Hydrophobic zeolite membranes

A- HYDROPHILIC ZEOLITE MEMBRANE B- HYDROPHOBIC ZEOLITE MEMBRANE... 

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. 

Weyd, M., Richter, H., Ray, P., Voigt, I., Hamel, C., Seidel-Morgenstem, A. (2008). Transport of binary water-ethanol mixtures through a multilayer hydrophobic zeolite membrane. Journal of Membrane Science, 307, 239—248. 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

Generally speaking, two terms, hydrophobic and hydrophilic, are employed in pervaporation in zeolite membranes, to refer to the affinity of organophUic and water molecules, respectively, toward the zeolite. In this way, a hydrophUic zeohte adsorbs and preferentially permeates water. Gyaya et al. [118] defined a hydrophobicity index (HI), which is the ratio between the amount of organic and the amount of water that a sohd adsorbs ... 

Yang et al. (2001) studied PV properties of different zeolite-filled PDMS membranes. They reported that the incorporation of hydrophobic zeolites into PDMS enhances the permeation selectivity toward the VOCs, but decreases the permeation rate in the corresponding membranes. The decrease in the permeation rate results from the cross-linking effect of the zeolite particles and also from the increase in the diffusion path the zeolite particles act as solvent reservoirs in the sorption, but as obstacles for the permeation diffusion. Table 9.5 shows the flux and selectivity of filled and unfilled PDMS membranes in the PV of 1.23% EA-water mixture at 50°C. 

Adnadjevid et al. (1997) studied the effect of three different types of hydrophobic zeolites (ultrastable zeolite type Y, pentasyl-type zeolite (ZSM-5), and ALPO-5 type zeolite) on the PV properties of zeolite-tilled PDMS membranes. The physiochemi-cal properties of the zeolite used, primarily the degree of hydrophobicity, as well as the sorption capacity for EtOH, the specific pore volume, specific area, and mean crystallite size of the zeolite, significantly influence the membrane s PV properties. An increase in the zeolite content results in an increase in both membrane permeability and membrane selectivity, while an increase in the PV temperature results in an increase in the permeability and a decrease in the selectivity, as opposed to the effect of membrane thickness. 

Sikdar et al. (2000) developed adsorbent-filled PV membranes for removing VOCs from waste water. These membranes were prepared by dispersing at least one hydrophobic adsorbent uniformly into a polymer matrix. Polymeric membrane was made of rubbery polymer selected from the group consisting of PDMSs, PTMSP, PUs, polycarbonates (PCs), PE-block-polyamides, silicon PCs, styrene butadiene rubber, nitrile butadiene rubber, and ethane-propene terpolymer. The hydrophobic adsorbent was selected from the group consisting of hydrophobic zeolites, hydrophobic molecular sieves, activated carbon, hydrophobic polymer resin adsorbents, and mixtures thereof. 

Yang et al. (1999) prepared two types of hydrophobic zeolites Y by treating the NaY-type zeolite with SiCl4, with or without subsequent hydrothermal treatment. It was reported that the hydrophobic zeolite Y as filler had a significant effect on the silicone membrane properties, even at a filler content level as low as 5 wt%. In addition, the ester sorption, permeation selectivity, and flux of the filled membranes increased with the filler Si/Al ratio in the EA extraction from water by PV. 

Adnadjevic, B., Jonaovic, J. and Gajinov, S. 1997. Effect of different physicochemical properties of hydrophobic zeolites on the pervaporatin properties of PDMS-membrane.. 1. Memh. Sd. 136 173-179. 

Another type of inorganic membranes used to the PV separation is a zeolite membrane. Na-type zeolite membranes have been applied for dehydration of aqueous alcohol. Kita et al. [9] reported that a permeation flux of 3kgm h and separation factor (a) over 10 000 isopropyl alcohol aqueous solution (90wt% isopropyl alcohol), which corresponds to much larger flux and selectivity compared with polymeric membranes (normally a 1000 flux <0.1 kgm h h) On the other hand, a silicalite membrane, which is hydrophobic, preferentially permeates alcohol over water, showing a selectivity of 60 and flux of 0.8kgm h - at 5wt% of ethanol at60°C [8). 

Intrinsic material selection aspects can guide construction of mixed matrix membranes with desirable performance characteristics. The molecular sieving phase must accurately discriminate between the size and shape differences of spherocylindrical O2 and N2 molecules. Silicalite, commonly used in reported mixed matrix membrane studies, is a hydrophobic zeolite possessing channels with dimensions between 5.2 and 5.8 A. Likewise, zeolite 13X possesses an aperture of 10 A (P). Clearly, these materials are not molecular sieves for O2 and N2 molecules with lengths of 3.75 and 4.07 A, respectively. Rather, an effective molecular sieve would possess an aperture size between the molecular dimensions of the two molecules. Zeolite 4A possesses an eight sided aperture with an effective aperture size of 3.8 A and is appropriate for this application. 

Another potential application for zeolite/polymer mixed-matrix membranes is the separation of various liquid chemical mixtures via pervaporation. Pervapora-tion is a promising membrane-based technique for the separation of liquid chemical mixtures, especially in azeotropic or close-boihng solutions. Polydime thy 1-siloxane (PDMS), which is a hydrophobic polymer, has been widely used as the continuous polymer matrix for preparing hydrophobic mixed-matrix membranes. To achieve good compatibility and adhesion between the zeolite particles and the PDMS polymer, ZSM-5 was incorporated into the PDMS polymer matrix, the resulting ZS M -5/ P DM S mixed-matrix membranes showed simultaneous enhancement in selectivity and flux for the separation of isopropyl alcohol from water. It was demonstrated that the separation performance of these membranes was affected by the concentration of the isopropyl alcohol in the feed [96]. 

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