Intracrystalline transport resistances measurements

The existence of intracrystalline transport resistances has been confirmed by PFG NMR self-diffusion measurements of short-chain length alkanes in MFI-type zeolites [216,217] with varying observation time. Figure 25 presents the relevant data obtained with n-butane as a probe molecule. Here, the diffusivities are plotted in a way, which is made possible by the special features of PFG NMR, viz. as a function of the displacements over which the molecular diffusion paths (giving rise to the plotted diffusivities) have been measured. This is achieved on the basis of Eq. 7 by which the measured diffusivities maybe transferred into the mean square displacements covered by the molecules during the observation time. Obviously, in the case of ordinary diffusion, i.e. in the original notion of Eq. 7, the diffusivity depends on neither... 

This case study clearly illustrates the usefulness of the ZLD-TEOM technique in determining intracrystalline diffusivities in zeolites, provided that effects of other transport resistances such as the surface barrier are eliminated by varying the crystal size of the zeolites. The measured steady-state diffusivity can be directly used for predicting effects of diffusion in reactions catalyzed by zeolites. More important, the TEOM makes it possible to distinguish the deactivation caused by blockage of the active sites and by increased diffusion resistance caused by blockage of cavities or channels by coke. 

If the calculated value of is equal to the measured intracrystalline lifetime, Tinira, the rate of molecular exchange between different crystals is controlled by the intracrystalline self-diffusion as the rate-limiting process. Any increase of Timn, in comparison with Tf,j L indicates the existence of transport resistances different from intracrystalline mass transport. Under the conditions of TD NMR one has A r. > Antra, thus these resistances can only be brought about by sur ce barriers. The ratio Timra/Tfn L represents, therefore, a direct measure of the influence of surface barriers on molecular transport. 

In addition to the conventional application of nmr pulsed field gradient experiments to self-diffusion studies, it is also possible to determine the intracrystalline molecular life times. Referring to the corresponding classical experiment, this method has been termed nmr tracer desorption technique (7). Together with the self-diffusion measurements it provides an excellent tool for characterizing the transport properties in the intra- and intercrystalline spaces, as well as at the interface between them. So far, the nmr techniques provide the only possibility for a direct determination of the existence and of the intensity of transport resistances at this... 

Figure 6 provides a comparison between measured spectra and theoretical spectra calculated under the assumption that the adsorption/desorption process is controlled by either intracrystalline diffusion (Fig. 6a) or external transport resistances such as surface barriers (Fig. 6b). For simplicity in the calculations, the crystallites have been assumed to be of nearly spherical shape with a concentration-independent transport diffusivity Dj or surface permeability a, respectively. Values of the intracrystalline mean lifetime are therefore given by...

If molecular exchange is controlled by intracrystalline diffusion, then the intracrystalline mean lifetime is given by Eq. (2), where it is assumed that the crystallites may be approximated by spheres (Sec. II.A.). Clearly, coincides with the directly measured Tj ,ra if desorption is controlled by intracrystalline diffusion. If, however, the rate of molecular exchange is additionally reduced by transport resistances at the crystallite boundary (so-called surface barriers), Tji,ra may be much greater than ... 

Although the systems investigated here exhibited predominantly macropore control (at least those with pellet diameters exceeding 1/8" or 0.32 cm), there is no reason to believe that surface diffusion effects would not be exhibited in systems in which micropore (intracrystalline) resistances are important as well. In fact, this apparent surface diffusion effect may be responsible for the differences in zeolitic diffusion coefficients obtained by different methods of analysis (13). However, due to the complex interaction of various factors in the anlaysis of mass transport in zeolitic media, including instabilities due to heat effects, the presence of multimodal pore size distribution in pelleted media, and the uncertainties involved in the measurement of diffusion coefficients in multi-component systems, further research is necessary to effect a resolution of these discrepancies. 

The combined application of PFG NMR self-diffusion and tracer desorption experiments has thus proved to be an effective tool for studying the hydrothermal stability of A-type zeolites with respect to their transport properties [186]. It turns out that with commercial adsorbent samples there are considerable variations in hydrothermal stability between different batches of product and even between different pellets from the same batch. As an example. Fig. 24 shows the distribution curves [A(Tin,ra) versus Ti ,r.j] measured with ethane as a probe molecule at 293 K for two different samples of commercial 5A zeolites. Evidently batch 1 is more resistant to hydrothermal deterioration, because the lengthening of Tjn,ra is less dramatic than with batch 2. Since the intracrystalline diffusivity was the same for all samples, the deterioration can be attributed to the formation of a surface barrier. 

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