Zeolite highly-siliceous

MFl-type zeolites are highly siliceous with Si/Al ratios from about 10 to infinity generally, the higher the Si/Al ratio the easier the synthesis. MFI zeolites are reported to be hydrophobic and organophilic and for that reason useful for removing organics from water streams and stable for separations and catalysis in the presence of water. 

Anderson, M.W., Klinowski,)., and Xinsheng, L. (1984) Alumination of Highly Siliceous Zeolites. J. Chem. Soc. Chem. Commun., 1596-1597. 

On a highly siliceous external surface, the pore-opening is not readily narrowed because of the similarity between the zeolite and silica deposited. With increasing the aluminum content, silica grows in a somewhat different manner, with different bond length and bond angle from those of zeolites. As a result, the protrudent slloxane bonds reduce the pore-opening size. 

We saw that zeolite A has a Si/Al ratio of 1. Some zeolites have quite high Si/Al ratios zeolite ZK-4 (LTA), with the same framework structure as zeolite A, has a ratio of 2.5. Many of the new synthetic zeolites that have been developed for catalysis are highly siliceous ZSM-5 (MFI) can have a Si/Al ratio which lies between 20 and oo (the latter, called silicalite (see Section 7.2.2) being virtually pure Si02) this far outstrips the ratio of... 

Clearly, changing the Si/Al ratio of a zeolite also changes its cation content the fewer aluminium atoms, the fewer cations will be present to balance the charges. The highly siliceous zeolites are inherently hydrophobic in character, and their affinity is for hydrocarbons. 

An important feature of Si MAS-NMR spectra is that measurement of the intensity of the observed peaks allows the Si/Al ratio of the framework of the sample to be calculated. This can be extremely useful when developing new zeolites for catalysis, as much research has concentrated on making highly siliceous varieties by replacing the Al in the framework. Conventional chemical analysis only gives an overall Si/Al ratio, which includes trapped octahedral Al species and [AlCf], which has not been washed away. At high Si/Al ratios, Al MAS-NMR results are more sensitive and accurate and so are preferred. 

The infrared stretching frequency of the hydroxyl associated with the Bronsted sites in decationized zeolites, fails in the range 3600 to 3660 cm. As the Si/Al ratio in the framework increases, this frequency tends to decrease. What does this suggest about the acidity of the highly siliceous zeolites ... 

The ZSM series (for Zeolite Socony Mobil) of highly siliceous zeolites is prepared from solutions containing alkylammonium bases. 

In the light of what has been said in Sections III,B-III,D about the appearance and interpretation of 29Si MAS NMR spectra of zeolites, one might expect the spectrum of a highly siliceous zeolite to be uncomplicated displaying a single Si(4Si) signal, sometimes with a smaller Si(3Si) resonance,... 

However, apart from silicalite, the spectra of synthetic zeolites do not reveal signals that may be assigned to nonequivalent silicon sites. In particular, no fine detail is observed in the spectra of zeolite ZSM-5 (Si/Al ratio typically ca. SO), which is isostructural with silicalite, and of similar degree of crystallinity (see Fig. 20). However, studies of highly siliceous... 

At 104.22 MHz the linewidths in all zeolite samples were substantially the same, and all chemical shifts fell within the 50-70 ppm range reported for four-cordinated A1 by Muller et al. (139) and by Freude and Behrens (J51). Chemical shifts in zeolites X and Y remains virtually constant while the Al content is halved on the other hand shifts in different zeolites with the same Al content are sometimes quite different, although highly siliceous materials correspond in general to low values of chemical shift. 

We further illustrate the approach by reference to studies (331) of two related zeolite structures zeolite ZK-4 (isostructural with Linde A) and the highly siliceous analogue of sodalite known as TMA-sodalite. As was shown earlier (Sections III,A and III,D), the structure of zeolite A consists of a cubic array of / -cages linked through double four-membered rings so as to form larger polyhedral a-cages. The sodalite structure (Fig. 7) consists of a dense,... 

Alkylation [14] zeolites, clays, silicates C3—C5 alkanes C7—C9 isoalkanes high-octane fuel... 

We report (i) isomorphous substitution of boron, by secondary synthesis, into silicalite and into highly siliceous (Si/Al>400) ZSM-5 and (ii) an improved direct synthesis of zeolite (Si,B) -ZSM-5. The chemical status of B in die boronated products depends upon reaction conditions. Careful control of the concentration of the base, the borate species and of die duration of treatment, allows the preparation of samples containing only 4-coordinated B or a mixture of 3- and 4-coordinated B in various relative concentrations. The products were characterized by magic-angle-spinning (MAS) NMR and infrared (IR) spectroscopies and by powder x-ray diffraction (XRD). 

The synthetic zeolite, ZSM-5, is a highly siliceous material with a Si/Al ratio from 25 up to 2000 [32]. As shown in Fig. 2.7, it consists of a three-dimensional network of two intersecting 10-ring tunnel systems of 0.55-0.6 nm diameter. One of these resembles the 12-ring tunnels passing through mordenite (see... 

Most data agree that when the A1 content is relatively low, the amount of Bronsted sites in zeolites actually strictly depends on A1 concentrahon, according to theory. The ratio between catalytically active sites and A1 ions ranges apparently from 80 to 100% for highly siliceous extraframework-species-free zeolites. 

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