Ammonium-exchanged Y zeolites

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). 

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.

Two sets of chemically and hydrothermally dealuminated zeolites were prepared from separate sources of partially ammonium-exchanged Y zeolite. The first set of samples, designated USY-1 and AFS-1, were prepared from Davison ammonium-exchanged Y zeolite (Y-l). The second set of samples, designated USY-2 and AFS-2, were prepared from Linde LZ-Y62 zeolite (Y-2). Typical physical and chemical properties of the two starting materials are compared in Table I the primary difference between these materials is the extent of soda removal by ammonium exchange. 

Figure 5.13. NMR spectra of thermally-dealuminated ammonium-exchanged Y-zeolite. A. 14.1T MAS NMR spectrum, B. 11.7T DOR spectrum, from Ray and Samoson (1993), by...

Figure 3. Spectral simulation of the hydrated 477. ammonium exchanged Y zeolite shown in Figure 2.

As mentioned, NH Y zeolites must also undergo thermal treatments to generate Broensted sites upon calcination. Calcined ammonium-exchanged Y-zeolites are oflen known as USY (ultrastable Y-zeolites). Calcination is, however, a very complex step as several reactions may take place, as shown in Fig. 6. Furthermore, zeohtes undergo structural modifications upon calcination. 

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

The preparation of ammonium ion exchanged Y zeolites has long been known to be a precursor step in the preparation of H Y zeolites. The former materials have been characterized by thermo-gravimetric experiments, while the latter zeolites that contain the H atoms as hydroxyl groups have been intensively examined by infrared spectroscopy (1 .2 A systematic description of the... 

The lanthanum exchanged Y zeolite (LaY) was made by contacting an ammonium Y (Linde type 31-200 powder) with an aqueous solution of lanthanum chloride. Approximately 60-70 percent of the ammonium ions were exchanged in the procedure. The resulting LaY powder was pressed into tablets, crushed and sieved to -60+80 mesh. 

NaY from Katallstiks (Si/Al=2.56) has been used for the preparation of ammonium, potassium and cesium exchanged Y-zeolites. Three series of NH4NaY zeolites differing in the degree of exchange hav e been prepared by the multiple exchange of NaY with NH Cl solutions and denoted hereafter as 1, 2, 3. Stabilized forms of Y-zeolitea (USY) were obtained from 1, 2 and 3 by deep-bed hydrothermal treatment at 875 K for... 

The investigation of the acidic properties of zeolites started with ammonium ion-exchanged Y-zeolites (NH4Y). Ammonium ion-exchanged Y-zeolite evolves ammonia and water by heat-treatment at 650 — 600 K, and 770 — 820 K, respectively. The transformation of NH4Y can be schematically expressed as follows. 

G. Derouane, M. Mestdagh, and L. Vielvoye, "EPR Study of Nature and Removal of Iron(III) Impurities in Ammonium-Exchanged Nay-Zeolite," y. Catal, 33 [2], 169-75 (1974). 

Fig. 40. High-resolution 29Si MAS NMR study of progressive ultrastahilization of zeolite Y (Si/Al = 2.37) (165). Upper spectra without, lower spectra with cross-polarization, (a) and (b). Zeolite Na-Y (sample 1) (c) and (d), sample 1 after 50% NH exchange (sample 2) (e) and (0, sample 2 after DB treatment at 540°C for 3 hr (sample 3) (g) and (h), sample 3 after extraction with 0.1 M HC1 for 3.5 hr at 100°C (sample 4) (i) and (k), sample 3 after twofold ammonium exchange and DB treatment at 815°C for 3 hr (sample 5) (1) and (m), sample 5 after extraction with 0.1 M HC1 for 3.5 hr at 100°C (sample 6).

When zeolite NH4-Na-Y was treated at 400°C under DB conditions a decrease in the number of observable Al atoms was found as the degree of ammonium exchange increased from 0 to 90 %. In the latter case, only ca. of Al present in the zeolite is observed by 27A1 NMR (see Table XIV). The authors estimate vQ > 1.2 MHz for the unobservable Al. However, extralattice Al can be detected by contacting the zeolite with a 38% solution of acetylacetone (Hacac) in ethanol, whereupon mobile Al(acac)3 complexes are formed, and a very narrow 27A1 NMR line results the solution does not affect framework aluminum. It was found that the amount of six-coordinated (i.e., extra-framework) Al increases from 5 % in 84 De Na-Y 300 SB zeolite to 50% in 84 De Na-Y 500 DB zeolite (in this notation the first number refers to the... 

Figure 1. The extent of ammonium ion exchange of Na Y zeolite (A) at 23°C and at (O) 85°C. Equilibrations of the solutions with v/m = 20 cm3/g were carried out for 4 hr each.

Figure 2. The degree of ammonium ion exchange of Na Y zeolite following triple equilibrations at selected temperatures. Points C and F correspond to the designated data points in Figure 1.

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