Zeolite-based membranes

Controlled removal of the template is especially important when zeolite based membranes are involved consisting of a continuous MFI layer on a ceramic or sintered metal support (ref. 14). In these novel composite ceramic membranes the formation of cracks during template removal would be detrimental. The unique properties (ref. 14) of metal-supported MFl-layer membranes prove that indeed crack formation can be essentially prevented. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

ZEOLITE-BASED MEMBRANES PREPARATION, PERFORMANCE AND PROSPECTS... 

Following the above introductory rranarks a discusaon will be givoi on the dynamics of zeolite pores and the access of molecules into than. Then the state of the art of zeolite-based membranes will be overviewed in the sequence ... 

Small pore (6- and 8-ring) zeolite-based membranes might be used in industrial processes involving hydrogen, in air separation or in separation of linear and branched alkanes,. plying small pore apertures might lead to high separation/selectivity. 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

Focus in the field of medium pore (ten-membered oxygen ring) zeolite-based membranes has been almost entirely on MFI-type zeolites. This zeolite and its all-silica form. 

LARGE PORE ZEOLITE-BASED MEMBRANES (12 membered oxygen ring)... 

Chapter 12. Zeolite-based Membranes, Preparation, Performance and Prospects... 

The first reported zeolite-based membranes were composed of zeolite-filled polymers [3-9]. The incorporation of zeolite crystals into these polymers resulted in a change of both permeation behavior and selectivity, due to the alteration of the affinity of the membrane for the components studied. Up to now, most known inorganic, zeolitic membranes have consisted of supported or unsupported ZSM-5 or silicalite [10-27]. Other reported membranes are prepared from zeolite-X [21], zeolite-A [21,28], or AIPO4-5 [29]. The materials used as support arc metals, glass, or alumina. The membrane configurations employed are flat sheet modules and annular tubes. 

One of the first zeolite based membranes were composite membranes, obtained by dispersion of zeolite crystals in dense polymeric films in order to make zeolite filled polymeric membranes [59,60,61], These membranes have been developed at the end of the 80 s for both gas separation and pervaporation. The clogging of zeolite pores by the matrix and the quality of the interface between the zeolite crystals and the polymer matrix (non-selective diffusion pathways) were key points. 

Isomerization of xylene is also a very attractive application for MFI zeolite membranes (Fig. 10). The commercialisation of a new zeolite-based membrane is being planned by NGK Insulators for /i-xylene production from other xylene isomers, p-xylene molecules which are smaller than those of m- or o-xylene, are sieved from the mixture using the membrane, thus cutting significantly the production costs [157]. 

One of the main problems limiting the industrial development of zeolite membranes is their low fluxes. Typically a few m m 2.h l.bar 1 are achieved by most of the zeolite-based membranes whose thickness has to be sufficient (currently in the range 0.5-5 pm), in order to limit the influence of non-zeolite pores. An other problem is related to the chemical stability of membranes in both acidic (Al is leached) and basic media (Si is leached). The recent developments of both MOR [31] and ETS-10 131,32] membranes will contribute to solve this latter problem. 

Zeolites are erystaUine nanoporous materials with uniform nanosized pores (<1 nm) (Fig. 9.3). Selective permeation in zeolite membranes is based on molecular sieving and selective adsorption. Zeolite membranes have drawn attention as suitable membranes for DH applications due to their high thermal and chemical stability. When supported (Fig. 9.3), zeolite-based membranes also offer excellent mechanical strength, which is an important feature for DH applications. The permeation of single compounds in zeolitic membranes depends on the kinetic diameter of the molecule and size selectivity and they exhibit moderate selectivities to hydrogen. 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

Fabrication of MMs has been studied by many authors, who have investigated Pd and zeolite based membranes on Si substrates. Generally, MMRs are fabricated using silicon (Si) wafer substrates for micro-fuel... 

As can be seen in Table 19.2, and with some more details in Table 19.3 (this last table shows a short list of some relevant membranes used in the ethyl acetate production research together with separation factors and fluxes obtained), zeolite-based membranes (mordenite and zeolite A) were also tested by De La Iglesia et al. (2007) in an ISU-type continuous membrane reactor packed with Amberlyst 15. Both membranes were capable of shifting the equilibrium (in <1 day) and, in particular, mordenite membranes allowed conversions of approximately 90% and high separation factors of H20/ethanol and H20/acetic acid (>170). Moreover, because of the lower content in aluminum, under acid conditions, mordenite membranes were more stable than zeolite A. Hence, mordenite was also used by De La Iglesia et al. (2006), in another work, to prepare two-layered mordenite-ZSM-5 composite membranes, as shown in Figure 19.15. A tubular alumina tube was used as support. As a result, the feasibility of coupling the separation characteristics of the mordenite layer with the catalytic behavior of the H-ZSM-5 layer was demonstrated. 

In all cases, if the cost of the octane number was low (3.1 euros/RON point/ t), addition of a MFI zeolite-based membrane separation unit was not financially justified. Under these conditions, in fact, either the energy consumption related to the vaporization of the feed weighed too heavily on the operating costs (membrane temperature 300 °C), or the margin cleared by the gain in octane number was not sufficient to make the construction of the membrane separation unit profitable, since it was too large (area > 10 000 m ). 

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