Direct Evidence of the Evolutionary Mechanism of Zeolite Monolayers

Jan 12, 2010 - State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China. § Department of Chemical Engineering, ...
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Direct Evidence of the Evolutionary Mechanism of Zeolite Monolayers on the Substrate Surface in a Hydrothermal Reaction Lin Lang,† Xiufeng Liu,*,†,‡ Haiyang Jiang,† Jerry Y. S. Lin,§ and Baoquan Zhang*,† †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China, ‡State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China, and §Department of Chemical Engineering, Arizona State University, Tempe, Arizona 85287 Received October 10, 2009. Revised Manuscript Received November 15, 2009 The microstructure control and optimization of zeolite films and membranes is an indispensable challenge for various innovative applications. It can be steered by understanding the formation process. Here we design an unprecedented strategy to uncover direct evidence via the hydrothermal synthesis of chitosan-supported zeolite monolayers. The chitosan-supported layer involved in the hydrothermal reaction is observed using SEM, AFM, EPMA, and HRTEM while nucleation and crystal growth in the bulk synthesis solution are pursued with HRTEM, DLS, and SEM. The direct HRTEM observation is achieved on the chitosan-supported layer by peeling chitosan from its support. It reveals that a gel layer is initially formed on the chitosan layer where the subsequent crystal growth is fatally restrained. Our own experimental evidence and the literature reports clearly demonstrate that the formation mechanism is homogeneous for severely reduced crystal growth on the substrate but is heterogeneous when crystal growth on the substrate is significantly enhanced.

I. Introduction As advanced functional materials, zeolite thin films and membranes have been applied to selectively separate hydrogen, methane, and carbon dioxide from gas mixtures,1-3 to remove water, alcohols, and other organics from liquid mixtures via pervaporation,2,3 and even to intensify a variety of catalytic reactions including hydrogenation, dehydrogenation, partial oxidation, and esterification.2,4 In addition, zeolite thin films can be employed as imaging and data storage media, light-energyharvesting devices, and electrode modifiers.5,6 The available publications in the past decade have demonstrated that the microstructure of a zeolite thin film or membrane substantially influences its performances in separation7,8 and/or reaction.9,10 Furthermore, second-harmonic-generation materials can be developed using oriented zeolite films for the aligned inclusion of hemicyanine dyes.11-13 The oriented zeolite monolayer loaded *Authors to whom correspondence should be addressed. Tel: þ86 22 27405165. Fax: þ86 22 87898959. E-mail: [email protected], bqzhang@ tju.edu.cn.

(1) Snyder, M. A.; Tsapatsis, M. Angew. Chem., Int. Ed. 2007, 46, 7560–7573. (2) Caro, J.; Noack, M. Microporous Mesoporous Mater. 2008, 115, 215–233. (3) Bowen, T. C.; Noble, R. D.; Falconer, J. L. J. Membr. Sci. 2004, 245, 1–33. (4) McLeary, E. E.; Jansen, J. C.; Kapteijn, F. Microporous Mesoporous Mater. 2006, 90, 198–220. (5) Yoon, K. B. Acc. Chem. Res. 2007, 40, 29–40. (6) Zhou, M.; Zhang, B. Q.; Liu, X. F. Chin. Sci. Bull. 2008, 53, 801–816. (7) Lai, Z. P.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300, 456–460. (8) Davis, M. E. Science 2003, 300, 438–439. (9) Lang, L.; Liu, X. F.; Hu, M. L.; Zhang, B. Q. ChemCatChem 2009, 1, 472-478. (10) Wang, X. D.; Zhang, B. Q.; Liu, X. F.; Lin, J. Y. S. Adv. Mater. 2006, 18, 3261–3265. (11) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y.-J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673–682. (12) Kim, H. S.; Sohn, K. W.; Jeon, Y.; Min, H.; Kim, D.; Yoon, K. B. Adv. Mater. 2007, 19, 260–263. (13) Kim, H. S.; Pham, T. T.; Yoon, K. B. J. Am. Chem. Soc. 2008, 130, 2134– 2135.

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with functional dyes ensures the fast transport of excitation energy in photonic devices.14-17 Besides, oriented silica MFI films are ideal low-k materials.18,19 Until now, hydrothermal reactions have been overwhelmingly used for the synthesis of zeolite thin films and membranes. The preferentially oriented thin films or membranes can be fabricated using several synthesis strategies such as the designed assembly of postsynthetic crystals on a substrate surface,5,6,20-23 controlled growth over a well-oriented seed layer,7-9,24 and in-situ-oriented nucleation and growth on a structure-directing matrix.10,25-27 With regard to the third strategy, Lee et al. used the poly(ethylene oxide) monolayer on a glass plate as a 2D template to direct the nucleation and subsequent oriented growth of TS-1 thin films.25 Similarly, a thin chitosan film residing on a porous alumina substrate was applied to fabricate the highly b-oriented TS-1 (14) Mass, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2002, 41, 2284–2288. (15) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem., Int. Ed. 2003, 42, 3732–3758. (16) Ruiz, A. Z.; Li, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2006, 45, 5282– 5287. (17) Huber, S.; Ruiz, A. Z.; Li, H. R.; Patrinoiu, G.; Botta, C.; Calzaferri, G. Inorg. Chim. Acta 2007, 360, 869–875. (18) Li, Z. J.; Johnson, M. C.; Sun, M. W.; Ryan, E. T.; Earl, D. J.; Maichen, W.; Martin, J. I.; Li, S.; Lew, C. W.; Wang, J. L.; Deem, M. W.; Davis, M. E.; Yan, Y. S. Angew. Chem., Int. Ed. 2006, 45, 6329–6332. (19) Li, Z. J.; Li, S.; Luo, H. M.; Yan, Y. S. Adv. Funct. Mater. 2004, 14, 1019– 1024. (20) Lee, J. S.; Ha, K.; Lee, Y.-J.; Yoon, K. B. Adv. Mater. 2005, 17, 837–841. (21) Zhang, B. Q.; Zhou, M.; Liu, X. F. Adv. Mater. 2008, 20, 2183–2189. (22) Lee, J. S.; Kim, J. H.; Lee, Y.-J.; Jeong, N. C.; Yoon, K. B. Angew. Chem., Int. Ed. 2007, 46, 3087–3090. (23) Zhou, M.; Liu, X. F.; Zhang, B. Q.; Zhu, H. M. Langmuir 2008, 24, 11942– 11946. (24) Ghoi, J.; Ghosh, S.; Lai, Z. P.; Tsapatsis, M. Angew. Chem., Int. Ed. 2006, 45, 1154–1158. (25) Lee, Y.; Ryu, W.; Kim, S. S.; Shul, Y.; Je, J. H.; Cho, G. Langmuir 2005, 21, 5651–5654. (26) Lee, J. S.; Lee, Y.-J.; Tae, E. L.; Park, Y. S.; Yoon, K. B. Science 2003, 301, 818–821. (27) Yang, W. S.; Liu, X. F.; Zhang, L.; Zhang, B. Q. Langmuir 2009, 25, 2271– 2277.

Published on Web 01/12/2010

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thin film.10 Yoon and co-workers proposed and developed an ingenious method to acquire highly aligned and preferentially oriented films of silicalite-1 crystals on glass plates.26 The crystal orientation in silicalite-1 films could be accurately manipulated by regulating the chemical and/or geometric nature of a polyurethane layer. It is very interesting that the polyurethane layer would disappear from the substrate surface after hydrothermal reaction, so there was no longer any need to remove the matrices. In the same way, a chitosan thin layer precoated onto a porous alumina substrate was used to manipulate the microstructure of SAPO-5 films.27 The chitosan thin layer also disappeared from the substrate surface after hydrothermal reaction because of the gradual dissolution of chitosan in the acid synthesis solution. Unfortunately, the available synthesis strategies for controlling the chemical and/or geometric microstructure of zeolite thin films and membranes during hydrothermal reaction are all based on trial and error because of the lack of a thorough understanding of the true formation mechanism. The actual formation process of supported zeolite films or membranes in a hydrothermal reaction involves nucleation and crystal growth via the scale-dependent self-assembly of building blocks in the bulk synthesis solution or on the substrate.28,29 The formation mechanism of supported zeolite thin films and membranes has been researched since the late 1990s. Currently, there are two main formation mechanisms for the hydrothermal synthesis of supported zeolite thin films and membranes. A formation mechanism named the heterogeneous nucleation model (HTN model) was established a decade ago.30,31 The core of the HTN model was the formation of a gel layer on the substrate surface, followed by nucleation and crystal growth at the interface of the gel layer and the synthesis solution. The HTN model was proposed on the basis of the analysis of supported pure silica MFI zeolite thin films acquired via hydrothermal reaction by using scanning electron microscopy (SEM) imaging together with corresponding X-ray diffraction (XRD) data30 or through in situ optical observation with SEM imaging.31 It can explain the fact that b-oriented crystals are formed on both horizontally and vertically placed substrates. Owing to the fact that not all of the gel layer in between could be completely consumed each time, it does not work when explaining the excellent adhesion between the zeolite thin film and the substrate surface. To contradict this model, a-oriented crystal grains could be easily observed on the monolayer. In particular, the monolayer usually consists of relatively uniform crystal grains that ought to be rarely formed on the interface, sometimes accompanied by the formation of the second-layer or multilayer structure. The SEM observations and analysis with respect to film evolution revealed that nucleation and crystal growth could occur both at the gel-solution interface and within the gel layer.32 Recently, Yan and co-workers proposed another formation mechanism, named the homogeneous nucleation model (HMN model), according to the growth of b-oriented pure silica MFI films on porous stainless steel substrates at different time intervals during hydrothermal reaction.33 On the basis of this model, the b-oriented MFI films were formed through homogeneous nucleation and crystal growth in the bulk synthesis solution, followed by the self-assembly of crystal grains onto the substrate surface, leading to the formation of a (28) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (29) Glotzer, S. C.; Solomon, M. J.; Kotov, N. A. AIChE J. 2004, 50, 2978–2985. (30) den Exter, M. J.; van Bekkum, H.; Rijn, C. J. M.; Kapteijn, F.; Moulijn, J. A.; Schellevis, H.; Beenakker, C. I. N. Zeolites 1997, 19, 13–20. (31) Koegler, J. H.; van Bekkum, H.; Jansen, J. C. Zeolites 1997, 19, 262–269. (32) Lai, R.; Yan, Y. S.; Gavalas, G. R. Microporous Mesoporous Mater. 2000, 37, 9–19. (33) Wang, Z. B.; Yan, Y. S. Microporous Mesoporous Mater. 2001, 48, 229–238.

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2D closely packed entity. Because the nucleation and crystal growth were only within the bulk synthesis solution, the constituent crystal grains in the MFI film were almost equally sized. The HMN model has a profound advantage in explaining the uniform size and patterned arrangement of zeolite crystals through the high-resolution transmission electron microscopy (HRTEM) observation of self-supporting MFI film samples and the backward analysis of film formation.34 However, the HMN model encounters unavoidable difficulty in explaining how the crystals can be tightly linked to the substrate via self-assembly. More important, the experimental results upon which this model was proposed did not provide direct evidence to prove that the nucleation and growth of zeolite crystals occurred only in the bulk synthesis solution. Besides, no experimental results indicate whether a gel layer existed. Thus far, all of the evidence in support of either the HTN model or the HMN model is circumstantial, and some important evidence is worthy of in-depth investigations. For example, information on the nucleation and crystal growth in the bulk synthesis solution ought to be very important in order to compare with that on supported zeolite monolayers and thin films. Direct observation of the evolution of supported zeolite monolayers on the subnanometer- or nanometer-scale level by using HRTEM is also critical in clarifying if the gel layer exists and what has really happened on it during the hydrothermal reaction. Although the direct observation of nucleation in the bulk synthesis solution or on the substrate surface is impossible at present, an accurate indication of the nucleation time during the hydrothermal reaction is definitely desirable. We therefore designed a set of systematic observations using atomic force microscopy (AFM), HRTEM, SEM, XRD, dynamic light scattering (DLS), electron probe microanalysis (EPMA), and high-pressure differential scanning calorimetry (HP-DSC). Direct HRTEM observation could be made on chitosan-supported gel layers and zeolite layers that were made by peeling the chitosan layer off of the substrate surface. The DSC data of the synthesis system could provide unambiguous evidence of the nucleation occurring during the hydrothermal reaction. The establishment and application of this well-designed strategy are unprecedented and supply insight into a more comprehensive formation mechanism of supported zeolite monolayers. Once the mechanism controlling the self-ordering process is fully understood, the crystal growth and self-assembly can be steered to engineer the microstructure of supported zeolite thin films and membranes.

II. Experimental Section Materials. All chemical reagents were analytical grade and used as purchased without further purification. The substrates used in our experiments were porous R-Al2O3 disks prepared inhouse.10 One side of the substrate was precoated with a thin layer of silica-zirconia (Si-ZrO2, ca. 3 μm thickness) that has an average pore size of ca. 8 nm (Supporting Information, Figure S6).35 Prior to use, the R-Al2O3/Si-ZrO2 substrate was boiled in acetone for 15 min and rinsed with doubly distilled water. After being dried in a clean atmosphere for 1 day, it was ready to use. The thin chitosan layer on the R-Al2O3/Si-ZrO2 substrate was prepared by using a spin-coater (KW-4A, Microelectric Institute of Chinese Academy of Sciences) and a ca. 1% (wt %) aqueous chitosan solution, which was made by dissolving chitosan (34) Li, S.; Li, Z. J.; Bozhilov, K. N.; Chen, Z. W.; Yan, Y. S. J. Am. Chem. Soc. 2004, 126, 10732–10737. (35) Liu, W.; Zhang, B. Q.; Liu, X. F.; Xu, L. M. Chin. J. Chem. Eng. 2006, 14, 31–36.

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Article Table 1. Preparation and Characterization of Collected Liquid, Powdery, and Film-Based Samples

samplesa

state

crystallization time (h)

preparation method

characterization

A30-L clear liquid direct collection DLS, HRTEM A30-H1-L clear liquid 1 direct collection DLS, HRTEM b DLS A30-H2-L white liquid 2 deposition A30-P yellow solid freeze drying XRD A30-H1-P white solid 1 freeze drying XRD A30-H2-P white solid 2 freeze drying SEM, XRD FS film spin coating AFM, SEM, XRD CFS film spin coating AFM, SEM, XRD A30-H1-CFS film 1 slow drying AFM, EPMA, SEM, XRD A30-H2-CFS film 2 slow drying SEM, XRD A30-H1-CF film 1 peeling HRTEM A30-H2-CF film 2 peeling HRTEM a Abbreviations: A30 = aged for 30 h; H1, H2 = hydrothermally treated for 1 and 2 h, respectively; L = liquid sample; P = powdery sample; FS = R-Al2O3/Si-ZrO2 substrate; CFS = chitosan layer on the R-Al2O3/Si-ZrO2 substrate; CF = chitosan layer peeled off of the CFS. b Sample A30-H2-L was the clear solution after the deposition of the collected synthesis solution for 6 h.

(deacetylation g90%, Zhenjiang Aoxing Chitin Company) in a 2% acetic acid solution.23 Synthesis. A clear precursor solution was used in this study. It was prepared by adding tetraethylorthosilicate (TEOS, 98%, Aldrich) to an aqueous solution of tetrapropylammonium hydroxide (TPAOH, 20%, Aldrich) dropwise under vigorous stirring, followed by aging under stirring at 298 K for 30 h. The final molar composition of the synthesis solution was 1:0.2:90 TEOS/ TPAOH/H2O. Then, 25 mL of the synthesis solution was loaded into a 50 mL Teflon-lined stainless steel autoclave. The substrate was held horizontally inside the synthesis solution by using a selfmade Teflon holder. In situ crystallization was carried out at 443 K for 1 and 2 h. After hydrothermal reaction, the sample was taken out of the autoclave and quenched to room temperature. The film-based sample was removed and thoroughly washed with doubly distilled water. Finally, the sample was dried at room temperature and 60% relative humidity for ca. 12 h and at 313 K in the oven overnight. Sampling. Both the synthesis solution and the synthesized film at each stage were collected as samples to follow the mechanistic evolution of zeolite monolayers on the substrate surface. The synthesis solution before and after hydrothermal reaction was collected in order to obtain both liquid and powdery samples; therefore, the nucleation and crystal growth in the bulk synthesis solution could be followed. For TEM investigation, the thin chitosan layer-supported zeolite monolayer was peeled off of the R-Al2O3/Si-ZrO2 substrate by drying the as-synthesized film at 333 K overnight after being rinsed with doubly distilled water. All of the samples collected at different stages are listed in Table 1. Instrumentation. The AFM analyses were performed on a Nanoscope III equipped with a 1553D scanner (Digital). Tapping mode in air was used to take the AFM images. The DLS analyses were carried out on a BI-200SM laser light scattering system (Brookhaven). DSC analyses were conducted with a high-pressure differential scanning calorimeter (DSC 204 HP, Netzsch). The composition of both C and Si along the film thickness was measured using EPMA (EPMA-1600, Shimadzu) with a secondary electron resolution of 6 nm. The vitrified specimens of liquid samples A30-L and A30-H1-L and the chitosan-supported zeolite films were imaged using HRTEM (Tecnai G2 F20, FEI). The SEM images were recorded on an XL30ESEM (Philips). XRD data were obtained on a Panalytical X’Pert Pro diffractometer using Co KR radiation (Philips). Detailed information has been given in the Supporting Information (section 6).

III. Results and Discussion The detection of the nucleation and crystal growth in the bulk synthesis solution was performed with DLS, HRTEM, and SEM. The direct observation of supported zeolite layers on each scale was achieved using AFM, EPMA, HRTEM, and SEM. The direct HRTEM observation could be made on chitosan-supported Langmuir 2010, 26(8), 5895–5900

gel layers and zeolite layers by peeling the chitosan layer off of the substrate surface. The experimental data obtained on a highpressure DSC were designed to provide evidence of nucleation during the hydrothermal reaction. The synthesis solution was aged for 30 h (A30-L or A30-P) or, after further hydrothermal reaction for 1 h (A30-H1-L or A30-H1-P) and 2 h (A30-H2-L or A30-H2-P), was collected to examine the formation of colloidal precursors, nucleation, and crystal growth in the bulk synthesis solution via HRTEM or SEM observations together with DLS and XRD measurements (Figure 1). After being aged for 30 h, amorphous and subcolloidal particles between ca. 2 and 20 nm in size in the synthesis solution are observed (Figure 1a), which is consistent with the corresponding DLS measurement (Figure 1e). According to the XRD data of powdery sample A30-P, the characteristic peaks of the MFI structure did not appear for the subcolloidal particles (Supporting Information, Figure S1a). After hydrothermal reaction for 1 h, subcolloidal particles in the synthesis solution are agglomerated into colloidal particles (Figure 1b,c). Some small crystallites are embedded in amorphous silica aggregates that are 20-50 nm in size (Figure 1b). Fully crystalline particles with MFI zeolite fringes (ca. 150 to 250 nm) in the synthesis solution can be observed with HRTEM (Figure 1c) and confirmed by XRD analysis (Supporting Information, Figure S1b). The DLS results indicate that the colloidal particles are distributed in a bidisperse fashion at around 30 and 200 nm (Figure 1e). After hydrothermal reaction for 2 h, zeolite crystals ca. 450 nm in size are detected in the clear solution (Figure 1d), which possess the MFI structure (Supporting Information, Figure S1c). Consistent with the SEM observation (Figure 1d), the DLS data of sample A30-H2-L show that the peak indicative of colloidal particles ca. 40 nm in size is dramatically decreased and the peak representing the generation of zeolite crystals ca. 450 nm in size becomes dominant (Figure 1e). The chitosan layer during the hydrothermal reaction was also checked using AFM, HRTEM, and SEM observations together with XRD analyses. The roughness of the R-Al2O3/Si-ZrO2supported chitosan layer is within (8 nm (Figure 2a). The chitosan layer becomes much smoother after hydrothermal reaction for 1 h (Figure 2b). The EPMA analysis of C and Si contents along the film thickness confirms the existence of a gel layer (Figure 2c). Because a fresh chitosan layer does not contain elemental Si, the Si distribution along the thickness implies that subcolloidal precursors in the synthesis solution have penetrated the chitosan layer, leading to the formation of the gel-chitosan layer. The XRD pattern of sample A30-H1-CFS shows that there are no Bragg reflections correspond to the MFI structure DOI: 10.1021/la903850r

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Figure 2. Evolution of the chitosan layer during the hydrothermal reaction. (a) SEM and AFM top views of the fresh R-Al2O3/ Si-ZrO2-supported chitosan layer (CFS). (b) SEM and AFM top views of the R-Al2O3/Si-ZrO2-supported chitosan layer after hydrothermal reaction for 1 h (A30-H1-CFS). (c) EPMA measurements of the C and Si contents with respect to the thickness of sample A30-H1-CFS. (d) HRTEM image of the chitosan layer peeled off of the R-Al2O3/Si-ZrO2 substrate (A30-H1-CF).

Figure 1. Evolution of zeolite crystals in the bulk synthesis solution. (a) HRTEM image of subcolloidal particles in the synthesis solution after being aged for 30 h (A30-L). (b, c) HRTEM images of colloidal particles in the synthesis solution after hydrothermal reaction for 1 h (A30-H1-L). (d) SEM image of crystal grains collected from the bottom of the autoclave after hydrothermal reaction for 2 h (A30-H2-P). (e) DLS data of the synthesis solution after being aged for 30 h (A30-L), followed by in situ crystallization for 1 h (A30-H1-L) and 2 h (A30-H2-L).

(Supporting Information, Figure S2b). Similar to that in the synthesis solution (Figure 1b,c), nanoscale crystallites on the gel-chitosan layer can be observed using HRTEM after hydrothermal reaction for 1 h (Figure 2d). Without being rinsed, a few submicrometer crystals are observed on the chitosan surface (Supporting Information, Figure S3). The solid hydrogel transformation has occurred within the gel-chitosan layer (Figure 2d), but it is rather slow compared to the crystal growth in the bulk synthesis solution (Figure 1b,c). The monolayer closely packed with b-oriented crystal grains is formed after hydrothermal reaction for 2 h (Figure 3a). Only (0k0) lines are present in the XRD pattern (Supporting Information, Figure S2c). As shown in Figures 1d and 3a, the zeolite crystals on the chitosan layer are roughly equal in size to those collected from the synthesis solution (sample A30-H2-L). To detect what had happened to the gel-chitosan layer after hydrothermal reaction for 2 h, the uncovered area was observed with HRTEM. As shown in Figure 3b, the amorphous gel on the gel-chitosan layer in Figure 2b,c was almost consumed in the hydrothermal reaction. The nanoscale crystallites embedded in the gel-chitosan layer have been transformed to zeolite crystals of ca. 10 nm size, which are much smaller than the crystal grains 5898 DOI: 10.1021/la903850r

Figure 3. Zeolite monolayer on the gel-chitosan layer. (a) SEM image of the zeolite monolayer on the Al2O3/Si-ZrO2-supported chitosan layer after hydrothermal reaction for 2 h (A30-H2-CFS). (b) HRTEM image of the gel-chitosan layer through the crystal boundaries in image a (A30-H2-CF).

collected from the synthesis solution after hydrothermal reaction for 2 h. It can be concluded that the zeolite monolayer is formed by self-assembling crystal grains onto the gel-chitosan surface under the prerequisite that there is no nucleation during the second 1 h hydrothermal reaction. Calorimetry provides a direct way to link a chemical event on the molecular scale to a macroscopically measurable property. The principal advantage of calorimetric methods is to make sure that the system is undisturbed, and the heat effects can be clearly recognized to indicate if nucleation is involved in the system. In situ calorimetry was employed to investigate the crystallization of TPA-silicalite-1 from the initially clear synthesis solution at 368 K.36-38 It has been demonstrated that the crystallization is transformed from exothermic to endothermic and that this (36) Yang, S. Y.; Navrotsky, A. Chem. Mater. 2002, 14, 2803–2811. (37) Yang, S. Y.; Navrotsky, A.; Wesolowski, D. J.; Pople, J. A. Chem. Mater. 2004, 16, 210–219. (38) Navrotsky, A. Curr. Opin. Colloid Interface Sci. 2005, 10, 195–202.

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Figure 4. DSC monitoring of in situ crystallization. DSC curve of the hydrothermal reaction, which is analogous to that in the Teflon-lined stainless steel autoclave. The open circles refer to the actual temperature change in the Teflon-lined stainless steel autoclave, and the blue line is the programmed temperature change during the DSC measurement.

exo-endo switch is associated with the alkalinity change in the synthesis solution. Although in situ calorimetry was used to study the crystallization of zeolite crystals only below 373 K, it is could also be used to reveal if there is nucleation during the second 1 h hydrothermal reaction. The temperature variation of the synthesis solution in a selfmade Teflon-lined stainless steel autoclave during the hydrothermal reaction was recorded using a computer-aided system. After hydrothermal reaction for 2 h, the actual temperature in the synthesis solution is ca. 433 K, 10 K lower than desired (Supporting Information, Figure S4). The in situ calorimetric process was applied here to follow the actual crystallization in the autoclave (Figure 4), where the temperature change (solid blue line) is closely comparable to the actual situation (open circles). It is reported that the clear synthesis solution yields a metastable nanoparticle phase at low temperature, enters a transition region with the increase in temperature, and eventually reaches the bulk phase with zeolite crystals.38,39 The experimental data demonstrate that the transition region also exists in the in situ crystallization of the supported zeolite monolayer (Figure 4). A very broad endothermic peak between 310 and 380 K is attributed to the enthalpy of water vaporization. The acute endothermic peak at 392 K (corresponding to ca. 47 min of crystallization time) is caused by the phase transition in the bulk synthesis solution because there is no obvious transition on the substrate surface after crystallization for 1 h (Figure 2). The endothermic peak decreases back to the baseline at ca. 405 K (corresponding to ca. 60 min crystallization time). This implies that the nucleation occurred only between 47 and 60 min. The exothermic peak indicating the onset of self-assembling primary nanoparticles into macroparticles is weak, whereas the endothermic peak due to the release of OH- into solution during crystallization is remarkable. This may be due to the formation of macroparticles during aging or the easier endothermic reactions at high temperatures. Similarly, the alkalinity change in the synthesis solution associated with the exo-endo switch can be observed as well (Supporting Information, Figure S5). The formation of supported MFI zeolite monolayers during the hydrothermal reaction is illustrated in Figure 5. Colloidal particles and submicrometer crystals were formed in the bulk synthesis solution initially, and a gel-chitosan layer was formed (39) Mintova, S.; Olson, N. H.; Senker, J.; Bein, T. Angew. Chem., Int. Ed. 2002, 41, 2558–2561.

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Figure 5. Evolutionary mechanism of the zeolite monolayer with experimental evidence. Formation mechanism of the zeolite monolayer on the Al2O3/Si-ZrO2-supported chitosan layer during the hydrothermal reaction.

on the substrate surface with only a few submicrometer crystals physically adsorbed on the gel-chitosan layer. As zeolite crystals were formed in the bulk synthesis solution, the solid hydrogel transformation occurred at the gel-chitosan layer. When the submicrometer crystals grew up to disk-shaped grains ca. 450 nm in size in the bulk synthesis solution, a large number of crystal grains were deposited onto the gel-chitosan layer via selfassembly because of the existence of amino and hydroxyl groups together with ether linkages,10 leading to the formation of the continuous monolayer. Because MFI zeolite crystals were diskshaped with the largest surface area in the (010) plane, they would be attached with their b axes parallel to the substrate surface. The crystals were further fastened onto the substrate surface by the solid hydrogel transformation on the gel-chitosan layer. Direct evidence allows a closer examination of previous models. Similar to the HTN model,30,31 a gel layer was initially formed on the chitosan layer in this study, but the crystal growth on the chitosan surface was dramatically restrained. Except for the existence of the gel layer and the subsequent formation of nanoscale crystals on it, our experimental evidence agreed with the HMN model.33,34 The homogeneous nature of monolayer formation is based on the fact that the crystal growth on the chitosan surface is fatally restrained compared with that in the bulk synthesis solution. On the contrary, when the crystal growth on the substrate surface is significantly enhanced, such as in the synthesis of structured silicalite-1 catalyst pellets aided with polyurethane foams40,41 or the fabrication of b-oriented TS-1 films using chitosan as the structure-directing matrix,42 the formation mechanism follows the HTN model. It could be concluded that whether the HMN or the HTN model applies to the evolutionary formation of a supported zeolite film is directly related to the growth rate of zeolite crystals on the substrate. When the crystal growth on the substrate is fatally restrained, the HMN model applies. Otherwise, the HTN model follows if the growth rate on the substrate is significantly increased. It should be noted that the synthesis of powdery zeolite crystals differs largely from that of supported zeolite films and membranes in the formation mechanism. To date, the formation mechanism (40) Lee, Y.-J.; Lee, J. S.; Park, Y. S.; Yoon, K. B. Adv. Mater. 2001, 13, 1259– 1263. (41) Lee, Y.-J.; Yoon, K. B. Microporous Mesoporous Mater. 2006, 88, 176–186. (42) Liu, X. F.; Wang, X. D.; Na, P.; Jiang, H. Y.; Lang, L.; Zhao, H. S.; Zhang, B. Q. Chin. Sci. Bull., in press. (43) Knight, C. T. G.; Kinrade, S. D. J. Phys. Chem. B 2002, 106, 3329–3332.

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of powdery zeolite crystals has been intensively researched.39,43-45 The formation mechanism of supported zeolite films and membranes is much more complicated because of the presence of the substrate surface. In particular, the strong influence of the substrate surface on the formation of zeolite films and membranes hurdles the structure control.6,10,27,46 Our experimental evidence and analysis can serve as a knowledge base by which to steer the crystal growth and self-assembly in the hydrothermal reaction aimed at engineering the microstructure of zeolite films and membranes.

IV. Conclusion The microstructure control and optimization of zeolite films and membranes as an indispensable challenge for various innovative applications can be steered by understanding the formation process. Here we have designed an unprecedented strategy to uncover direct evidence pertaining to the hydrothermal synthesis of chitosan-supported zeolite monolayers. The chitosan-supported layer in the hydrothermal reaction is observed using SEM, AFM, EPMA, and HRTEM, and the nucleation and crystal growth in the bulk synthesis solution are pursued with (44) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 3333–3334. (45) Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J. S.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; Miccormick, A. V.; Penn, R. L.; Tsapatsis, M. Nat. Mater. 2006, 5, 400–408. (46) Lang, L.; Liu, X. F.; Zhang, B. Q. Appl. Surf. Sci. 2009, 255, 4886–4890.

5900 DOI: 10.1021/la903850r

HRTEM, DLS, and SEM. The direct HRTEM observation is achieved on the chitosan-supported layer by peeling chitosan off of its support. This has revealed that a gel layer is initially formed on the chitosan layer where the subsequent crystal growth is fatally restrained. Our own experimental evidence and literature reports clearly demonstrate that the formation mechanism is homogeneous for the severely reduced crystal growth on substrate but it is heterogeneous when the crystal growth on a substrate is significantly enhanced. Our findings can serve as a knowledge base by which to steer crystal growth and self-assembly in the hydrothermal reaction aimed at engineering the microstructure of zeolite films and membranes. Acknowledgment. This research was supported by the National Natural Science Foundation of China (grant nos. 20636030, 20776100, and 20776108) and the National Basic Research Program of China (grant no. 2009CB623403). Supporting Information Available: The crystal structure of powdery samples and supported films, the surface morphology of the Al2O3/Si-ZrO2-supported chitosan layer after hydrothermal reaction for 1 h, the DSC study conducted during the hydrothermal reaction, the morphology of the R-Al2O3-supported silica-zirconia layer, and detailed information on sample characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(8), 5895–5900