Synthesis and Characterization of a Layered Silicogermanate PKU-22 and Its Topotactic Condensation to a Three-Dimensional STI-type Zeolite

A new layered silicogermanate, PKU-22, was hydrothermally synthesized under fluoride conditions using the tetraethylammonium (TEA + ) cation as the structure directing agent (SDA). The crystal structure was determined by single crystal X-ray diffraction. Structure analysis reveals that PKU-22 is constructed by sti layers stacking along the [100] direction in an ···AAAA··· manner, with TEA + cations occurring in the interlayer spaces and F – anions residing within the layer and connecting to Ge atoms, which also act as the charge compensation species. In situ temperature-variable powder X-ray diffraction results indicated that PKU-22 could be transferred into a three-dimensional STI -type zeolite PKU-22a on heating. It is interesting that the TEA + cations remain intact in the structure of the condensed product. Solid-state NMR, inductively coupled plasma atomic emission spectroscopy, thermogravimetric-differential scanning calorimetry, and carbon, hydrogen, and nitrogen elemental analyses were applied to aid the structure analysis of PKU-22 and illustrate its transformation. The scheme of the topotactic condensation of PKU-22 to PKU-22a is proposed.