Framework aluminum atoms

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. 

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]. 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

For weak acceptors like charge balancing cations (Na, K, etc.), the interaction with the lone pair on the nitrogen is weak and results in a absorbance band around 1440cm . For stronger acceptors like extra-framework aluminum atoms (Lewis... 

Numbers of OH Groups per Cavity (4 of Unit Celt), Framework Aluminum Atoms (Al, ), Extra-Framework Aluminum (A/A — -4/gS J, and Aluminum in the Form of Mobile Hydrated Complexes... 

Al(acac)a is the number of extra-framework aluminum atoms per cavity which can be extended using an acetylacetone/ethanol solution. 

NAl is the number of framework aluminum atoms per unit cell, that is, per 192T-atoms per unit cell in the FAU framework of the X and Y zeolites... 

The crystal structures of 4 ammonium exchanged, heat-treated faujasites were determined from x-ray powder data. Structure I, often called decationated Y, has lost 15 framework aluminum atoms and 21 framework 0(1) atoms (bridging oxygen atoms) per unit cell, and 15 Al(OH)2+ ions are present in the sodalite cages. Structure 11, called ammonium-aluminum Y hydrate, shows a complete rehydroxyla-tion of the vacant 0(1) positions. Structure III, called ultrastable Y, shows the same 15 framework aluminum atoms absent, and the removal of 25 0(3) and 13 0(h) framework oxygen atoms. Structure TV, which is a repetitive exchanged and heat-treated version of Structure 111, has a mean Si-O bond length of 1.610 A, which indicates that little framework aluminum is present. 

For AIPO4 molecular sieves, the A1 spectra provide a different type of information. In certain cases, some of the framework aluminum atoms can become hydrated, changing coordination from four to six and topology from... 

As shown in Fig. 11, the decrease in the Al MAS NMR signal of tetrahedrally coordinated framework aluminum at about 60 ppm is accompanied by the appearance of signals at about 0 ppm and 30 ppm. Furthermore, in the spectra recorded by the application of the cross-polarization technique (right-hand side), the intensities of the signals at 0 ppm and 30 ppm increase inversely to the intensity of the signal at 60 ppm. Therefore, the authors suggested that H- Al CP/MAS NMR experiments preferentially monitor non-framework aluminum atoms. In addition, this observation indicates that the 30-ppm line is an independent signal [90]. 

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