Zeolite micropores

Encapsulation in Y zeohte was also the method chosen to immobihze Mn complexes of C2-symmetric tetradentate hgands (Fig. 24) [75]. These materials were used as catalysts for the enantioselective oxidation of sulfides to sulfoxides with NaOCl. The lack of activity when the larger io-dosylbenzene was used as an oxidant was interpreted as an indication that the reaction took place inside the zeolite microporous system. Both the chemo- and enantioselectivity were dependent on the structure of the sulfide. (2-Ethylbutyl)phenylsulfide led to better results than methylphenylsulfide, although in all cases the enantioselectivity was low (up to 21% ee). 

As a further illustration of the compensation effect, we use solid-acid-catalyzed hydrocarbon activation by microporous zeolites. A classical issue in zeolite catalysis is the relationship between overall rate of a catalytic reaction and the match of shape and size between adsorbate and zeolite micropore. 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

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]. 

Figure 6. Translational molecular mobility Dt of neopentane and methane phases confined in model AlP04-5 zeolite micropores.

During evacuation at elevated temperature the XRD pattern of the CdCl2/NaY mixture did not remain unchanged. The observed intensity decay of the CdCl2 reflection at 20=15.1° can be interpreted as the loss of the long-range order of the salt crystals due their distribution in the zeolite micropores... 

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%). 

A.Corma, Zeolite Microporous Solids synthesis, structure and reactivity, NATO ASI Series C, 352 (1992) 373... 

C. R. Jacob, S. R. Varkey, and R. Ratnasamy, Selective oxidation over copper and manganese salens encapsulated in zeolites, Microporous Mesoporous Mater. 22, 465 74 (1998). 

There are four widely accepted theories of shape selectivity reactant shape selectivity (RSS), product shape selectivity (PSS), transition state selectivity (TSS) (Figure 12.2), and concentration effect all of them are based on the hypothesis that the reactions occur within the zeolite micropores only. As indicated earlier, this hypothesis is often verified, the external surface area of the commonly used zeolites being much lower (one to two orders of magnitude) than their internal surface area. ... 

Kinetic studies of the acetylation of several arylethers were carried out over HBEA zeolites. The main conclusion is that the rate and stability of the reactions are determined by the competition between reactant(s) and product(s) molecules for adsorption within the zeolite micropores. This competition shows that the autoinhibition of arene acetylation, that is, the inhibition by the acetylated products, and also by the very polar acetic acid product is generally observed. This effect is much more pronounced with hydrophobic substrates such as methyl and fluoro aromatics than with hydrophilic substrates because of the larger difference in polarities between substrate and product molecules. 

Wang, J., Feng, S., and Xu, R. (1989) Synthesis and characterization of zeolitic microporous alumino-borates, in Zeolites Facts, Figures, Future (eds P.A. Jacobs, and R.A. van Santen), Stud. Surf. Sd. Catak, vol. 49A, Elsevier, Amsterdam, pp. 143-150. 

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... 

Krause, 0. (2004) Role of addity in hydrogenation of dnnamaldehyde on platinum beta zeolite. Micropor. 

Stocker, M. (2005) Gas phase catalysis by zeolites. Micropor. Mesopor. Mater., 82, 257-292. 

Sato, M. and Uehara, H. (1997). Ab initio computer modelling of zeolite fameworks I. Modelling the basic clusters. Prog. Zeolite Microporous Mater. 105, 2299-306. 

In Table 4.3, the basic characteristics of the most important zeolite molecular sieve species are presented. Zeolite micropore openings are of the same order of magnitude as... 

It has been stated that zeolites exhibit a distribution of adsorptive energies. This is due to the complex structure within the zeolitic micropores and the strong dependence of electrostatic energies upon structure geometry. A discrete number of types of adsorption sites can be considered the finite number is dependent upon electrostatic and steric considerations. The most strongly adsorbing sites correspond to locations near the cations (SII, Sill, etc.). After these positions are filled, the adsorbate molecules seek positions in the framework structure to minimize repulsive forces between them on the zeolite surface, and at the same time to maximize adsorptive forces with the framework. 

Parton, R., de Vos, D. and Jacobs, P.A. (1992) Proceedings of the NATO Advanced Study on Zeolite Microporous Solids Synthesis, Structure and Reactivity (eds E.G. Derouane, F. Lemos, C. Naccache and F. Ribeiro), Kluwer, Dordrecht, p. 555. 

The set of catalysts selected for the dehydration of 2-butanol was also tested for the Friedel-Crafts acylation of anisole [69, 70]. The catalytic test was performed in the liquid phase due to the high boiling points of the reactants and products of this reaction. Anisole was reacted with acetic anhydride at 120 °C in the absence of solvent. In principle, acylation can occur on both the ortho and para positions of anisole. The main product (>99%) over all catalysts in this study was para-methoxyacetophenone, indicating that the reaction predominantly takes place inside the zeolite micropores. The same trend in catalytic activity as in the 2-buta-nol dehydration reaction is observed the conversion of anisole into para-nicihoxy-acetophenone increases upon increasing Ge content of the catalyst (Fig. 9.17) [67]. The main cause of deactivation for this reaction is accumulation of the reaction products inside the micropores of the zeolite. The different behavior of Ge-ZSM-5, compared with ZSM-5, may therefore be due to improved diffusional properties of the former, as the presence of additional meso- and macropores allows for... 

J. B. Nagy, P. Bodart, I. Hannus and I. Kiricsi, Synthesis, Characterisation and Use of Zeolitic Microporous Materials, DecaGen, Szeged, Hungary, 1998. 

Analysis of Fractions. Surface areas and pore size distributions for both coked and regenerated catalyst fractions were determined by low temperature (Digisorb) N2 adsorption isotherms. Relative zeolite (micropore volume) and matrix (external surface area) contributions to the BET surface area were determined by t-plot analyses (3). Carbon and hydrogen on catalyst were determined using a Perkin Elmer 240 C instrument. Unit cell size and crystallinity for the molecular zeolite component were determined for coked and for regenerated catalyst fractions by x-ray diffraction. Elemental compositions for Ni, Fe, and V on each fraction were determined by ICP. Regeneration of coked catalyst fractions was accomplished in an air muffle furnace heated to 538°C at 2.8°C/min and held at that temperature for 6 hr. 

Loss of micropore area/zeolite crystallinity is a slow process in comparison with dealumination. Following an initial decline in micropore area of 17% upon catalyst addition (Tables IV, VI), zeolite micropore area/crystallinity decreases by only an additional 3% over the next 26 days (Fraction B, Table IV,... 

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