Abstract
The hydrothermal transformation at 95°C of galliosilicate hydrogels with molar oxide composition Ga 2O 3 : 10SiO 2 : 4.5 Na 2O : 150 H 2O, prepared from colloidal silica and sodium gallate mixtures, yields crystals with the faujasite or natrolite structure if the crystallization reaction is performed in the absence or presence of stirring. Raising the crystallization temperature to 150°C causes the formation of analcime crystals. Moreover, when 10% of the hydrogel Ga 2O 3 content is added in the form of a seeding solution with composition Ga 2O 3 : 15SiO 2 : 10 Na 2O : 400 H 2O, the crystallization time for GaY formation decreases by about 50% and Ga incorporation in the crystals increases. In either case, the crystallization reaction is not stoichiometric and the crystals' silica to gallia molar ratio (SGR) is always less then that in the parent hydrogel. The calcined GaY crystals have surface areas in the 500–550 m 2 g −1 range and, for a given hydrogel composition, the SGR decreases to 3.7 from 4.4 when a seeding solution is used. The thermal stability of GaY can be drastically increased by the introduction of rare earth ions such as La. In fact, 71Ga MAS NMR spectra have shown that in both GaY and H(Ga,La)Y, no extraframework Ga(VI) species are present. In addition, 29Si NMR spectra yield an SGR molar ratio in agreement with chemical analysis. FTIR results from pyridine thermodesorption, together with microcalorimetry experiments with NH 3, indicate that the isomorphous substitution of Al with Ga in the faujasite framework reduces acid site strength, density and distribution.
Published Version
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