Zeolitic gallosilicates

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 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... 

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. 

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. 

The discovery of aluminum phosphate molecular sieves has fueled the search for new sieve like materials. This search has been amply rewarded. Newsom and Vaughan [13] were able to prepare new gallosilicate phases and gallophosphates have also been prepared [14]. One of the most interesting and startling discoveries is that of zincophosphate and beryllophosphate molecular sieves [15]. They can be prepared under very mild conditions [16] and have structures which closely resemble those of aluminosilicate zeolites [16-17]. These compounds have frameworks [(Zn02)(P02)] which are isoelectronic with the aluminosilicate framework... 

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. 

Temperature. Temperature influences the polymerization-depolymerization equilibrium. Higher temperatures cause denser materials to crystallize. Selbin and Mason (23) reported that they had to use a lower temperature to obtain the gallosilicate analog of zeolite X. Temperatures above 70 °C caused a gallosilicate of sodalite structure to crystallize. Countless examples for the temperature effect are given in the literature (see, e.g., Ref. 2). 

Hydrothermal syntheses of a series of gallosilicate molecular sieves, denoted TNU-w (Taejon National University Number n, where n = 1-7) are presented. The introduction of Ga into silicate frameworks was found to be a viable route to the discovery of novel low-silica zeolite structures. Among the materials prepared here, TNU-3 and TNU-4 with the NAT topology are characterized as members of a family of materials in which the two end members have random and non-random distributions of Si and Ga atoms over the available T-sites in the framework, respectively. 

The large pore zeolite MCM-58 has been synthesized with nsi/nAi-ratios varying from 15 to 35 and also in its new gallosilicate form [Ga]MCM-58 with a ns/noa ratio of 15. The catalytic properties of these materials were characterized using the disproportionation of ethylbenzene and the hydroconversion of n-decane as catalytic tests. MCM-58 and Pt-loaded MCM-58, respectively, proved to be highly active catalysts in these reactions. 

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. 

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