An X-ray absorption fine structure study of amorphous precursors of a gallium silicate zeolite

Abstract

Amorphous gallium silicates isolated in the early stages of the hydrothermal synthesis of the gallium silicate zeolite hydroxosodalite have been studied at both the Ga K and Si K absorption edges by extended X-ray absorption-fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. A series of crystalline gallium oxide materials were also studied for comparison; this demonstrated that octahedral and tetrahedral gallium can be unequivocally distinguished using both EXAFS and XANES. The local structure of the amorphous phases is found to resemble the structure of the zeolite. Gallium is found in six-coordinate sites in the amorphous gallium oxide starting material used, but even after the shortest heating periods it converts to a solely four-coordinate environment. Both gallium and silicon K-edge EXAFS indicate some degree of medium-range order in the amorphous materials (i.e. beyond the first coordination shell). This is indicative of an extended network constructed from alternating Ga(OSi) 4 and Si(OGa) 4 units forming the structure of the amorphous materials, reminiscent of the structure of the zeolite.