(653b) Investigations Of Ge,Al-Zsm-5; Influence Of Heteroatoms On Growth Rates And Local Structural Ordering Of Ge And Al
AIChE Annual Meeting
Friday, November 9, 2007 - 8:48am to 9:06am
This talk will provide an overview of two recent, and complimentary, research efforts to understand heteroatom inclusion in zeolites. The first set of results will describe the effect of germanium and aluminum inclusion in silicalite-1 clear solution syntheses. A series of Ge-silicalite-1, Al-ZSM-5, and Al,Ge-ZSM-5 samples have been prepared and characterized using diffraction, microscopy, porosimetry and IR/NMR spectroscopy, and their growth kinetics studied using small-angle X-ray scattering (SAXS). It is observed that the presence of germanium significantly increases crystal growth rates, whereas aluminum inclusion leads to significant depression of growth. X-ray fluorescence (XRF) is used to determine the Si/Ge ratio and indicates that germanium inclusion is typically 30-50% of that in the actual mixture and samples of Al-ZSM-5 possess slightly lower Si/Al ratios than the initial synthesis mixtures. In the case of simultaneous incorporation of germanium and aluminum, the final materials have a slightly lower Si/Al ratio than the synthesis mixture but a much higher Si/Ge ratio, indicating the aluminum is more readily incorporated in the zeolite as compared to germanium. This result is consistent with studies of the individual heteroatom substitution behavior. Germanium incorporation in the final material increases at higher heteroatom contents (Si/(Ge+Al) = 50 and 25). The promoting effect of germanium on the growth rate of silicalite-1 dominates at low heteroatom content (Si/(Ge+Al) = 100), leading to enhanced zeolite growth rates as compared to pure silicalite-1. This promoting effect is insensitive to the Ge/Al ratio at a Si/(Ge+Al) = 100. The influence of aluminum on the growth rate, as well as the crystallinity of final materials, becomes observable when the heteroatom content is increased (Si/(Ge+Al) = 50 and 25). This is the first study we are aware of that reports the synthesis of Ge,Al-substituted silicalite-1 phases formed in hydroxide media or from clear solutions, and has implications for the synthesis of nanoparticulate zeolitic materials for catalysis.
We will also present preliminary results, from powder and single-crystal X-ray diffraction studies, in regards to the preferential ordering of germanium in Ge-silicalite-1 materials as well as well Al,Ge-ZSM-5 phases. Of particular interest will be comparisons between samples made using fluoride versus hydroxide mediated syntheses, and in the case of fluoride syntheses, if there are correlations between the location of the fluorine anion and the germanium in the zeolite lattice.