μ-Tiles and mortar approach: A simple technique for the facile fabrication of continuous b-oriented MFI silicalite-1 thin films

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Abstract

Here we report a simple method, what we call a μ-tiles and mortar method, to grow highly b-oriented MFI thin films on substrates in a facile manner. The method involves three main steps: (1) rapidly assembling b-oriented MFI micro-crystal layers on porous substrates using manual assembly (i.e., direct assembly by rubbing crystals on functionalized surfaces), (2) passivating the flat crystal faces by sputtering a thin gold/palladium layer to prevent the nucleation and growth of crystals along the out-of-plane direction while allowing the growth of crystals along the in-plane direction, and followed by (3) filling the gaps between the MFI micro-crystals by allowing the crystals to grow only in the plane while preventing the out-of-plane growth of the crystals. It is shown that the key to the successful fabrication of oriented MFI thin films is to promote the in-plane (i.e., parallel to the support) growth of oriented zeolite tiles on substrates during the secondary growth step so as to eliminate the intercrystal gaps and generate a continuous film while preventing the out-of-plane (i.e., perpendicular to the support) growth and nucleation of mis-oriented grains. Using the μ-tiles and mortar method, we have demonstrated that continuous b-oriented MFI thin films can be readily made with controllable microstructure such as membrane thickness, size of single crystal domain, and density of grain boundary. We have also made, for the first time, MFI thin films that have preferentially oriented crystal grains with randomly oriented grain boundary structure. The method reported here has potentials to address some of the current engineering challenges for practical applications of zeolite membranes by enabling the fabrication of oriented zeolite membranes in a simple manner.

Introduction

Zeolites are crystalline materials with compositions and nanoporous structures that can be fine-tuned for many important applications such as catalysis, adsorption, and ion exchange [1], [2]. Apart from the use of zeolites in powder form, thin films of zeolites have a wide range of applications such as separation membranes [3], [4], [5], corrosion-protective coatings [6], sensors [7], and low dielectric constant films [8]. Zeolite thin films as high resolution selective membranes are of particular commercial interest due to their potential as energy-efficient alternatives to the current processes of distillation, crystallization, and others. Zeolite molecular sieve membranes can be used in a wide range of operating conditions such as high temperatures, high pressures, and in reactive environments, while allowing for membrane regeneration by aggressive treatments. Due to these superior properties of zeolite membranes, there is a wide range of zeolites that have been prepared in the form of membranes: MFI, zeolite A, faujasite (X and Y forms), mordenite, ferrierite, MEL, zeolite P, chabazite, SAPO-34, DDR, and a few other zeolites [3], [4], [9].

Due to the simplicity of its synthesis and its potential in the separation of light hydrocarbons such as xylene isomers, the structure-type MFI (also known as ZSM-5) [10] has attracted a great deal of interest as a model zeolite system for the development of a generalized membrane fabrication method [11]. The pore structure of the MFI zeolite is shown in Fig. 1. It has straight channels with an approximate pore opening of 0.56 nm × 0.54 nm along its b-axis and sinusoidal channels with an estimated pore opening of 0.51 nm × 0.55 nm along its a-axis. Though there are no direct channels along its c-axis, diffusion along the c-axis is still possible since the channels along the a- and b-axes are interconnected.

Since zeolite membranes are polycrystalline in nature, their microstructure plays an important role in determining their performance as membranes [5]. Therefore, controlling the microstructure of zeolite membranes such as the size and orientation of the grain and grain boundary structures has great implications [3], [4], [11]. It has both experimentally [5], [12] and computationally [13] been demonstrated that b-oriented MFI membranes show superior performance for organic vapor separations. Lai et al. [5] experimentally showed that the selectivity of p-xylene (kinetic diameter ∼0.58 nm) over o-xylene (kinetic diameter ∼0.68 nm) through their b-oriented MFI membranes was unprecedented (above 500) while the permeance of the membranes approached to that of the porous alumina support. This unprecedented increase in the membrane performance has been attributed to two main factors: (1) channels are straight along the b-axis, thus effectively enhancing selective intracrystalline diffusion of molecules and (2) b-oriented membranes result in better grain structures as compared to those of membranes with other orientation, thus effectively reducing non-selective intercrystalline diffusion of molecules through the grain boundary. In addition, better grain structures result in improved resistance to crack formation during the organic template removal step via calcination.

Several groups have reported b-oriented ZSM-5 membranes by both in situ and secondary growth methods [14], [15], [16], [17], [18], [19]. However, Tsapatsis’ group is the first to report the permeation results for b-oriented MFI membranes which were prepared by a secondary growth (i.e., seeded growth) method [20], [21]. In order to make their b-oriented MFI membranes, they first prepared b-oriented seed layers on mesoporous silica-coated α-Al2O3 substrates using a chemical interaction-based method developed by Ha et al. [22]. They then engineered relative growth rates along each crystallographic direction by using custom-made organic templates (so called, dimer-TPA and trimer-TPA) [23], such that the growth rate along the b-axis is comparable to that along the c-axis while significantly faster than that along the a-axis. By doing so, they were able to preserve the crystallographic orientation of the zeolite seed crystals during the secondary growth process. Later, the same group reported a-oriented MFI membranes using the same strategy [24]. Despite the remarkable performance of their b-oriented MFI membranes, the complexity of their manufacturing process (i.e., time-consuming preparation steps and the need for organic solvents and template synthesis) makes it difficult to produce high performance b-oriented MFI membranes in a reproducible and cost-effective way for the commercial application.

Apart from control of the preferred crystal orientation, there are a number of challenges that need to be addressed to find more commercial applications of zeolite membranes, particularly MFI membranes [3], [4]. Some of these challenges include: (1) synthesis of membranes with high permeability and selectivity (i.e., oriented, thin layers and small effective thicknesses) in a commercially viable manner, (2) reproducibility and long-term stability of membrane performance, (3) scaling-up of membrane modules, (4) cost of membranes (i.e., manufacturing cost and substrate cost), and (5) easy crack healing during membrane operation.

Here we report a novel method, which we call the μ-tiles and mortar approach, that enables the fabrication of b-oriented MFI thin films and membranes in a simple manner with the potential to address some of the current engineering challenges listed above. The proposed fabrication method is analogous to a macroscopic tiling process, i.e., assembling tiles and jointing them with mortar [11]. The proposed method (see Fig. 2) involves three main steps: (1) rapidly assembling b-oriented MFI micro-crystals on porous substrates using manual assembly [25] (i.e., direct assembly by rubbing crystals on functionalized surfaces), (2) passivating the out-of-plane crystal faces, i.e., the (0 1 0) faces, to prevent the nucleation and growth of the crystals in the out-of-plane direction while allowing the growth of the crystals in the in-plane direction, and lastly (3) filling the gaps between the MFI micro-tiles by allowing the crystals to grow only in the plane while preventing out-of-plane growth. The key to successful secondary growth is to promote the in-plane (parallel to the support) growth of the zeolite tiles so as to eliminate intercrystal gaps and generate a continuous film while preventing the out-of-plane (perpendicular to the support) growth and nucleation of mis-oriented grains. In addition, since one can calcine templates before the formation of continuous thin films, our method does not require the template removal step using a high temperature calcination process, thus avoiding the potential crack formation during this process.

Section snippets

Materials

All chemicals were used as received without further purification. Tetraethyl orthosilicate (TEOS, 98%, Sigma–Aldrich), tetrapropylammonium hydroxide (TPAOH, 1.0 M in H2O, Sigma–Aldrich), poly(ethyleneimine) (PEI, Mw = 25,000, Sigma–Aldrich) and ethanol (99.5%, Acros) were used as supplied. Deionized water (DI water) was used throughout the study. Silicalite-1 seeds with the average size of 0.8 × 0.7 × 0.4 μm3 and 2.5 × 1.8 × 0.7 μm3 were synthesized from a gel with the molar ratio of 5TEOS:1TPAOH:500H2O [21]

Results and discussion

Fig. 2 illustrates the procedure to synthesize b-oriented MFI thin films on glass or silicon wafer substrates. First, plate-like MFI seed crystals were hydrothermally synthesized following procedures reported in the literature [2], [21], [25]. To achieve b-oriented seed crystal layers, the seed crystals were then manually assembled onto substrates (tethered with poly(ethyleneimine) (PEI), a polymeric electrolyte, as a linker) by simply rubbing the crystals onto the substrates following the

Conclusions

In summary, we have developed a simple method based on a secondary growth technique, which we call the μ-tiles and mortar approach, to rapidly fabricate highly oriented zeolite thin films. The method enabled us to fabricate b-oriented MFI silicalite-1 thin films within several hours including the template removal process. The key step in our method is to passivate the flat facets of the seed crystals of the oriented seed layer, thus preventing the nucleation and growth of the crystals from the

Acknowledgments

H.-K.J. thanks the financial support from the Artie McFerrin Department of Chemical Engineering at Texas A&M University and Texas Engineering Experiment Station through new faculty startup. J.L.B. thanks the financial support from National Science Foundation through a REU Program (EEC0552655).

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