Distribution of aluminum and its influence on the acid strength of Y zeolite

A density functional theory (DFT) study has been conducted to investigate the distribution of Al atom in the framework of Y zeolite and thereby the relationship between siting position of Al atoms and the acid strength of zeolite. Structure properties of seven different Si/Al molar ratio of Y zeolite are calculated. The study shows that Al atoms are regularly distribution in the framework. And they are more easily distributed in the diagonal position of the four-ring. With the decreased Si/Al molar ratio, Al atoms are prior to distribute in the form of a trigonal symmetrical. From the whole distributions, Al atoms are trying to concentrate in the hemisphere of sodalite cage. The deprotonation energies of these seven cluster models are calculated to illustrate the change of Brönsted acid strength after the replacement. The result shows that the Brönsted acid strength is related to the Si/Al molar ratio of zeolite, as well as the distance between Al atoms. The substitution of Si located in the next-nearest-neighboring (NNN) of Al decreases the Brönsted acid strength of zeolite. As the Si/Al molar ratio decreases, the Brönsted acid strength decreased gradually. The difficulties of Al substitution are congruous with the change of Brönsted acid strength of A(1) site. The research on the relationship between Al siting place and Brönsted acid strength could provide theoretical information for the development of synthetic method to meet the specific requirement of Y zeolite catalysts, as well as the influence on the reaction process.

The figure describes the distributions of Al in the framework of Y zeolite, which influence the acidity and catalyst activity of zeolite in the hydrocracking process.Figure optionsDownload as PowerPoint slideHighlights
► Two Al atoms prefer to distribute in the diagonal position of four-ring.
► Al atoms concentrate in the hemisphere of sodalite cage.
► Brönsted acid strength is related to the Si/Al molar ratio of zeolite.
► The difficulties of Al substitution are consistent with the change of Brönsted acid strength.