High silica, preparation

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

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). 

The aluminosilicate versions of SSZ-31 and NCL-1 have been prepared with Si/Al ratios from 20 to The borosilicate was most conveniently prepared by using zeolite boron Beta as the source of boron. Recently, it was shown that a sodium borate source could be used with boron Beta seeds if the crystallization was conducted in sealed quartz tubes (39). In related work, a high silica Beta was converted... 

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. 

High-silica, aluminum-deficient zeolites have been prepared by the following methods (Table I) ... 

The study of obsidian by NAA has proved to be particularly fruitful because of the relatively limited number of sources and the extent to which it was traded (Beardsley et al. 1996, Cook 1995, Darling and Hayashida 1995, Kuzmin et al. 2002, Leach 1996). Studies have also extended to include other volcanic materials such as pumice (Bichler et al. 1997, Peltz et al. 1999). NAA has also been used for the analysis of flint as OES is insensitive and not reproducible due to the effect of the high silica content, and AAS requires significant sample preparation (Aspinall and Feather 1972). The wide range of appropriate materials extends to organic materials such as human bone (Farnum et al. 1995), and its exceptional sensitivity to trace elements has led to its wide use in geochemistry (for example in determining trace [ppb] contaminants in waters) and more recently in forensic chemistry. 

McDaniel, C.V. and Duecker, H.C. (1971) Process for preparing high silica faujasite. US Patent 3,574,538. 

Arika,)., Aimoto, M., and Miyazaki, H. (1986) Process for preparation of high-silica faujasite type zeolite. US Patent 4,587,115. 

Ebery, P.E., Laurent, S.M., and Robson, H.E. (1970) High silica crystalline zeolites and process for their preparation. US Patent 3,506,400. 

Ethene, propene, 1-butene, Isobutene, and 1-hexene were used as alkylating agents. H-Zeolltes, HY, HL, and HM (supplied by TOSOH Corp.) were used as catalysts. SI/AI2 ratios were 9.0-220 for HM, 5.8 for HY, and G.l for HL. High silica mordenltes above 23 of S1/A12 were prepared by dealuminatlon. Silica-alumina (SA, Sl/Al2=4.3) and HZSM-5 (Sl/Al2=50) were also used. SI/AI2... 

Surfactant extractions have been attempted previously, but mainly for HMS materials (mesoporous silicas, prepared using neutral amines as the surfactant). Due to the much weaker S°I° interaction, compared to the S+I interaction, these surfactants can be extracted relatively easily [6], Some reports have been published on the extraction of the cationic surfactant, but the resulting material is mostly inferior to the calcined one and in most cases, still a posttreatment at high temperatures is required [7], We describe an extraction procedure for MCM-48, that does not require a post-treatment and that produces materials with a better quality than the calcined ones. 

Figures 4a and 4b depict selected Nitrogen adsorption-desoprtion isotherms and pore size distributions (PSDs) for the same series of samples. As seen here and also in Table 1, all hexagonal phases exhibited pore sizes mostly above 5 nm, while typical pore sizes of MCM-41 silica prepared in the presence of CTAB under more common temperatures, i.e., 80 - 120 °C, have 3.5 to 4 nm pores [5, 19]. Earlier work showed that direct synthesis or postsynthesis hydrothermal restructuring in the mother liquor at high temperature, e.g. 150 °C gave rise to...

Roberto, D., Cariati, E., Ugo, R., and Psaro, R., Surface-mediated organometallic synthesis High-yield preparations of neutral and anionic osmium carbonyl clusters by controlled reduction of silica-supported [Os(CO)3Cl2]2 and OsClj in the presence of Na2C03 or K2C03. Inorg. Chem. 35,2311 (1996). 

Amine Additives. It is precisely in this area, suppressing an undesired product in favor of the desired one, that additives can be most useful. High silica zeolites have been formed in the presence of amine additives. Vaughan (16) has prepared faujasite with 7.0 SiC>2/Al2(>3 (24.52 A0) by addition of bis-(2-hydroxyethyl) dimethyl ammonium chloride in a slurry composition whose cation composition is 69% Na and 31% organic template (T). To scale this product up to commercial synthesis would require almost total recovery of the organic template. But its silica content makes it an interesting candidate for catalytic testing. 

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