Membranes of zeolites

For a zeolite T (OFF stmcture, 0.68 nm XRD pore diameter), Tanaka et al. [131], observed that the separation factor of a water/acetic acid (50/50 wt%) measured at 75°C decreased monotonically after the immersion of the membrane into the acetic acid mixmre. Initially, the separation factor and water flux were 182 and 1.46 kg/m h, respectively, and after 32 h these values changed to 86 and 1.77 kg/m h, showing a deterioration of the membrane. Cui et al. [130] also smdied the stability of crystals and membranes of zeolite T in acid medium. The powders were immersed in a 50/50 wt% water/acetic acid mixture for 7 days at 75°C and also in HCl solutions 0.5 and 1 M for 1 h at 50°C. The analysis of the samples after the treatment by ICP and XRD indicated that the sample treated in the acetic acid solution maintained its original Si/Al ratio equal to 4 however, the hydrochloridric acid treatment with the 1 M solution destroyed the zeolite stmcmre and the 0.5 M solution dealuminated the zeolite to a Si/Al equal to 8.9 and the XRD analysis corresponded to zeolite T. The membrane performance, after being used for 1 week at different water/acetic acid concentrations, remains almost unchanged and the separation factor of the membrane treated in HCl dramatically decreased as was expected. 

For a zeolite T (OFF structure, 0.68 nm XRD pore diameter), Tanaka et al. [183] observed that the separation factor of a water/acetic acid (50/50 wt.%) measured at 75°C decreased monotonically after the immersion of the membrane into the acetic acid mixture. Initially, the separation factor and water flux were 182 and 1.46 kg/(m h), respectively, and after 32 h, these values changed to 86 and 1.77 kg/(m h), showing a deterioration of the membrane. Cui et al. [182] also studied the stability of crystals and membranes of zeolite T in an... 

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

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

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 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. 

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. 

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. 

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. 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

As is obvious, many potential hurdles discussed in the previous sections do not apply to appHcation of zeolite membranes at the micro- and particle levels. Issues Hke scale-up and high-temperature sealing do not play a role here. Additionally, coated catalyst particles do not require a change of reactor, but only replacement of the catalyst. Application of zeoHte membranes at these levels is therefore considered to be easier and their implementation will probably occur earlier. 

When using the microporous zeolite membrane (curve 3) the N2 permeance decreases when the pressure increases such a behaviour can be accounted for by activated diffusion mechanisms [21], which are typical of zeolite microporous systems. In such systems the difflisivity depends on the nature and on the concentration of the diffusing molecule which interacts with the surface of the pore. For gases with low activation energies of diffusion, a decrease of the permeability can be observed [22]. 

Gavriilidis, a., Yeung, K. L., Design and fabrication of zeolite-based microreactors and membrane microseparators, Micropor. Mesopor. Mater. 42 (2001)... 

Li, Y. and Yang, W. (2008) Microwave synthesis of zeolite membranes a review. Journal ofMemhrance Science, 316, 3-17. 

Recently the synthesis of zeolitic membranes was reported by Suzuki... 

Microwave Synthesis of Zeolites and Molecular Sieves The use of microwaves holds promise for efficiency improvements in zeolite synthesis due to the rapid heating possible when using microwave radiation [166], The first report of microwave synthesis of zeolites was by Mobil Oil in 1988, which broadly claimed the synthesis of zeolite materials in the presence of a microwave-sympathetic material, such as water or other pro tic component [167]. A number of reports have appeared since, including synthesis of zeolites Y, ZSM-5 [168] and metaUoaluminophosphate-type materials, such as MAPO-5 [169], There have also been extensive investigations in using microwaves for zeoHte membrane synthesis. Recent reviews discuss the progress in microwave zeoHte synthesis [170, 171]. 

Bronid, J., Subotid, B., and Skreblin, M. (1999) Investigation of the influence of seeding on the crystallization of zeolite A in the membrane-type reactor micropor. Mesopor. Mater., 28, 73-82. 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

The present review of zeolite membrane technology covers synthesis and characterization methods as well as the theoretical aspects of transport and separation mechanisms. Special attention is focused on the performance of zeolite membranes in a variety of applications including liquid-liquid, gas/vapor and reactive... 

PolycrystalHne membrane growth proceeds by initial formation of a gel layer on the surface of the support crystallization takes place at the interface between the bulk Hquid phase and the gel layer, resulting in deposition of zeolite nuclei and crystals formed [8]. Concurrently, the crystals deposited onto the support surface continue to grow, eventually resulting in a continuous membrane layer. Postsynthesis treatment is necessary when a template is used in synthesis to activate the zeolite and open the pores. Usually this is accomplished through calcination or burn out of the organic molecule. 

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