Use of large pore zeolites

Oligomerization of butene over HMordenite has been carried out at 5 MPa and 523 K. The reaction gave mainly dimers and trimers with a minor fraction of tetramers and pentamers.[13] In contrast, oligomerization of butene over solid phosphoric acid catalyst gave mainly dimers. 

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The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

Chlorination of toluene with tert-butyl hypochlorite in the presence of silica gel gave a mixture of chlorotoluenes in which the ortho isomer predominated. Rutming this reaction using a large pore zeolite, H-X, gave p-chlorotoluene in... 

The pore size of large-pore zeolite HB is controlled by an improved chemical vapor deposition (CVD). In this method, ammonia or tripropylamine was used to protect the acidic sites in zeolite before deposition of (CH3)3Si-0-Si(CH3)3, differently from the conventional CVD. The pore volume of zeolite HB was reduced and the pore size was narrowed upon this modification. The results from IR spectra and catalytic decomposition of isopropanol demonstrated that the modified samples showed stronger acidity than that prepared from conventional CVD. In the transformation of trimethylbenzenes (TMB), conversion of 1,3,5-TMB was suppressed while conversion of 1,2,3-TMB was almost unaffected on the improved CVD samples the selectivity of 1,2,4-TMB on these samples was increased in the alkylation reaction of m-xylene with methanol. 

Studies of this type have been used by Smit and co-workers to explain the so-called inverse shape selectivity observed in the conversion of long chain w-alkanes over acid zeolites. In such reactions, product distributions are found to depend on the pore structure, particularly for medium-pore zeolites such as ZSM-5. In some cases branched alkanes are favoured over linear alkanes in the products of medium-pore zeolites compared to the reaction selectivities of large-pore zeolites such as zeolite Y. For example, doubly branched isomers are favoured over ZSM-5. This is in contrast with what would be expected from dilfusion rates and is attributed to the enhanced thermodynamic stability of some branched intermediates in the medium-pore zeolites that is predicted by configurational bias GCMC. 

The adsorption capacities of large pore zeolites for benzene and pyridine vapors are practically equal. The adsorption rate of pyridine is substantially lower than the adsorption rate of benzene due to its electronic structure. A comparison of the adsorption heats of pyridine and benzene makes it possible to conclude that the unshared pair of electrons and the large dipole moment of pyridine both make an important contribution to the energy of reaction of pyridine with zeolites [26]. Most linear and branched amines have also been used to titrate the acid sites in zeolites. 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

Large pore zeolites have been used for selective alkylation of diphenyl with propylene to 4,4 -diisc>propyl diphenyl with good selectivity. Similarly, naphthalene gives to 2,6 derivative. 

An amorphous component such as silica-alumina is added to the catalyst, for a sort of pre-cracking of the large molecules (greater than about C25), which cannot enter the zeolite pores. The smaller fragments may then react in the zeolite. Middle distillates maximum yield is achieved by the use of dealuminated Y zeolites. 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

For n-decane isomerization, when a good balance between the metal phase and the acidic phase of the catalysts is reached, the isomerization and cracking yield curves of the catalysts are a unique function of the conversion, meaning that these yields do not depends on the porosity nor the acidity of large pore materials. Formation of the most bulky isomers, such as 4-propylheptane and 3-ethyl-3-methylheptane was favored in mesoporous solids (figure 1). Criteria based on the formation of these particular isomers are linked with mesoporosity and could be useful to discriminate between zeolites catalysts with and without mesopores. 

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... 

In another article by Corma et al. (178), ITQ-7, a three-dimensional large-pore zeolite, was tested as an alkylation catalyst and compared with a BEA sample of comparable Si/Al ratio and crystal size. The ratio of the selectivities to 2,2,4-TMP and 2,2,3-TMP, which have the largest kinetic diameter of the TMPs, and 2,3,3-TMP and 2,3,4-TMP, which have the lowest kinetic diameter, was used as a measure of the influence of the pore structure. Lower (2,2,4-TMP + 2,2,3-TMP)/ (2,3,3-TMP + 2,3,4-TMP) ratios in ITQ-7 were attributed to its smaller pore diameter. The bulky isomers have more spacious transition states, so that their formation in narrow pores is hindered moreover, their diffusion is slower. The hydride transfer activity, estimated by the dimethylhexane/dimethylhexene ratio,... 

While medium pore zeolites such as ZSM-5 do not deactivate significantly during hexane cracking at 538°C, large pore zeolites usually do. For maximum accuracy of results in these cases we found it advisable to use a low hexane partial pressure of about 10 torr. This not only completely eliminates catalyst deactivation during the test (Fig. 7),... 

The authors suggest that the new method could be of some value for titanium containing zeolites with structures different from silicalite, for instance large pore zeolites which could be useful in the oxidation of large molecules which cannot be oxidized with TS-1. 

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... 

Concentrated sulfuric acid and hydrogen fluoride are still mainly used in commercial isoalkane-alkene alkylation processes.353 Because of the difficulties associated with these liquid acid catalysts (see Section 5.1.1), considerable research efforts are still devoted to find suitable solid acid catalysts for replacement.354-356 Various large-pore zeolites, mainly X and Y, and more recently zeolite Beta were studied in this reaction. Considering the reaction scheme [(Eqs (5.3)—(5.5) and Scheme 5.1)] it is obvious that the large-pore zeolitic structure is a prerequisite, since many of the reaction steps involve bimolecular bulky intermediates. In addition, the fast and easy desorption of highly branched bulky products, such as trimethylpentanes, also requires sufficient and adequate pore size. Experiments showed that even with large-pore zeolite Y, alkylation is severely diffusion limited under liquid-phase conditions.357... 

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