Spatial constraints, zeolite structures

On the other hand, a remarkable difference between catalysis by Y and 13 zeolites has been found for the Claisen-Sohmidt condensation of acetophenone and benzaldehyde (Table 5). When the cross aldolic reaction is carried out in the presence of HY, together with the expected trans and ois chalcones 5, the 3,3-diphenylpropiophenone 6 is also formed, this product being not detected on 13 zeolites. A likely explanation for the absence of 6 using zeolite beta is that the crystalline structure of this zeolite exerte a spatial constraint making difficult the formation of a big size molecule like 6, especially in the smaller channel. Similar effects due steno limitations on 6 catalysis have been found for the formation of multi-branched products during the cracking of alkanes (ref 8). 

A class of aluminosilicates called zeolites offer a number of chemical, physical, and structural characteristics that merit use as electrode modifiers (7,101-106). The chemical composition of zeolites is similar to that of clays however, zeohtes are three-dimensional aluminosilicate crystals rather than two-dimensional sheet structures. This distinction imparts unique chemical and physical properties that can be exploited for preconcentration, size and shape selectivity, and catalysis. However, molecules that can be incorporated into zeohtes are more limited due to spatial constraints of the zeolite cavity. 

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