Thermal zeolite membranes

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. 

Although hollow fibers are thought to be an excellent candidate to be used as support-they are cheap and have a very high surface area to volume (>1000 m m ) - very few reports on hollow-fiber-supported zeolite membranes exist in the open literature. For zeohte membranes, ceramic hollow fibers are preferred because of their mechanical and thermal stability. Recently, Alshebani... 

A zeolite membrane, where the pores originate from the structure, presents only one type of (ultramicro)pore and therefore seems to be a good candidate for CMRs application. Moreover the structural origin of the pores should induce a much better thermal stability of the... 

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. 

The use of zeolitic membranes in separation or combined reaction and separation processes is very appealing. Advantages of using this type of membrane include not only their ability to discriminate between molecules based on molecular size but also their thermal stability. The large variety of zeolite types could provide a tailor-made separation medium for specific processes. Moreover, the properties of zeolites are often easily adjustable (ion exchange, Si/Al ratio, etc.). This makes zeolitic membranes also very promising for use as catalytic membranes. 

The combination of reaction and separation in one multifunctional membrane reactor is an interesting option. In such a reactor the membrane could be catalytically active itself, or it could serve only as a separation medium. There are several types of operation for such a reactor [33]. It could be used to separate the formed products from the reaction mixture. In this way it is possible to overcome equilibrium limitations or to improve the selectivity of the reaction. Another possibility is the controlled addition of reactant via the membrane, which might be of use in, for example, oxidation reactions or sequential reactions. The advantage of using zeolitic membranes in a membrane reactor is that they have a high thermal stability and exhibit a good selectivity. Moreover, they can be made catalytically active. 

Application at high temperature requires robust and thermostable systems. Both for ceramically and stainless-steel-supported systems the thermostability has been demonstrated. So, in spite of the different thermal expansion coefficients, the asymmetric membrane remains intact. However, there are no data available on the resistance of zeolitic membranes to thermal stresses, as a result of, for example, large sudden changes in temperature. The siainless-steel-supported system seems the most promising configuration... 

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. 

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). 

When the membrane synthesis requires a template, it has to be removed from the zeolite pores. Chemical leaching or ion exchange with ammonium ions is only possible for big pore sizes (e.g. MOR, Beta, MCM). In most cases, and namely for the MFI structure, the template is removed by thermal treatment in strictly controlled conditions (atmosphere, heating rate, temperature and duration). We have to note that an efficient and gentle low temperature ozone treatment was recently proposed for organic template removal from MFI zeolite membranes [117]. 

Industrial applications of zeolite membranes can be considered only for separations where they offer some unique advantage in terms of flux, selectivity, or thermal and chemical stability. The very high fluxes obtained with LTA membranes (typically 100 times higher than those obtained with a polyacrylonitrile membrane at the same FLO/alcohol selectivity) explain the rapid expansion of this type of application at the end of the 90s [8J. 

Zeolites are used as detergent builders, adsorbents, and catalysts. In the past decade, we saw the development of a variety of zeoiite membranes, and a number of investigators reported on the preparation of such membranes and their applications to a variety of separation systems. These research activities are motivated by features common to inorganic membranes, such as thermal resistance and resistance to organic solvents, and features unique to zeolite materials, such as molecular sieving, selective adsorption, and catalytic activity.In this article, the discussion will be restricted to zeolite membranes for use in separation and catalysis. First, an overview is presented on recent progress in zeolite membranes, followed by a discussion of our research activities. 

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