Silicalite-1 membranes organic separations

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Nevertheless, if mixtures of gas and vapour of higher molecular mass species, or liquid mixtures of two species with different volatility and surface tension, are considered, the separation factors and permeation fluxes can be very interesting, but such separations cannot be predicted from the pure gas permeance. Silicalite membranes are hydrophobic and preferentially adsorb organic molecules that are small enough to enter the pore openings. Therefore, they can be used to separate hydrocarbon mixtures with relatively high separation factors. The selectivity for n-heptane isooctane has a maximum of 138 at 373 K for the ternary mixture of isooctane, n-heptane and n-hexane (Funke et al, 1996). 

Highly hydrophobic zeolite membranes, such as silicalite-1 (Sano et al. 1994), Ge-ZSM-5 (Li et al. 2001a), and p-type (Tuan et al. 2003), have been used for separating organic compounds from water. Matsuda et al. (2001) developed silicate PV membranes with a high separation factor for an EtOH-water system. The silicate membranes were prepared on porous supports of sintered stainless steel (2 pm) by hydrothermal synthesis. 

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