Journal of Catalysis 213 (2003) 321–323 www.elsevier.com/locate/jcat Research Note PFG NMR observation of an extremely strong dependence of the ammonia self-diffusivity on its loading in H-ZSM-5 O. Geier, S. Vasenkov, ∗ D. Freude, and J. Kärger Fakultät fur Physik und Geowissenschaften, Universität Leipzig, Linnéstrasse 5, 04103 Leipzig, Germany Received 15 July 2002; revised 29 August 2002; accepted 19 September 2002 Abstract Ammonia diffusion in H-ZSM-5 (Si/Al = 20) has been studied by the PFG NMR method for different ammonia loadings at 298 K. The results obtained indicate that the self-diffusion coefficient of ammonia changes by up to two orders of magnitude if ammonia loading is varied by a factor of only 2 between 1.6 and 0.8 mmol/g. This unusually strong loading dependence is attributed to the strong interaction of ammonia molecules with the limited number of acid sites of the H-ZSM-5 framework.  2002 Elsevier Science (USA). All rights reserved. Keywords: Ammonia; Diffusion; PFG-NMR; H-ZSM-5 1. Introduction Framework acidity is of crucial importance for many cat- alytic applications of microporous materials. Although the mechanism of chemical transformations on acid sites in ze- olites is sufficiently well understood for many industrially important reactions, studies of the influence of these sites on the intracrystalline molecular transport still remain one of the challenging subjects in zeolite research [1,2]. Such stud- ies are particularly important in view of recent data suggest- ing that the rates of some important chemical reactions may be limited by the rates of intracrystalline molecular trans- port [3]. NH 3 is routinely used as a probe molecule for acid sites. Hence, inrracrystalline diffusion of ammonia can be expected to depend significantly on the presence of such sites. This makes ammonia an ideal probe for the investigation of the influence of acid sites on intracrystalline transport. Understanding of the mechanism of ammonia transport in acidic zeolites is also important because the temperature-programmed desorption (TPD) of ammonia has become a standard method for probing acid sites [4]. The competition between ammonia desorption from acid sites and the intracrystalline diffusion of these molecules during TPD was a subject of very recent studies [2]. * Corresponding author. E-mail address: vasenkov@physik.uni-leipzig.de (S. Vasenkov). In this note we report for the first time the coefficients of intracrystalline self-diffusion of ammonia in H-ZSM-5 at different loadings. The diffusion measurements were performed using the pulsed field gradient (PFG) NMR technique [5]. The observed strong loading dependence of ammonia diffusivity is quantitatively correlated with the interaction between ammonia and the limited number of acid sites present in the H-ZSM-5 framework. 2. Experimental The measurements were carried out using the homebuilt PFG NMR spectrometer FEGRIS 400 NT operating at a 1 H resonance frequency of 400 MHz [6]. For all measurements the 13-interval pulse sequence [7] was employed. This sequence suppresses distortions of PFG NMR diffusion results by internal magnetic field inhomogeneities induced by susceptibility variations in a sample. To obtain the diffusivity, the attenuation of the PFG NMR spin echo signal (Ψ ) was measured as a function of the amplitude of the applied field gradient (g). This amplitude was varied from 0.05 T/m to a maximum value of 20 T/m for samples with different ammonia loadings. Under our measurement conditions the molecular displacements were in all cases sufficiently small in comparison with the smallest dimension of the crystals to ensure that the measured diffusivities represent intracrystalline diffusion coefficients. 0021-9517/02/$ – see front matter  2002 Elsevier Science (USA). All rights reserved. PII:S0021-9517(02)00018-0