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Water-assisted ionic conductance of zeolites with ZSM-5 structure
MICROPOROUS MATERIALS Microporous Materials 11 (1997) 3744
Water-assisted ionic conductance of zeolites with ZSM-5 structure S.D. Mikhailenko
a,*, S. Kaliaguine
a, E. Ghali b
a Dkpartement de Gtkie Chimique, Universitc? Laval, Sainte Foy, Qutbec, Canada GIK 7P4 b Dkpartement de Mines et Metallurgic, Universitk Laval, Sainte Foy, Qu&ec, Canada GIK 7p4 Received 23 December
1996; accepted 10 March
1997
Abstract The electrical conductivity (0) of compressed powders of an isostructural series of zeolites with MFI topology’ was measured using AC impedance spectroscopy. Silicalite-1 and ZSM-5 with different Si/Al ratios containing varying numbers of Na+ and/or Hi were studied at 100% relative humidity and immersed in liquid water. In the latter case, B is l-2 orders of magnitude larger than that in saturated water vapour. The value of r~ for silicalite immersed in water was found to be higher than that of water before and after the test. It was assumed that this is due to enhanced water dissociation on the silicalite surface. Both in water and at 100% RH the dependence of G on cation content follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in the repulsive interaction between ions in the conductive water layer on the zeolite channel walls. 0 1997 Elsevier Science B.V. Keywords:
Impedance;
Ionic conduction;
Water adsorption;
1. Introduction The investigation of the electrical properties of zeolites has attracted considerable attention over the last two decades [l-12]. These studies are significant as they not only provide information on the solid structure [5] and data related to surface ionic mobility [lo], but they also open up the possibility of using molecular sieves as solid electrolytes in some electrochemical applications, such as hydrogen sensors [ 111 and separators in fuel cells [6,12] or in galvanic cells [3]. Among
the great number
of zeolites,
the electri-
* Corresponding author. ‘According to the nomenclature of the Structure of the International Zeolite Association. 0927-6513/97/$17.00 0 1997 Elsevier PII SO927-6513(97)00023-O
Commission
Science B.V. All rights reserved.
Zeolite
cal properties of which have been studied, the ZSM-5 family has received less attention, in contrast to the broad worldwide discussion of their structure and other physicochemical properties. This gap is only partly covered by two works [8,9], where the electrical properties of HZSM-5 were investigated over the temperature range from ambient to 400°C. In these investigations, impedance spectroscopy (IS) was used over a rather narrow frequency range (0.1-100 kHz), which is far from the values associated with the dielectric relaxation times of solids (w > 10’ Hz) [ 131. This evidently affects the accuracy of the results. Moreover, the measurements were carried out in a room atmosphere, and are consequently related to materials with various degreesof dehydration. It is well established [4,8-l 0] that the conductiv-
38
SD.
Mikhailenko
et al. / Microporous
ity of zeolites varies strongly depending on the degree of hydration. It varies in the range from cr=10m8 S cm-’ in dry conditions up to as much as 10-3-10-2 S cm-’ even at room temperature [6] for fully hydrated specimens. Sorbed water plays an important role in conduction, bridging particles of zeolite, and assisting the ion-hopping process. It influences interactions between cations and the negative framework, and provides additional charge carriers (H+, OH-). So far, the electrical properties of zeolites have been studied at a relative humidity (RH) varying from 0 to 100%. However, some applications involve a direct contact of the material with liquid water, such as in the direct methanol fuel cell (DMFC). Moreover, conductivity measurements at RH = O-100% can lack precision since the equilibration of a compressed porous pellet in water vapour is slow, sometimes taking up to several days. In this work we report the electrical properties of some molecular sieves with the MFI structure (ZSM-5) both at 100% RH and immersed in water. The impedance technique was employed to study samples of Na-ZSM-5 with various Si/Al ratios. For comparison, HZSM-5 and silicalite were also tested.
2. Experimental Three zeolites with different Al contents and silicalite- 1 were synthesized and characterized as described elsewhere [ 14,151. X-ray powder diffraction patterns (Philips diffractometer, Cu Kcr radiation) were quite sharp and intense, and were consistent with the literature [ 141, indicating well crystallized ZSM-5 phases. Elemental compositions were determined by atomic absorption spectroscopy ( Perkin-Elmer), and Si/Al = 8,3 1 and 218 were found for Na-ZSM-5 samples. The sodium form of the sample with Si/Al = 31 was ion-exchanged into the ammonium form and then decomposed into HZSM-5 by calcination at 450°C. We also used HCl acid ion exchange at room temperature for the preparation of two partially protonated samples from zeolites with Si/Al=S and 3 1. This treatment changed the Si/Al ratio of
Materials
11 (1997)
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the former sample to Si/Al= 10. The sodium form of this new sample was obtained by back-exchange with NaCl. The specific area and the volume of the micropores were measured using N, adsorption at 77 K and capillary condensation. The results of elemental analysis, S,,, and Vmicropore for all the samples are reported in Table 1. It is clear from the low values of the micropore volume observed at low Si/Al ratios that in this case, part of the zeolite pores are obstructed by extra-framework aluminium species. The specimens were washed with deionised water until the conductivity of the solution was not higher than 5 x 10m6 S cm-‘. This was usually achieved by washing ten times in 200 ml of water (precipitation by centrifuging) and two final washings of 24 h each. The zeolites were then dried and ground into a fine powder, and 0.1 g of each was compressed in a 13 mm die to give pellets ca. 0.5 mm thick. Two kinds of measurements were carried out in this work. In the first set of measurements, the pellet was coated with carbon black and placed in a RHand temperature-controlled parallel-plate electrode test cell where it was clamped between two stainless-steel electrodes with springs, providing a permanent pressure of about 1 kg cm-2. In some experiments silver paste was used instead of carbon black to cover the pellet surfaces as blocking electrodes, and this was found to give the same conductivity results. However, the time of equilibration was longer. RH in these measurements was maintained at 100% by enclosing water at the bottom of the cell. In the second set of tests, two electrodes with a pellet between them were placed in a thermostated glass cell where they were covered with 15 ml of demineralized water. The conductivity of this water was measured before and after zeolite tests, and was always found to be not higher than 5 x 10m6 S cm-‘. These measurements were performed as blank experiments using the same arrangement with a 0.5 mm Teflon spacer instead of the pellet between the electrodes. Electrical impedance measurements were taken over the frequency range l-lo7 Hz with an oscillating voltage of 100mV using an SI 1260 impedance/gain-phase analyzer (Solartron), con-
S.D. Mikhailenko Table 1 Zeolite characteristics: No.
composition
Si/Al
8 10” lob 31 31” 35b 218 Silicalite-1 “Prepared “Prepared “HZSM-5.
by ion exchange by ion exchange
expressed
et al. / Microporous
as atoms
Materials
II (1997)
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39
per unit cell
Si
Al
Na
85.3 87.3 87.3 93.0 93.0 93.3 95.6 96.0
10.7 8.7 8.7 3.0 3.0 2.1 0.4
10.7 4.5 8.7 3.0 0.1 1.7 0.4
with HCI from the Na precursors with NaCl from sample no. 2.
H
4.2
2.9 1.0
with Si/Al=
trolled by a PC through a GPIB interface. Acquisition and analysis of the impedance/ admittance spectra were performed employing the Z-60 and Z-view software, provided by Solartron. The impedance data were corrected for the contribution from the empty and short-circuited cell. The conductivity 0 was calculated from the resistance R by c= R x S/d, where S is the pellet face area and d its thickness. The resistance R was derived from the low intersect of a semi-circle fit on a complex impedance plane with the Re (2) axis. A depression angle 8 (the angle between the Re (2) axis and a line drawn between the low intercept and the center of the semi-circle) was also determined from this fit. Water adsorption experiments were carried out using an AD-2 microbalance (Perkin-Elmer) equipped with temperature/atmosphere-controlled cell. Pellets prepared for the conductivity experiments were pretreated at 300°C in vacuum and then water was introduced into the cell. When a constant weight was attained, the pellet was brought into direct contact with liquid water and then weighed again. The last procedure was repeated three times and the average of these results was derived.
SBET Cm’ g - ‘1
V mlcropre (ml g-‘)
290 360 360 570 420 410 450 430
0.105 0.139 0.137 0.142 0.153 0.158 0.180 0.177
8 and 31, respectively.
water are shown in Figs. 1 and 2, respectively. At 100% RH (Fig. l), these measurements were repeated twice a day until the results stopped changing. Usually it took 4-8 days to equilibrate the sample. It is worth noting that equilibration was reached very slowly while the same pellets attained constant weight in saturated water vapour in l-2 h (Fig. 3). This suggeststhat in addition to the significant impact of the water content on the electrical properties, some rearrangement of the solid particles in the pellet when contacted with water also affects its ionic conductivity.
z
: !$&t~etd
= 12.4 CieQ)
-+-
2 days 5 days
= 12.4 = 12.3
meta meta
deQ)
deQ)
3. Results and discussion 3.1. Effect of water
Typical complex-plane impedance plots of zeolites conditioned at 2 1“C at 100% RH and in liquid
P (Ohm XUII) Fig. 1. Complex plane impedance of Na-ZSM-5 (Si/Al = 31) at 21”C, conditioned at 100% RH for various periods of time (19 values obtained by fitting).
S. D. Mikhailenko
et al. 1 Microporous
i -800 -
0
4ocl
800
p (Ohm
1200
sm)
Fig. 2. Complex plane impedance of Na-ZSM-S (Si/AI=31) 21”C, dipped in liquid water for various periods of time values obtained by fitting).
at (0
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I1 (1997)
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[2,3,6,8,9] that the conductivity increases with increasing water content. In the present work, an attempt was made to study how the conductivity was affected by a variation in the content of water absorbed by zeolites. With this aim in view, pellets were compressed at different pressures, which should provide them with different void ratios. Fig. 4 shows the decrease in water uptake at saturation as the compacting pressure is increased, which reflects this decreased macroporosity. Fig. 5 reports the results for the dependence of the conductivity on the compacting pressure botb at 100% RH (Fig. 5(a)) and in water (Fig. 5(b)). From a comparison of the results in Figs. 4 and 5, it may be concluded that (T is inversely proportional to the pellet void fraction, even when filled with water. This can be seen from Fig. 6, where the conductivity in the presence of liquid water is plotted against the pellet void fraction expressed as the water uptake. For all samples tested, when the pellet was immersed in water, it was observed that the complex impedance response (Fig. 2) changed with time. For all the solids reported in Table 1, the conductivity was found to decrease and reach a constant value over a period of l-2 h. To underA silbw=a v
.Si/Al=31 -7 SiiAI=218
+
Siiiilite
l
41,,
/ /,
A
- Si/Al=8
0
-Si/Al=31
1
, , , , / 1’1
0 2 4 6 8 10121416
c
1820
Time ( h ) Fig. 3. Water
adsorption
at 21°C and 100% RH for Na-ZSM-5.
5 Surprisingly high conductivity (about two orders of magnitude higher than at 100% RH) was observed when the pellet was immersed in water. It was reported in many previous papers
6 Pressure
7
8
9
(ton/cm2)
Fig. 4. Uptake of liquid water by pellets of Na-ZSM-5 silicalite compressed at various pressures at 21°C.
and
SD.
-4*o
Mikhailenko
et al. / Microporous
I A
-si/A,=~
V
-SilAI=218
1
I 0 -0
-Si/AI=31 - HZSM-5
0
-Silicalite
-
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11 (1997)
l.OOOe-3
37-44
41
4
Si/AI=31
V
Si/AI=218
i I
4
5
6 Pressure,
1 A
g
n
-3.0
4
-Si/AI=8 - HZSM-5
5
7
8
9
ton/cm*
V
-Si/AI=218
l
- Silicalite
/
Si/Al=31
6
7
8
9
Pressure, ton/cm* Fig. 5. Electrical conductivity of Na-ZSM-5, HZSM-5 calite compressed using various pressures, conditioned RH (a) and dipped in water (b) at 21°C.
and siliat 100%
stand this behaviour, as well as the inverse dependence of S on pellet porosity, the explanation proposed in Ref. [ 161 can be adopted. According to this work, increasing the porosity has a pronounced effect on the distribution of relaxation times. This brings about an increase in the depression angle 6’and the resistanceR due to weakening of the interparticle contacts. From Fig. 2, one can see that the depression angle in water increases
I
I
I
I
I
I
25 30 35 40 45 50 55 60 65 Water uptake, wt.% Fig. 6. Electrical conductivity of Na-ZSM-5 and silicalite immersed in water versus water uptake by pellets, compressed using pressures of 5.1, 6.8 and 8.6 ton cmm2.
during equilibration, while at 100% RH 19does not change when R decreases(Fig. 1). This meansthat in the zeolite pellet, water splits particles apart, thus decreasing the particle-particle contact area. This interpretation is further confirmed by the observation of an increased pellet thickness (by about 10%) upon water immersion. Exposing the sample to a water-saturated atmosphere (100% RH), however, does not bring about such an effect becausethe amount of water adsorbed is 3-5 times less (Figs. 3 and 4), and this thinner layer does not seemto affect the compactness of the pellet. For H-ZSM-5 the measured conductivity is much lower than that of the Na form of the zeolite with the same Si/Al ratio (Fig. 5). This is usually accounted for by the low mobility of protons due to their strong interaction with framework oxygens. A low conductivity of the proton form of zeolites compared to their cationic forms is generally observed for other zeolite structures, although these results are usually obtained at drier conditions [ 171. At the sametime, ~~uz~~-) is higher than that of silicalite. The mechanism of conductivity in silicalite is not fully understood. At 100% RH (Fig. 5(a)), CT is lower than that of free water. It would therefore be natural to assumethat the measured conductivity corresponds to the conductivity of the water
42
SD.
Mikhailenko
et al. / Microporous
occluded within the inert silicalite. In the presence of liquid water, however, 0 increases by one order of magnitude (Fig. 5(b)). It can then be assumed then that water dissociation increases due to the presence of surface silanols, which leads to the formation of a protonic space charge with increased charge-carrier concentration near the surface. This protonic conductivity is probably intrinsic to water-silicate systems in general, in contrast to the presumed Na+ conductivity of cationic aluminosilicates. A study of the temperature dependence of g was carried out in a temperature range limited to 60°C since at higher temperatures condensation of the water from the cell bottom onto the electrodes was observed. Measurements were performed in steps of 10°C with a 1 h dwell prior to each point. The temperature dependence of the ionic conductivity usually follows the Arrhenius equation (straight lines in log(oT) versus l/T). The calculated apparent activation energies E are presented in Table 2. Compared with previous results [7, lo], the values of El,,% RH are the same as found for hydrated zeolites [lo], and somewhat lower than values obtained at 30% RH [7]. In Ref. [lo] the observed trend was a decrease in activation energy as the degree of hydration was raised. In the presence of liquid water (Table 2), E decreases still further towards values comparable with the enthalpy of water self-diffusion (H=4.8 kJ mol-’
[171).
II
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nated as the liquid-like mechanism [ 181. Models proposed for compressed powders and sinters are not appropriate for water-solid systems, since such materials cannot be described as a discontinuous stacking of particles, but as a single, interstitial, continuous liquid medium. Conduction in composite water-solid samples probably occurs via the translative motion of charge carriers in a thin uniform water layer over the solid surface. The distribution of relaxation times should be much more narrow in this case, as was observed in our experiments. 3.2. EfSect of cation concentration From the results shown in Figs. 5(a) and 5(b), the conductivity of zeolites is strongly affected by their composition. This is not unexpected, since 0 must depend on the concentration of charge carriers. In the present case, these carriers may be assumed to be the mobile Na+ counter ions of the zeolite, the concentration (n) of which is inversely proportional to the Si/Al ratio. In Fig. 7 the measured conductivities (T,,~== and oloow RH are reported as functions of the number of Na+ ions per unit cell. We assume that the conductivity related to proton motion in partially protonated samples with Si/Al = 10 and 35 (Table 1) is negligibly small in comparison with 0 related to NaC. This follows in particular from the comparison of a764
The very low activation energy, as well low value of the suppression angle 0 (Figs. 2), suggests that in the presence of liquid the conduction behaviour is dictated by a phase mediated charge transport, usually Table 2 Apparent activation energies of ionic conductivity perature range from ambient to 60°C Zeolite
Na-ZSM-5 Na-ZSM-5 Na-ZSM-5 Silicalite
Materials
($/Al = 8) (Si/A1=31) (Si/Al=218)
2
as the 1 and water, liquiddesig-
over the tem-
E (kJ mol-‘) at lOO”/o RH
E (kJ mol-‘) in water
25.1 28.8 34.0 33.4
9.9 5.6 5.5 4.9
0
2
4
6
8
10
12
No. of Na’ per U.C. Fig. 7. Conductivity of Na-ZSM-5 Na+ ions per unit cell. Numbers sample numbers in Table 1.
at 21°C versus number of on top correspond to the
SD.
Mikhailenko
et al. I Microporous
c for the Na and H forms of the zeolites in Fig. 5, and was also observed, for example, in Refs. [ 2,111. From Fig. 7 one can see that the conductivities both in water and at 100% RH go through a maximum as n is increased. In the field of liquid aqueous electrolytes, the conductivity is expressed as 0 = kn,~ (p is the cation mobility, k is a constant) at low cation concentrations, but it goes through a maximum as the density of charge carriers is increased and interionic repulsion forces become significant. Typically this maximum is reached at values of n higher than 10m3 N. The results in Fig. 7 suggest that similar phenomena take place for Naf cations in zeolite pores. In Ref. [lo], the effect of decreasing electrical conductivity with an increasing cation concentration was also observed in NH, faujasites. The authors explained this behaviour by a diminishing charge-carrier mobility due to the decrease of the cation hydration number. This explanation seems reasonable, since their experiments were conducted at a constant number of water molecules per unit cell and for various cationic contents. In the present study the experimental conditions were different, the measurements were carried out in the presence of an excess of water ( 16-29 molecules per ion when dipped in liquid water), and the cation hydration number was not variable. Thus the behaviour observed in Ref. [lo] is probably a particular case of a more general dependence where the conductivity first increases then passes through a maximum before it decreases as the ion concentration is raised. This dependence appears as a common feature of the conductivity in both liquid and solid electrolytes. The location of the maximum on the curve of 0 versus n is determined by the zeolite framework structure and the nature and size of the charge carriers. In Ref. [lo] only the decreasing part of this curve is observed due to the high cationic concentration (number of cations per unit cell ranging from 50 to 90 compared those in the present work, n=O-10.7). The higher ionic diameter of NH: (2.8 A) compared to Naf (2.0 A) might also be a factor. In Ref. [l] the conductivity of dry Na faujasites was found to increase with the cation concen-
Materials
II (1997)
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43
tration, and thus the left side of the curve described above was observed. As the cation diameter was increased (K+ and Rb+) the 0 versus n plots showed a tendency to reach a maximum for lower n values. Therefore, the different behaviours found in Refs. [ 1, lo] for faujasites with similar cationic concentrations may suggest that in anhydrous conditions, repulsion forces play a lesser role. Another relevant observation was made in Ref. [9], where dealumination enhanced the protonic conductivity (measured at 200°C) by l-2 orders of magnitude. The author explained this change by an increase of proton mobility due to the appearance of silanol-bearing defects. Admitting that the conductance mechanism in the case of anhydrous HZSM-5 can strongly differ from that relevant to cationic transport in the presence of water, we can, however note that the decreased repulsion forces may also contribute to the increase in G reported in Ref. [9]. It is worth noting that the apparent activation energy &ater in Table 2 is discernibly higher for Na-ZSM-5 with Si/Al=8, the conductivity of which is partly suppressed by the excess of charge carriers. It is also possible that the presence of extraframework Al in zeolite channels can affect the charge transport, making it more difficult. Thus, the drastic increase in the conductivity of the zeolite with Si/Al=8 after HCl acid ion-exchange (samples 1 and 2 in Fig. 7) could be associated with the increase of S,,, and the micropore volume. However, the same treatment of the zeolite with Si/Al= 31 (samples 4 and 6 in Table 1) also increased the micropore volume, but resulted in a decrease in conductivity (Fig. 7). Hence the contribution of this factor is not predominant. Moreover, back-exchanging sample 2 with Na+ ions to yield sample 3 with no change in the micropore volume led to decreased conductivity. We have already noted that the conductivity increases with compacting pressure (Figs. 5(a), 5(b) and 6) due to a better interparticle contact, which promotes cationic transport over long distances. It is also worth noting that this is true only for the cationic forms of zeolites. As can be seen from Figs. 5 and 6, the conductivity of the watersilicalite composite does not depend on the pressure
44
S. D. Mikhailenko
et al. J Microporous
applied when preparing the pellets. Consequently, conductivity induced by water dissociation is affected to much a lesser extent by the solid structure than the conductivity attributed to the transport of sodium counterions.
4. Conclusions We have studied electrical conductivity variations of zeolite ZSM-5 in dependence on its chemical nature (content of Na cations, exchange of Na+ for Hf) at 100% RH and in liquid water. It has been found that in the latter case the conductivity is 1-2 orders of magnitude higher than in saturated water vapour. It is assumed that this is due to the formation of an uninterrupted adsorbed water phase which provides a pathway to charge carriers. This phase itself contributes to the protonic conductance due to water dissociation at the solid surface. Both at 100% RH and in water, the dependence of cr on cation content follows a curve with a maximum. A decrease in conductivity at high concentrations of charge carriers was ascribed to an increased repulsive interaction between ions in the conductive adsorbed layer on the channel walls in zeolites.
Materials
II
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References [I]
F.J. Jansen, R.A. Schoonheydt, J. Chem. Sot., Faraday Trans. 69 (1973) 1338. [2] M. Lal, C.M. Johnson, A.T. Howe, Solid State Ionics 5 (1981) 451. [3] K.D. Kreuer, W. Weppner, A. Rabenau, Mater. Res. Bull. 17 (1982) 501. [4] A.K. Aboul-Gheit, A.M. Summan, M.A. Ahmed, M.A. Mousa, Thermochim. Acta 158 (1990) 53. [5] J.C. Mougenel, H. Kessler, Zeolites 11 (1991) 81. [6] N. Knudsen, E.K. Andersen, I.G.K. Andersen, P. Norby, E. Skou, Solid State Ionics 38 (1990) 207. [7] R.C.T. Slade, H. Jinku, G.B. Hix, Solid State tonics 57 (1992) 177. [S] A.A. Higazy, M.E. Kassem, M.B. Sayed, J. Phys. Chem. Solids 53 (1992) 549. [9] M.B. Sayed, Zeolites 16 (1996) 157. lo] N.H. Mogensen, E. Skou, Solid State Ionics 77 (1995) 51. 111 M. Dekker, I. ‘t Zand, J. Schram, J. Schoonman, Solid State Ionics 35 (1989) 157. 121 J. Kjaer, S. Yde-Andersen, N.A. Knudsen, E. Skou, Solid State lonics 46 (1991) 169. 131 J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987. 141 Z. Gabelica, E.G. Derouane, N. Blom, Appl. Catal. 5 (1983) 227. [ 151 A. Mahay, G. Lemay, A. Adnot, I.M. Szoghy, S. Kaliaguine, J. Catal. 103 (1987) 480. [ 161 E.M. Skou, T. Jakobsen, Appl. Phys. 49 (1989) 117. [17] K.D. Kreuer, Chem. Mater. 8 (1996) 610. [IS] S. Chandra, N. Singh, A. Hashmi, Proc. Indian Nat. Sci. Acad. 52 (1986) 338.
Water-assisted ionic conductance of zeolites with ZSM-5 structure S.D. Mikhailenko
a,*, S. Kaliaguine
a, E. Ghali b
a Dkpartement de Gtkie Chimique, Universitc? Laval, Sainte Foy, Qutbec, Canada GIK 7P4 b Dkpartement de Mines et Metallurgic, Universitk Laval, Sainte Foy, Qu&ec, Canada GIK 7p4 Received 23 December
1996; accepted 10 March
1997
Abstract The electrical conductivity (0) of compressed powders of an isostructural series of zeolites with MFI topology’ was measured using AC impedance spectroscopy. Silicalite-1 and ZSM-5 with different Si/Al ratios containing varying numbers of Na+ and/or Hi were studied at 100% relative humidity and immersed in liquid water. In the latter case, B is l-2 orders of magnitude larger than that in saturated water vapour. The value of r~ for silicalite immersed in water was found to be higher than that of water before and after the test. It was assumed that this is due to enhanced water dissociation on the silicalite surface. Both in water and at 100% RH the dependence of G on cation content follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in the repulsive interaction between ions in the conductive water layer on the zeolite channel walls. 0 1997 Elsevier Science B.V. Keywords:
Impedance;
Ionic conduction;
Water adsorption;
1. Introduction The investigation of the electrical properties of zeolites has attracted considerable attention over the last two decades [l-12]. These studies are significant as they not only provide information on the solid structure [5] and data related to surface ionic mobility [lo], but they also open up the possibility of using molecular sieves as solid electrolytes in some electrochemical applications, such as hydrogen sensors [ 111 and separators in fuel cells [6,12] or in galvanic cells [3]. Among
the great number
of zeolites,
the electri-
* Corresponding author. ‘According to the nomenclature of the Structure of the International Zeolite Association. 0927-6513/97/$17.00 0 1997 Elsevier PII SO927-6513(97)00023-O
Commission
Science B.V. All rights reserved.
Zeolite
cal properties of which have been studied, the ZSM-5 family has received less attention, in contrast to the broad worldwide discussion of their structure and other physicochemical properties. This gap is only partly covered by two works [8,9], where the electrical properties of HZSM-5 were investigated over the temperature range from ambient to 400°C. In these investigations, impedance spectroscopy (IS) was used over a rather narrow frequency range (0.1-100 kHz), which is far from the values associated with the dielectric relaxation times of solids (w > 10’ Hz) [ 131. This evidently affects the accuracy of the results. Moreover, the measurements were carried out in a room atmosphere, and are consequently related to materials with various degreesof dehydration. It is well established [4,8-l 0] that the conductiv-
38
SD.
Mikhailenko
et al. / Microporous
ity of zeolites varies strongly depending on the degree of hydration. It varies in the range from cr=10m8 S cm-’ in dry conditions up to as much as 10-3-10-2 S cm-’ even at room temperature [6] for fully hydrated specimens. Sorbed water plays an important role in conduction, bridging particles of zeolite, and assisting the ion-hopping process. It influences interactions between cations and the negative framework, and provides additional charge carriers (H+, OH-). So far, the electrical properties of zeolites have been studied at a relative humidity (RH) varying from 0 to 100%. However, some applications involve a direct contact of the material with liquid water, such as in the direct methanol fuel cell (DMFC). Moreover, conductivity measurements at RH = O-100% can lack precision since the equilibration of a compressed porous pellet in water vapour is slow, sometimes taking up to several days. In this work we report the electrical properties of some molecular sieves with the MFI structure (ZSM-5) both at 100% RH and immersed in water. The impedance technique was employed to study samples of Na-ZSM-5 with various Si/Al ratios. For comparison, HZSM-5 and silicalite were also tested.
2. Experimental Three zeolites with different Al contents and silicalite- 1 were synthesized and characterized as described elsewhere [ 14,151. X-ray powder diffraction patterns (Philips diffractometer, Cu Kcr radiation) were quite sharp and intense, and were consistent with the literature [ 141, indicating well crystallized ZSM-5 phases. Elemental compositions were determined by atomic absorption spectroscopy ( Perkin-Elmer), and Si/Al = 8,3 1 and 218 were found for Na-ZSM-5 samples. The sodium form of the sample with Si/Al = 31 was ion-exchanged into the ammonium form and then decomposed into HZSM-5 by calcination at 450°C. We also used HCl acid ion exchange at room temperature for the preparation of two partially protonated samples from zeolites with Si/Al=S and 3 1. This treatment changed the Si/Al ratio of
Materials
11 (1997)
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the former sample to Si/Al= 10. The sodium form of this new sample was obtained by back-exchange with NaCl. The specific area and the volume of the micropores were measured using N, adsorption at 77 K and capillary condensation. The results of elemental analysis, S,,, and Vmicropore for all the samples are reported in Table 1. It is clear from the low values of the micropore volume observed at low Si/Al ratios that in this case, part of the zeolite pores are obstructed by extra-framework aluminium species. The specimens were washed with deionised water until the conductivity of the solution was not higher than 5 x 10m6 S cm-‘. This was usually achieved by washing ten times in 200 ml of water (precipitation by centrifuging) and two final washings of 24 h each. The zeolites were then dried and ground into a fine powder, and 0.1 g of each was compressed in a 13 mm die to give pellets ca. 0.5 mm thick. Two kinds of measurements were carried out in this work. In the first set of measurements, the pellet was coated with carbon black and placed in a RHand temperature-controlled parallel-plate electrode test cell where it was clamped between two stainless-steel electrodes with springs, providing a permanent pressure of about 1 kg cm-2. In some experiments silver paste was used instead of carbon black to cover the pellet surfaces as blocking electrodes, and this was found to give the same conductivity results. However, the time of equilibration was longer. RH in these measurements was maintained at 100% by enclosing water at the bottom of the cell. In the second set of tests, two electrodes with a pellet between them were placed in a thermostated glass cell where they were covered with 15 ml of demineralized water. The conductivity of this water was measured before and after zeolite tests, and was always found to be not higher than 5 x 10m6 S cm-‘. These measurements were performed as blank experiments using the same arrangement with a 0.5 mm Teflon spacer instead of the pellet between the electrodes. Electrical impedance measurements were taken over the frequency range l-lo7 Hz with an oscillating voltage of 100mV using an SI 1260 impedance/gain-phase analyzer (Solartron), con-
S.D. Mikhailenko Table 1 Zeolite characteristics: No.
composition
Si/Al
8 10” lob 31 31” 35b 218 Silicalite-1 “Prepared “Prepared “HZSM-5.
by ion exchange by ion exchange
expressed
et al. / Microporous
as atoms
Materials
II (1997)
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39
per unit cell
Si
Al
Na
85.3 87.3 87.3 93.0 93.0 93.3 95.6 96.0
10.7 8.7 8.7 3.0 3.0 2.1 0.4
10.7 4.5 8.7 3.0 0.1 1.7 0.4
with HCI from the Na precursors with NaCl from sample no. 2.
H
4.2
2.9 1.0
with Si/Al=
trolled by a PC through a GPIB interface. Acquisition and analysis of the impedance/ admittance spectra were performed employing the Z-60 and Z-view software, provided by Solartron. The impedance data were corrected for the contribution from the empty and short-circuited cell. The conductivity 0 was calculated from the resistance R by c= R x S/d, where S is the pellet face area and d its thickness. The resistance R was derived from the low intersect of a semi-circle fit on a complex impedance plane with the Re (2) axis. A depression angle 8 (the angle between the Re (2) axis and a line drawn between the low intercept and the center of the semi-circle) was also determined from this fit. Water adsorption experiments were carried out using an AD-2 microbalance (Perkin-Elmer) equipped with temperature/atmosphere-controlled cell. Pellets prepared for the conductivity experiments were pretreated at 300°C in vacuum and then water was introduced into the cell. When a constant weight was attained, the pellet was brought into direct contact with liquid water and then weighed again. The last procedure was repeated three times and the average of these results was derived.
SBET Cm’ g - ‘1
V mlcropre (ml g-‘)
290 360 360 570 420 410 450 430
0.105 0.139 0.137 0.142 0.153 0.158 0.180 0.177
8 and 31, respectively.
water are shown in Figs. 1 and 2, respectively. At 100% RH (Fig. l), these measurements were repeated twice a day until the results stopped changing. Usually it took 4-8 days to equilibrate the sample. It is worth noting that equilibration was reached very slowly while the same pellets attained constant weight in saturated water vapour in l-2 h (Fig. 3). This suggeststhat in addition to the significant impact of the water content on the electrical properties, some rearrangement of the solid particles in the pellet when contacted with water also affects its ionic conductivity.
z
: !$&t~etd
= 12.4 CieQ)
-+-
2 days 5 days
= 12.4 = 12.3
meta meta
deQ)
deQ)
3. Results and discussion 3.1. Effect of water
Typical complex-plane impedance plots of zeolites conditioned at 2 1“C at 100% RH and in liquid
P (Ohm XUII) Fig. 1. Complex plane impedance of Na-ZSM-5 (Si/Al = 31) at 21”C, conditioned at 100% RH for various periods of time (19 values obtained by fitting).
S. D. Mikhailenko
et al. 1 Microporous
i -800 -
0
4ocl
800
p (Ohm
1200
sm)
Fig. 2. Complex plane impedance of Na-ZSM-S (Si/AI=31) 21”C, dipped in liquid water for various periods of time values obtained by fitting).
at (0
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[2,3,6,8,9] that the conductivity increases with increasing water content. In the present work, an attempt was made to study how the conductivity was affected by a variation in the content of water absorbed by zeolites. With this aim in view, pellets were compressed at different pressures, which should provide them with different void ratios. Fig. 4 shows the decrease in water uptake at saturation as the compacting pressure is increased, which reflects this decreased macroporosity. Fig. 5 reports the results for the dependence of the conductivity on the compacting pressure botb at 100% RH (Fig. 5(a)) and in water (Fig. 5(b)). From a comparison of the results in Figs. 4 and 5, it may be concluded that (T is inversely proportional to the pellet void fraction, even when filled with water. This can be seen from Fig. 6, where the conductivity in the presence of liquid water is plotted against the pellet void fraction expressed as the water uptake. For all samples tested, when the pellet was immersed in water, it was observed that the complex impedance response (Fig. 2) changed with time. For all the solids reported in Table 1, the conductivity was found to decrease and reach a constant value over a period of l-2 h. To underA silbw=a v
.Si/Al=31 -7 SiiAI=218
+
Siiiilite
l
41,,
/ /,
A
- Si/Al=8
0
-Si/Al=31
1
, , , , / 1’1
0 2 4 6 8 10121416
c
1820
Time ( h ) Fig. 3. Water
adsorption
at 21°C and 100% RH for Na-ZSM-5.
5 Surprisingly high conductivity (about two orders of magnitude higher than at 100% RH) was observed when the pellet was immersed in water. It was reported in many previous papers
6 Pressure
7
8
9
(ton/cm2)
Fig. 4. Uptake of liquid water by pellets of Na-ZSM-5 silicalite compressed at various pressures at 21°C.
and
SD.
-4*o
Mikhailenko
et al. / Microporous
I A
-si/A,=~
V
-SilAI=218
1
I 0 -0
-Si/AI=31 - HZSM-5
0
-Silicalite
-
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37-44
41
4
Si/AI=31
V
Si/AI=218
i I
4
5
6 Pressure,
1 A
g
n
-3.0
4
-Si/AI=8 - HZSM-5
5
7
8
9
ton/cm*
V
-Si/AI=218
l
- Silicalite
/
Si/Al=31
6
7
8
9
Pressure, ton/cm* Fig. 5. Electrical conductivity of Na-ZSM-5, HZSM-5 calite compressed using various pressures, conditioned RH (a) and dipped in water (b) at 21°C.
and siliat 100%
stand this behaviour, as well as the inverse dependence of S on pellet porosity, the explanation proposed in Ref. [ 161 can be adopted. According to this work, increasing the porosity has a pronounced effect on the distribution of relaxation times. This brings about an increase in the depression angle 6’and the resistanceR due to weakening of the interparticle contacts. From Fig. 2, one can see that the depression angle in water increases
I
I
I
I
I
I
25 30 35 40 45 50 55 60 65 Water uptake, wt.% Fig. 6. Electrical conductivity of Na-ZSM-5 and silicalite immersed in water versus water uptake by pellets, compressed using pressures of 5.1, 6.8 and 8.6 ton cmm2.
during equilibration, while at 100% RH 19does not change when R decreases(Fig. 1). This meansthat in the zeolite pellet, water splits particles apart, thus decreasing the particle-particle contact area. This interpretation is further confirmed by the observation of an increased pellet thickness (by about 10%) upon water immersion. Exposing the sample to a water-saturated atmosphere (100% RH), however, does not bring about such an effect becausethe amount of water adsorbed is 3-5 times less (Figs. 3 and 4), and this thinner layer does not seemto affect the compactness of the pellet. For H-ZSM-5 the measured conductivity is much lower than that of the Na form of the zeolite with the same Si/Al ratio (Fig. 5). This is usually accounted for by the low mobility of protons due to their strong interaction with framework oxygens. A low conductivity of the proton form of zeolites compared to their cationic forms is generally observed for other zeolite structures, although these results are usually obtained at drier conditions [ 171. At the sametime, ~~uz~~-) is higher than that of silicalite. The mechanism of conductivity in silicalite is not fully understood. At 100% RH (Fig. 5(a)), CT is lower than that of free water. It would therefore be natural to assumethat the measured conductivity corresponds to the conductivity of the water
42
SD.
Mikhailenko
et al. / Microporous
occluded within the inert silicalite. In the presence of liquid water, however, 0 increases by one order of magnitude (Fig. 5(b)). It can then be assumed then that water dissociation increases due to the presence of surface silanols, which leads to the formation of a protonic space charge with increased charge-carrier concentration near the surface. This protonic conductivity is probably intrinsic to water-silicate systems in general, in contrast to the presumed Na+ conductivity of cationic aluminosilicates. A study of the temperature dependence of g was carried out in a temperature range limited to 60°C since at higher temperatures condensation of the water from the cell bottom onto the electrodes was observed. Measurements were performed in steps of 10°C with a 1 h dwell prior to each point. The temperature dependence of the ionic conductivity usually follows the Arrhenius equation (straight lines in log(oT) versus l/T). The calculated apparent activation energies E are presented in Table 2. Compared with previous results [7, lo], the values of El,,% RH are the same as found for hydrated zeolites [lo], and somewhat lower than values obtained at 30% RH [7]. In Ref. [lo] the observed trend was a decrease in activation energy as the degree of hydration was raised. In the presence of liquid water (Table 2), E decreases still further towards values comparable with the enthalpy of water self-diffusion (H=4.8 kJ mol-’
[171).
II
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nated as the liquid-like mechanism [ 181. Models proposed for compressed powders and sinters are not appropriate for water-solid systems, since such materials cannot be described as a discontinuous stacking of particles, but as a single, interstitial, continuous liquid medium. Conduction in composite water-solid samples probably occurs via the translative motion of charge carriers in a thin uniform water layer over the solid surface. The distribution of relaxation times should be much more narrow in this case, as was observed in our experiments. 3.2. EfSect of cation concentration From the results shown in Figs. 5(a) and 5(b), the conductivity of zeolites is strongly affected by their composition. This is not unexpected, since 0 must depend on the concentration of charge carriers. In the present case, these carriers may be assumed to be the mobile Na+ counter ions of the zeolite, the concentration (n) of which is inversely proportional to the Si/Al ratio. In Fig. 7 the measured conductivities (T,,~== and oloow RH are reported as functions of the number of Na+ ions per unit cell. We assume that the conductivity related to proton motion in partially protonated samples with Si/Al = 10 and 35 (Table 1) is negligibly small in comparison with 0 related to NaC. This follows in particular from the comparison of a764
The very low activation energy, as well low value of the suppression angle 0 (Figs. 2), suggests that in the presence of liquid the conduction behaviour is dictated by a phase mediated charge transport, usually Table 2 Apparent activation energies of ionic conductivity perature range from ambient to 60°C Zeolite
Na-ZSM-5 Na-ZSM-5 Na-ZSM-5 Silicalite
Materials
($/Al = 8) (Si/A1=31) (Si/Al=218)
2
as the 1 and water, liquiddesig-
over the tem-
E (kJ mol-‘) at lOO”/o RH
E (kJ mol-‘) in water
25.1 28.8 34.0 33.4
9.9 5.6 5.5 4.9
0
2
4
6
8
10
12
No. of Na’ per U.C. Fig. 7. Conductivity of Na-ZSM-5 Na+ ions per unit cell. Numbers sample numbers in Table 1.
at 21°C versus number of on top correspond to the
SD.
Mikhailenko
et al. I Microporous
c for the Na and H forms of the zeolites in Fig. 5, and was also observed, for example, in Refs. [ 2,111. From Fig. 7 one can see that the conductivities both in water and at 100% RH go through a maximum as n is increased. In the field of liquid aqueous electrolytes, the conductivity is expressed as 0 = kn,~ (p is the cation mobility, k is a constant) at low cation concentrations, but it goes through a maximum as the density of charge carriers is increased and interionic repulsion forces become significant. Typically this maximum is reached at values of n higher than 10m3 N. The results in Fig. 7 suggest that similar phenomena take place for Naf cations in zeolite pores. In Ref. [lo], the effect of decreasing electrical conductivity with an increasing cation concentration was also observed in NH, faujasites. The authors explained this behaviour by a diminishing charge-carrier mobility due to the decrease of the cation hydration number. This explanation seems reasonable, since their experiments were conducted at a constant number of water molecules per unit cell and for various cationic contents. In the present study the experimental conditions were different, the measurements were carried out in the presence of an excess of water ( 16-29 molecules per ion when dipped in liquid water), and the cation hydration number was not variable. Thus the behaviour observed in Ref. [lo] is probably a particular case of a more general dependence where the conductivity first increases then passes through a maximum before it decreases as the ion concentration is raised. This dependence appears as a common feature of the conductivity in both liquid and solid electrolytes. The location of the maximum on the curve of 0 versus n is determined by the zeolite framework structure and the nature and size of the charge carriers. In Ref. [lo] only the decreasing part of this curve is observed due to the high cationic concentration (number of cations per unit cell ranging from 50 to 90 compared those in the present work, n=O-10.7). The higher ionic diameter of NH: (2.8 A) compared to Naf (2.0 A) might also be a factor. In Ref. [l] the conductivity of dry Na faujasites was found to increase with the cation concen-
Materials
II (1997)
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43
tration, and thus the left side of the curve described above was observed. As the cation diameter was increased (K+ and Rb+) the 0 versus n plots showed a tendency to reach a maximum for lower n values. Therefore, the different behaviours found in Refs. [ 1, lo] for faujasites with similar cationic concentrations may suggest that in anhydrous conditions, repulsion forces play a lesser role. Another relevant observation was made in Ref. [9], where dealumination enhanced the protonic conductivity (measured at 200°C) by l-2 orders of magnitude. The author explained this change by an increase of proton mobility due to the appearance of silanol-bearing defects. Admitting that the conductance mechanism in the case of anhydrous HZSM-5 can strongly differ from that relevant to cationic transport in the presence of water, we can, however note that the decreased repulsion forces may also contribute to the increase in G reported in Ref. [9]. It is worth noting that the apparent activation energy &ater in Table 2 is discernibly higher for Na-ZSM-5 with Si/Al=8, the conductivity of which is partly suppressed by the excess of charge carriers. It is also possible that the presence of extraframework Al in zeolite channels can affect the charge transport, making it more difficult. Thus, the drastic increase in the conductivity of the zeolite with Si/Al=8 after HCl acid ion-exchange (samples 1 and 2 in Fig. 7) could be associated with the increase of S,,, and the micropore volume. However, the same treatment of the zeolite with Si/Al= 31 (samples 4 and 6 in Table 1) also increased the micropore volume, but resulted in a decrease in conductivity (Fig. 7). Hence the contribution of this factor is not predominant. Moreover, back-exchanging sample 2 with Na+ ions to yield sample 3 with no change in the micropore volume led to decreased conductivity. We have already noted that the conductivity increases with compacting pressure (Figs. 5(a), 5(b) and 6) due to a better interparticle contact, which promotes cationic transport over long distances. It is also worth noting that this is true only for the cationic forms of zeolites. As can be seen from Figs. 5 and 6, the conductivity of the watersilicalite composite does not depend on the pressure
44
S. D. Mikhailenko
et al. J Microporous
applied when preparing the pellets. Consequently, conductivity induced by water dissociation is affected to much a lesser extent by the solid structure than the conductivity attributed to the transport of sodium counterions.
4. Conclusions We have studied electrical conductivity variations of zeolite ZSM-5 in dependence on its chemical nature (content of Na cations, exchange of Na+ for Hf) at 100% RH and in liquid water. It has been found that in the latter case the conductivity is 1-2 orders of magnitude higher than in saturated water vapour. It is assumed that this is due to the formation of an uninterrupted adsorbed water phase which provides a pathway to charge carriers. This phase itself contributes to the protonic conductance due to water dissociation at the solid surface. Both at 100% RH and in water, the dependence of cr on cation content follows a curve with a maximum. A decrease in conductivity at high concentrations of charge carriers was ascribed to an increased repulsive interaction between ions in the conductive adsorbed layer on the channel walls in zeolites.
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References [I]
F.J. Jansen, R.A. Schoonheydt, J. Chem. Sot., Faraday Trans. 69 (1973) 1338. [2] M. Lal, C.M. Johnson, A.T. Howe, Solid State Ionics 5 (1981) 451. [3] K.D. Kreuer, W. Weppner, A. Rabenau, Mater. Res. Bull. 17 (1982) 501. [4] A.K. Aboul-Gheit, A.M. Summan, M.A. Ahmed, M.A. Mousa, Thermochim. Acta 158 (1990) 53. [5] J.C. Mougenel, H. Kessler, Zeolites 11 (1991) 81. [6] N. Knudsen, E.K. Andersen, I.G.K. Andersen, P. Norby, E. Skou, Solid State Ionics 38 (1990) 207. [7] R.C.T. Slade, H. Jinku, G.B. Hix, Solid State tonics 57 (1992) 177. [S] A.A. Higazy, M.E. Kassem, M.B. Sayed, J. Phys. Chem. Solids 53 (1992) 549. [9] M.B. Sayed, Zeolites 16 (1996) 157. lo] N.H. Mogensen, E. Skou, Solid State Ionics 77 (1995) 51. 111 M. Dekker, I. ‘t Zand, J. Schram, J. Schoonman, Solid State Ionics 35 (1989) 157. 121 J. Kjaer, S. Yde-Andersen, N.A. Knudsen, E. Skou, Solid State lonics 46 (1991) 169. 131 J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987. 141 Z. Gabelica, E.G. Derouane, N. Blom, Appl. Catal. 5 (1983) 227. [ 151 A. Mahay, G. Lemay, A. Adnot, I.M. Szoghy, S. Kaliaguine, J. Catal. 103 (1987) 480. [ 161 E.M. Skou, T. Jakobsen, Appl. Phys. 49 (1989) 117. [17] K.D. Kreuer, Chem. Mater. 8 (1996) 610. [IS] S. Chandra, N. Singh, A. Hashmi, Proc. Indian Nat. Sci. Acad. 52 (1986) 338.