Silica zeolites

Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites.

Freyhardt C C, Tsapatsis M, Lobe R F, Balkus Jr K J and Davis M E 1996 A high-silica zeolite with a 14-tetrahedral-atom pore opening Nature 381 295-8... 

Wagner P, Yoshikawa M, Lovallo M, Tsu]i K, Tsapatsis and Davis M E 1997 CIT-5 a high-silica zeolite with 14-ring pores Chem. Commun. 2179-80... 

A comprehensive summary of the preparation of high-silica zeolites. 

A fourth option is to apply high silica zeolites (de-aluminated mordenite, US-Y or beta) to cope with the HBr formed. 

Le Eebre, R.A. (1989) High-silica zeolites and their use as catalysts in organic chemistry , Ph.D. Thesis, Delft University of Technology. 

Regeneration of high silica zeolite used for Beckmann rearrangement... 

Wichterlova, B., Sobalik, Z. and Dedecek, J. (1997) Cu ion siting in high silica zeolites. Spectroscopy and redox properties, Catal. Today, 38, 199. 

Sefcik J, Demiralp E, Cagin T, Goddard WA (2002) Dynamic charge equilibration-morse stretch force field application to energetics of pure silica zeolites. J Comput Chem 23(16) 1507-1514... 

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts... 

Keywords pure silica zeolites, solid state NMR. 

Table 1 19F chemical shifts and fluoride location in high-silica zeolites cages are indicated by the... 

Marquez, F., Zicovich-Wilson, C.M., Corma, A., Palomares, E. and Garcia, H. (2001). Naphthalene included within all-silica zeolites influence of the host on the naphthalene photophysics. J. Phys. Chem. B 105, 9973-9979... 

Pure silica zeolites or molecular sieves are metastable with regards to the thermodynamic stable polymorph at ambient conditions, a-quartz. However, they are... 

Fig. 2. Potential energy profile and structure of final alkoxide for the adsorption of isobutylene on a high-silica zeolite according to Ref. (29).

If one examines the evolution of new zeolite structures over the past decade the most interesting discoveries have been made with high silica compositions. Many of these phases can be prepared in essentially all silica forms. Purists would prefer to classify such molecular sieves as organosilicates or porosils (1), in part because the physical properties differ from more classical low Si/Al ratio zeolites. In particular, the high silica zeolites tend to be more thermally stable and chemically robust. Additionally, the higher the Si/Al ratio the more hydrophobic the zeolite. These features are desirable for catalysts that may be used in catalytic processes such as cracking (3). 

Low pressure low density polyethylene (LDPE), 70 595 Low pressure tanks, 24 288 Low Q-state, 74 674 Low rank coal, defined, 6 828 Low resistivity joints, 23 847 Low silica zeolites, 76 833... 

The discovery of the new class of high-silica zeolites "pentasil" during the last decade has attracted considerable interest due to the important applications of these zeolites in catalysis. The best known member of this family of zeolites is ZSM-5, developed in the Mobil laboratories. The unusual properties of pentasil zeolites have rekindled the interest in other high-silica zeolites, prepared by dea-lumination of low-silica zeolites. In this paper we shall review the preparation methods of aluminum-deficient zeolites, and shall discuss the properties of these materials, with emphasis on recent advances in their characterization. 

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