Framework aluminum

Not only the concentration but also the nature of the aluminum and its location (next section) plays a role in desilication by alkaline treatment. The presence of a 

Most of the papers available on desilication are devoted to the alkaline treatment of ZSM-5 zeolites. Since a large number of zeolites consist of aluminosilicate frameworks and framework Al plays a crucial role in the mesoporosity development during desilication, this methodology should be suitable for extrapolation to other 

5 Framework Trivalent Cations Different from Aluminum 

The aim of the newly introduced mesoporosity is to enhance the utilization of the microporous network by improved accessibihty of the active sites that are mostly present in the micropores. Although numerous papers have reported on the improved catalytic performance of desihcated zeolites in catalysis (details in Section 2.4.5), only few works are available that reaUy tackle the hierarchical nature of the desihcated zeohtes and demonstrate that selective sihcon removal leads to an enhanced physical transport in the zeohte crystals. 

1 Characterization of Hierarchical Structures from Argon Adsorption to MIP 

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Figure 2.5 N2 adsorption isotherms and schematized silicon dissolution (inset) upon alkaline treatment ofZSM-5 zeolites with different framework Si/AI ratios, highlighting the crucial role of framework aluminum.

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Figure 2 clearly indicates a large variability in both the sites occupied by Al atoms and the concentration of the Al atoms in these sites for the ZSM-5 samples studied. Even samples with similar framework aluminum concentration exhibit very different Al sitings as well as Al concentrations in the individual occupied T sites. This fact clearly rules out that the Al siting in the zeolite framework is random or is controlled by thermodynamic stability. It rather shows that the conditions of the synthesis are responsible for the Al siting in the framework. 

Dealuminated samples were obtained by hydrothermal treatment of calcined MCM-22 (Si/Al = 15) at different temperatures (673, 773, 873 K) for 2-24 h under a saturated flow of a nitrogen/steam mixture (flow rate of 200 ml min"1). These steamed samples were further treated with 6N HNO3 solution at 353 K for 4 h in order to remove the extra-framework aluminum species. 

SiC>2N2 were clearly assigned. Since NH substitution for O in zeolite Y occurs preferably near A1 [70-72], a substitution mechanism may be surmised which involves Lewis acidic framework aluminum which reacts with the Lewis base NH3. 

The intrazeolite cations necessary to balance the negative charge on the framework aluminum atoms are poorly shielded and as a result high electric (electrostatic) fields on the order of 1-10 V/nm are found in their vicinity. The magnitudes of the electric fields can be calculated from measured effects on the vibrational frequencies or intensities of IR bands of small diatomics such as CO or N2.24 They can also be determined from difference electron density maps determined by X-ray diffraction methods.25 These high electric fields can dramatically influence the stabilities of transition states with significant charge separations. 

III.B.3. Lewis Acid Sites and Extra-Framework Aluminum. 260... 

Nevertheless, the adsorption of alkenes can differ substantially from one zeolite to another, even for one type of zeolite, depending on the concentration of framework aluminum and the modification procedure (70). 

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. 

Table II. FRAMEWORK and NON-FRAMEWORK ALUMINUM SPECIES in USY ZEOLITES...

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

In reaction (b) an ionic exchange and solubilization of the aluminum species takes place. In reaction (c) only the solubilization of neutral aluminum species takes place. If the chemical treatment with acid in the two-step process also involves the solubilization of framework aluminum, reaction (a) takes place. 

When the two-step process is repeated on the same material, the thermal treatment following the chemical dealumina t ion results in further expulsion of aluminum from the framework into zeolite cages or channels. The solubilization of non-framework aluminum during the first chemical treatment appears to facilitate further framework dealumination during the subsequent thermal treatment due to the altered steric and electrostatic parameters in the zeolite channels. The newly formed non-framework aluminum species can be readily solubilized by acid treatment. This cyclic method has allowed the almost total removal of aluminum from mordenite (5). 

The unusually high stability of DAY zeolites prepared from USY-B and having SiO /Al O ratios over 100 indicates that the non-framework aluminum species present in USY-B play no role in enhancing the stability of this zeolite. It is the highly silicious framework, in which most of the aluminum has been replaced by silicon atoms, that is responsible for the high stability of USY-B zeolites and of corresponding DAY zeolites. In zeolites with a lesser degree of framework dealumination (i.e. in USY-A), the non-framework aluminum species appear to play a role in the stabilization of the zeolites, since their removal results in materials of lesser stability (28). 

B., and Hunger, M. (1992) Mordenite acidity dependence on the silicon/ aluminum ratio and the framework aluminum topology. 1. Sample preparation and physicochemical characterization. /. Phys. Chem., 96 (21), 8473-8479. 

Figure 4.19 Frequency dependence on mole fraction of framework aluminum for various vibrational modes ofX and Y zeolites. D6R means double six-ring, and Uj are the...

Another limitation is that sensitivity of the technique for detecting changes in the aluminum content of the framework varies depending on the Si/Al content of the sample. In low aluminum content zeolites (Si/Al2 > 60), there is very little aluminum in the framework. Since there are very few aluminum atoms per unit cell to begin with, a loss of some of these aluminum atoms from the framework will have a very small effect on the overall structural T-O-T bond angles. This effect is shown in Figure 4.20 for a series of NHJ form MFI zeolites where the number of framework aluminum atoms per unit cell varies between 0.7 and 8.0. The... 

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]. 

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