Zeolite ammonium-exchanged

Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica 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). 

Calcined and steamed FAU samples also have complex hydroxyl IR spectra. Figure 4.25 shows the difference between an ammonium ion-exchanged FAU before and after steaming and calcination. The very simple, easily interpretable hydroxyl spectrum of the ammonium exchanged FAU sample is transformed into a complex series of overlapping hydroxyl bands due to contributions from framework and non-framework aluminum atoms in the zeolite resulting from the hydrothermal treatment conditions [101]. 

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 starting NaY zeolite was an SK-40 from Union Carbide with a framework Si/Al ratio of 2.4. Ultrastable HY zeolites (HYUS) were prepared by steam-calcination of partially ammonium exchanged zeolites at atmospheric pressure and 550-750 °C during 3-20 hours. After dealumination they were exchanged twice with an NH solution at 80 C for one hour and then calcined at 550 °C for 3 hours. In this way dealuminated samples containing less than 2% of the original Na were obtained. One of these (HYUS-8) was subjected to different treatments (1) washed with a solution of citric acid or HCl (pH=3) at 25 °C for one hour (samples HYUSAC and HYUSl, respectively) (2) washed with a solution 0.1 M of NaOH at 40 °C for one hour (HYUSN), and (3) washed with a 38% v/v solution of acetylacetone in ethanol at 20 °C for 2 hours (HYUSA). 

The tetraethylammonium-Beta (TEA-3) zeolites used in this work have been synthesized following the procedure described in the literature (5). Samples with Si/Al ratios between 7 and 106 (as measured by chemical analysis) and crystallite sizes in the range of 0.2-0.9 ym (as measured by scanning microscopy) were obtained. The H-form of these zeolites was prepared in the following way the TEA-3 samples were heated at 550 C for 3 hours by slowly increasing the calcination temperature (5°C min l), with one-hour intermediate steps at 350 and 450 C. After this treatment all TEA molecules had been removed from the zeolite (as monitored by IR spectroscopy). In a second step, the zeolite was exchanged with 1 M ammonium acetate solution and then heated at 550°C for 3 hours as described. 

Preparation, characterization, and photoreactivity of titanium (IV) oxide encapsulated inside sodium or ammonium exchanged zeolite, mordenite, or potassium zeolites were described 111... 

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... 

It is also of interest to use MAS NMR for the study of the thermal treatment of zeolites which are not in the ammonium-exchanged form. In an X-ray study, Pluth and Smith (179) found electron density at the center of the sodalite cages in dehydrated zeolites Ca-A and Sr-A and attributed this to a partial occupancy of these sites by a four-coordinated aluminous species. No such effect was found in zeolite A exchanged with monovalent cations. Corbin et al. (180) used 27A1 MAS NMR to examine commercial samples of K-A, Na-A and (Ca,Na)-A, as received (see Fig. 41). For K-A and Na-A, only framework tetrahedral Al species were observed, with chemical shifts of 57 and 52 ppm respectively. However, in (Ca,Na)-A an additional intense resonance at 78 ppm, typical of AlfOH) but definitely not due to framework aluminum, was also found (see Fig. 41). A much weaker signal, also at 78 ppm, was detected in zeolite Sr-A its intensity increased greatly on heating the sample to 550°C. Freude et al. (183) came to very similar conclusions in their NMR study of heat-treated zeolite Ca-A. They found that maximum framework dealumination occurs at 500°C and corresponds to ac. 17% of total Al. 

A1 being removed from the samples. Despite repeated ammonium exchange followed by hydrothermal treatments, the bulk of the Al, as much as 58.8% for A8, is still in the zeolite interstices. For all the samples the framework-Si/Al ratio as determined by NMR is considerably higher than the Si/Al ratio measured by elemental analysis, in agreement with the presence of extra-framework Al. 

Sufficient (NH4)2TiF6 was added to the ammonium exchanged ZSM-5 to replace 50% of the framework A1 with Ti. The LZ-241 product, Sample C, contained 8.9 wt.% Ti02. The zeolite was 48% dealuminated after reflux for 28 hours. All of the Ti was incorporated into the zeolite. There was a small decrease in the amount of defects, a feature observed with zeolites that are not readily susceptible to acid attack and have a high silica content. This has been attributed to "etching" of the surface of the zeolite crystals by the combination of acid and fluoride, and transport of the silicon atoms in solution to fill other defect sites in the framework <1). 

The as-prepared ZSM-48 was calcined in air at 500°C for 48 hrs to give H,Na-ZSM-48. This was washed with 1.0 M HC1 at 25°C for 3 hrs to give H-ZSM-48. The material was then heated at 450°C for 30 hrs in a tubular quartz furnace (Hamdan, H. Sulikowski, B. Klinowski, J. T. Phvs. Chem.. in press) with water being injected at a rate of 12 ml per hour into the tube by a peristaltic pump so that the partial pressure of H2O above the sample was 1 atmosphere. It was then exchanged with 1 M NH4NO3 and steamed at 650°C for a further 16 hrs. The ammonium exchange was repeated and the zeolite steamed at 650°C for 16 hrs and at 800°C for 8 hrs. XRD diffraction patterns (not shown) did not indicate any loss of crystallinity upon thermal treatment. 

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. 

The work discussed here compares the relative site occupancies for three series of hydrated Y zeolites ammonium, calcium and lanthanum exchanged. The data for these same three series of samples in the dehydrated forms are also presented and the assignment of the observed resonances is discussed. 

Materials Used. The NaY zeolite and an ion-exchanged form of it, SK-500, were supplied by Union Carbide Corp., Linde Division, in the form of uncalcined powder. The SK-500 (Lot Number 12506-39) is a rare earth-ammonium exchanged type Y zeolite and had not been activated previously or calcined in its preparation. The calculated unit cell formula was... 

Hydrogen ions may be exchanged into zeolites directly from acidic solution. They may be generated within zeolites by exchange with ammonium ions followed by heating and... 

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