Zeolites gallosilicate

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

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). 

X.S. Liu and J.M. Thomas, Gallosilicate Zeolites from Porotectosilicate Precursors the Ready Preparation of Gallo-ZSM-11 from Silicalite-II. J. Chem. Soc., Chem. Commun., 1985, 1544-1545. 

P-10 - Utilization of dry-gel conversion method for the synthesis of gallosilicate zeolites beta, ZSM-5 and ZSM-12... 

Synthesis of gallosilicate zeolites [Ga]-beta, [Ga]-ZSM-5 and [Ga]-ZSM-12 was performed by dry-gel conversion (DGC) method. The crystallization of the dry gel was performed in presence of small amount of water, without which the crystallization failed. The method was convenient and as effective as conventional hydrothermal method. The samples were pure and highly crystalline, and showed characteristic of typical gallosilicate zeolites. 

Structural chemical zoning in this gallosilicate zeolite, i.e. an alternation of Ga-rich MAZ and Ga-poor MOR layers in its framework, probably due to the local fluctuation of composition of the mother liquor caused by the crystallisation of the previous layer. 

Park MB, Cho SJ, Hong SB. Synthesis of aluminosilicate and gallosilicate zeolites via a charge density mismatch approach and their characterization. J Am Chem Soc 2011 133 1917-34. 

In spite of the variety of methods now available for the synthesis of alkali metal clusters in zeolites, the range of zeolites in which clusters have been identified remains small. On the basis of diffuse reflectance spectra, Liu and Thomas [36] reported the presence of K + in potassium zeolite L (LTL), and NaJ and Na + in both sodium mordenite (MOR) and the clay laponite, after irradiation with far-UV light. Ikemoto et al. [37] used microoptical spectroscopy to examine the adsorption of potassium in potassium mordenite. With regard to frameworks other than aluminosilicates, Na has been reported in aluminogermanate and gallosilicate sodalites (SOD) [38,39] and in gallosilicate zeolite Y (FAU) [36]. 

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... 

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 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. 

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... 

For silica hinder, Choudhary and his co-workers[1,2] showed in the case of H-gallosilicate that inter-crystalline and intra-crystalline acidity decreased appreciably, resulting in a decrease in total acidity. As in the case of alumina binder, Sousa-Aguiar et al.[7] found a similar interaction of silica binder with ultra-stable Y zeolite, generating an acidic silica-alumina compound. 

Influence of the addition of silica, as a binder at a concentration of 10 or 50 wt%, to H-gallosilicate (MFI) zeolite on its inter- and intracrystalline acidity, initial activity, product selectivity and distribution of aromatics formed in the propane amortization (at 550°C) and also on its deactivation due to coking in the aromatization process has been thoroughly investigated. Silica binder caused an appreciable decrease in the zeolitic acidity (both external and intracrystalline acid sites) and also in the propane conversion/aromatization activity. Because of it, the deactivation due to coking of the zeolite in the propane aromatization is, however, decreased. The deactivation rate constant for the initial fast deactivation is decreased but that for the later slow deactivation is increased because of the binder. The aromatics selectivity for aromatics and para shape selectivity of the zeolite, particularly at lower conversions, are increased but the propylene selectivity and dehydrogenation/cracking activity ratio are decreased due to the presence of binder in the zeolite catalyst. 

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