Zeolites configurational diffusion

For high-quality MFI-type zeolite membranes either Knudsen or configurational diffusion is representative of the flow. Knudsen diffusion occurs in pores where... 

Extensive manifestations of configurational diffusion can be seen in catalytic zeolites. The landmark measurements by Gorring [16] of the diffusion coefficients of alkanes (in potassium T zeolites) as a function of their carbon number are shown in Fig. 4, indicating over two orders of magnitude of change in diffusivity, with a minimum at C-8 and a maximum at C-12 for unexplained reasons. Similarly, spectacular effects for more complex molecules have been observed by Haag and Chen [17] and are shown in Fig. 5. Although we do not yet have a workable correlation between zeo-... 

In zeolitic materials, where the molecules are of pore size dimensions, activated configurational diffusion... 

The discovery of a new family of shape selective zeolites by Mobil (239) has now extended the range of pore sizes and thus the accessible range of configurational diffusion. To date the majority of studies have been carried out... 

Fresh Zeolite. The diffusivities within usual porous catalyst (pore radius a few nm) can be estimated by the parallel pore model (18) or the random pore model (19). However, configurational diffusion occurs within the pores of zeolites (pore diameter < 1 nm) and there are only a few reports on the measurement or estimation methods of the diffusivities of zeolites, especially at higher temperature range (20,21). Here we will review the results of ZSM-5, which first explains the diffusivity of fresh ZSM-5, then the results of coke loaded ZSM-5. 

The activation energy for gas translational diffusion represents the energy barrier that a molecule has to overcome upon moving from one channel intersection to the other. The order of magnitude can be calculated from the difference in potential field in a channel and in an intersection. The transition from Knudsen to configurational diffusion depends on the size of the molecule with respect to the zeolite channels and the proportions of the molecule (ratio of length to diameter), as is shown in Fig. 11 for ZSM-S and zeolite-A. 

Figure 11 Transition from Knudsen diffusion to configurational diffusion in ZSM-5 and 5A as a function of the ratio between the minimum kinetic diameter of the molecule and the maximum channel diameter of the zeolite at 300 K. The ratio between the length and the kinetic diameter of the diffusing molecule was taken to be 1.25. (Adapted from Ref. 37.)...

The third step, migration inside the micropore, is also denoted as intracrystalline zeolite diffusion or configurational diffusion. 

Activated surface diffusion or configurational diffusion occurs in molecules with a diameter larger than 60% of the pore diameter, as is the case in pervaporation with zeolite membranes [113]. Diffusion is an activated process and the diffusivity follows an Arrhenius-type equation ... 

Fig. 7. Schematic evolution of the permeance of single gas through MFI membranes, as a function of temperature. ABC Activated configurational diffusion, AB the mobility of adsorbed species increases and occupancy decreases, B the increase in mobility cannot compensate the decrease in occupancy. C adsorption is negligible. CD, and CD2 activated transport through zeolite pores and/or through grain boundaries ( ).

On the other hand, our knowledge of configurational diffusion in zeolites is far from being adequate. While Fick s law on diffusion has been commonly used to obtain diffusion coefficients in zeolites, there is experimental data such as the window or cage effect observed in erionite which cannot be interpreted by such equations. Data on diffusion of high molecular weight molecules in zeolites are almost nonexistent. Discrepancies also remain unresolved between diffusion coefficients determined by NMR and uptake data (22-23). Needless to say, much remains to be investigated, Uoth in theory (24-25) and in experimental measurement (26). 

Molecular and Knudsen diffusion will be discussed first. Configurational diffusion, mainly encountered with zeolite catalysts, will be discussed separately in Section 3.5.4. 

Steps 1 and 5 depend on the following factors permeation conditions (temperature and partial pressures), the nature of the chemical species and the type of crystalline material. Steps 1 and 5 are, generally, assumed to be fast processes. Steps 2, 3 and 4 are usually activated processes (Barrer, 1990). Intra-crystalline zeolite diffusion is described as configurational diffusion. 

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