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Separation Science and Technology, 40: 1555–1566, 2005Copyright # Taylor & Francis, Inc.
ISSN 0149-6395 print/1520-5754 onlineDOI: 10.1081/SS-200056001 Department of Chemical and Biomolecular Engineering, GlaxoSmithKline Australia Ltd., Port Fairy, Australia Department of Chemical and Biomolecular Engineering, Abstract: Ionic liquids have been proposed as replacements for volatile organicsolvents (VOSs) by a range of authors, due to their very low vapor pressure, abilityto dissolve a range of organic, inorganic, and organometallic compounds, immiscibilitywith water, and ability to form biphasic systems depending on the choice of cation/anion combination making up the ionic liquid. In this study the room temperatureionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] was syn-thesized and a range of physical properties including the interfacial tension, viscosity,and density determined. The distribution of tyramine and 2-methoxyphenethylamine(MPEA) as a function of pH was determined for the [bmim][PF6] system. This wascompared to distribution data obtained for these solutes in conventional organic Received August 26, 2004, Accepted January 9, 2005Address correspondence to Geoffrey W. Stevens, Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia.
E-mail: [email protected] solvent/water systems: xylene/tributylphosphate (TBP)/water and xylene/Benzylalcohol (BA)/water.
Keywords: 1-Butyl-3-methylimidazolium hexafluorophosphate, ionic liquid, organicsolvents, physical properties, Tyramine, 2-methoxyphenethylamine, Solvent extraction Many industries, including the pharmaceutical industry, utilize organicsolvents to facilitate separation, purification, and concentration of naturalproducts or compounds of interest. In the insecticide industry, methanol isused for the extraction of pyrethrum (1); in the fragrance industry, hexaneis used to extract essential oils (2), and in the extraction of estrogens, organicsolvents such as ethyl acetate, acetone, tetrahydrofuran, acetonitrile, andcyclohexanol are used (3). However, increasing environmental andoccupational health and safety concerns have prompted a search for new,benign solvents in the hope of replacing the highly volatile and toxiccompounds currently in use.
Room temperature ionic liquids (RTILs) that consist of organic cations and inorganic anions are being explored as potential environmentally benignsolvents (4 – 6). Their negligible vapor pressure (7) is of interest as this wouldsignificantly reduce the risk for operators and losses due to solvent evapo-ration. 1-Butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] hasbeen successfully used for the extraction of erythromycin-A and for theRhodococcus R312 catalyzed biotransformation of 1,3-dicyanobenzene(1,3-DCB) in a liquid – liquid two-phase system (6). It has also been studiedwith water as a biphasic extraction system involving partitioning of chargedand uncharged aryl moieties as compared to their partitioning in traditionalorganic solvent-water system (8). 1-alkyl-3-methylimidazolium hexafluoro-phosphate ([Cnmim][PF6], n ¼ 4, 6, 8) RTILs have also been investigatedfor the extraction of Naþ, Csþ and Sr2þ from aqueous solutions, in the hopeof reducing the use of large quantities of volatile organic solvents (VOSs)in the hydrometallurgy industry (9).
There are many ionic liquid systems, depending on the combination of cation and anion. Cations such as 1,3-dialkylimidazolium, alkylpyridiniumor tetraalkylammonium can pair up with anions such as hexafluorophosphate,tetrafluoroborate, or bis(trifluoromethylsulfonyl)imide, which are only a fewexamples, to form different ionic liquids. Varying the alkyl substituents inthe cation that is paired to any particular anion would yet again introduceother ionic liquids. Physical properties such as viscosity, density, meltingpoint, and water miscibility alter, depending on the combination of cation andanion (10).
In this work, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim] [PF6] was chosen for investigation because it was initially reported to be water Partitioning Behavior of Tyramine and 2-Methoxyphenethylamine immiscible (11, 12) (since this study was undertaken water solubilities of up to2.3 wt% (13) have been reported), liquid at room temperature (14, 15), stable(15 – 17), and less expensive than other water immiscible ionic liquids (18).
Some researchers have since then shown that this material is unstable inwater (19, 20) and this paper will present further results on the instability ofthis particular RTIL. The aim of this work is to also measure the distributionratio [bmim][PF6]/water system and compare the data with conventional systemssuch as xylene/TBP and xylene/BA.
Tyramine (99% pure) and 2-methoxyphenethylamine (MPEA) (98% pure),anhydrous methanol (99.8% pure) from Sigma – Aldrich were used asreceived in this work. Chemicals used for the synthesis of [bmim][PF6]were either AR grade or HPLC grade and were also obtained from Sigma –Aldrich, except for the high-purity nitrogen from BOC and silver nitratefrom MERCK. Xylene, tributylphosphate (TBP) and benzyl alcohol (BA) wereall LR grade from MERCK. Hydranalw-Composite 5 from Riedel-de Hae¨nwas used as received.
The 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] was synthesized by the method described by Marsh (21). The RTIL waswashed several times with water, to reduce the chloride content. It wasconfirmed that no precipitation, i.e., AgCl, formed on adding AgNO3 to thefinal wash water. The purity of the precursor [bmim][Cl] and final product[bmim][PF6] was confirmed by 1H (399.75 MHz) and 13C NMR(100.52 MHz) analyses. 19F NMR (376.14 MHz) analysis was also carriedout for the final product. Degradation of the RTIL was determined bymeasuring the fluoride content of water that is continuously in contact withthe IL. Readings were taken in triplicate, once a week for 7 weeks. Prior tomeasurements, the electrode (Model 96 – 09 Fluoride ionplus Sure – Floww)was calibrated using Total Ionic Strength Adjustment Buffer (TISAB) IIwith 1,2-cyclohexane diaminetetraacetic acid (CDTA) and fluoride standardsolutions made up by diluting the ORION Fluoride Standard 100 ppm NaFsolution. The meter used was a Model EA940 Multi-Channel BenchtopMeter and the filling solution for the electrode was a double junctionreference electrode outer fill solution (Catalog No. 900003).
A Karl Fischer titrator (Metrohm 701 KF Titrino, Switzerland) was used with Hydranalw-Composite 5 and anhydrous methanol for the titrations todetermine the water content of [bmim][PF6], xylene/TBP (1:1v/v) andxylene/BA (1:1v/v). The IL content in water was determined by first vigo-rously agitating equal volumes of [bmim][PF6] and water for 24 h, followedby the separation and weighing of the two phases. After evaporating asmuch water as possible from the aqueous phase, the small amount of IL left behind was placed under vacuum for further drying until no bubbles wereobserved. This was then weighed again to determine the IL content that hadtransferred to the aqueous phase.
Densities were determined directly from the pycnometer volume and the mass of the sample. Interfacial tensions were measured using the First TenA ˚ ngstrom (FTA˚200) instrument (USA), which was operated with the FTA˚ Video Drop Shape Software, and viscosities were determined at 218C usingthe Rheometric Scientific SR5 rheometer.
The distribution ratios were obtained by performing shake-up tests, which involved the mixing of equal volumes of the aqueous phase containing thesolute and the solvent on an orbital shaker (OM6 Orbital Mixer, Ratek Instru-ment Pty. Ltd., Australia) at 130 rpm until the system reached equilibrium(3 – 4 h). These experiments were performed at 21 + 18C. After separationof the two phases, the aqueous phase was analyzed using HPLC. The equip-ment consisted of Waters Model 590 pump, Waters 712 WISP injector, WatersM-490 programmable multi-wavelength detector and Varian 4270 integrator.
The column used was Waters Symmetryw C18 5 mL 3.9 Â 150 mm and thesonicator for degassing the mobile phase was a Soniclean 120HT. Mobilephase was made up of Milli-Q water, acetonitrile (HPLC grade from EMScience), glacial acetic acid (HPLC grade from Science Supply), trifluoroace-tic acid (100% HiPerSolve from MERCK), and ammonium acetate (AR grade98.0% pure from MERCK). Water was distilled and deionized using aMilli-Q water filtration system from Millipore. All chemicals were used asreceived without further purification. The flow rate for the mobile phasewas 1 mL/min and detection for both tyramine and MPEA was at 280 nm.
The solute in the solvent from all systems was back-extracted into 10% v/vglacial acetic acid in one wash, which was then analyzed quantitatively byHPLC.
The physical properties of [bmim][PF6], xylene/TBP and xylene/BA arepresented in Table 1. The 1H NMR spectra confirmed the cation in theprecursor [bmim][Cl] and final product [bmim][PF6] and showed thatthere was no or very limited amount of residual water in the ionic liquid.
19F NMR was also used to confirm [bmim][PF6]. The chemical shifts fromthese NMR analyses were in agreement with Huddleston (22). NMR, however,does not detect trace levels of chlorine anion (23), and Seddon et al. (11) drewattention to the fact that low concentrations of chloride in the ionic liquidscould increase the viscosity significantly. Therefore to ensure chloride con-tamination was essentially negligible or minimal in the final product, AgNO3was added in the last wash of the synthesis to ensure that no visible precipitate(AgCl) was present. It has, however, been reported that a chloride content of1 – 6% w/w is present in the ionic liquid if the preparation method involves Partitioning Behavior of Tyramine and 2-Methoxyphenethylamine Physical properties of [bmim][PF6], xylene/TBP, and xylene/BA systems (all results were obtained from experiments conducted at 20 + 18C) aReference (40).
bReference Fine Chemicals in UK (40).
cReference (41), obtained at 208C.
dReference (26); no temperature was reported, but authors noted that the value was susceptible to change due to the presence of adsorbed moisture from the air which cansignificantly decrease the viscosity.
eReference (6); obtained at 308C.
fReference (14).
gReference (21).
hReference (13).
iReference (11).
ÃDmax cannot be determined when MPEA is the solute for the pH range studied.
†Cone and plate (Since [bmim][PF6] was much more viscous that VOSs, it was possible to avoid using bob and cup, which required a lot more of sample to obtainaccurate measurements.) metathesis from the corresponding chloride prescursor (23). Hence, eventhough AgCl precipitate was not observed in the final wash, there still maybe trace levels of chloride anions, and as Marcus (24) pointed out for anymaterial, absolute purity is difficult to achieve.
In liquid – liquid extraction systems, high interfacial tension is desired as it enhances coalescence and reduces loss of the aqueous phase in thesolvent and vice versa. Comparing the three systems, xylene/TBP hadthe highest interfacial tension (13.3 mN/m) followed by [bmim][PF6](10.6 mN/m) then xylene/BA (7.2 mN/m), which implied that phase separ-ation between xylene/TBP and water would occur quickest. The viscosityof [bmim][PF6] was approximately 200 times that of xylene/TBP(1.61 cP) and xylene/BA (1.85 cP), which made the IL more difficult tohandle.
It was important to determine the water content of the samples since water can effectively change the physical and chemical properties of the solvents(11, 24 – 27), especially in the case of [bmim][PF6] because of its hygroscopi-city. It was also necessary to know the solubility of the solvents in the aqueousphase because if the solubility was high there would be limitations to theextraction of solutes in the solvent phase. Hence, mutual solubilities were con-sidered in these studies. In comparison to Anthony et al. (2.0 wt.%@258C)(13) and Marsh et al. (2.7 wt.%@188C) (21) the solubility of [bmim][PF6]in water obtained was 2.2 wt.%@218C, which was reasonable consideringthe difference in temperature conditions. The VOSs were virtually insolublein water except for benzyl alcohol which on its own had a solubility of4 wt.% (28). The water content in the IL was found to be 0.17 wt.% aftersynthesis, but after saturation the water content increased to 2.04 wt.%. Thewater content of saturated ionic liquid was in agreement with Marsh et al.
(21), but about 0.5 wt.% higher than Seddon et al. (11) and 0.3 wt.% lowerthan Anthony et al. (13). To obtain the water content of saturated ionicliquid, it was necessary to vigorously agitate the ionic liquid and water phasesfor several hours in order to achieve equilibrium before measurement. Anthonyand coworkers (13) mentioned that even stirring the two phases for 24 hwithout interrupting the interphase would be insufficient to reach equilibrium.
Hence, the relatively small difference (0.24 wt.%) between Anthony’s groupand ours confirms that at equilibrium [bmim][PF6] can absorb about 2.0 wt.%water. Similar to the ionic liquid, the two organic systems also increasedin water content after saturation. However, there was no indication that thewater in these organics would cause any instability to the solvents. Severalresearchers (9, 19, 20, 29) observed that [bmim][PF6] was unstable in thepresence of water or nitric acid. Out of interest, we investigated its instabilityby measuring the fluoride concentration of the aqueous phase for both washedand unwashed samples of [bmim][PF6] using a fluoride electrode and discov-ered that the fluoride concentration increased over time (Fig. 1). Thisconfirmed that when [bmim][PF6] remained in contact with water, it wasunstable.
Partitioning Behavior of Tyramine and 2-Methoxyphenethylamine Fluoride concentration in water, which had been continuously shaken with washed and unwashed [bmim][PF6] samples over time.
The partitioning behavior of tyramine and MPEA as a function of pH was determined in the three different liquid – liquid systems, and this was expressedin terms of distribution ratios (D). Distribution ratio is defined as follows: Concentration of solute in solvent phase (C Þ Concentration of solute in aqueous phase (C Þ Tyramine and MPEA (see Figs. 2 and 3 for structures) were used as solutes because they are representative of compounds used in some of thenatural products industries. The systems studied were (1) [bmim][PF6], (2)xylene/TBP, and (3) xylene/BA as the nonaqueous phase and water,sometimes with small amounts of sodium hydroxide or hydrochloric acid asthe aqueous phase. The results are summarized in Figs. 2 and 3 and Table 1.
In the organic systems using tyramine as solute, the maximum distri- bution ratio, Dmax % 1.07 in xylene/TBP and Dmax % 3.16 in xylene/BA, occurred at about pH 10. In [bmim][PF6], Dmax % 0.53 was displayedat around pH 9. In both xylene/TBP and xylene/BA systems, the distributionratio decreased to essentially zero in pHs below 7 and pHs above 13. This wasdue to tyramine having two ionizable groups, an amine group and a phenolicgroup. Various researchers have determined the pK values of p-tyramine.
There are a few discrepancies with the pK values assigned to their respectiveamine and phenolic groups amongst the various authors. Despite the conflic-ting designation of pK values to the groups, both values remain relativelyclose for example, 9.3 and 10.9 (30), 9.77 or 9.53 (depending on method),and 10.78 (31), 9.37 and 10.70 (32), 9.74 and 10.53 (33), 9.3 and 10.9 (34).
When the pH is 9.3 or lower and 10.7 or higher the tyramine molecule is Distribution ratio vs. pH with tyramine as solute in [bmim][PF6]/water, xylene/TBP/water and xylene/BA/water systems.
ionized, and thus has a high solubility in the aqueous phase, resulting indecreasing distribution ratios on either side of pH 10.
The inability to load tyramine appreciably into [bmim][PF6], and the tendency for tyramine to remain dissolved in [bmim][PF6] at pH , 7, makethe ionic liquid unsuitable for extracting tyramine. The distribution ratiosremained at 0.27 at pH , 7, indicating that at acidic pHs it is possiblethat there was a cation – exchange occurring in which the positively chargedtyramine molecule partitioned into the IL and the dialkylimdazolim[bmim]þ into the aqueous phase, a mechanism that has been suggested in lit-erature (35 – 37). In this mechanism the counter ion in the aqueous phase atthese lower pHs would be the chloride anion from the HCl added for pHadjustment. In addition to ion-exchange considerations, studies (9, 38) haveshown that in the presence of acids such as HNO3, leaching of [bmim]þ tothe aqueous phase can occur, especially at high concentrations (8M). However,Visser and coworkers (9) have reported that the use of HCl resulted in nodetection of [bmim]þ in the aqueous phase, even at a concentration of 8M.
Hence, we can assume that using HCl (0.05 M) for pH adjustments in ourstudies would cause minimal leaching.
Figure 1 shows a comparison of all the solvent systems using tyramine as solute. The error bars in the plots from Fig. 2 were obtained from triplicateHPLC readings for each of the shake-up tests for the varying pHs. When afew of the experiments were repeated for the VOS systems with back extrac-tion, the error ranged from 1.1 – 3.2%.
Partitioning Behavior of Tyramine and 2-Methoxyphenethylamine Distribution ratio vs. pH with MPEA as solute in [bmim][PF6]/water, xylene/TBP/water and xylene/BA/water systems.
Similarly, when MPEA was used as a solute, some partitioned in the ionic liquid in the acidic pHs. However, since MPEA has only one ionizable group,as pH increased MPEA became less ionized and partitioned into thenonaqueous phase. The pK value of MPEA is reported as 10.20 at 258C(33). Hence, with all systems studied, MPEA displayed increasing distributionratios as pH increased, especially when pH . pK  10. From Fig. 3, it can beobserved that regardless of pH in the [bmim][PF6] system, MPEA cannot becompletely back extracted into the aqueous phase with only one washbecause of its solubility in the ionic liquid at pH , 7, similar to thepossible cation-exchange mechanism as mentioned for the tyramine solute.
Hence, several washing stages will be necessary to recover the MPEA fromany loaded in the ionic liquid system, compared to the VOS systems.
Using either tyramine or MPEA as the solute, the following systems seemed most effective for solvent extraction in decreasing order: xylene/BA . xylene/TBP . [bmim][PF6]. Generally these systems displayedhigher distribution ratios for MPEA than tyramine. Stripping of tyramine andMPEA from xylene/BA and xylene/TBP systems were attained with a0.1 – 7.4% error in the material balance.
In this work, the low distribution ratios obtained from [bmim][PF6] forextracting tyramine and MPEA from aqueous solution suggest that it is notideal for replacing the VOSs. Also, the high viscosity of the ionic liquid as compared to the VOSs, makes it difficult to handle. Modifying the anioncounterpart in the ionic liquid to say, bis((trifluoromethyl)sulfonyl)imide[Tf2N], would reduce the solvent viscosity (39), which might increase itspotential as a solvent extractant. However, an investigation to lowerviscosity ionic liquids is required to confirm this. The instability of[bmim][PF6] when contacted with water as reported by various researchers(9, 19, 29) is also of concern since it produces volatiles such as HF, POF3,which can etch or dissolve glassware and corrode equipment. Studies intoother ionic liquids that are immiscible with water but remain stable areessential if these nonvolatile, nonflammable liquids are to be used forliquid – liquid extraction processes.
This work was made possible from the financial support of the AustralianResearch Council (ARC), GlaxoSmithKline Australia Ltd. (GSK), and Parti-culate Fluids Processing Centre (PFPC). Dr. Peter Scammels and coworkers atVictorian College of Pharmacy at Monash University assisted in the synthesisof the ionic liquid, which was greatly appreciated.
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