Stability of Zeolite MCM-22 with Varying Si/Al Ratios: A Solid-State NMR Investigation

Abstract

Determination of the local environment of aluminum in zeolites after various post-synthesis treatments is important for understanding structure-performance relationships. MCM-22 zeolites, synthesized with different silicon-to-aluminum ratios, were treated under various conditions: direct calcination in air, calcination in ammonia and oxygen, and ammonia treatment after calcination. The aluminum co-ordination in a zeolitic framework is highly sensitive to the treatments. The aluminum distribution over crystallographic T-sites in the zeolitic framework was followed by Al-27 MAS and Al-27 MQ-MAS NMR experiments. Loss of tetrahedrally co-ordinated aluminum on several T-sites and appearance of octahedrally coordinated aluminum was observed after calcination, Higher Si/Al ratios result in less octahedrally co-ordinated aluminum. This partial structural collapse can be restored by treatment with a base such as ammonia. An increased disorder in the framework is found after the conversion of octahedrally co-ordinated aluminum to tetrahedrally co-ordinated aluminum compared to the starting material.