Zeolites zeolite membranes

Key words zeolite, zeolite membranes, zeolite membrane reactors. 

Mizumaki F 1999 Application of zeolite membranes, films and coatings Stud. Surf. Sol. Catal. 125 1-12... 

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. 

Zeolite Membranes in Catalysis What Is New and How Bright Is the Future ... 

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. 

Figure 10.2 NaA zeolite membrane (a) cross-section and (b) top view [11]. (Reprinted with permission of Elsevier.)...

The principles of application of zeolite membranes at the microlevel can be very similar to those on the particle level, but now at the crystal (micrometer) scale, enclosing the active catalytic material. 

In this section, an attempt is made to sketch the current status of zeohte MRs with respect to specific applications. The application of zeohte MRs is strongly related to the development status of zeolite membranes. Topics that are discussed are the most often studied reactions for zeolite membrane apphcations dewatering. 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

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. 

Table 10.1 A comparison of recent examples of zeolite membranes in dehydrogenation reactions. 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

The following are some of the reasons that microreactors can be be used (i) reduced mass and heat transfer limitations, (ii) high area to volume ratio, (iii) safer operation, and (iv) ease of seating up by numbering out. The advantages of scaling down zeolite membranes are that it could be easier to create defect-free membranes and... 

Table 10.2 Performance of several zeolite membrane reactors in the xylene isomerization reaction. 

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

The considerations above apply to zeolite membranes as applied on the macrolevel (e.g., PBMR). Zeohte membranes apphed on the particle level or smaller might lead to a more optimistic outlook since this type of application neither involves expensive modules and supports nor expensive sealing material. 

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... 

An important driver for zeolite membrane apphcations has been the commercialization of the NaA membranes for dehydration. However, for these membranes, the quality required is not as high as compared to gas-phase molecular sieving... 

The reproducibility of zeolite membranes is sometimes questioned. But, taking into account the large surface areas that are produced for the mentioned application examples, for well-studied zeolite membrane syntheses this does not appear to be a limiting factor. 

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