Microporous highly selective

Both materials were tested as catalysts in the anisole acylation (Scheme 1). The conventional Beta sample showed a slightly higher activity than the Beta (PHAPTMS). At 3 hours, the conversions were 26.8 and 22.8 % for the conventional and seed silanized catalysts, respectively. This behavior is explained as a consequence of the relatively small size of the anisole molecule, which allows this compound to diffuse without significant hindrances through the zeolitic micropores, and of the slightly weaker acidity of the Beta (PHAPTMS) sample. In both cases, p-methoxyacetophenone (p-MAP) was the main reaction product, being obtained with a high selectivity (> 97%). 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

Microporous membranes (pore radius less than 10 A) are ideal materials to be used as separators in membrane reactor processes. Microporous membranes also combine the high selectivities to certain components with high permeation rates. The high selectivities mean that maximum conversions (and thus equilibria shifts) higher than those achieved by porous membranes can be attained, while the high permeation rates allow for high reaction rates... 

Unfortunately due to time limitations, there was no time to coat these steam stable mesoporous membranes with a microporous silica toplayer. Microporous (doped) silica membranes have, however, been applied on conventional mesoporous Y-AI2O3 membranes which were not stable under SASRA conditions. On these membranes permeance measurements have been performed which showed that these membranes could be prepared with a very high selectivity under cleanroom conditions. Stability measurements on (doped) silica bulk material sintered at a high temperature (600-800°C) showed no change in the specific surface area during SASRA treatment. This is an indication that it should be possible to prepare a silica membrane that shows complete stability towards SASRA conditions. 

The aluminosilicate zeolites may be regarded as the most important and well-established members of a special class of microporous adsorbents in which the porosity is intra-crystalline. Although zeolites have been known for over 200 years, their potential value as highly selective adsorbents was first realized about 50 years ago (Barrer, 1945, 1978). Interest was further stimulated by die announcement by Breck et al. (1956) of the synthesis of the hitherto unknown zeolite A (i.e. Linde sieve A). Since then several hundred new porous zeolites have been synthesized. 

The ZSM-5 catalyst shows quite high selectivity in the formation of paraffins and olefins and branched hydrocarbons, while the yield of gases is also high. Both high yields of gases and lighter liquids are the consequence of the large microporous surface area. 

Application Qearly one important application of microporous materials in which the effectiveness is critically dependent on the monodispersity of the pores is the sieving of proteins. In order that an ultrafiltration membrane have high selectivity for proteins on the basis of size, the pore dimensions must first of all be on the order of 25 - ioOA, which is the size range provided by typical cubic phases. In addition to this, one important goal in the field of microporous matmals is the attainment of the narrowest possible pore size distribution, enabling isloation of proteins of a very specific molecular weight, for example. Applications in which separation of proteins by molecular weight are of proven or potential importance are immunoadsorption process, hemodialysis, purification of proteins, and microencapsulation of functionally-specific cells. 

Ideally, the zeolite membranes must be continuous with good cross-linking between crystals and free of pinholes and cracks to get high selectivities. However, most of the synthesis procedures render membranes with some intercrystaUine gaps and defects. The amount of these membranes and their sizes play an important role in the overall quahty of the membrane. Therefore, it has been considered illustrative to explain briefly the transport regimes in porous materials whatever the pore size, after which the mass transport mechanisms through microporous media will be fully described. 

Zeosils are microporous solids with tetrahedral frameworks, which are similar to those of aluminosilicate zeolites, but which are built from pure SiOz [1, 2]. With their neutral frameworks, zeosils do not show the typical properties of zeolites such as ion exchange, hydrophilicity, and catalytic activity instead, these materials are hydrophobic and non-reactive. Zeosils find their main qrplications as highly selective adsorbents for sorbing nonpolar molecules from wet gas streams or aqueous solutions. 

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