A study of Silicalite-1 and Al-ZSM-5 membrane synthesis on stainless steel supports

Microporous and Mesoporous Materials 75 (2004) 209–220 www.elsevier.com/locate/micromeso

A study of Silicalite-1 and Al-ZSM-5 membrane synthesis on stainless steel supports G.T.P. Mabande a, G. Pradhan a, W. Schwieger a,*, M. Hanebuth b, R. Dittmeyer b, T. Selvam a, A. Zampieri a, H. Baser a, R. Herrmann a a

Institute of Chemical Reaction Engineering, University Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany b Karl-Winnacker-Institut, DECHEMA e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany Received 2 March 2004; received in revised form 6 July 2004; accepted 9 July 2004

Abstract Silicalite-1 and Al-ZSM-5 membranes were prepared by combining one-step in situ seeding with one secondary membrane growth step. For membranes prepared under similar conditions, Al-ZSM-5 membrane growth was found to be considerably slower than Silicalite-1 growth leading to a significantly higher preferred axial orientation of the Silicalite-1 membranes. H2/SF6 single gas permeation measurements revealed that for both membrane types a correlation existed between the H2/SF6 ideal perm-selectivities and the zeolite layer thickness. Above a certain thickness the defect density is independent of layer thickness. However, Al-ZSM-5 membrane perm-selectivities were higher, despite lower membrane thicknesses and comparable hydrogen permeation rates. For example, room temperature ideal H2/SF6 perm-selectivities of about 136 and 65 were measured for a 7.5 lm thick Al-ZSM-5 and a 45 lm thick Silicalite-1 membrane, respectively. Comparison to literature data revealed that the membranes were of good quality. Moreover, reproducibility of the membrane synthesis procedure proved to be good.
2004 Elsevier Inc. All rights reserved. Keywords: MFI membrane; Single gas permeation; Seeding; Stainless steel support

1. Introduction Materials with uniform microporous architecture, such as zeolites, are traditionally of great scientific and industrial importance in the areas of catalysis, adsorption (separation) and ion exchange [1]. The synthesis of zeolites as films in recent years has targeted a potential broadening of the areas of application of zeolites to low-k dielectric films, selective membranes, sensors and optical films, etc. [2–5]. Zeolite membranes have become an important class of inorganic membranes which combine temperature stability and solvent resistance with * Corresponding author. Tel.: +49 9131 852 8910; fax: +49 9131 852 7421. E-mail address: [email protected] (W. Schwieger).

1387-1811/$ - see front matter
2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.009

diverse fine-tunable zeolite characteristics, such as molecular sieving on the subnanometer size scale, hydrophobic/hydrophilic nature and catalytic activity [6,7]. Appropriate synthesis techniques allow for the preparation of zeolite layers with the ability to separate different molecules based on different diffusivities, adsorption characteristics and capillary condensation effects [8,9]. Separation based mainly on hydrophilic– hydrophobic interaction has already gained commercial importance in the case of zeolite A membranes for pervaporative solvent dehydration [10,11]. Researchers continue to explore different methods of zeolite membrane fabrication, mainly one-step in situ hydrothermal synthesis, dry gel conversion and seeded membrane synthesis. Most work has focused on the synthesis and characterisation of MFI membranes [12]. Thin continuous layers can be achieved by growing

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small, well-intergrown crystals when using direct and secondary synthesis of membranes. Large, well-shaped crystals grown under conditions of high crystal growth rate (e.g. high temperature) tend to form thick layers with distinct interlayer cavities [13]. Since its introduction in the early 90 s [14,15], the seeding technique has established itself as a robust and reliable means of synthesis of thin molecular sieve layers. Nucleation and crystal growth are decoupled, allowing for an individual optimisation of the two processes. The influence of the surface chemistry of the support is reduced [18], reproducibility is improved [16] and detrimental sedimentation of bulk phase crystals can easily be avoided [17]. Mostly ex situ crystallised seeds are used. These are attached to the support surface, e.g. using slip casting [18], electrostatic attraction [19,20], dip coating [21] and pulsed laser ablation [22]. The seeds can also be crystallised directly onto the support [23,24]. It has been demonstrated that for MFI membrane synthesis, the synthesis conditions (e.g. initial seed orientation, support roughness, template type and synthesis time) determine the orientation of crystals in the final membrane layer [3,24–26]. Diverse planar and tubular macroporous bodies have been used as supports for zeolite membrane synthesis. The most widely used support material is alumina [6]. This is probably mainly due to the availability of highquality micro-, nano- and ultra-filtration ceramics with relatively smooth top surfaces and small average pore sizes, e.g. down to 5 nm pore size for c-Al2O3 [7]. Some research groups make use of home-made ceramic supports [3,27]. A smooth top surface is an important requirement for the preparation of thin continuous zeolite layers. For example, micro-filtration a-Al2O3 membranes with a 100 nm pore size top coating were used to prepare high-flux MFI membranes with 500 nm thickness [19]. Stainless steel supports generally have rougher surfaces and larger pore sizes (>100 nm) [17,28,29]. Another disadvantage of stainless steel is the higher thermal expansion coefficient of 15–19 · 10 6 K 1 compared to 2–7 · 10 6 K 1 for alumina [12]. During membrane calcination (after synthesis) the zeolite layer shrinks due to template burn-off and in some cases transitions in the orthorhombic and monoclinic phases [30–32]. Thus stainless-steel-supported zeolite membranes can be more susceptible to thermally induced cracks and to adhesion problems. The problem of thermal expansion coefficient differences can be ameliated by the introduction of a ceramic intermediate layer between zeolite layer and support, e.g. TiO2 [33]. The above-mentioned factors make it harder to produce thin defect-free zeolite membranes directly on stainless steel compared to ceramic supports. However, stainless steel possesses the important advantage of ductility and is compatible with the most commonly used

plant equipment parts. Its better heat conductivity can be advantageous for membrane reactor applications involving heat transfer to or from the reaction region. Due to their brittleness, ceramic materials also possess fewer possibilities for sealing, especially at high temperature. Saracco et al. [34] identified the issue of sealing at high temperatures as a major hurdle for the industrial application of membrane reactors. H2/SF6 single gas permeation measurements are commonly used to characterise MFI membranes [6]. SF6 has a kinetic diameter of 0.55 nm, which is a size comparable to the pore dimensions of MFI [35,36]. The kinetic diameter of H2 is significantly smaller (0.29 nm). Thus a membrane with high ideal perm-selectivity, i.e. a high ratio of H2 to SF6 permeance, is considered to be of good quality (low number of defects). However, the permeances of H2 and SF6 possess different dependencies on temperature [17,37], upstream/downstream pressure [18,38], the presence of sweep gas [39–41] and adsorbed molecules in the zeolite layer [42]. Thus, only permselectivities measured under the same conditions should be compared. In the present study, we show that in addition to permeance measurement conditions, the membrane zeolite layer thickness has to be taken into consideration. This is because an increased zeolite layer thickness results in a stronger increase of transport resistance for SF6 than for H2. The advantages of stainless steel make the development of high-quality stainless-steel-supported zeolite membranes for high-temperature applications desirable, despite the above-mentioned preparative difficulties. Comparing membranes characterised under similar conditions, the H2/SF6 ideal perm-selectivities of the Silicalite-1 membranes produced in this work are comparable to those of ceramic-supported membranes reported in the literature, whereas the perm-selectivities of Al-ZSM-5 membranes are even higher for some membranes.

2. Experimental 2.1. Materials The porous, asymmetric 316L stainless steel support discs (18 mm diameter and 270 nm average pore size according to capillary flow porometry) used in this study were purchased from GKN Sinter Metals Filters GmbH. Tetraethyl orthosilicate (TEOS, 98 wt.%) and tetrapropylammonium hydroxide (TPAOH, 1.0 M) were obtained from Aldrich. The aluminium nitrate (Al(NO)3 Æ 9H2O, 95 wt.%) used as an aluminium source for Al-ZSM-5 synthesis was purchased from Merck. Permeation experiments were carried out using hydrogen (purity 5.0) and sulphur hexafluoride (purity 3.0) from Linde.

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2.2. In situ seeding The synthesis mixture used for in situ seeding had the molar composition 1TEOS:0.32TPAOH:29H2O. After the supports had been cleaned with boiling water they were placed at the bottom of 60 ml Teflon-lined steel autoclaves with the fine surface to be coated facing upwards. About 15 g of (clear) synthesis mixture were charged into each autoclave on top of the supports. The autoclaves were then placed in a pre-heated convection oven for crystallisation at 433 K for 48 h. After synthesis the seeded supports were washed in stirred boiling water in a beaker for 15 min. They were then rinsed in ethanol and stored in a dry desiccator with sulphuric acid as drying agent for later use. The excess powder products (formed within the liquid phase and on Teflon surfaces within the autoclave) were centrifuged, washed with water to neutrality and dried at 373 K for at least 12 h. 2.3. Membrane synthesis and post-treatment The membranes were synthesised in the same autoclaves used for seeding. Home-made Teflon holders were used to suspend the seeded supports in the autoclaves with the fine side facing downwards to avoid sedimentation of homogeneously nucleated crystals onto the membrane surface during crystallisation. The support discs were slightly slanted in order to avoid the trapping of bubbles formed in the reaction mixture on the disc underside during the synthesis [19]. Teflon discs covered the coarse side of the supports to hinder crystallisation there. The composition of the (alkali-free) synthesis mixture was 1TEOS:xAl2O3:0.11TPAOH:110H2O, with x = 0 for the preparation of Silicalite-1 and x = 0.0175 for AlZSM-5 membranes. About 20 g of synthesis mixture were used for each membrane. The membranes were crystallised at three different crystallisation temperatures (438, 448 and 458 K) and the crystallisation time varied between 2 and 48 h to study the membrane growth kinetics. Excess products were filtered, washed with water to neutrality and then dried at 373 K for at least 12 h. After washing with boiling water for 1 h, the membrane samples were rinsed in ethanol and then left to dry in a desiccator at least 12 h before further processing. Membrane calcination was performed at 673 K for 16 h using a heating rate of 0.5 K min 1 and a cooling rate of 1.0 K min 1. A sweep stream of nitrogen was passed over the membranes during calcination to avoid adsorption of moisture and other laboratory air contaminants during cooling and after the calcination procedure.

211

X-ray diffractometer using Cu-Ka radiation. Scanning electron microscopy (SEM) pictures of calcined membrane samples were taken using a JEOL JSM 6400 after carbon or gold coating, at an acceleration voltage of 20 kV and a working distance of 10 mm. After SEM analysis of the top surface, the cross-sections of all the membranes were prepared by embedding the membranes in epoxy resin, cutting, grinding and polishing. Excess zeolite powder formed in the bulk phase during the seeding procedure was also analysed by thermogravimetric analysis (TGA) using an SDT 2960 Simultaneous DSC–TGA from TA Instruments to determine the template and moisture content. About 10 mg of sample were heated at 10 K min 1 to 1273 K in a 120 ml min 1 synthetic air stream. The particle size distribution of the excess product obtained during seeding was determined using photon correlation spectroscopy (Malvern Zetasizer 3000). Calcined excess powder products obtained during membrane synthesis were also analysed by N2 adsorption after conditioning at 573 K using an automated nitrogen adsorption analyzer (Micromeritics, ASAP 2010). The elemental compositions of excess Al-ZSM-5 powders were obtained using atomic emission spectroscopy inductively coupled plasma analysis (AES–ICP, Perkin-Elmer 400). 2.5. Permeation experiments Single gas permeation measurements were carried out after calcination as described above. The gas permeances were measured at room temperature and at 1 bar trans-membrane pressure difference using the dead-end method. Fig. 1 shows a sketch of the permeation cell. The sealing concept used an EPDM O-ring in contact with the membrane disc edge. Placing the zeolite layer on the high-pressure side of the permeation cell insured that the layer-support attachment was not put under undue strain. The volume flow rate of hydrogen was measured before that of the stronger adsorbing sulphur

2.4. Physicochemical characterisation The membranes and excess products were analysed by X-ray diffraction (XRD) with a Philips Analytical

Fig. 1. Sketch of the permeation cell used for single gas permeation measurements.

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hexafluoride. Soap film flow meters attached to the permeation cell outlet were used. Before the values were measured, the permeation apparatus was purged with the respective probe gas, even on the downstream side, to avoid back-permeation of air during permeation measurements. For both gases, the flux was allowed to stabilise for 1 h before permeation readings were taken.

2.6. Zeolite layer adhesion tests Air jet and thermal shock tests were carried out on three calcined Silicalite-1 membranes synthesised for 48 h at 448 K using the procedure described above. This was done to obtain a qualitative measure of the adhesion of the zeolite layers to the supports. Before carrying out the air jet tests [43] the membranes were allowed to equilibrate under ambient conditions. After no more weight change could be observed their initial weights were determined on a precision scale. They were then exposed to a pressured air stream (6 bars up-stream pressure, 295 K) from a flexible tube (di = 4.6 mm) placed 2 cm in front of the membrane surface for 5 min. The membranes were then allowed to re-equilibrate under ambient conditions before being weighed again. Silicalite-1 membranes were used in order to minimise falsification of results due to adsorbed moisture from the laboratory air (this is more pronounced in hydrophilic Al-ZSM-5 membranes). Prior to carrying out temperature shock tests H2/SF6 single gas permeation rates were measured as described above. The membranes were then heated rapidly to 773 K in a muffle oven. The hot membranes were quickly removed from the muffle oven and quenched immediately with running water at 288 K. After 10 h of re-calcination at 573 K with 1 K min 1 ramp and N2 sweep gas, H2/SF6 single gas permeation rates were again measured. The membranes were also analysed by SEM to evaluate thermally induced defects.

3. Results and discussion 3.1. Seeding After in situ seeding and washing, the support surface (Fig. 2a) is densely covered with small Silicalite-1 crystals (Fig. 2b). The crystals possess a predominantly cubic morphology as is commonly reported for concentrated MFI synthesis mixtures [44]. Excess product formed in the liquid bulk sediments onto the supports but can be easily washed off after synthesis due to a lack of crystal intergrowth. This lack of intergrowth is probably due to the high synthesis mixture concentration which results in the formation of a large number of nuclei per unit volume. The growing crystals rapidly consume nutrients from the surrounding liquid phase, to such an extent that no nutrients remain for extensive crystal growth and intergrowth. After washing off loosely attached crystals, the remaining zeolite seeds on the support surface shown in Fig. 2b are not visible to the naked eye. The number-based particle size distribution of the product excess obtained from photon correlation spectroscopy measurements showed a maximum at about 400 nm, in good agreement with SEM observations on the seeded support surface (Fig. 2b). As discussed above, the fairly small crystal size and extensive support coverage are a result of the high template concentration and low water content used during seeding. A total zeolite yield of 91 wt.%, in relation to the silica used for synthesis, is obtained. According to XRD analyses the resulting zeolitic phase is composed of pure MFI, both on the support surface as well as in the liquid bulk (excess product), as we reported previously [24]. 3.2. Membrane growth In both Silicalite-1 and Al-ZSM-5 membranes the resulting zeolite layers are optically transparent and typically no zeolite sediment is observed on the support

Fig. 2. SEM surface images of the (a) support surface prior to seeding, (b) seeded support surface.

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surface. Optical transparency is attributed to a lack of light-scattering defects in the zeolite layer [45] and is a rough indication of good membrane quality. Figs. 3 and 4 show SEM micrographs of calcined membranes synthesised at 448 K and for various crystallisation times for Silicalite-1 and Al-ZSM-5 respectively. The densely packed seeds initially present on the support surface (Fig. 2b) undergo secondary growth during the second synthesis cycle. Crystal size gradually increases and intergrowth between neighbouring crystals inevitably results due to space limitations. A distinct seed layer is not observed in the cross-sections of the composite membranes showing that all the seed crystals either participate in secondary growth or are dissolved in an

213

Ostwald ripening process. No cracks in the zeolite layers (due to calcination) were observed. The gaps between the zeolite layers and the epoxy resin visible in the cross-section micrographs are due to shrinkage of the epoxy resin used for sample embedding while it dries during sample preparation. The final zeolite layers are almost monolithic in nature, more so in the case of the Al-ZSM-5 membranes than the Silicalite-1 membranes, as can be seen in the cross-section view of the membranes in Figs. 3 and 4d–f. Although some isolated laterally oriented crystals are observed up to 4 h of crystallisation time, probably due to secondary nucleation (see Fig. 4a), preferred axial orientation of the MFI crystals (0 0 h and h 0 h directions

Fig. 3. SEM surface (a–c) and cross-section (d–f) images of Silicalite-1 membranes synthesised at 448 K after (a and d) 4 h, (b and e) 12 h, (c and f) 24 h of crystallisation.

Fig. 4. SEM surface (a–c) and cross-section (d–f) images of Al-ZSM-5 membranes synthesised at 448 K after (a and d) 4 h, (b and e) 12 h, (c and f) 24 h of crystallisation.

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(002)

(020) (200)

(101)

9

10

11

12

13

48 h

Linear scale

perpendicular to the support surface) develops at a fairly early stage in the membrane crystallisation and progressively becomes more pronounced. This is commonly observed in seeded MFI membrane synthesis and is attributed to the relatively fast growth of the c-direction in comparison to the other crystallographic directions under the prevailing conditions [8]. Thus the surfaces of the final membranes are almost wholly composed of intergrown 101 crystal faces (Figs. 3 and 4a–c). The surface views also show that the Silicalite-1 membranes are characterised by a more angular surface morphology (well-shaped MFI crystals) than the Al-ZSM-5 membranes. Fig. 5 shows that the zeolite layer growth rate is constant up to about 12 h crystallisation time. The zeolite top layer initially grows at 1.5 lm h 1 for the Silicalite1 membranes compared to 0.5 lm h 1 for Al-ZSM-5. The slower growth rate of the Al-ZSM-5 membranes can be attributed to the lower pH of the Al-ZSM-5 synthesis mixture of about 8.3 compared to 10.9 for the Silicalite-1 synthesis mixture resulting in a lower solubility of the silica source. The presence of aluminium is also known to retard silica dissolution [46] and zeolitisation [47]. With progressive crystallisation time the layer growth rates for both membrane types decrease, probably due to nutrient exhaustion and re-dissolution phenomena. The XRD patterns of the surfaces of the membranes (Figs. 6 and 7) confirm the development of the crystal orientation observed with SEM. The intensity of the 0 0 2 peak (2h  13.3) continuously increases with crystallisation time. However, the 1 0 1 peak intensity (2h  8.1) goes through a maximum which is shifted towards a shorter crystallisation time for the faster growing Silicalite-1 membranes. The Silicalite-1 membranes also attain a more pronounced preferred axial orientation. For example, the ratio of the 0 0 2 peak height

24 h 12 h 8h 4h 2h

5

10

15

20

25

30

2θ [°] Fig. 6. XRD patterns of the surfaces of calcined Silicalite-1 membranes synthesised at 448 K after varying crystallisation times.

(002)

(200) (020) (101)

9

10

11

12

13

48 h 24 h

Linear scale

214

12 h 8h 4h 2h

5

10

15

20

25

30

2θ [°] 50

Fig. 7. XRD patterns of the surfaces of calcined Al-ZSM-5 membranes synthesised at 448 K after varying crystallisation times.

Silicalite-1 Al-ZSM-5

membrane thickness [ µm]

40

30

20

10

0 0

10

20

30

40

50

crystallisation time [h]

Fig. 5. Membrane thickness development as observed with crosssectional SEM.

(2h  13.3) to the sum of the 0 2 0 and 2 0 0 peak heights (2h  8.8 and 8.9 respectively) for membranes synthesised for 48 h at 448 K is 35 and 2 for Silicalite-1 and for Al-ZSM-5, respectively. SEM observations of excess powder products (Fig. 8) obtained during membrane synthesis show that the Silicalite-1 crystals are much more elongated (higher c/a and c/b ratios) than the AlZSM-5 crystals. This comparison demonstrates the influence of the relative growth rates of the different crystallographic directions on the crystal orientation in the final membrane. A comparison of SEM views of the membranes (Figs. 3 and 4) with measurements of the excess products (Fig.

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215

Fig. 8. SEM images of excess powder product obtained during membrane synthesis at 448 K for Silicalite-1 after (a) 4 h, (b) 12 h and (c) 24 h of crystallisation time and for Al-ZSM-5 after (d) 4 h, (e) 12 h and (f) 24 h of crystallisation time.

Crystallisation temperature:

10-5

Permeance [mol s-1 m-2 Pa-1]

8) reveals that especially for the Al-ZSM-5 synthesis procedure, crystal growth within the bulk phase is significantly slower. XRD measurements on the excess products confirm these results (figures not shown). Unlike membrane growth (Fig. 5) excess product crystallisation possesses a considerable incubation phase, especially for Al-ZSM-5. This is probably due to the absence of seeds in the bulk phase, except for a few crystals that detach from the support surface during membrane preparation. Atomic emission spectroscopy (AES–ICP) measurements carried out on the excess Al-ZSM-5 products (24 h crystallisation at 448 K) resulted in a Si/Al ratio of 50.

438 K 448 K 458 K

10-6

H2

10-7

10-8

SF6

10-9 0

10

20

30

40

50

Crystallisation time [h]

3.3. H2/SF6 single gas permeation and N2 adsorption Fig. 9 shows the H2 and SF6 permeances of Al-ZSM5 membranes synthesised for different crystallisation times and at three different temperatures. The permeances of both gases decrease with increasing crystallisation time as a result of the increasing zeolite layer thickness and/or intergrowth and, consequently, increasing transmembrane transport resistance. However, in relative terms (note the logarithmic scale of Fig. 9), there is a stronger decrease in the permeance of SF6 than of H2. SF6 is more affected by membrane thickness and crystal intergrowth than H2 since its kinetic diameter is comparable to the pore size of MFI. Hydrogen, however, has a significantly smaller kinetic diameter and its permeance is not as drastically affected by increasing membrane thickness and intergrowth (increasing crystallisation time and temperature). This translates to an increase of ideal perm-selectivity with increasing crystal-

Fig. 9. H2/SF6 single gas permeances of Al-ZSM-5 membranes synthesised at 438, 448 and 458 K in dependence of crystallisation time.

lisation time and temperature (Fig. 10). The H2/SF6 ideal perm-selectivity reaches a value of 356 for a membrane crystallised for 48 h at 458 K. This high value is probably due to both the relatively high layer thickness (10 lm according to cross-sectional SEM) and low number of defects. Another factor that probably contributes to the difference in permeances calculated for SF6 and H2 is the difference in the adsorption behaviour between the two probe molecules. Thus an analysis of the adsorption isotherm for SF6 on Silicalite-1 powders at room temperature shows that the adsorbent is nearly saturated at 1 bar (absolute) SF6 pressure, as we showed in a previous report [17]. Separate measurements showed that the same applies for Al-ZSM-5 powders (figure not shown),

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H2/SF6 Perm-Selectivity [-]

400

Permeance [mol-1sm-2 Pa-1]

Crystallisation temperature: 458 K 448 K 438 K

300

200

100

10

-5

10

-6

10

-7

10

-8

10

-9

H2 SF6 PS

70 60 50 40 30 20 10

0

10

20

30

40

50

H2/SF6 Perm-Selectivity [-]

216

0

Crystallisation time [h]

0 0

10

20

30

40

50

Crystallisation time [h]

Fig. 10. H2/SF6 ideal perm-selectivities of Al-ZSM-5 membranes synthesised at 438, 448 and 458 K in dependence of crystallisation time.

albeit at a lower maximum adsorption capacity. Under the conditions used for permeation in this work (1 bar above atmospheric pressure on the high pressure side and atmospheric pressure on the downstream side), the upstream membrane side is likely to have only a slightly higher SF6 loading than the downstream side. Thus the effective driving force for permeation of SF6 is not likely to be very high, except for membranes with large defects. However, the SF6 loadings of the membranes characterised in this work should be comparable since all membranes were characterised under the same permeation conditions. Thus, the drastic decrease of SF6 permeance with increasing membrane crystallisation time and temperature discussed above and demonstrated in Figs. 10 and 11 is not caused by differences in SF6 loadings in the membrane layers. For comparison, the H2 and SF6 single gas permeation characteristics of Silicalite-1 membranes synthesised at 448 K are shown in Fig. 11. The H2 permeances are comparable to those of the Al-ZSM-5 membranes. However, the SF6 permeances are higher, leading to lower H2/SF6 perm-selectivities. This is somewhat surprising, since the Silicalite-1 membranes are actually thicker than the Al-ZSM-5 membranes (Figs. 3 and 4d–f). The lower perm-selectivities measured on the Silicalite-1 membranes imply higher defect densities in these membranes, perhaps due to imperfect crystal intergrowth and/or intra-crystalline defects. The Al-ZSM-5 layers indeed appear to be more intergrown than the Silicalite-1 membrane layers in the SEM micrographs shown in Figs. 4 and 3 respectively. However, it cannot be ruled out that differences in adsorption characteristics between Silicalite-1 and Al-ZSM-5, as well as the presence of charge balancing cations within the Al-ZSM-5 pores can have an effect on SF6 permeation.

Fig. 11. Single gas permeances and ideal perm-selectivities (PS) of Silicalite-1 membranes synthesised at 448 K in dependence of crystallisation time.

N2 adsorption measurements carried out on the excess powder products obtained during membrane crystallisation are in line with the results of the gas permeation and SEM measurements discussed above. Fig. 12a shows the N2 adsorption–desorption isotherms obtained for excess Silicalite-1 and Al-ZSM-5 powders (obtained during membrane synthesis) respectively, crystallised for 48 h at 448 K. The Silicalite-1 adsorption–desorption isotherm shows a hysteresis loop between the adsorption and the desorption curves in the region P/P0  0.15–0.3. The occurrence of a hysteresis loop indicates the presence of cavities larger than the zeolite pores. These can be intra- [48] or inter-crystalline [49]. Such a distinct hysteresis is not visible in the isotherm of the Al-ZSM-5 sample. A comparison of the mesopore distribution of the samples obtained with the Barret, Joyner and Halenda (BJH) method in Fig. 12b shows the presence of mesoporous cavities in the Silicalite-1 powder with a maximum at about 2.1 nm. There is hardly any mesoporosity determined with the BJH method in the Al-ZSM-5 sample. It is pertinent to assume that the Silicalite-1 membrane layers also contain the (mesoporous) defects observed in the excess powder products, leading to a lowering of H2/SF6 ideal perm-selectivities measured in these membranes. Fig. 13 compares the dependence of the H2/SF6 ideal perm-selectivity on the zeolite layer thickness for Al-ZSM-5 and Silicalite-1 membranes synthesised at 448 K. Above a certain membrane thickness, the permselectivity increases linearly with the zeolite layer thickness for both membrane types. The Silicalite-1 membrane perm-selectivity line has a lower slope. The fact that the perm-selectivities of both membrane types show a linear dependency on the zeolite layer thickness implies that, for both membrane types, the above-mentioned defect density does not vary with membrane thickness.

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217

Fig. 12. N2 adsorption on the excess powder products obtained during Al-ZSM-5 and Silicalite-1 membrane syntheses; 48 h crystallisation at 448 K: (a) adsorption isotherms (77 K) (b) mesopore distribution according to the Barret, Joyner and Halenda (BJH) method.

H2/SF6 Perm-Selectivity [-]

150

Al-ZSM-5 Silicalite-1

125 100 75 50 25 0 0

10

20

30

40

50

Zeolite layer thickness [µm] Fig. 13. H2/SF6 ideal perm-selectivities of Silicalite-1 and Al-ZSM-5 membranes synthesised at 448 K as a function of SEM zeolite layer thickness.

Table 1 shows H2/SF6 permeation results of membranes from the literature with comparable MFI layer thicknesses (according to SEM). Caution has to be exercised when comparing perm-selectivity values measured by different groups in the literature since the driving force for gas permeation differs according to the measurement conditions, especially for the stronger adsorbing SF6, as discussed in the introduction. The permeation measurement results summarised in Table 1 were obtained under similar conditions to those used in this work. The Silicalite-1 membranes prepared in this work have thickness-normalised ideal H2/SF6 permselectivities (PS/dMFI) comparable to those of Silicalite1 membranes reported in the literature. The 7.5 lm thick Al-ZSM-5 membrane shows an even higher normalised perm-selectivity value (PS/dMFI = 18 lm 1) than the 10 lm ceramic-supported membranes reported by Bonhomme et al. (PS/dMFI = 5 lm 1) [50]. This emphasizes the high quality of the membranes produced in this

Table 1 H2 permeances and H2/SF6 ideal perm-selectivities (PS) of disc membranes from the literature with different SEM zeolite layer thicknesses (dMFI) in comparison to the data of membranes prepared in this work (448 K crystallisation temperature) Membrane Silicalite-1/a-Al2O3 Silicalite-1/a-Al2O3 Silicalite-1/stainless steel Al-ZSM-5/stainless steel Silicalite-1/a-Al2O3 Silicalite-1/stainless steel Al-ZSM-5/stainless steel

dMFI (lm)

H2 permeance · 107 (mol m 2 Pa

0.5 1 0.5 1 10 10 7.5

219 27.7 13.8 9.62 2.40 7.25 2.16

1

s 1)

H2/SF6-PS (–)

PS/dMFI (lm 1)

Ref.

17 14 9 20 48 40 136

34 14 18 20 5 4 18

[19] 50(2 + 1) in [23] This work (2 h crystallisation) This work (4 h crystallisation) 18 A in [50] This work (8 h crystallisation) This work (48 h crystallisation)

Measurements were carried out at room temperature with 1 bar pressure drop across the membrane, atmospheric pressure on the downstream side and no sweep gas.

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Table 2 H2 permeance and H2/SF6 ideal perm-selectivity measurements of identically prepared Al-ZSM-5 membranes synthesised at 448 K for 24 h Membrane H2 permeance · 107 (mol m H2/SF6 perm-selectivity (–)

2

Pa

1

s 1)

1

2

3

4

5

6

3.0 117

4.6 122

4.8 137

3.7 176

3.7 174

6.7 120

work. The H2 permeances of our membranes are also comparable with most literature reports. The much higher H2 permeance of the membrane reported by Hedlund et al. [19] is probably due to the support masking technique used during membrane preparation to avoid zeolite formation within the support pores. 3.4. Membrane synthesis reproducibility Reproducibility can be a problem in zeolite membrane preparation. Ideal perm-selectivity values reported in the literature often vary dramatically, even for membranes synthesised under identical conditions. Lassinantti et al. [51], for example, reported N2/SF6 ideal perm-selectivities ranging from 4.4 to >2000 for a set of membranes synthesised under identical conditions. They attributed these variations to synthesis reproducibility difficulties and the influence of adsorbed species present in the Al-ZSM-5 pores. The reproducibility of the membrane synthesis procedure used in this work is demonstrated in Table 2. Six Al-ZSM-5 membranes prepared using 24 h of crystallisation at 448 K have H2 permeances in the range of 3.0–6.7 · 10 7 mol Pa 1 s 1 m 2 and ideal H2/SF6 perm-selectivities in the range of 117–176. 3.5. Zeolite layer adhesion No weight loss is detected on any of the membranes after the air jet tests. Visual observations and SEM measurements show that the thermal shock tests do not result in any peeling off of the zeolite layer, despite the large difference in thermal expansion coefficients between the zeolite layers and the stainless steel supports. These results confirm strong adhesion to the stainless steel support and good intergrowth within the zeolite layers as already indicated by the SEM observations in

Figs. 3 and 4. This strong adhesion is probably due to good mechanical anchoring of the zeolite layer on the support caused by the rough nature of the support surface and zeolite crystallisation within the support. Table 3 shows the permeance of the three Silicalite-1 membranes used for the air jet and temperature shock tests before and after these tests. The permeances of H2 and SF6 increase because of the treatment and the perm-selectivities mostly decrease. This should be due to the formation of thermally induced defects. The fact that the perm-selectivities of the first two membranes did not decrease significantly is probably due to the high thickness of the membranes used (45 lm) which means that the transport resistance for SF6 permeation remains considerably high, even after the thermal shock treatment. The slight increase in the perm-selectivity of membrane 2 in Table 3 lies within the error margins of the H2/SF6 permeance measurements (about ±10% cf. [52]).

4. Conclusion The growth of Al-ZSM-5 and Silicalite-1 membranes on relatively rough porous stainless steel supports (dP  270 nm) was studied. Much higher H2/SF6 ideal perm-selectivities were achieved for Al-ZSM-5 membranes compared to Silicalite-1 membranes synthesised under similar conditions, despite the higher thickness of the Silicalite-1 layers. This was perhaps due to the detrimental effect of excessively fast crystal growth during Silicalite-1 synthesis resulting in elongated, well-shaped crystals with distinct inter-/intra-crystalline cavities present in the membrane layers. SEM, permeation experiments and adsorption measurements indicated that these layers had higher defect densities than the Al-ZSM-5 layers. Different adsorption behaviours and the presence of charge balancing cations in

Table 3 H2 permeance and H2/SF6 ideal perm-selectivity values of identically prepared Silicalite-1 membranes (synthesised at 448 K, 48 h) before and after thermal shock tests Membrane

1 2 3

Before temperature shock

After temperature shock

H2 permeance · 107 (mol m 2 Pa 1 s 1)

H2/SF6 perm-selectivity (–)

H2 permeance · 107 (mol m 2 Pa 1 s 1)

H2/SF6 perm-selectivity (–)

5.1 3.5 3.7

70 49 57

5.5 5.4 6.0

55 54 5

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Al-ZSM-5 can, however, not be ruled out as having an influence on SF6 permeance. Temperature shock tests showed that the zeolite layers were well attached to the supports. The reproducibility of the membrane synthesis was also evaluated and found to be good. A high H2/SF6 ideal perm-selectivity value of 356 was achieved for a 10 lm thick Al-ZSM-5 membrane crystallised for 48 h at 458 K. Comparison of H2/SF6 permeation results with literature data showed that the membranes prepared in this work are of good quality.

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[25]

Acknowledgments The authors wish to thank the Deutsche Forschungsgemeinschaft (projects SCHW 478/11-3 and DI 696/4-3) for financing this work and Prof. Dr. J. Caro (Institut fu¨r Physikalische Chemie und Elektrochemie, Universita¨t Hannover), Dr. Noack (Institut fu¨r Angewandte Chemie Berlin-Adlershof e.V.) and Prof. Tsapatsis (University of Minnesota) for advice. Special thanks go to Mr. P. Widlok for adsorption measurements.

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