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Preparation of chabazite membranes by secondary growth using zeolite-T-directed chabazite seeds
Microporous and Mesoporous Materials 179 (2013) 128–135
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Preparation of chabazite membranes by secondary growth using zeolite-T-directed chabazite seeds Rongfei Zhou a, Yuqin Li a, Bo Liu a, Na Hu a, Xiangshu Chen a,⇑, Hidetoshi Kita b,⇑ a b
Jiangxi Inorganic Membrane Materials Engineering Research Centre, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
a r t i c l e
i n f o
Article history: Received 30 January 2013 Received in revised form 13 May 2013 Accepted 6 June 2013 Available online 15 June 2013 Keywords: Chabazite membrane Induction Pervaporation Macroporous support
a b s t r a c t Chabazite crystals were directly prepared from template-free aluminosilicate gel with the addition of nano-sized zeolite T (100 nm). This new route requires only one thirtieth seeds amount for a certain product, and one twentieth synthesis time compared with the classic one by the conversion of zeolite Y crystals. A pure and dense chabazite membrane was prepared on macroporous supports by secondary growth. The membrane synthesis parameters such as seeds, supports and synthesis time on membrane formation and pervaporation performance were also studied. Eight chabazite membranes prepared on macroporous stainless-steel supports showed a high average flux of 3.4 kg m 2 h 1 and high average selectivity of 2100 with low deviations on the flux and selectivity for a water/ethanol (10/90 wt%) mixture at 75 °C, indicating membrane synthesis using stainless-steel supports had a good reproducibility. The as-synthesized chabazite membranes also showed good hydrothermal stability under pervaporation test in a water-rich ethanol aqueous solution. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Zeolite membranes are considered to be the most promising candidates for pervaporation (PV) [1–7] and gas separation [8–10] because of the unique characteristics of zeolite crystals such as well-defined pore structures, selective adsorption, and high thermal stability. Zeolite NaA membranes with the excellent PV performance have been successfully applied for dehydration of some organic solvents on a large scale [1]. However, zeolite NaA membranes are unstable in the solutions containing even a trace of acid or high content of water by strong dealumination since its framework has the lowest Si/Al ratio of 1 [11]. Chabazite (CHA framework) owns similar pore properties (3-dimeteral and interconnected small pores and relative low framework identity), but an increased crystal Si/Al ratio (2–5) compared with zeolite NaA (LTA framework). The 8-ring channels have a pore size of 0.38 nm which is smaller than most alcohols but larger than water. The pore properties together with the medium crystal Si/Al ratio allow chabazite membrane becoming a candidate of high-performance dehydration membrane. The membrane has been applied in water-rich and acidic surroundings where NaA membrane is not stable [2,3]. There were only a few literatures involved in chabazite membrane preparation [2–4]. Hasegawa et al. ⇑ Corresponding authors. Tel.: +86 791 8120 533; fax: +86 791 8120 843 (X. Chen). E-mail addresses: [email protected] (X. Chen), [email protected] (H. Kita). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.06.003
[2,3] prepared chabazite membranes in a synthesis gel containing strontium cation, which worked as a pseudo-template for crystal growth of the CHA-type zeolite. The best membranes always contained a few MER-type crystals [2,3]. Li et al. [4] reported that chabazite membrane was synthesized in the presence only potassium cation. The seeds for secondary growth of these membranes [2–4] were all prepared by the conversion of Y-type crystal. It is the standard way for the preparation of chabazite crystals [12]. In our present study, chabazite seeds were directly prepared from aluminosilicate gel with the addition of a little amount of nanosized zeolite T. The yield and crystallization rate of chabazite were greatly improved. High-silica CHA-type (SSZ-13) seeds and membranes were prepared in the gel precursor containing expensive organic structure-directing agents (OSDAs) of N,N,N-trimethyl-1-adamantammonium cation (TMAda+) [12–14]. A SSZ-13 membrane with Si/ Al ratio of 13.4 showed high stability in 69.5 wt% nitric acid aqueous solution but a poor water selectivity over nitric acid [13]. An improved water selectivity of 1600 over isopropyl alcohol was obtained through a CHA-type zeolite membrane with Si/Al ratio of 8 by Sato et al. [14]. The usage of OSDA increased the cost of membrane preparation and the uncertainty during the additional calcination process. The crystallization and separation performance of supported zeolite layers were affected by the properties of porous supports including the composition, pore size and porosity and so on. Hasegawa et al. [3] reported that a-alumina support with high porosity
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of 50–68% and small pore sizes of 0.14–0.31 lm will resulted in a high-performance chabazite membrane, while the membrane prepared on macroporous supports (2.5 lm a-alumina and 1.7 lm mullite supports) had both low flux and poor water selectivity over ethanol. CO2 permanence of SAPO-34 membrane was also improved by a factor of 4 when asymmetrical a-alumina nanofilters (0.1–0.2 lm) support [15] instead of symmetrical stainless steel (0.3 lm) support [16] for CO2/CH4 separation. On the other hand, seed morphology for membrane secondary growth is another important factor on membrane growth [16,17]. In our present study, a new route was developed for the preparation of chabazite seeds by interzeolite conversion of zeolite T, in which nano-sized zeolite T (100 nm) induced the template-free amorphous gel to form the new chabazite phase. The pure (MERphase-free) chabazite membranes with high pervaporation performance were successfully obtained upon inexpensive symmetrical macroporous supports when the zeolite-T-directed chabazite seeds were used for seeding. 2. Experimental 2.1. Chabazite seed preparation All chemicals were purchased from Sigma–Aldrich Company for both seeds and membrane preparation except for precipitated silica that was purchased from Degussa Company. In a typical synthesis, 6.2 g aluminum hydroxide was dissolved in a base aqueous solution containing 18.8 g sodium hydroxide, 10.3 g potassium hydroxide and 217 g DI water while heating. 1 g nano-sized seeds were added in the boiling base solution. Nano-sized zeolite T was prepared following our previous work [18]. The nano-sized zeolite T showed a spherical morphology with approximately 100 nm diameters (Supplementary Fig. S1). After 0.5-h stirring, the hot cloudy solution was cooled down to room temperature. Zeolite T crystals should be chipped by strong dissolution of the boiling base solution since the colloidal seeded solution was hardly separated by increased rotation speed from 10,000 run per minute (RPM) to the limited 15,000 RPM. 47.4 g precipitated silica was then added in the solution slowly. A milk-like gel (Gel 1) was obtained after gel aging overnight and had a mole ratio of SiO2:Al2O3:Na2O:K2O:H2O = 1:0.05:0.285:0.095:16. The aged gel was placed in a Teflonlined autoclave. The hydrothermal synthesis was carried out at 150 °C for 8 h. The product was washed by DI water, separated and dried. For comparison, chabazite seeds were also prepared by the reported interzeolite conversion of zeolite HY, in which all silica and alumina sources came from crystalline zeolite HY powder [2,3,14]. The base solution (Gel 2) of 48 ml 1 mol l 1 potassium hydroxide solution (K2O/H2O = 0.009) was mixed with 4 g zeolite HY powder in a 250 ml polypropylene bottle. The crystallization was carried out at 90 °C for 7 d. The resulted crystals were separated from base solution and dried at 100 °C overnight. Chabazite crystals were also attempted to prepare by interzeolite conversion of zeolite HY using the aluminosilicate gel (Gel 1), and of nanosized zeolite T using base solution (Gel 2). To evaluate the induction efficiency of seeds, Seed Efficiency Index (SEI) was defined as the weight ratio of added seeds with respect to the obtained chabazite. 2.2. Chabazite membrane preparation The outer surface of porous 10-cm-long mullite and stainless steel supports were rub-coated with water slurry of chabazite seeds. The stainless steel tubes from Pall Company had the internal diameter of 10 mm, the outer diameter of 12 mm, the porosity of
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48% and the average pore diameter of 1.8 lm. And the mullite support from Nikkato Corporation had the internal diameter of 9 mm, the outer diameter of 12 mm, the porosity of 43% and the average pore diameter of 1.3 lm. Two kinds of seeds (S1 and S3) were seeded on the porous support using the same rub-coating procedure. Seed slurry was prepared by mixing 5 g seed powders and 20 g water and sonicated for 30 min. All the outside surface of the tubes was rubbed back and forth by hand for a total time of approximately 1 min. The seeded tubes were dried at 80 °C for 1 h. The excessive seeds on the surface were then removed by a cotton swab slowly. A modified gel preparation from the Ref. [2] was carried out by adjusting gel SiO2/Al2O3 ratio and alumina source. The gel was prepared by mixing aluminum isopropoxide, potassium hydroxide, colloidal silica, strontium nitrate and deionized water. The resulted precursor gel had a molar ratio of SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65. After 6 h aging, 270 g gel was placed in an autoclave and a seeded support was vertically immersed into the gel. The hydrothermal synthesis was carried out at 150 °C for a given time. After synthesis, the autoclave was cooled down to room temperature and the as-synthesized membranes were washed using running taping water for 15 min, and soaked into DI water for several batches until the solution become neutral, and dried at 80 °C overnight. 2.3. Characterization and PV test The crystalline phases for the powdery and membranous crystals were characterized by X-ray diffraction (XRD, Ultima IV) with a Cu-Ka radiation source using a Rigaku Ultima IV machine. The morphology of the seeds and membrane were observed using scanning electron microscopy (SEM) (Tescan Vega3). For the SEM samples’ preparation of stainless steel supported membranes, the membrane was soaked in the liquid-nitrogen bath in order to embrittle the supports before test. Element analyst was carried out by energy dispersive X-ray analysis (EDX) using a Hitachi S-3400N machine. Nitrogen adsorption and desorption isomers were tested on a Micrometer ASAP 2200 machine. The as-synthesized chabazite crystals and the Ca2+-ion-exchanged samples were used for the adsorption–desorption test. The ion-exchanged process was followed by Goto et al. [19]. Ion exchange was carried out using 3 times of the calcined as-synthesized chabazite with 1 M CaCl2 aqueous solution at 60 °C for 2 h, and the solids were washed to remove the chloride anions. The 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the powders were recorded at 104.2 MHz and 79.5 MHz, respectively, using a zirconia rotor of 4 mm diameter, on a Bruker Advance III 400 WB spectrometer. The rotor was spun with dry air at 4 kHz for 29Si MAS NMR and at 15 kHz for 27Al MAS NMR. The spectra were accumulated with 3 ls pulses, 2 s recycle delay and 1024 scans for 27Al MAS NMR, and 4.5 ls pulses, 40 s recycle delay and 1024 scans for 29Si MAS NMR. Al(NO3)39H2O and Si(CH3)4 (TMS) were used as chemical shift reference for 27Al MAS NMR and 29Si MAS NMR, respectively. PV performance test for a water/ethanol (10/90 wt%) mixture was carried out at 75 °C using a PV experimental apparatus described elsewhere [20]. The effective membrane area was approximately 27 cm2 and the permeation side was kept under vacuum of 20 Pa. The membrane separation performance is evaluated by the permeation flux (kg m 2 h 1) and the separation factor (a). The permeation flux is calculated by the mass of the permeation, which is collected by a liquid-nitrogen trap. Composition analysis of the feed and permeation were performed using a gas chromatograph (Shimadzu GC-14C) equipped with a TCD detector. The separation factor is defined as a = (YA/YB)/(XA/XB), where XA, XB, YA, and YB denote the mass fractions of components A (water) and B (alcohols) in the feed and permeation sides, respectively.
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3. Results and discussion 3.1. Synthesis of chabazite seeds Fig. 1 shows the morphologies of the as-synthesized chabazite crystals. The crystals S1 prepared by interzeolite conversion of zeolite T (Fig. 1a) displayed a walnut shape which was a typical chabazite morphology reported in the literatures [2,3]. The 2– 4 lm walnut-like crystals were composited with hundreds of sub-micrometer crystals with good self-assembly. The crystals S3 prepared by interzeolite conversion of zeolite HY (Fig. 1b) had a small ellipsoidal shape with an average size of 0.5 lm. Fig. 2 shows XRD patterns of the as-synthesized chabazite crystals. Both XRD patterns in Fig. 2b and c for the products S1 and S3 were consistent with the CHA-type structure [2,14] and the simulated one in Fig. 2a. The low baseline and the absence of non-zeolite-CHA peaks indicate that the products were free from impurities. Crystals S1 had the broad and low peaks with similar peak width of the nano-sized zeolite T (100 nm) (Fig. 2d) due to size effects. Those sub-micrometer polycrystalline crystals observed in Fig. 1a could be responsible for its broad XRD peaks in Fig. 2b. 27 Al and 29Si MAS NMR spectra were used to study the chemical state of aluminate and silicate in zeolite-T-directed chabazite. Only one peak at approximate 59 ppm was found in 27Al MAS NMR
Fig. 1. SEM images of chabazite seeds (a) S1 and (b) S3.
Fig. 2. XRD patterns for (a) simulated CHA framework, (b) seeds S1 prepared by conversion of zeolite T, (c) seeds S3 prepared by conversion of zeolite HY, (d) nanosized zeolite T and (e) zeolite HY.
spectrum in Fig. 3a, corresponding to tetrahedrally-coordinated aluminum in framework. There was no extraframework aluminum since no peak was observed at 0 ppm. The 29Si MAS NMR spectrum of as-synthesized chabazite crystals S1 in Fig. 3b showed four peaks at 109, 104, 98 and 93 ppm. These peaks are assigned to Si(0Al), Si(1Al), Si(2Al) and Si(3Al) species, respectively [19,21]. An additional peak at 88 ppm assigning to Si(4Al) was observed for a LEV-transformed chabazite [19], but was absent in our crystals. Nitrogen adsorption and desorption isomers for Ca2+-ion-exchanged crystals S1 and S3 and as-synthesized crystals S1 were present in Fig. 4. The as-synthesized Na+ + K+-form S1 adsorbed
Fig. 3. Solid NMR spectra of zeolite-T-directed chabazite (a) 27Al MAS NMR and (b) 29 Si MAS NMR.
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only a small amount of nitrogen (Fig. 4a) as well as K+-form S3 (Supplementary Fig. S2). The adsorption amount increased by one order of magnitude after Ca2+-ion exchange and their adsorption–desorption curves (Fig. 4b and c) were classified as typical Type I curves according to the IUPAC classification, which indicates their micropore properties. The low adsorption amount for the assynthesized powders is probably because of alkali cations (Na+ and/or K+) as charge balanced ion exited at particular position inhabiting the diffusion of nitrogen into the pore. It was also in consistence of Na-chabazite [19]. Table 1 summarizes the properties of the products obtained by interzeolite conversion. The ion-exchanged crystals S1 had a larger BET surface area than ion-exchanged crystals S3 and the reported ones [19]. In addition, the Si/Al ratio in crystals S1 was a little lower than that in crystals S3. The consumption of nano-sized zeolite T for crystals S1 preparation is one thirtieth for that of zeolite HY in conversion to chabazite based on a certain amount of the product. The reason is that the content of Si in framework for crystals S1 is >92% in mole from amorphous gel. In contrast, all components in crystals S3 totally derived from the zeolite HY crystals themselves. It also suggested that zeolite T seeds supplied the nuclei to induce amorphous gel arranging to CHA framework since chabazite crystals were not gained from the zeolite-T-free gel (Gel 1) as shown in Supplementary Fig. S3. Interestingly, chabazite crystals cannot be obtained by interzeolite conversion of either zeolite HY in the aluminosilicate-gel (Gel 1) hydrothermal system or zeolite T in the base-solution (Gel 2) one. The zeolite-T-directed crystallization was studied as shown in Supplementary Fig. S4. The metastable CHA phase occurred and the crystallinity increased with the increased hydrothermal time from 4 to 8 h (Supplementary Fig. S4d and e). A stable PHI phase occurred in the product after hydrothermal synthesis of 16 h (Supplementary Fig. S4f). The competitive formation of zeolite phases was discussed in the Supporting information. Several kinds of zeolites were also successfully synthesized by interzeolite conversion. The hydrothermal conversion of zeolite A (LTA) into sodalite (SOD) was reported by Subotic´ et al. [22]. For a DON-to-AFI conversion in the presence of OSDA, a computer modeling showed that the (0 0 1) cleavage plane in DON framework fits perfectly to the (1 1 0) in AFI structure [23]. Sano group successfully prepared pure BEA, RUT, MTN and CHA from zeolite FAU in the presence of different OSDAs [24–27]. The effects of heterophasic crystals included [27]: (1) the increase surface area due to the addition of seeds resulted in a faster consumption of reagents
Fig. 4. Nitrogen adsorption–desorption isomers for (a) as-synthesized crystals S1, (b) ion-exchanged crystals S3 and (c) ion-exchanged crystals S1. Closed symbols represent adsorption data and open symbols do desorption data.
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and (2) seeds promoted nucleation through some secondary nucleation mechanisms. Recently, this group reported a template-free preparation of LEV-transformed CHA zeolite [19].The authors supposed that the dissolved/decomposed LEV-type zeolite keeps locally ordered aluminosilicate species named nanoparts. Taking into account the fact that LEV- and CHA-type zeolites have similar composite building units such as double 6-ring (D6R), the decomposed nanoparts D6R from LEV-type zeolite results in the selective formation of CHA-type zeolite. In the above studies, zeolite was used as the sole silica and alumina sources for interzeolite conversion to the desired one. In the present study, a small amount of zeolite T induced the large number of aluminosilicate gel to form the CHA framework in the absence of OSDA. Based on our results above and the data from Sano group [19,27], we can figure out the process of the conversion starting from mixed zeolite T and amorphous gel. We also consider that the nutrition from decomposed zeolite T crystals is different to that from the aluminosilicate gel, and contains some special species (nanoparts) since the seed-free gel via the same hydrothermal synthesis fails to gain chabazite crystals (Supplementary Fig. S3). Although these nanoparts were not observed by XRD, they are considered to be/contain probably composite building units such as D6R. Due to D6R being also the main building unit for chabazite, the decomposed zeolite T is easy to convert into chabazite crystals. Besides the transformation of zeolite T themselves, these special species derived from zeolite T seeds in Gel 1 also help the large amount of amorphous gel forming the CHA framework, probably due to their nuclei role. In view of the fact of competitive crystallization of CHA and PHI phases occurred into zeolite-T-seeded gel shown in Supplementary Fig. S4, we consider that the phase selection to the metastable phase (CHA) other than stable phase (PHI) is because that the induction of decomposed zeolite T advantaged the crystallization of chabazite. The composite building unit (D6R) of zeolite T is the same with that of CHA framework, but different from those (S4R and S8R) of PHI framework. The decomposed parts from zeolite T are considered to induce the formation of metastable CHA phase selectively for the first stage of 8 h of synthesis time, indicating a kinetics-control process. The crystallization tends to thermodynamics control at the additional stage of 8 h, and results in the formation of a stable PHI phase. Our results indicate that the interzeolite conversion is an effective way to fabricate metastable zeolite phases that were difficult to make directly from amorphous gel. In addition, the FAU-to-CHA conversion process for the synthesis of crystals S3 could also accord with the above explanation since D6R is the common composite building unit for FAU and CHA framework in Gel 2 system. However, it was interesting that the FAU seeds failed to induce the amorphous aluminosilicate gel (Gel 1) to chabazite crystals, although its framework also owns the same D6R composite building unit. It suggested that hydrothermal conditions including the solvent environment are the other important effect for the selective induction and/or conversion by heterophasic crystals. The Gel 1 system had a higher alkali concentration (M2O/H2O = 0.024) and was carried out at higher hydrothermal temperature (150 °C) compared with the Gel 2 system (K2O/H2O = 0.009 and synthesis temperature of 100 °C). Meanwhile, the Si/Al ratio in the framework of our zeolite T seeds is 3.4 which is higher than that in HY seeds (Si/Al = 1.8). From the fact that zeolite T crystals were stable in Gel 2 system but zeolite HY crystals were dissolved as shown in Table 1, we consider that the D6R in the zeolite T framework has a higher Si content and therefore shows a better hydrothermal stability than that in the zeolite HY framework. The low-silica D6R decomposed from the HY seeds could not be stable in high-corrosive Gel 1 system and result in the loss of the induction effect of HY seeds, as a consequence, an amorphous product was obtained after 8-h synthesis. Due to Gel 2
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Table 1 Summation of product properties prepared under different conditions. No.
S1 S2 S3 S4
Seeds
Zeolite Zeolite Zeolite Zeolite
Si source from seeds (wt%)
T HY HY T
2 2 100 100
Gel
Gel Gel Gel Gel
Product
1 1 2 2
Seed efficiency index a
b
2
Crystal phase
Particle size (lm)
Si/Al in crystal
SBET (m g
Chabazite Am Chabazite Zeolite T
4.3 – 0.5 –
2.4 – 2.6 –
320 – 250 –
1
) 14 – 0.45 –
Am = Amorphous phase. a Average particle size was tested by laser scattering method. b BET surface area test after CaCl2 ion exchange.
system being carried out at moderate hydrothermal conditions, zeolite T seeds was not dissolved and not transformed to the new chabazite phase. The more detailed research on investigating the interaction process of gel-seeds is underway. 3.2. Synthesis of chabazite membranes 3.2.1. Effect of chabazite seeds For membrane secondary growth, the microstructure and quality of resulted membrane was strongly affected by the surface seeds. Membrane synthesis was carried out under the same synthesis conditions except for seeding with different seeds (seeds S1 and S3). Fig. 5 shows SEM images of as-synthesized stainless steel supported membranes using the two kinds of seeds. The seeds S1 induced the formation of pure polycrystalline chabazite with the uniform rhombic morphology (Fig. 5a). However, some column-like impurity crystals were observed (Fig. 5b) for the membrane prepared using seeds S3. Two chabazite membranes prepared with different seeds had similar membrane thickness of 6–8 lm (Fig. 5c and d). The cracks in the membranes (Fig. 5) probably came from the soaking process in liquid-nitrogen bath for sample preparation. The column-like crystals were further verified as MER phase by XRD (Fig. 6c). Fig. 6b confirmed that the membrane prepared with seeds S1 was pure CHA phase and free of impurity phase. Table 2 presents the PV performance for a water/ethanol (10/ 90 wt%) mixture at 75 °C through chabazite membranes prepared using both kinds of seeds. A high selectivity of 2200 was obtained for chabazite membrane prepared using seeds S1, which is higher than that of the other membrane C2 (a = 370) done by seeds S3. Poor selectivity for membrane C2 was resulted from the non-zeolite pores among the MER-type zeolite and chabazite crystals since the two kinds of crystals displayed different shapes and sizes. Hasegawa et al. [2,3] also found that the impurity phase of MER crystal was always embedded in the crystal layer. The seeds were prepared by interzeolite conversion of zeolite HY as our seeds S3. However, dense zeolite layers with pure CHA phase were obtained when our seeds S1 were used. The results indicated that the crystallization of membrane was affected by surface seeds. Although the crystallization mechanism of the secondary growth of membranous chabazite was not clear at present, the properties of seeds S1 including higher surface area, smaller primary crystal size and larger adsorption capacity may contribute to the formation of pure chabazite layCC. 3.2.2. Effect of supports Table 3 presents the PV performance of chabazite membranes using different supports. In our system, chabazite membrane prepared on macroporous stainless steel support displayed a higher flux and higher water/ethanol selectivity than those prepared on the macroporous mullite support. Hasegawa et al. [3] also reported that the usage of macroporous mullite and alumina supports caused the bad chabazite membranes. The alumina nanofilter
(140–310 nm) with high porosity (51–68%) instead of macroporous support however yielded the high-performance chabazite membranes. The best membrane displayed a high flux of 14 kg m 2 h 1 that was 4 times higher than that of macroporous alumina supported membrane together with a high separation factor of 10,000 for a 90 wt% ethanol solution at 75 °C. The authors regarded that the increase of flux for the nanofilter-supported membrane was mainly due to the decrease of mass transfer resistance of support layer since the nanofilter had a higher porosity [3]. Our results together with the above data [3] suggest that the pore properties of the supports have great effects on the membrane properties. For our stainless-steel supported chabazite membrane, the high selectivity might be due to its surface effects that benefit the nucleation and growth of chabazite crystal upon the support surface. Jansen et al. [28] found the density of OH on stainless steel support is higher than ceramic alumina support. The high OH density of support surface should accelerate the concentration reaction of OHs between the support and crystal/nuclei, which result in dense zeolite layers.
3.2.3. Effect of synthesis time Fig. 7 illustrates the SEM images of stainless steel supported chabazite membranes with different synthesis time. During the initial 8 h of synthesis time, the support surface was covered by 3–4 lm coil-like polycrystalline chabazite crystals that comprised with many sub-micrometer disk-shaped crystals, but the obvious voids among crystals were observed (Fig. 7a). The coil-like and disk-like shapes were also reported as typical morphologies of chabazite [2]. The crystal phase was also verified by XRD (Supplementary Fig. S5). As the synthesis time increased to 16 h, the support surface was fully covered with rhombic chabazite crystals (Fig. 7b). However, the impurity MER crystal with column-like morphology increased in the membrane layer with the increase of synthesis time, as shown in Fig. 7. The corresponding PV performance of the resulted membranes for different synthesis time was presented in Table 4. Chabazite membrane C9 with impurity MER phase showed low water/ethanol selectivities compared with the pure chabazite membrane C8. The fact that the prolonged synthesis time resulted in the formation of MER phase was also illustrated in Ref. [3]. The authors however claimed the best membrane was the mixed MER/CHA membrane other than the pure CHA-type one.
3.2.4. Reproducibility of membrane preparation Eight membranes were repeatedly prepared under optimized conditions to investigate the reproducibility of membrane preparation. Their PV performance was presented in Table 5. These membranes showed an average flux of 3.4 kg m 2 h 1 and an average selectivity of 2100. The deviation for the flux and selectivity was as low as 15% and 19%, respectively. Those results suggest that our preparation of chabazite membrane using macroporous stainless steel support show good reproducibility.
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Fig. 5. SEM images of chabazite membranes on stainless steel tubes prepared using (a) and (c) seeds S1 and (b) and (d) seeds S3.
3.2.5. Hydrothermal stability Hydrothermal stability is an important issue for dehydration membrane. The stainless steel supported membrane (C8) had been tested in a water-rich ethanol solution at 75 °C for a week. Fig. 8 shows the time-course variations in flux and selectivity. As expected, the flux and selectivity of chabazite membrane was not obviously changed. The flux was stable at approximately 6 kg m 2 h 1 after a slight decrease in the initial stabilization period (the first 20 h), and the selectivity was stable at approximately 1100 (water content of 99.79 wt%). The SEM observation in Supplementary Fig. S6 shows that the morphologies of the crystals in the membrane after the test are typical chabazite’s shape and similar to those of the fresh membrane. The results indicated that the as-synthesized membrane showed a good hydrothermal stability even in the solution containing 30 wt% water.
4. Conclusions Fig. 6. XRD patterns for (a) porous stainless steel tubes, and stainless-steel supported chabazite membrane prepared by (b) seeds S1 and (c) seeds S3.
Chabazite crystals were directly prepared from a template-free aluminosilicate gel with the addition of a small amount of
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Table 2 PV performances for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different chabazite seeds. No.
Seeds
Total flux (kg m
C1 C2
S1 S3
3.50 4.60
2
h
1
)
Water content in permeation (wt%)
Separation factor
99.59 97.63
2200 370
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h; support: stainless steel.
Table 3 PV performances for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared using different porous supports. Support
OD (mm)
Thickness (mm)
Porosity (%)
Average pore diameter (lm)
Total flux (kg m 2 h
Alumina Alumina Mullite Mullite C3
2.2 6.1 3.0 12
0.28 1.61 0.50 1.50
68 51 45 43
0.31 1.2 1.9 1.3
Stainless steel C4
12
1.00
48
1.8
Water content in permeation (wt%)
Separation factor
Ref.
14 2.5 1.7 2.7
>99.92 90.52 97.69 98.73
>10,000 86 380 700
3.7
99.57
2100
[3] [3] [3] This work This work
1
)
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h, seeds: S1.
Fig. 7. Surface morphologies of the chabazite membranes on porous stainless steel tubular supports prepared for different crystallization time. (a) 8 h, (b) 16 h, (c) 20 h and (d) 24 h.
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R. Zhou et al. / Microporous and Mesoporous Materials 179 (2013) 128–135 Table 4 PV performance for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different synthesis time. No.
Synthesis time (h)
Total flux (kg m
C5 C6 C7 C8 C9
8 12 16 20 24
10.0 5.6 4.0 3.0 3.5
2
h
1
)
Water content in permeation (wt%)
Separation factor
87.84 98.52 99.40 99.64 99.19
65 600 1500 2500 1100
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; seeds: S1; support: stainless steel.
Table 5 PV performance for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different synthesis time. No.
Flux (kg m
C1 C4 C8 C10 C11 C12 C13 C14 Average
3.5 3.7 3.0 3.6 3.8 3.2 2.6 4.0 3.4 ± 0.5
2
h
1
)
Separation factor 2200 2100 2500 1800 1900 1910 3000 1700 2100 ± 400
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h; support: stainless steel; seeds: S1.
Fig. 8. Time-course of total flux and separation factor for chabazite membrane prepared using stainless steel supports.
nano-sized zeolite T. The seed-to-crystal yield was increased by a factor of 30 compared with the classical route in which zeolite HY was used as the sole silica and alumina sources. Pure and intergrowth chabazite layers were formed on macroporous supports by secondary growth using the zeolite-T-directed chabazite seeds. Eight stainless-steel supported chabazite membranes showed an average flux of 3.4 kg m 2 h 1 and an average separation factor of 2100 with low deviations for a water/ethanol (10/90 wt%) mixture at 75 °C. The resulted chabazite membrane displayed a good hydrothermal stability at 30 wt% water content. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (2009AA034801),
the International Science and Technology Cooperation Program of China (2010DFB52950) and the National Scientific Foundation of China (20906042 and 20966003). And we are grateful to Dr. S.L. Chen for his assistance of SEM measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 06.003. References [1] Y. Morigami, M. Kondo, J. Abe, H. Kita, Sep. Purif. Technol. 25 (2001) 251. [2] Y. Hasegawa, H. Hotta, K. Sato, T. Nagase, F. Mizukami, J. Membr. Sci. 347 (2010) 193. [3] Y. Hasegawa, C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka, J. Membr. Sci. 364 (2010) 318. [4] X. Li, H. Kita, H. Zhu, Z. Zhang, K. Tanaka, K.I. Okamoto, Microporous Mesoporous Mater. 143 (2011) 270. [5] Q.Q. Ge, Z.B. Wang, Y.S. Yan, J. Am. Chem. Soc. 131 (2009) 17056. [6] W.H. Yuan, Y.S. Lin, W.S. Yang, J. Am. Chem. Soc. 126 (2004) 4776. [7] W. Xiao, J.H. Yang, J.M. Lu, J.Q. Wang, J. Membr. Sci. 345 (2009) 183. [8] H.B. Wang, Y.S. Lin, J. Membr. Sci. 396 (2012) 128. [9] E.W. Ping, H.H. Funke, J.L. Falconer, R.D. Noble, J. Membr. Sci. 415 (2012) 770. [10] M. Yu, H.H. Funke, R.D. Noble, J.L. Falconer, J. Am. Chem. Soc. 133 (2011) 1748. [11] R.A. Rakoczy, Y. Traa, Microporous Mesoporous Mater. 60 (2003) 69. [12] S.I. Zones, J. Chem. Soc., Faraday Trans. 87 (1991) 3709. [13] H. Kalipcilar, T.C. Bowen, R.D. Noble, J.L. Falconer, Chem. Mater. 14 (2002) 3458. [14] K. Sato, K. Sugimoto, N. Shimotsuma, T. Kikuchi, T. Kyotani, T. Kurata, J. Membr. Sci. 409–410 (2012) 82. [15] M.A. Carreon, S. Li, J.L. Falconer, R.D. Noble, J. Am. Chem. Soc. 130 (2008) 5412. [16] M.A. Carreon, S.G. Li, J.L. Falconer, R.D. Noble, Adv. Mater. 20 (2008) 729. [17] Z.P. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Science 300 (2003) 456. [18] R.F. Zhou, F. Zhang, N. Hu, X.S. Chen, H. Kita, Chem. Lett. 40 (2011) 1383. [19] G. Ikuhiro, I. Masaya, S. Syohei, H. Koutaro, I. Yusuke, S. Masahiro, T. Sano, Microporous Mesoporous Mater. 158 (2012) 117. [20] R.F. Zhou, Z.L. Hu, N. Hu, L.Q. Duan, X.S. Chen, H. Kita, Microporous Mesoporous Mater. 156 (2012) 166. ˇ as, C. Baerlocher, J. Phys. Chem. B 102 [21] M.A. Camblor, A. Corma, M.-J. Diaz-Caban (1998) 44. [22] B. Subotic´, L. Sekovanic´, J. Cryst. Growth 75 (1986) 561. [23] V. Valtchev, J.L. Paillaud, H. Kessler, E.J. Creyghton, Microporous Mesoporous Mater. 33 (1999) 143. [24] H. Jon, K. Nakahata, B. Lu, Y. Oumi, T. Sano, Microporous Mesoporous Mater. 96 (2006) 72. [25] H. Jon, S. Takahashi, H. Sasaki, Y. Oumi, T. Sano, Microporous Mesoporous Mater. 113 (2008) 56. [26] H. Sasaki, H. Jon, M. Itakura, T. Inoue, T. Ikeda, Y. Oumi, T. Sano, J. Porous Mater. 16 (2009) 465. [27] M. Itakura, I. Goto, A. Takahashi, T. Fujitani, Y. Ide, M. Sadakane, T. Sano, Microporous Mesoporous Mater. 144 (2011) 91. [28] J.C. Jansen, J.H. Koegler, H. van Bekkum, H.P.A. Calis, C.M. van den Bleek, F. Kaptejin, J.A. Moulijn, E.R. Geus, N. van der Puil, Microporous Mesoporous Mater. 21 (1998) 213.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of chabazite membranes by secondary growth using zeolite-T-directed chabazite seeds Rongfei Zhou a, Yuqin Li a, Bo Liu a, Na Hu a, Xiangshu Chen a,⇑, Hidetoshi Kita b,⇑ a b
Jiangxi Inorganic Membrane Materials Engineering Research Centre, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
a r t i c l e
i n f o
Article history: Received 30 January 2013 Received in revised form 13 May 2013 Accepted 6 June 2013 Available online 15 June 2013 Keywords: Chabazite membrane Induction Pervaporation Macroporous support
a b s t r a c t Chabazite crystals were directly prepared from template-free aluminosilicate gel with the addition of nano-sized zeolite T (100 nm). This new route requires only one thirtieth seeds amount for a certain product, and one twentieth synthesis time compared with the classic one by the conversion of zeolite Y crystals. A pure and dense chabazite membrane was prepared on macroporous supports by secondary growth. The membrane synthesis parameters such as seeds, supports and synthesis time on membrane formation and pervaporation performance were also studied. Eight chabazite membranes prepared on macroporous stainless-steel supports showed a high average flux of 3.4 kg m 2 h 1 and high average selectivity of 2100 with low deviations on the flux and selectivity for a water/ethanol (10/90 wt%) mixture at 75 °C, indicating membrane synthesis using stainless-steel supports had a good reproducibility. The as-synthesized chabazite membranes also showed good hydrothermal stability under pervaporation test in a water-rich ethanol aqueous solution. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Zeolite membranes are considered to be the most promising candidates for pervaporation (PV) [1–7] and gas separation [8–10] because of the unique characteristics of zeolite crystals such as well-defined pore structures, selective adsorption, and high thermal stability. Zeolite NaA membranes with the excellent PV performance have been successfully applied for dehydration of some organic solvents on a large scale [1]. However, zeolite NaA membranes are unstable in the solutions containing even a trace of acid or high content of water by strong dealumination since its framework has the lowest Si/Al ratio of 1 [11]. Chabazite (CHA framework) owns similar pore properties (3-dimeteral and interconnected small pores and relative low framework identity), but an increased crystal Si/Al ratio (2–5) compared with zeolite NaA (LTA framework). The 8-ring channels have a pore size of 0.38 nm which is smaller than most alcohols but larger than water. The pore properties together with the medium crystal Si/Al ratio allow chabazite membrane becoming a candidate of high-performance dehydration membrane. The membrane has been applied in water-rich and acidic surroundings where NaA membrane is not stable [2,3]. There were only a few literatures involved in chabazite membrane preparation [2–4]. Hasegawa et al. ⇑ Corresponding authors. Tel.: +86 791 8120 533; fax: +86 791 8120 843 (X. Chen). E-mail addresses: [email protected] (X. Chen), [email protected] (H. Kita). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.06.003
[2,3] prepared chabazite membranes in a synthesis gel containing strontium cation, which worked as a pseudo-template for crystal growth of the CHA-type zeolite. The best membranes always contained a few MER-type crystals [2,3]. Li et al. [4] reported that chabazite membrane was synthesized in the presence only potassium cation. The seeds for secondary growth of these membranes [2–4] were all prepared by the conversion of Y-type crystal. It is the standard way for the preparation of chabazite crystals [12]. In our present study, chabazite seeds were directly prepared from aluminosilicate gel with the addition of a little amount of nanosized zeolite T. The yield and crystallization rate of chabazite were greatly improved. High-silica CHA-type (SSZ-13) seeds and membranes were prepared in the gel precursor containing expensive organic structure-directing agents (OSDAs) of N,N,N-trimethyl-1-adamantammonium cation (TMAda+) [12–14]. A SSZ-13 membrane with Si/ Al ratio of 13.4 showed high stability in 69.5 wt% nitric acid aqueous solution but a poor water selectivity over nitric acid [13]. An improved water selectivity of 1600 over isopropyl alcohol was obtained through a CHA-type zeolite membrane with Si/Al ratio of 8 by Sato et al. [14]. The usage of OSDA increased the cost of membrane preparation and the uncertainty during the additional calcination process. The crystallization and separation performance of supported zeolite layers were affected by the properties of porous supports including the composition, pore size and porosity and so on. Hasegawa et al. [3] reported that a-alumina support with high porosity
R. Zhou et al. / Microporous and Mesoporous Materials 179 (2013) 128–135
of 50–68% and small pore sizes of 0.14–0.31 lm will resulted in a high-performance chabazite membrane, while the membrane prepared on macroporous supports (2.5 lm a-alumina and 1.7 lm mullite supports) had both low flux and poor water selectivity over ethanol. CO2 permanence of SAPO-34 membrane was also improved by a factor of 4 when asymmetrical a-alumina nanofilters (0.1–0.2 lm) support [15] instead of symmetrical stainless steel (0.3 lm) support [16] for CO2/CH4 separation. On the other hand, seed morphology for membrane secondary growth is another important factor on membrane growth [16,17]. In our present study, a new route was developed for the preparation of chabazite seeds by interzeolite conversion of zeolite T, in which nano-sized zeolite T (100 nm) induced the template-free amorphous gel to form the new chabazite phase. The pure (MERphase-free) chabazite membranes with high pervaporation performance were successfully obtained upon inexpensive symmetrical macroporous supports when the zeolite-T-directed chabazite seeds were used for seeding. 2. Experimental 2.1. Chabazite seed preparation All chemicals were purchased from Sigma–Aldrich Company for both seeds and membrane preparation except for precipitated silica that was purchased from Degussa Company. In a typical synthesis, 6.2 g aluminum hydroxide was dissolved in a base aqueous solution containing 18.8 g sodium hydroxide, 10.3 g potassium hydroxide and 217 g DI water while heating. 1 g nano-sized seeds were added in the boiling base solution. Nano-sized zeolite T was prepared following our previous work [18]. The nano-sized zeolite T showed a spherical morphology with approximately 100 nm diameters (Supplementary Fig. S1). After 0.5-h stirring, the hot cloudy solution was cooled down to room temperature. Zeolite T crystals should be chipped by strong dissolution of the boiling base solution since the colloidal seeded solution was hardly separated by increased rotation speed from 10,000 run per minute (RPM) to the limited 15,000 RPM. 47.4 g precipitated silica was then added in the solution slowly. A milk-like gel (Gel 1) was obtained after gel aging overnight and had a mole ratio of SiO2:Al2O3:Na2O:K2O:H2O = 1:0.05:0.285:0.095:16. The aged gel was placed in a Teflonlined autoclave. The hydrothermal synthesis was carried out at 150 °C for 8 h. The product was washed by DI water, separated and dried. For comparison, chabazite seeds were also prepared by the reported interzeolite conversion of zeolite HY, in which all silica and alumina sources came from crystalline zeolite HY powder [2,3,14]. The base solution (Gel 2) of 48 ml 1 mol l 1 potassium hydroxide solution (K2O/H2O = 0.009) was mixed with 4 g zeolite HY powder in a 250 ml polypropylene bottle. The crystallization was carried out at 90 °C for 7 d. The resulted crystals were separated from base solution and dried at 100 °C overnight. Chabazite crystals were also attempted to prepare by interzeolite conversion of zeolite HY using the aluminosilicate gel (Gel 1), and of nanosized zeolite T using base solution (Gel 2). To evaluate the induction efficiency of seeds, Seed Efficiency Index (SEI) was defined as the weight ratio of added seeds with respect to the obtained chabazite. 2.2. Chabazite membrane preparation The outer surface of porous 10-cm-long mullite and stainless steel supports were rub-coated with water slurry of chabazite seeds. The stainless steel tubes from Pall Company had the internal diameter of 10 mm, the outer diameter of 12 mm, the porosity of
129
48% and the average pore diameter of 1.8 lm. And the mullite support from Nikkato Corporation had the internal diameter of 9 mm, the outer diameter of 12 mm, the porosity of 43% and the average pore diameter of 1.3 lm. Two kinds of seeds (S1 and S3) were seeded on the porous support using the same rub-coating procedure. Seed slurry was prepared by mixing 5 g seed powders and 20 g water and sonicated for 30 min. All the outside surface of the tubes was rubbed back and forth by hand for a total time of approximately 1 min. The seeded tubes were dried at 80 °C for 1 h. The excessive seeds on the surface were then removed by a cotton swab slowly. A modified gel preparation from the Ref. [2] was carried out by adjusting gel SiO2/Al2O3 ratio and alumina source. The gel was prepared by mixing aluminum isopropoxide, potassium hydroxide, colloidal silica, strontium nitrate and deionized water. The resulted precursor gel had a molar ratio of SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65. After 6 h aging, 270 g gel was placed in an autoclave and a seeded support was vertically immersed into the gel. The hydrothermal synthesis was carried out at 150 °C for a given time. After synthesis, the autoclave was cooled down to room temperature and the as-synthesized membranes were washed using running taping water for 15 min, and soaked into DI water for several batches until the solution become neutral, and dried at 80 °C overnight. 2.3. Characterization and PV test The crystalline phases for the powdery and membranous crystals were characterized by X-ray diffraction (XRD, Ultima IV) with a Cu-Ka radiation source using a Rigaku Ultima IV machine. The morphology of the seeds and membrane were observed using scanning electron microscopy (SEM) (Tescan Vega3). For the SEM samples’ preparation of stainless steel supported membranes, the membrane was soaked in the liquid-nitrogen bath in order to embrittle the supports before test. Element analyst was carried out by energy dispersive X-ray analysis (EDX) using a Hitachi S-3400N machine. Nitrogen adsorption and desorption isomers were tested on a Micrometer ASAP 2200 machine. The as-synthesized chabazite crystals and the Ca2+-ion-exchanged samples were used for the adsorption–desorption test. The ion-exchanged process was followed by Goto et al. [19]. Ion exchange was carried out using 3 times of the calcined as-synthesized chabazite with 1 M CaCl2 aqueous solution at 60 °C for 2 h, and the solids were washed to remove the chloride anions. The 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the powders were recorded at 104.2 MHz and 79.5 MHz, respectively, using a zirconia rotor of 4 mm diameter, on a Bruker Advance III 400 WB spectrometer. The rotor was spun with dry air at 4 kHz for 29Si MAS NMR and at 15 kHz for 27Al MAS NMR. The spectra were accumulated with 3 ls pulses, 2 s recycle delay and 1024 scans for 27Al MAS NMR, and 4.5 ls pulses, 40 s recycle delay and 1024 scans for 29Si MAS NMR. Al(NO3)39H2O and Si(CH3)4 (TMS) were used as chemical shift reference for 27Al MAS NMR and 29Si MAS NMR, respectively. PV performance test for a water/ethanol (10/90 wt%) mixture was carried out at 75 °C using a PV experimental apparatus described elsewhere [20]. The effective membrane area was approximately 27 cm2 and the permeation side was kept under vacuum of 20 Pa. The membrane separation performance is evaluated by the permeation flux (kg m 2 h 1) and the separation factor (a). The permeation flux is calculated by the mass of the permeation, which is collected by a liquid-nitrogen trap. Composition analysis of the feed and permeation were performed using a gas chromatograph (Shimadzu GC-14C) equipped with a TCD detector. The separation factor is defined as a = (YA/YB)/(XA/XB), where XA, XB, YA, and YB denote the mass fractions of components A (water) and B (alcohols) in the feed and permeation sides, respectively.
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3. Results and discussion 3.1. Synthesis of chabazite seeds Fig. 1 shows the morphologies of the as-synthesized chabazite crystals. The crystals S1 prepared by interzeolite conversion of zeolite T (Fig. 1a) displayed a walnut shape which was a typical chabazite morphology reported in the literatures [2,3]. The 2– 4 lm walnut-like crystals were composited with hundreds of sub-micrometer crystals with good self-assembly. The crystals S3 prepared by interzeolite conversion of zeolite HY (Fig. 1b) had a small ellipsoidal shape with an average size of 0.5 lm. Fig. 2 shows XRD patterns of the as-synthesized chabazite crystals. Both XRD patterns in Fig. 2b and c for the products S1 and S3 were consistent with the CHA-type structure [2,14] and the simulated one in Fig. 2a. The low baseline and the absence of non-zeolite-CHA peaks indicate that the products were free from impurities. Crystals S1 had the broad and low peaks with similar peak width of the nano-sized zeolite T (100 nm) (Fig. 2d) due to size effects. Those sub-micrometer polycrystalline crystals observed in Fig. 1a could be responsible for its broad XRD peaks in Fig. 2b. 27 Al and 29Si MAS NMR spectra were used to study the chemical state of aluminate and silicate in zeolite-T-directed chabazite. Only one peak at approximate 59 ppm was found in 27Al MAS NMR
Fig. 1. SEM images of chabazite seeds (a) S1 and (b) S3.
Fig. 2. XRD patterns for (a) simulated CHA framework, (b) seeds S1 prepared by conversion of zeolite T, (c) seeds S3 prepared by conversion of zeolite HY, (d) nanosized zeolite T and (e) zeolite HY.
spectrum in Fig. 3a, corresponding to tetrahedrally-coordinated aluminum in framework. There was no extraframework aluminum since no peak was observed at 0 ppm. The 29Si MAS NMR spectrum of as-synthesized chabazite crystals S1 in Fig. 3b showed four peaks at 109, 104, 98 and 93 ppm. These peaks are assigned to Si(0Al), Si(1Al), Si(2Al) and Si(3Al) species, respectively [19,21]. An additional peak at 88 ppm assigning to Si(4Al) was observed for a LEV-transformed chabazite [19], but was absent in our crystals. Nitrogen adsorption and desorption isomers for Ca2+-ion-exchanged crystals S1 and S3 and as-synthesized crystals S1 were present in Fig. 4. The as-synthesized Na+ + K+-form S1 adsorbed
Fig. 3. Solid NMR spectra of zeolite-T-directed chabazite (a) 27Al MAS NMR and (b) 29 Si MAS NMR.
R. Zhou et al. / Microporous and Mesoporous Materials 179 (2013) 128–135
only a small amount of nitrogen (Fig. 4a) as well as K+-form S3 (Supplementary Fig. S2). The adsorption amount increased by one order of magnitude after Ca2+-ion exchange and their adsorption–desorption curves (Fig. 4b and c) were classified as typical Type I curves according to the IUPAC classification, which indicates their micropore properties. The low adsorption amount for the assynthesized powders is probably because of alkali cations (Na+ and/or K+) as charge balanced ion exited at particular position inhabiting the diffusion of nitrogen into the pore. It was also in consistence of Na-chabazite [19]. Table 1 summarizes the properties of the products obtained by interzeolite conversion. The ion-exchanged crystals S1 had a larger BET surface area than ion-exchanged crystals S3 and the reported ones [19]. In addition, the Si/Al ratio in crystals S1 was a little lower than that in crystals S3. The consumption of nano-sized zeolite T for crystals S1 preparation is one thirtieth for that of zeolite HY in conversion to chabazite based on a certain amount of the product. The reason is that the content of Si in framework for crystals S1 is >92% in mole from amorphous gel. In contrast, all components in crystals S3 totally derived from the zeolite HY crystals themselves. It also suggested that zeolite T seeds supplied the nuclei to induce amorphous gel arranging to CHA framework since chabazite crystals were not gained from the zeolite-T-free gel (Gel 1) as shown in Supplementary Fig. S3. Interestingly, chabazite crystals cannot be obtained by interzeolite conversion of either zeolite HY in the aluminosilicate-gel (Gel 1) hydrothermal system or zeolite T in the base-solution (Gel 2) one. The zeolite-T-directed crystallization was studied as shown in Supplementary Fig. S4. The metastable CHA phase occurred and the crystallinity increased with the increased hydrothermal time from 4 to 8 h (Supplementary Fig. S4d and e). A stable PHI phase occurred in the product after hydrothermal synthesis of 16 h (Supplementary Fig. S4f). The competitive formation of zeolite phases was discussed in the Supporting information. Several kinds of zeolites were also successfully synthesized by interzeolite conversion. The hydrothermal conversion of zeolite A (LTA) into sodalite (SOD) was reported by Subotic´ et al. [22]. For a DON-to-AFI conversion in the presence of OSDA, a computer modeling showed that the (0 0 1) cleavage plane in DON framework fits perfectly to the (1 1 0) in AFI structure [23]. Sano group successfully prepared pure BEA, RUT, MTN and CHA from zeolite FAU in the presence of different OSDAs [24–27]. The effects of heterophasic crystals included [27]: (1) the increase surface area due to the addition of seeds resulted in a faster consumption of reagents
Fig. 4. Nitrogen adsorption–desorption isomers for (a) as-synthesized crystals S1, (b) ion-exchanged crystals S3 and (c) ion-exchanged crystals S1. Closed symbols represent adsorption data and open symbols do desorption data.
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and (2) seeds promoted nucleation through some secondary nucleation mechanisms. Recently, this group reported a template-free preparation of LEV-transformed CHA zeolite [19].The authors supposed that the dissolved/decomposed LEV-type zeolite keeps locally ordered aluminosilicate species named nanoparts. Taking into account the fact that LEV- and CHA-type zeolites have similar composite building units such as double 6-ring (D6R), the decomposed nanoparts D6R from LEV-type zeolite results in the selective formation of CHA-type zeolite. In the above studies, zeolite was used as the sole silica and alumina sources for interzeolite conversion to the desired one. In the present study, a small amount of zeolite T induced the large number of aluminosilicate gel to form the CHA framework in the absence of OSDA. Based on our results above and the data from Sano group [19,27], we can figure out the process of the conversion starting from mixed zeolite T and amorphous gel. We also consider that the nutrition from decomposed zeolite T crystals is different to that from the aluminosilicate gel, and contains some special species (nanoparts) since the seed-free gel via the same hydrothermal synthesis fails to gain chabazite crystals (Supplementary Fig. S3). Although these nanoparts were not observed by XRD, they are considered to be/contain probably composite building units such as D6R. Due to D6R being also the main building unit for chabazite, the decomposed zeolite T is easy to convert into chabazite crystals. Besides the transformation of zeolite T themselves, these special species derived from zeolite T seeds in Gel 1 also help the large amount of amorphous gel forming the CHA framework, probably due to their nuclei role. In view of the fact of competitive crystallization of CHA and PHI phases occurred into zeolite-T-seeded gel shown in Supplementary Fig. S4, we consider that the phase selection to the metastable phase (CHA) other than stable phase (PHI) is because that the induction of decomposed zeolite T advantaged the crystallization of chabazite. The composite building unit (D6R) of zeolite T is the same with that of CHA framework, but different from those (S4R and S8R) of PHI framework. The decomposed parts from zeolite T are considered to induce the formation of metastable CHA phase selectively for the first stage of 8 h of synthesis time, indicating a kinetics-control process. The crystallization tends to thermodynamics control at the additional stage of 8 h, and results in the formation of a stable PHI phase. Our results indicate that the interzeolite conversion is an effective way to fabricate metastable zeolite phases that were difficult to make directly from amorphous gel. In addition, the FAU-to-CHA conversion process for the synthesis of crystals S3 could also accord with the above explanation since D6R is the common composite building unit for FAU and CHA framework in Gel 2 system. However, it was interesting that the FAU seeds failed to induce the amorphous aluminosilicate gel (Gel 1) to chabazite crystals, although its framework also owns the same D6R composite building unit. It suggested that hydrothermal conditions including the solvent environment are the other important effect for the selective induction and/or conversion by heterophasic crystals. The Gel 1 system had a higher alkali concentration (M2O/H2O = 0.024) and was carried out at higher hydrothermal temperature (150 °C) compared with the Gel 2 system (K2O/H2O = 0.009 and synthesis temperature of 100 °C). Meanwhile, the Si/Al ratio in the framework of our zeolite T seeds is 3.4 which is higher than that in HY seeds (Si/Al = 1.8). From the fact that zeolite T crystals were stable in Gel 2 system but zeolite HY crystals were dissolved as shown in Table 1, we consider that the D6R in the zeolite T framework has a higher Si content and therefore shows a better hydrothermal stability than that in the zeolite HY framework. The low-silica D6R decomposed from the HY seeds could not be stable in high-corrosive Gel 1 system and result in the loss of the induction effect of HY seeds, as a consequence, an amorphous product was obtained after 8-h synthesis. Due to Gel 2
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Table 1 Summation of product properties prepared under different conditions. No.
S1 S2 S3 S4
Seeds
Zeolite Zeolite Zeolite Zeolite
Si source from seeds (wt%)
T HY HY T
2 2 100 100
Gel
Gel Gel Gel Gel
Product
1 1 2 2
Seed efficiency index a
b
2
Crystal phase
Particle size (lm)
Si/Al in crystal
SBET (m g
Chabazite Am Chabazite Zeolite T
4.3 – 0.5 –
2.4 – 2.6 –
320 – 250 –
1
) 14 – 0.45 –
Am = Amorphous phase. a Average particle size was tested by laser scattering method. b BET surface area test after CaCl2 ion exchange.
system being carried out at moderate hydrothermal conditions, zeolite T seeds was not dissolved and not transformed to the new chabazite phase. The more detailed research on investigating the interaction process of gel-seeds is underway. 3.2. Synthesis of chabazite membranes 3.2.1. Effect of chabazite seeds For membrane secondary growth, the microstructure and quality of resulted membrane was strongly affected by the surface seeds. Membrane synthesis was carried out under the same synthesis conditions except for seeding with different seeds (seeds S1 and S3). Fig. 5 shows SEM images of as-synthesized stainless steel supported membranes using the two kinds of seeds. The seeds S1 induced the formation of pure polycrystalline chabazite with the uniform rhombic morphology (Fig. 5a). However, some column-like impurity crystals were observed (Fig. 5b) for the membrane prepared using seeds S3. Two chabazite membranes prepared with different seeds had similar membrane thickness of 6–8 lm (Fig. 5c and d). The cracks in the membranes (Fig. 5) probably came from the soaking process in liquid-nitrogen bath for sample preparation. The column-like crystals were further verified as MER phase by XRD (Fig. 6c). Fig. 6b confirmed that the membrane prepared with seeds S1 was pure CHA phase and free of impurity phase. Table 2 presents the PV performance for a water/ethanol (10/ 90 wt%) mixture at 75 °C through chabazite membranes prepared using both kinds of seeds. A high selectivity of 2200 was obtained for chabazite membrane prepared using seeds S1, which is higher than that of the other membrane C2 (a = 370) done by seeds S3. Poor selectivity for membrane C2 was resulted from the non-zeolite pores among the MER-type zeolite and chabazite crystals since the two kinds of crystals displayed different shapes and sizes. Hasegawa et al. [2,3] also found that the impurity phase of MER crystal was always embedded in the crystal layer. The seeds were prepared by interzeolite conversion of zeolite HY as our seeds S3. However, dense zeolite layers with pure CHA phase were obtained when our seeds S1 were used. The results indicated that the crystallization of membrane was affected by surface seeds. Although the crystallization mechanism of the secondary growth of membranous chabazite was not clear at present, the properties of seeds S1 including higher surface area, smaller primary crystal size and larger adsorption capacity may contribute to the formation of pure chabazite layCC. 3.2.2. Effect of supports Table 3 presents the PV performance of chabazite membranes using different supports. In our system, chabazite membrane prepared on macroporous stainless steel support displayed a higher flux and higher water/ethanol selectivity than those prepared on the macroporous mullite support. Hasegawa et al. [3] also reported that the usage of macroporous mullite and alumina supports caused the bad chabazite membranes. The alumina nanofilter
(140–310 nm) with high porosity (51–68%) instead of macroporous support however yielded the high-performance chabazite membranes. The best membrane displayed a high flux of 14 kg m 2 h 1 that was 4 times higher than that of macroporous alumina supported membrane together with a high separation factor of 10,000 for a 90 wt% ethanol solution at 75 °C. The authors regarded that the increase of flux for the nanofilter-supported membrane was mainly due to the decrease of mass transfer resistance of support layer since the nanofilter had a higher porosity [3]. Our results together with the above data [3] suggest that the pore properties of the supports have great effects on the membrane properties. For our stainless-steel supported chabazite membrane, the high selectivity might be due to its surface effects that benefit the nucleation and growth of chabazite crystal upon the support surface. Jansen et al. [28] found the density of OH on stainless steel support is higher than ceramic alumina support. The high OH density of support surface should accelerate the concentration reaction of OHs between the support and crystal/nuclei, which result in dense zeolite layers.
3.2.3. Effect of synthesis time Fig. 7 illustrates the SEM images of stainless steel supported chabazite membranes with different synthesis time. During the initial 8 h of synthesis time, the support surface was covered by 3–4 lm coil-like polycrystalline chabazite crystals that comprised with many sub-micrometer disk-shaped crystals, but the obvious voids among crystals were observed (Fig. 7a). The coil-like and disk-like shapes were also reported as typical morphologies of chabazite [2]. The crystal phase was also verified by XRD (Supplementary Fig. S5). As the synthesis time increased to 16 h, the support surface was fully covered with rhombic chabazite crystals (Fig. 7b). However, the impurity MER crystal with column-like morphology increased in the membrane layer with the increase of synthesis time, as shown in Fig. 7. The corresponding PV performance of the resulted membranes for different synthesis time was presented in Table 4. Chabazite membrane C9 with impurity MER phase showed low water/ethanol selectivities compared with the pure chabazite membrane C8. The fact that the prolonged synthesis time resulted in the formation of MER phase was also illustrated in Ref. [3]. The authors however claimed the best membrane was the mixed MER/CHA membrane other than the pure CHA-type one.
3.2.4. Reproducibility of membrane preparation Eight membranes were repeatedly prepared under optimized conditions to investigate the reproducibility of membrane preparation. Their PV performance was presented in Table 5. These membranes showed an average flux of 3.4 kg m 2 h 1 and an average selectivity of 2100. The deviation for the flux and selectivity was as low as 15% and 19%, respectively. Those results suggest that our preparation of chabazite membrane using macroporous stainless steel support show good reproducibility.
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Fig. 5. SEM images of chabazite membranes on stainless steel tubes prepared using (a) and (c) seeds S1 and (b) and (d) seeds S3.
3.2.5. Hydrothermal stability Hydrothermal stability is an important issue for dehydration membrane. The stainless steel supported membrane (C8) had been tested in a water-rich ethanol solution at 75 °C for a week. Fig. 8 shows the time-course variations in flux and selectivity. As expected, the flux and selectivity of chabazite membrane was not obviously changed. The flux was stable at approximately 6 kg m 2 h 1 after a slight decrease in the initial stabilization period (the first 20 h), and the selectivity was stable at approximately 1100 (water content of 99.79 wt%). The SEM observation in Supplementary Fig. S6 shows that the morphologies of the crystals in the membrane after the test are typical chabazite’s shape and similar to those of the fresh membrane. The results indicated that the as-synthesized membrane showed a good hydrothermal stability even in the solution containing 30 wt% water.
4. Conclusions Fig. 6. XRD patterns for (a) porous stainless steel tubes, and stainless-steel supported chabazite membrane prepared by (b) seeds S1 and (c) seeds S3.
Chabazite crystals were directly prepared from a template-free aluminosilicate gel with the addition of a small amount of
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Table 2 PV performances for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different chabazite seeds. No.
Seeds
Total flux (kg m
C1 C2
S1 S3
3.50 4.60
2
h
1
)
Water content in permeation (wt%)
Separation factor
99.59 97.63
2200 370
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h; support: stainless steel.
Table 3 PV performances for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared using different porous supports. Support
OD (mm)
Thickness (mm)
Porosity (%)
Average pore diameter (lm)
Total flux (kg m 2 h
Alumina Alumina Mullite Mullite C3
2.2 6.1 3.0 12
0.28 1.61 0.50 1.50
68 51 45 43
0.31 1.2 1.9 1.3
Stainless steel C4
12
1.00
48
1.8
Water content in permeation (wt%)
Separation factor
Ref.
14 2.5 1.7 2.7
>99.92 90.52 97.69 98.73
>10,000 86 380 700
3.7
99.57
2100
[3] [3] [3] This work This work
1
)
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h, seeds: S1.
Fig. 7. Surface morphologies of the chabazite membranes on porous stainless steel tubular supports prepared for different crystallization time. (a) 8 h, (b) 16 h, (c) 20 h and (d) 24 h.
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R. Zhou et al. / Microporous and Mesoporous Materials 179 (2013) 128–135 Table 4 PV performance for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different synthesis time. No.
Synthesis time (h)
Total flux (kg m
C5 C6 C7 C8 C9
8 12 16 20 24
10.0 5.6 4.0 3.0 3.5
2
h
1
)
Water content in permeation (wt%)
Separation factor
87.84 98.52 99.40 99.64 99.19
65 600 1500 2500 1100
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; seeds: S1; support: stainless steel.
Table 5 PV performance for a water/ethanol (10/90 wt%) mixture at 75 °C through chabazite membranes prepared with different synthesis time. No.
Flux (kg m
C1 C4 C8 C10 C11 C12 C13 C14 Average
3.5 3.7 3.0 3.6 3.8 3.2 2.6 4.0 3.4 ± 0.5
2
h
1
)
Separation factor 2200 2100 2500 1800 1900 1910 3000 1700 2100 ± 400
Note: membrane gel composition: SiO2:Al2O3:K2O:SrO:KNO3:H2O = 1:0.2:0.17:0.08:0.67:65; synthesis temperature: 150 °C; synthesis time: 16 h; support: stainless steel; seeds: S1.
Fig. 8. Time-course of total flux and separation factor for chabazite membrane prepared using stainless steel supports.
nano-sized zeolite T. The seed-to-crystal yield was increased by a factor of 30 compared with the classical route in which zeolite HY was used as the sole silica and alumina sources. Pure and intergrowth chabazite layers were formed on macroporous supports by secondary growth using the zeolite-T-directed chabazite seeds. Eight stainless-steel supported chabazite membranes showed an average flux of 3.4 kg m 2 h 1 and an average separation factor of 2100 with low deviations for a water/ethanol (10/90 wt%) mixture at 75 °C. The resulted chabazite membrane displayed a good hydrothermal stability at 30 wt% water content. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (2009AA034801),
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