Lewis basic and Lewis acidic sites in zeolites

Abstract

This chapter describes Lewis basic and Lewis acidic sites in zeolites. Acidic and basic sites should be regarded as paired concepts always conjugated to each other. This general relationship should be also true in the case of zeolites. The extra frame work protons and cations in zeolites are the Br6nsted and Lewis acid sites, respectively, while the framework oxygens are their conjugated basic sites. The alkali-exchanged zeolites are designated as basic zeolites. Indeed, the acid–base pairs in cation exchanged zeolites are Lewis acids (cations) and Lewis bases (framework oxygens) pairs. Many factors are known to affect the Lewis acidity and Lewis basicity in cation exchanged zeolites. They are the bond angles and bond lengths, the location of A1, the crystallographic sitting of the oxygen, the framework Si/A1 ratio, the electronegativity of both the framework and extra framework atoms, and so on.