Towards the preparation of binderless ZSM-5 zeolite catalysts: the crucial role of silanol nests

Abstract

Zeolites, containing abundant silanol nests, are often called defective zeolites. Zeolite-based catalysts are shaped to be used effectively in industrial applications. Binders, which are catalytically inert, are indispensable components in the forming process. However, massive amounts of binders (usually 30–50 wt%) may cover the surface of the zeolites, thereby limiting the access to catalytic sites. In the present work, two types of binderless zeolites were prepared by the alkali post-treatment using a defective ZSM-5 zeolite as the starting material, namely monolithic binderless ZSM-5 zeolites dominated by micropores and hierarchical monolithic binderless ZSM-5 zeolites. It has been found that the silica binder can be converted into the zeolite phase in the binderless defective ZSM-5 zeolites. Moreover, an additional aluminum source can be incorporated into the zeolitic framework upon alkaline treatment. However, if the zeolite does not contain silanol nests, the Al incorporation is impossible. Combining with various characterization results, we proposed the formation mechanisms of binderless ZSM-5 zeolites, including re-crystallization and alumination. As expected, the as-treated binderless zeolites exhibit higher activity and selectivity than ordinary shaped zeolites with a binder in the catalytic cracking of n-hexane. This work provides new insight into the preparation and optimization of more monolithic zeolite catalysts from a practical point of view.