Inorganic zeolites

This chapter provides a brief introduction to polymer and inorganic zeolite membranes and a comprehensive introduction to zeolite/polymer mixed-matrix membranes. It covers the materials, separation mechanism, methods, structures, properties and anticipated potential applications of the zeolite/polymer mixed-matrix membranes. 

Endowing these polymolecular entities with recognition units and reactive functional groups may lead to systems performing molecular recognition or supramolecular catalysis on external or internal surfaces of organic (molecular layers, membranes, vesicles, polymers, etc.) [7.1-7.13, A.41] or inorganic (zeolites, clays, sol-gel preparations, etc.) [7.14-7.20] materials. 

There are essentially two approaches one may employ a traditionally porous material such as inorganic zeolite, in which the framework remains essentially unchanged during the absorption and desorption of the various guest molecules, and whose selectivity is governed by the channel patterns and dimensions [1-3]. Alternatively, one may utilise organic or metal-organic compounds as hosts, and... 

The stable porosity of such MOFs is attributable to the structural properties of the metalcarboxylate clusters, where each metal ion is locked into place by the carboxylates, to create rigid units of simple geometry, referred to as SBUs [232], similarly to the case of inorganic zeolites. 

Naturally-occurring inorganic zeolites are also used for this purpose. 

Geus E.R., Preparation and characterisation of composite inorganic zeolite membranes with moleculare sieve properties. Ph.D. Dissertation, Technical University of Delft, The Netherlands (1993). 

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. 

The above-mentioned studies reveal several features that determine the permeation through zeolitic membranes as well as their selectivity. Apart from size exclusion due to molecular sieving, both the affinity of the membranes for a given component and the mobility of that component in the pore network of the zeolite play a major role. In this section the importance of these features is shown on the basis of several examples. The emphasis will be on inorganic zeolitic membranes. 

E.R. Geus. Inorganic Zeolite membranes, Ph.D. thesis. Delft University of Technology, 1993, The Netherlands. 

Inorganic zeolites (synthetic or naturally occurring aluminosilicates) later found widespread application in softening hard water, i.e., removal of dissolved polyvalent cations, principally calcium and magnesium, through exchange with sodium. 

Walton (W2), and Kunin and Myers (K7), have reviewed the empirical equations that have been proposed to fit ion-exchange equilibria many of these relate specifically to inorganic zeolites rather than to the synthetic-resin exchangers. 

For exchange in inorganic zeolites, the same rate relations should apply as for adsorption from a liquid phase. 

Organic zeolite analogues are commonly referred to as porous solids. These materials promise a new range of applications, e.g., in pharmaceutical manufacture and in molecular sieves, sensors, and devices. They are crystalline or amorphous materials that permit the reversible passage of molecules through holes on their surface. Porous solids are classified according to pore diameter nanoporous or microporous (< 15 A), mesoporous (15—500 A) and macroporous (>500 A). The natural and synthetic inorganic zeolites with uniform pore sizes of 10-20 A are the classical examples of microporous materials with widespread use in industry. 

Giving a concise definition for an organic zeolite remains an ambiguous task. In inorganic zeolites, the crystal structure is determined by ionic and covalent bonding. Directional and nondirectional forces play the important role for design of organic crystal structures. 

In comparison to inorganic zeolites, organic porous networks show a considerably higher uptake of different guests. CuSiF6(bipy)2 features a higher affinity to methane than that found for the inorganic zeolite 5A (Fig. 6c). 

Although very many carboxylate and amine-based hybrid frameworks have been prepared, solid state NMR has not been used as a characterisation tool in the same way as it has for inorganic zeolitic frameworks. This is probably because they are commonly prepared as crystals suitable for single crystal diffraction, and often with paramagnetic metals. It is likely, however, that NMR will reveal important structural details as these materials are studied more carefully, particularly in monitoring interactions and framework rearrangements upon adsorption of molecules.Studies of framework and sorbate motion by deuterium NMR are also highly relevant here (Chapter 7). 

Urbiztondo MA, Peralta A, Pellejero I, Ses J, Pina MP, Dufour I, Santamarta J. Detection of organic vapours with Si cantilevers coated with inorganic (zeolites) or organic (polymer) layers. Sens Actuators B 2012 171-172 822-831. 

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