Gallium substitution in the alumosilicate framework: synthesis and structural studies of hydro sodalites

Abstract

A series of hydro sodalites of composition Na 6 [Al 1Ày Ga y SiO 4 ] 6 (H 2 O) 8 (0 y 1) have been prepared from Na 6þx [Al 1Ày Ga y SiO 4 ] 6 (OH Á H 2 O) x (H 2 O) 8À4x sodalites by NaOH/H 2 O exchange methods carried out in acidic aqueous media. One of the end members (y ¼ 1) Na 6 [GaSiO 4 ] 6 (H 2 O) 8 is obtained from Na 8 [GaSiO 4 ] 6 I 2 by a hydrothermal transformation process. The polycrystalline samples are characterized by Rietveld refinements using X-ray powder diffraction data. The cell parameter fluctuates slightly from the [AlSiO 4 ] 6 to the [GaSiO 4 ] 6 framework with an average value of 884.8 pm. The occurrence of an almost constant cell parameter has been explained as a combined effect of hydrogen bonding between the oxygen atoms of the entrapped water (or the occluded [Na 3 & (H 2 O) 4 ] 3þ complex in the b-cage as a whole), and the framework together with six-ring window deformation. During XRD refinements both aluminium and gallium atoms have been converged to a single crystallographic site. The Al/Ga--O bond distance therefore indicates the mean of Al--O and Ga--O distances, which increases with increasing gallium content in the framework. However, the Si--O distance remains almost constant at 162.9 pm at any Ga/Al ratio. The coupled twisting of the (Al,Ga)O 4 and SiO 4 tetrahedra confirms the increase of tilt angles and decrease of six-ring pore dimension upon aluminium replacement by gallium in the framework. Results of 1 H and 29 Si MAS NMR as well as FTIR spectroscopy are also discussed in terms of successive gallium incorporation in the alumosilicate hydro sodalite frameworks.

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

“…1), which is in good agreement with previous reports [8,27]. At the initial stage (up to y $ 0.35), the gallium content in the framework is very close to the feed composition.…”

“…Afterwards, the gallium concentration in the framework returns towards the equivalence line. The discrepancy of gallium concentration between the feed stoichiometry and the sodalite framework has been discussed in a recent report [8]. A similar behaviour was observed in preparing other sodalites [27] and zeolites [30].…”

“…Sodalites, therefore, have been regarded as a model compound to study the discreet b-cages of larger and more complicated zeolitic systems. Typical zeolitic ion exchanges are also known to occur in sodalites in aqueous media [8][9][10].…”

“…In this paper, we have studied a series of nitrite sodalites with a composition of Na 8 [Al 1Ày Ga y SiO 4 ] 6 (NO 2 ) 2 (y ¼ 0-1) following our recent works on halide [27] and hydro [8,28] sodalites. We will report on how aluminium atoms in the T 1 -sites can be replaced by larger gallium atoms thereby changing the framework properties; with particular emphasis on the synthesis, structural and spectroscopic features of Al/Ga miscibility at the mid-range gallium concentration (y $ 0.5) of the [Al 1Ày Ga y SiO 4 ] framework.…”

“…The reason for the selection of NaAlO 2 and NaGaO 2 rather than the binary oxide sources (e.g. g-Al 2 O 3 and Ga 2 O 3 ) for a simultaneous occupation of aluminium and gallium in the sodalite framework matrix [Al 1Ày Ga y SiO 4 ] 6 is discussed elsewhere [8].…”