Study of Dealumination from Frameworks of Zeolite-Y during Protonation

Abstract

The dealumination from the framework of zeolite-Y during the protonation was studied by using a high-resolution solid-state MAS (Magic-Angle Spinning) NMR spectroscopy. In addition, the acidic properties were investigated. During the process from the Na-type to the NH₄-type, no remarkable changes were observed in the microenvironments of the framework, i. e., the distribution of the Si(nAl) configuration and the Al-O coordination states. When the NH₄-type was calcined in air at 550°C, the dealumination from the framework appreciably occurred. The dealumination results from the change of four-coordinated aluminum Al(tet) to six-coordinated aluminum Al(oct) and from the decrease in the concentration of Si(2 Al) and Si(3 Al) configuration.
The dealumination was contro lled by the atmosphere of calcination. It was inhibited in the calcination in vacuo, that is, in the low partial pressure of water. It was also. inhibited by the exchange from the NH₄-type to the type of the rare earth elements (La). There was no appreciable difference in the ratio of Al(tet) to Al(oct) between the H(vac)-type and La-type. However, the content of Brφnsted acid was larger than Lewis acid in the La-type. Therefore, the electronic influence by the presence of La³⁺ ion to the aluminum microenvironments is considered. Not only the La-exchange inhibit the dealumination, but also it may change the acidic properties.
The amount of the acid sites was related to the total amount of aluminum including dealuminated species from the framework. But no good correlation was found between the amount of acid and the Al/Si atomic ratio of the framework.