Characterization of the acid sites in MCM-41-type materials by spectroscopic and catalytic techniques

Temperature-programmed desorption of pyridine, FTIR and multinuclear NMR spectroscopy and catalytic test reaction have been applied to investigate acid sites in mesoporous materials with silicon/aluminum ratios of 266 and 20 and in an amorphous aluminosilicate with n(Si)/n(Al) = 5.4. The results obtained by these methods indicate the presence of weak Bronsted acid sites in the aluminosilicate samples. Based on the spectroscopic data, a model of Bronsted acid sites in the aluminum-containing MCM-41 is proposed which consists of silanol groups with 3-4 framework aluminum atoms in their local structure. In the dehydrated ammonium form of the mesoporous aluminosilicate, these framework aluminum atoms have a tetrahedral oxygen coordination. Deammoniation leads to a strong broadening of the (27)Al echo NMR signal due to framework aluminum atoms which indicates a variation of their local geometry and/or oxygen coordination. (27)Al echo NMR studies of calcined and subsequently reammoniated samples indicate that this variation in the bonding state of framework aluminum atoms is a reversible process. (C) 1999 Elsevier Science B.V. All rights reserved.