14 May 1999 Ž . Chemical Physics Letters 305 1999 39–43 Time-dependent xenon diffusion measurements in microporous silicon imidonitrides by pulsed field gradient NMR spectroscopy F. Junker, W.S. Veeman ) Institut fur Physikalische und Theoretische Chemie, Gerhard-Mercator-UniÕersitat Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany ¨ ¨ Received 12 February 1999; in final form 12 February 1999 Abstract We present time-dependent xenon diffusion measurements over a wide range of diffusion times in three different silicon imidonitrides, a new class of microporous catalysts, by PFG NMR spectroscopy. Our experimental data show that for short diffusion times D a sharp decrease of the diffusion coefficient D with increasing D occurs, while in the long-time regime D approaches a constant value. We correlate the differences in the diffusional behaviour with the pore structure of the material. Additionally, we found that for these materials there is a linear correlation between the chemical shift of the 129 Xe resonances and the corresponding long-time diffusion coefficient. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Over the years, 129 Xe-NMR has been used exten- sively to characterize porous materials, like zeolites w x w x w x 1,2 , clathrates 3,4 and polymers 5–10 . The ad- vantage of xenon in comparison to other commonly used adsorbates like water or organic substances is the large polarizability of its electron cloud, which can lead to large differences in the chemical shift of the 129 Xe-NMR resonances. For most materials, the chemical shift of adsorbed xenon is found to be between 0 and 250 ppm, when the chemical shift of free xenon gas is taken as the reference at 0 ppm. Several authors have proposed empirical relations and theoretical models, which relate the chemical shift of 129 Xe, adsorbed in pores of the material, with ) Corresponding author. E-mail: w.s.veeman@uni-duisburg.de Ž w x the dimensions of the pore pore diameter 11 , wall w x w x. curvature 12 , surface-to-volume ratio 13,14 . An additional source of information about the structure of the porous material in which the xenon is located, can be obtained from time-dependent xenon diffusion measurements. It is known that the self-diffusion coefficient of a fluid in a solid porous w x medium is a function of the diffusion time 13 . For short diffusion times D, the diffusion of the majority of the molecules is not hindered by the pore walls and the diffusion coefficient almost equals that of free diffusion, as used in Fick’s law. The restrictions the pore structure imposes on the self-diffusion gain importance for longer diffusion times. Mitra et al. w x 14 have shown that the short-time behaviour of the diffusion coefficient D as a function of the diffusion time D leads to information about the pore wall surface to pore volume ratio and the long-time be- haviour of D as a function of D gives information about the tortuosity of the material. The experimental 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00333-4