Accessibility of the acid sites in dealuminated small-port mordenites studied by FTIR of co-adsorbed alkylpyridines and CO N.S. Nesterenko a , F. Thibault-Starzyk b , V. Montouillout b , V.V. Yuschenko a , C. Fernandez b , J.-P. Gilson b , F. Fajula c , I.I. Ivanova a, * a Department of Chemistry, Moscow State University, Lenin Hills, 119892 Moscow, Russia b Laboratoire Catalyse et Spectrochimie, UMR 6506 CNRS-ENSICaen, 6 Bvd Mar echal Juin, F-14050 Caen Cedex, France c Laboratoire de Mat eriaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France Received 16 January 2004; received in revised form 16 January 2004; accepted 26 March 2004 Available online 7 May 2004 Abstract A novel methodology for characterization and quantification the acidic sites of different accessibility has been developed for dealuminated zeolite catalysts. The method is based on probing the accessibility of acid sites by IR spectroscopy of adsorbed al- kylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of subsequently adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-by-step characterization of the nature and strength of sites with different accessibility. The approach proposed was tested on the series of small-port mordenites dealuminated up to Si/Al ratios of 11, 12, 14 and 18 and characterized by various techniques including XRD, TPD-NH 3 , nitrogen adsorption–desorption, 1 H MAS NMR and IR spectroscopy. It has been demonstrated that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) the acid sites in the side pockets become accessible for such probe molecules as pyridine due to the partial destruction of side pockets upon dealumination (ii) subsequently, the formation of secondary mes- opore system due to further dealumination makes the zeolite crystals completely accessible to relatively bulky molecules such as lutidine, collidine, 2,6-di-tert-butylpyridine. Ó 2004 Published by Elsevier Inc. Keywords: IR spectroscopy of probe molecules; Dealuminated mordenites; Acidity of zeolites; Alkylpyridines 1. Introduction The rational design of efficient heterogeneous cata- lysts requires a detailed knowledge of the active site(s). In the case of zeolites, much work has been devoted to a detailed description of the active site in its nanoscale (micro-porosity) environment. It has also been repeat- edly demonstrated [1–4] that mass transfer limitations play an important role in many of their industrial applications. Creation of controlled mesoporosity has been shown to minimize or even cancel these limitations [5,6]. There is a need and a strong incentive to further improve the active site description of zeolitic catalysts and in particular, the accessibility to the active site de- serves more attention. FTIR spectroscopy of adsorbed molecules is a well- known powerful tool to investigate the acid sites of zeolite catalysts, in particular, their nature, number and strength [7–10]. Studies on the location and accessibility of the zeolitic acid sites are available but less wide- spread. For instance, the distribution of active sites between the external and internal zeolite surface was studied by adsorbing bulky probe molecules, unable to enter zeo- litic pores. Corma et al. [11] used 2,6-di-tert-butylpyri- dine for the investigation of the external surface acidity of MFI and MOR zeolites. This probe molecule, how- ever, was able to penetrate easily into the pore system of 3-dimensional BEA zeolite. The intensity of the N–H þ stretching band of adsorbed 2,6-di-tert-butylpyridine was shown to correlate with the catalytic activity of * Corresponding author. Fax: +7-95-9328846. E-mail addresses: fajula@cit.enscm.fr (F. Fajula), iiivanova@ phys.chem.msu.ru (I.I. Ivanova). 1387-1811/$ - see front matter Ó 2004 Published by Elsevier Inc. doi:10.1016/j.micromeso.2004.03.028 www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 71 (2004) 157–166