J. of Supercritical Fluids 37 (2006) 384–389

Enantioseparation of ibuprofen by supercriticalfluid extraction

Peter Molnar a, Edit Szekely a, Bela Simandi a,∗,Sandor Keszei b, Judit Lovasz a, Elemer Fogassy b

a Department of Chemical Engineering, Budapest University of Technology and Economics, H-1521 Budapest, Hungaryb Department of Organic Chemical Technology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary

Received 15 June 2005; received in revised form 24 August 2005; accepted 17 October 2005

Abstract

In this work influencing factors of enantioseparation of racemic ibuprofen via partial diastereomeric salt formation with R-(+)-phenylethylamineand subsequent supercritical fluid extraction (SFE) of the unreacted enantiomers were studied. It was demonstrated that conditions for diastereomericsalt formation and sample preparation (amount of resolution agent and achiral support, quality of solvent) have deep effects on SFE and onaiuar©

K

1

poiataierhDso[s

0d

chievable resolution efficiency (F). Optimal value of the resolution agent/racemic ibuprofen molar ratio is 0.5. Addition of achiral supportmproves significantly the quality of the extraction bed and reduces the extraction time, but it has negligible effect on F. Solvent effects were studiedsing different organic solvents (acetone, dichloromethane, ethyl alcohol, ethyl acetate, methyl alcohol, methyl ethyl ketone and tetrachloromethane)nd were compared to resolution in melt and suspension. It was demonstrated that solvents have important effect on F as well as on initial dissolutionates. Among SFE control parameters only pressure has significant effect on F precluding the possibility of pure density dependence.

2006 Elsevier B.V. All rights reserved.

eywords: Enantioseparation; Supercritical fluid extraction; Ibuprofen; Carbon dioxide; Sample preparation

. Introduction

Chiral molecules called enantiomers rotate the angle of plane-olarized light in diverse direction; moreover, many of theseptically active molecules may have radically different biolog-cal activities. Breaking the racemic composition (consisting ofdverse enantiomers in ratio of 1:1) and producing pure enan-iomers are essential to reduce the chemical loading affect, and tovoid the possible harmful impact of the unwanted one. Accord-ng to FDA, since 1992, almost no racemates are allowed tonter the market as new medicines [1]. Since the first sepa-ation experiments made by Pasteur [2], numerous techniquesave been developed for the resolution of racemic compounds.espite this, worldwide, 55% of chiral products in 2002 were

till generated by traditional technologies, mainly by diastere-meric salt formation followed by fractionated crystallization3]. Using supercritical fluid extraction (SFE) for enantiomereparation of racemic compounds is a sufficiently new way to

∗ Corresponding author.

obtain optically active mixtures. The first experiments werecarried out by Hungarian engineers [4] who recognized thatunreacted enantiomers could be dissolved from beside par-tially created diastereomeric salts by SFE. This method wasapplied for the resolution of several chiral acid–base pairs [5],moreover, for the resolution of molecules creating complexeswith the resolution agent [6]. The use of carbon dioxide assupercritical solvent has the advantage of avoiding hazardous,organic solvents. Producing absolutely solvent free enantiomersmakes the resolution–extraction technique a great tool of greenchemistry.

Ibuprofen (IBU), as non-steroidal anti-inflammatory drug, isgenerally used as active compound against fever, inflammation,etc.; however, S-(+)-IBU is much more effective than R-(−)-IBU [7]. Several methods can be found in literature to producehigh purity S-ibuprofen, like enantioselective catalysis or enzy-matic reactions [8]. In situ chiral derivatization of ibuprofen toobtain diastereomeric amides and their subsequent SFE was alsoinvestigated by Spanish scientists [9].

In this work (±)-ibuprofen was chosen as a model compoundto study the influence of different sample preparing methods and

E-mail address: simandi@mail.bme.hu (B. Simandi). SFE conditions on the extraction and separation efficiency.

896-8446/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2005.10.009

P. Molnar et al. / J. of Supercritical Fluids 37 (2006) 384–389 385

Nomenclature

a, b, c constantsee enantiomeric excess (Eq. (1))F resolution efficiency (Eq. (3))FEA R-(+)-phenylethylamineIBU ibuprofenmCO2 relative amount of carbon dioxide (g) to racemic

IBU (g)mrel relative amount of IBU (g) to the support surface

(m2)mr molar ratio (Eq. (4))P pressure (bar)R R-enantiomerscCO2 supercritical carbon dioxidess specific dissolution rate (Eq. (7))S S-enantiomerT temperature (◦C)Y yield (Eq. (2))

SubscriptsE extractR raffinate

2. Materials and methods

2.1. Materials

Racemic ibuprofen, 2-(4-isobutylphenyl)-propionic acid(IBU), was given by Sanofi-Synthelabo Ltd. The solubility ofpure IBU in scCO2 at the pressure of 150 bar and temperature of33 ◦C was estimated 20 mg IBU/g CO2, according to Hirunsit etal. [10].

Resolving agent, R-(+)-phenylethylamine (FEA), was pur-chased from Merck, Hohenbrunn, Germany.

CO2 was 99.5% (w/w) pure, supplied by Linde Ltd.,Budapest, Hungary.

Organic solvents (acetone (AC), dichloromethane (DCM),ethyl alcohol (EtOH), ethyl acetate (EtAC), methyl alcohol(MeOH), methyl ethyl ketone (MEK) and tetrachloromethane

(TCM)) were delivered by Reanal Ltd., Budapest, Hungary.The solvents were used to create mixture of free enantiomersand diastereomeric salts: clear solutions of IBU and FEA wereprepared in each solvents separately and mixed just before evap-oration of the solvent.

Support, Perfil 100TM (expanded and milled perlite for use asa filtering aid, specific surface area is 2.89 m2/g), was obtainedfrom Baumit Co. (Budapest), Hungary.

2.2. Experimental techniques

Calculated amount of IBU and FEA were separately dis-solved in selected solvent and the clear solutions were mixed.Perfil 100TM was added and the solvent evaporated in vacuum.The sample was dried at room temperature overnight and super-critical fluid extraction was performed with CO2 in a laboratoryscale supercritical unit (Fig. 1). Average CO2 mass flow rate was0.6 g/s. Inner volumes of extractor vessel and separator vessel are25 cm3 each. The extracts containing S-(+)-IBU in excess werecollected from separator. R-(−)-IBU rich enantiomeric mixtureswere retrieved from the raffinate. Liquid–liquid extraction wasperformed to decompose the diastereomeric salts. Yield was cal-culated afterwards (Eq. (1)).

Ymass of IBU in extract or in raffinate (g)

OE(vE

e

w

F

V

F at excg

ig. 1. High pressure extraction unit. A, CO2 vessel; C, cooler; P, pump; H, heas meter.

(%) =mass of racemic IBU used in the experiment (g)

×100

(1)

ptical rotation measurements were carried out on a Perkin-lmer 241 polarimeter, and enantiomeric excess (ee) values (Eq.

2)) of extracts and raffinates were determined from rotationalues according to prior calibration. Resolution efficiency (F,q. (3)) was calculated.

e = R − S

R + S, (2)

here R and S are the single enantiomers, and R > S

= YE × eeE + YR × eeR (3)

alues of F are between 0 < F < 1.

hanger; E, extractor vessel; PV, pressure control valve, S, separator vessel; G,

386 P. Molnar et al. / J. of Supercritical Fluids 37 (2006) 384–389

Fig. 2. Effect of molar ratio (mr) on F. P = 150 bar and T = 33 ◦C (experimentaldata were fitted by a second order polynomial).

3. Results and discussions

3.1. Determination of optimal molar ratio

Operating conditions of pressure and temperature during SFEwere set, respectively, at 150 bar and 33 ◦C. Ethanol was used assolvent at the sample preparation and 1.0 g Perfil 100TM/g (±)-IBU was added as support. As regards the effect of resolutionagent/racemic compound molar ratio (mr), defined in Eq. (4),on F, two adverse influences must be taken into account. At lowamount of FEA, the mixture of raffinate enantiomers containingR-(−)-IBU can be obtained with high purity, however with lowyield (enantiomer excess of S-(+)-IBU tends to the racemic com-position). Increasing the amount of FEA, the purity of extractedS-(+)-IBU gets higher with recessing extraction yield and eevalues of R-(−)-IBU. As a result of these opposite effects, theresolution efficiency passes through an optional maximum curveas a function of mr, shown in Fig. 2. The optimal value of mr toachieve the highest F-parameter is mr = 0.5.

mr = amount of FEA (moles)

amount of racemic IBU (moles)(4)

3.2. Description of extraction curves

nasocselnc(s

Y

Fig. 3. Kinetics of supercritical fluid extraction of a sample prepared in EtOH.mr = 0.5, P = 150 bar and T = 33 ◦C (experimental data were fitted by Eq. (5),R2 = 0.9998).

3.3. Optimal settings of extraction operational parameters

The advantage of extraction with supercritical solvent lies inthe changeable solvent power that can be varied in relativelywide range by tuning the operational conditions during extrac-tion. At the same time the efficiency of separation is directlyinfluenced by these parameters. A 32 experimental design withthree levels and two factors (pressure and temperature) wascompleted with repetitions at the center point (125 bar, 39 ◦C),mr = 0.5. Experimental results are shown in Table 1. Pareto chartof the evaluated experimental design is depicted in Fig. 4 show-ing the linear (L) and quadratic (Q) dependence of F on pressureand temperature and their interactions. Extraction pressure is theonly parameter which significantly influences F at the signifi-cance level of 95%. Eq. (6) is the regression function fitted onthe experiments. These results mean that increasing of pressurecauses increase on the value of F.

F = 0.0548 + 0.00247 × P (6)

This function can also be shown on fitted surface diagram inFig. 5. In summary, a pressure of 150 bar and temperature of33 ◦C has been demonstrated to be the best parameter values asregards the resolution of (±)-IBU.

Table 1E

P

11111111111

Non ideal behavior of this resolution system was observed:ot only IBU can be dissolved in scCO2, since FEA traces werelso detected in the extract. There is supposed to have a slightolubility of the diastereomeric salts (the detected solubilitiesf diastereomeric salts in supercritical CO2, at the experimentalonditions used, are not higher than 0.02%, w/w). Therefore, amall amount of diastereomeric salts continuously pollute thextracted free enantiomers, which have to be decomposed byiquid–liquid extraction to get the final product. The fitted expo-ential extraction curve, introduced by Brunner [11], must beorrected by adding a linear dependent term in the equation (Eq.5)) to take into account the dissolving process of diastereomericalts, depicted in Fig. 3. The corrected kinetic function is:

E = a × (1 − e−b×mCO2 ) + c × mCO2 (5)

xperimental data of the investigated experimental design

(bar) T (◦C) F

00 33 0.29025 33 0.37050 33 0.42000 39 0.30025 39 0.38025 39 0.38525 39 0.35550 39 0.41000 45 0.29525 45 0.36550 45 0.425

P. Molnar et al. / J. of Supercritical Fluids 37 (2006) 384–389 387

Fig. 4. Pareto chart of 32 experimental design; dashed line represents p = 0.05(significance level of 95%).

Fig. 5. Fitted surface diagram of the experimental design.

3.4. Study of different sample preparation methodsaffecting the extraction behavior

As regards of sample preparation, factors like molar ratio,crystallization time and temperature, amount of used inert (achi-ral) support, or used different solvents, etc., may have deepeffect on the extraction rates and resolution efficiencies. Thisphenomenon is confirmed beyond. To reduce the pollution of

Fig. 6. Extraction curves of ibuprofen using different organic solvents for samplepreparation; mr = 0.5, P = 150 bar and T = 33 ◦C.

the extract by the diastereomeric salts, it is necessary to find thequickest way of retrieving almost all (99%) of the free enan-tiomers by supercritical extraction.

3.4.1. Effect of different solventsIt is demonstrated in this section that using different organic

solvents, as a medium wherein IBU and FEA can react anddiastereomeric salts can be precipitated, has effects on the yieldof extracted material at any chosen CO2 amount and on theextraction rates. Fig. 6 qualitatively shows the margins amongyields with different solvents. Table 2 gives a summary of reso-lution experiments started from different organic solvents. Theamount of used CO2 to reach 50% yield of racemic IBU (theo-retical amount of free enantiomers based on the applied molarratio: 0.5) was calculated by corrected Brunner’s equation. Spe-cific dissolution rate (ss), relevant for the solubility of IBU at theinitial point of extraction, can be determined in the case of fixedextraction pressure, temperature and CO2 mass flow. Differentvalues of ss can be calculated by Eq. (7). These values of differ-ent solvents are comparable to each other and were calculatedby the multiplication of a and b constants from the derived formof extraction equation. Factor c describes the solubilization ofdiastereomeric salts and it is supposed to be constant during eachexperiment, thus it can be neglected from the comparison. The

Table 2Yield and resolution efficiency influencing effects of different solvents, mr = 0.5, P =

Solvent CO2 [g] to reachYE = 50.0%

Precipitation occurs

DCM 120 QuicklyTCM 141 At the beginning of evaporationMEK 352 QuicklyEtAC 430 ImmediatelyMeOH 438 During solvent evaporationAC 613 ImmediatelyEtOH 621 At the end of evaporation

150 bar and T = 33 ◦C

ss [10 mg IBU/CO2 g] F R2 values of fittedextraction curves

1.25 0.404 0.9911.06 0.403 0.9990.58 0.354 0.9990.66 0.362 0.9990.64 0.381 0.9990.74 0.427 0.9960.38 0.353 0.999

388 P. Molnar et al. / J. of Supercritical Fluids 37 (2006) 384–389

specific dissolution rate has the dimension of [10 mg extract/gCO2].

ss = Y ′(0) = [a · (1 − e−b×mCO2 ) + c × mCO2 ]′mCO2 =0

= a × b + c (7)

The fastest extractions were achieved in the case of chlorinatedmethane solvents while extraction of sample prepared fromEtOH needed relatively the biggest amount of carbon dioxide. Insome cases, after pouring the solution of FEA into the solutionof IBU immediate salt precipitation was experienced before sol-vent evaporation. The slowest precipitation was observed fromEtOH. Considering the values of F, it can be clearly noticedthat selection of solvents significantly affects the resolution effi-ciency and the enantioselectivity as well.

3.4.2. Resolution without organic solventIn order to check the possibility to exploit SFE as green sepa-

ration technique [12], without solvent, different samples of IBUand FEA were prepared in absence of organic solvent. Differentways of reacting IBU and FEA were investigated:

(a) mixing the powder IBU with liquid FEA directly (roomtemperature, 1 day crystallization);

(b) mixing liquid FEA in melted IBU (room temperature, 1 day

(

aoebmtodwrf

TRimum

E

((((

Table 4Yield influencing effect of used amount of Perfil 100TM as support, mr = 0.5,P = 150 bar and T = 33 ◦C

mrel [g/m2] ss [% IBU/CO2 g] F R2

0.28 >2 0.396 0.9800.49 >2 0.381 0.9472.10 1.82 0.389 0.9993.98 0.41 0.344 0.999∞ 0.38 0.353 0.999

lies in the difficulty of making an uniform dispersion of liquidFEA into the solid/melted IBU.

3.4.3. Effect of used amount of supportThe effect of mrel (the mass of IBU/surface of the support at

resolution experiments) on the chiral recognition was investi-gated in our former study [13]. It was found that a small amountof added inert support (Perfil 100TM, active carbon) can sig-nificantly affect the enantiomeric excess of extracted mixtureresolving racemic tetramisole. The possibility of enantioselec-tive precipitation controlled by kinetics must not be left out ofconsiderations [14]. As concerns the resolution of IBU usingethanol solvent, changes in enantioselective precipitation werenot observed using Perfil 100TM, however we found observablechanges in ss, shown in Table 4. The solubility of pure IBUin scCO2 at the pressure of 150 bar and temperature of 33 ◦Cwas estimated 2 × 10 mg IBU/g CO2, according to Hirunsit etal. [10]. While ss decreases with mrel, the used Perfil 100TM

support is supposed to have an extraction-surface booster effect.The more added Perfil 100TM, the faster the extraction of solublematerials.

4. Conclusions

The traditional, Pope-Peachy type, resolution technique ofibuprofen with R-(+)-phenylethylamine has been combined withsacttpobIuptttthifipIt

crystallization);(c) mixing liquid FEA in melted IBU (room temperature, 29

days crystallization);d) mixing liquid FEA in melted IBU (55 ◦C, 1 day crystalliza-

tion).

Results are shown in Table 3. Comparing experiments (a)nd (b), actually the same enantioseparation efficiencies werebtained with same specific dissolution rates. Experiment (c)mphasizes the importance of crystallization time in comparisony (b). It suggests that formation of chemical equilibrium needsore time and results in favorable enantioselectivity. At the crys-

allization temperature of 55 ◦C (slightly above the melting pointf racemic IBU) the same F-value was achieved within only 1ay crystallization (d): the favorable diastereomeric equilibriumas achieved much faster. The enantioselectivity and dissolution

ates are comparable to those obtained after sample preparationrom organic solvents. However, the limitation of this process

able 3esolution without organic solvent, mr = 0.5, P = 150 bar and T = 33 ◦C; (a) mix-

ng the powder IBU with liquid FEA directly (room temperature, 1 day); (b)ixing liquid FEA in melted IBU (room temperature, 1 day); (c) mixing liq-

id FEA in melted IBU (room temperature, 29 days); (d) mixing liquid FEA inelted IBU (55 ◦C, 1 day)

xperiment ss F R2

a) 0.37 0.315 0.999b) 0.32 0.314 0.999c) 0.92 0.348 0.999d) 1.78 0.340 0.999

upercritical fluid carbon dioxide extraction to separate the unre-cted enantiomers and the diasteromeric salts. The extractsontaining S-(+)-IBU in excess were collected from separa-or. R-(−)-IBU rich enantiomer mixtures were retrieved fromhe raffinate. Liquid–liquid extraction was performed to decom-ose the diastereomeric salts. A molar ratio of 0.5, a pressuref 150 bar and temperature of 33 ◦C have been demonstrated toe the best parameter values as regards the resolution of (±)-BU. Sample preparation for resolution has been investigatedsing different organic solvents and different amounts of sup-ort. Salt precipitations from chlorinated methane solvents ledo the highest specific dissolution rates (ss values), while extrac-ions performed on samples prepared in ethanol were the slowesto reach 50.0% yield. Selection of solvents significantly affectshe resolution efficiency and the enantioselectivity as well. Itas been also demonstrated that amount of support is anothermportant factor influencing the ss values. The more added Per-l 100TM, the faster the extraction of soluble materials. Samplereparation without organic solvent over the melting point ofBU resulted in good enantioselectivity and satisfying extrac-ion bed structure as well. This method for producing optically

P. Molnar et al. / J. of Supercritical Fluids 37 (2006) 384–389 389

active mixtures opens a novel route for developing an environ-mentally clean technology for the resolution of ibuprofen.

Acknowledgement

This research work was supported by the Ministry of Educa-tion of Hungary, OTKA (T0402805).

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