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Texture of MFI films grown from seeds
Current Opinion in Colloid & Interface Science 10 (2005) 226 – 232 www.elsevier.com/locate/cocis
Texture of MFI films grown from seeds Jonas Hedlund *, Fredrik Jareman Division of Chemical Technology, Lulea˚ University of Technology, S 97187 Lulea˚, Sweden Available online 27 October 2005
Abstract This review describes how the texture of MFI films grown from seed crystal is developed during film preparation. Reports published during the last 5 years are in focus. Relative growth rates in various crystallographic directions, competitive growth, properties of the seed layer, defects, grain boundaries and other parameters influencing the film properties are discussed. Mathematical models describing competitive growth are also discussed. Suitable characterization methods for defects are described. The last part of the review is devoted to diffusion. Diffusion models accounting for texture in MFI films and the influence of texture on diffussion are discussed. D 2005 Elsevier Ltd. All rights reserved.
1. Major recent advances During the last 5 years, the following major advances (in chronological order) have been reported in the field of MFI films grown from seeds. A novel seeding method, by chemical anchoring of the seeds, has been described. Fluorescence confocal microscopy and permporosimetry have been proven very useful tools for characterization of defects and grain boundaries in MFI membranes. The successful preparation and evaluation of 0.5-Am MFI membranes on graded alumina supports and 1-Am b-oriented membranes on non-graded alumina supports have been reported. 2. Introduction Zeolite films have great potential in a number of technologically sophisticated applications, such as membranes, sensors, catalyst and adsorbents [1 –4]. MFI zeolite has pores close to the kinetic diameter of several industrially important hydrocarbons, which explains the great scientific interest for this particular zeolite. The MFI structure is non-isotropic with sinusoidal pores ˚ ) running in the a-direction and slightly (diameter ca. 5.5 A wider straight pores running in the b-direction and connections between the two pore systems create a three-dimensional network of pores (see Fig. 1a).
* Corresponding author. Tel.: +46 920 492105; fax: +46 920 491199. E-mail address: [email protected] (J. Hedlund). 1359-0294/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2005.09.009
Supported MFI films can be grown by several techniques [1]. Film growth by the in situ method and methods involving pre-seeding of the support are most frequently reported. Preparation of supported films by the in situ method is accomplished by hydrothermal treatment of the support in a synthesis mixture. During hydrothermal treatment, crystals nucleate and grow simultaneously on the support and a dense film may form. In methods involving pre-seeding of the support, the support is seeded prior to film growth. The nucleation and crystal growth are carried out in separate steps and may thus be controlled separately, which may be an advantage. The texture of films grown from seeded supports is developed during seeding and film growth. Cracks and other defects may form in the film during or after film growth. Since the texture has a significant impact on the performance of the film in the applications mentioned above, this review article starts with a discussion on texture in MFI films grown on preseeded supports. The following part describes techniques for detection of defects since these will also affect the film properties. Diffusion in MFI films grown on seeded supports is discussed in the last part. This review will focus on work reported during the last 5 years. 3. Crystal habit MFI crystals display a number of crystals shapes or habits (see Fig. 1). The different habits result from differences in growth rates in varying directions of the crystal. The growth rate is highest in the c-direction in many systems, resulting in
J. Hedlund, F. Jareman / Current Opinion in Colloid & Interface Science 10 (2005) 226 – 232
Fig. 1. A schematic drawing of an MFI crystal with a coffin habit with the pore structure and crystallographic axes and planes indicated (a) and a corresponding SEM image (b). A schematic drawing of a MFI twin crystal (c) and an SEM image of such crystals (d).
elongated crystals. If the {h0h} faces are well developed, crystals with a habit referred to as Fcoffin shape_ result (see Fig. 1a and b). The crystals are thinnest in the b-direction and the ratio of the lengths in the c- and a-directions can vary, resulting in wider or narrower coffins. Often, no well-defined {h0h} faces are formed, but rounded crystals are observed. In these systems, Ftwin_ crystals, or more precisely F90- rotational intergrowths,_ are common (see Fig. 1c and d). High-silica MFI crystals often display this habit. It is also possible to grow MFI crystals with no or few twins and no well-defined {h0h} faces, i.e., with a Fpill_ habit (see Fig. 16D in Ref. [5&&]). The possibility to control the habit of MFI crystals by applying various structure-directing agents and amines has been explored since several years ago.1 A recent example of habit control by the addition of amines is the work by Ban et al. [6&]. Bonilla et al. [7&] recently published a follow-up study of habit control by using dimers and trimers of TPAOH. By this approach, it is possible to grow MFI crystals with drastically different habits compared to the crystals grown using TPAOH monomer. For instance, it is possible to grow single crystals (without twins) that are relatively thick in the b-direction, which may be utilized to grow b-oriented films (see below). When seed crystals are dispersed in a synthesis mixture and hydrothermally treated, the habit of the resulting crystals is dictated by the relative growth rates during that particular hydrothermal treatment. Fig. 1b and d show relatively large crystals with coffin and twin habit, respectively, grown by two completely different hydrothermal treatments of the same silicalite-1 seeds [8&]. 4. Seed layer A number of methods for deposition of MFI seeds in monoor multilayers have been developed. Deposition of monolayers 1
See Ref. [6&] and [7&].
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may rely on electrostatic attraction or chemical bonds between seeds and support. For many applications, such as sensors and membranes, very thin films are advantageous. The response time of a sensor and the flux in a membrane are to a first approximation proportional and inversely proportional, respectively, to the film thickness. The thickness of the seed layer sets the lower limit for the film thickness. A monolayer of seeds is the thinnest possible seed layer, which may be beneficial. One method for monolayer deposition has been developed and patented by Sterte et al. [9]. This method relies on electrostatic attraction of the seeds to the surface by cationic polymer molecules. This method may produce a monolayer of seeds, which is smoother than the support surface [10&&] (see Fig. 2a and b). By repeating polymer and seed adsorption, it is possible to deposit bi-layers [8&] or even multi-layers [11]. This method is flexible and has been used to coat many types of supports with varying chemical composition and geometry. Ha et al. [12&&] have developed a method for chemical anchoring of zeolite crystals to a support. In this method, a halopropylsilyl compound is used to attach seeds to the support by covalent bonds. The halopropylsilyl compound is very sensitive to moisture and the work must be carried out in a dry environment [5&&], which may be a complication. This method has been used by Lai et al. [5&&] to deposit monolayers of boriented MFI crystals with the pill habit. Exxon Chemical [13,14] and Lovallo and Tsapatsis [15] have independently developed methods for deposition of multilayers of MFI seeds relying on dip- or spin-coating techniques. One of these methods has been used frequently by Tsapatsis et al. [15 – 17] and by several other groups [18,19&,20– 23&] for the preparation of MFI membranes. The thickness of the seed layer can be controlled by varying the concentration of the seed sol or the slip-casting time [24]. It should be noted that Yan et al. have proposed a mechanism [25&&,26] for growth of MFI films by the in situ method. In this case, colloidal crystals first nucleate and grow in the synthesis mixture. A film is formed subsequently by deposition and self-assembly of these crystals on the support. The crystals grow and re-orient to form a dense film. According to this mechanism, the in situ method and methods relying on seeding are actually very similar. 5. Growth of MFI films on seeded supports MFI films can grow on a support seeded with MFI crystals in a synthesis solution free from organic template molecules in which no zeolite forms in the absence of seeds [8&]. This shows that the seeds may control the phase of the growing film. Another example is that the same hydrothermal treatment can be used to grow MOR and MFI films by applying the appropriate seed crystals [19&]. If secondary nucleation is low and crystals are not attached from the bulk of the synthesis mixture, the competitive growth mechanism [27] will control the growth of the zeolite film. Competitive growth will lead to funnel-shaped crystals extending from the seed layer to the film top surface and encapsulation of other crystals (see Fig. 2c).
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Fig. 2. SEM top view images of a porous a-alumina support prior to (a) and after seeding with a monolayer of 60-nm silicalite-1 crystals (b). A SEM side view image of a silicalite-1 film comprised of funnel-shaped crystals extending from the support to the film surface and embedded crystals (c).
The habit of the seeds will influence the preferred orientation of the crystals in the film [8&]. By combining the effects of oriented seed layers and competitive growth, it is possible to obtain MFI films with almost any orientation. A ca. 1-Am-thick a-oriented (or (h00) oriented) film [8&] can be
grown by treatment of a mono-layer of MFI twin crystals in a synthesis mixture where the fastest growing direction is the aV-direction of the twin (see Fig. 3I). This approach was used [10&&] to grow 500-nm MFI membranes with very high flux on graded alumina supports. However, at this film thickness,
Fig. 3. A schematic drawing of an a-oriented film grown from a monolayer of twin crystals (seeds) (I). Most of the larger parts of the seeds are b-oriented. The crystallographic directions, habit of seeds grown in the synthesis mixture and film thickness are indicated. A b-oriented film grown from seeds with pill habit (II). A thick c-oriented film grown from a monolayer of twin crystals (III). A film with oblique orientation forms earlier in the competitive growth process (IV).
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the film is not yet fully a-oriented since it is still early in the competitive growth process. If the seeds have a pill habit, the seed crystals will mostly be b-oriented. If the crystals in this layer are grown without the formation of twins or attachment of differently oriented crystals, a b-oriented (or (0k0) oriented) film [5&&,28&,29&] will form as illustrated in Fig. 3II. Tsapatsis et al. has used this approach to grow 1-Am boriented MFI membranes on non-graded alumina supports. These membranes had high flux of p-xylene and very high pxylene/o-xylene selectivity. In systems where crystals elongated in the c-direction are formed in the bulk of the synthesis mixture, thick films of c-oriented (or (00l) oriented) crystals will develop even from multilayers of randomly oriented seeds [16]. As illustrated in Fig. 3III, c-oriented films should also develop from monolayers of weakly oriented seeds provided that sufficiently elongated crystals are formed. Films with {h0h} preferred orientation, appropriately denoted Foblique_ orientation by Bons and Bons [30&&], will develop earlier in the competitive growth process [16], before the formation of a c-oriented layer (see Fig. 3IV). Bonilla [31&] and Bons [30&&] have numerically modeled competitive growth resulting in c- and oblique orientation. Both groups have used a similar approach, i.e., a marker particle front tracking technique, although Bons model also allowed curved interfaces between crystals. Crystals with the coffin habit are assumed to grow in the system and random orientation of the seed crystals was also assumed. It was shown that a corientation will develop in thick films (30 Am) and that oblique orientation may dominate in thinner films (10 Am), especially when Fwider_ coffins with a larger a/c ratio are formed in the bulk, as indicated in Fig. 3IV. Numerous experimental studies of the growth of MFI films on seeded supports have been reported [8&,24,32&] and some examples are given in the reference list. The incorporation of aluminum in MFI films is difficult to control. The composition of the synthesis mixture will affect the concentration of aluminum in the zeolite film. This well-known fact has been explored in order to prepare compositionally zoned MFI films with varying aluminum concentration [33&]. It is also well known that the aluminum concentration may vary along the radius of crystals grown in a synthesis mixture or along the thickness of a film, since the composition of the mixture may vary during synthesis. Several groups have also reported that aluminum may leach from alumina supports and incorporate in the growing MFI film [21,34,35]. Leaching may be reduced by support masking [10&&,36&&] or by coating the support with a layer of mesoporous silica [5&&,28&]. These methods also reduce the growth of zeolite into the pores of the support [5&&,36&&], which may be denoted as Fsupport invasion_ [36&&]. Support invasion may increase the amount of defects in the zeolite film and/or reduce the flux in zeolite membranes [36&&]. 6. Defects Defects in MFI films and membranes may be defined as alternative transport pathways with a diameter exceeding the
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diameter of zeolite pores. Although quite dense films may be grown by hydrothermal treatment of seeded supports, defects may form during the subsequent calcination. Defects are formed during calcination of MFI membranes most likely due to a thermal expansion mismatch between the MFI film and the support and several mechanisms for defect formation in MFI membranes have been proposed [37,38&,39]. Defects in MFI membranes are difficult to study by conventional techniques. Only relatively large defects (say >5 nm) may be detected by SEM due to the limited resolution of most instruments. Furthermore, only the width at the surface of the film may be measured by SEM in combination with nondestructive sample preparation techniques. TEM instruments have higher resolution, but new defects may be introduced during sample preparation. However, the use of two new nondestructive techniques for characterization of defects in MFI films has been reported recently. One technique is fluorescence confocal microscopy (FCOM) reported by Vlachos and Tsapatsis et al. [40&&,41&]. In FCOM, defects and grain boundaries are impregnated with a fluorescent dye and viewed along the thickness of the film by confocal microscopy. An advantage with this technique is that objects as small as a few nanometers can be viewed. A disadvantage is the limited lateral and axial resolution of about 0.25 and 0.5 Am, respectively, which is close to or larger than the grain size and thickness of thin zeolite membranes and films. n-Hexane porosimetry [10&&,36&&,42&&] has been applied for characterization of flow-through defects in MFI membranes. A defect distribution can be estimated from porosimetry data [43&&]. An advantage with this method is that larger (in the mesopore range) as well as very small (in the micropore range) flow-through defects may be quantified. A disadvantage is that a number of assumptions and approximations are necessary in order to estimate the defect distribution from experimental data. 7. Diffusion in MFI films In view of the discussion above, diffusion in MFI films will now be considered. A number of mathematical models for diffusion have been adopted and applied to diffusion in zeolite crystals [44 – 46]. However, as shown above, MFI films have a complicated texture: The films are polycrystalline, grain boundaries are inevitable, the crystals are often funnel shaped and some are buried, the concentration of aluminum in the MFI film may be affected by the support, etc. Knudsen diffusion will prevail in open grain boundaries with a width smaller than the mean free path (ca 140 nm for He at STP) of the diffusing molecule. The diffusivity in MFI pores is several orders of magnitudes lower than Knudsen diffusivity for larger molecules such as hydrocarbons. Open grain boundaries may thus increase the effective diffusivity in the films, especially in the case of larger molecules. Closed grain boundaries may act as diffusion barriers between channels in adjacent grains [28&,47]. Furthermore, the support may affect the diffusion in zeolite membranes significantly [43&&,48,49,50]. Mathematical models for diffusion in perfect MFI crystals are thus inadequate to describe diffusion in MFI
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Table 1 Commonly used probe molecules for characterization of MFI membranes Molecule
˚) Diameter (A
Hydrogen Nitrogen SF6 n-Butane i-Butane n-Hexane 2,2-DMB p-Xylene o-Xylene m-Xylene
2.89 3.64 5.5 4.3 5.0 4.3 6.2 5.85 6.8 6.8
The critical diameter [55,56] of each molecule is included for reference.
films and membranes. In an attempt to account for the effect of defects and support, Jareman et al. have developed and presented a simple model for diffusion in zeolite membranes [43&&,51&]. We believe that many conclusions drawn from separation data for MFI membranes are erroneous. First of all, we would like to point out that a high separation factor is not equal to high membrane quality. A typical example is the N2/SF6 single gas permeance ratio. Jareman et al. have shown [36&&,51&] that this ratio depends on texture (film thickness and preferred orientation) and experimental conditions and is not suitable for assessment of quality of membranes with different textures. In fact, most single gas ratios and separation factors from binary experiments depends on texture and test conditions [51&,52]. The SF6 permeance is often used as an indication of the amount of defects in MFI membranes. We wish to emphasize that SF6 has a critical diameter (see Table 1) which is smaller than that for p-xylene, a molecule which is commonly believed to access the MFI pores. The SF6 diffusivity is consequently quite high in MFI zeolite at room temperature [43&&,53]. The SF6 permeance is thus inappropriate for estimation of the amount of defects in MFI films. Quality of MFI membranes can probably better be estimated by confocal microscopy or porosimetry or by permeation measurements with larger probe molecules such as tri-isopropyl-benzene (TiPB). Furthermore, the possible effects on permeance of Si/ Al ratio (aluminum leaching from support), counter-ions and adsorbed molecules in MFI membranes are often neglected. An extremely low SF6 permeance for MFI membranes is sometimes an indication of blocked pores or thick film rather than an indication of high quality [54]. Finally, we wish to point out that the texture of MFI membranes should be optimized for each separation task. A texture which is optimal for one separation application may be inappropriate for another separation task. The optimal texture can be determined from separation data if the defect distribution of the film is known.
Dr. Anton-Jan Bons are acknowledged for valuable discussions during the preparation of this review. References and recommended readings [1] Nair S, Tsapatsis M. Synthesis and properties of zeolitic membranes. In: Auerbach S, Carrado K, Dutta P, editors. Handbook of zeolite science and technology. Marcel Dekker; 2003. p. 867 – 919. [2] Tosheva L, Valtchev VP. Nanozeolites, synthesis, crystallization mechanism, and applications. Chem Mater 2005;17:2494 – 513. [3] Cundy CS, Cox PA. The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism. Microporous Mesoporous Mater 2005;82:1 – 78. [4] Yan Y, Wang H. Nanostructured zeolite films. Encycl Nanostruct Mater 2004;7:763 – 81. [5] Lai ZP, Tsapatsis M, Nicolich JR. Siliceous ZSM-5 membranes by secondary && growth of b-oriented seed layers. Adv Funct Mater 2004;14:716 – 29. This paper describes the preparation of b-oriented MFI films on porous alumina supports in detail and is of high significance for the present review. [ 6 ] Ban T, Mitaku H, Suzuki C, Matsuba J, Ohya Y, Takahashi Y. & Crystallization and crystal morphology of silicalite-1 prepared from silica gel using different amines as a base. J Cryst Growth 2005;274:594 – 602. Control of relative growth rates is necessary in order to influence the preferred orientation in the resulting films. [ 7 ] Bonilla G, Diaz I, Tsapatsis M, Jeong HK, Lee Y, Vlachos DG. Zeolite & (MFI) crystal morphology control using organic structure-directing agents. Chem Mater 2004;6:5697 – 705. The influence of monomer-, dimer and trimer-TPA on the relative growth rates is investigated. This is important to tailor the preferred orientation. [ 8 ] Hedlund J. Control of the preferred orientation in MFI films synthesized & by seeding. J Porous Mater 2000;7:455 – 64. The influence of seed size, habit and competitive growth on preferred orientation is discussed. [9] Sterte J, Hedlund J, Schoeman B. Procedure for preparing molecular sieve films. ExxonMobil US6177373. [10] Hedlund J, Sterte J, Anthonis M, Bons AJ, Carstensen B, Corcoran EW, et && al. High flux MFI membranes. Microporous Mesoporous Mater 2002;53:179 – 89. The preparation of high flux MFI membranes with a novel method involving seeding and support masking was described for the first time. This work is considered as a major breakthrough. [11] Valtchev V, Mintova S. Layer-by-layer preparation of zeolite coatings of nanosized crystals. Microporous Mesoporous Mater 2001;43:41 – 9. [12] Ha K, Lee YJ, Jung DY, Lee JH, Yoon KB. Micropatterning of oriented && zeolite monolayers on glass by covalent linkage. Adv Mater 2000;12:1614 – 7. This paper describes a novel method for covalently anchoring zeolite crystals on a support surface for the first time. This method has subsequently been utilized for membrane preparation. This work is considered as a major breakthrough. [13] Verduijn JP, Bons AJ, Anthonis MH, Czarnetzki LR, Mortier WJ. Molecular sieves and processes for their manufacture. US Pat. 6090289. Exxon Research and Engineering Company; 2000. [14] Lai WF, Deckman HW, McHenry JA, Verduijn JP. Supported zeolite membranes with controlled width and preferred orientation grown on a growth enhancing layer. US Pat. 5871650. Exxon Research and Engineering Company; 1999. [15] Lovallo MC, Tsapatsis M. Preferentially oriented submicron silicalite membranes. AIChE J 1996;42:3020 – 9. [16] Xomeritakis G, Lai Z, Tsapatsis M. Separation of xylene isomer vapors with oriented MFI membranes made by seeded growth. Ind Eng Chem Res 2001;40:544 – 52.
Acknowledgements The authors acknowledge the Swedish research council for financial support of this work. Professor Michael Tsapatsis and
& of special interest. && of outstanding interest.
J. Hedlund, F. Jareman / Current Opinion in Colloid & Interface Science 10 (2005) 226 – 232 [17] Nair S, Lai ZP, Nikolakis V, Xomeritakis G, Bonilla G, Tsapatsis M. Separation of close-boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth. Microporous Mesoporous Mater 2001;48:219 – 28. [18] Bonhomme F, Welk ME, Nenoff TM. CO2 selectivity and lifetimes of high silica ZSM-5 membranes. Microporous Mesoporous Mater 2003;66:181 – 8. [19] Li G, Kikuchi E, Matsukata M. The control of phase and orientation in & zeolite membranes by the secondary growth method. Microporous Mesoporous Mater 2003;62:211 – 20. This paper shows that the phase of the film can be controlled by the phase of the seeds. [20] Lin X, Kita H, Okamoto K. Silicalite membrane preparation, characterization, and separation performance. Ind Eng Chem Res 2001;40: 4069 – 4078. [21] Pan M, Lin YS. Template-free secondary growth synthesis of MFI type zeolite membranes. Microporous Mesoporous Mater 2001;43: 319 – 327. [22] Takata Y, Tsuru T, Yoshioka T, Asaeda M. Gas permeation properties of MFI zeolite membranes prepared by the secondary growth of colloidal silicalite and application to the methylation of toluene. Microporous Mesoporous Mater 2002;54:257 – 68. [23] Wong WC, Au LTY, Lau PPS, Ariso CT, Yeung KL. Effects of synthesis & parameters on the zeolite membrane morphology. J Membr Sci 2001;193:141 – 61. This is a very detailed study on the influence of synthesis parameters on the properties of the resulting zeolite film. [24] Wong WC, Au LTY, Ariso CT, Yeung KL. Effects of synthesis parameters on the zeolite membrane growth. J Membr Sci 2001;191:143 – 63. [25] Li SA, Li ZJ, Bozhilov KN, Chen ZW, Yan YS. TEM investigation of && formation mechanism of monocrystal-thick b-oriented pure silica zeolite MFI film. J Am Chem Soc 2004;126:10732 – 7. This paper suggests that growth mechanism during film preparation with methods involving seeds and the in situ method are similar. [26] Wang ZB, Yan YS. Controlling crystal orientation in zeolite MFI thin films by direct in situ crystallization. Chem Mater 2001;13: 1101 – 1107. [27] Mu¨gge O. Ueber die Enstehung faseriger und ihrer Aggregationsformen. In: Brauns R, et al, editors. Neues Jahrbuch fu¨r Mineralogie Geologie und Pala¨ontologie, p. 303 – 48. [28] Lai ZP, Bonilla G, Diaz I, Nery JG, Sujaoti K, Amat MA, et al. & Microstructural optimization of a zeolite membrane for organic vapor separation. Science 2003;300:456 – 60. The preparation of b-oriented membranes is described for the first time. This work is considered as a major breakthrough. [29 ] Lai ZP, Tsapatsis M. Gas and organic vapor permeation through b& oriented MFI membranes. Ind Eng Chem Res 2004;43:3000 – 7. A detailed permeation study of b-oriented MFI membranes is reported in this paper. [30] Bons AJ, Bons PD. The development of oblique preferred orientations in && zeolite films and membranes. Microporous Mesoporous Mater 2003;62:9 – 16. Simulation of competitive growth is used to explain the development of oblique-orientation. [31] Bonilla G, Vlachos DG, Tsapatsis M. Simulations and experiments on & the growth and microstructure of zeolite MFI films and membranes made by secondary growth. Microporous Mesoporous Mater 2001;42: 191 – 203. Simulation of competitive growth is used to explain the development of different textures, such as crystal orientation and voids, in MFI films. [32 ] Lai SM, Au LTY, Yeung KL. Influence of the synthesis conditions and & growth environment on MFI zeolite film orientation. Microporous Mesoporous Mater 2002;54:63 – 77. This is a very detailed study on the influence of synthesis parameters on the properties of the resulting zeolite film. [33 ] Li Q, Hedlund J, Sterte J, Creaser D, Bons AJ. Synthesis and & characterization of zoned MFI films by seeded growth. Microporous Mesoporous Mater 2002;56:291 – 302.
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The preparation of MFI films with aluminum gradient is described in detail for the first time. [34] Noack M, Kolsch P, Seefeld V, Toussaint P, Georgi G, Caro J. Influence of the Si/Al-ratio on the permeation properties of MFI-membranes. Microporous Mesoporous Mater 2005;79:329 – 37. [35] van der Puil N, Dautzenberg FM, van Bekkum H, Jansen JC. Preparation and catalytic testing of zeolite coatings on preshaped alumina supports. Microporous Mesoporous Mater 1999;27:95 – 106. [36] Hedlund J, Jareman F, Bons AJ, Anthonis MH. A masking technique to && improve thin MFI membranes. J Membr Sci 2003;222:163 – 79. The influence of support masking on membrane quality is described in this paper for the first time. [37] den Exter MJ, van Bekkum H, Rijn CJM, Kapteijn F, Moulijn JA, Schellevis H, et al. Stability of oriented silicalite-1 films in view of zeolite membrane preparation. Zeolites 1997;19:13 – 20. [38 ] Dong J, Lin YS, Hu MZC, Peascoe RA, Payzant EA. Template& removal-associated microstructural development of porous-ceramicsupported MFI zeolite membrane. Microporous Mesoporous Mater 2000;34:241 – 53. New defect formation mechanisms in MFI films on porous supports are proposed in this paper. [39] Geus ER, van Bekkum H. Calcination of large MFI-type single-crystals: 2. Crack formation and thermomechanical properties in view of the preparation of zeolite membranes. Zeolites 1995;15:333 – 41. [40] Bonilla G, Tsapatsis M, Vlachos DG, Xomeritakis G. Fluorescence && confocal optical microscopy imaging of the grain boundary structure of zeolite MFI membranes made by secondary (seeded) growth. J Membr Sci 2001;182:103 – 9. The authors report a non destructive method (Fluorescence confocal optical microscopy, FCOM) to characterize defects in zeolite films for the first time. This work is considered as a major breakthrough. [41 ] Snyder MA, Lai Z, Tsapatsis M, Vlachos DG. Combining simultaneous & reflectance and fluorescence imaging with SEM for conclusive identification of polycrystalline features of MFI membranes. Microporous Mesoporous Mater 2004;76:29 – 33. This is a more detailed study of the FCOM method. [42] Deckman HW, Cox DM, Bons AJ, Carstensen B, Chance RR, Corcoran && EW, et al. Characterization of zeolite membrane quality and structure. IWZMM2001 Book of Abstracts. p. 9 – 12. The porosimetry experiment for assessment of membrane quality is described for the first time. This work is considered as a major breakthrough. [43] Jareman F, Hedlund J, Creaser D, Sterte J. Modelling of single gas && permeation in real MFI membranes. J Membr Sci 2004;236:81 – 9. A permeation model that accounts for film thickness, defect distribution and support (i.e., membrane texture) is reported for the first time. [44] Ka¨rger J, Ruthven DM. Diffusion in zeolites and other microporous solids. New York’ Wiley-Interscience; 1992. [45] Keil FJ, Krishna R, Coppens MO. Modeling of diffusion in zeolites. Rev Chem Eng 2000;16:71 – 197. [46] Krishna R, Baur R. Modelling issues in zeolite based separation processes. Sep Purif Technol 2003;33:213 – 54. [47] Caro J, Noack M, Ko¨lsch P, Scha¨fer R. Zeolite membranes—state of their development and perspective. Microporous Mesoporous Mater 2000; 38:3 – 24. [48] de Bruijn FT, Sun L, Olujic Z, Jansens PJ, Kapteijn F. Influence of the support layer on the flux limitation in pervaporation. J Membr Sci 2003;223:141 – 56. [49] Gardner TQ, Falconer JL, Noble RD, Zieverink MMP. Analysis of transient permeation fluxes into and out of membranes for adsorption measurements. Chem Eng Sci 2003;58:2103 – 12. [50] Skoulidas AI, Sholl DS. Multiscale models of sweep gas and porous support effects on zeolite membranes. AIChE J 2005;51:867 – 77. [51 ] Jareman F, Hedlund J. Single gas permeance ratios in MFI membranes: & effects of material properties and experimental conditions. Microporous Mesoporous Mater 2005;82:201 – 7. This paper shows that single gas permeance ratios are not useful for assessment of membrane quality. It also shows that these ratios are highly dependent of membrane texture and test conditions.
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Texture of MFI films grown from seeds Jonas Hedlund *, Fredrik Jareman Division of Chemical Technology, Lulea˚ University of Technology, S 97187 Lulea˚, Sweden Available online 27 October 2005
Abstract This review describes how the texture of MFI films grown from seed crystal is developed during film preparation. Reports published during the last 5 years are in focus. Relative growth rates in various crystallographic directions, competitive growth, properties of the seed layer, defects, grain boundaries and other parameters influencing the film properties are discussed. Mathematical models describing competitive growth are also discussed. Suitable characterization methods for defects are described. The last part of the review is devoted to diffusion. Diffusion models accounting for texture in MFI films and the influence of texture on diffussion are discussed. D 2005 Elsevier Ltd. All rights reserved.
1. Major recent advances During the last 5 years, the following major advances (in chronological order) have been reported in the field of MFI films grown from seeds. A novel seeding method, by chemical anchoring of the seeds, has been described. Fluorescence confocal microscopy and permporosimetry have been proven very useful tools for characterization of defects and grain boundaries in MFI membranes. The successful preparation and evaluation of 0.5-Am MFI membranes on graded alumina supports and 1-Am b-oriented membranes on non-graded alumina supports have been reported. 2. Introduction Zeolite films have great potential in a number of technologically sophisticated applications, such as membranes, sensors, catalyst and adsorbents [1 –4]. MFI zeolite has pores close to the kinetic diameter of several industrially important hydrocarbons, which explains the great scientific interest for this particular zeolite. The MFI structure is non-isotropic with sinusoidal pores ˚ ) running in the a-direction and slightly (diameter ca. 5.5 A wider straight pores running in the b-direction and connections between the two pore systems create a three-dimensional network of pores (see Fig. 1a).
* Corresponding author. Tel.: +46 920 492105; fax: +46 920 491199. E-mail address: [email protected] (J. Hedlund). 1359-0294/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2005.09.009
Supported MFI films can be grown by several techniques [1]. Film growth by the in situ method and methods involving pre-seeding of the support are most frequently reported. Preparation of supported films by the in situ method is accomplished by hydrothermal treatment of the support in a synthesis mixture. During hydrothermal treatment, crystals nucleate and grow simultaneously on the support and a dense film may form. In methods involving pre-seeding of the support, the support is seeded prior to film growth. The nucleation and crystal growth are carried out in separate steps and may thus be controlled separately, which may be an advantage. The texture of films grown from seeded supports is developed during seeding and film growth. Cracks and other defects may form in the film during or after film growth. Since the texture has a significant impact on the performance of the film in the applications mentioned above, this review article starts with a discussion on texture in MFI films grown on preseeded supports. The following part describes techniques for detection of defects since these will also affect the film properties. Diffusion in MFI films grown on seeded supports is discussed in the last part. This review will focus on work reported during the last 5 years. 3. Crystal habit MFI crystals display a number of crystals shapes or habits (see Fig. 1). The different habits result from differences in growth rates in varying directions of the crystal. The growth rate is highest in the c-direction in many systems, resulting in
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Fig. 1. A schematic drawing of an MFI crystal with a coffin habit with the pore structure and crystallographic axes and planes indicated (a) and a corresponding SEM image (b). A schematic drawing of a MFI twin crystal (c) and an SEM image of such crystals (d).
elongated crystals. If the {h0h} faces are well developed, crystals with a habit referred to as Fcoffin shape_ result (see Fig. 1a and b). The crystals are thinnest in the b-direction and the ratio of the lengths in the c- and a-directions can vary, resulting in wider or narrower coffins. Often, no well-defined {h0h} faces are formed, but rounded crystals are observed. In these systems, Ftwin_ crystals, or more precisely F90- rotational intergrowths,_ are common (see Fig. 1c and d). High-silica MFI crystals often display this habit. It is also possible to grow MFI crystals with no or few twins and no well-defined {h0h} faces, i.e., with a Fpill_ habit (see Fig. 16D in Ref. [5&&]). The possibility to control the habit of MFI crystals by applying various structure-directing agents and amines has been explored since several years ago.1 A recent example of habit control by the addition of amines is the work by Ban et al. [6&]. Bonilla et al. [7&] recently published a follow-up study of habit control by using dimers and trimers of TPAOH. By this approach, it is possible to grow MFI crystals with drastically different habits compared to the crystals grown using TPAOH monomer. For instance, it is possible to grow single crystals (without twins) that are relatively thick in the b-direction, which may be utilized to grow b-oriented films (see below). When seed crystals are dispersed in a synthesis mixture and hydrothermally treated, the habit of the resulting crystals is dictated by the relative growth rates during that particular hydrothermal treatment. Fig. 1b and d show relatively large crystals with coffin and twin habit, respectively, grown by two completely different hydrothermal treatments of the same silicalite-1 seeds [8&]. 4. Seed layer A number of methods for deposition of MFI seeds in monoor multilayers have been developed. Deposition of monolayers 1
See Ref. [6&] and [7&].
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may rely on electrostatic attraction or chemical bonds between seeds and support. For many applications, such as sensors and membranes, very thin films are advantageous. The response time of a sensor and the flux in a membrane are to a first approximation proportional and inversely proportional, respectively, to the film thickness. The thickness of the seed layer sets the lower limit for the film thickness. A monolayer of seeds is the thinnest possible seed layer, which may be beneficial. One method for monolayer deposition has been developed and patented by Sterte et al. [9]. This method relies on electrostatic attraction of the seeds to the surface by cationic polymer molecules. This method may produce a monolayer of seeds, which is smoother than the support surface [10&&] (see Fig. 2a and b). By repeating polymer and seed adsorption, it is possible to deposit bi-layers [8&] or even multi-layers [11]. This method is flexible and has been used to coat many types of supports with varying chemical composition and geometry. Ha et al. [12&&] have developed a method for chemical anchoring of zeolite crystals to a support. In this method, a halopropylsilyl compound is used to attach seeds to the support by covalent bonds. The halopropylsilyl compound is very sensitive to moisture and the work must be carried out in a dry environment [5&&], which may be a complication. This method has been used by Lai et al. [5&&] to deposit monolayers of boriented MFI crystals with the pill habit. Exxon Chemical [13,14] and Lovallo and Tsapatsis [15] have independently developed methods for deposition of multilayers of MFI seeds relying on dip- or spin-coating techniques. One of these methods has been used frequently by Tsapatsis et al. [15 – 17] and by several other groups [18,19&,20– 23&] for the preparation of MFI membranes. The thickness of the seed layer can be controlled by varying the concentration of the seed sol or the slip-casting time [24]. It should be noted that Yan et al. have proposed a mechanism [25&&,26] for growth of MFI films by the in situ method. In this case, colloidal crystals first nucleate and grow in the synthesis mixture. A film is formed subsequently by deposition and self-assembly of these crystals on the support. The crystals grow and re-orient to form a dense film. According to this mechanism, the in situ method and methods relying on seeding are actually very similar. 5. Growth of MFI films on seeded supports MFI films can grow on a support seeded with MFI crystals in a synthesis solution free from organic template molecules in which no zeolite forms in the absence of seeds [8&]. This shows that the seeds may control the phase of the growing film. Another example is that the same hydrothermal treatment can be used to grow MOR and MFI films by applying the appropriate seed crystals [19&]. If secondary nucleation is low and crystals are not attached from the bulk of the synthesis mixture, the competitive growth mechanism [27] will control the growth of the zeolite film. Competitive growth will lead to funnel-shaped crystals extending from the seed layer to the film top surface and encapsulation of other crystals (see Fig. 2c).
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Fig. 2. SEM top view images of a porous a-alumina support prior to (a) and after seeding with a monolayer of 60-nm silicalite-1 crystals (b). A SEM side view image of a silicalite-1 film comprised of funnel-shaped crystals extending from the support to the film surface and embedded crystals (c).
The habit of the seeds will influence the preferred orientation of the crystals in the film [8&]. By combining the effects of oriented seed layers and competitive growth, it is possible to obtain MFI films with almost any orientation. A ca. 1-Am-thick a-oriented (or (h00) oriented) film [8&] can be
grown by treatment of a mono-layer of MFI twin crystals in a synthesis mixture where the fastest growing direction is the aV-direction of the twin (see Fig. 3I). This approach was used [10&&] to grow 500-nm MFI membranes with very high flux on graded alumina supports. However, at this film thickness,
Fig. 3. A schematic drawing of an a-oriented film grown from a monolayer of twin crystals (seeds) (I). Most of the larger parts of the seeds are b-oriented. The crystallographic directions, habit of seeds grown in the synthesis mixture and film thickness are indicated. A b-oriented film grown from seeds with pill habit (II). A thick c-oriented film grown from a monolayer of twin crystals (III). A film with oblique orientation forms earlier in the competitive growth process (IV).
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the film is not yet fully a-oriented since it is still early in the competitive growth process. If the seeds have a pill habit, the seed crystals will mostly be b-oriented. If the crystals in this layer are grown without the formation of twins or attachment of differently oriented crystals, a b-oriented (or (0k0) oriented) film [5&&,28&,29&] will form as illustrated in Fig. 3II. Tsapatsis et al. has used this approach to grow 1-Am boriented MFI membranes on non-graded alumina supports. These membranes had high flux of p-xylene and very high pxylene/o-xylene selectivity. In systems where crystals elongated in the c-direction are formed in the bulk of the synthesis mixture, thick films of c-oriented (or (00l) oriented) crystals will develop even from multilayers of randomly oriented seeds [16]. As illustrated in Fig. 3III, c-oriented films should also develop from monolayers of weakly oriented seeds provided that sufficiently elongated crystals are formed. Films with {h0h} preferred orientation, appropriately denoted Foblique_ orientation by Bons and Bons [30&&], will develop earlier in the competitive growth process [16], before the formation of a c-oriented layer (see Fig. 3IV). Bonilla [31&] and Bons [30&&] have numerically modeled competitive growth resulting in c- and oblique orientation. Both groups have used a similar approach, i.e., a marker particle front tracking technique, although Bons model also allowed curved interfaces between crystals. Crystals with the coffin habit are assumed to grow in the system and random orientation of the seed crystals was also assumed. It was shown that a corientation will develop in thick films (30 Am) and that oblique orientation may dominate in thinner films (10 Am), especially when Fwider_ coffins with a larger a/c ratio are formed in the bulk, as indicated in Fig. 3IV. Numerous experimental studies of the growth of MFI films on seeded supports have been reported [8&,24,32&] and some examples are given in the reference list. The incorporation of aluminum in MFI films is difficult to control. The composition of the synthesis mixture will affect the concentration of aluminum in the zeolite film. This well-known fact has been explored in order to prepare compositionally zoned MFI films with varying aluminum concentration [33&]. It is also well known that the aluminum concentration may vary along the radius of crystals grown in a synthesis mixture or along the thickness of a film, since the composition of the mixture may vary during synthesis. Several groups have also reported that aluminum may leach from alumina supports and incorporate in the growing MFI film [21,34,35]. Leaching may be reduced by support masking [10&&,36&&] or by coating the support with a layer of mesoporous silica [5&&,28&]. These methods also reduce the growth of zeolite into the pores of the support [5&&,36&&], which may be denoted as Fsupport invasion_ [36&&]. Support invasion may increase the amount of defects in the zeolite film and/or reduce the flux in zeolite membranes [36&&]. 6. Defects Defects in MFI films and membranes may be defined as alternative transport pathways with a diameter exceeding the
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diameter of zeolite pores. Although quite dense films may be grown by hydrothermal treatment of seeded supports, defects may form during the subsequent calcination. Defects are formed during calcination of MFI membranes most likely due to a thermal expansion mismatch between the MFI film and the support and several mechanisms for defect formation in MFI membranes have been proposed [37,38&,39]. Defects in MFI membranes are difficult to study by conventional techniques. Only relatively large defects (say >5 nm) may be detected by SEM due to the limited resolution of most instruments. Furthermore, only the width at the surface of the film may be measured by SEM in combination with nondestructive sample preparation techniques. TEM instruments have higher resolution, but new defects may be introduced during sample preparation. However, the use of two new nondestructive techniques for characterization of defects in MFI films has been reported recently. One technique is fluorescence confocal microscopy (FCOM) reported by Vlachos and Tsapatsis et al. [40&&,41&]. In FCOM, defects and grain boundaries are impregnated with a fluorescent dye and viewed along the thickness of the film by confocal microscopy. An advantage with this technique is that objects as small as a few nanometers can be viewed. A disadvantage is the limited lateral and axial resolution of about 0.25 and 0.5 Am, respectively, which is close to or larger than the grain size and thickness of thin zeolite membranes and films. n-Hexane porosimetry [10&&,36&&,42&&] has been applied for characterization of flow-through defects in MFI membranes. A defect distribution can be estimated from porosimetry data [43&&]. An advantage with this method is that larger (in the mesopore range) as well as very small (in the micropore range) flow-through defects may be quantified. A disadvantage is that a number of assumptions and approximations are necessary in order to estimate the defect distribution from experimental data. 7. Diffusion in MFI films In view of the discussion above, diffusion in MFI films will now be considered. A number of mathematical models for diffusion have been adopted and applied to diffusion in zeolite crystals [44 – 46]. However, as shown above, MFI films have a complicated texture: The films are polycrystalline, grain boundaries are inevitable, the crystals are often funnel shaped and some are buried, the concentration of aluminum in the MFI film may be affected by the support, etc. Knudsen diffusion will prevail in open grain boundaries with a width smaller than the mean free path (ca 140 nm for He at STP) of the diffusing molecule. The diffusivity in MFI pores is several orders of magnitudes lower than Knudsen diffusivity for larger molecules such as hydrocarbons. Open grain boundaries may thus increase the effective diffusivity in the films, especially in the case of larger molecules. Closed grain boundaries may act as diffusion barriers between channels in adjacent grains [28&,47]. Furthermore, the support may affect the diffusion in zeolite membranes significantly [43&&,48,49,50]. Mathematical models for diffusion in perfect MFI crystals are thus inadequate to describe diffusion in MFI
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Table 1 Commonly used probe molecules for characterization of MFI membranes Molecule
˚) Diameter (A
Hydrogen Nitrogen SF6 n-Butane i-Butane n-Hexane 2,2-DMB p-Xylene o-Xylene m-Xylene
2.89 3.64 5.5 4.3 5.0 4.3 6.2 5.85 6.8 6.8
The critical diameter [55,56] of each molecule is included for reference.
films and membranes. In an attempt to account for the effect of defects and support, Jareman et al. have developed and presented a simple model for diffusion in zeolite membranes [43&&,51&]. We believe that many conclusions drawn from separation data for MFI membranes are erroneous. First of all, we would like to point out that a high separation factor is not equal to high membrane quality. A typical example is the N2/SF6 single gas permeance ratio. Jareman et al. have shown [36&&,51&] that this ratio depends on texture (film thickness and preferred orientation) and experimental conditions and is not suitable for assessment of quality of membranes with different textures. In fact, most single gas ratios and separation factors from binary experiments depends on texture and test conditions [51&,52]. The SF6 permeance is often used as an indication of the amount of defects in MFI membranes. We wish to emphasize that SF6 has a critical diameter (see Table 1) which is smaller than that for p-xylene, a molecule which is commonly believed to access the MFI pores. The SF6 diffusivity is consequently quite high in MFI zeolite at room temperature [43&&,53]. The SF6 permeance is thus inappropriate for estimation of the amount of defects in MFI films. Quality of MFI membranes can probably better be estimated by confocal microscopy or porosimetry or by permeation measurements with larger probe molecules such as tri-isopropyl-benzene (TiPB). Furthermore, the possible effects on permeance of Si/ Al ratio (aluminum leaching from support), counter-ions and adsorbed molecules in MFI membranes are often neglected. An extremely low SF6 permeance for MFI membranes is sometimes an indication of blocked pores or thick film rather than an indication of high quality [54]. Finally, we wish to point out that the texture of MFI membranes should be optimized for each separation task. A texture which is optimal for one separation application may be inappropriate for another separation task. The optimal texture can be determined from separation data if the defect distribution of the film is known.
Dr. Anton-Jan Bons are acknowledged for valuable discussions during the preparation of this review. References and recommended readings [1] Nair S, Tsapatsis M. Synthesis and properties of zeolitic membranes. In: Auerbach S, Carrado K, Dutta P, editors. Handbook of zeolite science and technology. Marcel Dekker; 2003. p. 867 – 919. [2] Tosheva L, Valtchev VP. Nanozeolites, synthesis, crystallization mechanism, and applications. Chem Mater 2005;17:2494 – 513. [3] Cundy CS, Cox PA. The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism. Microporous Mesoporous Mater 2005;82:1 – 78. [4] Yan Y, Wang H. Nanostructured zeolite films. Encycl Nanostruct Mater 2004;7:763 – 81. [5] Lai ZP, Tsapatsis M, Nicolich JR. Siliceous ZSM-5 membranes by secondary && growth of b-oriented seed layers. Adv Funct Mater 2004;14:716 – 29. This paper describes the preparation of b-oriented MFI films on porous alumina supports in detail and is of high significance for the present review. [ 6 ] Ban T, Mitaku H, Suzuki C, Matsuba J, Ohya Y, Takahashi Y. & Crystallization and crystal morphology of silicalite-1 prepared from silica gel using different amines as a base. J Cryst Growth 2005;274:594 – 602. Control of relative growth rates is necessary in order to influence the preferred orientation in the resulting films. [ 7 ] Bonilla G, Diaz I, Tsapatsis M, Jeong HK, Lee Y, Vlachos DG. Zeolite & (MFI) crystal morphology control using organic structure-directing agents. Chem Mater 2004;6:5697 – 705. The influence of monomer-, dimer and trimer-TPA on the relative growth rates is investigated. This is important to tailor the preferred orientation. [ 8 ] Hedlund J. Control of the preferred orientation in MFI films synthesized & by seeding. J Porous Mater 2000;7:455 – 64. The influence of seed size, habit and competitive growth on preferred orientation is discussed. [9] Sterte J, Hedlund J, Schoeman B. Procedure for preparing molecular sieve films. ExxonMobil US6177373. [10] Hedlund J, Sterte J, Anthonis M, Bons AJ, Carstensen B, Corcoran EW, et && al. High flux MFI membranes. Microporous Mesoporous Mater 2002;53:179 – 89. The preparation of high flux MFI membranes with a novel method involving seeding and support masking was described for the first time. This work is considered as a major breakthrough. [11] Valtchev V, Mintova S. Layer-by-layer preparation of zeolite coatings of nanosized crystals. Microporous Mesoporous Mater 2001;43:41 – 9. [12] Ha K, Lee YJ, Jung DY, Lee JH, Yoon KB. Micropatterning of oriented && zeolite monolayers on glass by covalent linkage. Adv Mater 2000;12:1614 – 7. This paper describes a novel method for covalently anchoring zeolite crystals on a support surface for the first time. This method has subsequently been utilized for membrane preparation. This work is considered as a major breakthrough. [13] Verduijn JP, Bons AJ, Anthonis MH, Czarnetzki LR, Mortier WJ. Molecular sieves and processes for their manufacture. US Pat. 6090289. Exxon Research and Engineering Company; 2000. [14] Lai WF, Deckman HW, McHenry JA, Verduijn JP. Supported zeolite membranes with controlled width and preferred orientation grown on a growth enhancing layer. US Pat. 5871650. Exxon Research and Engineering Company; 1999. [15] Lovallo MC, Tsapatsis M. Preferentially oriented submicron silicalite membranes. AIChE J 1996;42:3020 – 9. [16] Xomeritakis G, Lai Z, Tsapatsis M. Separation of xylene isomer vapors with oriented MFI membranes made by seeded growth. Ind Eng Chem Res 2001;40:544 – 52.
Acknowledgements The authors acknowledge the Swedish research council for financial support of this work. Professor Michael Tsapatsis and
& of special interest. && of outstanding interest.
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The preparation of MFI films with aluminum gradient is described in detail for the first time. [34] Noack M, Kolsch P, Seefeld V, Toussaint P, Georgi G, Caro J. Influence of the Si/Al-ratio on the permeation properties of MFI-membranes. Microporous Mesoporous Mater 2005;79:329 – 37. [35] van der Puil N, Dautzenberg FM, van Bekkum H, Jansen JC. Preparation and catalytic testing of zeolite coatings on preshaped alumina supports. Microporous Mesoporous Mater 1999;27:95 – 106. [36] Hedlund J, Jareman F, Bons AJ, Anthonis MH. A masking technique to && improve thin MFI membranes. J Membr Sci 2003;222:163 – 79. The influence of support masking on membrane quality is described in this paper for the first time. [37] den Exter MJ, van Bekkum H, Rijn CJM, Kapteijn F, Moulijn JA, Schellevis H, et al. Stability of oriented silicalite-1 films in view of zeolite membrane preparation. Zeolites 1997;19:13 – 20. [38 ] Dong J, Lin YS, Hu MZC, Peascoe RA, Payzant EA. Template& removal-associated microstructural development of porous-ceramicsupported MFI zeolite membrane. Microporous Mesoporous Mater 2000;34:241 – 53. New defect formation mechanisms in MFI films on porous supports are proposed in this paper. [39] Geus ER, van Bekkum H. Calcination of large MFI-type single-crystals: 2. Crack formation and thermomechanical properties in view of the preparation of zeolite membranes. Zeolites 1995;15:333 – 41. [40] Bonilla G, Tsapatsis M, Vlachos DG, Xomeritakis G. Fluorescence && confocal optical microscopy imaging of the grain boundary structure of zeolite MFI membranes made by secondary (seeded) growth. J Membr Sci 2001;182:103 – 9. The authors report a non destructive method (Fluorescence confocal optical microscopy, FCOM) to characterize defects in zeolite films for the first time. This work is considered as a major breakthrough. [41 ] Snyder MA, Lai Z, Tsapatsis M, Vlachos DG. Combining simultaneous & reflectance and fluorescence imaging with SEM for conclusive identification of polycrystalline features of MFI membranes. Microporous Mesoporous Mater 2004;76:29 – 33. This is a more detailed study of the FCOM method. [42] Deckman HW, Cox DM, Bons AJ, Carstensen B, Chance RR, Corcoran && EW, et al. Characterization of zeolite membrane quality and structure. IWZMM2001 Book of Abstracts. p. 9 – 12. The porosimetry experiment for assessment of membrane quality is described for the first time. This work is considered as a major breakthrough. [43] Jareman F, Hedlund J, Creaser D, Sterte J. Modelling of single gas && permeation in real MFI membranes. J Membr Sci 2004;236:81 – 9. A permeation model that accounts for film thickness, defect distribution and support (i.e., membrane texture) is reported for the first time. [44] Ka¨rger J, Ruthven DM. Diffusion in zeolites and other microporous solids. New York’ Wiley-Interscience; 1992. [45] Keil FJ, Krishna R, Coppens MO. Modeling of diffusion in zeolites. Rev Chem Eng 2000;16:71 – 197. [46] Krishna R, Baur R. Modelling issues in zeolite based separation processes. Sep Purif Technol 2003;33:213 – 54. [47] Caro J, Noack M, Ko¨lsch P, Scha¨fer R. Zeolite membranes—state of their development and perspective. Microporous Mesoporous Mater 2000; 38:3 – 24. [48] de Bruijn FT, Sun L, Olujic Z, Jansens PJ, Kapteijn F. Influence of the support layer on the flux limitation in pervaporation. J Membr Sci 2003;223:141 – 56. [49] Gardner TQ, Falconer JL, Noble RD, Zieverink MMP. Analysis of transient permeation fluxes into and out of membranes for adsorption measurements. Chem Eng Sci 2003;58:2103 – 12. [50] Skoulidas AI, Sholl DS. Multiscale models of sweep gas and porous support effects on zeolite membranes. AIChE J 2005;51:867 – 77. [51 ] Jareman F, Hedlund J. Single gas permeance ratios in MFI membranes: & effects of material properties and experimental conditions. Microporous Mesoporous Mater 2005;82:201 – 7. This paper shows that single gas permeance ratios are not useful for assessment of membrane quality. It also shows that these ratios are highly dependent of membrane texture and test conditions.
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