Verified Syntheses of Zeolitic Materials
2nd Revised Edition
Preparation of zeolite membranes
Valentin Valtchev
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
1. Introduction
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. [7]
Zeolite-containing composite membranes have been prepared by a vapor phase transport
method called Îdry synthesis.â 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. [4]
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.
References
[1] R. D. Noble, J. L Falconer, Catal. Today 25 (1995) 209
[2] T. Bein, Chem. Mater. 8 (1996) 1636
[3] M. J. den Exter, J. C. Jansen, J. M van de Graaf, F. Kapteijn, J. A. Mouliun,
H. van Bekkum, in Stud. Surf. Sci. Catal. 102, H. Chon, S. I. Woo, S.-F.. Park
(eds.), Fisevier, Amsterdam, 1996, p. 413
[4] M. Matsukata, F. Kikuchi, Bull. Chem. Soc. Jpn. 70 (1997) 2341
[5] J. Coronas, J. Santamaria, Separation and Purification Methods 28 (1999)127
[6] A. Tavolaro, E Drioli, Adv. Mater. 11 (1999) 975
[7] J. Caro, M. Noack, P. Kolsch, R Schafer, Micropor. Mesopor. Mater 38 (2000)
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