Separation and Purication Technology 57 (2007) 483494Extraction
of lactic acid by phosphonium ionic liquidsJ an Mart ak,Stefan
SchlosserInstitute of Chemical and Environmental Engineering,
Faculty of Chemical and Food Technology,Slovak University of
Technology, Radlinsk eho 9, 81237 Bratislava,
SlovakiaAbstractTetraalkylphosphonium ionic liquid (IL) with a bis
2,4,4-trimethylpentylphosphinic anion (Cyphos IL-104) is an
effective extractant of lacticacid (LA) achieving at low LA
concentrations the distribution coefcients for aqueous systems
above 40. L/L equilibrium data for pure CyphosIL-104 and its
solutions in n-dodecane have been measured. With increasing acid
concentration the value of the distribution coefcient of
LAdecreases. Cyphos IL-104 extracts only undissociated molecules of
lactic acid (LAH) via H-bonding. Increase in the concentration of
IL-104in n-dodecane results in increasing distribution coefcient of
LA and the water solubility. The high water content in the solvents
with CyphosIL-104 is connected with the formation of reverse
micelles. An interesting phenomenon of the liberation of water from
the solvent in extractionof LA has been observed. It is suggested
that splitting of the reverse micelles due to the formation of
LAH/IL complexes occurs. With increasingtemperature the values of
the distribution coefcients of LA practically does not change or
only slightly increases. In contrary to this, the increasein
temperature decreased the water solubility in solvents containing
IL-104. This is interpreted by the lower stability of reverse
micelles at highertemperatures. Based on experimental equilibrium
data the formation of stoichiometrically dened complexes with the
structure (LAH)p(IL)(H2O)2and (p, 1, 2) stoichiometry, where p is
in the interval from 1 to 3, is suggested. The proposed model ts
the equilibrium data well and indicates thedomination of the (2, 1,
2) complex at medium aqueous acid concentrations in the interval
from 0.2 to 2 kmol m3. There are two mechanisms ofwater extraction
into the solvents with Cyphos IL-104: the formation of reverse
micelles, and the formation of hydrated complexes of LAH withIL. A
sharp decrease in the viscosity of solvents with IL-104 with
increasing concentration of water or LA was observed. 2006 Elsevier
B.V. All rights reserved.Keywords: Extraction; Lactic acid;
Phosphonium ionic liquids; L/L equilibrium; Modelling1.
IntroductionThere is a continuing interest in application of ionic
liquids(ILs) in extractive separations documented in papers [14].
ILsare composed of an organic cation and either an organic or
aninorganic anion. They remain liquid over a wide range of
tem-peratures, including roomtemperature [5,6]. ILs are a
newgroupof solvents of great interest, which have been recently
studiedas potential green solvents. Practically zero vapour
pressureof the most ILs and temperature stability make them
attractivesolvents in many applications, e.g. in synthesis. Ahigher
viscos-Abbreviations: IL, ionic liquid; LA, lactic acid; LAH,
undissociatedmoleculeof lacticacid; (p, 1, 2), stoichiometryof
thecomplexinthesol-vent phasecontainingpmoleculesofLAH,
1moleculeofionicliquidand2moleculesofwater; TOMAC,
trioctylmethylammoniumchloride;
TIOPO,tris(2,4,4-trimethylpentyl)phosphine oxide, which is in fact
triisooctylphosphineoxideCorresponding author. Tel.: +421
252496743; fax: +421 252496920.E-mail addresses:
[email protected] (J. Mart ak),[email protected] (S.
Schlosser).ity at room temperatures could be their less favourable
property[7]. In the most papers on extraction separations with ILs
areused ILs with imidazolium cation. In metal extractions ILs
arefrequentlyusedonlyasadiluentfortheextractant[810]asshown also in
the review [3]. Also in some papers on extractionof organic acids
ILs served as diluents [11,12]. A new group ofILs with
tetraalkylphosphoniumcation has been developed [13].Apotential of
their application in extraction of organic acids wasshown in papers
[1,1417].Solvent extraction of organic acids by ionic liquids was
stud-ied by Huddleston et al. [18] and in papers [1,1417], of
ery-thromycin by Cull et al. [19], and of chlorophenols by Bekouet
al. [20]. Extraction of several amino acids studied Smirnovaet al.
[12]andWanget al. [21]. Separationofaromaticandaliphatic
hydrocarbons by imidazolium and pyridinium ILs hasbeen tested by
Meindersma et al. [22,23]. Eight phosphoniumand imidazolium ILs, or
solvents containing ILs were tested forthe extraction of several
organic acids in paper [14].
Extractionoflacticacid(LA)hasbeenstudiedinpapers[1417].Mat-sumotoetal.
[11]investigatedextractionofcarboxylicacidsandLAbytributylphosphatedilutedinimidazoliumILsand1383-5866/$
see front matter 2006 Elsevier B.V. All rights
reserved.doi:10.1016/j.seppur.2006.09.013484 J. Mart ak,S.
Schlosser / Separation and Purication Technology 57 (2007)
483494Nomenclaturec molar concentration of the solute (lactic acid)
inundissociated form (kmol m3)caanalytical (overall) molar
concentrationof thesolute (LA) considering both dissociated
andundissociated forms (kmol m3)cp,1molar concentration of the
complex(LAH, IL) with stoichiometry (p, 1) in the solvent(kmol
m3)c0,1molar concentration of the free ionic liquid in thesolvent
(kmol m3)cIoverall molar concentration of the ionic liquid inall
forms in the solvent (kmol m3)D distribution coefcient, dened by
Eq. (9)Dadistribution coefcient, dened for analyticalaqueous
concentration of LA in the aqueousphase, caFH enthalpy (J
mol1)Kp,1extraction equilibrium constant of the (p, 1)complex
(m3pkmolp)m mass (kg)M molar mass (kg kmol1)p number of solute
molecules (lactic acid) in (p, 1)acid/IL complexR gas constant
(8.314 J K1mol1)S entropy (J K1mol1)T temperature (K)V volume (m3)w
mass fractionz loading of the ionic liquid dened by Eq. (7)Greek
symbols mass balance ratio of acid dened by Eq. (10) dynamic
viscosity (mPa s) density (kg m3)Indicesdry dry solventF feed
(aqueous) phaseH2O overall waterhydrat H2Ohydration water in the
(LAH, IL, H2O)complexI ionic liquidk solute (acid)o initial valueS
solvent (organic) phase* at equilibriumtested also solvent toxicity
for bacteria. Toxicity of ionic
liquidsandorganicsolventstolacticacid-producingbacteriastudiedMatsumoto
et al. [24].The phosphonium IL with the
2,4,4-trimethylpentyl-phosphinic anion (Cyphos IL-104) is a new
effective extractantof LA [14,17] with a considerably higher value
of the distribu-tion coefcient comparing to liquid extractants
described earlier.Mainly the synergetic effect of the anion is
responsible for theincreased distribution coefcient of LA by Cyphos
IL-104. Theextraction of LA with this IL is carried out by a
coordinationmechanism. The presented results document a potential
in thedesigning of ionic liquids with specic properties via a
combi-nation of the cation and the anion exhibiting synergy effect.
Ahigh solubility of water in the studied phosphonium ILs is
con-nected with the formation of reverse micelles in the solvent.
Adual mechanism of water extraction to hydrophobic phospho-nium ILs
was identied. This involves incorporation of
waterintoreversemicelles,andformationofthehydratedcomplexof LA with
IL. The extraction of LA causes splitting of reversemicelles with
liberation of water from the solvent. In the sat-urated solvent
only hydration water remains in the complex ofLA with IL with the
dominating structure (LAH)2(IL)(H2O)2[17].Extraction of azo dyes
fromwaste solutions of leatherindustry by N-butyl,N-methyl
pyrrolidinium
bis(triuoro-methanesulfonyl)amideILisreportedinpaper[25].Alargeeffort
is devoted to heavy and radioactive metals extraction withsolvents
based on IL diluents as reviewed in paper [3].Transport of amines
and neutral organic substances throughliquid membranes from IL is
of concern in the papers of Fortu-nato [26], and Branco [27].
Pertraction of organic acids throughliquid membranes was described
in papers [15,16], where pro-longed operation of phosphonium IL was
documented. Trans-port of organic acids facilitated by enzymatic
reactions on L/Linterfaces, using IL as a liquid membrane, has been
studied byMiyako et al. [28,29].The aimof this work was a more
detailed study of the solventproperties of the phosphonium ionic
liquid Cyphos IL-104
withbis2,4,4-trimethylpentylphosphinateanionwhosestructureisshown
in Fig. 1 in extraction of lactic acid. Pure IL-104 and
itssolutions in n-dodecane have been studied.2. TheoryPhosphonium
ILs with a hydrophobic anion form complexeswith LA and other acids
via a coordinating mechanism showninworks[14,16,17].
Thisfactwillbesupportedwithfurtherdata presented in this paper.
Monobasic carboxylic acids usu-ally form in reactive extraction
complexes containing a
variablenumberofacidmoleculesandonlyonemoleculeofextrac-tant, the
so called (p, 1) complexes. This has been found, forexample in the
extraction of carboxylic acids by trialkylamines[30,31]. Based on
the data presented in this paper, as discussedin Section 4.2, the
formation of stoichiometrically dened (p,1, 2) complexes with the
structure (LAH)p(IL)(H2O)2is sug-gested wherep is from the interval
from 1 to 3. A simpliednotationof thestoichiometry(p, 1) will
alsobeused. Thereactiveextractionof
LAbytheionicliquidIL-104canbedescribedbythefollowingreactionof
ahydratedcomplexformationpLAH +2H2O +IL (LAH)p(IL)(H2O)2(1)J. Mart
ak,S. Schlosser / Separation and Purication Technology 57 (2007)
483494 485Fig. 1. Structure of trihexyl(tetradecyl)phosphonium bis
2,4,4-trimethylpentylphosphinate (Cyphos IL-104).as will be
discussed in Section 4.2. The stripping of acid fromIL to an
aqueous alkaline solution follows the
reaction(LAH)p(IL)(H2O)2+pOHp(LA)+IL +(p +2)H2O(2)These reactions
proceed on the water/solvent or sol-vent/stripping solution
interfaces. The overbar designatesspecies in the solvent phase. The
extraction constants of individ-ual complexes based on reaction (1)
are dened by the followingrelationKp,1
=[(LAH)P(IL)(H2O)2][LAH]p[IL]=cp,1(cF)pc0,1(3)where c0,1 is the
concentration of the free ionic liquid. Becauseof an excess of
water in the aqueous phase its concentration isconsidered to be
constant and is included in the extraction con-stant. The stripping
reaction (2) proceeds quantitatively whenan excess of alkali is
maintained, as shown at the end of Section3.2. From this follows
that the value of equilibrium constant ofthe stripping reaction is
approaching innity.For the concentration of the complex in the
solvent can bederived from Eq. (3)cp,1 = c0,1Kp,1(cF)p(4)The
concentration of the free ionic liquid can be calculated fromthe
total IL concentration in the solvent after subtraction of theionic
liquid in the complexesc0,1 = cIc0,1i
p=1Kp,1(cF)p=cI1 +
ip=1Kp,1(cF)p(5)The concentration of LAin the solvent phase is
equal to the sumof concentrations of complexes multiplied by the
number of LAmolecules in the complexcS =cI
ip=1pKp,1(cF)p1 +
ip=1Kp,1(cF)p(6)The loading of IL by LA is dened asz =cScI=
ip=1pKp,1(cF)p1 +
ip=1Kp,1(cF)p(7)For the correlation of experimental data three
complexes with(LAH, IL) stoichiometry (1, 1), (2, 1), and (3, 1)
were consideredand the following equation was usedz
=K1,1cF+2K2,1(cF)2+3K3,1(cF)31 +K1,1cF+K2,1(cF)2+K3,1(cF)3(8)in
estimation of the extraction constants. Eq. (7) shows that load-ing
is independent of the IL concentration. For the
distributioncoefcient follows relationD =cScF=
ip=1pKp,1(cF)p11 +
ip=1Kp,1(cF)pcI(9)Fromthis relationresults that
thedistributioncoefcient isdependent onboththe aqueous
concentrationof undissoci-ated acidcFand the overall concentration
of IL in the solventcI.3. Experimental3.1. MaterialsCyphos IL-104
(Cytec, CA) is an ionic liquid,
whichistrihexyl-(tetradecyl)phosphoniumbis2,4,4-trimethylpentyl-phosphinate
witha puritybetter than95%. Its structure isshown in Fig. 1. In the
technical product of Cyphos IL-104,
tris(2,4,4-trimethylpentyl)phosphine oxide (TIOPO) ispresent as an
impurity in a concentration up to 5 mass%what isaround0.11 kmol m3.
CyphosIL-104ishydropho-bic what is documented by its low solubility
in water (Table 1)andiscompletelymisciblewitharomatichydrocarbons,
iso-propanol, diethylether andpartlymisciblewithalkanes,
tri-octylamineandDEHPA. CyphosIL-101(Cytec)
istrihexyl-(tetradecyl)phosphoniumchloride with a purity of 96 to97
mass%. Cyanex 272 (Cytec) is a widely used metal extrac-tant
bis-(2,4,4-trimethylpentyl)phosphinic acid with a purity ofabout 85
mass% and it is a protonated form of the Cyphos IL-104 anion.
Cyanex 923 (Cytec) is a mixture of trialkylphosphineoxides
withC6andC8alkyls witha purityof 93%. The solubilityof water in
pure Cyphos IL-104 and its solutions in n-dodecaneand in other
compounds studied together with the density of drysolvents at 25C
are presented in Table 1.All solvents have been conditioned before
experiments.Cyphos IL-101 was washed with equal volume of 0.5 kmol
m3486 J. Mart ak,S. Schlosser / Separation and Purication
Technology 57 (2007) 483494Table 1Solubility of water in pure
Cyphos IL-104 and its solutions in n-dodecane and in other
compounds studied together with the density and viscosity of dry
solvents at25CSolvent cIo,dry(kmol m3)Molar mass(g mol1)I,dry (kg
m3) I,dry (mPa s) Solubility inwater (g m3)ws,
H2O102(mass%)cS,H2O(kmol m3)Pure Cyphos IL-104 1.102 773.3 885.67
707,2 9.1 14.4 7.28Cyphos IL-104 inn-dodecane0.724 837.81 49.4a
9.24 4.360.324 788.98 3.55 1.58Pure Cyphos IL-101 1.661 519.3
889.36b 14.4 7.21Pure Cyanex 272 2.716 290.4 917.17b16 at pH
2.6c;38 at pH 3.7c1.28 0.653Pure Cyphos IL-105 1.581 549.9
896.54b72.7d3.31 1.69Pure Cyanex 923 2.369 344.6 877.56b
