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

Abstract

A series of hydro sodalites of composition Na₆[Al_1−yGa_ySiO₄]₆(H₂O)₈ (0 ≤ y ≤ 1) have been prepared from Na_6+x[Al_1−yGa_ySiO₄]₆(OH · H₂O)_x(H₂O)_8−4x sodalites by NaOH/H₂O exchange methods carried out in acidic aqueous media. One of the end members (y = 1) Na₆[GaSiO₄]₆(H₂O)₈ is obtained from Na₈[GaSiO₄]₆I₂ 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 [AlSiO₄]₆ to the [GaSiO₄]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 [Na₃□(H₂O)₄]³⁺ complex in the β-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)O₄ and SiO₄ tetrahedra confirms the increase of tilt angles and decrease of six-ring pore dimension upon aluminium replacement by gallium in the framework. Results of ¹H and ²⁹Si MAS NMR as well as FTIR spectroscopy are also discussed in terms of successive gallium incorporation in the alumosilicate hydro sodalite frameworks.