Verified Syntheses of Zeolitic Materials
2nd Revised Edition
Preparation of zeolite membranes
Laboratoire de Matériaux, Minéreaux
U.P.R.E.S.-A-CNRS 7016, ENSCMu,
Université de Haute Alsace
3, rue Alfred Werner, 6803, Mulhouse Cedex, France
Separation processes are widely used in industry since the chemical conversions are often incomplete. Membrane technique is one of the most attractive separation methods because of its low cost and high selectivity. A membrane is an intervening phase acting as an active or passive barrier between phases adjacent to it under a driving force. Zeolitic membranes have gained considerable attention during the last decade. Detailed information can be found in the current literature and in several excellent reviews dealing with the subject of zeolitic membranes which have appeared over the last five years. [1-7]
The regularly structured pores and cages make the zeolites a unique material for designing thin films, coatings and membranes that can be utilized for a variety of purposes. Since the early 1990s, intensive research efforts have been underway to develop the synthesis and separation applications of zeolitic membranes. The specific properties of zeolite membranes which have attracted the attention of academic and applications scientists are: (i) long-term stability at high temperature, (ii) resistance to harsh environments, (iii) resistance of high pressure drops, (iv) inertness to microbiological degradations, and (v) easy cleanability and catalytic activation.
One of the most challenging problems in the preparation of zeolitic membranes is the complete exclusion of pinholes from the membranes, particularly under conditions of severe thermal cycling.
2. Preparation of zeolitic membranes
Zeolitic films and membranes are completely different from simple crystalline zeolite powders and their preparation requires new strategies. Methods which have been developed for the preparation of zeolite membranes are as follows:
2.1. Zeolite-filled polymeric membranes
One of the most direct methods of preparation of zeolite-containing membranes is to embed zeolite crystals in a matrix. [2,6] Sealing the gaps between zeolite crystals with a gas-tight matrix can provide a membrane configuration. The application of this method of membrane preparation, however, is limited. The clogging of zeolite pores by the matrix is a serious concern. Furthermore, gaps between binder and zeolite or a porosity in the matrix could introduce nonselective diffusion pathways.
2.2. Free-standing zeolite films
For molecular sieving applications a dense, pin-hole free zeolite film with limited thickness (<1 m m) would be an ideal configuration. Such films have been grown on temporary supports like Teflon and cellulose or at the interface between two phases. [2,6] This route for the preparation of zeolitic membranes is abandoned now because of the fragility of self-supported zeolite membranes.
2.3. Supported zeolite membranes
The most frequently used and probably the most promising seems
to be the so-called composite membranes. This type of membrane is prepared by
in situ hydrothermal synthesis. A relatively thin zeolite layer is crystallized
on the surface or in the pores of a pre-shaped porous support. Among different
types of inorganic materials, like ceramics, metal glasses, carbon used assupports,
porous alumina has been the most popular for these preparations. The nucleation
and crystal growth on the support can be self-induced or induced by attachment
of seed crystals on the substrate. The latter procedure requires a pretreatment
of the support before the hydrothermal synthesis. 
Zeolite-containing composite membranes have been prepared by a vapor phase transport method called ╬dry synthesis.Ô The zeolite layer in this case is prepared by conversion of a previously deposited silica or silica-alumina layer under joint action of vapors containing water and a structure-directing agent. 
The zeolite type prepared most often as a membrane is MFI, which is interesting for industrial applications with its suitable pore diameter, high thermal and chemical stability, easy synthesis and modification of the chemical composition. The experience gained in the preparation of MFI and other zeolitic membranes has shown that as well as the pin-holes there are many factors critical for the performance of the composite membranes. Some of them are (i) the adhesion of the zeolite layer on the support surface, (ii) the difference of the thermal expansion coefficients of support and zeolite, (iii) the orientation of zeolite crystals, (iv) the thickness of the zeolite layer, (v) the anisotropy of mass transport due to an anisotropic pore geometry, and (vi) the influence of crystal boundaries on the permeation properties.
3. Concluding remarks
The first zeolitic membranes are already on the market. Nevertheless, the control and fine tuning of the properties of the zeolite-containing membrane configurations remains a challenge. A common problem is that, despite the use of a pre-defined methodology, it is difficult to obtain membranes with consistent and predictable properties. However, the steeply increasing interest in this field suggests that zeolitic membranes with excellent separation properties will soon be available.
Lately the zeolitic membranes have attracted considerable attention for catalytic membrane reactors, where the zeolite phase can carry the dual function of separator and catalyst.
Other potential applications of zeolite film and layers include chemical sensors, zeolite electrodes, solar energy conversion, zeolite batteries, optical and data storage materials.
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