APPLIED CHEMISTRY One-Way Extraction of a Chemical Potential through a Liquid Membrane: Concept Demonstration and Applications Zohar Lavie, † Gadi Rothenberg,* ,‡ and Yoel Sasson* ,† Casali Institute of Applied Chemistry, Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Chemical Engineering Department, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands A novel concept for the preparation of hydroxide compounds using liquid/liquid anion exchange is presented. The exchange is accomplished by loading a chemical potential into a liquid organic membrane and subsequently releasing it from the membrane using an onium-alcoholate complex as the potential carrier. Accordingly, this method requires no electricity and no energy input other than a concentration gradient. The influence of the alcohol concentration and structure on the extraction equilibrium is studied. Alcohol acidity and stabilization of the alcoholate- quat ion pair are the key factors in determining the transfer efficiency. For example, diols are good coextractants, probably as a result of intramolecular hydrogen bonding that stabilizes the complex. The synthetic application of this concept to make CsOH, LiOH, KOH, and water-soluble tetraalkylonium hydroxides is examined. The extraction mechanism and the function of the organic membrane as a one-way transport medium and a “water pump” are discussed. Introduction Uncommon hydroxide bases such as cesium hydrox- ide, rubidium hydroxide, and short-chained onium hydroxides are gaining interest in the high-technology industry. For example, tetramethylammonium hydrox- ide is applied as an anisotropic silicon etchant in silicon micromachining because of it’s etch selectivity to mask- ing layers and relatively low toxicity. 1 CsOH and RbOH are also efficient etching agents for silicon wafers. 2,3 In addition, being strong bases, they are used as base catalysts in numerous organic reactions. 4 The high solubility of hydrophilic quaternary onium hydroxides (with C 1 -C 4 alkyl chains) in both aqueous and organic media lends them to a variety of applica- tions in catalysis and materials science. Their unique feature of decomposition into all-gaseous products ren- ders them ideal structure-directing agents (e.g., in zeolite synthesis). 5 They are used as developers for photoresist films on printed circuit boards, 6 as basic titrants in nonaqueous media, 7 as alkylating agents, 8 and as phase-transfer catalysts. 9 These salts can be prepared by various electrolytic or precipitation meth- ods. 10 The main production method for alkali hydroxide salts is based on the electrolysis of the corresponding chlo- rides, as in eq 1 Three different types of electrolytic cells are currently employed: diaphragm, mercury, and membrane cells. 11 Mercury and membrane cells are most often used, depending on the local costs of steam and electrical energy. Regardless of cell type, however, the electro- chemical production plants are capital-intensive and sensitive to scale, and they incur high operating costs. The hazard associated with any process that uses large amounts of mercury is an additional disadvantage. 12 It is well-known that direct liquid/liquid extraction of hydroxide ions into organic media using onium salts is extremely difficult. 13 This is a direct reflection of the miniscule K sel values for the hydroxide ion. 14 Conse- quently, the vast majority of base-initiated phase- transfer catalysis (PTC) reactions are believed to pro- ceed according to Makosza’s interfacial mechanism, where no direct extraction of OH - is required, rather than via the extraction mechanism offered by Starks. 15 Previously, we have reported that hydroxide salts can be obtained via nonelectrolytic solid/liquid anion ex- change. 16 However, the sequential evaporation and filtration steps required encumbered the process. In addition, we have demonstrated that alcoholic coextrac- tants can enhance basicity extraction into organic media. 17 Here, we show that the combination of quaternary onium halide salts and lipophilic alcohols creates a liquid membrane that can selectively extract and release a basic potential between two separate aqueous phases. 18 This enables the preparation of various hydroxide salts, in good yields and purities, without the physical extrac- tion of any hydroxide ions through the membrane. Results Concept. The extraction of a chemical potential through a liquid membrane is essentially a two-stage * Authors to whom correspondence should be addressed. E-mail: gadi@science.uva.nl, ysasson@vms.huji.ac.il. † Hebrew University of Jerusalem. ‡ Universiteit van Amsterdam. 2MCl + 2H 2 O 9 8 electricity 2MOH + Cl 2 + H 2 (1) 6045 Ind. Eng. Chem. Res. 2001, 40, 6045-6050 10.1021/ie0104161 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001