Synthesis and Characterization of Ge,Al-ZSM-5 Made in Alkaline Media

The synthesis and characterization of a series of Ge,Al-ZSM-5 samples made in hydroxide media is reported. X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS) indicate there is a germanium-rich overlayer on these crystals. Electron microscopy results validate this conclusion, and also indicate that the aluminum distribution through the samples is uniform. While powder X-ray diffraction does not reveal the presence of an impurity, the nitrogen adsorption isotherms show a reduced nitrogen uptake before the surface overlayer material is removed. The crystal size and the germanium uptake appear to correlate strongly with pH, as lower pH values lead to higher germanium uptake and larger crystal size. X-ray absorption spectroscopy indicates that the germanium is tetrahedrally coordinated in the zeolite framework, and that GeO₂ is not present in these samples after they have been ion exchanged with sodium nitrate solutions. This work highlights some of the complexities involved in characterizing zeolite crystals with multiple heteroatoms and verifying that the materials are in fact phase pure.