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Preparation of b-Oriented MFI Films on Porous Stainless Steel Substrates Godwin T. P. Mabande,†,§ Shubhajit Ghosh,‡ Zhiping Lai,‡ Wilhelm Schwieger,† and Michael Tsapatsis*,‡ Institute of Chemical Reaction Engineering, University Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany, and Department of Chemical Engineering and Materials Science, University of Minnesota, 151Amundson Hall, 421 Washington Avenue SE, Minneapolis, Minnesota 55455-0132
The preparation of smooth mesoporous silica layers on rough, porous stainless steel supports and the subsequent synthesis of well-intergrown b-oriented MFI films on these intermediate layers were investigated. Parameters such as surfactant-silica sol dilution, number of coating cycles, drying, and calcination were systematically varied to optimize mesoporous silica deposition results. Multiple slip- and aerosol-coating cycles with intermediate drying/calcination steps had to be employed to prepare mesoporous layers with adequate thickness and minimal cracking. b-Oriented MFI seed monolayers could be covalently attached to these mesoporous layers with high coverage using 3-chloropropyltrimethoxysilane as the coupling agent. Sonication during the final step of seed deposition was found to be necessary to achieve high seed coverage and orientation. Finally, highly b-oriented and intergrown MFI films could be produced by secondary growth of b-oriented seed layers using trimer tetrapropylammonium iodide as the structuredirecting agent. 1. Introduction The organization of zeolite microcrystals with controlled orientation on substrates has been a subject of scientific interest in recent years. Oriented zeolite films have been investigated in such applications as selective sensors,1,2 templates for nanowire arrays,3,4 catalysts,5-7 and membranes.8-10 The highly ordered environment created by oriented zeolite films means that their characteristics are more precisely defined than those of randomly oriented layers and film properties can be better tailored to specific functions. The surface roughness of porous stainless steel supports poses a challenge for the preparation of thin, oriented, integral zeolite films. Several methods exist for the application of a mesoporous intermediate silica layer that smoothens the support surface. For example, sol-gel dip coating,11-13 pressurized gel coating,14 and aerosol-assisted deposition15 have been applied to coat porous substrates with amorphous silica layers. Alternatively, TiO2 layers deposited on stainless steel supports have been used as substrates for zeolite membranes.16 Due to the high thermal expansion coefficient of stainless steel, which is higher than that of ceramic supports,17 zeolite films coated on stainless steel substrates are particularly susceptible to cracking during temperature cycling. The use of intermediate silica layers counterbalances thermal stresses that lead to the formation of cracks. The use of 3-halopropyl reagents as covalent linkers is an effective method for assembling oriented zeolite microcrystal monolayers on silica surfaces.18 The hy* To whom correspondence should be addressed. Tel.: (612) 626-0920. Fax: (612) 626-7246. E-mail:
[email protected]. † University Erlangen-Nuremberg. ‡ University of Minnesota. § Current address: BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany.
droxyl groups present on the surfaces facilitate the tethering of silane coupling agents such as 3-chloropropyltrimethoxysilane. Sonication as a method of agitation and reaction promotion during zeolite crystal deposition onto 3-chloropropyl-tethered glass substrates has recently been shown to increase the coating efficiency compared to crystal attachment under reflux conditions.19 During MFI crystal deposition onto 3-chloropropyltethered silica surfaces, crystals with large b-faces will tend to attach to the surface with their b-axes perpendicular to the support. We utilized such layers as seed layers for b-oriented MFI zeolite membrane synthesis. The use of trimer tetrapropylammonium hydroxide (trimer TPAOH, instead of the conventionally used monomer TPAOH) for secondary growth was effective in maintaining the initial orientation of the seeds due to the promotion of crystal growth in the b-direction and suppression of twinning.20 Our b-oriented MFI membranes possess organic vapor separation capabilities superior to those of previously reported membranes. The high membrane fluxes achieved on these microstructurally optimized membranes were only limited by the transport resistance of the R-alumina supports,21 and thus there is still potential to achieve even better performance with higher flux supports. Motivated by this potential and the wide range of advantages of metallic supports for commercial membrane applications,22 we investigate here the preparation of b-oriented MFI films on modified surfaces of asymmetric porous stainless steel supports. Yan and co-workers have reported the synthesis of b-oriented MFI films on stainless steel substrates using direct in situ crystallization.8,23-25 However, all these investigations were carried out on smooth polished stainless steel surfaces. To our knowledge, this is the first report on the synthesis of highly b-oriented inter-
10.1021/ie050668s CCC: $30.25 © 2005 American Chemical Society Published on Web 10/26/2005
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9087 Scheme 1. Overview of the Steps Involved in the Preparation of b-Oriented MFI Films and the Parameters (in Brackets) Varied in This Work
grown MFI films on porous stainless steel substrates that are suitable for membrane applications. 2. Experimental Section b-Oriented MFI films were prepared on the basis of our previous work on ceramic supports.10,20 However, several modifications had to be made to the individual preparatory steps due to the different characteristics of the stainless steel supports used in this work. Scheme 1 gives an overview of the individual preparation steps and the parameter variations investigated in this work. The support surfaces were smoothed by the application of mesoporous silica layers using slip coating11,20 or aerosol deposition15 and combinations of these deposition techniques. Flat MFI seeds were prepared using a procedure that we reported in our earlier work.20 These were then covalently bonded to the mesoporous silicacoated support surfaces using 3-chloropropyltrimethoxysilane as a coupling agent and with the aid of sonication to improve seed coverage.19 Bis-N,N-(tripropylammoniumhexamethylene)di-N,N-propylammonium triiodide
(trimer TPAI) was prepared on the basis of literature reports26,27 and our previous work,20 but with some improvements. Finally, secondary growth was investigated using trimer TPAI as the structure-directing agent. Details are given below. 2.1. Materials. Porous, asymmetric 316L stainless steel support disks (SIKA-F-AS; 18 mm diameter and ∼270 nm average through pore size according to capillary flow porometry) were purchased from GKN Sinter Metals Filters GmbH. These were cleaned in boiling water for 1 h, soaked in acetone for several days, and then dried in an oven (80 °C) before use. Tetraethyl orthosilicate (TEOS, 98 wt %), tetrapropylammonium hydroxide (TPAOH, 1.0 M), and potassium hydroxide (KOH, 1 N) were obtained from Aldrich. Anhydrous ethanol (99.5%) for the preparation of surfactant-templated silica sol for slip coating was purchased from Fluka. Cetyltrimethylammonium bromide (CTAB) and Brij-56 (C16H33(OCH2CH2)10OH) from Aldrich were used as surfactants for the preparation of the mesoporous silica layers. For the preparation of the structure-
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directing agent trimer TPAI for b-oriented film growth, bis(hexamethylene)triamine, 1-iodopropane, 2-butanone, anhydrous potassium carbonate, and ethyl acetate from Aldrich were used. 2.2. Mesoporous Silica Layers via Slip Coating. First, TEOS was mixed with anhydrous ethanol, water, and 0.07 M hydrochloric acid in a round-bottomed flask and the mixture was stirred for 2 h at 60 °C to give a clear sol. The molar composition of this mixture was 1 SiO2:8.1 EtOH:1.3 H2O:5.3 × 10-5 HCl. A water bath was used for heating, and a water-cooled condenser (15 °C) was attached to the flask during hydrolysis. Cetyltrimethylammonium bromide (CTAB) surfactant and more 0.07 M hydrochloric acid were then added to 40 mL of this sol; in this way the molar composition was adjusted to 1 SiO2:8.1 EtOH:4.9 H2O:4.5 × 10-3 HCl: 0.10 CTAB. The mixture was stirred until the CTAB had dissolved and then aged without stirring at ∼50 °C for 58.5 h. It was further aged at 20 °C for 3 days until it had almost gelled; aging at 20 °C slowed the aging process and made it easier to stop the process before total gelation. The sol was then diluted with anhydrous ethanol to give sols of dilutions ×1/4, ×1/8, ×1/16, and ×1/128. These were stored in a refrigerator for continued use over several months. The slip-coating procedure was carried out in a laminar flow hood to avoid dust contamination. The support was suspended in an inverted position with a pair of tweezers attached to a clamp fixed loosely enough to a stand to allow for rotation of the tweezers. The fine side was brought into contact with the sol. The disk was kept in this half-immersed position for 20 s, slipped away horizontally by rotating the tweezers, and then dried in a vertical position for 40 s. Excess sol was wiped from the underside of the disk. For multiple coatings the support was turned 90° each time an additional coating was performed to avoid accumulation of mesoporous silica on one side of the disk, i.e., the underside during drying. The layers were dried for 1 h at ambient and for 1 h at 120 °C and then allowed to cool before each additional slip-coating step. After completion of the coating procedure the coated supports were allowed to dry for at least a day at ambient before further processing. 2.3. Mesoporous Silica Layers via Aerosol Coating. Unlike the slip-coating procedure, a sol had to be freshly prepared each time aerosol coating was carried out since the sol continually aged during the deposition process. Brij-56 was mixed with water, TEOS, and 0.07 M HNO3 in a capped Pyrex bottle to give a molar composition of 1 SiO2:4 EtOH:120 H2O:0.14 Brij-56: 0.0060 HNO3. The mixture (325 g) was sonicated (Branson 1210, 50/60 Hz) for 3 h, resulting in a turbid but homogeneous sol. This sol was aged for about 1.5 h and then charged into a humidifier (kaz). The sol had a pH of 2.5. The supports were placed horizontally onto a wire gauze (fine side up) and exposed to an aerosol cloud generated by the underlying humidifier into an enclosed space that housed the gauze and supports. This allowed heavy aerosol droplets to settle on the support surfaces. In preliminary experiments it was found that the support surface becomes visibly flooded with sol after a deposition time of 10 min. It is recommended to operate using 1/4-1/5 of this “flooding time” for deposition to allow sufficient drying and solvent-evaporation-induced self-assembly of liquid crystalline mesophases between
deposition periods.28 In this work a deposition period of 1-2 min was used and the support was dried for at least 45 min in a laminar flow hood between depositions. Depositions had to be repeated several times to obtain significant layer thicknesses on the support surfaces. After the coatings, the supports were left to dry for at least 2 days at ambient before further processing. 2.4. Seed Preparation. Silicalite-1 seeds were prepared using a synthesis solution with the molar composition 1 TEOS:0.214 TPAOH:104 H2O. The template and water were mixed, and then the TEOS was added slowly. The TEOS was hydrolyzed at room temperature under rigorous stirring for 1 day in a closed plastic bottle. About 38 g of synthesis solution was charged into each of eight 45 mL Teflon-lined autoclaves (Parr). The autoclaves were placed in a preheated oven at 130 °C for 12 h under rotation at ∼12 rpm. After synthesis the seeds were repeatedly centrifuged until the washing water reached a pH of 7.7. The seeds were then dried at 90 °C for 18 h and then calcined at 525 °C in air for 10 h using a ramp rate of 0.5 K‚min-1. 2.5. Seed Deposition Using 3-Chloropropyltrimethoxysilane. Prior to seed deposition the surfactant-silica-coated supports were calcined at 450 °C for 4 h in air using heating and cooling rates of 0.2 and 0.5 K‚min-1, respectively. Precalcination of the substrates resulted in improved coverage compared to seed deposition attempts on noncalcined supports. The calcined supports were stored in a 120 °C drying oven, and all apparatus was dried at 80 °C at least overnight prior to the experiments. The substrate which was supported by a Teflon holder was loaded vertically into a specially designed glass reactor which was under continuous purge with argon. First 40 mL of dry toluene and then 4 mL of 3-chloropropyltrimethoxysilane were quickly transferred to the reactor from argon-purged containers. The reactor was then closed and connected to a watercooled condenser (15 °C). The solution was refluxed for 3 h under a gentle argon stream. After this the support was taken out, washed in fresh toluene and dried for about 10 min at 120 °C. In a separate step a magnetic stirrer bar, 0.05 g of seeds, and 40 mL of dry toluene were charged into a dry reactor under argon flow. The suspension was sonicated for 10 min (seed dispersion), after which the functionalized support was then quickly introduced into this suspension under argon purge. This time the support was positioned horizontally using a Teflon holder with the functionalized silica layer facing upward. The mixture was stirred at 250 rpm during reflux for 3.5 h. The reactor was then closed and placed in a sonicator (Branson 1210, 50/60 Hz) for 2 min sonication. Subsequently the seeded support was removed from the seed suspension, further sonicated for 5-10 s, and rinsed in fresh toluene to remove multilayered crystals. Finally, the seeded supports were calcined in air at 450 °C for 4 h using a ramp rate of 0.5 K‚min-1. 2.6. Synthesis of Trimer TPAI. Trimer TPAI was prepared by the exhaustive alkylation of bis(hexamethylene)triamine with iodopropane. A 450 mL volume of 2-butanone, 72.6 g of anhydrous potassium carbonate, and 27.9 g of bis(hexamethylene)triamine were added to a dry three-necked round-bottomed 1 L flask equipped with an argon supply, an oil bath, a 60 mL addition funnel, a magnetic stirrer, and a reflux condenser. The reaction flask and addition funnel were covered with aluminum foil to avoid iodide oxidation. Argon was
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Figure 1. Bare stainless steel support surface (a) and surfactant-silica layers on the support surface after slip coatings with surfactantsilica sols of varying dilutionss(b) ×1/128, (c) ×1/64, (d) ×1/16, (e) ×1/4sand drying at 120 °C for at least 12 h.
passed through the flask and vented through a bubbler continuously. The flask contents were heated to 35 °C for 30 min. A 101 mL volume of 1-iodopropane was quickly transferred to the addition funnel and added dropwise to the stirring reaction mixture as it was simultaneously heated to 85 °C. The reaction mixture was maintained at this temperature for 15 h. After reaction the flask contents were allowed to cool. The solid fraction containing mainly potassium carbonate, most of the trimer TPAI, and some potassium iodide was separated from the liquid fraction and purified by repeated vacuum filtration and washing with fresh n-butanone (3 × 15 mL). The filter cake was then recovered and stirred in 250 mL ethanol for 4 h to dissolve the trimer TPAI. The solution containing the trimer TPAI and trace KI was recovered by vacuum filtration, and the ethanol was subsequently removed by rotary evaporation. KI was removed by repeated dissolution of the product in smaller amounts of ethanol (∼60 mL), filtration of undissolved KI, and evaporation of ethanol until no KI was detectable in the product by X-ray diffraction (XRD). Further purification was by
recrystallization in equal amounts of cold 2-butanone and ethyl acetate. The crude product was mixed with 250 mL of 2-butanone and stirred for 1 h. An equal amount of ethyl acetate was added under stirring, and the mixture was left to stand for at least 10 h. The purified solid product could then be recovered by vacuum filtration and dried. A product yield of 50 g or 41% was typically achieved. Product purity was confirmed by elemental analysis (C, H, N) and 13C NMR spectroscopy. 2.7. Secondary Growth of b-Oriented Films. The molar synthesis mixture composition for secondary growth was 1 TEOS:x KOH:0.13 trimer TPAI:238 H2O with x ) 0.20 and 0.58. Trimer TPAI was dissolved in deionized water before KOH and TEOS were added. The mixture was then homogenized under rigorous stirring for 5 h. The supports were loaded into 45 mL Teflonlined autoclaves (Parr) and positioned in an almost vertical but slightly tilted position, the seeded surface facing downward, with the help of a Teflon holder. Approximately 20 g of synthesis solution was filtered (Qualitative P8, Fischer Scientific) into each autoclave.
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Figure 2. Surfactant-silica layers on stainless steel support surface after multiple slip coatings with surfactant-silica sol of dilution ×1/16 after two (a) and three (b-d) coatings and drying at 120 °C for at least 12 h.
Figure 3. Silica layers on stainless steel support surface following 8 × 2 min aerosol deposition and after (a) drying at room temperature for 2 days and (b) further drying at 120 °C for 32 h with heating ramp rate 0.1 K‚min-1 and calcination in air at 480 °C for 4 h with heating and cooling ramp rates of 0.3 and 0.6 K‚min-1, respectively.
The loading of the supports and synthesis solution into the autoclaves was performed in a laminar flow hood. The crystallization was at 175 °C for a duration of 20-24 h. Secondary growth was terminated by quenching the autoclaves in running water. 3. Results and Discussion 3.1. Mesoporous Silica Layers via Slip Coating. Figure 1 shows scanning electron microscopic (SEM) images of the surfaces of slip-coated supports after coatings with surfactant-silica sols of different dilutions. The use of high sol dilutions such as those used in our previous work20 for porous R-alumina supports leads to an inadequate coverage of the stainless steel support surfaces due to the much larger pore sizes and higher surface roughness of the latter. Whereas a sol dilution of ×1/256 was adequate to obtain a smooth mesoporous silica layer on the surface of R-alumina supports, even a sol dilution of ×1/128 resulted in no noticeable surface smoothening of the stainless steel
supports used in this work. The SEM image shown in Figure 1b resembles very much that of the original support in Figure 1a.22 More concentrated sols give rise to better coverage and smoothness of the support surface. However, because of differential drying due to solvent gradients in the layers,29 the thicker mesoporous coatings also have a higher tendency to crack, as Figure 1e shows for a layer produced with a ×1/4 dilution sol. Cracking is reduced when multiple slip-coating procedures are applied with intermediate dilution sols, as demonstrated in Figure 2 for multiple coatings with dilution ×1/16. This is due to the fact that the drying process of the individual thinner layers is associated with less cracking (cf. Figure 1d). In addition to this, subsequent coatings also cover the cracks within underlying layers. As Figure 2b shows, a very good degree of smoothing of sections of the support surface with no observable cracking can be achieved using three coatings of the ×1/ 16 dilution sol. However, some areas on the support
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Figure 4. Mesoporous silica coatings on stainless steel support surface after calcination in air at 450 °C for 4 h with heating and cooling ramp rates of 0.2 and 0.5 K‚min-1, respectively. The layers were prepared using (a) 4 × 2 min aerosol deposition and (b) 6 × 2 min, 7 × 2 min, 5 × 1 min, and 4 × 1 min aerosol deposition with drying at room temperature for 2 days and calcination at 450 °C for 4 h between the depositions.
Figure 5. Mesoporous silica coatings on stainless steel support surface after calcination in air at 450 °C for 4 h with heating and cooling ramp rates of 0.2 and 0.5 K‚min-1, respectively. The layers were prepared using (a) two slip coatings with surfactant-silica sol dilution ×1/16 followed by 6 × 1 min aerosol deposition and (b) three slip coatings with surfactant-silica sol dilution ×1/16 followed by 6 × 1 min aerosol deposition.
surface have less silica deposited, as shown in Figure 2c. Other areas have excessive silica deposited, leading to substantial cracking in the top layer (Figure 2d). Nevertheless, cracks do not propagate to the stainless steel support surface as they are arrested by the underlying layers. Inhomogeneities in silica layer thickness are a result of an uneven distribution of sol on the support surface during drying due to its nonlateral position at this stage. According to XRD measurements (not shown) the calcined mesoporous layers have a d-spacing of about 3 nm. 3.2. Mesoporous Silica Layers via Aerosol Coating. Mesoporous silica layers deposited by the aerosol deposition technique are more uniform than slip-coated layers due to the more even distribution of surfactantsilica sol on the support surface during the aerosol deposition process. This is demonstrated in Figure 3a, in which the silica layer is uniformly thick and smooth after drying at room temperature. However, these thick layers readily form large cracks upon calcination even when low ramp rates and intermediate drying steps are employed (Figure 3b). A 60% weight loss in the temperature range 25-480 °C was determined in thermogravimetric measurements performed on powder samples that were scratched off from glass slides after aerosol deposition. The calcination process is associated with considerable shrinkage in the silica layer due to water, ethanol, and surfactant removal. This shrinkage not only occurs out of plane but also to a large extent in plane inevitably leading to crack-inducing stresses.
Thinner silica layers have a better balance of in-plane and out-of-plane shrinkage upon calcination and only have small cracks (
