Zeolite micropore diffusion

Numerical values for solid diffusivities D,j in adsorbents are sparse and disperse. Moreover, they may be strongly dependent on the adsorbed phase concentration of solute. Hence, locally conducted experiments and interpretation must be used to a great extent. Summaries of available data for surface diffusivities in activated carbon and other adsorbent materials and for micropore diffusivities in zeolites are given in Ruthven, Yang, Suzuki, and Karger and Ruthven (gen. refs.). 

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]. 

Both materials were tested as catalysts in the anisole acylation (Scheme 1). The conventional Beta sample showed a slightly higher activity than the Beta (PHAPTMS). At 3 hours, the conversions were 26.8 and 22.8 % for the conventional and seed silanized catalysts, respectively. This behavior is explained as a consequence of the relatively small size of the anisole molecule, which allows this compound to diffuse without significant hindrances through the zeolitic micropores, and of the slightly weaker acidity of the Beta (PHAPTMS) sample. In both cases, p-methoxyacetophenone (p-MAP) was the main reaction product, being obtained with a high selectivity (> 97%). 

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

Micropore diffusion Diffusion within the small micropores of the adsorbent which are of a size comparable with the molecular diameter of the sorbate. Under these conditions the diffusing molecule never escapes from the force field of the solid surface and steric hindrance is important. For zeolites the terms micropore diffusion and intracrystalline diffusion are synonymous. Raffinate Product stream containing the less strongly adsorbed species. 

The foregoing discussion refers solely to intraparticle diffusivity (micropore diffusion) as distinct from interparticle effects (macropore diffusion). Since a practical zeolite catalyst will consist of composite particles, each containing a large number of individual zeolite crystals, it is important to make a clear distinction between these two types of diffusion. In some cases macropore diffusion may be important in determining the overall reaction kinetics but will obviously not introduce or affect shape selectivity in any way. 

Ctace a reactant molecule has adsorbed within the zeolite mouth, it needs to diffuse toward the active sites in contact of which reactions will occur. This diffusion can be very dependent on the size and shape of the zeolite micropores as well as on the size of the reactants or products. This becomes especially true when the reactants or products have similar size to the micropores diameter. After reaction, it is the turn of the products to diffuse away from the micropores. 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

Kiselev et al. defined a classical force field for which zeoUte silicon atoms are not explicitly considered. The dynamic simulations they performed using this force field succeeded well in describing molecules diffusion within zeolite micropores. ... 

The study of reactivity by QM calculations concerns only a small part of a catalytic event, as phenomena such as macro and micro diffusion of the reactants and products outside and inside the zeolite micropores cannot be investigated. On the other hand, QM methods are the only theoretical methods available that provide information on the reactivity. 

The dynamics of methane, propane, isobutane, neopentane and acetylene transport was studied in zeolites H-ZSM-5 and Na-X by the batch frequency response (FR) method. In the applied temperature range of 273-473 K no catalytic conversion of the hydrocarbons occurred. Texturally homogeneous zeolite samples of close to uniform particle shape and size were used. The rate of diffusion in the zeolitic micropores determined the transport rate of alkanes. In contrast, acetylene is a suitable sorptive for probing the acid sites. The diffusion coefficients and the activation energy of isobutane diffusion in H-ZSM-5 were determined. 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

The table shows the remarkable decrease in the micropore diffusivity of a gas when its molecular diameter approaches that of the zeolite pore. The temperature coefficients of and Dm are given by the Arrhenius relationship [ >s, Dm = D exp(— // 7)] because these diffusions are activated processes. E is the activation energy for the diffusion process and D° is a constant. These diffusivities can also be complex functions of adsorbate loadings and compositions. ... 

It has been explained in a previous section that diffusion of hexane and isohexane can be considered to be fast. From a combination of theoretical and experimental data, a reaction energy scheme corresponding to the catalytic reaction cycle that converts zz-hexene into z-hexene has been deduced. This is shown in Fig. 17 [136, 137]. As in Section V, adsorption on the siliceous part of the zeolite micropore is considered to be independent of proton activation. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

They are of great interest in catalysis because of their uniform and large pores, which allow sterically hindered molecules (i.e., unable to diffuse effectively through the smaller microporous chaimels of zeolites) to diffuse easily to internal active sites. These properties could also be interesting for application in gas sorption processes. 

Species formed from acetylene (Ay) adsorbed in zeolite Y, mordenite, beta and ZSM-5 have been studied by IR spectroscopy. The dynamics of Ay physisorption has been characterized by the frequency response method (FR). The rate of micropore diffusion governed the transport in Na-mordenite, while sorption was the rate limiting process step for all the H-zeolites. The equilibrium constants (Ka) of Ay sorption have been determined applying the Langmuir rate equation to describe the pressure dependence of the sorption time constants. The -octane hydroconversion activity of Pt/H-zeolites was found to increase linearly with the Ka of Ay sorption on the H-zeoIites. 

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