Facile Fabrication of Ultrathin Tungsten-MFI Zeolite Films with

Jun 12, 2019 - Ultrathin and highly b-oriented tungsten-MFI (W-MFI) films are successfully synthesized by secondary growth in alkaline media. The effe...
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Facile Fabrication of Ultrathin Tungsten-MFI Zeolite Films with Enhanced Hydrophobicity Yue Dai, Yun Li, and Baoquan Zhang*

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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China ABSTRACT: Ultrathin and highly b-oriented tungsten-MFI (W-MFI) films are successfully synthesized by secondary growth in alkaline media. The effect of W species on the film formation is investigated. The synthesized films are characterized by using scanning electron microscopy, atomic force microscopy, X-ray diffraction, diffuse reflectance UV/Vis, Fourier transform infrared spectroscopy, and contact angle measurements. It has been demonstrated that the synthesized W-MFI films with a thickness of ca. 320 nm are smooth and highly b-oriented with a tiny roughness. The incorporation of W onto the framework of MFI zeolites could promote in-plane growth and whittle framework defects during secondary growth. The W-MFI films with a smooth outer surface exhibit a much higher water contact angle of 128.2° over the Si-MFI films synthesized either in alkaline media or in fluoridecontaining neutral media, leading to significantly enhanced hydrophobicity on the film surface. The W-MFI film free of silanol would have broad perspects in catalysis and separation.

1. INTRODUCTION Zeolite films and membranes, owing to their precisely defined pore structure, a tunable composition, and excellent stabilities, are particularly promising in applications as separation, catalysis, corrosion protection, and low-k materials.1−5 Of the reported zeolite films, ultrathin and b-oriented pure silica MFI (Si-MFI) films are of great interest due to their short diffusion pathway as well as high hydrophobicity. Compared with in situ crystallization, secondary growth has been considered as an effective method to fabricate b-oriented Si-MFI films owing to its higher reproducibility and lower constraints with supports. However, the in-plane growth is always accompanied by serious out-of-plane growth when tetrapropylammonium hydroxide (TPAOH) is used as the structure-directing agent (SDA), which would weaken the oriented growth and increase the film thickness. Presently, various optimization methods have been proposed to suppress the out-of-plane growth during secondary growth. Altering the composition of the synthesis solution such as SDA concentration,6 water amount,7 and silica species2 is a simple way to inhibit the formation of twin crystals. Microwave irradiation heating was also pioneered to prepare ultrathin MFI zeolite films with preferential b-orientation in a reduced crystallization time.8,9 Crystallization-mediating agents such as ammonium salts and aluminum sulfate were also used to promote the in-plane growth of crystals within the film.10,11 In addition, the replacement of TPAOH by trimer-TPAOH1 and tetraethylammonium hydroxide12 had been investigated to trigger in-plane growth. Despite the success achieved in fabrication of b-oriented MFI zeolite films with minimized thickness, these new attempts have some limitations. For instance, in an optimized synthesis solution, the growth of twin crystals could not be completely suppressed when the synthesis time is in a reasonable range. The synthesis of trimer-TPAOH © 2019 American Chemical Society

needs strict reaction media and multistep procedures. It should be noted that the Si-MFI films synthesized in alkaline media bring a less than desired level of framework defects, resulting in adverse impacts on their surface wettability and separation performances.13−16 Zhou et al. synthesized thin and oriented Si-MFI zeolite membranes in neutral and fluoride-containing media to alleviate the framework defects on the membrane surface.16 Recently, Lu et al. developed a solvent-free secondary growth by covering the seeded silica wafer with ground powder of silica gel, tetrapropylammonium bromide and NH4F to fabricate ultrathin and b-oriented MFI zeolite films with fewer framework defects.17 Although the synthesized Si-MFI films in fluoride media contain fewer framework defects, the synthesis solution is severely corrosive and toxic, which gives rise to significant security, economic, and ecological issues for possible commercial use. Therefore, it is desirable to synthesize ultrathin and perfectly b-oriented MFI films with superior hydrophobicity in alkaline medium. By introducing heteroatoms with different oxidation states and electronegativity, the isomorphous substitution of silicon has been playing a critical role in modifying the wettability and separation performances of zeolite films.18−20 Recently, Grand et al. reported the synthesis of tungsten-substituted MFI zeolites (W-MFI) in alkaline media to prevent the formation of silanol by introducing framework W.21 Inspired by their novel outcome, it could be expected that the W-MFI film would demonstrate enhanced hydrophobicity over Si-MFI ones with an application potential in separation. In this contribution, the ultrathin and highly b-oriented W-MFI films with enhanced hydrophobicity are synthesized in alkaline synthesis solutions Received: March 5, 2019 Revised: May 7, 2019 Published: June 12, 2019 4521

DOI: 10.1021/acs.cgd.9b00281 Cryst. Growth Des. 2019, 19, 4521−4525

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by the seeded growth on b-oriented seed layers. The effects of W species on the crystal growth and film formation will be discussed.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Sodium tungstate dihydrate (Na 2 WO 4 ·2H 2 O, Aldrich), tetrapropylammonium hydroxide (TPAOH, 40%, Aldrich), and tetraethylorthosilicate (TEOS, 98%, Aldrich) were used as supplied. Glass plates (20 mm × 20 mm) were treated with hydrogen peroxide solution for 30 min, followed by sonication in acetone, and rinsed with deionized water (DI H2O) before use. 2.2. Preparation of b-Oriented Seed Monolayers. Coffinshaped Si-MFI crystals with an average size of 0.55 × 0.25 × 0.72 μm3 were synthesized as described previously.9 By slowly adding TEOS to the solution containing TPAOH and H2O under stirring, a clear synthesis solution with a ratio of 1TEOS/0.2TPAOH/90H2O could be obtained. Then, the clear synthesis solution was transferred to a Teflon-lined autoclave, and the reaction was carried out at 130 °C for 8 h with stirring. The resulting crystals were thoroughly washed by centrifugation and dried at 60 °C. Before seeding, the SDA included in the crystals was removed at 500 °C with a ramp rate of 1 °C·min−1. Highly b-oriented Si-MFI seed monolayers were fabricated by directly rubbing seed crystals on clean glass plates.22 2.3. Fabrication of MFI Zeolite Films. W-MFI and Si-MFI zeolite films were fabricated through secondary growth in ultradilute solutions with TPAOH as a template.9 The seeded glass plate was horizontally placed in a 50 mL Teflon-lined autoclave and immersed into the synthesis solution with the ratio of 1TEOS/0.2TPAOH/ 800H2O/(0−0.01)Na2WO4. Subsequently, the autoclave was placed in an electric oven at 120 °C for 16 h. The resultant sample was rinsed with DI H2O and dried at 60 °C after synthesis. 2.4. Characterization. The coordination state of W species in the zeolitic structure was analyzed by using diffuse reflectance UV/vis (DR-UV/vis, PerkinElmer Lambda 850, USA) spectra with diffuse reflectance integrating sphere. The chemical structures of zeolite crystals were investigated by a Fourier transform infrared spectrometer (FTIR, Nicolet 6700, USA) in the region of 400 to 4000 cm−1. The orientation of the as-synthesized zeolite monolayers and films was confirmed by X-ray diffraction measurements (XRD, D8-Focus, Bruker, Germany) using Cu Kα radiation. The morphologies of the seed layers and films were recorded on a field emission scanning electron microscopy (FESEM, Hitachi, S-4800, Japan). The roughness of the film surface was measured using atomic force microscopy (AFM, Multimode 8, Bruker Nano Inc., USA) with the noncontact tapping mode. The static contact angle of water (WCA) was conducted by a DropMeter A-200 contact angle system (MAIST VisionInspection & Measurement Co. Ltd., China) at room temperature with a total drop size of 2 μL.

Figure 1. Top and cross-sectional SEM images of seed layer (a, b), SiMFI films (c, d), and W-MFI films synthesized from the synthesis solution with a W/Si ratio of 0.005 (e, f), 0.0075 (g, h) and 0.01 (i, j).

suggesting that the incorporation of W plays a critical role in suppressing undesired twin growth during the seeded growth. The thickness of all W-MFI films is ca. 320 nm. Furthermore, the surface roughness of the Si-MFI film and W-MFI film with the W/Si of 0.0075 is detected by using AFM measurements (Figure 2). By comparison, the surface of the W-MFI film contains well-defined nearly oriented nanoscale hills and valleys with an average roughness of approximately 6 nm (Figure 2a,b), whereas hillocks with an average height of 27 nm are observed for the synthesized Si-MFI film (Figure 2c,d). The smoother surface on the W-MFI film indicates a better in-plane intergrowth with much restricted formation of twin crystals compared with that of the Si-MFI film, just as already demonstrated in the above SEM observations. Figure 3 shows the XRD patterns of the seed layer, Si-MFI, and W-MFI films. Obviously, the seed layer is highly boriented (trace a of Figure 3). Only five distinct diffraction peaks at around 8.8°, 17.7°, 26.8°, 36.0°, and 45.5° associated with (020), (040), (060), (080), and (0100) reflections are observed for W-MFI films (trace c−e of Figure 3), revealing

3. RESULTS AND DISCUSSION 3.1. Inhibited Out-of-Plane Growth by W Species. The uniform and closely packed seed monolayers were obtained by manual assembly, which are conducive to the formation of compact and b-oriented MFI films (Figure 1a,b). After the seeded growth in the pure silica synthesis solution, a top layer with substantial twin crystals is formed on the seed layer with a total thickness of 460 nm (Figure 1c,d). Similar observations were also reported in the literature.7,10 With respect to the seeded growth using W-containing synthesis solutions, the synthesized films are shown in Figure 1e−j. The introduction of W species could dramatically suppress the generation of twin crystals (Figure 1e,f). When the W/Si ratio in the synthesis solution is increased to 0.0075 or more, the compact b-oriented W-MFI films with a smooth surface are observed with no twin crystals on the upper surface (Figure 1g−j), 4522

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peaks are observed for the Si-MFI film (trace a of Figure 4). On the contrary, the synthesized W-MFI films exhibit two

Figure 2. AFM images of W-MFI (a, b) and Si-MFI (c, d) films.

Figure 4. DR-UV/vis spectra of Si-MFI film (trace a), and W-MFI films with the W/Si ratio of 0.005 (trace b), 0.0075 (trace c), and 0.01 (trace d).

intense bands at 207 and 260 nm (trace b−d of Figure 4). The band at 207 nm is assigned to the ligand-to-metal charge transfer, which derives from the tetrahedral configuration of isolated [WO4]2− species. The band at 260 nm is assigned to O2− → W6+ charge transfer, which correlates with octahedral polytungstate species. The above bands are red-shifted with the increase of W loading, which agrees with the results reported elsewhere.23−25 The band assigned to WO3 (370 nm) is not observed. These results give strong evidence that W is successfully incorporated into the framework of MFI zeolite. 3.2. Enhanced Hydrophobicity. The surface wettability of the as-synthesized MFI films is evaluated based on WCA measurements. The as-synthesized Si-MFI film surface exhibits a WCA of ca. 70.8° (Figure 5a). By contrast, the synthesized

Figure 5. WCAs of as-synthesized Si-MFI (a) and W-MFI (b) films. Figure 3. XRD patterns (bottom figure) and magnified images (upper figure) of seed layer (trace a), Si-MFI film (trace b), and W-MFI films synthesized from the synthesis solution with a W/Si ratio of 0.005 (trace c), 0.0075 (trace d), and 0.01 (trace e).

W-MFI film exhibits a much higher WCA of 128.2° as illustrated in Figure 5b. This change in surface wettability could be attributed to the formation of flexible W−O−Si bridges instead of Si−O−Si, preventing the occurrence of the silanol group.21 Farzaneh et al. demonstrated that the Si-MFI films synthesized in fluoride-containing media possessed a WCA of ca. 85°.13 Therefore, the synthesized W-MFI film exhibits significantly enhanced hydrophobicity over both SiMFI films synthesized either in alkaline media or in fluoridecontaining neutral media. The elimination of silanol group in the MFI zeolitic structure after the incorporation of W species is further verified by FTIR measurement. To evade the influence of substrates, the FTIR spectra of zeolite crystals are measured.

that the synthesized W-MFI films are indeed perfectly boriented. For the synthesized Si-MFI film, the extra shoulder peaks at around 35.7° and 45.1° assigned to (800) and (1000) reflections are observed (trace b of Figure 3), indicating the occurrence of twin crystals. This again proves that undesired twin growth over seed layers can be effectively inhibited with the help of incorporated W atoms during the seeded growth, which is in accordance with SEM observations as above. The incorporation state of W is confirmed by DR-UV/vis spectra with a region of 200−600 nm. No obvious absorption 4523

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species onto the framework of MFI crystals would whittle framework defects by building flexible W−O−Si bridges during the seeded growth. Compared with Si-MFI films synthesized either in alkaline media or in fluoride-containing neutral media, the synthesized W-MFI zeolite films exhibited a much higher WCA of 128.2°. The fabrication in the presence of W species in alkaline media ensured the formation of ultrathin and highly b-oriented MFI films with very enhanced hydrophobicity.

As shown in Figure 6, apart from the characteristic framework bands of MFI zeolites, extra two bands at ca. 960 and 3500

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 2285356517. ORCID

Baoquan Zhang: 0000-0001-7571-8103 Notes

The authors declare no competing financial interest.

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Figure 6. FTIR spectra of zeolite crystals for Si-MFI (trace a) and WMFI (trace b).

ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 21136008)

cm−1 are observed for the Si-MFI zeolite crystals (trace a of Figure 6). The band at 960 cm−1 is associated with the stretching mode of O3Si−OH group, and the band at 3500 cm−1 corresponds to the H-bonded silanols on internal defects.21,26−28 Both absorptions indicate the existence of framework defects in Si-MFI zeolite crystals. By contrast, the absence of these two bands in W-MFI zeolite crystals suggests that the W-MFI zeolitic structure is free of framework defects (trace b of Figure 6), which contributes to the higher hydrophobicity of W-MFI zeolite films. 3.3. Role of W Species in b-Oriented Films. As what is known, the attachment of new MFI nuclei in the bulk solution to the (0k0) faces of pristine MFI seeds is the main reason for twin growth during the seeded growth.29,30 When W species is introduced, the formation of new nuclei is dramatically suppressed. In addition, the WVI species could coordinate with neighboring Si atoms to form more energetically favorable Si−O−W bonds, leading to the prevention of silanol formation.21 The absence of framework defects inhibits the attachment of nuclei onto the surface of the seed layer and improves the hydrophobicity of W-MFI films. Furthermore, W-MFI zeolites are also synthesized to illustrate the effect of W on crystal growth in this study. The growth rate of W-MFI crystals along three axes follows a:b:c = 2:1:2, indicating that the growth of W-MFI crystals along a- and c-axis dominates over that in b-axis. As a result, the incorporation of W could promote the in-plane growth within the b-oriented seed layer and is conducive to the fabrication of ultrathin and b-oriented W-MFI films. Therefore, a conclusion can be drawn that the synergistic effect of enhanced in-plane growth and the suppressive adherence of new MFI nuclei leads to the formation of highly b-oriented W-MFI films.

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REFERENCES

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4. CONCLUSIONS The ultrathin and highly b-oriented W-MFI zeolite films with a thickness of 320 nm were synthesized by using the seeded growth method in alkaline media. The incorporation of W species could promote the in-plane growth and suppress the attachment of nuclei on original MFI seeds, leading to the formation of ultrathin and perfectly b-oriented W-MFI films with a pretty low surface roughness. The incorporated W 4524

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