Gallosilicate

Contributions of three types of Ga sites to propane conversion into aromatics were examined by using model catalysts, i.e., gallosilicate of MOR structure with deposited GaaOs particles. The rates of propane conversion and aromatics formation were correlated with the densities of three types of Ga sites determined by NH3-TPD, and it was shown that the propane conversion and the aromatics formation were limited by Ga sites on Ga20j surface. 

Zeolite structures typically consist of silicon and aluminum finked by tetrahedrally coordinating oxygen atoms. However, similar structures as found for these aluminosilicates can be formed by substitution of the aluminum by other elements (e.g., Ga in gallosilicates or Ti in titanosilicates). Even the substitution of both Si and A1 is possible, as for example in aluminophosphates or... 

Gadolinia, conversion rates, 27 35 Gallium arsenide, high Miller index, 26 12 Gallosilicate zeolite, Si MAS NMR studies of, 33 233-236... 

Figure 2. Representations of the three distinct topologies derived by simulated annealing based on data for a lithium gallosilicate (orthorhombic, Pna2i a = 18.5A, c = 7.5 A, 2 unique T-sites, 8 T-atoms in the unit cell) [66], The correct model (which proves isotopological with the parent zeolite Li-A(BW)) is (c).

In a study the direct aromatization of natural gas over H-gallosilicate, H-gallo-aluminosilicate (Ga-H-[Al]ZSM-5) and Ga-H-ZSM-5 zeolites were compared.412 Natural gas containing 27.3 wt% C2+ hydrocarbons can be converted over Ga-H-[Al]ZSM-5 to aromatics with very high selectivity ( 90%) at a high conversion (70%) at 600°C. This catalyst was found to be the best choice for aromatization of propane413 and n-hexane.414 This results from the uniform distribution of extraframework Ga-oxide species in the zeolite channel.415... 

The obvious case to be considered first is that of synthetic faujasites, which come in a range of compositions, and for which a considerable amount of spectral information is available. Evidence of Si, A1 ordering in zeolites X and Y is provided by the presence of discontinuities in the plot of the (cubic) lattice parameter versus the Si/Al ratio (60), which indicates stepwise rather than gradual change in Si, A1 distribution. This effect is even more pronounced in synthetic faujasitic gallosilicates (61). 

The 71Ga MAS NMR spectra of gallosilicate zeolites are rather too broad [full width at half maximum (FWHM) ca. 60 ppm] for ready interpretation (77). 

The syntheses of novel molecular sieves such as aluminophosphates, silicoaluminophosphates (SAPO), gallosilicates, aluminogermanates, ferro-silicates, borosilicates, and chromosilicates, clearly open new vistas for the... 

The weaker acidity of gallosilicates (compared to aluminosilicates) was confirmed by the lower frequency shift caused by adsorption of weak bases (e.g. CO) and by the lower activity for model reactions. 

CAN Cancrinite Nacancrinite hydrate, gallosilicate cancrinite, alumino-germanate, cancrinite... 

Y. Uemichi and T. Suzuki, H-gallosilicate-catalyzed degradation of polyethylene into aromatic hydrocarbons using different types of reactors, Chem. Lett., 1137 (1999). 

K. Taknma, Y. Uemichi and A. Ayame, Product distribution from catalytic degradation of polyethylene over H-gallosilicate, Applied Catalysis A, 192, 273-280 (2000). 

The microtomy-based preparation can be used to show the heterogeneities in composition and texture in zeolites and co-precipitates, such as mixed oxides. Figure 9.18 shows the image of a crystal (BF), and maps of the distribution of Si, Ga and the Si/Ga ratio in a gallosilicate crystal. The excess concentration level of Ga in the heart of the crystal is clear. 

Mazzite MAZ 7.4 1 LZ-217 TMA-0 Omega zsmI LZ-202 Gallosilicate mazzite Gallosilicate L... 

Studies on crystalline gallosilicate molecular sieves with the beta structure (Occelli et al. 1999) have led to the suggestion that substitution of Al by Ga in these structures leads to a change of 3 ppm towards higher frequencies, but a systematic determination of the range of shifts in gallosilicate compounds has yet to be made. 

Table 6.10. O NMR interaction parameters in crystalline alumino- and gallosilicates.

Delmotte et al. (1990) used F NMR to study crystalline microporous alumino- and gallosilicates synthesised in a fluoride-containing medium. The spectra contained a resonance at - 129 ppm, assigned to Si-F species. The spectra of samples prior to calcination showed a peak at — 155 ppm, as was also found in a calcined gallium-containing sample. A signal observed in the as-synthesised materials at — 64 to — 75 ppm was attributed to ion-paired free fluoride. 

A series of crystalline gallosilicate molecular sieves with the zeolite P structure synthesised by a rapid method from alkali-free hydrogels have been studied by NMR methods, including Ga MAS NMR (Occelli et al. 1999). The Ga spectra (Figure 10.12B) show that most samples contain only tetrahedral Ga in framework... 

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