Zeolitization in General

The carbeniiun ion so formed then reacts in the ICC 1 manner except perhaps for not abstracting a hydride ion from another alkane. Although initial views that zeolites in general were super acids have come into question, definite super acids have been found such as calcined H2S04 Zr(0H)4 which catalyze the isomerization of alkanes at low T. 

These are some of the more important factors that should be considered in order to explain the catalytic activity of aluminum-deficient mordenite zeolites. In general, the same factors will affect the catalytic properties of aluminum-deficient Y zeolites, although fewer data are available with regard to the catalytic properties of these materials. 

The catalysts are predominantly modified ZSM-5 zeolite. In general, the modifications are intended to restrict pore mouth size to promote the shape selective production of para-xylene within the microporous structure. The same modifications also serve to remove external acid sites and eliminate the consecutive isomerization of para-xylene. Methods used to modify the zeolite pore openings have included silation [50], incorporation of metal oxides such as MgO, ZnO and P2O5 [51, 52], steaming and the combination of steaming and chemical modification [53]. 

The pyrolysis gas chromatogram of ABS at 550°C changes considerably when the pyrolysis products are passed over zeolite catalysts. The specific activity towards certain reactions, e.g., cycliza-tion, aromatization, or chain cleavage is somewhat dependent on the nature of the individual zeolite. In general, enhanced benzene, toluene, ethylbenzene at the cost of dimer, trimer formation is observed. Nitrogen containing compounds do not appear in the pyrolysis oil after catalytic conversion. However, the product gas is rich in nitriles (132). 

This approach is extremely powerful and endowed with the ability to reach near-atomic resolution. But it is a difficult one to apply in routine analyses because it demands electron microscopic expertise of a high order. It is also a destructive procedure, and not easily adapted for rapid detection and characterization of intergrowths in as-prepared zeolites in general. 

It is known that Silicalite in particular, and zeolites in general contain several different sites (24) the inhomogeneity of adsorption sites is largely responsible for the non-exponential decay law. Figure 2 illustrates the decay observed in the case of 8-phenylpropiophenone in Silicalite. 

There are bibliographic reviews on catalysis with zeolites 62, 94, 100) as well as on zeolites in general 7,19, 76, 85) covering the literature... 

Ammonia TPD patterns from zeolites can exhibit multiple desorption peaks similar to that shown in Figure 2 for ammonia sorbed on an H-Y zeolite. In general, at least... 

It is not proposed to review the properties of zeolites in general. Structural aspects related to catalysis are, however, discussed in detail. Accessibility of sites and other related factors basic to an understanding of zeolite catalytic behavior are also examined. Considerable emphasis is given to the chemistry of activation processes, the characterization of acid zeolite catalysts, and the physicochemical interactions of adsorbed molecules with zeolite surfaces. 

Deuterium NMR spectroscopy has been employed to probe the interaction between perdeuteriated n-alkanes and host molecules such as liquid crystals, urea crystals, lipid bilayers or zeolites. In general, the temperature-dependent quadrupolar splitting of the deuterium signals is interpreted in terms of conformation and ordering of the alkane chains. For lipid bilayers these studies are of interest in connection with the anaesthetic properties of alkanes. Phospatidylcholine bilayer membranes were chosen as a model. The solubilities of n-alkanes as determined by NMR were found to be dependent on both the membrane- and alkane-chain length ". The complex signal patterns which show the dynamic processes of perdeuteriated hexane in a multilayer is reproduced in Figure 3 ... 

A few studies have been made on PV with acidic feeds using zeolite membranes. Hydrophilic zeolites, in general, are not suitable in low pH environments because acid leaches Al from the framework. Zeolite manbranes used for low pH PV, therefore, need to have relatively high Si/Al ratios so that the framework is not destroyed when Al is removed. Stainlesssteel supports are usually used for these applications because AI2O3 supports are susceptible to degradation by acids. A Ge-ZSM-5 membrane removed acetic acid from a 5 wt % acetic acid-water mixture at 363 K with a = 14 and a 16.8 mol/m h flux (Tan et al. 2006). 

Clays are aluminosilicate structures often having close similarities to the zeolites. Montmorillonite, for example, may be regarded as the counterpart of the zeolites heulandite and stilbite. Like heulandite it swells greatly upon hydration (4), as is indicated by Fig. 21. The clays also show base-fexchange properties like the zeolites. In general they contain larger and less definite channels. 

The crystalline lattice structure of zeolites consists of exceptional lattice stability by virtue of which they facilitate considerable freedom of ion-exchange and reversible dehydration. Zeolites can accommodate new cations (mainly sodium, potassium, magnesium and calcium), water molecules and even small organic molecules. Furthermore, ions and molecules in the cages are loosely bound so that they can be removed or exchanged without destroying the zeolitic firamework. However, this depends on the chemical composition and the crystalline structures of a specified zeolite. In general, zeolite minerals have been classified into various families as presented in Table 2.1 [2, 7, 8, 14, 15]. 

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