Isomorphous framework cation substitution in the alumosilicatesodalites: synthesis, structural and spectroscopic studies of nitritecontaining phases

Abstract

Partial substitution of aluminium with gallium in the Na 8 [Al 1Ày Ga y SiO 4 ] 6 (NO 2 ) 2 sodalite system (y ¼ 0-1) revealed an extensive isomorphous miscibility. Each composition (y ¼ 0-1) is isotypic to either the alumosilicate or gallosilicate end members (y ¼ 0 or 1, respectively). The Al/Ga ratio was controlled by selective initial stoichiometries in the direct hydrothermal syntheses. The samples were investigated by X-ray powder diffraction, SEM, FTIR and 29 Si MAS NMR spectroscopies. The gallium concentration in the framework was calculated using the X-ray data Rietveld refinement, which was always lower than the feed concentration in each composition. The cell parameter and tilt angle increased, while the T--O--T angle and six-ring window dimension decreased with increasing gallium content in the framework. The Al/Ga--O distance is the mean of Al--O and Ga--O distances and increases with increasing gallium content. In contrast, the Si--O distance fluctuates close to 163 pm with increasing gallium insertion. The broadening of the XRD peaks in the patterns with mid-range gallium concentrations (y ¼ 0.35-0.70) is associated with the formation of Al-rich and Garich domains, which is explained in terms of Al/Ga--O distances and average crystal size with respect to Al/Ga ratio in the framework. The domains are assumed to be smaller than the coherence length of the X-ray radiation but large enough to detect by MAS NMR techniques.

Synthesis, crystal structure and temperature-dependent properties of gallogermanate nitrite sodalite |Na8(NO2)2|[GaGeO4]6

“…The structure of gallosilicate nitrite sodalite [21] modified by changing Si 4+ against Ge 4+ served as starting model for the refinement of the isotypic gallogermanate phase. Sieger et al [20] established two borderline models to obtain a more detailed account of the 12-fold NO 2 À orientation disorder in the sodalite cage with respect to the first approach given by Kempa et al [19]: One model describes the nitrogen atom on an 8e (x, x, x) position with an occupancy of 1 = 4 , the other model places the nitrogen atom on the P 43n 12f position with an occupancy of 1/6.…”

“…Sieger et al [20] established two borderline models to obtain a more detailed account of the 12-fold NO 2 À orientation disorder in the sodalite cage with respect to the first approach given by Kempa et al [19]: One model describes the nitrogen atom on an 8e (x, x, x) position with an occupancy of 1 = 4 , the other model places the nitrogen atom on the P 43n 12f position with an occupancy of 1/6. The true nitrogen positions were assumed to be somewhere in-between these approximated positions [20], although a precise answer for the true nitrogen position is lacking so far [19][20][21][22]27]. The refinement with both borderline models did not lead to significant differences in structural results before [20], but for the gallosilicate nitrite sodalite [21,22] better results were observed with nitrogen refined on the P 43n 8e position.…”

“…The true nitrogen positions were assumed to be somewhere in-between these approximated positions [20], although a precise answer for the true nitrogen position is lacking so far [19][20][21][22]27]. The refinement with both borderline models did not lead to significant differences in structural results before [20], but for the gallosilicate nitrite sodalite [21,22] better results were observed with nitrogen refined on the P 43n 8e position. Hence we decided for our model to use the 8e nitrogen atomic site and the corresponding fractional coordinates.…”

“…Seven years later, the crystal structure was published by Kempa et al [19] and revised by Sieger et al [20] with respect to the 12-fold orientation disorder of the dumbbell-shaped NO 2 À anion in the sodalite cage. Isomorphous framework cation substitution of Al 3+ by Ga 3+ displayed a complete solid solution series of aluminosilicate and gallosilicate nitrite sodalites [21,22].…”

“…Organic molecules as structure directing agents led to the formation of gallogermanate sodalites with incorporated tetrametylammonium or ethylenediamine [26]. On one hand the gallogermanate host framework shows this flexibility with regard to the size of the incorporated templates, on the other hand nitrite enclathrated sodalites are only directly hydrothermal accessible in one-step-syntheses from kaolin, NaAlGeO 4 or NaGaO 2 in the aluminosilicate [16,17,19], aluminogermanate [27,28] gallosilicate [21] system. With respect to these results it is surprising, that the gallogermanate nitrite sodalite could only be observed with quite low phase fractions in all of our one-step-synthesis.…”