Solvent-Free Secondary Growth of Highly b-Oriented MFI Zeolite Films

Feb 11, 2019 - Department of Chemical and Biomolecular Engineering, University of Delaware , Newark , DE19716 , United States. J. Am. Chem. Soc. , 201...
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Solvent-free Secondary Growth of Highly b-Oriented MFI Zeolite Films from Anhydrous Synthetic Powder Xiaofei Lu, Yanwei Yang, Junjia Zhang, Yushan Yan, and Zhengbao Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00018 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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Solvent-free Secondary Growth of Highly b-Oriented MFI Zeolite Films from Anhydrous Synthetic Powder Xiaofei Lu,† Yanwei Yang,† Junjia Zhang,† Yushan Yan†,‡and Zhengbao Wang*,† †Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China ‡ Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE19716, USA Supporting Information Placeholder ABSTRACT: Up to date, zeolite films have been mainly fabricated by in situ crystallization, secondary growth in a solution/hydrogel, or occasionally by vapor phase transformation of dry gel. Here we demonstrate for the first time a solvent-free secondary growth method for b-oriented silica MFI zeolite films using the synthetic powder from ground anhydrous raw solids in the presence of NH4F. Typically, precisely b-oriented MFI zeolite films are synthesized from seed layers of highly b-oriented MFI zeolite crystals in the synthetic powder of 1SiO2:0.035TPABr: 0.05NH4F at 175 oC for 6 h. If needed, b-oriented MFI zeolite multilayer films can be acquired by changing the synthesis time or the amount of NH4F in the synthetic powder. Compared with the traditional hydrothermal synthesis, the approach developed here may provide a new avenue for fabricating high quality zeolite films/membranes.

Continuous zeolite films/membranes grown on different supports are attracting much research interest because of their wide applications as membranes, chemical sensors, low-k dielectrics, catalyst and optical materials.[1] Among them, the random-oriented films are intrinsically inclined to crack during calcining to remove the templates owing to the different thermal expansion coefficients along each principal axis, which imposes challenges in practical applications.[2] Compared with randomoriented zeolite membranes, uniformly oriented ones benefit from alignment of the integrated channels, which can improve the separation performance by decreasing the diffusion lengths, reducing the grain boundary defects and having ordered arrangement of nanopores.[2-3] The superior performance of b-oriented pure silica MFI zeolite membranes with straight channels vertical to the surfaces of porous supports have been demonstrated in isomer mixtures separation such as i-/n-butane and p-/o-xylene, and CO2 separation from natural or synthesis gas.[2-4] For example, Tsapatsis et al. designed a structure directing agent (SDA) of trimer-TPAOH (TPAOH= tetrapropylammonium hydroxide) and synthesized a highly b-oriented MFI membrane, which showed a high separation factor of ˃480 for the p-/o-xylene separation.[2a] By introducing the (NH4)2SiF6 and TEAOH (tetraethylammonium hydroxide) into the synthesis hydrogel, a perfectly b-oriented membrane was obtained by Yoon et al., which showed an exceptionally high separation factor over 1000 for the p-/o-xylene separation.[3a] Although b-oriented MFI zeolite films were first prepared by in situ crystallization in the solution,[5] the availability of suitable

supports is limited. Therefore, MFI films/membranes with borientation have been commonly fabricated by secondary growth in the solution/gel.[1] Except for the above mentioned methods, various other synthetic approaches have been explored to restrain the growth of twin crystals when using TPA+ as the SDA.[6] For example, Yang et al. reported hydrothermal pretreated[6a] and microwave-assisted methods.[6b] Hedlund et al. used TPA++HF as SDAs.[4] Our group also developed many methods such as lowering TPAOH concentration,[6c] adding ammonium salts,[6d] neutral synthesis[6e] and ultra-diluting synthesis solution.[6f] The above mentioned methods for producing b-oriented MFI films/membranes are all based on synthesis solutions/gels, while Yoon’s group reported a gel-free secondary growth,[3b] which is actually a kind of steam-assisted crystallization method. Oriented MFI zeolite membranes were also fabricated by gel-less secondary growth of MFI-nanosheet layers in Tsapatsis’s group.[7] A silica layer is usually needed for this method. The use of solvents in the alkaline synthesis solution could cause safety and environment issues. Therefore, the development of a new crystallization method without the use of solvent is highly desirable. The previous studies showed that the Fcontaining route leads to zeolite membranes with a low density of lattice defects, pinholes or cracks.[3a, 4] To obtain the defect-free zeolite film with the channels vertically oriented, a synthesis technique with the help of F- is an attractive alternative. Here we demonstrate for the first time that a highly b-oriented MFI zeolite film can be produced by a facile solvent-free secondary growth method. As shown in Scheme 1, silica (solid silica gel), TPABr and NH4F with a certain ratio were ground in a mortar, and then the obtained synthetic powder was transferred to cover the seeded substrate in a stainless steel autoclave with a Teflon-liner for secondary growth. An MFI film with high borientation can be acquired after heating at 175 oC for a few hours. Compared with other preparation strategies, with the use of fluoride as the mineralizing agent, b-oriented MFI films can be acquired in a synthesis mixture with dry reagents in the form of powder completely without a liquid phase. Water in the reaction was produced in the chemical reaction of solids. A closely-packed and b-oriented monolayer of MFI zeolite seeds was produced on a silicon wafer by manual rubbing (Figure 1a). The silicon wafer was horizontally placed in the autoclave and covered with finely ground powder of solid silica gel/TPABr/ NH4F (1:0.035:0.05). After secondary growth at 175 oC for 6 h, a continuous film with no noticeable twin crystals was formed (Figure 1b). The cross-sectional view further indicated that the prepared film was ~0.4 μm thick (Figure 1c). The film XRD pattern (Figure 1d) just exhibits the diffraction peaks from the

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planes of (020), (040), (060), (080), and (0100), indicating the borientation of the film. It is worth noting that the synthesis time for obtaining continuous films with high b-orientation for the solvent-free secondary growth method is much shorter than that used for secondary growth in the solution/semisolid gel employed fluoride media.[3a, 4]

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x=0.05 and 0.15 for the same crystallization time (6-10 h) (Figure S2). The crystals always grew faster for the high NH4F amount (x=0.15) when the synthesis time is the same.

Figure 1. SEM images of (a) seed layer of b-oriented MFI zeolite crystals on silicon wafer, (b, c) MFI zeolite film prepared by solvent-free secondary growth: (b) surface view and (c) cross-sectional view; (d) XRD patterns of MFI zeolite seed layer and the obtained MFI zeolite film (*silicon wafer substrate).

Scheme 1. Schematic illustration of the zeolite film fabrication process via the solvent-free secondary growth approach.

As a method for zeolite membrane synthesis, the so-called “dry gel conversion” (DGC) has been employed for transforming a dry gel layer to a zeolitic membrane under vapors.[8] The solvent-free route demonstrated here is easily distinguishable from “DGC”, where the organic SDAs or water was transported to the gel layer through the vapor phase to assist the crystallization. Our solventfree method does not add any solvent to the powder in the whole process. The manufacture of b-oriented MFI zeolite films usually needs a large amount of water (H2O/SiO2 =150-3000.[6] One way to form MFI membranes with high b-orientation using a small amount of water (semisolid, H2O/SiO2=10) was introducing the Fand OH- to the precursor as mineralizer, but in this case the quantity of water was still high enough to follow the classical mechanism of crystallization and usually a long synthesis time (e.g., one week) was needed.[3a] Therefore, we hypothesize that a different crystallization mechanism exists for the solvent-free secondary growth demonstrated here from the traditional hydrothermal synthesis in a solution. To better understand the growth process, effects of the composition of the synthetic powder and crystallization time have been investigated. The effect of NH4F amount (x in 1SiO2:0.035TPABr: xNH4F) on the quality of the produced zeolite films was investigated. Clearly, the seed crystals grew bigger even when a small amount of NH4F (x=0.05) was used for 2 h, but significant gaps existed between the crystals (Figure 2a). A nearly continuous film could be obtained when the synthesis time was prolonged to 4 h (Figure 2b). When x>0.05, the synthesis time for achieving continuous films could be dramatically shortened. When x=0.25, for instance, a continuous MFI film formed for only 2 h (Figure 2c). A few new b-oriented MFI crystals emerged simultaneously and attached on the continuous b-oriented film for 4 h (Figure 2d). The XRD patterns also demonstrated no change of the orientation using different amounts of NH4F in the synthetic powder (Figure S1). Moreover, we also compared the SEM images of the films obtained at

Figure 2. Effects of NH4F amount in the synthetic powder: SEM images of MFI zeolite films produced by solvent-free secondary growth on seeded silicon wafers at 175 oC in the synthetic powder (1SiO2:0.035TPABr:xNH4F), (a, b) x=0.05 and (c, d) x=0.25 for (a, c) 2 h and (b, d) 4 h.

From Figures 1 and 2, it is clear that the uniformly b-oriented MFI film was contaminated with some small random-oriented (mainly c-oriented) crystals as the NH4F/SiO2 ratio (x) is 0.05. This was discussed in Supporting Information (See effects of the amount of NH4F and Figure S3). The effects of the amount of TPABr (y in 1SiO2:yTPABr: 0.05NH4F) were also investigated. It is found that there is no significant difference between the films obtained at y=0.01 and 0.05 (Figure S4). A nearly continuous b-oriented zeolite film was obtained at a very low TPABr amount (y=0.01), and the orientation is still very well when y=0.05. Under hydrothermal synthesis of uniformly b-oriented MFI zeolite films in the secondary growth solution, it is well known that inhibiting the yield of new zeolitic crystals from the solution is the key to suppressing the formation of twin crystals in the boriented zeolite film.[3a] In the case of solvent-free secondary growth, the synthetic powder with a low amount of F- (e.g.,

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Journal of the American Chemical Society x=0.05 and 0.15) rarely form new MFI crystals during reaction periods monitored for up to 10 h (Figure S5 a and b). Therefore, it is reasonable that there is almost no formation of twin crystals in the b-oriented MFI films in the synthetic powder with x=0.05 or 0.15 (Figure S2). It is known that MFI zeolite crystals formed in the powder even after the 4 h synthesis (Figure S5c) and almost entirely crystallized after the synthesis of 24 h (Figure S5f) when x=0.25. However, a perfectly b-oriented continuous multilayer film without twin crystals was obtained after the synthesis of 24 h when x=0.25. The orientation of the film was also evidenced by SEM images (Figure S6) and the XRD pattern (Figure S7). The film thickness is ~1.5 μm, indicating that the thickness of boriented MFI zeolite films can be easily regulated by controlling the synthesis time and the amount of NH4F. Even though selfcrystallization happened in the powder with a high concentration of F-, the nucleation and growth of new crystals lie on the top of the dense zeolite film along the a & c-direction. This is very different from the phenomenon observed in the manufacture of boriented MFI zeolite films by the liquid phase secondary growth, which is a very interesting result deserving further investigation in the future. The effects of the amount of synthetic powder, the position of substrate and the type of substrate were discussed in Supporting Information. Our above results suggested a different crystallization mechanism from conventional hydrothermal synthesis. Schüth et al. reported the crystallization of zeolite crystals from a dry powder.[9] Based on the following reaction, they suggested a mass transfer process of vapor phase with SiF4 as the mobile species between the formed zeolite crystals and the solid phase of amorphous particles. SiO2 + 4 NH4F  SiF4+ 4 NH3+2H2O (1) This equilibrium at room temperature is on the left side, but shifts to the right side above 100 oC. Xiao et al. reported a solvent-free synthesis of pure silica MFI zeolite crystals from anhydrous raw solid reagents in the presence of NH4F.[10] They found that a weak XRD peak at 18.2°, which is assigned to ammonium hexafluorosilicate, appeared after grinding the synthetic power at room temperature. We also observed a weak XRD peak at 18.2° for the synthetic powders even after the synthesis (Figure S5). Based on these facts, we consider that (NH4)2SiF6 exists at room temperature and SiF4 exists at the synthetic temperature (e.g., 175 oC) in the synthetic powder (Scheme 1). We thus suggest a mass transfer process of vapor phase with SiF4 as the mobile silica species and water vapor produced in Reaction 1 as the reaction media during the epitaxial growth of seed crystals on the substrate and crystallization of new crystals. These can explain why the amounts of NH4F and the synthetic powder are very sensitive to the growth of b-oriented MFI zeolite films. The mechanism for the multi b-oriented layer formation is still not very clear, which deserves further research. Compared to the traditional secondary growth in the liquid phase, the solvent-free secondary growth route here has the obvious advantages as follows (Details are in Supporting Information): (1) a significant reduction of pollutants, especially waste water; (2) a more green process: saving time, energy and space; (3) a broad operation window of the synthetic composition; (4) simple procedures: seeding, grinding and heating; (5) a low cost process. These features make this route highly attractive from a good perspective. In summary, we now report an unprecedented solvent-free secondary growth in the synthetic powder of solid silica gel /TPABr/NH4F for the fabrication of precisely b-oriented MFI zeolite films. The composition of the synthetic powder with fluoride as mineralizer is broad, for instance, 1SiO2:0.010.05TPABr: 0.05-0.25NH4F. Moreover, the solvent-free route can

effectively suppress the twin crystal formation to obtain the multilayer b-oriented MFI zeolite films even though the selfcrystallization exists in the synthetic powder. This would open a completely new way for fabrication of zeolite films/membranes, which may be used as membranes, sensors or optical devices.

ASSOCIATED CONTENT Supporting Information Synthesis method, characterization data and some discussions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21676238 and 21236006).

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