Si,Al ordering and basicity clusters in faujasites.
The basicity of cation exchange faujasites is described in terms of basicity clusters. Those clusters are based on both the Al location in the framework and the faujasite topology. They are built from an aluminum taken as origin (Al0) surrounded by neighboring layers (0-3) of TO4 tetrahedra where an inductive influence of Al is effective through the framework. These clusters allow for the calculation of the basic strength and of the estimated number of basic sites. The framework oxygens are stronger, for the strong or the global basicity, when they link an Al atom in a layer n to two Si in the layer n - 1 of the cluster. This is the basis of a framework-induced basicity. It follows that in X, Y, or dealuminated Y various schemes show how much the Si,Al ordering plays a major role in the basicity. A combination of the different possible clusters in a crystal is suggested to take into account the experimental evidence of a range of basic strengths. On average, X zeolite not only contains more and stronger sites than Y but it also shows the highest ratio of strong to global basicity. The weak basicity of cationic dealuminated Y is connected through the clusters to its strong acidity in the protonic form. The clusters, more generally called topological clusters, may be applied to the case of properties other than basicity (like the redox behavior when transition metal cations are present) and to porous materials other than zeolites.
Barthomeuf
Denise
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