Acid sites in zeolite Beta: effects of ammonium exchange and steaming

Zeolite Beta samples that had undergone calcination and/or steam deactivation were examined by temperature-programmed ammonia desorption (TPAD), Al-27 magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and Si-29 MAS NMR, in order to evaluate the nature and concentration of acid sites formed upon ammonium exchange or steaming. We observe octahedral aluminum species connected to the framework structure of zeolite Beta, and these aluminum sites exhibit characteristics of Lewis acid sites. Such aluminum sites are created by partial hydrolysis of framework Si-O-Al bonds; they convert reversibly to tetrahedral aluminum sites upon aqueous NH4+ exchange or reaction with NH3 gas. When zeolite Beta is steamed severely, a substantial portion of the framework aluminum is completely hydrolyzed, and non-framework aluminum species are generated. The high-temperature shoulder observed in some TPAD profiles is caused by liberation of ammonia from Lewis acid sites. This ammonia had previously been released from Bronsted acid sites and was temporarily re-adsorbed on Lewis acid sites. The evidence presented suggests that these Lewis sites are located in the zeolite framework. (C) 2000 Elsevier Science B.V. All rights reserved.