Medium and large-pore zeolites

The disproportionation of ethylbenzene (EB) to benzene (B) and diethylbenzenes (DEB) serves as a case study. This reaction has been introduced by Karge et al. and Weitkamp et al. to compare the activity of different zeolite catalysts [58, 59], and to distinguish between medium- and large-pore zeolites [112, 60], Moreover, since 1994, it has been further developed as a standardized test reaction by the International Zeolite Association (IZA) [55], A general representation of the individual transformations which occur in this type of reaction, is given in the following scheme ... 

Cracking catalysts using combinations of medium and large-pore zeolites in order to maximize the production of products to be used in reformulated gasoline has been r orted [23]. In a study of the craclmg of n-heptane over MCM-22, ZSM-5 and Beta it was shown that the yield of propene was greatest in the case of MCM-22 and the overall alkane/alkene ratio of the products lay between ZSM-5 (0.94) and Beta (1.17). 

Zeolites PER, ZSM-48, EUO, MFI zeolites (small-, medium-, and large-pore zeolites) CATALYSIS CVFF and cff91... 

VOC removal during automotive cold starts High-silica, hydrophobic medium-and large-pore zeolites Achieved 35-70% reduction... 

J. Marchi at Ryksuniversiteit Gent (Belgium) provide a comprehensive review of zeolite catalysis for the methanol-to-olefin reaction. He examines small, medium, and large-pore zeolites, the important role of acidity and shape selectivity on product distribution, and, like several of the other Reporters, the importance of understanding the coke-forming deactivation processes. 

The methylation of naphthalene and methylnaphthalene with zeolitic catalysts was studied. The main purpose was to achieve a selective synthesis of 2,6-dimethylnaphthalene (2,6-dmn). The alkylation of naphthalene with methanol didn t give interesting results. Best performances were obtained with 1,2,4-trimethylbenzene as solvent/reagent, together with methanol. In such chemical system, among several medium and large pore zeolites, MTW stands out for both activity and selectivity to 2,6-dmn. As transalkylation is the main reaction, this behavior can be explained if a restricted transition state shape selectivity takes place. 

Many zeolites have been evaluated as supports for Cu in SCR catalysis. Almost all the earlier work, however, is limited to medium- and large-pore zeolites, such as ZSM-5 (MFI, 10-ring), ferrierite (FER, 10-ring), mordenite (MOR, 12-ring), Y (FAU, 12-ring), and beta (BEA, 12-ring). Among them, Cu/ZSM-5 and Cu/beta are the two most studied systems. Of the two, Cu/beta catalysts show better hydrothermal stability and were favored by industry, while Cu/ZSM-5 catalysts were primarily studied by academia. It was not until recently that both industry and academia shifted their interests to small-pore zeolites as supports of Cu SCR catalysts. 

Early in the 1980s, Breck and Skeels developed a new method for the dealumination of medium- and large-pore zeolites. It was first described in a patent [180] assigned to the Union Carbide Corp. (application filed in 1981) and then presented at the 6th International Zeolite Conference in 1984 [181]. Their fundamental idea was to treat a zeolite slurried in water with an aqueous solution of an agent which extracts aluminum from the framework, provides ligands for the formation of a thermodynamically strongly favored, soluble aluminum complex and serves as an extraneous source of silicon atoms filling up the framework vacancies formed upon extraction of aluminum. Breck and Skeels realized that only soluble hexafluorosilicate salts, especially the ammonium and lithium salts, meet the requirements of such a process. The overall process of this dealumination process can be described by Eq. (6). 

With this purpose, several different types of solid acid catalysts have been investigated for the acylation of aromatics, but the best performances have been obtained with medium-pore and large-pore zeolites (3-9). In general, however, the use of acylating agents other then halides, e.g., anhydrides or acids, is limited to the transformation of aromatic substrates highly activated towards electrophilic substitution. In a previous work (10), we investigated the benzoylation of resorcinol (1,3-dihydroxybenzene), catalyzed by acid clays. It was found that the reaction mechanism consists of the direct 0-benzoylation with formation of resorcinol monobenzoate, while no primary formation of the product of C-benzoylation (2,4-dihydroxybenzophenone) occurred. The latter product formed exclusively by... 

Large changes in relative energy are found upon variation of the Al/Si ratio. Medium- and small-pore zeolites are much more sensitive to an increase in aluminum content than the wide-pore material. This should be ascribed to stacking of the cations in the channels of the zeolite. The implications of these results for zeolite synthesis are discussed. 

Pore size may also affect the reaction order. Cracking of small (i.e., less than C ) paraffins over amorphous acid catalysts and large-pore zeolites may proceed either by a bimolecular or by a monomolecular mechanism. In medium- and small-pore zeolites the space is insufficient to form bulky bimolecular transition states. This makes a monomolecular path more likely. Low reactant partial pressure, low acid site density, and high temperatures (above 450-500 C) also favor the monomolecular mechanism. According to Haag and Dessau [24] and Kranilla, Haag, and Gates [25], the transition state of the monomolecular reaction involves a penta-coordinated carbonium ion. 

Zeolites are crystalUne alumino silicates with a highly ordered crystalline structure. Cavities of a definitive size are formed in three-dimensional network composed of Si04 and AIO4 tetrahedra. The lattice contains cavities of varying diameters, depending on the type of zeohtes. A distinction is made between large-, medium- and small-pore zeolites. 

A distinction is made between wide, medium and narrow-pored zeolites. In wide-pored Y-zeolites, the structures leading to the large voids are formed by 12 Si04 or AIO4 tetrahedra. These apertures have a diameter of 7.4 A the largest voids have a diameter of around 13 A. 

The discovery and the commercialization of small-pore zeolite supported Cu SCR catalysts raise a fundamental question why can small-pore zeolites enhance the hydrothermal stability of the Cu/zeohte catalysts, while medium- and large-pore... 

However, catalysts with Cu supported on SSZ-13, a material from the alumi-nosihcate family, also exhibit good hydrothermal stability. The acid/exchange sites in an SSZ-13 still originate from the aluminum sites, just like the other aluminosilicate medium- or large-pore zeolites. Under hydrothermal aging conditions, these sites are accessible to H2O and are potentially still subject to the dealumination process and the undesirable C11/AI2O3 interaction as discussed previously. 

Compared to the medium- or large-pore zeolite supported Cu catalysts, small-pore zeolite supported Cu catalysts exhibit superior hydrothermal stability, excellent SCR activity, and selectivity with very low N2O formation. They are less susceptible to HC inhibition or poisoning. AU of these desirable features can be attributed to the narrow openings in the framework structure of the smaU-pore zeolites. With all these improvements, small-pore zeolite supported Cu SCR catalysts have been successfully commercialized and applied on diesel powered vehicles meeting the stringent US EPA 2010 or European EU5 emission standards. 

The last two contributions show the controversy about the mechanism and involvement of acid sites in the hydride transfer reactions over zeohtes. If the hydride transfer activity depends on the hydrophobicity, the rate should correlate to the acid strength of the sites, but not to the acid site density, i.e., the concentration of adjacent active sites. If the reaction is expected to proceed via two carbenium ions adsorbed on adjacent sites or one adsorbed carbenium ion on one site and a feed molecule influenced by the second adjacent site, the acid site density would be probed, but the information about the acid strength would be less obvious. However, in both cases it does not seem that these test reactions can be applied to compare large, medium, and small pore zeolite structures, due to the large (bimolecular) transition state proposed for hydride transfer reactions (unless very small molecules are used [203]). 

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

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