(110b) Sorption and Diffusion of Ethanol and Water in Fluoride Mediated Silicalite-1

Sorption and diffusion of ethanol and water have been studied in silicalite-1 crystals synthesized via a fluoride mediated route. The near-perfect hydrophobic silicalite (F^-) crystals have very few internal silanol defects and show unusually attractive ethanol and water transport behavior. Full isotherms (0.05 – 0.95 activity) over the range of 25- 55 ^oC are presented. At a water activity level of 0.95 (35 ^oC), the total water uptake by silicalite-1 (F^-) was found to be 0.263 mmol/g, a factor of 8 lower water uptake than in silicalite-1 (OH^-), thereby giving an ideal sorption selectivity of 35 for 5 mol% ethanol solutions. Uptake rates for water and ethanol have been measured in silicalite-1 (F^-) over a wide range of activities between 25-45 ^oC. The transient uptake curves are consistent with intra-crystalline diffusion control and yield transport diffusivities (D_t) of order 10^-9-10^-10 cm²s^-1 for ethanol and water, with the values for ethanol being about three times larger than those for water at comparable low activity conditions. Thermodynamically corrected diffusivities (D_o) for ethanol, which reflect the intrinsic mobility, are almost an order of magnitude smaller than the values for water, as is expected from the difference in molecular size. Such results yield ethanol/water permselectivities of 7-27 for 1-5 mol% ethanol solutions and estimated ideal separation factors of 130-310 over the same concentration range. For binary ethanol-water mixture tests with competitive sorption, lower actual separation factors are expected, since the sorbed ethanol molecules may act as water sorption site “seeds” to decrease ethanol/water sorption selectivity. However it is clear that silicalite (F^-) is promising for ethanol extraction from water due to its significantly depressed water adsorption compared to silicalite (OH^-).