Microporous and Mesoporous Materials 27 ( 1999) 261–271 Characterization of the acid sites in MCM-41-type materials by spectroscopic and catalytic techniques M. Hunger, U. Schenk, M. Breuninger, R. Gla ¨ser, J. Weitkamp * Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany Received 7 November 1997; received in revised form 27 January 1998; accepted 27 January 1998 Abstract 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 Brønsted acid sites in the aluminosilicate samples. Based on the spectroscopic data, a model of Brønsted 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 27Al echo NMR signal due to framework aluminum atoms which indicates a variation of their local geometry and/or oxygen coordination. 27Al 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. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Brønsted acid sites; Catalytic test reactions; FTIR spectroscopy; MCM-41; Solid-state NMR spectroscopy 1. Introduction a number of publications were dealing with the synthesis of MCM-41 [3–5], its sorptive properties [6–8], its application as a host material for various MCM-41, a member of the M41S family, pos- guest compounds [9–11], and the behavior of sesses a hexagonal arrangement of uniformly sized isomorphously substituted mesoporous materials mesopores with diameters in the range 1.5–10 nm. [ 12–14 ]. For the application of MCM-41 in cataly- With the advent of these materials, the pore size sis, a detailed knowledge is desirable concerning range of inorganic molecular sieves has been dra- the acid sites existing in the mesopores of the matically expanded. Since the re-discovery of aluminum-containing material. Even though these MCM-41 by scientists of Mobil Oil Corporation sites were investigated earlier by different spectro- in 1992 [1] (it appears that practically the same scopic techniques [15–21], many questions material has been described as early as 1969 [2]) remained open and their chemical nature is largely unknown. FTIR spectra of calcined MCM-41 con- sist of bands at 3745 cm-1 due to isolated silanol * Corresponding author. Tel: +49 711 685 4060; Fax: +49 711 685 4065; E-mail: jens.weitkamp@po.uni-stuttgart.de groups and at 3715 and 3530 cm-1 caused 1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1387-1811(98)00260-1