Silicalite/poly(dimethylsiloxane) nanocomposite pervaporation membranes for acetic acid/water separation
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Composite separation membranes of poly(dimethylsiloxane) (PDMS) containing well-dispersed silicalite particles of 50 nm were successfully prepared and applied to the preferential pervaporation of acetic acid over water. The nanocomposite membranes showed improvement on both separation factor and permeation flux for the pervaporation process, as compared to plain PDMS membranes and composite membranes containing silicalite particles of 5 m. The improvement can be attributed to higher readily accessible specific surface area and higher sorption selectivity toward acetic acid of the nano-size silicalite particles. Improvement on membrane thermal stability was also acquired through incorporation of nano-size silicalite particles.
I. INTRODUCTION
Membrane pervaporation is an important low-cost separation process, especially for mixtures of close volatility and mixtures of high thermal or chemical sensitivity.1 Its applications can be found in (a) dehydration of organic/water (azeotropic) mixtures, (b) removal/ recovery of organic compounds from water, and (c) separation of organic/organic mixtures. Traditional materials for separation membranes have been mainly plain polymers.1 On the other hand, the recent development for pervaporation using adsorbent-filled membranes2 has fostered a great number of possibilities in creating new, improved composite separation membranes. For organophilic pervaporation, it has been proven useful to incorporate microporous adsorbents of high sorption capacity of selected organic into a suitably chosen host polymeric matrix for preferential pervaporation of the selected organic from relevant mixtures. For example, silicalite-filled polymer membranes have been used in pervaporation studies of various organic/water and organic/organic mixtures, such as acetic acid/water,3 methanol/toluene, 4 propanol/water, 5 and alcohol/ water.6 –8 These works used micron-size absorbent particles, which serve as an easy passageway for a selected organic but act as an obstacle for other mixture components, thus enhancing the selectivity of the selected organic. It is well accepted that pervaporation process involves three fundamental mechanisms. Basically, mixture components absorb onto the feed side of the membrane, a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 11, Nov 2001
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diffuse across the membrane, and finally desorb near the exit side and vaporize into the downstream of the membrane. The well-dispersed selective absorbent particles enhance the diffusion of the selected organic but hinder that of other mixture components. This difference in diffusion, effected by the absorbent particles, can be increased if the specific outer surface area of the absorbent particle is increased, thus increasing the selectivity. For microporous absorbents, the selected organic, in theory, can access and pass through the whole absorbent particle volume, and the absorbent particle thus exerts a volumetric e
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