Zeolite is a generic term referring to a group of crystalline aluminosilicates. Because the sites around aluminum in the framework of aluminosilicate have negative charges, cations for charge balancing are present in the pores, and the remaining space in the pores is usually filled with water molecules. The structure, shape and size of the three-dimensional pores of zeolite vary depending on the type of zeolite, but the diameter of the pores usually corresponds to the molecular size. Thus, zeolite is also called “a molecular sieve”, because it has size selectivity or shape selectivity for molecules entering the pores depending on the type of zeolite.
Meanwhile, zeotype molecular sieves in which silicon (Si) and aluminum (Al) atoms constituting the framework structure of zeolite are partially or wholly replaced by various other elements have been known. Examples of the known zeotype molecular sieves include porous silicalite-based molecular sieves free of aluminum, AlPO4-based molecular sieves in which silicon is replaced by phosphorus (P), and other zeotype molecular sieves obtained by substituting a portion of the framework of such zeolite and zeotype molecular sieves with various metal atoms such as Ti, Mn, Co, Fe and Zn. These zeotype molecular sieves, being materials derived from zeolites, do not belong to the zeolite group based on the mineralogical classification but are commonly known as zeolites in the art.
Accordingly, the term “zeolite” as used herein is meant to include the above-mentioned zeotype molecular sieves in a broad sense.
Meanwhile, zeolites having an MFI structure are most commonly used in the art and include the following types:                1) ZSM-5: MFI zeolite in which silicon and aluminum are present in a specific ratio;        2) Silicalite-1: zeolite composed only of silica; and        3) TS-1: MFI zeolite in which aluminum sites are partially replaced by titanium (Ti).        
The structure of an MFI zeolite is shown in FIG. 1. In the MFI zeolite, elliptical pores (0.51 nm×0.55 nm) are connected in a zigzag configuration to form channels extending in the a-axis direction, and substantially circular pores (0.54 nm×0.56 nm) linearly extend in the b-axis direction to form linear channels. No channels remain open in the c-axis direction.
Powdery MFI zeolites are very widely used in household and industrial applications, including petroleum cracking catalysts, adsorbents, dehydrating agents, ion exchangers, gas purifiers, etc. MFI zeolite thin films formed on porous substrates, such as porous alumina, are widely used as membranes through which molecules can be separated on the basis of size. Furthermore, MFI zeolite thin films can be used in a wide range of applications, including second- and third-order nonlinear optical thin films, three-dimensional memory materials, solar energy storage devices, electrode auxiliary materials, carriers of semiconductor quantum dots and quantum wires, molecular circuits, photosensitive devices, luminescent materials, low dielectric constant (k) thin films, anti-rusting coatings, etc.
As described above, the pore shape, size and channel structure of MFI zeolites vary depending on the crystal direction.
Meanwhile, methods for producing MFI zeolite thin films on substrates such as glass plates are broadly divided into a primary growth method and a secondary growth method. According to the primary growth method, a substrate is soaked in a gel for the synthesis of MFI zeolite without any pretreatment, and then spontaneous growth of an MFI zeolite film on the substrate is induced. Herein, the gel for synthesis contains tetrapropylammonium hydroxide (TPAOH) as a structure-directing agent or an organic template. In this case, b-axis-oriented MFI zeolite crystals grow perpendicular to the substrate at the initial stage of the reaction. At this time, a-axis oriented crystals begin to grow parasitically from central portions of most of the crystals grown on the glass plate. In addition, with the passage of time, the crystals grow in various directions, and as a result, the final thin film has various orientations. The randomly oriented MFI zeolite thin film is useful in some applications, but its applicability is limited. Particularly, when the randomly oriented MFI zeolite thin film is applied as a membrane for the separation of molecules, the molecular permeability, which is one of the most important factors in molecular separation, is markedly reduced. When structure-directing agents other than TPAOH are used in the primary growth method, no MFI zeolite thin film grows on the substrate. To overcome such problems, the secondary growth method is used.
In the secondary growth method, a substrate having MFI zeolite crystals attached thereto is soaked in an MFI zeolite synthesis gel, and then allowed to react to form an MFI zeolite thin film. Herein, the MFI zeolite crystals attached to the substrate act as seeds. The orientation of the MFI zeolite crystals attached on the substrate plays a very important role in determining the orientation of the MFI zeolite thin film to be produced later. For example, when the a-axes of the MFI zeolite seed crystals are oriented perpendicular to the substrate, the a-axes of the MFI zeolite thin film formed therefrom tend to be oriented perpendicular to the substrate, and when the b-axes of the MFI zeolite seed crystals are oriented perpendicular to the substrate, the b-axes of the MFI zeolite thin film formed therefrom tend to be oriented perpendicular to the substrate.
However, the orientation of the resulting zeolite thin film is highly sensitive to a structure-directing agent contained in the synthesis gel added to form the thin film rather than to the orientation of the seed crystals. For example, the MFI synthesis gel which has been used in the secondary growth method usually contains TPAOH. In this case, even when the MFI zeolite seed crystals are attached to the substrate such that the a- or b-axis is oriented perpendicular to the substrate, the orientation of the resulting MFI zeolite thin film changes randomly.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present disclosure.
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