The acid properties of H-ZSM-5 as studied by NH 3 -TPD and 27 Al-MAS-NMR spectroscopy Luz Rodrı ´guez-Gonza ´lez a , Florian Hermes a , Marko Bertmer b , Enrique Rodrı ´guez-Castello ´n c , Antonio Jime ´nez-Lo ´pez c , Ulrich Simon a, * a RWTH Aachen University, Institute of Inorganic Chemistry, Landoltweg, 1, D-52056 Aachen, Germany b RWTH Aachen University, Institute of Technical and Macromolecular Chemistry, Worringer Weg 1, D-52056 Aachen, Germany c Faculty of Sciences, Department of Inorganic Chemistry, Crystallography and Mineralogy, University of Ma ´laga, Campus de Teatinos s/n 29071, Ma ´laga, Spain Received 16 January 2007; received in revised form 21 May 2007; accepted 3 June 2007 Available online 13 June 2007 Abstract The acid properties of H-ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratios (30, 50, 80, 150, 280 and 1000) were examined by means of the temperature programmed desorption of ammonia (NH 3 -TPD). Different pretreatments together with a comparison of different curve-fitting methods after desorption of ammonia were used to differentiate between different adsorption sites. 27 Al-MAS-NMR spectroscopy was applied to distinguish the framework (Al F ) from the extra-framework aluminum (Al EF ). Both techniques reveal that the concentration of Brønsted acid sites is lower than expected when considering the aluminum content. The strength of the Brønsted acid sites, determined by a theoretical model assuming the free readsorption of ammonia, is found to be in the range of DH = 129–161 kJ mol 1 and shows variations in the acid strength to be less than s = 12 kJ mol 1 . # 2007 Elsevier B.V. All rights reserved. Keywords: NH 3 -TPD; H-ZSM-5; 27 Al-MAS-NMR; Brønsted acidity; Zeolite 1. Introduction The interest in zeolite acidity is motivated by its application to solid acid catalysts [1] as well as in gas sensing [2]. There are several methods available to determine the properties of a solid acid, which can reveal its concentration, strength and type (Brønsted or Lewis type). Traditional methods such as the indicator method [3] or the amine titration method [4] may give the acid strength and concentration but they are complicated by the presence of water and/or slow diffusion into the micropores. Another method of characterization is infrared spectroscopy (IR), which yields the nature and concentration of acid sites as well as of adsorbed basic molecules [5]. Nuclear magnetic resonance (NMR) can be used to differentiate between different Si and Al species [6]. Furthermore, NMR spectroscopy accompanied by theoretical studies has been used to analyze the proton transfer from the Brønsted site to adsorbate molecules, such as H 2 O or H 2 S [7]. By calorimetric measurements, for example, differential scanning calorimetry, not only the acid strength of zeolites can be determined, but also information about the strength distribution can be obtained [8–10]. The temperature programmed desorption of a basic molecule such as ammonia (NH 3 -TPD) is one of the most commonly used methods used for measuring the surface acidity of porous materials such as zeolites, clays or mesoporous silica [11–14]. It is widely used because of its simplicity and it is a likewise inexpensive method [15]. However, a detailed analysis of the TPD-profile may be complicated by several factors. Whereas the total acid amount can be more or less easily determined, the interpretation of the peak shape and position is complicated by its dependence on the experimental conditions, like the amount of sample or the gas flow, as well as by kinetic phenomena such as readsorption and/or slow diffusion. These problems can be overcome using well-selected experimental conditions and applying a mathematical description for the desorption peaks observed, as has been shown by different www.elsevier.com/locate/apcata Applied Catalysis A: General 328 (2007) 174–182 * Corresponding author. E-mail address: ulrich.simon@ac.rwth-aachen.de (U. Simon). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.06.003