Gallium faujasites

High Resolution Silicon-29 NMR Studies of Gallium Faujasites and a Gallium Sodalite... 

Chemical Shifts and Relative Intensities in the Silicon NMR Spectra of Gallium Faujasites... 

In Fig. 1, the spectra for ZK-4 (21), Na-X (22) and gallium sodalite, all with similar compositions, are compared. Relative to Na-X, the Si-nAl peaks for ZK-4 are shielded by an amount which varies from 4.8 ppm for Si-4A1 to 7.8 ppm for the Si-OAl peak. In comparing Na-X with gallium-sodalite, however, the opposite effect is observed the Si-4Ga peak is deshielded by 8.4 ppm whereas the Si-OGa peak is only deshielded by 1.3 ppm. The first case illustrates the structure sensitivity of the chemical shift, whereas in the second case, substitution in the first coordination sphere by gallium evidently is the dominant effect. The comparison of aluminum and gallium faujasites below further illustrates this point. 

Finally, we note that the relative peak intensities for the three gallium faujasites are closely similar to those of their aluminum analogues Indicating that the distributions of T atoms are similar in the two sets of materials. 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Other framework structures based on zeolites have also been synthesized which contain atoms other than aluminium and silicon, such as boron, gallium, germanium, and phosphorus, which are tetrahedrally coordinated by oxygen. Such compounds are known as zeotypes. Pure aluminium phosphate, commonly called ALPO, and its derivatives, can take the same structural forms as some of the zeolites such as sodalite (SOD), faujasite (FAU), and chabazite (CHA) (e.g., ALPO-20 is isostructural... 

In order to further extend our understanding of the origin of chemical shift variations in zeolites, we have examined the effects of gallium substitution for aluminum in the framework on the silicon NMR spectra of sodalite and faujasite. Gallium substitution in zeolites is well known though most preparations... 

The NMR results for three sodium faujasites and a gallium sodalite are shown in Figs. 1 and 2. The chemical shifts and relative peak intensities are given in Tables III and IV. The relative peak intensities are normalized to E I(Si-nT) = 1.0. 

It is also of interest to compare the relative peak intensities for these three materials. The differences between the peak intensities for Na-X and Y and ZK-4 as a function of composition have previously been discussed (22). Gallium sodalite with Si/Ga - 1.26 has a different intensity pattern to either faujasite or ZK-4. A comparison of the ratios of I(Si-4T)/I(Si-3T) and I(Si-3T)/I(Si-2T) for the three compounds shows the differences. The corresponding values are 1.23, 1.74, 1.25 and 3.18, 2.16, 1.98 for ZK-4, Na-X and gallium sodalite, respectively. Further discussion of the Si/Ga distribution in gallium sodalite requires data over a range of composition and this not yet available. 

Source of Activity in other Siliceous Catalysts.—Although various oxides can be combined with silica to give amorphous, acidic catalysts, the replacement of aluminium in zeolites (specially non-faujasitic zeolites) has proved to be very difficult with any element other than gallium. Materials of ZSM-5 structure with iron or boron in place of aluminium have been claimed recently, but it is not yet certain that either iron or boron is part of the zeolite lattice or that the catalytic activity observed is not due to residual lattice aluminium. 

Thomsonlte samples in which Si was replaced by Ge in the zeolite framework. Again in the fifties appeared the remarkable work of Barrer et al. [3] Thomsonite, zeolite A, Faujasite and Harmotome were obtained having gallium and/or germanium in the lattice. These works were followed by a worldwide scientific effort to identify zeolites in which isomorphous substitution could give rise to ... 

The zeolite A structure has not been observed in gallosilicates. Possibly, a stable precursor of the cube type cannot be formed because of the larger gallium. Instead, the trimer is formed immediately, leading to the faujasite structure. 

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