Low-silica faujasite

G.H. Kuhl, Crystallization of Low-Silica Faujasite (SiO2/Al2O3-Approximately-2.0). Zeolites, 1987, 7, 451 457. 

Mobil Oil Corporation. 1980. Manufacture of low-silica faujasites. GB1580928. Mobil Oil Corporation. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Caglione, A.)., Cannan, T.R., Greenlay, N., and Hinchey, R.). (1994) Process for preparing low silica forms of zeolites having the faujasite type structure. US Patent 5,366,720. 

For zeolites composed only of clusters of the third type, [Al2Si2AlSi3]n, the ratio Si/Al is 1.7. This is just the border between Y- and X-type faujasites. Therefore, for low-silica-containing Y zeolites and X zeolites, one might expect the presence of both OH-III and OH-II groups. The former should predominate in X-type zeolites. 

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. 

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.). ... 

However, due to their poor solubUity in aqueous media as compared to alkali metals their use has been limited. Synthesis periods for low-silica materials range from several hours to several days. Several low-sUica zeolites, including zeolite A (LTA), faujasite (FAU), zeolite L (LTL), and zeohte P (GIS), have found use industrially. 

The first natural zeolites were discovered more than 200 years ago, and to date over 40 types of natural zeolites have been found [94]. Because natural zeohtes cannot meet the huge industrial demand, the synthesis of zeolite was started at the end of the 1940s. Low silica zeolites, such as zeolites A (Hnde type A, LTA) and X (faujasite, FAU) were first synthesized, and by 1954 began to be produced... 

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]. 

There are three different kinds of octane catalysts in current use. Some are based in part on an active non-zeolite matrix composed of a porous silica/alumina component. Others are based on low cell size (2.425-2.428 nm) ultra stable faujasite (USY), a catalyst composition developed in 1975 (2) for the purpose of octane enhancement. A third catalyst system makes use of a small amount (1-2%) of ZSM-5 as an additive. While the net effect in all cases is an increase in the measured octane number, each of the three catalytic systems have different characteristic effects on the composition and yield of the gasoline. The effects of the ZSM-5 component on cracking is described in other papers of this symposium and will not be discussed here. 

Zeolites. Unlike silica and clay, zeolites possess interior structures that are uniform and well defined in shape and size. In spite of this, inhomogeneity in the microenvironment around a guest included in a faujasite zeolite may arise for two reasons variation in the occupancy number within a cage and the presence of sites of varying microenvironment. Even at low loading levels, the... 

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. 

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. 

When calcined at high temperatures (540° to 760°C/5h) in flowing air, HY type crystals (Linde s LZY-82) are stable even in the presence of 4% V. With 5% V, the faujasite structure collapses only when the calcination temperature is rmsed from 540°C to 760°C, forming mullite and some silica [22]. Recently, Marchal and coworkers [39] have reported that V2O5, can interact with NaY crystals even at low (410° to 480°C) temperatures and that when the V/(Si + Al) atomic ratio reaches 0.2, a collapse of the faujasite structure occurs with formation of a sodium-vanadate-like phase. Thus, even in the absence of steam, V-loaded Y zeolites collapse when calcined in air with an ease dependent largely on calcination temperatures, Na and V levels. 

Zeolitization reactions have been found to predominate in the low-temperature hydrothermal reactions of metakaolinite, Al2Si207. The reactions have been carried out with and without additions of silica by use of bases LiOH, NaOH, RbOH, and CsOH and also the mixtures NaOH + LiOH, NaOH + KOH, KOH + LiOH, and NaOH -I- Me4NOH. The zeolites produced, which have been characterized by a number of physicochemical techniques, have been listed in the section on aluminosilicates (Table 40). The synthesis of the zeolites Linde A and faujasite has been studied using phosphorescence spectroscopy and laser Raman spectroscopy the results obtained were indicative of a zeolite crystallization in the solid gel phase via condensation between hydroxylated Si— Al tetrahedra. 

Using TST we calculated in 1967 ( ) that the site density of si ica alumina for the cracking of cumene might be as low as 10 cm. For t-butylbenzene cracking, Bourne and coworkers in 1971 ( 56) showed using TST tha th ir silica-alumina catalyst had a site density of about 10 cm. For cumene cracking over some metal faujasites gich rdson calculated in 1967 site... 

Furfural is formed by dehydration of pentose. Xylose is a major aldopentose and is involved as a form of xylan in hemicelluloses. Unlike glucose, furfural can be formed from xylose by Bronsted acids alone at high temperature, although the furfural selectivity is low. A variety of Bronsted acid catalysts have been examined for furfural synthesis and they are H-type zeolites such as H-mordenite and H-Y faujasite [183], delaminated zeolite [184], H-MCM-22 [185], ion-exchange resins [186], sulfonated porous silicas [187-189], porous niobium silicate [190], metal oxide nanosheets [51], and sulfated zirconia [191]. Sulfated tin oxide (S04 /Sn02) is an effective catalyst for furfural formation [192] because of the combination of Lewis acid and Bronsted acid properties, as well as HMF synthesis. 

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