SU-57 – An aluminosilicogermanate with a DFT topology and variable composition

An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170 °C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P42/n with estimated a ≈ 10.38–10.46 Å, c ≈ 8.88–8.91 Å, V ≈ 957–975 Å3. It has variable Al–Si–Ge composition with an approximate formula |C2H10N2| [Al(SixGe1−x)O4]2 (x ≈ 0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO4 and (Si, Ge)O4 tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al + Si + Ge)) is nearly constant and slightly less than 50 at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by 27Al and 29Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375 °C in N2 atmosphere.