Zeolite geometry

The results obtained with the slit pore model are not reported since the calculated values do not agree with the experiment [97], This is an expected outcome, since the zeolite geometry can be modeled with the cylindrical pore or the spherical pore geometries, but not with the slit pore geometry. 

There is ample spectroscopic evidence that this occluded complex has exclusively cis-symmetry (see model), allowing Mn coordination with the organic ligands as well as with the oxygens of the zeolite cage walls via Me solvatation. Entrapment shows a clear adaptation of the complex to the zeolite geometry and transforms the peroxide decomposition catalyst into... 

We chose to model the zeolite with only the oxygen atoms in it at the appropriate positions in the zeolite geometry. As described before, methane was modeled using the UA potential, whereas we chose to model butane using the PCFF description (Hill and Sauer 1994). The interaction parameters of the UA force field are shown in Table 10.1. 

It can be concluded that the heat curves should be similar only when the solids contain exlusively isolated sites where the next-nearest neighbors (NNN) are identical. It appears that the NNN determines Bronsted acidity, in fact, it can be said that zeolite geometry has an influence on acidity due to long-range effects as well as on the local structure of the acid sites. 

Spectroscopy at variable temperatures enables us to reveal linkage isomerism of adsorption, when certain molecule fonu with the same site two or more complexes with different geometry and chemical properties. The most studied so far is the case of CO in zeolites, when besides the usual C-bonded complexes with the cations or OH-groups, energetically unfavorable O-bonded complexes ai e formed. 

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

Usually, the zeolite inner surface characteristics are rather complex as a consequence of the (3D) character of the porous topologies of most of the zeolite types. The porous framework is a (3D) organization of cavities connected by channels. Inner surfaces are composed of several sorption sites characterized by their local geometry and curvature. Illustrative examples of such inner surface complexity are represented on Figures 1 and 2 they concern the Faujasite and Silicalite-I inner surfaces respectively. 

Ray, S. and Vasudevan, S. (2003). Encapsulation of cobalt phthalocyanine in zeolite-Y evidence for nonplanar geometry. Inorg. Chem. 42, 1711-1719... 

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

The bi-functional conversion of 2,2,4-trimethylpentane over Pt/DAY has been recently reported by Jacobs et al. (104). It was compared to the corresponding conversion over Pt/H-ZSM-5 and Pt/H-ZSM-11. All three zeolites had the same chemical composition. The authors found that 2,2,4-trime-thylpentane underwent 3-scission over Pt/DAY, while the formation of feed isomers was favored over the other two catalysts. The differences in reaction products were related to differences in the pore geometry of the zeolites. A similar study was carried out with n-decane. 

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