(445d) Synthesis and Characterization of Ge-ZSM-5 and Sn,Ge-ZSM-5

The catalytic properties of zeolites are often highly dependent on the number and nature of the heteroatoms in the zeolite framework. While a wide number of studies have focused on single element substitution (e.g. Al+3, Ti+4, Fe+3), there have been relatively few reports on the incorporation of multiple heteroatoms simultaneously. Recently our labs have been interested in investigating zeolites containing both germanium and aluminum [1, 2].

In the first part of the talk we will summarize a series of Ge,Al-ZSM-5 samples made in hydroxide media. X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS) indicate that 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 the powder X-ray diffraction data does not reveal the presence of an impurity, the nitrogen adsorption data shows a reduced nitrogen uptake before the surface germanate 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 results 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.

The second part of the talk will focus on more recent efforts in our lab making both Sn-silicalite-1 and Sn,Ge-MFI materials. The talk will specifically focus on the competitive growth effects in synthesis mixtures that contain Sn, Al, and Ge and compare inclusion to cases where just single heteroatoms (e.g. Sn-silicalite-1) are used in the synthesis preparation.

References

1. Cheng, C.-H; Juttu, G.; Mitchell, S.; Shantz, D. F. ?Synthesis, Characterization, and Growth Rates of Germanium Silicalite-1 Grown from Clear Solutions? J. Phys. Chem. B 2006, 110 (43), 21430-21437.

2. Ghosh, A.; Garcia Vargas, N.; Mitchell, S. F.; Stevenson, S.; Shantz, D. F. ?Synthesis and Characterization of Ge,Al-ZSM-5 Made in Alkaline Media?, J. Phys. Chem. C, In revision.