Ultra-stable zeolites

J. W. Ward It was previously shown (Ward, /. Catalysis 1968, 11, 251, Nature of Active Sites on Zeolites VI) that alkaline earth ions stabilized the hydrogen-Y zeolite. Examination of the infrared spectra, unit cell constant, ion exchange capacity, thermal analysis data, and other properties indicates that this material is not the so-called ultra-stable zeolite. 

Nearly all syntheses of zeolites and microporous aluminophosphates have limitations to gel composition and other parameters. For example, some zeolites with special compositions such as high-silica Y zeolite and low-silica ZSM-5 cannot be directly synthesized. A secondary framework modification is necessary for their preparation. For instance, dealuminization, isomorphous substitution of extraneous silicon for aluminum, and removal of the sodium process in Y zeolite are necessary to prepare ultra-stable zeolite Y (USY) isomorphous replacement of framework atoms of boron with aluminum in a presynthesized silicon-boron structure is often used to prepare some specific aluminosilicate zeolites that cannot be directly synthesized, such as Al-SSZ-24 (AFI) and Al-CIT-1. Secondary synthesis (post-treatment) will be discussed in detail in Chapter 6. 

In 2003, Ganapathy et al. published a comprehensive study using Al, Si, and Ti SSNMR experiments to investigate the structure of titanium substituted ultra-stable zeolite Y (Ti-USY) [102]. Using a magnetic field of 11.7 T, static SSNMR Hahn-echo experiments were per-... 

The aluminum defects (III) in the above scheme created by dealumination may be eliminated by silicon species from the zeolite structure of amorphous silica contained in the material. The dealumination can be achieved also by the reaction with silicon tetrachloride. In this case, no vacancies are formed since aluminum in the structure is directly substituted by silicon. Since the thermal stability of zeolites increases with increasing Si/Al ratio, zeolites become thermally more stable after dealumination. In the case of Y-zeolites, the stabilized zeolites are called ultra-stable zeolites. 

Isomerization of a-pinene epoxide to campholenic aldehyde, an intermediate for perfumery chemicals, has been carried out elegantly with ultra stable Y-zeolite. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

An example of such thermal dealumination is the formation of ultra-stable Y zeolites (USY zeolites). McDaniel and Maher (6) reported the preparation of two types of ultrastable Y zeolites (a) one type prepared by the hydrothermal... 

The left panel of Figure 4 in Ref. 214, shown on the right, displays 27 A1 MAS NMR data from an ultra-stable Y (USY) zeolite sample after several treatments with nitric acid solutions. 

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. 

USY (ultra-stable type Y) is a good material which has served us well but which has probably been pushed to its limit (10). In simplified terms, as Al3 is eliminated from the T-positions in the structure by thermal treatment in the presence of H2O, they are replaced by Si4 from some other portion of the crystal. Table n compares a typical USY (LZ-Y82) to the parent material, NaY. The Si02/Al203 ratio (5.77) probably understates the transformation because of non-framework alumina retained in the structure. Reduced crystallinity is evidence of structural damage this same effect would be expected to reduce the zeolite character of its sorption properties. The reduction in cation content (0.38 Na/Al) renders it unsuitable for an alkaline application such as the ELF-Aquitaine aromatization catalyst... 

Zeolites Y, MOR and all silica-BEA showed no activity, although activity is shown by US-Y (a dealuminated, ultra stable form of Y), which contains some extra framework A1 and the less crystalline MCM-41, which react with the alcohol to form an activated alkoxide species. 

Various kinds of sorbents were investigated in this work. Four as-received sorbents Na-type Y-zeolite (Si/Al=2.43, Strem Chemical), H-type ultra-stable Y-zeolite (Si/Al=195, TOSOH Corporation), activated carbon (Type PCB, Calgon Carbon Corporation) and modified activated alumina (Selexsorb CDX, Alcoa Industrial Chemical), were used in this study. According to the product datasheets, Selexsorb CDX is formulated for adsorption of sulfur-based molecules, nitrogen-based molecules, and oxygenated hydrocarbon molecules. Na-Y and H-USY were in powder form (binderless). Since activated carbon was in granular form and activated alumina was in pellet form, they were crushed into powder form for evaluation. 

Acid sites in mesoporous silicates can be generated either by isomorphous substitution of trivalent cations such as A1 or B for Si, or by adding an acidic component such as a heteropolyacid, an ultra stable Y (USY) or a A1 containing ZSM-5 zeolite. 

The process of obtaining of the ultra stable form of NH4Y zeolite by dealumination by an aqueous solution of hexafluorosilicate ammonium with alternating steps of cation exchanges and calcinations is investigated. Structure change of a sample during its preparation is studied. 

In the present paper the change of cry stalline structure and adsorption ability of NaY zeolite is investigated with the purpose of obtaining from it the ultra stable form. An aqueous solution hexafluorosilicate is used as the agent of dealumination. 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

These dealumination procedures result in Al deficient zeolites of the high thermal stability. In the case of faujasite Y, the so called ultra-stable Y zeolite (US-Y) is used as a catalyst for cracking catalysis (47). It was reported that the total acidity of (48) Al-deficient zeoITtes was less than that of the parent zeolite buT with stronger acidic sites (49). For mordenite, this acidity decreases linearly with Al content (50, 51). However calorimetric measurement of the NH- heat of adsorption has shown that when the total number of acidic sites decreases regularly with dealumination, as could be reasonably expected, the strength of the strongest acid sites is enhanced (52). 

For silica hinder, Choudhary and his co-workers[1,2] showed in the case of H-gallosilicate that inter-crystalline and intra-crystalline acidity decreased appreciably, resulting in a decrease in total acidity. As in the case of alumina binder, Sousa-Aguiar et al.[7] found a similar interaction of silica binder with ultra-stable Y zeolite, generating an acidic silica-alumina compound. 

It was early realised that the acidity of zeolites is affected by their composition. Zeolites with a low aluminium content, hence a low density of lattice anions, present isolated acidic sites, which strength is not diminished by mutual interaction [30]. In this way, catalytic applications usually demand zeolites with a lower aluminium content than applications in water softening or in the separation of air gases, for which a high density of cations is demanded. The aluminium content of zeolites can be controlled by post-synthesis treatment, like in the dealumination treatments which lead to the ultra-stable Y (USY) used in the TCC catalysts [31]. The aluminium content can also be controlled by modifying the conditions of synthesis, albeit each zeolite structure presents a preferential field of composition. It was early shown that the Si/AI ratio is affected by the pi I of the synthesis system, the lowest Si/Al ratio being obtained in the most alkaline systems, in which silica is largely depolymerised and is incorporated as isolated tetrahedra [32],... 

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