Synthesis, characterization, and growth rates of germanium silicalite-1 grown from clear solutions

The synthesis, characterization, and growth of Ge-silicalite-1 from optically clear solutions are reported. Ge-silicalite-1 is readily formed from optically clear solutions of TEOS, TPAOH, water, and a germanium source at 368 K. 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. Adsorption, power X-ray diffraction (PXRD), and 29Si NMR indicate the materials are crystalline and microporous. In situ small-angle X-ray scattering (SAXS) is applied to investigate the influences of germanium source (GeO2 and Ge(OC2H5)4) and content (Si/Ge 100:5) on the growth of Ge-silicalite-1 from clear solutions at 368 K. The in situ SAXS investigations show that for solutions with Si/Ge ratios of 100, 50, and 25 using Ge(OC2H5)4 the induction periods are approximately 6 h and the particle growth rates are 1.82 +/- 0.04, 2.52 +/- 0.13, and 2.85 +/- 0.08 nm/h, respectively, at 368 K, compared to those of pure silicalite-1 (6 h induction period, 1.93 +/- 0.1 nm/h growth rate). Further increasing the Si/Ge ratio to 15 and 5 shortens the induction period to approximately 4.5 h, and the growth rates are 3.07 +/- 0.16 and 2.05 +/- 0.10 nm/h, respectively, indicating the Si/Ge ratio that maximizes Ge-silicalite-1 growth is between 25 and 15. Similar trends are obtained with germanium oxide; however, the growth rates are all consistently larger than those for syntheses with Ge(OC2H5)4. The results indicate that Ge-silicalite-1 growth rates in the presence of germanium are increased as compared to those of pure-silica syntheses.