Towards the Preparation of Binderless ZSM-5 Zeolite Catalyst: the Crucial Role of Silanol Nest
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 amount 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 defective ZSM-5 zeolite as the starting materials, 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, the additional aluminum source can be incorporated into 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 the ordinary shaped zeolite with the binder in the catalytic cracking of n-hexane. This work provides new insight into the preparation and optimization of more monolithic zeolite catalysts from practical point of view.