Zeolites jump diffusion mechanism

In the case of H-SSZ-24, the values of the pre-exponential factor experimentally obtained (see Table 5.4) do not agree with the values theoretically predicted by the equation for a jump diffusion mechanism of transport in zeolites with linear channels, in the case of mobile adsorption [6,26], Furthermore, the values obtained for the activation energies are not representative of the jump diffusion mechanism. As a result, the jump diffusion mechanism is not established for H-SSZ-24. This affirmation is related to the fact that in the H-SSZ-24 zeolite Bronsted acid sites were not clearly found (see Figure 4.4.) consequently p- and o-xylene do not experience a strong acid-base interaction with acid sites during the diffusion process in the H-SSZ-24 channels, and, therefore, the hopping between sites is not produced. 

Pore diffusion may occur by several different mechanisms depending on the pore size, the sorbate concentration, and other conditions. In fine micro-pores such as the intracrystalline pores of zeolites, the diffusing molecule never escapes from the force field of the adsorbent surface and transport occurs by an activated process involving jumps between adsorption sites. Such a process is often called surface diffusion, but the implication of a two-dimensional surface is unnecessarily restrictive since the micropore structure in a zeolite crystal is often three-dimensional. The more general terms micropore or intracrystalline diffusion are therefore used here to describe transport in such systems, while diffusion in larger pores such that the diffusing molecule escapes from the surface field is referred to as macropore diffusion. This distinction between micropore and macropore diffusion is useful since, in a zeolitic adsorbent, the diameter of the intracrystalline... 

When the proton is attached to a mobile ion, two modes of transport have been proposed. (1) Polyatomic ions like H3O+ or NH4+ may migrate by a simple jump from site to site in the lattice, as has been claimed in the /3-aluminas and in the zeolites. (2) A more involved mechanism based on the simultaneous diffusion of two types of polyatomic units, the so-called vehicular mechanism, was suggested... 

Abstract Neutron scattering was first used to derive the self-diffusivities of hydrocarbons in zeolites, but transport diffusivities of deuterated molecules and of molecules which do not contain hydrogen atoms can now be measured. The technique allows one to probe diffusion over space scales ranging from a few A to hundreds of A. The mechanism of diffusion can, thus, be followed from the elementary jumps between adsorption sites to Lickian diffusion. The neutron spin-echo technique pushes down the lower limit of diffusion coefficients, traditionally accessible by neutron methods, by two orders of magnitude. The neutron scattering results indicate that the corrected diffusivity is rarely constant and that it follows neither the Darken approximation nor the lattice gas model. The clear minimum and maximum in diffusivity observed by neutron spin-echo for n-alkanes in 5A zeolite is reminiscent of the controversial window effect . 

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