One-Way Extraction of a Chemical Potential through a Liquid

Zohar Lavie, Gadi Rothenberg*, and Yoel Sasson*. Casali Institute of Applied Chemistry, Hebrew University of Jerusalem, Jerusalem 91904, Israel, and ...
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Ind. Eng. Chem. Res. 2001, 40, 6045-6050

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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 alcoholatequat 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 hydroxide, rubidium hydroxide, and short-chained onium hydroxides are gaining interest in the high-technology industry. For example, tetramethylammonium hydroxide is applied as an anisotropic silicon etchant in silicon micromachining because of it’s etch selectivity to masking 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 C1-C4 alkyl chains) in both aqueous and organic media lends them to a variety of applications in catalysis and materials science. Their unique feature of decomposition into all-gaseous products renders 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 methods.10 The main production method for alkali hydroxide salts is based on the electrolysis of the corresponding chlorides, as in eq 1 electricity

2MCl + 2H2O 98 2MOH + Cl2 + H2

(1)

Three different types of electrolytic cells are currently * Authors to whom correspondence should be addressed. E-mail: [email protected], [email protected]. † Hebrew University of Jerusalem. ‡ Universiteit van Amsterdam.

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 electrochemical 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 Ksel values for the hydroxide ion.14 Consequently, the vast majority of base-initiated phasetransfer catalysis (PTC) reactions are believed to proceed 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 exchange.16 However, the sequential evaporation and filtration steps required encumbered the process. In addition, we have demonstrated that alcoholic coextractants 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 extraction of any hydroxide ions through the membrane. Results Concept. The extraction of a chemical potential through a liquid membrane is essentially a two-stage

10.1021/ie0104161 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001

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Scheme 1

Table 1. Bacisity Extraction Results under Various Conditionsa

entry

anion-exchange process (Scheme 1). In the first stage, the basicity is extracted into an organic solution (eq 2). This reaction, in itself, comprises two processes in one pot, wherein, first, the alcohol deprotonates at the interface through an acid-base reaction (eq 3) and then the lipophilic ion pair Q+RO- forms in the bulk of the organic solution (eq 4). The equilibrium constant for eq 2 is known as the acidity-selectivity constant: Ks(RO-/Cl-) ) Ka × Ksel(RO-/Cl-).14b

NaOH(aq) + QCl(org) + ROH(org) h NaCl(aq) + QOR(org) + H2O(aq) (2) K3

ROH(org) + NaOH(aq) y\z Na+OR-(int) + H2O (3) K4

Na+OR-(int) + H2O y\z NaCl(aq) + QOR(org)

(4)

K5

MCl(aq) + QOR(org) + H2O(aq) y\z MOH(aq) + QCl(org) + ROH(org) (5) In the second stage, the basicity is released into the other aqueous phase (the destination phase), where the hydroxide salt is formed (eq 5). The recovered organic phase is then recycled back to the first stage. Two-Stage Liquid/Liquid Hydroxide Extraction. In a typical system, a concentrated donor aqueous solution of NaOH is contacted in a separation funnel with an organic solution of equivalent weight. The organic solvent (n-hexane) contains a phase-transfer reagent and a lipophilic alcohol as extracting agents. The phases are separated, and the loaded organic phase is contacted with a destination aqueous solution of the alkali (or hydrophilic quaternary onium) chloride. After each extraction cycle, [OH-] and [Cl-] are analyzed in the destination phase, and the organic phase is recycled back to the first stage with no further treatment. Control experiments using xylenes instead of n-hexane confirmed that replacing the solvent does not affect the system. The effects of various system parameters on the hydroxide concentration in the destination phase are shown in Table 1. (For clarity, results are presented for the first extraction cycle only.) The phase-transfer agent plays a crucial role in that it permits the alkoxide anion, formed at the interface, to enter the bulk of the organic phase by ion pair formation (Table 1, entry 1). High concentrations of extracting agents yielded better results (Table 1, entries 2-4) but also increased the viscosity of the organic

parameter changed

% KOH formed

extractant capacityb

extracted hydroxide concentration

1 2e 3e 4e

Aliquat 336 concentrationc,d none 0.5 0.5 mol kg-1 4.5 18 -1 1.0 mol kg 7.7 15 1.5 mol kg-1 13.2 17

0.01 0.09 0.15 0.26

5f 6f 7 8

NaOH concentrationd 10.0 mol kg-1 4.5 18 12.5 mol kg-1 4.6 18 10.0 mol kg-1 7.7 15 12.5 mol kg-1 8.5 17

0.09 0.09 0.15 0.17

9 10 11

1-hexanol 2-octanol none

alcohole 7.7 4.5 4.1

15 11 8

0.15 0.11 0.08

12 13

2-octanol 2-pentanol

alcohole,f 2.5 2.5

10 10

0.05 0.05

14 15 16 17

1-hexanol 1-octanol pinacol 1,2-octanediol

alcohol 8.5 8.6 19.2 16.4

17 17 38 33

0.17 0.17 0.38 0.33

18 19 20 21h

1-hexanol 1-nonanol pinacol 1,2-decanediol

alcoholg 9.0 2.6 9.3 8.8

55 15 54 51

0.18 0.05 0.18 0.17

22 23

KOH TEAH

productd,g 9.0 9.1

55 51

0.18 0.17

a Reaction conditions (unless noted otherwise): 12.5 mol kg-1 NaOH, 2 mol kg-1 KCl, 1 mol kg-1 extracting agent in n-hexane. b Extracted hydroxide concentration relative to the theoretical hydroxide concentration, which would have been obtained if the extraction and release equilibria (eqs 2 and 5) were shifted completely to the right. c Tricaprylmethyl ammonium chloride.d 1Hexanol was used. e 10 mol kg-1 NaOH. f 0.5 mol kg-1 extracting agent. g 1 mol kg-1 alcohol and 0.33 mol kg-1 Aliquat 336. h Xylene was used instead of n-hexane.

phase. The optimal extractant concentration (balancing efficacy and viscosity) was found to be ca. 1 gmol kg-1. No major changes were observed when the NaOH concentration was increased from 40 to 50 wt % (Table 1, entries 5-8), indicating that these concentrations are within the saturation plateau of the distribution curve.19 Higher concentrations were avoided because the solution was nearly saturated. Structure-Activity Relationships for Alcohol Coextractants. A series of experiments was carried out to examine the activity of primary and secondary alcohols as coextractants. Alcohol acidity and structure were found to be the key parameters. Primary alcohols were found to be approximately twice as effective as secondary ones20 (cf. pKa ) 16 and pKa ) 17 for primary and secondary alcohols, respectively21). In fact, secondary alcohols gave results that were comparable to those obtained for extraction in the absence of alcohols altogether. As shown by the results obtained using 2-octanol and 2-pentanol (Table 1, entries 12 and 13), changes in the carbon chain length did not influence the extraction capacity. The presence of alcohols, and especially diols, is known to change the performance of base-catalyzed PTC systems. Diols are reputedly better as basicity coextrac-

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tants because of their higher acidities and the possible formation of intramolecular hydrogen-bonded complexes.17 It is interesting to compare the increased activity attributed to diols in those base-catalyzed reactions to our experimental results. The acceleration of HX bis eliminations from 1,2-dihalides is taken as an example. It was reported that pinacol was four times better in basicity extraction than 1-hexanol and that primary diols were five times more active than the corresponding mono-alcohols.22 We observed that replacing 1-hexanol with pinacol only doubled the hydroxide concentration in the destination phase. Furthermore, 1,2-octanediol was less effective than pinacol (Table 1, entries 14-16), although one would expect it to be the best coextractant. This reflects the fact that our system consists of the extraction and release of basicity, so that a highly stable Q+RO- complex interferes with the overall extraction efficacy. Indeed, 1,2-octanediol is a stronger acid than pinacol (pKa ) 15 and 18, respectively23), making K3,octanediol > K3,pinacol, but because K5,octanediol < K5,pinacol, the overall effect is that pinacol (and generally bistertiary diols) is the better coextractant. The unexpected behavior of the system in the absence of alcohols encouraged us to try different molar ratios of extracting agents. Based on earlier studies,23 quat/ alcohol molar ratios of 1:3 were employed using various alcohols and diols (Table 1, entries 18-21). Remarkably, in the case of 1-hexanol (cf. entries 14 and 18), the same percentage of hydroxide salt was formed in the destination phase when 1:1 and 1/3:1 quat/alcohol ratios were used, even though in the second experiment, the amount of quat was reduced by 66%. (This experiment was repeated seven times under various conditions, and the results remained consistent.) The cation in the destination phase did not influence the extraction behavior (Table 1, entries 22 and 23), emphasizing the generality of this process and its potential synthetic applications. Furthermore, replacing the KCl with KI did not affect the first cycle. This implies that the concentration gradient between the aqueous phases is such that the second-stage equilibrium is shifted completely to the products (at least in the first cycle), and this negates the influence of the selectivity constant for the different anions. Multistage Processing. Successive extraction cycles increased OH-/Cl- ratios in the destination solution until an equilibrium was reached (Figure 1). The final composition for this experimental system was found to be 97 mol % KOH and 3 mol % KCl. Removal of the remaining Cl- ions can be achieved by evaporation and filtration.24 A curious “self-concentration” of the destination phase was observed in the multistage experiments. This phenomenon can be attributed to water transport from the dilute destination phase, through the organic solution, into the concentrated donor phase. A series of Karl Fischer titration experiments (Table 2) validated this hypothesis. Discussion The overall process in this system is NaOH(aq) + KCl(aq) S NaCl(aq) + KOH(aq). Obviously, when the salts are mixed together in one aqueous phase, the separation problem precludes any practical application. Dividing the system into separate phases enables one to, in effect, “make one hydroxide salt from another”.

Figure 1. Successive cycles of basicity extraction from a source solution of 12.5 gmol kg-1 NaOH into a destination solution of 2.0 gmol kg-1 KCl (see Experimental Section for details). Table 2. Karl Fischer Experimentsa entry

contacted phase

wt % of H2O in organic phase

1 2 3 4

none donor destination donor (second cycle)

0.35 0.21 1.00 0.29

a Reaction conditions: 12.5 mol kg-1 NaOH, 2 mol kg-1 KCl, 1 mol kg-1 1-hexanol and 0.33 mol kg-1 Aliquat 336 (tricaprylmethyl ammonium chloride) in n-hexane.

The final composition of the destination phase is determined by the equilibrium constant of the overall reaction25

Ktotal )

[NaCl][MOH] [NaOH][MCl]

(6)

Thus, it is dependent on the salts’ concentration gradient alone, regardless of the composition of the organic phase. After each cycle, the composition of the destination phase (represented by each point in Figure 1) is dictated by the product K4K5. For the nth cycle, this product is expressed by

Kn )

)

[NaCl]n+1[QOR]n+1[H2O]n+1 [NaOH]n[QCl]n[ROH]n [MOH]n+1[QCl]n+1[ROH]n+1 [NaCl]n+1[H2O]n+1

[MCl]n[QOR]n+1[H2O]n

[NaOH]n[QCl]n[ROH]n [MOH]n+1[QCl]n+1[ROH]n+1 [MCl]n[H2O]n

(7)

The values [Q+RO-] can be canceled because they stand for the same loaded organic solution in both cases, but

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the values {[QCl][ROH]}n and {[QCl][ROH]}n+1 do not cancel because they represent the pre-extraction and the post-release solutions, respectively. Similarly, [H2O] should not be disregarded because the concentrations of the destination and the source solutions differ, and both solutions are concentrated. Kn is highly dependent on the organic phase composition and, more specifically, on its recovery ability (i.e., the number of cycles needed to attain the final equilibrium in the destination phase). In the ideal theoretical process, different organic solutions would differ in their extraction profiles but would reach the same final equilibrium. In this case, it would be correct to cancel {[QCl][ROH]}n and {[QCl][ROH]}n+1, transforming eq 7 into the simpler eq 6 and, in essence, equating the two-stage extraction with the monophasic aqueous mixing of NaOH(aq) and KCl(aq). Anion Exchange vs Interfacial Mechanisms. Insight into the elementary reactions that comprise the overall extraction process can be gained by examining the efficacy of different quat/alcohol ratios. If Q+RO- is the main species responsible for basicity extraction, a quat/alcohol molar ratio of 1:1 should yield optimal performance, in accordance with the interfacial mechanism proposed by Makosza.15a However, Starks’ extraction mechanism should not be ruled out, because some KOH (albeit a very small amount) was formed in the destination solution even in the absence of an alcohol. Early reports by Agarwal and Diamond claimed that Q+OH- species can be stabilized by hydrogen bonding to three alcohol molecules in their first solvation shell.23 In our system, this might be the case when 1-hexanol is used as the coextractant, as the optimal quat/alcohol ratio was found to be 1:3. Probably, steric interference prevents the stabilization of the {Q+OH-(ROH)3} complexes when diols or long-chain alcohols are used.26 A One-Way Liquid Membrane. The organic phase can be considered as a liquid membrane connecting the two aqueous phases. No hydroxide ions are transported through this membrane (except perhaps in the case of 1-hexanol). Rather, it is the chemical potential for the formation of hydroxide ions that is being transferred. The formation of Q+RO- in the first stage does not involve the entrance of species from the NaOH aqueous phase into the organic membrane (see Scheme 1). Thus, hydroxide ions that are formed in the destination phase must originate from the water in this solution. The transfer of chemical potential shifts the equilibrium H2O S H+ + OH- to the right. Because no species enter the destination phase, the KOH formed there is actually produced from the KCl and water that were there to begin with. The membrane effects selective HCl transport from the destination phase into the NaOH donor phase. This organic membrane also functions as a “water pump”, because it concentrates the destination phase and dilutes the donor phase with each cycle. Upon contact with the highly concentrated donor phase, the organic membrane loses hydration water that it has absorbed from the dilute destination phase. Karl Fischer titrations show the expected differences in the amount of water according to the aqueous phase last contacted. The mechanism of water transport stems from the strong hydrogen bonding between the anion and the water molecules, forming complexes with the general formula {Q+RO-[H2O]n}.27-29 Indeed, this self-concen-

tration is advantageous, as evaporation is an integral part of product purification operations. Conclusions We have demonstrated here a new concept for the synthesis of water-soluble hydroxide salts, based on the extraction of a chemical potential through a liquid membrane that is selective for unidirectional HX transport. The concentration gradient between the aqueous phases is the driving force for the entire process and, at the same time, determines the final composition of the product solution. The composition of the organic membrane controls the number of cycles needed for the system to reach equilibrium. The optimal organic phase has to combine good basicity extraction from the donor aqueous phase with efficient release of that basicity into the destination aqueous phase. The concept is applicable for any water-soluble hydroxide salt and has been successfully used in the syntheses of commodities (e.g., KOH) as well as speciality chemicals (CsOH, LiOH, or Et4NOH). Experimental Section Volumetric titrations of OH- ions (0.01 gmol kg-1 HNO3 as the titrant and 0.5% w/w phenolphthalein in 1:1 EtOH/water as the indicator) and Cl- ions (0.05 N AgNO3 as the titrant and 5% w/w K2CrO4 as the indicator) were performed using glass titrators. The concentration of Na+ was measured using a single-beam atomic absorption spectrometer (GBC 903). Karl Fischer experiments were performed on a Mettler DL35 automatic titrator. Ion concentrations in the destination solution were confirmed by ion chromatography (Alltech ion chromatograph with a conductivity detector, using a 100 × 4.6 mm universal cation column and 3 mM methanesulfonic acid as the eluent for Na+ and K+ and an Allsep 100 × 4.6 mm anion 7 µ column and 0.85 mM NaHCO3/0.9 mM Na2CO3 as the eluent for Cl-). Chemicals were purchased from commercial firms (>98% pure) and used without further purification. All experiments were carried out in glass apparatus at 25 °C. General Laboratory Experimental Procedure. Example: KOH from KCl and NaOH. A source solution A was prepared by dissolving 50 g of NaOH in 50 g of water (50% w/w soln, 12.5 gmol kg-1 NaOH). An extractant solution B was prepared by dissolving 40 g of Aliquat-336 and 12 g of pinacol in 48 g of hexane (1.0 gmol kg-1 A-336 and 1.0 gmol kg-1 pinacol). A destination solution C was prepared by dissolving 15 g of KCl in 85 g of water (15% w/w soln, 2.0 gmol kg-1 KCl). A and B were contacted in a 1-L separatory funnel, and following phase separation, the organic (lighter) phase B was contacted with C. The phases were separated, and C was analyzed for the concentrations of Cl- and OH- (vide supra). After the first cycle, the KCl solution contained ca. 20 mol % KOH. Different parameters in this procedure were changed, and their effects were studied (see Table 1). Multistage Preparative Procedure. Example: Tetraethylammonium hydroxide from tetraethylammonium chloride and NaOH. A source solution A was prepared by dissolving 100 g of NaOH in 100 g of water (50% w/w soln, 12.5 gmol kg-1 NaOH). An extractant solution B was prepared by dissolving 27 g of Aliquat-336 and 20 g of 1-hexanol in 153 g of hexane (0.3 gmol kg-1 A-336 and 1.0 gmol kg-1 1-hexanol). A destination solution C

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was prepared by dissolving 33 g of tetraethylammonium chloride in 67 g of water (33% w/w soln, 2.0 gmol kg-1 TEAC). A and B were contacted, and following phase separation, the organic (lighter) phase B was contacted with C. The phases were separated, and the organic phase was contacted with A (starting the next cycle). The process was repeated until analysis of the Cl- and OH- concentrations in C showed that equilibrium had been attained. Evaporation and filtration of C precipitated the remaining TEAC to yield a pure TEAH soln. KOH, LiOH, and CsOH were prepared in a similar manner from KCl, LiCl, and CsCl, respectively. Acknowledgment G.R. thanks the European Commission for a Marie Curie Individual Research Fellowship. Supporting Information Available: Ternary phase diagram for KCl-KOH-water. This material is available free of charge at http://www.pubs.acs.org. Literature Cited (1) Paranjape, M.; Pandy, A.; Brida, S.; Landsberger, L.; Kahrizi, M.; Zen, M. Dual-doped TMAH Silicon Etchant for Microelectromechanical Structures and Systems Applications. J. Vac. Sci. Technol. A 2000, 18, 738. (2) (a) Edell, D. J.; Clark, L. D., Jr. Cesium Hydroxide Etch of a Semiconductor Crystal. U.S. Patent 5,116,464, 1990. (b) Wang, T.; Surve, S.; Hesketh, P. J. Anisotropic Etching of Silicon in Rubidium Hydroxide. J. Electrochem. Soc. 1994, 141, 2493. (3) Cesium and rubidium hydroxides can also be manufactured from cesium aluminum or rubidium aluminum, respectively, by ion exchange. The products are obtained free of aluminum, but it is a cumbersome multicomponent process. See: (a) Mueller, A.; Emons, H. H.; Walter, H.; Horlbeck, W.; Pollmer, K.; Hofmann, A.; Sommer, K. Manufacture of Cesium Hydroxide from Cesium Alum by Ion Exchange. East German Patent DD268,925, 1986. (b) Scholz, E.; Sievert, W. Cesium Hydroxide or Rubidium Hydroxide. German Patent DE2,651,228, 1976. For LiOH, see also: (c) Bauer, R. J. In Ullmann’s Encyclopedia of Industrial Chemistry; Elvers, B., Hawkins, S., Schultz, G., Eds.; Verlag Chemie: Weinheim, Germany, 1990; Vol. A15, p 409. (d) Bu¨chner, W.; Schliebs, R.; Winter, G.; Bu¨chel, K. H. Industrial Inorganic Chemistry; Verlag Chemie: Weinheim, Germany, 1989; p 217. (4) (a) Tam, M. S.; Craciun, R.; Miller, D. J.; Jackson, J. E. Reaction and Kinetic Studies of Lactic Acid Conversion over AlkaliMetal Salts. Ind. Eng. Chem. Res. 1998, 37, 2360. (b) Tzalis, D.; Knochel, P. Cesium Hydroxide: A Superior Base for the Catalytic Alkynylation of Aldehydes and Ketones and Catalytic Alkenylation of Nitriles. Angew. Chem., Int. Ed. 1999, 38, 1463. (c) Salvatore, R. N.; Nagle, A. S.; Schmidt, S. E.; Jung, K. W. Cesium Hydroxide Promoted Chemoselective N-Alylation for the Generally Efficient Synthesis of Secondary Amines. Org. Lett. 1999, 1, 1893. (5) (a) Knight, C. T. G.; Syvitski, R. T.; Kinrade, S. D. In Zeolites: A Refined Tool for Designing Catalytic Sites; Bonneviot, L., Kaliaguine, S., Eds.; Elsevier Science: Amsterdam, 1995; pp 483-488. (b) Tuel, A.; Ben Taarit, Y.; Naccache, C. Characterization of TS-1 Synthesized Using Mixtures of Tetrabuthyl and Tetraethyl Ammonium Hydroxides. Zeolites 1993, 13, 454. (6) Vidusek, D. A.; Legenza, M.; Vincent, J. L. Positive Acting Bilayer Photoresist Development. U.S. Patent 4,806,453, 1989. (7) Harlow, G. A.; Noble, C. M.; Wyld, G. E. A. Potentiometric Titration of Very Weak Acids: Tetrabuthylammonium Hydroxide as Titrant in Nonaqueous Media. Anal. Chem. 1956, 28, 787. (8) Burke, D. G.; Halpern, B. Quaternary Ammonium Salts for Butylation and Mass Spectral Identification of Volatile Organic Acids. Anal. Chem. 1983, 55, 822. (9) Starks, C. M.; Liotta C. L.; Halpern, M. Phase Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives; Chapman & Hall: New York, 1994. (10) (a) Walker, L. E. Quaternary Ammonium Hydroxides Compositions and Preparation Thereof. U.S. Patent 5,559,155, 1994. (b) Cundiff, R. H.; Markunas, P. C. Tetrabythylammonium

Hydroxide as Titrant for Acids in Nonaqueous Solutions. Anal. Chem. 1956, 28, 792. (c) Shimizu, S.; Cho T.; Yagi, O. Method for Production of Aqueous Quaternary Ammonium Hydroxide Solution. U.S. Patent 4,634,509, 1987. See also ref 7. (11) (a) Minz, F. R. In Ullmann’s Encyclopedia of Industrial Chemistry; Elvers, B., Hawkins, S., Russey, W., Schultz, G., Eds.; Verlag Chemie: Weinheim, Germany, 1993; Vol. A24, pp 347354. (b) Curlin, L. C.; Bommaraju, T. V.; Hansson, C. B. In KirkOthmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Howe-Grant, M., Eds.; Wiley: New York, 1991; Vol. 1, pp 946-1021. (c) Bu¨chner, W.; Schliebs, R.; Winter, G.; Bu¨chel, K. H. Industrial Inorganic Chemistry; Verlag Chemie: Weinheim, Germany, 1989; pp 152-162. (12) (a) Freilich, M. B.; Petersen, R. L. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., HoweGrant, M., Eds.; Wiley: New York, 1996; Vol. 19, pp 1083-1084. (b) Johnson, J. The Mercury Conundrum. Chem. Eng. News 2001, 79, 21. (c) Short, P. L. Negotiating Away Mercury Emissions. Chem. Eng. News 2001, 79, 21. (13) (a) See ref 9, pp 23-46. Hydroxide extraction under special conditions has been reported, see: (b) Herriott, A. W.; Picker, D. On the Mechanism of Phase Transfer Catalysis. Tetrahedron Lett. 1972, 44, 4521. (c) Dehmlow, E. V.; Slopianka, M.; Heider, J. Phase Transfer Catalysis in Strongly Alkaline Media: Notes on the Extractability of Hydroxyl Ions and on the Stability of Catalysts. Tetrahedron Lett. 1977, 27, 2361. (14) For a given pair of anions Y- and Z-, the selectivity constant, Ksel(Z/Y) is defined as the equilibrium constant for the biphasic exchange reaction QY(org) + Z-(aq) S QZ(org) + Y-(aq), i.e., Ksel(Z/Y) ) {[Y-(aq)][QZ(org)]}/{[Z-(aq)][QY(org)]}. For a discussion on Ksel values, see: (a) Gordon, J. E.; Kutina, R. E. On the Theory of Phase-Transfer Catalysis. J. Am. Chem. Soc. 1977, 99, 3903. (b) de la Zerda, J.; Sasson, Y. Kinetic Determination of Selectivity Constants of Anion Extraction in Phase-Transfer Catalytic Systems. J. Chem. Soc., Perkin Trans. 2 1987, 1147. (15) (a) Makosza, M. Two-Phase Reactions in the Chemistry of Carbanions and HalocarbenessA Useful Tool in Organic Synthesis. Pure Appl. Chem. 1975, 43, 439. (b) Starks, C. M. Phase-Transfer Catalysis. I. Heterogeneous Reactions Involving Anion Transfer by Quaternary Ammonium and Phosphonium Salts. J. Am. Chem. Soc. 1971, 93, 195. (16) Toib, L.; Dermaik, S.; Michman, M.; Sasson, Y. Preparation of Quaternary Ammonium Hydroxides via a Two-Stage Anion Exchange Process. Synlett 1995, 245. (17) Dehmlow, E. V.; Thieser, R.; Sasson, Y.; Pross, E. The Extraction of Alkoxide Anions by Quaternary Ammonium Phase Transfer Catalysis. Tetrahedron 1985, 41, 2927. (18) (a) Preliminary communication: Rothenberg, G.; Wiener, H.; Lavie, Z.; Sasson, Y. Novel Synthesis of Alkali and Quaternary Onium Hydroxides via Liquid Anion Exchange: An Alternative Concept for the Manufacture of KOH and Other Hydroxide Salts. Chem. Commun. 2000, 1293. (b) Rothenberg, G.; Wiener, H.; Sasson Y.; Lavie, Z. Process for the Preparation of Metal and Quaternary Onium Hydroxides. Isr. Pat. Appl. 133622 (Dec 20, 1999). (19) The distribution curve depicts the partitioning of a compound between two phases. It represents the concentration of the chemical species in one phase vs its concentration in the other. When an increase in the concentration of the chemical species in one phase does not influence its concentration in the other, the phase is said to be saturated, and the curve displays a saturation plateau. (20) Cf. ref 17, where primary alcohols enhanced basicity extraction three and four times better than secondary alcohols. (21) Murto, J. In The Chemistry of the Hydroxyl Group; Patai, S., Ed.; Wiley-Interscience: New York, 1971; p 1106. (22) Dehmlow, E. V.; Thieser, R.; Sasson, Y.; Neumann, R. Diols as Effective Cocatalysts in the Phase Transfer Catalysed Preparation of 1-Alkynes from 1,2-Dihalides. Tetrahedron 1986, 42, 3569. (23) Agarwal, B. R.; Diamond, R. M. The Extraction of Tetraalkylammonium Hydroxides and the Solvation of the Hydroxide Ion. J. Phys. Chem. 1963, 67, 2785. (24) Please refer to the Supporting Information for the solubility and phase data for the KCl/KOH/water system. (25) Naturally, with the discussed concentrated solutions, the equilibrium constants are treated using activities rather than concentrations. (26) When a quat/alcohol ratio of 1:1 was used, the extractant capacity was not limited by the alcohol concentration, and the main

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extracted species was indeed Q+RO-. The slight increase in extractant capacity shown by the diols can be attributed to additional solvation of the Q+RO- ion pair. (27) Sasson, Y.; Neumann, R.In Handbook of Phase Transfer Catalysis; Sasson, Y., Neumann, R., Eds.; Chapman & Hall: New York, 1997; pp 510-512. (28) A° kerlof, G.; Short, O. Solubility of Sodium and Potassium Chlorides in Corresponding Hydroxide Solutions at 25 °C. J. Am. Chem. Soc. 1937, 59, 1912.

(29) For a discussion, see: Moore, W. J. Physical Chemistry, 4th ed.; Prentice Hall: Englewood Cliffs, NJ, 1962; pp 157-158.

Received for review May 9, 2001 Revised manuscript received September 10, 2001 Accepted September 14, 2001 IE0104161