Subject zeolite membranes

The specific properties of zeolites, coupled with the separation properties of membranes, open the field to many areas of research for the future. This explains why the preparation and application of zeolite membranes is the subject of intensive research. By combining their adsorption and molecular sieving properties, zeolite membranes have been used for the separation of mixtures containing nonadsorbing molecules, different organic compounds, permanent gas-vapor mixtures, or water-organic mixtures. 

Many publications refer to pervaporation with zeolite membranes and recently the group of Falconer and Noble has presented an excellent review paper [113] about this subject. In a bibliographic search about pervaporation and zeolites for the last 10 years we find that many efforts, 28% of the publications, pertain to composite zeolite-polymer membranes, and the rest correspond to different zeolite structures that are summarized in the pie chart shown in Figure 10.19. The most studied structure... 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

A number of novel applications of zeolites depend on the ability to create thin, adhesive films on various substrates. While zeolite films or layers are commonly prepared on dense substrates such as silicon wafers, zeolite membranes are made on porous supports in order to permit permeation through the zeolite layer. Numerous synthetic studies have addressed the goal of obtaining adhesive layers of zeolites on various substrates such as noble and nonnoble metals, glass, ceramics, silicon, and even biological substrates such as cellulose fibers. For a more detailed discussion of zeolite membranes the reader is referred to the article by Julbe in this book. Pertinent reviews to this subject are given in the following references.[57,58]... 

Among the publications related to this subject, Jafar et al. [268] used a tubular NaA zeolite membrane prepared on a carbon-zirconia support in the vapor space of an esterification reactor to remove the product, water, formed in the reaction of lactic acid with ethanol catalyzed by p-toluenesulfonic acid. This protected the sensitive zeolite A membrane from acid attack and still allowed high permeation fluxes responsible for the enhanced yields of ethyl lactate. The same group [269] applied a... 

Steady state flow of molecule through a zeolite membrane is a constant. It can be obtained by integrating eq. (10.2-8) subject to constant boundary conditions at two ends of the membrane ... 

Mixed-matrix membranes have been a subject of research interest for more than 15 years [28-33], The concept is illustrated in Figure 8.10. At relatively low loadings of zeolite particles, permeation occurs by a combination of diffusion through the polymer phase and diffusion through the permeable zeolite particles. The relative permeation rates through the two phases are determined by their permeabilities. At low loadings of zeolite, the effect of the permeable zeolite particles on permeation can be expressed mathematically by the expression shown below, first developed by Maxwell in the 1870s [34],... 

The in situ membrane growth technique cannot be applied using the zeolite-based ceramic porous membrane as support, under hydrothermal conditions in a solution containing sodium hydroxide. The high pH conditions will cause membrane amorphization and lead to final dissolution. Therefore, we tried to synthesize an aluminophosphate zeolite such as AlP04-5 [105] over a zeolite porous ceramic membrane. For the synthesis of the AlP04-5-zeolite-based porous membrane composite, the in situ membrane growth technique [7,13,22] was chosen. Then, the support, that is, the zeolite-based porous ceramic membrane, was placed in contact with the synthesis mixture and, subsequently, subjected to a hydrothermal synthesis process [18]. The batch preparation was as follows [106] ... 

On the other hand, the oxidative coupling reaction of CH4 in the presence of O2, even when performed in membrane type reactors,188 is mainly catalysed by metal oxides catalysts.185 Also, oligomerisation, aromatisa-tion, and the partial oxidation apply non-metallic heterogeneous catalysts (such as zeolites). The reader is therefore directed to some excellent reviews on these subjects.189,190 At this point, it is perhaps relevant to introduce the formation of carbon nanofibres or nanotubes from methane, these being catalysed by metal nanoparticles, but at this moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process.191 192... 

Zeolites are crystalline aluminosiUcates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al, 2006 Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). 

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