Framework induced basicity in zeolites

Abstract

The basicity of oxygens of zeolites (intrinsic basicity) is considered to derive from specific Al location in the T sites. Basicity clusters are defined as topological domains around a central Al atom and are limited by the second (strong basicity) or the third (medium and weak basicity) layers of TO 4 tetrahedra. The framework topology directs the number of TO 4 neighbours. This determines in a given framework several opportunities for the basicity strength and the amount of sites depending on which T site is occupied by Al. A topological bond distance is suggested to characterize closeness of Al atoms in the framework. This is detailed for the case of faujasite where sites of the second layer may be singly or doubly bonded to the central Al atom generating stronger basicity than in ferrierite where all these sites are singly bonded. As the framework topological density becomes higher, with a lower degree of connectivities between atoms, the basicity trends are, for a fixed Al molar fraction (or Si/Al) to increase the number of basic sites and/or, for a given number of Al located in the basicity cluster to decrease the mean basic strength. The topological density is calculated for the basicity clusters. Its order is FAU < LTL < BEA < MWW < MOR < MFI < FER and the expected changes in basicity are described for this series. Experimental basicity results are in line with this approach of the impact of the framework topology on the basicity.