Y zeolite ammonium

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

Stability. Ultrastable Y zeolites, prepared by the hydrothermal treatment of ammonium Y zeolites, have considerable thermal and hydrothermal stability (6) The high... 

Preparation of Copper(II)-Exchanged Y Zeolites from Sodium and Ammonium Y Zeolites... 

The noble metal component may be either palladium or platinum the effect of the concentration of both metals on methylpentane as well as on dimethylbutane selectivity in C6 hydroisomerization on lanthanum and ammonium Y-zeolite with Si/Al of 2.5 has been studied by M.A. Lanewala et al. (5). They found an optimum of metal content for that reaction between 0.1 and 0.4 wt.-%. The noble metal has several functions (i) to increase the isomerization activity of the zeolite (ii) to support the saturation of the coke precursors and hence prevent deactivation, as was shown by H.W. Kouvenhoven et al. (6) for platinum on hydrogen mordenite (iii) to support the hydrodesulfurization activity of the catalysts in sulfur containing feedstocks. 

Two main components were used in the model catalysts described in this paper. One component was a europium exchanged ammonium Y zeolite (EuNH-Y). The other component was an amorphous aluminosilicate containing about 75% Si 0 and 25% Al203 (AAA-alumina). All materials were artificially V-contaminated by impregnation with vanadyl naphthenate solutions in benzene. Tetraphenyl tin (in hot toluene) was the passivating agent used. It was added either before or after loading vanadium on the zeolite (EuNH-Y), on the gel or on a gel-zeolite mixture. 

Kerr (d) has shown that during the thermal decomposition of ammonium Y zeolite at 500 °C there is a loss of aluminum if a deep bed geometry is employed. In the particular material that Kerr studied, sodium hydroxide treatment of the deep bed sample led to the increase... 

This type of equilibrium is supported by the known details of the dehydration of hydrogen Y and the reconstitution of ammonium Y zeolite. The decrease in frequencies with increasing temperature is probably an indication of the increasing interaction between neighboring atoms in the structure. The situation is not completely clear since if the proton is visualized as moving from oxygen to oxygen atom, the lifetime... 

Ward, J.W. (1970) "Thermal decomposition of ammonium Y zeolite", J. Catal.,... 

Fig. 14.4. Conductivity of ammonium Y zeolite exposed to various gases. Symbols +, Ar , hydrogen + ammonia x, hydrogen A, hydrogen + water (with permission).

Hydrothermal treatment can however be used constructively to modify the properties of an adsorbent. Perhaps the best example is the formation of ultrastable Y by hydrothermal treatment of sodium ammonium Y zeolite. The change is accompanied by a contraction in the unit cell parameter and an increase in the Si/Al ratio due to elimination of aluminum from the lattice. The resulting product shows greater thermal stability than Y zeolites of similar composition which have not been subjected to the hydrothermal treatment. However, with X zeolite the usual result is a loss of crystallinity with attendant deterioration of the adsorptive properties while with A zeolite a more subtle effect referred to as pore closure occurs. Hydrothermally treated 4A zeolite behaves as if the window aperture is somewhat smaller than in normal 4A sieve. This effect can be useful since a pore-closed 4A does not admit chlorinated hydrocarbons and is therefore useful for drying freon refrigerants. If a wider pore sieve is used for this purpose premature breakdown and loss of capacity may occur due to formation of HF and/or HCl by hydrolysis. The precise mechanism of pore closure has not yet been established and it remains uncertain whether it involves a true modification of the ciystal structure or merely a rearrangement of the surface layers. 

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). 

The ACH process has recently been improved, as stated by Mitsubishi Gas. Acetone-cyanohydrin is first hydrolized to 2-hydroxyisobutylamide with an Mn02 catalyst the amide is then reacted with methylformiate to produce the methyl ester of 2-hydroxyisobutyric acid, with coproduction of formamide (this reaction is catalyzed by Na methoxide). The ester is finally dehydrated with an Na-Y zeolite to methylmethacrylate. Formamide is converted to cyanhydric acid, which is used to produce acetone-cyanohydrin by reaction with acetone. The process is very elegant, since it avoids the coproduction of ammonium bisulphate, and there is no net income of HCN. Problems may derive from the many synthetic steps involved, and from the high energy consumption. 

Figure 4.41 Isotherm for ammonium exchanged Y-zeolite showing type I and steamed Y-zeolite showing a type iV isotherm.

Figure 4.42 Calculated mesopore size distribution for the steamed Y-zeolite based on the BET adsorption. The ammonium exchanged Y-zeolite has no mesopores.

The capability of NO to reduce nitrates, providing a pathway for the production of ammonium nitrite and thus of nitrogen, has also been demonstrated recently by Weitz and co-workers, mainly on the basis of IR data collected over BaNa-Y zeolite [75] however, according to a parallel additional route NO would also react with NO2 to form N2O3 and then nitrogen [reactions (13.25) and (13.26)], as already discussed. The oxidation of NO by surface nitrates over a Pt-Ba/Al2O3 catalyst has also been reported by Olsson et al. [76], whereas the formation of surface nitrates from NO2 on bare AI2O3 has been reported by Apostolescu et al. [77] and previously observed in our laboratories also [78]. 

Ambs and Flank correctly observed that variables can be introduced into the calcination of ammonium Y so that a variable series of products can be obtained 33). However, there is no doubt that the normal hydrogen zeolite can be obtained from the ammonium form by carefully controlled calcination. In addition, carefully controlled calcination of the acid yields the dehydroxylated form. The ultrastable form, which can be prepared by a number of procedures described below, differs drastically in stability and composition from the other two forms. That it may contain some sites similar to, or perhaps identical with, sites in the hydrogen and dehydroxylated forms cannot be refuted. Unquestionably, however, the ultrastable form differs significantly from the other two forms. 

Y. Kerr showed that about one-third of the ammonium and aluminum could be removed from ammonium Y using H4EDTA 25). Carefully controlled calcination of this material (under conditions which yield the relatively unstable, normal hydrogen form from the normal ammonium form) yielded a hydrogen zeolite of very high stability. Kerr proposed the following reaction steps to explain the stability 23,25). 

The intensity of an NMR signal is directly proportional to the total number of protons. It is possible to measure a minimum value of 1020 hydroxyl groups per cm3 with an accuracy of 10% (1). Table I gives the number of OH groups per supercage for various zeolites as determined from the intensities of proton signals (14,15,16). For Y-zeolites no ammonium ions could be detected by both NMR or IR techniques after pretreatment... 

Linde Na-form Y zeolite (wt %, dry basis Na20, 12.9 AI2O3, 23.1 Si02, 64.0) was converted into the ammonium form using the method de-... 

The experimental procedures are those used previously (1). The following saturations were obtained 255.2 X 10 3 equivalent of ethyl-ammonium (EA), 141 X 10 3 equivalent of diethylammonium (DEA), and 65.5 X 10 3 equivalent of triethylammonium (TEA) per 100 grams of Y zeolite equilibrated at a relative humidity of 32%. Thus there is still an appreciable fraction of the lattice negative sites balanced by Na+ cations (6). The crystalline character of the zeolite lattice is not affected during these experiments. 

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