Highly siliceous

Pillay, A. E. and Punyadeera, C. (2001). An enhanced procedure for the rapid digestion of high silicate archeological specimens followed by ICP-MS determination of traces of rare earth elements (REEs). Journal of Trace and Microprobe Techniques 19 225-241. 

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 ... 

L. C. Roberts, S. J. Hug, T. Ruettimann, M. Billah, A. W. Khan, and M. T. Rahman, Arsenic Removal with Iron(II) and Iron(III) Waters with High Silicate and Phosphate Concentrations, Environ. Sci. Technol. 2004, 38, 307. 

Chemical Properties. The chemical durability is a function of the durability of the crystals and the residual glass. Generally, highly siliceous glass-ceramics with low alkali residual glasses, such as glass-ceramics based on quartz and (3-spodumene, have excellent chemical durability and corrosion resistance similar to that obtained in borosilicate glasses. 

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,... 

There are several explanations for these deviations. As Figure 11 indicates, it takes some time to establish the adsorption equilibrium. Thus, the theoretical assumption concerning the adsorption equilibrium and resulting in Equation 3 may not be satisfied adequately. Here a higher initial silicic acid concentration would result. On the other hand, at high silicic acid concentrations the Langmuir isotherm is an idealization which will not properly represent experimental data of extensive... 

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