Hierarchical zeolites Y obtained by desilication: Porosity, acidity and catalytic properties

The porosity, acidity and catalytic properties of zeolite Y desilicated in NaOH, tetrabutylammonium hydroxide (TBAOH) and NaOH/TBAOH mixture were investigated. The parent dealuminated zeolite Y of Si/Al = 31 contained very strongly acidic Si-OH-Al groups (more acidic than in other zeolites). These OH groups were also homogeneous. Desilication in NaOH solution extracted ca. 80% of Si but caused the amorphization of zeolite, loss of micropore system and of zeolitic acidity. On the other hand, TBAOH removed a small fraction of the Si, and preserved the crystallinity, porosity and acidity. Nevertheless, the NaOH/TBAOH treatment extracted about half of Si without a loss of crystallinity and zeolite microporosity. The mesopore volume and surface increased distinctly. Both concentration of Bronsted and Lewis acid sites also increased. Remarkably, the acid strength of the Si-OH-Al groups did not change upon desilication in the NaOH/TBAOH. The consequence of the enhanced mesoporosity, sites density, preservation of microporosity and very high acid strength was the catalytic activity in a-pinene isomerization that increased significantly.