MEL-type zeolite

O. Terasaki, T. Ohsuna, H. Sakuma, D. Watanabe, Y. Nakagawa, and R. C. Medrud, Chem. Mater., 8, 463 (1996). Direct Observation of Pure MEL Type Zeolite. 

At 1 kPa, for strongly adsorbing Cio alkanes in MEL-type zeolite, there is a large preference for adsorption of the linear alkane. This preference is much less than for the MFI zeolite. Differences appear at high micropore occupation. Competitive adsorption suppresses the formation of i-Cio in MEL owing to the difference in the channel cross-section geometry, where branched alkanes prefer to adsorb. As a consequence, the rate of n-Cio conversion is low towards i-Cio. The MFI zeolite, therefore has the superior rate since the rate of iCio formation is higher. The reaction products are the result of consecutive reactions of i-Cio. In contrast, as one notes from Fig. 4.39, in MEL at 10 kPa for C7 there is no such preference in adsorption for the n-C7 versus i-C7 molecule since under these conditions the adsorption concentration is still too low. 

Siliceous MEL-type (silicalite-2) zeolite nanocrystals with average particle size of less than 100 nm have been investigated by and Si MAS NMR. ... 

The first reaction, hydrogenation of the alkylanthraquinone, is catalyzed by Pd. The second, the epoxidation of propene by the HP generated by air oxidation of the RAHQ, is catalyzed by TS-1. This is possible because TS-1 activity is not affected by the polynuclear compounds forming the redox couple, since they do not enter the zeolite cages due to steric hindrance (the average diameter of the channel system of TS-1 and TS-2, with ME I and MEL type structures, respectively, is 0.55 nm the cross... 

Derouane on pentasil-type zeolites suggests that in ZSM-11 (MEL) there are more T-sites available for substitution by A1 than there are in ZSM-5(MFI). Their subsequent work suggested that random siting of aluminium could lead to the formation of Al-pairs... 

When zeolites are grown as films, zeolite membranes are formed. Efforts to prepare polycrystalline zeolite membranes started in the late 1980s, but not until the early 1990s were MFI-type zeolite membranes (ZSM-5 and silicalite-1) successfully prepared with very good permeation and separation properties [3]. Since then, zeolite membranes have constantly attracted considerable attention because of their unique properties in terms of size uniformity, shape selective separation behavior, and good thermal/chemical stabilities. So far, more than 20 different types of zeolite membranes have been prepared - such as LTA, FAU, MOR, FER, MEL, CHA, DDR, and AFI - with significant separation interest [4, 5]. Table 3.1 lists a few typical zeolite membranes and their potential applications for separation of fluid mixtures. 

Closely related zeolite framework structures often form under very similar conditions, and this can lead to the formation of stacking faults or intergrowth structures. For example, both ZSM-5 (MFI) and ZSM-11 (MEL) contain penlasil sheets. The only difference between the two is the linkage between adjacent sheets (they are related by a center of inversion in MFI and by a mirror plane in MEL, see sections 2.2.7 and 2.2.8), and it is not uncommon for an occasional stacking fault to occur [23]. If substantial domains of two framework types are formed and these domains share a common face, the material is referred to as an intergrowth. 

Metallosilicate molecular sieves result when the ions in zeolitic materials (aluminosilicate) are replaced (isomorphous substitution) by other ions. A large number of metal ions have reportedly been incorporated in zeolite lattices. However, doubts arise regarding the location of these metal ions in the framework in many cases. Detailed characterization of the metallosilicates is necessary to identity the nature and location of the metal ions. As an example, the types of V-ions present in vanadosilicate molecular sieves of MEL, MFI and BEA structure types are discussed based on detailed physicochemical characterization of these materials. Also, the influence of preparation methods on the type and location of the V-ions are reported. 

Generally zeolites of the MFI, MEL and BEA types are synthesized from alkaline media. Occasionally acidic media containing fluoride ions have also been used for the synthesis of zeolites [13]. We will now examine the influence of the mode of preparation (acidic or alkaline medium) on the nature of the V-ions in V-MFI molecular sieves. Two V-MFI samples, A from acidic and B from basic media were prepared. Analytical data of the precursor gels and the crystalline samples are presented in Table 1. 

Apart from the MFI and MEL structure type, there are not yet clear evidences about the possibility to Insert titanium atoms In other zeolite structures by direct synthesis. 

As for Ti-containing molecular sieves, the incorporation of V has been mainly attempted in pentasil-type frameworks (MFI and MFI/MEL), while little information is available about the possibility to synthesize vanadosilicates with the framework topology of other zeolites. Table 6 summarizes the data reported in the literature. Hydrothermal synthesis is the route used in most cases. 

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