High-silica zeolite faujasite

One gram of TMS-capped silica was treated with 15 mL of 0.25 M TMSI in dry acetonitrile at 70° C for a period of at least 12 h. Temperatures of less than40°C (which were sufficient to deprotect systems based on one benzyl carbamate per imprint molecule) did not show appreciable deprotection of material 1. Given the treatment necessary to achieve carbamate deprotection, it is important to emphasize that the TMSI procedure does not change the connectivity of the silica framework, as ascertained by studies on model crystalline materials such as a high-silica zeolite faujasite [42]. Following TMSI treatment, the silica was filtered and washed with acetonitrile, methanol, saturated aqueous sodium bicarbonate, methanol, and acetonitrile. The purpose of the aqueous treatment was hydrolysis of the silyl carbamate intermediate as shown in Fig. 6b-c. 

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. 

Nowadays many large pore zeolites are known (Table 1). However, only zeolite Beta seems to have the right overall characteristic for organic reactions. Beta is commercially available in various Si A1 ratios. The commercially available Faujasite, Mordenite and Linde type L all have low Si Al ratios, while the high-silica zeolite ZSM-12 has a parallel channel system giving rise to diffusional problems. The recently discovered zeolites DAF-1, CIT-1 and ITQ-7 require expensive templates and the synthesis is often quite delicate. 

Recently, we reported [5] on the calculation of the long-term stability of high-alumina zeolites in gas-desulphurization and gas-drying processes. The present contribution describes the chemical behaviour of different aluminosilicates under hydrothermal conditions and, especially, the prediction of the long-term stability of high-silica zeolites generated by the dealu-mination of faujasites. 

Low-silica zeolites such as sodium type A, having a molar ratio of Si Al near unity, contain the maximum number of cation exchange sites that balance the aluminum in the structure and thus have the highest possible cation exchange capacities [104,105,120]. Intermediate-silica zeolites, for example, of the faujasite type, have ratios of 2-5 and high silica zeolites, for example, ZSM types, have ratios of 10-50, respectively [104]. 

An example of an industritilly important synthetic zeolite is ZSM-5 This zeolite was first synthetized in 1976 at Mobil CORP. It is a high silica zeolite with structural and chemical properties rather different fixnn diat Faujasite. Its crystallographic unit cell stoichiometry is... 

On the other hand, a wide variation in Si/Al from 1 to 21, as noticed in Fig. 6.25a, reveals impure zeolites, which have less Al " and significantly increased Si", which can be in the forms of Quartz and MuUite, or a high silica zeolite p (not detected in the XRD) as exhibited in Fig. 6.22. On the contrary, such a combination of ions may commensurate with an impure form of zeolites, as well. Further from Fig. 6.25b it is apparent that O/T (i.e., oxygen atoms/sum of Si and A1 atoms) increases without any change in Si/Al ratio, at 1 and 2.5. This elucidates a situation when due to activation in three steps, more oxygen atoms are incorporated in the zeolites Na-Pl to Faujasite (refer Table 6.10) and a polymerization of [Si-O-Al] linkage occurs, which also indicate formation of more porous and pure zeolites, by undergoing several transitions in both the residue and the filtrate. 

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

Therefore, it seems that tridireotional zeolites HY and HB are the most promi sing ones for liquid phase reactions. In the case of HB zeolites, two items deserve special comments. Firstly, the yield (82%) of ethyl phenylacetate for the equimolar esterification of phenylacetic acid and ethanol in the presence of the fl-10 sample is substantially higher than that of the equilibrium (69%) at the same temperature and solvent (Table 3). Analogous results have been already observed with dealuminated acidic Y faujasites and can be due to zeolite water adsorption and/or to the hydrophobicity of the in surfaces (ref. 2). The hydrophobic character of high silica... 

Faujasite is a rare aluminosilicate mineral. Its synthetic high-aluminia counterpart, Linde X, was synthesized by Milton (2), and Linde Y, a high-silica/alumina synthetic faujasite, was synthesized by Breck (3). The framework of faujasite-type zeolites has been well established by Bergerhoff et al. (4) and Broussard and Shoemaker ( 5). 

At the same time, specific properties (primarily the Si/Al ratio) of a zeolite should be taken into account when discussing the mechanism of its dehy-droxylation. It is quite possible that the mechanism typical of H forms of faujasites would be completely improper for high-silica-containing zeolites. Thus, in their studies of dehydroxylated forms of ZSM-5 zeolite by means of IR spectroscopy of molecular hydrogen adsorbed at low temperatures, Kazansky et al. (72, 76) have demonstrated that the Uytterhoeven-Cristner-Hall scheme seems valid in this case. 

Blatter, F. Schumacher, E. The Preparation of Pure Zeolite NaY and its Conversion to High-Silica Faujasite, J. Chem. Educ. 1990, 67, 519-521. 

Compared with the high-alumina zeolites NaCaA or NaY, the relatively low hydrothermal stability of high-silica faujasites DAY-S and DAY-Tg result from the attack of water molecules at the silanol groups and the energy-rich Si-O-Si bonds of the crystal surface. The kinetics of this dissolution is significantly more rapid than that of the hydrolysis of aluminosilicates. 

An aluminosilicate layer at the crystal surface generated subsequently by an alumination of DAY-S zeolite or directly by the steaming of NaY in order to get DAY-T zeolite stabilizes the high-silica faujasites. The decomposition follows the acid hydrolysis of usual aluminosilicates. Consequently, in this case the kinetic model described in ref [5] is valid at least for the surface layer. 

In faujasite, the A1 content must necessarily be varied by dealumination of a low-silica zeolite, as no synthesis recipe for high-silica faujasite is available. Because the aim of this work was to study the acidity of the zeolite itself, the SiCl4 dealumination method was selected in order to minimise the formation of extralattice aluminum (24.). ... 

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