On the interpretation of the NH 3 -TPD patterns of H-ZSM-5 and H-mordenite Ferenc L onyi, J ozsef Valyon * Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary Received 4 April 2001; received in revised form 25 June 2001; accepted 29 June 2001 Abstract The species obtained from adsorption of ammonia over zeolite H-ZSM-5 and H-mordenite were characterized by temperature-programmed desorption NH 3 -TPD) and infrared spectroscopy IR). At ammonia loadings of NH 3 =Al > 1, the IR spectra indicated the formation of NH  4  nNH 3 associations, where n P 1. At loadings of NH 3 =Al < 1 spectra of NH  4 ions and Lewis-bound NH 3 were recorded. The decomposition of the NH  4 ions and desorption of NH 3 from strong Lewis acid sites gave rise to TPD peaks above about 673 K. The peaks at temperatures lower than about 673 K were assigned to ammonia released from protonated NH 3 associations. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Acidity; H-ZSM-5; H-mordenite; NH 3 -TPD; IR spectra of NH 3 1. Introduction The interest in zeolite acidity is motivated by the importance of zeolites as solid acid catalysts. Usually, the interaction of acid sites and basic probe molecules is studied to distinguish between Br onsted B)- and Lewis L)-type sites and to de- termine their amounts and strengths. Numerous theoretical and experimental studies have been devoted to the adsorption of NH 3 [1±28]. NH 3 - TPD is one of the most often used methods [7±19]. In principle, both the concentration of sites having similar acid strengths and the average adsorption heat or activation energy of NH 3 desorption can be determined using the TPD method. Often the temperature of maximum desorption rate, i.e., the temperature of a TPD peak, is used as a rough measure of the acid strength of the sorption sites. Among the limitations of the method is that it can distinguish sites by sorption strength only, but not L- from B-type sites. Moreover, desorption may proceed simultaneously from sites of dierent types resulting in more or less overlapping TPD peaks. In the NH 3 -TPD curves of H-zeolites, peaks are generally observed in two temperature re- gions. The regions below and above about 673 K are referred to as low-temperature LT) and high- temperature HT) region, respectively [7,8]. The peaks in the HT region can be attributed to the desorption of NH 3 from strong B and L sites, Microporous and Mesoporous Materials 47 2001) 293±301 www.elsevier.com/locate/micromeso * Corresponding author. Tel.: +36-1-438-4147; fax: +36-1- 325-7554. E-mail address: valyon@chemres.hu J. Valyon). 1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S1387-181101)00389-4