Key factors for the encapsulation of Keggin-type heteropoly acids in the supercages of Y-type zeolite

The influences of the properties of the zeolite support on the amount of encapsulation of Keggin-type heteropoly acid (HPA) molecules in the supercages of Y-type zeolite were investigated to clarify the key factors for the production of a solid acid catalyst which maintains activity in aqueous solutions. The investigated properties were the SiO2/Al2O3 ratio and the type and amount of counter cation occupying the cation sites of the zeolite support. It was found that in order to efficiently encapsulate HPA molecules, the zeolite support must have a moderate number of aluminum atoms in its framework, and cations which promote the formation of HPA precursors must occupy the cation sites induced by these aluminum atoms. It was also found that the degree of ion exchange of such cations must be high enough so that the cations can exist within the supercages, and therefore can contribute to the formation of HPAs. By controlling these factors, we succeeded in enhancing the catalytic activity of the encapsulated catalyst. (C) 2001 Elsevier Science Ltd. All rights reserved.