Layer-by-Layer Deposition of Barrier and Permselective c-Oriented

SEM images were taken from the center (a), bottom (b), and top (c) of the substrate and at high magnification (d). .... examined by X-ray diffraction ...
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Ind. Eng. Chem. Res. 2007, 46, 7096-7106

Layer-by-Layer Deposition of Barrier and Permselective c-Oriented-MCM-22/ Silica Composite Films Jungkyu Choi,†,‡ Zhiping Lai,†,‡,§ Shubhajit Ghosh,†,⊥ Derek E. Beving,| Yushan Yan,*,| and Michael Tsapatsis*,† Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 151 Amundson Hall, 421 Washington AVenue SE, Minneapolis, Minnesota 55455, and Department of Chemical and EnVironmental Engineering, UniVersity of California, RiVerside, Bourns Hall A242, RiVerside, California 92521

A layer-by-layer deposition method is presented for the fabrication of compact c-oriented-MCM-22/silica films on aluminum alloys and porous R-alumina discs. The film fabrication procedure combines deposition of platelike MCM-22 crystals on substrates by covalent attachment under reflux and/or by sonication-assisted covalent attachment (using the methods introduced by Yoon and co-workers and recently reviewed [Acc. Chem. Res. 2007, 40 (1), 29-40]) with evaporation-induced-self-assembly (EISA) of surfactant-templated silica. The composite c-oriented MCM-22/silica films exhibited corrosion resistance barrier properties comparable to commercial chromate conversion coatings. Moreover, they exhibited hydrogen ideal selectivities (e.g., H2/N2 ∼ 7) above those expected by Knudsen diffusion indicating molecular sieving potential. 1. Introduction Zeolite films have potential applications such as gas, vapor, or liquid separations,1,2 membrane reactors,3-5 chemical sensors,6-8 dielectric thin films,9 pollutant removal,10 corrosion protection,11,12 etc. Since most zeolites have anisotropic crystal structures, mechanical properties, thermal expansion coefficients, and adsorbate diffusivity are often strongly dependent on crystallographic direction, and consequently, orientation control is desirable. For example, in some cases, a properly oriented zeolite membrane will exhibit superior performance than others.13,14 Zeolite films can be synthesized by various methods, such as in situ, seeded (secondary) growth, vapor-phase transport, etc.2,15-17 Most of these methods are based on the hydrothermal synthesis process which involves exposure of the bare or seeded substrates to the synthesis liquid or vapor at elevated temperature. Despite the success of the above-mentioned synthesis methods for corrosion protection,11,12 hydrogen separation,18 and other applications, the hydrothermal synthesis process is difficult to scale up. Therefore, we have been investigating alternative synthesis methods that eliminate hydrothermal synthesis steps. Here, we report an alternative film microstructure consisting of oriented zeolite crystals in a silica matrix prepared using layer-by-layer deposition. In the layer-by-layer deposition method, introduced here, although zeolite particles are synthesized by the hydrothermal procedure, no hydrothermal synthesis is involved during the film fabrication. By choosing the appropriate morphology of particles, both deposit thickness and particle orientation in the direction perpendicular to the support (referred to as out-of-plane orientation) can be well controlled. * To whom correspondence should be addressed. Phone: 612-6260920. Fax: 612-626-7246. E-mail: [email protected] (M.T.). Phone: 951-827-2068. Fax: 951-827-5696. E-mail: [email protected] (Y.Y.). † University of Minnesota. ‡ These two authors contributed equally to this work. § Current address: Division of Chemical and Biomolecular Engineering, Nanyang Technological University, Singapore 637722. ⊥ Current address: Halliburton, Baroid Fluid Services, 3000 N. Sam Houston Pkwy E, Houston, Texas 77032. | University of California, Riverside.

Figure 1. Illustration of layer-by-layer deposition using layered material, MCM-22, in order to make MCM-22/silica films.

Layer-by-layer deposition for the fabrication of inorganic thin films has been reported before.19-23 In most of these previous reports, polyelectrolytes with opposite charges were used alternatively to link inorganic particles on the substrate. Here, we use a combination of deposition by chemical bonding (covalent attachment) between zeolite crystals and a substrate as developed by Yoon’s group24-26 and EISA silica coating methods introduced by Brinker’s group27,28 in order to fabricate compact thin films. MCM-22 (MWW structure type) is a high-silica zeolite that can be prepared as highly anisotropic plates with high thermal and chemical stability.29 Previously, preferentially a-out-of-plane oriented MCM-22 films were hydrothermally made by an insitu method30,31 and via pulsed laser deposition.32 As an application, liquid pervaporation measurements (water/ethanol) were reported for the a-oriented MCM-22 films made via pulsed laser deposition.32 However, these a-oriented MCM-22 mem-

10.1021/ie0706156 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7097 Scheme 1. Schematic Procedure Involved in Deposition under Reflux

Scheme 2. Schematic Procedure Involved in Sonication-Assisted Deposition

Scheme 3. Schematic Procedure for (a) Silica Dip Coating and (b) Silica Aerosol Coating

Na2O and ∼55% Al2O3), sodium hydroxide (Fisher, 99%), and hexamethyleneimine (HMI, Aldrich 99%). Brij 56 (Aldrich) and tetraethylorthosilicate (TEOS, Aldrich) were used to make a silica sol for aerosol coating, while TEOS, tetrapropylammonium hydroxide (TPAOH, Aldrich), and ethanol (Aldrich) were used to make the silica sol for dip coating. The surface of substrates was functionalized with 3-chloropropyl trimethoxysilane (3CPTMS, Aldrich). MCM-22 calcination was performed using acrylamide (AM, Aldrich), N,N′-methylenebisacrylamide (MBAM, Aldrich), and ammonium persulfate (Aldrich).35 All chemicals were used as received. 2.2. Substrate Pretreatment. Aluminum alloy sheets were cut into 20 × 20 mm square strips, while 3 in. × 6 in. sized aluminum alloy panels were used as received. The substrates were polished by sandpaper (grit 320, Buehler) until the surface was shining in order to obtain a smooth surface, washed with DI water in an ultrasonic bath for 5 min, and dried overnight.

branes are not suitable for hydrogen separation because the pore size along the a-axis is too large for the high-temperature molecular sieving separation of hydrogen from other small gas molecules. Similarly, we feel that the small pore opening along the c-axis is advantageous for corrosion protection barrier applications. Therefore, c-out-of-plane oriented MCM-22/silica films were evaluated for their potential in two applications: corrosion protection11,12 and hydrogen separation.33,34 2. Experimental Section The film fabrication procedure is illustrated in Figure 1. Aluminum alloy and R-alumina substrates were used depending on the application (corrosion protection and gas separations, respectively). First, all substrates were functionalized with a silane-coupling agent. Then, platelike MCM-22 crystals were deposited on the substrate surface followed by a silica coating. In the next step, the samples were calcined to increase the adhesion of MCM-22 crystals to the substrate and remove the surfactant template used to form the silica sol. MCM-22 deposition, silica coating, and calcination (from now on, referred to as the deposition cycle) were repeated several times. 2.1. Materials. Substrates used for corrosion protection in this study were aluminum alloy 2024-T3 purchased from McMaster-Carr Corp, while substrates for hydrogen separation were homemade R-alumina discs masde as decribed elsewhere.14 Chemicals used for MCM-22 synthesis were fumed silica (CabO-Sil M5, 99.8%), sodium aluminate (MP Biomedicals, ∼42%

Figure 2. Illustration of the pore structure of (a) as-synthesized MCM-22 (P) and (b) MCM-22 after calcination.

See section 2.8 for explanation of symbols. b SML: silica mesoporous layer. c DM: MCM-22 particle deposition method. d cal: calcination. e Calcination was done after the silica dip coating.

SD3

a

sonication-assisted deposition yes dip coating sonication-assisted deposition yes dip coating SD2

SD1

sample no.a

Scheme 4. Flowchart of Samples and Corresponding Preparation Procedures

yes dip coating

SML DM cal SML DM cal SML no SML

dip coatinge

substrate

R-alumina discs (22 mm in diam)

DM

yes sonication-assisted deposition no aluminum alloy (3 in. × 6 in. ) LS1

S1R2

S1R1

S1

R3

R2

AA R1

sonication-assisted deposition

aerosol coating

yes deposition under reflux aerosol coating yes sonication-assisted deposition

yes deposition under reflux aerosol coatinge no

no

aluminum alloy (22 × 22 mm) BA

cal

aerosol coating

yes aerosol coating

deposition under reflux

aerosol coating

aerosol coating

deposition under reflux

yes deposition under reflux

cal DM SML cal DM SML cald DMc SMLb substrate sample no.a

Table 1. Summary of the Procedures Used to Prepare the Various MCM-22/Silica Films in This Study

yes

aerosol coating

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SML

7098

One side of each R-alumina disc was polished by a sandpaper (grit 600, Buehler). 2.3. Synthesis of MCM-22 Precursor and MCM-22. MCM22 precursor, referred to as MCM-22 (P), with Si/Al ∼ 50, was synthesized based on a reported procedure.36 First, NaOH and sodium aluminate were added to DI water. Subsequently, hexamethyleneimine was mixed with the above solution. Finally, fumed silica was slowly added under continuous stirring. The mass composition of this mixture was 1 fumed silica:0.1 NaOH: 0.03 sodium aluminate:0.8 HMI:13.1 H2O. The mixture was stirred vigorously for 5 h. The viscous gel was transferred to 45 mL Teflon liners quickly, and the liners were mounted in autoclaves (Parr Instruments). The autoclaves were positioned and rotated in an oven preheated to 135 °C. After 11 days, the reaction was quenched with tap water and the solid was recovered from the mother liquid and washed by 5 repetitions of centrifugation and decanting. MCM-22 (P) was not dried. The wet cake was immediately calcined to avoid aggregation as follows. MCM-22 (P) was calcined in the presence of a polymer using the method reported in ref 35. About 5 g of MCM-22 (P) wet cake was added to 20 mL water in a glass jar. MCM-22 (P) in water was stirred for 2 h and sonicated for 1 h. A Branson 1210 sonicator, 50-60 Hz, was used for sonication unless otherwise indicated. Then, the suspension of MCM-22 (P) was filtered by a syringe equipped with a filter tip (POREX) that has 15 µm retention size. After that, 10 g of acrylamide, 0.1 g of N,N′methylenebisacrylamide, and 0.25 g of (NH4)2S2O8 were in turn added to a glass beaker while the MCM-22 (P) suspension was stirred. The glass beaker was immersed in a sonication bath and sonicated for 1 h. After sonication, a puddinglike polymer was formed, cut to pieces in a crystallization glass, and transferred to a furnace. The polymer, in which MCM-22 (P) was embedded, was dried overnight at 80 °C under a N2

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Figure 3. SEM image (a) and corresponding powder XRD pattern of as-synthesized MCM-22 (P) particles (b) along with SEM image (c) and corresponding powder XRD pattern (d) of calcined MCM-22.

Figure 4. SEM images (a and b) and XRD pattern (c) of MCM-22 deposits (R1) formed by deposition under reflux. An MCM-22 powder pattern is also given for comparison.

environment and heated to 550 °C under N2 flow with a 2 °C/ min ramp rate. After it was held for 2 h, the N2 flow was switched to air flow and the temperature was held for additional 12 h under air followed by slow cooling. After the calcination process, only white powder, i.e., MCM-22, was left and collected. 2.4. MCM-22 Particle Deposition by Covalent Attachment under Reflux. Each substrate held by a Teflon holder was first placed vertically in a glass reactor. About 30 mL dry toluene was added to the reactor and the substrate was completely immersed. Then, 3 mL of 3CP-TMS was added to the solution. The solution was refluxed at 110 °C for 3 h with dry argon purging. Meanwhile, 0.015 g of MCM-22 particles were dispersed into 40 mL of dry toluene by sonication for 1 h. After functionalization, the substrate was then taken out, rinsed by

fresh toluene, and dried at 120 °C before it was moved to the MCM-22 suspension. The dried and functionalized substrate laid on Teflon holder was quickly placed horizontally inside the MCM-22 suspension with the functionalized side facing up. The solution was again refluxed at 110 °C for 5 h with dry argon purging. The sample was taken out, rinsed by fresh toluene, and then sonicated for 1 min. The sample was further dried at 120 °C before calcination. A precise scheme for deposition under reflux is displayed in Scheme 1. 2.5. MCM-22 Particle Deposition by Sonication-Assisted Covalent Attachment. After substrate functionalization with 3CP-TMS was completed, the substrate was sandwiched between two cover glasses. The sandwiched substrate was quickly placed inside the MCM-22 suspension and held by a Teflon holder. The glassware was then placed inside an

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Figure 5. Top view (a) and tilted view (b) SEM images of three-times-coated MCM-22/silica films (R3).

Figure 6. MCM-22/silica film (S1) on aluminum alloy by sonication-assisted deposition. SEM images were taken from the center (a), bottom (b), and top (c) of the substrate and at high magnification (d).

Scheme 5. Experimental Setup for Single Gas Permeation Measurements

ultrasonicator (Branson 1210, 50-60 Hz for the small substrate, and Branson 5510, 135 W 42 kHz for the large panel). The sandwiched substrate was first placed vertically and sonicated for 4-10 min. The sandwiched substrates were then positioned horizontally and sonicated for another 2 min. Aluminum alloy strips were further placed in fresh toluene and sonicated for 3

s. At last, the sample was dried in an oven heated to around 120 °C. A flowchart for sonication-assisted deposition is illustrated in Scheme 2. 2.6. Silica Dip Coating. Silica dip coating was mainly done for films made on R-alumina discs. A silica sol was prepared following a method reported by Brinker and co-workers.27 The

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Figure 7. SEM images in low (a) and high magnification (b) of a onetime-coated MCM-22 film on an R-alumina disc by sonication-assisted deposition.

Figure 8. SEM images in low (a) and high magnification (b) of a twotimes-coated MCM-22/silica film on an R-alumina disc by sonicationassisted deposition.

silica sol was further 256:1 diluted by ethanol in order to make a thin silica layer. Detailed information about making the silica sol and the dip coating can be found elsewhere.14 We present here a scheme for synthesis of this silica sol as depicted in Scheme 3a. The surface of R-alumina discs was coated by this diluted silica sol before the first MCM-22 particle deposition. Each subsequent silica coating step was followed by a calcination step, heating samples up to 480 °C with a ramp rate of 1 °C/min, holding them for 4 h, and cooling them slowly. 2.7. Silica Aerosol Coating. The coating sol was prepared according to the literature.28 A 14.24 g portion of Brij 56, 308.64 g of DI water, 31.66 g of TEOS, and 12.77 g of nitric acid (0.07 M) were mixed together and sonicated for 3 h. A semitransparent solution was obtained after sonication. The sol was further aged for additional 1 h before it was transferred to an ultrasonic humidifier (Kaz Inc. Model 5520). The previously calcined MCM-22 deposited aluminum alloy substrate with the coated side facing upward was placed horizontally on a steel mesh in a container connected to the outlet of the humidifier. Calcination was done by heating samples up to 300 °C with the ramp rate of 5 °C/min, holding them at 300 °C for 1 h, and cooling them slowly. Then, the humidifier was run for about 1 min after the mist started to reach the sample. After that, the sample was taken out and dried in a laminar flow hood to avoid contamination from dust. This aerosol coating step is illustrated in Scheme 3b. 2.8. Sample Preparation by Deposition under Reflux and/ or Sonication-Assisted Methods. Several samples were prepared by deposition under reflux or sonication-assisted deposition. Some samples were made using both methods at different cycles. In general, there were three types of samples. 1. Only MCM-22 deposition under reflux along with a silica aerosol coating was repeated to make films on aluminum alloy. This type of sample was denoted by Ri, in which R stands for reflux and i represents the number of deposition cycles of the procedure. 2. The first layer was formed on aluminum alloy by sonication-assisted deposition and additional layers were deposited by deposition under reflux. This type of sample is

referred to as SiRj where S and R represent sonication and reflux, respectively, while i and j stand for the number of deposition cycles by sonication-assisted deposition and deposition under reflux, respectively. Especially, when j is equal to 0, the sample is designated by Si. Additionally, samples made on large aluminum alloy panels (3 in. × 6 in.) are called as LSi where L stands for large panel and i represents the number of repetitions of deposition cycle by sonication-assisted deposition. 3. Sonication-assisted deposition of MCM-22 along with silica dip coating cycles were repeated in order to make MCM-22/ silica films. These last types of films were fabricated on R-alumina discs. They are referred to as SDi, where S stands for sonication-assisted deposition, D represents dip coating, and i is the number of deposition cycles. In addition, BA and AA are bare aluminum alloy and silicaaerosol-coated aluminum alloy, respectively. All the samples are summarized in Table 1, and a flowchart for sample preparation is also shown in Scheme 4. 2.9. Characterization. The synthesized MCM-22 powder and films were examined by X-ray diffraction (XRD) using a Siemens D-5005 diffractometer with Cu KR radiation and scanning electron microscopy (SEM) using a field-emission gun scanning electron microscope JEOL 6500. Samples were coated by Pt sputtering before SEM imaging unless mentioned otherwise. 2.10. DC Polarization. Polarization test was carried out with a Solartron potentiostat SI 1287 in a three-electrode configuration with the zeolite-coated substrate as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The corrosive medium was contacted with the center of the sample using O-rings for sealing. The contact area is about 10 mm in diameter. The corrosive medium was contacted with the sample for 5 min prior to the polarization test. The temperature of the solution during polarization tests was maintained at 298 K. A sweep rate of 1 mV/s was applied, and all potentials were referred to the saturated calomel electrode (SCE). 2.11. Single Gas Permeation Measurement. The schematic of the single gas permeation setup is shown in Scheme 5. Rates of 450-500 mL/min of single gases (He, H2, O2, and N2), controlled by metering valves, were fed to one side (retentate) of the permeation cell, the other side (permeate) of which was under vacuum. Since the retentate side was open to ambient air, 1 atm pressure difference was maintained between the feed side and the permeate side during permeation measurements. The pressure in the permeate side was measured by use of a pressure transducer (Omega PX-303) and continuously recorded in a computer (Labview 7). The ideal gas law was used to estimate the molar flow rate across the membranes based on the recorded pressure. The permeance, P, was calculated according to the following equation,

P)

dp V 1 1 dt RTA∆p

(

in

mol sec‚m2‚Pa

)

where dp/dt is the rate of pressure change in the permeate side, V is the volume of the permeate side, R is the ideal gas constant, T is temperature, A is the cross-sectional area of the membrane, and ∆p is the pressure difference across the membrane. Ideal selectivity (IS) was obtained by taking a ratio of permeances of two gases,

IS )

PA PB

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Figure 9. SEM image (no Pt coated) (a) and XRD pattern (b) of three-times-coated MCM-22/silica film (S3DA) on R-alumina discs by sonication-assisted deposition. A simulated XRD pattern of MCM-22 powder using Mercury Software (Cambridge Crystallographic Data Centre) and an XRD pattern of calcined MCM-22 powder are also given in part b.

Figure 10. DC polarization results of BA (curve 2), AA (curve 1), R1 (curve 3), R2 (curve 4), R3 (curve 7), and S1 (curves 5 and 6) tested in 0.1 M sulfuric acid solution.

where, PA and PB are the permeances of certain gases A and B, respectively. 3. Results and Discussion 3.1. MCM-22 (P) and MCM-22 Structures and Potential Applications. The synthesis of MCM-22 crystals requires an intermediate synthesis of a layered alumino-silicate precursor, MCM-22 (P), which is hydrothermally obtained by using hexamethyleneimine (HMI). The structure of MCM-22 (P) is illustrated in Figure 2a. It consists of alumino-silicate layers linked together with HMI along the c-axis. The layers consist of a hexagonal array of “hourglass pockets” or “cups” on the (001) planes having 12-membered ring (12-MR) apertures on both sides of the layers. However, the limiting apertures for transport in a direction perpendicular to the layers, i.e., along the c-axis, are 6-MRs comprised of six interconnected SiO4 tetrahedra. Upon calcination, adjacent layers in MCM-22 (P) condense as HMI present in interlayers is removed. As a result, the three-dimensional crystalline MCM-22 structure is formed as shown in Figure 2b. Since the limiting pore opening along

Figure 11. DC polarization results of BA, S1, S1R1, and S1R2 tested in 0.5 M sulfuric acid solution.

the c-axis is a 6-MR in the MCM-22 structure, we hypothesize that a preferred c-out-of-plane oriented MCM-22/silica film can be, for any practical purposes, an impermeable barrier for most gases and liquids. This impermeable barrier is suitable for corrosion protection applications because of the limited access of corrosive chemicals. We also hypothesize that at elevated temperature small molecules like helium and hydrogen can pass through the 6-MR at a substantial rate37 and that a c-out-ofplane oriented MCM-22/silica film can be hydrogen permselective with respect to gas molecules like nitrogen, oxygen, and carbon dioxide. The as-synthesized MCM-22 (P) powder is shown in Figure 3a. The powder was confirmed to be pure MCM-22 (P) from its XRD pattern (Figure 3b). Most particles were platelike and have a uniform size with diameter around 1 µm and thickness less than 50 nm. The flat face of MCM-22 crystals is perpendicular to the [001] crystallographic direction (c-axis). Some MCM-22 particles were curled indicating that MCM-22 (P) particles are flexible due to the absence of a strong SiO-Si bond network extended along the c-axis. Also some degree of intergrowth is evident with two or more platelike particles emanating from a single nucleation center.

Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7103 Table 2. Kinetic Diameters and Molecular Weights of Gases Used for Permeation Measurement and Estimated Ideal Selectivities through Bare r-Alumina Discs Based on Knudsen Diffusion

moleculesa

kinetic diameterb (nm)

molecular weight

ideal selectivity

ideal Knudsen diffusion selectivityc

He H2 O2 N2

0.26 0.289 0.346 0.364

4 2 32 28

H2/He H2/O2 H2/N2 N2/O2

∼1.4 ∼4.0 ∼3.7 ∼1.1

a Molecules are listed in order of kinetic diameter. b Kinetic diameters were obtained from the literature.43 c Ideal selectivity was calculated based on Knudsen diffusion-limited transport.

Figure 12. DC polarization results of BA and LS1 tested in 0.5 M sulfuric acid solution. The results were obtained from three different locations in LS1: (red) 1 in. from top; (blue) 3 in. from top; (green) 1 in. from bottom.

Figure 13. (a) Permeances of single gases through a one-time-coated MCM22 film (SD1) and (b) corresponding ideal selectivities.

It is well-known that MCM-22 is obtained by calcination of MCM-22 (P) at 540 °C.36 However, direct calcination will lead to a severe agglomeration. We also found that long time storage of MCM-22 (P) may also result in the formation of big agglomerations with a donut shape similar to that reported in the literature.38 In order to avoid these issues, fresh prepared MCM-22 (P) was calcined in the presence of a polymer which plays the role of an agglomeration barrier to MCM-22 (P) particles. MCM-22 (P) was first dispersed into DI water by stirring. Next, the MCM-22 (P) suspension was sonicated and filtered with a sieve. The suspension was then polymerized into hydrogel and then calcined in the way described by Yan and co-workers.35 MCM-22 particles obtained in such a way are shown in Figure 3c. The powder was confirmed to be pure MCM-22 by XRD (Figure 3d). MCM-22 particles seemed well separated and appeared more flat than MCM-22 (P) particles, indicating that the particles become more rigid after calcination due to Si-O-Si bond network formation in all three directions. The MCM-22 particles are not agglomerated by calcination, but a certain number of intergrowths exist apparently originating from MCM-22 (P) intergrown particles. This calcined nonagglomerated powder was stored for use over several months. 3.2. MCM-22 Particle Deposition by Covalent Attachment under Reflux. MCM-22 crystals deposited on aluminum alloy substrates by deposition under reflux are shown in Figure 4a and b. The MCM-22 particles were attached with the large basal ab-plane being parallel to the substrate. The preferred c-outof-plane orientation was confirmed by XRD patterns (Figure 4c), in which the strongest peak is corresponding to the (002) plane. MCM-22 crystals in the first monolayer reacted to the silane-coupling agent and anchored to the surface, while other

top layers are physically/weakly attached to each other and can be further removed by a brief sonication in fresh solvent. As a result, MCM-22 crystal layers were obtained by deposition under reflux. An analogy can be drawn between this method and a tiling process, in which MCM-22 crystals, analogous to individual tiles, are deposited as monolayers on a support in a similar way to the deposition of tiles on a surface. In summary, platelike MCM-22 crystals that have a large width to thickness aspect ratio (like a tile) enabled the formation of compact MCM-22 layers by covalent bonding between the MCM-22 crystals and the substrate via a silane-coupling agent. However, there was a tradeoff between the sonication time and the uniformity of the MCM-22 deposits. Too long sonication removed some portion of the first layer and resulted in exposing bare substrate.39 It seemed that sonication around 1 min was enough in order to make uniform MCM-22 deposits which were mostly comprised of one to three layers of MCM-22 particles as shown in Figure 4b. Figure 5 shows the top and tilted view SEM images of a three-times-coated MCM-22/silica film (R3). A smooth silica layer completely covered all MCM-22 deposits. As evident in Figure 5b, MCM-22 deposits were clearly embedded inside a silica matrix and the film thickness was estimated to be about 1 µm after three cycles (R3). 3.3. MCM-22 Particle Deposition by Sonication-Assisted Covalent Attachment. (A) Aluminum Alloy Substrates. A MCM-22 layer on aluminum alloy, formed by sonicationassisted deposition, is shown in Figure 6. SEM images were taken from different places showing uniformly deposited MCM22 crystals. Compared to deposition under reflux, sonicationassisted deposition required a short time. A typical sonicationassisted-deposition procedure required only about 10 min, while deposition under reflux needed several hours. As verified by SEM imaging done in various locations (Figure 6a-c), the film appeared more compact than that obtained by deposition under reflux (see Figure 4). It appears that, during deposition by sonication, the cover glasses by sandwiching the substrates could limit the access of any large agglomerated particles from the substrate. Although we did not perform a systematic study, the deposition method is apparently selective in that it disfavors deposition of intergrown particles. Furthermore, sonicationassisted deposition was not restricted by the geometric shape of supports. Therefore, the technique is promising for scale-up provided that the appropriate sonication device can be fabricated. (B) a-Alumina Substrates. MCM-22/silica films were also fabricated on porous R-alumina discs by sonication-assisted deposition. The MCM-22/silica films after one, two, and three MCM-22 particle deposition (SD1, SD2, and SD3) are shown in Figures 7-9, respectively. In addition, the XRD pattern of SD3 film on an R-alumina disc is shown in Figure 9b. These results confirm the formation of a thin c-oriented MCM-22/ silica nanocomposite on porous R-alumina substrates.

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Table 3. Detailed Single Gas Permeation Information for MCM-22/Silica Films Shown in Figures 13-15 permeance (mol/m2‚sec‚Pa) × 1010 sample no. SD1

tempa

(°C)

25 50 100 150 200

ideal selectivity

He

H2

O2

N2

H2/He

H2/O2

H2/N2

N2/O2

30200 29100 26100 24600 22400

40600 39000 36200 33300 30600

11800 11400 9970 9300 8400

12600 11800 10400 9970 9030

1.3 1.3 1.4 1.4 1.4

3.5 3.4 3.6 3.6 3.6

3.2 3.3 3.5 3.3 3.4

1.1 1.0 1.1 1.1 1.1

permeance (mol/m2‚sec‚Pa) × 1010 temperatureb (°C) sample no. SD2

tempa

(°C)

25 50 100 150 200

SD3

25 50 100 150 200

ideal selectivityc

He

H2

O2

N2

H2/He

H2/O2

H2/N2

N2/O2

1000 25 1050 61 1130 106 1250 147 1710 191 860 29 730 53 670 105 640 155 810 208

1340 25 1330 66 1330 109 1410 151 1940 193 1100 31 910 54 780 107 620 154 690 209

340 25 330 62 320 105 320 147 430 190 260 29 220 54 160 105 120 158 120 206

350 25 340 63 310 105 300 146 390 190 280 29 230 56 180 102 110 159 90 205

1.3

3.9

3.8

1.0

1.3

4.0

3.9

1.0

1.2

4.2

4.2

1.1

1.1

4.4

4.7

0.9

1.1

4.5

5.0

0.9

1.3

4.3

4.0

1.1

1.3

4.2

4.0

1.0

1.2

4.7

4.3

1.1

1.0

5.1

5.6

0.9

0.8

5.8

7.5

0.8

a Furnace set temperature. b Permeation cell temperature. c Temperature for ideal selectivities used in Figures 14 and 15 was averaged by the measured temperature of two gases.

3.4. Corrosion Protection Performance. Corrosion resistance of a MCM-22/silica film was examined by DC polarization, a commonly used laboratory technique for corrosion studies. DC polarization utilizes a three electrode systems reference, working, and countersin which the working electrode (MCM-22/silica coated aluminum alloy) has its potential varied with respect to the reference electrode and the corresponding current passing through the working electrode is measured through the counter electrode. The rate of corrosion can mathematically be related to the obtained current. In a typical polarization curve, lower polarization current means high corrosion resistance and vice versa. Figure 10 shows the results from tests in dilute (0.1 M) sulfuric acid. Curve 1 was from a silica coated aluminum alloy substrate (AA), and curve 2 was from a bare aluminum alloy sample (BA). Curves 3, 4, and 7 from MCM-22/silica films were made by one, two, and three repetitions of deposition cycles by deposition under reflux (R1, R2, and R3), respectively. Finally, curves 5 and 6 were from one-time-coated MCM-22/silica films (S1) made by sonicationassisted deposition. The results showed that the silica layer alone generally did not protect the substrate from corrosion. The corrosion performance of MCM-22/silica films made by deposition under reflux was improved with the number of deposition cycles (R3 was better than R2 which was better than R1). Curve 7 is almost indistinguishable from curves 5 and 6. This indicated that even one-time-coated MCM-22/silica films (S1) made by sonication-assisted deposition are equally good as the threetimes-coated MCM-22/silica films (R3) made by deposition under reflux. Figure 11 shows the corrosion resistance results for S1, S1R1, and S1R2 tested in concentrated sulfuric acid (0.5 M) along with the result for BA. It was evident that additional deposition cycles lead to films exhibiting better corrosion protection

performance in the sequence of S1R2, S1R1, S1, and BA. In particular, S1R2 showed corrosion protection performance almost comparable or slightly superior to the standard chromate conversion coatings. In addition, as a part of the scale-up process, a large aluminum alloy panel was used to make MCM-22/silica films (LS1) by the sonication-assisted method. The corrosion protection performance of LS1 in 0.5 M sulfuric acid solution is shown in Figure 12. The corrosion peformance was uniform throughout the film. Besides, the corrosion protection results of LS1 were even superior to that of S1 indicating that sonication-assisted deposition can be scaled up. MCM-22/silica films after calcination are nontoxic, while the chromate conversion coating is known to release hexavalent chromium, a proven human carcinogen which has caused serious environmental and health safety concerns.40-42 Therefore, the MCM-22/silica films could serve as a promising alternative for corrosion protection. 3.5. Single Gas Permeation Measurement. Single gas permeation measurements through one-, two-, and three-timescoated MCM-22/silica films (SD1, SD2, and SD3, respectively) were done by the experimental setup illustrated in Scheme 5. First, a one-time coated MCM-22 film (SD1) exhibited almost the same behavior as a bare R-alumina disc (not shown here), with the only minor difference being the smaller permeances than those through supports (Figure 13a). Knudsen diffusion was dominant in this film, and permeances monotonically decreased with temperature. Ideal selectivities for SD1, plotted in Figure 13b and listed in Table 3, were as expected for Knudsen diffusion. After an additional deposition, sample SD2 showed a distinct permeation performance unlike SD1 (Figure 14). The permeance of helium almost caught up with that of hydrogen, and the permeance of oxygen became slightly higher

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Figure 14. (a) Permeances of single gases through a two-times-coated MCM-22/silica film (SD2) and (b) corresponding ideal selectivities.

conversion coatings. In addition, a uniform MCM-22/silica film made on a large aluminum alloy panel by sonication-assisted deposition showed comparable results to films fabricated on small substrates. These films can potentially serve as an environmental friendly corossion protecting layer on aluminum alloys. Good corrosion performance indicates that layer-by-layer deposition, especially by sonication-assisted deposition, is promising in order to prepare compact and preferentially oriented MCM-22/silica films on a large scale. MCM-22/silica films prepared by sonication-assisted deposition on R-alumina discs showed improved permeation performance favoring hydrogen (or helium) passage over larger gases (oxygen and nitrogen). In particular, the fact that ideal selectivity of hydrogen to larger gases grew monotonically with temperature is promising. Acknowledgment

Figure 15. (a) Permeances of single gases through a three-times-coated MCM-22/silica film (SD3) and (b) corresponding ideal selectivities.

than that of nitrogen as the temperature increased. The kinetic diameters of helium, hydrogen, oxygen, and nitrogen are 0.26, 0.289, 0.346, and 0.364 nm (Table 2),43 while the corresponding molecular weights of the gases are 4, 2, 32, and 28. Under the Knudsen diffusion regime, the permeance of hydrogen is the fastest among them due to the lowest molecular weight. It appears that as the influence of Knudsen diffusion on mass transport compared to molecular sieving through the MCM-22 pores is being weakened in SD2 films, the size of the gas started to play the important role in mass transport. Figure 15 shows the permeation performance of three-timescoated MCM-22/silica films (SD3). Permeances of all gases were decreased until about 150 °C indicating a dominant Knudsen contribution, but the degree of reduction of the permeance was less for smaller gases resulting in a monotonic increase of ideal selectivities of helium or hydrogen to other larger gases. After passing 150 °C, the deviation from the Knudsen diffusion regime became more evident in the sense that the permeances of helium and hydrogen started to increase, while those of other gases kept decreasing. As a result, hydrogen permeation performance for SD3 was improved exhibiting a H2/N2 ideal selectivity of ∼7. Further work to explore the potential of these films as high-temperature hydrogen selective membranes for a water-gas shift membrane reactor is underway. 4. Conclusions Compact c-oriented MCM-22/silica films were successfully prepared by layer-by-layer deposition. Platelike MCM-22 particles were used as flakes in a mesoporous silica matrix. The corrosion resistance of the MCM-22/silica films made by deposition under reflux was increased with the number of repetitions of deposition cycles. Three-times-coated MCM-22/ silica films by deposition under reflux and one-time-coated MCM-22/silica films by sonication-assisted deposition exhibited the most promising results in 0.1 M H2SO4 solution. Also, MCM-22/silica films made by a combination of sonicationassisted deposition and deposition under reflux showed a comparable corrosion performance to commercial chromate

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ReceiVed for reView April 30, 2007 ReVised manuscript receiVed July 18, 2007 Accepted July 26, 2007 IE0706156