Synthesis and structure analysis of zeolite AS-1 from HF–Al2O3–SiO2–ethylenediamine–H2O

Zeolite AS-1, an aluminosilicate analogue with a DFT framework, has been synthesized hydrothermally from a system of HF–Al(OH)3–SiO2–ethylenediamine–H2O and characterized by TG-DTA, NMR, and XRD techniques. The coexistence of fluoride anions and diprotonated ethylenediamine cations in the reaction system plays an important role in the formation of the AS-1 framework with Si/Al = 1. The 27Al NMR spectrum shows only one signal due to tetracoordinated aluminum at a chemical shift of ca. 57 ppm. In the 29Si NMR spectrum, there is only one main peak at ca. −89 ppm, which is assigned to the resonance of Si(4Al). The structure of AS-1 has been determined on the basis of powder X-ray diffraction data, using Rietveld analysis. AS-1 crystallizes in the space group P42/n with lattice parameters of a = 10.3022(1) Å and c = 8.8785(1) Å at 21 °C. Both the NMR spectra and the structure analysis data indicate that the distribution of Si and Al in the framework of AS-1 follows a strictly alternating pattern, in accordance with the Lowenstein avoidance rule. The framework of AS-1 can be represented by a cross-linking of bifurcated hexagonal-square (bhs) chains or by the connection of superimposed 4.82 nets.