Dealumination Techniques for Zeolites

The framework Si/Al atomic ratio of zeolites is an important parameter that exerts a strong influence on properties such as maximum ion-exchange capacity, thermal and hydrothermal stability, hydrophobicity, concentration and strength of acid sites of the Bronsted-type, which may be generated in zeolite structures, and catalytic activity and selectivity. Though in some processes, e. g., in ion-exchange procedures, low Si/Al ratios may be preferred, generally zeolites with low aluminum content (high Si/Al ratio) are more favorable, especially when applied as catalysts. However, the framework Si/Al ratio of zeolites prepared by direct synthesis, such as by direct hydrothermal crystallization in absence of templates, is generally restricted to more or less narrow limits. One of the technically most important members of the zeolite family, faujasite, cannot be directly synthesized with Si/Al ratios substantially higher than 2.5, at least not in economically reasonable crystallization times. It is, therefore, of great importance to find methods that can increase the Si/Al ratio by chemical post-synthesis modification of zeolite frameworks, i. e., by dealumination.