Pore structures of zeolites

Catalysis of 12-membered zeolites, H-mordenlte (HM), HY, and HL was studied In the alkylation of biphenyl. The para-selectlvltles were up to 70% for Isopropylblphenyl (IPBP), and 80% for dllsopropylblphenyl (DIBP) In HM catalyzed Isopropylatlon. Catalysis of HY and HL zeolites was nonselectlve. These differences depend on differences In pore structure of zeolites. Catalysis of HM to give the least bulky Isomer Is controlled shape-selectlvely by sterlc restriction of the transition state and by the entrance of IPBP Isomers. Alkylation with HY and HL Is controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealuminatlon of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. 

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

In several cases, such as the MPV reduction, the specific pore structures of zeolites lead to selectivities not realizable with other catalysts. Nevertheless, in some cases, a zeolite shows no advantage in activity over other solid acids, except for the superior regenerability provided by their thermally stable inorganic matrix. 

THE PORE STRUCTURES OF ZEOLITES AND THE MODES OF COKE DEPOSITION... 

Pore structure of zeolites of type Y and pentasil as the function conditions of preparation and methods of modification... 

Widely used as adsorbents and the components of catalysts synthetic zeolites are unique molecular sieve, which is defined by specify of their pore structure, namely, size and form of cavities of channels. During development of adsorbents and catalysts it is necessary to decide the tasks of saving of initial pore structure and connected with it adsorption ability of samples or of directed its changing with the purpose of increase of yield of necessary products, for example para - selectivity increase in the isomerisation of alkylaromatic compounds. The pore structure of zeolite-containing systems directly depends on cry stalline structure ones. 

The acidity and pore structure of zeolites play significant roles in their deactivation by carbonaceous deposits ("coke"). This is not surprising, as the formation of coke involves reactions cataly by acid sites located inside the pores and also requires the retention of coke molecules by adsorption on the acid sites or by condensation or by steric blockage in the pores. Although it is often difficult to estimate quantitatively and separately the impacts of the acidity and of pore structure, it is clear that it is the latter characteristic which plays the greater role. 

The pore structure of zeolite membranes is formed by arrays of intergrown zeolite particles or zeolite particle packings with interparticle pores filled with another material. The intracrystalline pores are a part of the crystallographic structure and are the ones which should be responsible for the selectivity. 

D. Uzio, J. Peureux, A.G. Fendler, J.-A. Dalmon and J. Ramsay, Formation and pore structure of zeolite membranes, in J. Rouquerol, F. Rodriguez-Reinoso, K.S. W. Sing and... 

Influence of the acidity and of the pore structure of zeolites on the alkylation of toluene hy 1-heptene. 

Zeolites are acidic catalysts. Their acidity and pore Table 9.3. Pore structure of zeolites (data from... 

Similarly, insight has been gained into the pore structure of zeolites of type MWW, ITQ-2 and IM-5 through microcalorimetric studies of adsorption of n-hexane, toluene, m- and o-xylenes, 1,2,4- and 1,3,5-trimethylbenzene [61,67]. Another study [68] has shown that the y-cages of H-ZK-5 and K-ZK-5 are the... 

While there are no commercial xenon-containing polymers, this nucleus finds applications as a probe molecule [17,18,20]. It has been successfully employed to investigate the pore structure of zeolites, and its use in polymer characterization is increasing. The chemical shift of Xe gas depends on the pressure or, more specifically, on the number of collisions per second,... 

TEM, SEM analysis. High-Resolution Transmission Electron Microscopy (HRTEM) is one of the effective techniques to study the sub-microscopic structure of solid materials. They can observe nano-scale structure of microcrystalline and its defects, the pore structure of zeolite, as well as layered materials. All these information can be hardly provided by other equipments. 

Metal colloids can be synthesized within the pore structure of zeolite. The coordination of metal atoms witii the fiamework of the zeolite leads to formatimi of stable nanoclusters with a Sructural geometry imprinted by the host matrix.. 

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