NMR tracer desorption

With the development of the fast tracer desorption NMR method a more detailed investigation of these systems became possible. A study of diffusion of C2H6 in 5A by this method showed no significant surface barrier, even when the sieve was dehydrated at 600°C under conditions similar to those used commercially(28,29). 

Molecular mean lifetime inside a crystal " intra Tracer desorption NMR 1.1 ms... 

Comparison between xf a as determined on the basis of Eq. (3.1.15) from the microscopically determined crystallite radius and the intracrystalline diffusivity measured by PFG NMR for sufficiently short observation times t (top left of Figure 3.1.1), with the actual exchange time xintra resulting from the NMR tracer desorption technique, provides a simple means for quantifying possible surface barriers. In the case of coinciding values, any substantial influence of the surface barriers can be excluded. Any enhancement of xintra in comparison with x a, on the other side, may be considered as a quantitative measure of the surface barriers. 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

It is worth noting that within a range of 20 %, five different methods of analyzing the crystallite size, viz., (a) microscopic inspection, (b) application of Eq. (3.1.7) for restricted diffusion in the limit of large observation times, (c) application of Eq. (3.1.15) to the results of the PFG NMR tracer desorption technique, and, finally, consideration of the limit of short observation times for (d) reflecting boundaries [Eq. (3.1.16)] and (e) absorbing boundaries [Eq. (3.1.17)], have led to results for the size of the crystallites under study that coincide. 

B. Self-Diffusion Measurements by the NMR Tracer Desorption Technique... 

From the NMR tracer desorption and self-diffusion data (second and third lines of Table I), one obtains the relation Timm > TmlL. In the example given, intercrystalline molecular exchange is limited, therefore, by transport resistances at the surface of the individual crystals. Combined NMR and high-resolution electron microscopy studies 54) suggest that such surface barriers are caused by a layer of reduced permeability rather than by a mere deposit of impenetrable material on the crystal surface, although that must not be the case in general. 

The basic principles of both the nmr pulsed field gradient and nmr tracer desorption techniques are presented. It is shewn that these methods allow the measurement of the coefficients of intracrystalline and of long-range self-diiiusion as well as or the intracrystalline mean life times of the adsorbate molecules. By combining this information, a unique possibility for the direct proof of the existence of surface barriers is provided. The nmr methods are applied to study the transport properties of adsorbate molecules in zeolite NaX, NaCaA and ZSlf-5, with particular emphasis on the existence of surface barriers. 

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... 

In general, nmr tracer desorption data are represented by the molecular Intracrystalline life time which is determined... 

With commercial samples, the range of measurement may be considerably enhanced by applying a large constant field gradient in addition to the pulsed field gradients. It has been shown (ID that in this case by the nmr tracer desorption technique a time constant t is... 

Figure 1 Analysis oi the nmr data for tracer desorption studies (in the example butane/HaX, lot) mgg, R = 25 pm.

D.i., . increases with increasing sorbate concentration (12). Optimum conditions for the application of the nmr tracer desorption technique to zeolite NaX are in general provided, therefore, at higher sorbate concentrations. The nmr tracer desorption experiments on zeolite NaX have been carried out with n-paraffins and with aromatic compounds as adsorbates. 

Figure 2a Intracrystalline near, life times determined from nmr tracer desorption studies on zeolite NaX for methane < , c = 94 ngg-, R = 25 pa) ethane t. 0, 113 mgg 25 urn n-butane (, 165 mgg 25 pm) and n-hexane (A, 165 mgg, ...

For a direct check of the existence or non-existence of surface barriers we have applied the nmr tracer desorption technique. For a few selected systems, Table I gives a comparison between the mtracrvstaiiine mean life times a,and the quantities T ,n., l J 1 calculated from the coefficients oi intra-crystalline diffusion on the basis of Equation 11. The order of - magnitude agreement between these quantities indicates that under the given conditions in fact a substantial influence of surface barriers on molecular transport may be excluded,... 

Figure 21 Analysis of PFG NMR data for NMR tracer desorption studies (butane/ NaX, 165 mgg R = 25 p.m, 353 K). (From Ref. 48.)...

The usual way to analyze the PFG NMR data for NMR tracer desorption measurements is illustrated in Fig. 21 [48] the intensity of the slowly decaying component (corresponding to the broader distribution within the propagator representation) coincides with the relative amount of molecules [l-7(t)j which, for the given observation time A, have remained in their crystallites (Fig. 21a,b). 

Equivalently, one may investigate the existence of a surface barrier by comparing the intracrystalline diffusivity as determined by PEG NMR with a quantity derived from the NMR tracer desorption curve assuming intracrystalline diffusion control. In the absence of significant surface barriers, one should find Dj .a > whereas the existence of a barrier will give Di ,ra > f des ... 

Figure 22 gives a comparison of the results of NMR tracer desorption studies and self-diffusion measurements on short chain length paraffins in zeolite NaX [48]. For illustration, the complete tracer desorption curves are also given at selected temperatures. Covering the range from — 140 to 200°C and chain lengths from one to six carbon atoms, the intracrystalline mean lifetimes are found to coincide with values of calculated via Eq. (2) from the NMR self-diffusion co-... 

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. 

Figure 28 Values for the coefficients of intracrystalline self-diffusion (A) and for the apparent diffusivity determined from NMR tracer desorption curves ( 1) at 293 K...

It is noteworthy that, in principle, each of these time constants may be determined by the PFG NMR method Xj-rys,, which is identical with Xi ,ra " and Xpciict directly from the measurement of the coefficients of intracrystalline and of long-range diffusion, and x harr by combining the result of the NMR tracer desorption measurements (Xjn, see Sec. III.C.) with Xj ,ra , using the equation Timra + x barr Since in the NMR measurements a wide range of... 

---