Zeolites diffusion mechanism

When using the microporous zeolite membrane (curve 3) the N2 permeance decreases when the pressure increases such a behaviour can be accounted for by activated diffusion mechanisms [21], which are typical of zeolite microporous systems. In such systems the difflisivity depends on the nature and on the concentration of the diffusing molecule which interacts with the surface of the pore. For gases with low activation energies of diffusion, a decrease of the permeability can be observed [22]. 

With the forgoing as background we are now prepared to discuss the various diffusion mechanisms that occur in formed zeolites. 

Membranes made from zeolite materials provide separahon properties mainly based on molecular sieving and/or surface diffusion mechanism. Separation with large pore zeolite membranes is mainly based on surface diffusion when their pore sizes are much larger than the molecules to be separated. Separation with small pore zeolite membranes is mainly based on molecular sieving when the pore sizes are smaller or similar to one molecule but are larger than other molecules in a mixture to be separated. 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

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. 

To explain this D(y) relation, it is necessary to assume overlapping of 2 diffusion mechanisms a "pore—fissure diffusion and a diffusion by simultaneous position interchange between cations. In both cases, cation motion would be along the walls of cavities, since cations are strongly attracted by the negatively charged zeolite lattice. Rate of diffusion is smaller in the interchange mechanisms than in pore-fissure diffusion. 

The application of the Maxwell-Stefan theory for diffusion in microporous media to permeation through zeolitic membranes implies that transport is assumed to occur only via the adsorbed phase (surface diffusion). Upon combination of surface diffusion according to the Maxwell-Stefan model (Eq. 20) with activated-gas translational diffusion (Eq. 12) for a one-component system, the temperature dependence of the flux shows a maximum and a minimum for a given set of parameters (Fig. 15). At low temperatures, surface diffusion is the most important diffusion mechanism. This type of diffusion is highly dependent on the concentration of adsorbed species in the membrane, which is calculated from the adsorption isotherm. At high temperatures, activated-gas translational diffusion takes over, causing an increase in the flux until it levels off at still-higher temperatures. 

The different mechanisms that operate in the separation of gases have been previously described in Section 10.4.1. In pervaporation, the transport mechanism can be described by an adsorption-diffusion mechanism [74,114] similar to one for polymeric membranes [115]. However, it is necessary to consider that the specific interactions between the permeating component and the zeolitic material are different in zeolites. Moreover, the diffusion through the ordered zeolite nanopores is different than in the dense organic matrix. 

Xiao J and Wei J. Diffusion mechanism of hydrocarbons in zeolites 2. Analysis of experimental-observations. Chem Eng Sci 1992 47 1143-1159. 

Force-field calculations could be simple energy minimization or advanced monte-carlo and molecular dynamics calculations. The major assumption here is the transferability of force-field parameters among the related materials. These calculations can provide wealth of information such as the relative ordering of adsorption sites on surface, diffusion mechanism of molecules particularly inside zeolites, energy barrier for difihision, diffusion coefficients, heats of adsorption and more importantly, the effect of temperature on all these properties. 

One of the most controversial issues in the field of diffusion in zeohtes is the so-called window effect . This term was coined by Gorring to interpret the anomalous transport results obtained for hnear alkanes in zeoHte T [46], but his experimental conditions have been criticized. More recent macroscopic studies could not reproduce the periodic variation in diffusivity, and a monotonic decline with carbon number was reported [47,48]. However, a microscopic technique, such as neutron scattering, is better suited to probe anomalous diffusion mechanisms on a molecular scale, since it is much less sensitive to the influence of defects or internal transport barriers within the zeolite crystals. 

Finally, it is important to notice the effect of the support in the pervaporation flux, analyzed by de Bruijn et al. [164] who proposed a model and evaluated the contribution of the support layer to the overall resistance for mass transfer in the selected literature data. They found that in many cases, the support is limiting the flux the permeation mechanism through the support corresponds to a Knudsen diffusion mechanism, which makes improvements in the porosity, tortuosity, pore diameter, and thickness necessary for an increase in the pervaporation flux. In fact, the researchers of Bussan Nanotech Research Institute Inc. (BNR), Sato et al. [165], designed and patented an appropriate asymmetric ceramic porous support to suppress pressure drop, and in this case, the water flux increased dramatically compared to previous reported results. Wang et al. [166] have clearly shown that the flux of the membranes increased with the porosity of the hollow fiber supports. In spite of the thin 1 pm zeolite layer, prepared by Zhou et al. [167], the flux enhancement compared to layers 10 times thicker [168] was not significant. 

This chapter has presented a number of adsorption models for homogeneous particles where parallel diffusion mechanism is operating. This type of mechanism is applicable to solids such as activated carbon. We will present in the next chapter a number of models for zeolite type solids where a bimodal diffusion mechanism is operating. 

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. 

To investigate the behaviour of the bimodal diffusion mechanism, we consider the linear isotherm first. In this case the mass balance equations become linear and hence they are amenable to linear analysis. With linear isotherm, the boundary condition at the zeolite pore mouth (eq. 10.4-7) becomes ... 

As usual with membrane separations, the membrane is critical for success. Currently, two different classes of membranes are used commercially for pervaporation. To remove traces of organics from water a hydrophobic membrane, most commonly silicone rubber is used. To remove traces of water from organic solvents a hydrophilic membrane such as cellulose acetate, ion exchange men )rane, polyacrylic acid, polysulfone, pol5 inyl alcohol, composite membrane, and ceramic zeolite is used. Both types of membranes are nonporous and operate by a solution-diffusion mechanism Selecting a membrane that will preferentially permeate the more dilute conponent will usually reduce the membrane area required. Membrane life is typically about four years tBaker. 20041. 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given... 

In bidisperse porous adsorbents such as zeolite pellets there are two diffusion mechanisms the macropore diffusion with time constant Rp /Dp and the micropore diffusion with time constant rc /Dc. Bidisperse porous models for ZLC desorption curves have been recently developed by Brandani [28] and Silva and Rodrigues [29]. In bidisperse porous adsorbents, it is important to carry out experiments in pellets with different sizes but with the same crystal size (different Rp, same rc) or pellets with the same size but with different crystals (same Rp, different rc). If macropore diffusion is controlling, time constants for diffusion should depend directly on pellet size and should be insensitive to crystal size changes. If micropore diffusion controls the reverse is true. The influence of temperature is also important when macropore diffusion is dominant the apparent time constant of diffusion defined by Rp2(H-K)/Dp is temperature dependent in the same order of K (directly related to the heat of adsorption) which is determined independently from the isotherm. The type of purge gas is... 

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