Zeolites Aluminosilicates specific types

The analysis of the kinetics of crystallization of different types of zeolites from aluminosilicate gels points to the conclusion that the crystallization takes place by the simultaneous growth of the constant number N0 of nuclei-I present in the system at the very start of the crystallization process and the number Na of nuclei-II released from the gel disolved during the crystallization process. Some characteristics of the crystallization systems such as the duration of the "induction period", the shortening of the "induction period" and the increase of the crystallization rate, respectively, with the gel ageing and the bimodal size distributions in the specific cases have been discussed and explained in relation to the ratio Na/N0 of particles (nuclei)-II and particles (nuclei)-I present in the crystallizing systems. 

In the sub-monolayer range, the amount adsorbed on the external area of a 1 pm cubic zeolite crystal is very small in comparison with the adsorption within the micropore structure (the intracrystalline sorption). Also, apart from a small multilayer adsorption on the external surface, there should be no additional uptake at higher pip0. However, there are three ways in which the non-zeolitic contribution may be increased (a) the binder may have a relatively large specific surface (b) the zeolite crystallite size may be much smaller than 1 pm and (c) the zeolite may contain some amorphous aluminosilicate or silica. In practice, one or more of these effects can result in a significant distortion from the classical form of the Type I isotherm (see Sayari etal., 1991). 

There is a specific interest in Ga-containing MTS, which stems from the high selectivity to aromatics shown by Ga-containing zeolites in the catalytic conversion of hydrocarbons [140-143]. For example, the commercial Cyclar process, where C3-C5 alkanes are dehydrocycHzed to yield aromatic hydrocarbons, proceeds over [Ga]-ZSM-5, whereas conventional ZSM-5 aluminosilicates mainly yield paraffins. It has also been shown that Ga-containing MCM-41 is very active for Friedel-Crafts type benzylation and acylation reactions [143,144]. 

Another type of widely occurring aluminosilicates is classified as zeolite. Zeolites have much more open structures than feldspars, and, as a result, they can take up loosely bound water or other small molecules into their cavity. Now zeolites are artificially created, which have specific chemical characteristic and holes that accommodate only a specific substance. 

The most important class of inorganic adsorbents is the zeolites, a subclass of molecular sieves. These are crystalline aluminosilicates with specific pore sizes located within small crystals. Two common classes have simple cubic crystals (type A) or body-centered cubic crystals (type X). Sometimes, the type is assigned a number equal to a nominal pore size in the crystals. For example, zeolite 5A with a nominal 5 A pore size is used to separate normal from branched paraffins. 

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