Zeolites inorganic membranes

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. 

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. 

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

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. 

Recent developments demonstrate possibilities for inorganic C02 selective membranes. Microporous membranes with strong C02 adsorption show C02 selectivity if other gas species are hindered in accessing the pores. For instance, at intermediate temperatures, limited C02 selectivity to N2 (to about 400 °C) and H2 (to about 200 °C) is reported for MFI zeolite membranes [96]. Also, at high pressure (10-15 bars) C02 selectivity has been demonstrated in MFI membranes (C02/N2 separation factor ... 

However, at least for separative applications, most hopes to find consistent application of inorganic-membrane reactors lie in the development of inorganic membranes having pores of molecular dimensions (<10 A, e.g., zeolitic membranes). Such membranes should moreover be thin enough to allow reasonable permeability, defect-free, resilient, and stable from the thermal, mechanical, and chemical standpoints. Such results should not be achieved only at a lab scale (a lot of promising literature has recently appeared in this context), but should also be reproducible at a large, industrial scale. Last, but not least, such membranes should not be unacceptably expensive, in both their initial and their replacement costs. 

Inorganic membranes are very resistant and quite stable at hard-operating conditions. Several materials are available. Different membranes have been successfully tested for separations involving supercritical fluids such as tubular carbon membranes [ 1 ], mbular silica membranes [2-5], silica hollow fibber membranes [6], zeolite membranes [7-10], titane-nafion membranes [11], polycarbonate membrane [12], nanofilter having a thin layer of Zr02-Ti02 [12], and silicalite membranes [4]. 

Zeolite membranes have the potential to selectively separate gas molecules in a mixture operating under steady state, unsteady state, or under cyclic conditions whereas fixed bed adsorbers are typically operated under transient conditions. In addition, because of the inorganic nature of zeolite membranes, they have higher mechanical strength and greater thermal and chemical stability than their polymeric counterparts. Also, their ability to operate under very different conditions (total pressure. 

Microporous inorganic membranes have pores that can be tuned to the molecular size. This enables zeolite membranes to carry out separations (i.e., the separation of isomer compounds) that are not possible with membranes in which only Knudsen selectivity is possible. Moreover, zeolite microporous membranes can compete with traditional energy costly separation methods, such us distillation of mixmres of close boiling point components, separation of mixtures of low concentration, and azeotropic distillation. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of PVA (polyvinyMcohol) have been widely employed for dehydratation and separation of organic mixmres however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixmre is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures they usually present a low selectivity. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

Nanoporous inorganic ceramic membranes show significant promise for hydrogen separation and purification, primarily due to the high selectivity that is afforded by this class of membranes. Development work has focused on zeolites although... 

Zeolite membranes form the most recent branch of the inorganic membrane field for which characterised and properly described real microporous zeolite... 

Z.A.E.P. Vroon, Synthesis and transport studies of thin ceramic supported zeolite MFI membranes. PhD Thesis 1995, University of Twente, Enschede, The Netherlands. Z.A.E.P Vroon, K. Keizer, H. Verweij and A.J. Burggraaf, Transport properties of a ceramic thin zeolite membrane, in Yi Hua Ma (Ed.), Proceedings of the 3rd International Conference on Inorganic Membranes 10-14 July 1994, Worcester. Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA, pp 503-508. 

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