis was conducted with SDT apparatus from TA Instruments at a heating rate of 20 K min-1, from room temperature to 450 C under dry nitrogen flow rate of 50 mL min-1. The apparatus used to acquire complex permittivity data is an integrated system, which consists of a Vector Network Analyzer (VNA) supplied by Agilent in PNA_L series N5230A associated with a high temperature open-ended coaxial probe supplied in 85070E32 probe kit by the same company with data acquisition software. The system is illustrated with diagrams in Figures 1 and 2. Dielectric measurements were conducted from 0.5 to 20 GHz. Technical specifications of this method suggest an accuracy limit for a low dissipation factor, tan δ = ε00 /ε0 < 0.05. In operation, the VNA scans the whole frequency range selected. Reflection coefficients of electromagnetic waves for each frequency with the material are determined. The material affects the phase and magnitude of the reflected power observed by the VNA, from which the complex permittivities are extracted. The numerical values of complex permittivities are calculated as dimensionless values relative to free space permittivity ε0, εr = ε/ε0.

’ RESULTS Dielectric analyses of zeolite powders were made. εr0 and εr00 spectra were then plotted for the different zeolites at various water loading, partially to fully saturated, from 500 MHz to 20 GHz at room temperature (20 C ( 1). Figures 3 and 4 show the results obtained for NaX and NaA zeolites, and FAU and LTA zeolites. The first basic observation concerns the change in permittivity with water loading: in all of the cases, permittivities increase with the amount of water molecules adsorbed. Highly polar molecules such as water possess high dielectric permittivities when they are in a condensed phase. Their ability to polarize and relax under the action of an alternative electromagnetic field is well-known as a major phenomenon in microwave energy conversion. The more water is adsorbed, the higher the permittivity of the loaded zeolite is, and the more it will absorb and convert electromagnetic energy into heat. For a given water content, the real part of the permittivity typically decreases with frequency. On the contrary, the profiles of the imaginary part exhibit one or several local maxima. As an example, Figures 3b and 4b show dissimilar trends, with the visible presence of two peaks in NaA contrary to NaX where only one is obvious. It appears in the middle range, near 7 GHz for every water loading. For NaA, a second one appears around 2 GHz. These maxima correspond

Figure 3. Relative dielectric spectra of NaX (εr0 and εr00 ) for various water loadings (H2O/u.c.). 3092


The Journal of Physical Chemistry C

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Figure 4. Relative dielectric spectra of NaA (εr0 and εr00 ) for various water loadings (H2O/u.c.).

Figure 5. Argand diagrams of slightly (a) and fully (b) hydrated zeolites with water under atmospheric humidity: NaA 66 and 201 H2O/u.c., NaK-LSX 62 and 247 H2O/u.c., NaX 43 and 238 H2O/u.c., NaY 56 and 227 H2O/u.c., cbv712 17 and 142 H2O/u.c., and cbv760 24 and 110 H2O/u.c.

to some particular frequencies for which dielectric relaxation phenomena are predominant. These curves already indicate the way in which microwaves interact with the zeolite is affected by the structure and nature of the zeolite. Additionally and more precisely for NaX zeolite, the influence of water loading on the increase of the dielectric losses is more significant in the low frequency range, while this enhancement does not seem to be qualitatively present for NaA. This increase of losses can probably be attributed to a conductivity phenomenon consequence of cations hopping diffusion along zeolite structure.16-18 The data for all the adsorbents, slightly hydrated and fully saturated with water, are plotted on Argand diagrams Figure 5a and b, respectively, showing the relationship between εr0 and εr00 . A minimum of one circular arc can be drawn for each solid. These results indicate the existence of relaxation processes. At low water loadings, Figure 5a, one or two relaxation phenomena seem obvious for materials. Dealuminated zeolites (DAYs: nSi/nAl = 6 and 30) have the lowest complex permittivities contrasting with ionic zeolites, which have high cation content as NaK-LSX, NaX, NaY, and NaA and higher permittivities. At saturated water loading Figure 5b, dielectric losses increase drastically, and a conductivity phenomenon is qualitatively detected by a strong increase in the real and imaginary parts corresponding to the low frequency scale for major ionic zeolites except NaA. On the contrary, dealuminated zeolites do not exhibit conductivity.

’ MODEL A separation of overlapped relaxations is needed to clarify the contribution of the various phenomena, which interact between microwaves and material. A single relaxation can be expressed

with the Debye formulation,33-35 which links the different parts of the complex permittivity as a function of frequency by the following equations: 8 εs - ε¥ 0 > < ε r ¼ ε¥ þ 1 þ ω 2 τ 2 ð2Þ ðεs - ε¥ Þωτ > : ε00 r ¼ 1 þ ω2 τ2 where ω (rad s-1) is the wave pulsation, ε¥ is the permittivity at infinite frequency, εs is the static permittivity, and τ (1/2πf) is the relaxation time. Argand diagrams over light, particularly for NaA zeolites, show two relaxations. One is observed at low microwave frequency (1-2 GHz), and a second is in the middle range (6-9 GHz), Figure 4b. These two relaxations are more observable for low water loading because no conductivity hides the phenomena. At high water content, nearly water saturation level at room atmospheric pressure, a second mechanism governs. This mechanism is assimilated to the ionic conductivity phenomenon, the consequence of cation movement between cationic sites. So an extended Debye model is suggested to describe spectra in the frequency range studied. Similar models describing multiple relaxation phenomena have already been used successfully, and their fitting is used for calibration of permittivity measurement apparatus.36 The model proposed follows the equations below: 8 P Δεi > 0 > < ε r ¼ ε¥ þ 2 2 i 1 þ ω τi ð3Þ Δεi ωτi σ > > þ : ε00 r ¼ 1 þ ω2 τi 2 ωε0 3093



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where τi are the relaxation times, and σ is the conductivity, only affecting the dielectric losses. Analyses of spectra were carried out by using eq 3, and the optimum values of the parameters were determined using the least-squares fitting present in Matlab Curve Fitting Toolbox. Parameters are given in Tables 2 and 3, respectively, for fully hydrated solids and for slightly hydrated ones. The validity of the model was judged by comparing the calculated spectra with the measured ones. The model described by eq 3 was found to fully satisfy experiments in cases of low (NaA and NaK-LSX) or intermediate (NaX and NaY) silica zeolites when two relaxation processes as suggested are used with a conductivity contribution to the imaginary part. Even if the two relaxations (Figure 3b) or ionic conductivity (Figure 4b) were not clearly visible on the spectra, modeling demonstrated the need to incorporate these two relaxations and ionic conductivity to fit the experimental values over the whole frequency range. A comparison example is shown in Figure 6 for NaX water saturated and in Figure 7 for NaA fully saturated. In the case of dealuminated zeolites (cbv760 and cbv712), no conductivity Table 2. Optimum Parameters of the Extended Debye Model for Fully Hydrated Zeolites cations zeolite

number/u.c. H2O/u.c.

σ (S/m) -2

f1 (GHz) f2 (GHz)

NaA

96

201

1.35 10

1.45

9.36

NaK-LSX

96

247

2.79 10-2

1.59

6.92

NaX

84.6

238

2.68 10-2

1.55

8.46

NaY

59.2

227

2.00 10-2

1.60

DAY cbv712

25

142

DAY cbv760

6.8

8.89 12.2

110

0.99

10.6

Table 3. Optimum Parameters of the Extended Debye Model for Slightly Hydrated Zeolites cations zeolite

number/u.c. H2O/u.c.

σ (S/m) -3

f1 (GHz) f2 (GHz)

NaA

96

66

8.7 10

1.45

NaK-LSX

96

44

1.27 10-2

1.59

NaX

84.6

43

2.7 10-3

1.58

NaY

59.2

56

5.0 10-3

1.59

DAY cbv712

25

41

DAY cbv760

6.8

43

9.36 6.92 11.5 8.79 10.4

1.04

9.88

contribution is required as judged beforehand by Argand diagrams, Figure 5.

’ DISCUSSION Dipolar relaxations and ionic conductivity are clearly coexisting mechanisms that govern the evolution of dielectric properties over the microwave frequency range. They are affected by a number of factors linked to the zeolite properties, the importance of which has to be examined. 1. Water Loading. The zeolites studied have high or low affinity with water: water molecules can be more or less present in the zeolite structure. Water molecules are known to interact with an alternative electric field by rotational polarization. The rotational process of water molecules is directly correlated to their physic state. In the water gas phase, the relaxation of molecules is very short, the molecules are dispersed, and they have no organization, so no constraint in rotation. In the condensed phase, water molecules are intimately and partially or fully coordinated to each other with weak hydrogen bonds. The relaxation phenomenon appears at 20 GHz for liquid water at 25 C (socalled free water relaxation34), but only at several kilohertz35 for crystallized water (100 Hz for -80 C crystallize ice V37). In our study, water molecules are adsorbed, so they are in a pseudocondensed phase. The major relaxation frequency f2 is considered to be due to the rotational polarization of adsorbed water, which is between 7 and 13 GHz, depending on the zeolite nature, and the water loading as shown in Tables 2 and 3. This frequency is at an intermediate value between the one for free water and for crystallized water and is the consequence of existing interaction forces. Molecular interactions take place between water and zeolites (through the exchangeable cations and the oxygen atoms of the framework), and between neighboring molecules of water (hydrogen bound). This relaxation frequency value is in a common order of magnitude for bound water molecules.38 According to the nature of the zeolite, the relaxation frequency shifts. DAYs zeolites show a relaxation at about 10 GHz, whereas values around 8 GHz are obtained for low and intermediate silica zeolite (see Table 2). In DAYs zeolites, cations are of acid form (Hþ); hydroxyl groups are formed by the protons bound to the framework oxygen atoms and cause Brønsted acidic site39 (Figure 8b). The OH bonds are strong covalent bonds superposed by small electrostatic interactions. Interactions with wateradsorbed molecules are then weak hydrogen bonds. Furthermore, dealuminated zeolites have low cation numbers. On the contrary, in the cases of alkali cations, strong electrostatic

Figure 6. Comparisons between the measured and calculated spectra for NaX zeolite 238 H2O/u.c.: gray 9, measured spectra; thick -, calculated spectra (sum curve); - -, conductivity contribution; --, relaxation 1; - -, relaxation 2; -, εinf. 3094



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Figure 7. Comparisons between the measured and calculated spectra for NaA zeolite 201 H2O/u.c.: gray 9, measured spectra; thick -, calculated spectra (sum curve); - -, conductivity contribution; --, relaxation 1; - -, relaxation 2; -, εinf.

Figure 8. Adsorption level of water molecules: (a) zeolite cationic site, (b) Brønsted acidic site, and (c) free silanol group.

interactions exist between the oxygen atoms of the framework, cations, and water molecules. For the low or intermediate alkali zeolites studied, with high cation numbers, the water molecules are not as free in their rotation movement and involve lower relaxation frequencies as compared to dealuminated ones. According to the adsorption locations of water molecules, a significant effect on their rotational polarization is suggested. In Figure 9, showing the evolution of the relaxation frequency of water molecules (f2) with water loading, opposite behaviors are observed depending on the nature of the zeolite. In the case of NaX (Figure 8b), relaxation frequency is close to 11 GHz at low water loading and decreases with water content, indicating that the more the NaX zeolite is hydrated, the stronger the water molecule interactions between each other are. Yet NaA, NaKLSX, and NaY show no influence on the relaxation frequency of the hydration level. These observations are limited to the hydration measurement domain. These behaviors can be attributed to the content and distribution of cations in the zeolites. For NaY, Naþ cations are located in SI and SII (Table 4) for the dehydrated state. Sites I are located in the hexagonal prisms connected to the sodalite cages, SI0 inside the sodalite cages facing SI, and SII at the border of the supercage and the sodalite cage.4 For low water content, simulated cation distribution44,45 and experimental works46 show that SII are first solvated to approximately 50 molecules per unit cell corresponding to about 1.5 water molecules per Naþ cation. Our first measurement already corresponds to this water loading for which the interaction strength of water molecules with cation in SII is of intermediate level. A cation redistribution then takes place between SI and SI0 .44,45 Sodium progressively moves from SI to SI0 in the sodalite cage, accompanied by a progressive occupancy of the sodalite cages by water molecules. Water molecules are adsorbed in SI0 , which is a site of higher interaction strength than SII (that is to say, of lower potential energy), but at the same time, water adsorption goes on with SII multiple solvation

corresponding to water-water interactions (of much lower interaction strength than SII). This second step results in a relative stability of the average interaction energy, and also of the relaxation frequency f2 (Figure 9 b). Another parameter, representative of the potential of interactions between the water molecules and their surrounding atoms, is the self-diffusion coefficient of water in the zeolites. The comparison of this parameter with the relaxation frequency (f2) demonstrates the same behavior. In the same range, up to 230 H2O/u.c., the self-diffusion coefficient is quite constant.45 In the case of NaA, the first water molecules coordinate SII cations and afterward SIII cations,25 and no cation redistribution was observed.47 Self-diffusion of water molecules in NaA unit cell is said to be reasonably stable47 for 70-220 H2O/u.c., with only a decrease at very high water content due to self-blocking of water molecules.47,48 As the self-diffusion coefficient, the relaxation frequency of water molecules, f2, is constant (Figure 9b). It is close to 9.5 GHz. For NaX, the cations are located in SI, SII, and SIII (Table 4). This FAU zeolite has SIII cations contrary to NaY. At low loading, water molecules are hardly affected in their free rotation, as shown by the high relaxation frequency f2 measured. SIII cations in FAU, which are present at the border of the supercages, are weakly fixed to the zeolite framework and have great mobility. They interact with the first water molecules adsorbed.44 The water-SIII potential energy is very low, leading to a very strong interaction. One can suppose that SIII cation and water create a new entity with a dipolar moment influenced by the electromagnetic field. Naþ in SIII is so mobile that rotation remains easy. When each cation in SIII becomes solvated by one H2O molecule, adsorption starts in the other cationic sites of higher energy levels (SII and SI0 ). Water molecules adsorbed in SII tend also to interact with SIII cations and are therefore more stabilized. At full loading, SIII cations migrate to site J, at the center of the supercage, improving water molecule stabilization with cationic interactions in contrast to water-water interactions taking place in FAU without SIII cations, like NaY.45 During hydration of NaX, molecules become progressively more and more strongly linked to the structure, confined in sodalite cages, and are hampered in their rotation. The relaxation frequency decreases to 8-9 GHz for saturated zeolites (Figure 9b). The self-diffusion coefficient47 of water molecules in NaX also tends to decrease between intermediate and saturation content in water molecules and seems to follow the same behavior again. 3095



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Figure 9. Relaxation frequency of water molecules, f2, with water loading.

Table 4. Theoretical Distribution of Cations between Zeolite Cationic Sites4,23 for Dehydrated Zeolites zeolite

cations number/u.c.

NaA

96

NaK-LSX

96

NaX NaY DAY cbv712 DAY cbv760

6.8

SI/I0 64 (SI only)

SII

SIII

24

8

32

32

32

84.6

32

32

19.6

59.2 25

24.5

32 25 6.8

In NaK-LSX, the increased number of cations and the presence of Kþ certainly limit the rotation of water molecules and cause a low relaxation frequency close to 7 GHz: Kþ has stronger polarizability ability due to its larger size as compared to Naþ. The interaction strength Kþ--H2O is stronger than that for Naþ-H2O, and more cations are in SIII2. Concerning dealuminated zeolites, different trends are observed for the water relaxation frequency. An increase in relaxation frequency with water loading appears from 10 to 12 GHz for cbv 712 in Figure 9a. These zeolites have no noticeable cation content as explained before. First, water molecules are adsorbed on the Brønsted sites or on the free silanol groups offered (these two zeolites present some mesopores). Water molecules are then linked together with hydrogen bonds. The average freedom of rotation of water molecules increases with the bond weakness, and therefore with water loading. 2. Cation Effects. As qualitatively revealed by Argand diagrams in Figure 4, and proved by the modeling of permittivity with frequency, for low (NaK-LSX, NaA) and intermediate (NaX and NaY) silica zeolites, cations imply permittivity enhancement at low microwave frequencies due to ionic conductivity. These zeolites hold alkali cations (Kþ, Naþ) from 59 to 96 per unit cell. In the case of DAYs, the number of cations is not sufficient to create noticeable conductivity. Furthermore, interactions with adsorbed molecules are weak, and acid cations have low mobility in the zeolite framework in contrast to alkali cations in the present study. Interactions between the oxygen atoms of the framework and Hþ are stronger in the dealuminated zeolites than the ones between O and the alkali cations, which are preferentially strongly bound by electrostatic interactions to water molecule. The increase in dielectric property values at low frequency is amplified with the water loading level. The hydration of zeolites involves cation solvation,49 which improves their mobility and

improves conductivity phenomena. The influence of cations on conductivity depends on their quantity and their solvation level (Figure 10a). At low solvation level, ionic conductivity directly follows the number of cations for equal structures (NaK-LSX > NaX > NaY). Kþ has stronger polarizability capacity due to its larger size as compared to Naþ. This explains in addition the higher conductivity values obtained for NaK-LSX, especially at low hydration level. At full solvation level, NaA, a LTA zeolite shows lower ionic conductivity in contrast to FAU zeolites. This observation is directly associated with the zeolite structure. In LTA zeolites, interconnected windows have lower size as compared to FAU zeolites (8 membered-ring vs 12 MR). This dissimilarity limits cation movement in structure and results in a lower ionic conductivity.15 In addition, conductivity also seems to depend on the cations location. As assumed, SIII cations are slightly fixed to the zeolite framework4 in FAU zeolites, SII and SIII in LTA zeolites.49 Their mobility with the electric field is facilitated. In Figure 10a, all the zeolites show, more or less, an exponential rise of the conductivity value for a define solvation number. This considerable rise occurs at a lower solvation number for zeolites holding more SIII cations than the others. In the case of NaY, containing only SI and SII cations, conductivity rises between 2.7 and 3 water molecules per cation. This number is similar to the primary solvation number calculated for sodium ions in aqueous solution.31,34 The primary solvation number refers to solvent molecules (water in this case), which are firmly associated with the ion by electrostatic attraction. They lose their translational degree of freedom and move as one entity with the ion during its Brownian motion. If only the strongly fixed cations are considered in the solvation number calculation, the rise of conductivity is translated to the primary solvation number (Figure 10b) for every zeolite. While static cations need to reach their primary solvation number (around 3 water molecules for Naþ) to become mobile and cause a rise in ionic conductivity, SIII cations in FAU, and SIII and SII in LTA, do not need to be solvated to involve a conductivity phenomenon. This observation confirms studies done on microwave heating of dehydrated LTA zeolites, where it was concluded that only zeolites with SIII cations involve high dielectric property enhancement with temperature,26 a consequence of the easy motion of SIII cations without solvation. The variation in conductivity values with NaK-LSX zeolites can be distinguished from the others (Figure 10), because 30% of the cations in NaK-LSX zeolites are potassium ones. The primary solvation number of these ions in solution is very low, between 3096



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Figure 10. Ionic conductivity with cations solvation level: considering the total number of cations (a) or considering only static cations (SI and SII in FAU, SI only in LTA) (b).

zero and one water molecule.34 This cation mixture in this zeolite involves a global linear growth in conductivity with a small enhancement between 2 and 3 H2O molecules per cations. Another effect seems to be the consequence of the presence of cations. The dielectric measurements of ionic zeolites at low water loading, represented on Argand diagrams in Figure 5a and spectra analysis, demonstrate the existence of a relaxation between 1.45 and 1.6 GHz (f1, Tables 3 and 4). A well-known phenomenon called space charge polarization11 can be produced by the separation of mobile positively and negatively charged particles under an applied electric field, which forms positive and negative space charges in the bulk of the material or at the interfaces between different materials. In the case of zeolites, extraframework cations and charged oxygen atoms of the framework are, respectively, the positive and the negative charges. The detected relaxation can be assumed to be an interfacial polarization in the intercrystalline void. It is the consequence of the accumulation of cations at the air/ zeolite interface. The substantial difference of dielectric properties between the particle and the medium involves this electrical polarization. Interfacial polarizations have even been detected for polycrystalline alumina with alkaline cation impurities, and a relaxation has been revealed close to 1 GHz.20,40 4A zeolites dispersed in silicon oil exhibit interfacial polarization41 in the same manner. 3. Silanol Group Polarization. Considering dealuminated zeolites, interfacial polarization has to be omitted because no mobile charges are present on the surface, or, if there are any, they are strongly fixed to the structure (Hþ). The dielectric properties of low dealuminated zeolite cbv712 are easily modeled with a single relaxation using Debye formulation corresponding to the water molecules relaxation. The same answer is not true for the largely dealuminated cbv760 zeolite. A secondary minor relaxation exists close to 1 GHz. For the reasons explained above, this relaxation is not an interfacial polarization. In dealuminated zeolites, vacancies of aluminum in the Si-O-Al framework introduce the presence of hydroxyl functions on silicon, Si-OH, called silanol groups. Water molecules interact with silanol by hydrogen bonds. These interactions are of different forms and involve cluster formation.42 As encountered in crystalline ice, water clusters are organized structures, which introduce around 1 GHz relaxation in dielectric spectra.20 The slight relaxation observed in the case of largely dealuminated zeolite cbv760 is therefore assimilated to rotational polarization of water molecules in cluster form with silanol groups.

processes and one conductivity contribution. A primary relaxation is caused by rotational polarization of water molecules, more or less troubled in their free rotation by zeolite framework and cationic interactions. This relaxation is detected between 6.9 and 9.5 GHz for ionic zeolites, and this value is the consequence of the existing interaction forces. The stronger are the interactions, the lower is the relaxation frequency. The location of cations is also very influent as shown by NaX behavior through its relaxation frequency with water loading. At low water loading below 100 water molecules per unit cell, relaxation is close to 11 GHz and decreases with increasing water content. In NaX, the water molecules are primarily trapped by weakly fixed SIII cations, whereas they are usually fixed preferentially on SII sites in other FAU. For dealuminated zeolites, this polarization appears at higher frequency and is related to the weak adsorption energy of many of the water molecules on high silica zeolites, universally called hydrophobic zeolites. However, several water molecules can be adsorbed on hydroxyl sites offered and caused by desalumination treatment of DAY zeolites. These adsorbed water molecules are very well organized and involve relaxation phenomenon near 1 GHz with cbv760 contrary to cbv712 weakly dealuminated. Concerning low and intermediate silica zeolites, a second orientational polarization takes place close to 1.5 GHz caused by the interfacial polarization of charges in the intercrystalline void. All these phenomena increase with water loading. Water loading particularly affects charge carriers mobility. Conductivity is improved by cation solvation by water molecules. The mobility is fully enhanced when cation solvation is close to the primary solvation number, between 2 and 3 water molecules per Naþ in a strongly fixed location (SI and SII in FAU, SI only in LTA). Kþ and SIII cations need a low solvation value to improve their mobility because they are weakly fixed to the zeolite framework and so already mobile without water. The contribution of cations to the dielectric properties of low silica zeolite is the major phenomenon at 2.45 GHz. As a consequence, the conventional microwave frequency of 2.45 GHz for heating applications of ionic zeolites has to be used with caution. The conductivity phenomenon is known to drastically increase with temperature and provoke thermal runaways.43 A frequency of 5.8 GHz would be a safer value in that case. Concerning dealuminated zeolites, microwave heating is safer but more difficult due to lower permittivities. It is very selective regarding water molecules adsorbed as compared to the low solid heating, the consequence of no mobile charges in the framework.

’ CONCLUSIONS In zeolites, FAU, and LTA, three phenomena are proved to occur under microwave frequencies range: two relaxation

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 3097



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Mechanisms Responsible for Dielectric Properties of Various

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