Ionic conductivity of single-crystal ferrierite

Ionic conductivity of single-crystal ferrierite

Microporous and Mesoporous Materials 40 (2000) 283±288 www.elsevier.nl/locate/micromeso Ionic conductivity of single-crystal ferrierite Naohide Yama...

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Microporous and Mesoporous Materials 40 (2000) 283±288

www.elsevier.nl/locate/micromeso

Ionic conductivity of single-crystal ferrierite Naohide Yamamoto, Tatsuya Okubo * Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 1 January 2000; received in revised form 6 June 2000; accepted 20 June 2000

Abstract Ionic conduction in a single-crystal zeolite is reported for the ®rst time. Sub-millimeter sized ferrierite is grown by an organothermal method, and the conduction of sodium cation is measured along [0 0 1] and [0 1 0] separately by a.c. impedance analysis. The main conduction along [0 0 1] is through ten-member ring channels, while that along [0 1 0] is through eight-member ring channels. The measurement at 673±873 K reveals that the conductivity along [0 1 0] is greater than along [0 0 1]. The activation energy along [0 1 0] (1.2 eV) was greater than that (0.84 eV) along [0 0 1]. These di€erences are discussed in view of the ferrierite framework structure. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Ionic conduction; Zeolite; Ferrierite; Single crystal; Sodium ion

1. Introducion Solid ionic conductors have been utilized increasingly in broad applications such as batteries, fuel cells, sensors, electrochromic displays, catalysts, oxygen pumps, and so on. Zeolites, constructed from TO4 tetrahedra (T ˆ tetrahedral species; Si or Al), are a class of microporous materials occluding molecular-sized spaces within, and have widely been used as ion exchangers as well as catalysts and adsorbents. More than 120 framework topologies have been recognized regarding zeolites and related microporous crystals

* Corresponding author. Tel.: +81-3-5841-7348; fax: +81-35800-3806. E-mail address: [email protected] (T. Okubo).

[1]. The pore structure spreads from one to three dimensions. Zeolites contain exchangeable nonframework cations to compensate for the negative charge of the framework caused by the di€erence in the valence between Si(IV) and Al(III). Therefore, zeolites can potentially be a candidate for novel ion-conducting materials, since the cation is mobile in the framework structure, which has been materialized as ion exchangers in an aqueous phase. So far, the ionic conduction in zeolites has been studied to understand the structure±property relationship during the last three decades [2±16]. In these works, however, pelletized powders were characterized as specimens, since zeoliltes have exclusively been used in powdery form as ion exchangers, catalysts and adsorbents. The studies on single crystals and thin ®lms have been, therefore, rather limited. Many zeolites have anisotropic structures, and the conductivity should depend on the crystal axis. Accordingly, the observed

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conductivity for the pellets is, more or less, an averaged value of the intrinsic ones for di€erent axes. In order to understand the structure±property relationship in detail, a measurement for a single crystal is the most straightforward approach, where the contribution of the grain boundary can be removed at the same time. In these years, several novel routes to obtain submillimeter-to-millimeter-sized single crystals were reported [17±22]. In this paper, we report, to the best of our knowledge, the ®rst measurement of the ionic conductivity in a zeolite single crystal. We obtained sub-millimeter-sized ferrierite single crystal by an organothermal technique [17±20], and the ionic conductivity of sodium was determined by a.c. impedance analysis.

2. Experimental Ferrierite belongs to the orthorhombic crystal system. Projections along [0 0 1] and [0 1 0] are shown in Fig. 1. Along [0 0 1] and [0 1 0], 10member rings (MRs) and 8-MRs are viewed, respectively, together with 5-MRs and 6-MRs. Along [1 0 0], only 5-MRs and 6-MRs are viewed, where the ionic conduction is rather hindered. The molar composition of the starting gel for the single crystal growth was 0.25Al2 O3 :2SiO2 : 2(Et 3 N  3HF):6H 2 O:0.5TPA-Br:16Py:2PrNH 2 , where Et3 N, TPA-Br, Py, and PrNH2 are triethylamine, tetrapropylammonium bromide, pyridine,

and propylamine, respectively. The reagent-grade organic chemicals were used without further puri®cation. First, pyridine (Wako) and propylamine (Wako) were mixed in a 90 ml Te¯onâ bottle, and a ¯uoride source, Et3 N  3HF (Aldrich), was added dropwise under stirring. Then, a spoonful, about 0.1 g, of silica (Carb-O-Sil M-5, Cabot) was added per every 30 s so as not to make the solution mixture too viscous. After all the silica was introduced, alumina (CATAPAL B, Vista) was added, and the mixture was kept stirring for >30 min, which was followed by the addition of tetrapropylammonium bromide (Wako). The gel was charged in a 21 ml Te¯onâ -lined stainless steel autoclave (Parrâ , acid digestion bomb #4749), which was placed stationary in an oven preheated at 453 K. In eight days, the autoclave was removed from the oven, and quenched with cold water. The product was recovered by ®ltration, and washed with distilled water and an organic solvent (acetone or ethanol). In order to remove the amorphous phase, an aqueous solution of NaOH (3%) was used. Finally, the product was treated under ultrasonic irradiation in NaOH solution, and then washed with distilled water again. As-synthesized ferrierite crystals occluded organic structure-directing agents (SDAs) in the structure. In order to remove the SDAs without damaging the framework, the crystals were calcined in the following steps. First, the crystals were heated to 973 K in 9 h under an inert gas (N2 or He) ¯ow, and held isothermal at 973 K for 10 h. Then, the temperature was raised from 973 to 1173

Fig. 1. Ferrierite crystal structure: (a) 10-MR channels viewed along [0 0 1]; and (b) 8-MR channels viewed along [0 1 0].

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K in 4 h under an air ¯ow, and held isothermal at 1173 K for 24 h. After calcination, non-framework cations were exchanged with an objective ion. A post-synthesis exchange of the cation is one of the unique features of zeolite among solid ionic conductors. In this study, sodium was selected as the conducting carrier since higher conductivity has been reported among alkali metals in zeolites [2]. First, samples calcined were converted to the NH‡ 4 form by immersing in an aqueous solution of ammonium nitrate (1 M) over night at room temperature. This procedure was repeated twice. The crystals were recovered by ®ltration, and then immersed in an aqueous solution of sodium nitrate (0.5 M) over night at 358 K to obtain the Na‡ form, which was repeated twice. The ion-exchanged crystals were ®nally recovered by ®ltration. The obtained crystals were characterized by powder X-ray diffractometry (PXRD, MAC Science MXP3TA), scanning electron microscopy (SEM, Hitachi S-2400), thermal analysis (TG-DTA, Rigaku TG8120), inductively coupled plasma mass spectroscopy (ICP-MS, Hewlett Packard HP 4500), and 29 Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR, JEOL JNM-EX270). A.c. impedance measurements were carried out using an impedance analyzer (Hioki Z Hi-tester 3531, or Hewlett Packard LF impedance analyzer HP4192) in the frequency range of 50 Hz to 5 MHz. The single crystal was placed on a quartz glass plate, and gold wires were connected on both sides of (0 0 1) or (0 1 0) plane with a silver paste (Nilaco, AG-40100). At the same time, both ends of the single crystal were ®xed on the plate. All the procedures were carried out under an optical microscope. The gold wires were lead to the impedance analyzer. The resistance of the specimen was determined from ArgandÕs plot, and the ionic conductivity was calculated using the sizes of the specimen. Ionic conductivity in zeolites was strongly in¯uenced by the amount of adsorbed water [23,24]. In order to avoid this e€ect, we pretreated zeolite samples at 873 K under an inert gas ¯ow for P6 h. The ionic conductivity measurement was performed under an inert gas ¯ow without exposure to the atmosphere.

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3. Results and discussion PXRD patterns of the products were well identi®ed with the standard one due to ferrierite [25]. After calcination, the ferrierite structure was kept unchanged without collapse. SDAs were removed by calcination, since no weight loss nor exothermic peak was detected by thermal analysis after the removal. An SEM picture of a typical ferrierite crystal is shown in Fig. 2. Most of the crystals had similar size (approximately 300 lm  100 lm  10 lm), and showed platelike rectangle shape. 10-MR channels run along [0 0 1], and 8-MR channels run along [0 1 0] [19,20] as shown in Figs. 1 and 2. The Si/Al ratio of the crystal determined from 29 Si-MAS-NMR spectrum was >60, which suggested limited conductivity. The Si/Al and the Na/Al ratios determined from ICP-MS measurement were 9 and 0.6, respectively. These results revealed that sodium existed in the crystal, and some of aluminum were located at non-framework position. Further characterization is underway. A.c. impedance measurement was performed along [0 0 1] and [0 1 0]. Along [1 0 0] no measurement was performed since only 6-MRs and 5-MRs are viewed, and the conductivity must be much lower. After the measurement, the crystal was removed, and the same procedure was repeated to evaluate the contribution of the quartz glass itself.

Fig. 2. An SEM picture of a typical ferrierite single crystal.

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Accordingly, we could determine the intrinsic ionic conductivity of ferrierite along each axis separately excluding a grain boundary e€ect. Assuming the parallel connection paths, the resistance value due to ferrierite was determined using the following equation: 1=Robs ˆ 1=RF ‡ 1=RQ

…1†

where Robs , RF , and RQ denote observed resistance due to a crystal ®xed on a quartz glass plate, intrinsic resistance of ferrierite, and resistance of quartz glass plate itself, respectively. In order to convert the resistance to the conductivity, the length and the cross-sectional area along each axis were determined from the SEM image. The ionic conduction through the zeolite framework is regarded as an activated process, and the conductivity, r, is expressed in the following form: Fig. 3. ArgandÕs plot obtained for the conduction measurement along [0 0 1] at 823 K. The results obtained in the course of the temperature decrease are shown as ``'', and those in the temperature increase are shown as ``´''.

A typical ArgandÕs plot for the a.c. impedance measurement along [0 0 1] at 823 K is illustrated in Fig. 3. The results obtained in the course of the temperature decrease are shown as ``'', and those in the temperature increase are shown as ``´''. Both agreed well with each other. The data obtained show a single semi-circle without another one at the high-resistance region (low-frequency region) which was mostly observed in pellet samples caused by a grain boundary e€ect [12]. At the higher-resistance region, however, the increase in the reactance is observed, which is caused by the interfacial e€ect between the specimen and the electrode. In this study, the resistance values were determined from the intersecting points of the semi-circle and the resistance axis. The ferrierite single crystals contained limited ion-exchange sites since the Si/Al ratio was relatively large. In addition, the prepared crystals had limited crosssectional area. Therefore, the resistance was anticipated to be high. Certainly, we could obtain meaningful information only at higher temperature, P673 K. The resistance obtained, however, was about 107 ±108 X at 673±873 K, which was 0.12±0.2 times of quartz glass plateÕs resistance.

rT ˆ A exp…ÿEa =kB T †

…2†

where T, Ea , A, and kB denote temperature, activation energy of ionic conduction, prefactor and BoltzmannÕs constant, respectively. ArrheniusÕ plots of the rT along [0 0 1] and [0 1 0] are shown in Fig. 4. The activation energy along [0 0 1] was 0.84 eV, while that along [0 1 0] was 1.2 eV. The re-

Fig. 4. ArrheniusÕ plots of rT for sodium along [0 0 1] and [0 1 0] of ferrierite single crystals.

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ported activation energy of sodium in several zeolite pellets was 0.7±1.0 eV [21] although there is no report on ferrierite. The activation energy along [0 1 0], that is, through 8-MRs, was slightly larger than the reported values. In the measurement of the pellets, the observed conductivity was the average value, and the contribution of faster transport was predominant when there existed parallel paths of faster and slower lanes. In other words, the phenomenon relating to the slower path was, thus, left behind in the case of pellet samples. Such an observation as shown here can be attained by using single crystals only. The higher activation energy through 8-MR channels might be due to the steric hindering e€ect due to the smaller ring [2], but the detailed mechanism is still under consideration. In the range of 673±873 K, the conductivity along [0 1 0] is larger than that along [0 0 1]. If the line is extrapolated, the values around room temperature are of the same order. The magnitude of the conductivity depends not only on the activation energy but also prefactor A in Eq. (2), which includes the several structural factors. The number density of 8-MRs per unit area on (0 1 0) planes is larger than 10-MRs density on (0 0 1) planes as shown in Fig. 1, which means that there is a di€erence in the e€ective path number. For further understanding, these structural factors should be included in the next step.

4. Conclusion Ferrierite single-crystals were synthesized, and a.c. impedance measurements were performed for di€erent axes of the single crystal. Ionic conductivities along [0 0 1] and [0 1 0] were determined separately. The activation energy (1.2 eV) along [0 1 0], through 8-MR channels, was greater than that (0.84 eV) along [0 0 1], through 10-MR channels. Acknowledgements The authors are grateful to Profs. T. Kudo, H. Takagi, S.-I. Nakao, T. Yamaguchi and Dr. M. Hibino (UT) for their help in the a.c. impedance

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measurement. We also wish to thank Dr. J. Plevert (UT) and Dr. H. Koller (Univ. M unster) for MAS-NMR measurement, and Mr. Y. Kawazu (UT) for ICP-MS measurement. This work was ®nancially supported by ``Kawasaki Steel 21st Century Foundation'', ``The Iwatani Naoji Foundation'', and ``The Toyota High-Tech Research Grant Program''.

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