Intracrystalline barriers

An absolutely different situation occurs in case of polycrystalline adsorbent treated at high temperature in air or in other oxygen containing medium. In this case the volt-ampere analysis exhibits sharply nonlinear VAC, deviations from the Ohm law being observed at anomalously low fields [47]. This indicates an existence of high intracrystalline barriers in such adsorbents. These barriers can be attributed to crys-... 

Abstract In this chapter the main macroscopic experimental methods for measuring diffusion in microporous solids are reviewed and the advantages and disadvantages of the various techniques are discussed. For several systems experimental measurements have been made by more than one technique, and in Part 3 the results of such comparative studies are reviewed. While in some cases the results show satisfactory consistency, there are also many systems for which the apparent intracrystaUine diffusivities derived from macroscopic measurements are substantially smaUer than the values from microscopic measurements such as PFG NMR. Recent measurements of the transient intracrystalline concentration profiles show that sirnface resistance and intracrystalline barriers are both... 

Introduction of PFG NMR to zeolite science and technology has revolutionized our understanding of intracrystalline diffusion [19]. In many cases, molecular uptake by beds of zeolites turned out to be limited by external processes such as resistances, surface barriers or the finite rate of sorbate supply, rather than by intracrystalline diffusion, as previously assumed [10, 20-24]. Thus, the magnitude of intracrystalline diffusivities had to be corrected by up to five orders of magnitude to higher values [25, 26],... 

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. 

Micropore mass transfer resistance of zeoUte crystals is quantified in units of time by r /Dc, where is the crystal radius and Dc is the intracrystalline diffusivity. In addition to micropore resistance, zeolitic catalysts may offer another type of resistance to mass transfer, that is resistance related to transport through the surface barrier at the outer layer of the zeoHte crystal. Finally, there is at least one additional resistance due to mass transfer, this time in mesopores and macropores Rp/Dp. Here Rp is the radius of the catalyst pellet and Dp is the effective mesopore and macropore diffusivity in the catalyst pellet [18]. 

The prerequisites of the evaluation of data characteristic of intracrystalline processes in the case of zeolite sorbents are discussed, along with the conditions under which diffusion can be compared to self-diffusion. Selected results of investigations carried out in the author s laboratory are given in order to demonstrate the consistency of sorption kinetic data with intracrystalline mobility data of single components on molecular sieves (HS). Various types of surface barrier which may influence the uptake rate are also described. 

The concept of transport resistances localized in the outermost regions of NS crystals was introduced in order to explain the differences between intracrystalline self-diffusion coefficients obtained by n.m.r methods and diffusion coefficients derived from non-equilibrium experiments based on the assumption that Intracrystalline transport is rate-limiting. This concept has been discussed during the past decade, cf. the pioneering work [79-81] and the reviews [2,7,8,23,32,82]. Nowadays, one can state that surface barriers do not occur necessarily in sorption uptake by NS crystals, but they may occur if the cross-sections of the sorbing molecular species and the micropore openings become comparable. For indication of their significance, careful analysis of... 

An intriguing aspect of these measurements is that the values of D determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10 5 cm2 sec-, and the value determined for sorption rates only 5 x 10"10 cm2 sec-1. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296) showed that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10-6 cm2 sec-1) can be reliably measured by sorption kinetics... 

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. 

The absence of additional diffusion barriers at the crystal surface (coke, modifiers, etc.) can be assumed if the experimentally determined mean intracrystalline lifetimes, Timra (measured by TD NMR), and the corresponding calculated data, rSitra [according to Eq. (10)], coincide. The rSim data are calculated by using the self-diffusion coefficients, Dimra (measured by PFG NMR), and crystal radii, R, assuming the adsorption/desorption process to be diffusion controlled. 

In contrast to HZSM-5 coked by n-hexane, for mesitylene-coked HZSM-5 (Fig. 39) the intracrystalline mobility, Dmtfa, of methane is nearly unaffected by coke deposits (42,131). On the other hand, the effective selfdiffusion coefficient, decreases continuously with increasing time onstream, i.e., Deff < Dintra. The conclusion is that mesitylene coking leads to the pronounced formation of a surface barrier, i.e., from the beginning of... 

These are all activated steps, which can be modeled assuming that molecules jump between low-energy sites. Each Jump can be correlated to an activation energy, and the net flow is calculated from the forward and reverse jumps. Obstructions in the pores can be modeled as occasional intracrystalline energy barriers. The rate-determining step in this model is dependent on operating conditions (temperature, partial pressure) and the characteristics of the molecule and the crystalline material. Step 1,2,4, and 5 are referred to as interfacial processes. The importance of these processes will be discussed in Section III.B. 

The control of reaction rates by a bulk difiusion process is not usually demonstrable by microscopic observations, but support may be obtained from measurements of diffusion coefficients of appropriate species within the structure concerned. This approach has been invaluable in formulating the mechanisms of oxidation of metals, where rates of reaction have been correlated with rates of transportation of ions across barrier layers of product. Sometimes the paths by which such movements occur correspond to regions of high difi isivity, involving imperfect zones within the barrier layer, compared with normal rates of intracrystalline diffusion across more perfect regions of material [63]. Difiusion measurements have been made for ions in nickel sulfide and it was concluded that the decomposition of NiS is diffiision controlled [50]. 

Zeolites, with their rigid crystal structure, pose special problems relevant to phenomena in soils. For example, the water content is found to change, in accordance with the space available for solvent in the intracrystalline cavities, when a counter ion is exchanged for another of different crystalline radius (Barrier, 1980). In turn, the change in water content affects the ionic mobilities. 

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. 

Hence, the nmr pulsed iield gradient technique could be applied to study the difiusion behaviour of a large variety of adsorbate molecules. The data for intracrystalline self-diffusion are summarized in the review <6), In the following, we shall restrict ourselves to the discussion of experimental results referring to the existence of surface barriers. 

Covering temperatures from -140 up to 200 C and chain lengths from one to six carbon atoms, the intracrystalline mean life times are found to coincide with the values oi rV 1 r calculated from the nmr self-diffusion coefficients. This clearly indicates that molecular exchange is controlled by intracrystalline self-diffusion, and that for the considered adsorbate-adsorbent systems there are no perceptible surface barriers. 

Aromatic Compounds in NaX. Molecular transport of aromatic compounds in zeolite NaX has been studied by both nmr and uptake measurements. On the basis of Equation 4,and if surface barriers are absent, both methods should lead to comparable results. Though uptake measurements by the variable - pressure, constant - volume method by Biilow and coworkers (16,17) apparently are in satisfactory agreement with the nmr data (18), extensive uptake measurements including chromatographic methods are continuously found to yield diffusivities of about two orders of magnitude below these values (19,20). In principle, this discrepancy might be explained by the existence of surface barriers, which remain invisible for nmr studies of intracrystalline diffusion, but which may control the uptake rate. 

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 ... 

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 ... 

Table 4 indicates that this effect is attributable primarily to the formation of surface barriers rather than to a significant change of the intracrystalline mobility. Studies of the surface composition of the zeolite crystallites by x-ray photoelectron spectroscopy show that the formation and enhancement of the surface barrier is accompanied by a decrease of the cation content in the surface layer, thereby indicating a structural collapse of the surface layer of the zeolite crystallite [182]. 

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. 

From Eq. (2), the measured diffusivities may be used to determine the mean lifetime of the reactant and product molecules within the individual crystallites under the assumption that the molecular exchange is exclusively controlled by intracrystalline diffusion. These values, being of the order of 30 ms, are found to agree with the real intracrystalline mean lifetime directly determined by NMR tracer desorption studies (208], so that any influence of crystallite surface barriers may be excluded. From an analysis of the time dependence of the intracrystalline concentration of the reactant and product molecules, the intrinsic reaction time constant is found to be on the order of 10 s. This value is much larger than the intracrystalline mean lifetimes determined by PFG NMR, and thus any limiting influence of mass transfer for the considered reaction may be excluded. In agreement with this conclusion, the size of the applied crystallites was found to have no influence on the conversion rates in measurements with a flow reactor (208]. 

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