Intracrystalline diffusion in zeolites

Jobic, H. and Methivier, A. (2005) Intracrystalline diffusion in zeolites studied by neutron scattering techniques. Oil Gas Sci. Technol., 60 (5) 815-830. 

Figure 13.14 was published as Figure 11 from the article Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques by H. Jobic and A. Methivier, Oil and Gas Science and Technology - Rev. IFP, Vol. 50 (2005),... 

A more sophisticated method which has found wide application in the study of intracrystalline diffusion in zeolites is the nuclear magnetic resonance (NMR) pulsed field gradient self-diffusion method. The method, which is limited to hydrocarbons and other sorbates with a sufficient density of unpaired nuclear spins, depends on measuring directly the mean square distance traveled by molecules,... 

Earlier studies of intracrystalline diffusion in zeolites were carried out almost exclusively by direct measurement of sorption rates but the limitations imposed by the intrusion of heat transfer and extra-crystalline mass transfer resistances were not always fully recognized. As a result the reported diffu-sivities showed many obvious inconsistencies such as differences in diffusivity between adsorption and desorption measurements(l-3), diffusivities which vary with fractional uptake (4) and large discrepancies between the values measured in different laboratories for apparently similar systems. More recently other experimental techniques have been applied, including chromatography and NMR methods. The latter have proved especially useful and have allowed the microdynamic behaviour of a number of important systems to be elucidated in considerable detail. In this paper the advantages and limitations of some of the common experimental techniques are considered and the results of studies of diffusion in A, X and Y zeolites, which have been the subject of several detailed investigations, are briefly reviewed. 

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. 

Tn most applications of zeolites, it is necessary for molecules to be able to diffuse into or out of their fine pore structure, and in many of these applications, particularly catalysis, the counterdiffusion of at least 2 different kinds of molecules occurs. The rates of these diffusion processes can have a profound effect upon the apparent activity and selectivity of zeolitic catalysts (21) and upon such characteristics as dispersion and sharpness of separation in the use of zeolites in separation and purification processes. The state of knowledge of intracrystalline diffusion in zeolites is reviewed by Barrer in a paper for this symposium (4). Little is known about unidirectional diffusion in zeolites of substances of indus-... 

The shape of this relation is found to be rather insensitive to the given geometry of the crystallites []]. In particular, the effectiveness factor is found to be equal to 1 for intracrystalline diffusion in zeolites under stationary conditions [4,5]. 

The ZLC method recently developed by Eic and Ruthven (5) is an important new technique for measuring the intracrystalline diffusion in Zeolites since only a very small sample of... 

Table 4 Classification of methods for measuring intracrystalline diffusion in zeolites...

For sufficiently small particles 0 0 and 1, so the measured rate constant approaches the intrinsic rate constant (k). By making replicate measurements under similar conditions, with different particle size fractions it is possible to determine both the intrinsic rate constant and the effective interparticle diffusivity. Haag [67] suggested that this approach could be used to determine intracrystalline diffusivities in zeolite crystals. A more complete experimental study in which the diffusivity of 2,2-dimethyl butane in HZSM-5 was determined both chromatographically and from measurements of the cracking rate under diffusion-limited conditions was reported by Post et al. [68] - see Fig. 11. This approach has the advantage that it makes steady-state rather than transient measurements, but it is limited to sorbates for which a suitable catalytic reaction occurs. 

Table 3. Experimental Methods for Measuring Intracrystalline Diffusion in Zeolites QENS...

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]. 

This methods depends on the implicit assumption that the uptake rate is controlled entirely by intracrystalline diffusion in an isothermal system, with all other resistances to either mass or heat transfer negligible. This is a valid approximation if diffusion is sufficiently slow or if the zeolite crystals are sufficiently large but the dominance of intracrystalline diffusional resistance should not be assumed without experimental verification. In many practical systems, particularly with small commercial zeolite crystals, the external heat and mass transfer resistances are in fact dominant. A detailed discussion of such effects has been given by Lee and Ruthven(5-7). 

Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. 

There are several models to describe intracrystalline diffusion (step 3) in microporous media. Diffusion in zeolites is extensively described in Ref. 30. For the modeling of permeation through zeolitic membranes, such a model should take the concentration dependence of zeolitic diffusion into account. Moreover, it should be easy applicable to multicomponent systems. In Section III.C, several models will be discussed. 

Because of the rigid crystal structure and small window size, ionic diffusion in zeolites is slow and the activation energy is high (Barrer, 1980). Except on samples of very fine particle size, the exchange rate is controlled by intracrystalline rather than liquid-phase mass transfer. 

Fig. 3 The ratio Dapp/Antra between the apparent and the intracrystalline diffusivity in PFG NMR experiments with zeolite crystallites of radius R as a function of the normalized observation time f = Dappf/R. From [69] with permission...

Studying molecular diffusion in zeolite crystallites is complicated by the small size of the objects of investigation. Inevitable deviations of the real structure of a sample from the ideal one lead to an additional complication of the situation. It is not unexpected, therefore, that in spite of considerable progress in the experimental techniques, there is still some controversy in the imder-standing of intracrystalline zeohtic diffusion (cf. the preceding chapters of this volume). 

The diffusion process in the macropore and mesopore follows the combination of the molecular and Knudsen mechanisms while the diffusion process inside the zeolite crystal follows an intracrystalline diffusion mechanism, which we have discussed in Section 10.2. The length scale of diffusion in the macropore is the dimension of the particle, while the length scale of diffusion in the micropore is the dimension of the zeolite crystal thus, although the magnitude of the intracrystalline diffusivity (in the order of 10 to 10 cmVsec) is very small compared to the diffusivity in the macropore the time scales of diffusion of these two pore systems could be comparable. 

When reactive hydrocarbons such as olefins are present, slow formation of polymeric species may occur within the zeolite crystals. On thermal regeneration these species are converted to coke, leading to a decline in the useful capacity of the adsorbent and in some cases also a decline in intracrystalline diffusivity. In the drying of cracked gas this problem may be avoided by using... 

Pfeifer. H., et al.. Concentration dependence of intracrystalline self-diffusion in zeolites, Adsorpt. Sci. Technol.. 2(4), 229-240(1985). 

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],... 

Fig. 3.1.6 Temperature dependence of the intraparticle diffusivity of n-octane in an FCC catalyst and the intracrystalline diffusivity of n-octane in large crystals of USY zeolite measured by PFG NMR. The concentration of n-octane in the samples was in all cases 0.62 mmol g 1. Lines show the results of the extrapolation of the intracrystalline diffusivity and of the intraparticle diffusivity of n-octane to higher temperatures, including in particular a temperature of 800 K, typical of FCC catalysis.

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