s

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vaporationugh

of sodiumerefore theis observedh a

Journal of Colloid and Interface Science 282 (2005) 478–485www.elsevier.com/locate/jcis

Continuous process for singletoxygenation of hydrophobic substratein microemulsion using a pervaporation membrane

Laurent Carona,∗, Véronique Nardelloa, José Muggeb, Erik Hovingb, Paul L. Alstersb,Jean-Marie Aubrya

a LCOM, Equipe “Oxydation & Formulation”, UMR CNRS 8009, École Nationale Supérieure de Chimie de Lille BP 108,F-59652 Villeneuve d’Ascq Cedex, France

b DSM Pharma Chemicals, Advanced Synthesis, Catalysis &Development, P.O. Box 18, 6160 MD Geleen, The Netherlands

Received 28 June 2004; accepted 17 August 2004

Available online 18 November 2004

Abstract

Chemically generated singlet oxygen (1O2, 1�g) is able to oxidize a great deal of hydrophobic substrates from molybdate-catahydrogen peroxide decomposition, provided a suitable reaction medium such as a microemulsion system is used. However, high substrconcentrations or poorly reactive organics require large amounts of H2O2 that generate high amounts of water and thus destabilize thsystem. We report results obtained on combining dark singlet oxygenation of hydrophobic substrates in microemulsions with a permembrane process. To avoid composition alterations after addition of H2O2 during the peroxidation, the reaction mixture circulates throa ceramic membrane module that enables a partial and selective dewatering of the microemulsion. Optimization phase diagramsmolybdate/water/alcohol/anionic surfactant/organic solvent have been elaborated to maximize the catalyst concentration and threaction rate. The membrane selectivity towards the mixture constituents has been investigated showing that a high retentionfor the catalyst, for organic solvents and hydrophobic substrates, but not forn-propanol (cosurfactant) and water. The efficiency of sucprocess is illustrated with the peroxidation of a poorly reactive substrate, viz.,β-pinene. 2004 Elsevier Inc. All rights reserved.

Keywords:Microemulsion; Oxidation; Singlet oxygen; Pervaporation; Ceramic membrane; Dewatering

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1. Introduction

Microemulsions are thermodynamically stable mixtuof organic compound(s) (oil), water, surfactant(s), ain most cases, cosurfactant(s)[1]. These colloidal dispersions consist of droplets of oil-in-water (O/W), water-in-(W/O), or bicontinuous microdomains ranging from ab10–100 nm in diameter, which results in a transpamedium. The thermodynamic stability of such systemachieved as a result of a surfactant/cosurfactant systemerally a short chain-length alcohol that decreases interfatension down to ultralow values.

* Corresponding author.E-mail address:l.caron@boursorama.com(L. Caron).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.08.156

-l

Over the past decade, much research has been deto microemulsions as reaction media[2,3] and their indus-trial applications are steadily increasing[4,5], e.g., polymer-ization[6,7], enzymatic[8], and oxidation[9,10] reactions.Given their specific microstructure, microemulsions are asuitable media for the “dark,” i.e., nonphotochemical, peridation of organic substrates with singlet oxygen,1O2 (1�g),generated by molybdate-catalyzed hydrogen peroxideproportionation,

2H2O2MoO2−

4−−−−→pH 9–12

2H2O + 1O2 (100%). (1)

Indeed, the average droplet size in microemulsionsmaller than the mean travel distance of1O2, which isabout 200 nm in water (Fig. 1) [10,11]. Thus,1O2, whichis formed in the aqueous droplet, can diffuse freely in

L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485 479

s

fore-

tes,the

of

cks.ac-ichad-d

rusere-

ox-

elyntherble

eon

tionre-

ion

s ton bemhasl use

s

ul-

inal,logyn of

f an ofem-aremi-eatede-re-

ngted

o-bo.

-m

o-reper-

out-n a

C18en,GasAgi-P-1

ora-ssel,nd a

ane

-sing

Fig. 1. Schematic representation of the oxidation of hydrophobic substrateby the hydrophilic chemical source of singlet oxygen, viz., H2O2/MoO2−

4 ,in a water-in-oil microemulsion.

organic phase and react with the organic substrate bedeactivation. Therefore, these media are particularly appropriate to the peroxidation of highly hydrophobic substrasuch as aromatic compounds, olefins, or dienes, bysystem H2O2/MoO2−

4 as a hydrophilic chemical source1O2 [10,12].

However, such systems present two main drawbaFirst, the addition of hydrogen peroxide during the retion results in an increase of the water proportion, whmodifies the composition of the microemulsion and grually causes decreasing performance and low reactor yiel(kg product per m3 reactor). Such a drawback limits the useof the microemulsion eitherto highly reactive substrates oto relatively low concentrations of substrate. Second, theof surfactant-containing reaction mixtures hampers facilecovery of the desired products. Actually, at the end of theidation process, the reaction medium is relatively complex,since it is made up of more than six constituents, namwater, oil, surfactant, cosurfactant, catalyst, and oxidatioproduct(s). Hence, isolation of the products requires eithe addition of an extra substance, which is not desirain many cases[13,14], the inactivation or destruction of thsurfactant[15], or a tedious treatment of the microemulsiitself [10,12].

Based on these considerations, a semibatch oxidaprocess in which water is continuously and selectivelymoved from the system to maintain the initial compositof the microemulsion appears particularly attractive from anindustrial point of view. The use of a membrane procesthis end is an ideal solution, since the membrane area caadapted to the desired water-removal flux, whereas the mbrane does not interfere with the reaction itself. Therebeen increasing interest in recent years in the industriaof pervaporation membrane separation techniques[16,17]thanks to new membranes withimproved chemical inertnesand selectivity in the separation of water from mixtures. Thecombination of a pervaporation process with a microem

e-

sion as reaction medium is, however, rather new and origsince, so far, the main applications of membrane technoto (micro)emulsion treatment concern the destabilizatiomicroemulsions[18–20].

In the present paper, we report the combination opervaporation process with the dark singlet oxygenatioorganic hydrophobic substrates. The choice of the mbrane and the formulation of the microemulsion systemdiscussed first. The membrane selectivity towards thecroemulsion components is then investigated and permfluxes are measured to check the proper working of thevice, the ultimate goal of the study being the selectivemoval of water from the reaction mixture under oxidiziconditions. The efficiency of the process is finally illustrawith the peroxidation of a poorly reactive substrate.

2. Materials and methods

2.1. Chemicals

Sodium molybdate dihydrate (99%), toluene,n-propanol,lauric acid, sodium hydroxide,α-terpinene,β-citronellol,and β-pinene were all purchased from Aldrich. Hydrgen peroxide 50% (17.5 M) was purchased from ProlaSodium laurate was prepared as a 1 mol kg−1 aqueous solution by adding lauric acid (Aldrich) to aqueous sodiuhydroxide.

2.2. Procedures

In 5-ml SVL tubes the appropriate amounts of oil, csurfactant, water, catalyst, and aqueous sodium laurate wemixed. The mixtures were maintained at a constant temature (25± 0.1◦C) and allowed to stabilize.

2.3. Instrumentation

UV/visible spectrophotometry analyses were carriedwith a Varian Cary 50 spectrometer. High-performance liquid chromatography (HPLC) analyses were performed oWaters 600 chromatograph equipped with a Novapak(4-µm) column and a prefilter.Solvents were HPLC gradCH3OH and Milli-Q water. For detection and quantitatioa Waters 490E multiwavelength UV detector was used.chromatography (GC) analyses were performed on alent 6890 N chromatograph equipped with an apolar H(60 m× 0.32 mm− 0.25 µm) column.

Pervaporation experiments were conducted in a labtory-scale unit: the experimental setup consists of a vea recirculation pump, a membrane unit, condensers, avacuum pump, as illustrated inFig. 2. The liquid is circu-lated in cross-flow over the outside of the tubular membr(Sulzer, type SMS), which contains an active SiO2 layer ontop of the ceramic (γ -Al2O3) support material. On the inside of the tubular membrane, a vacuum is applied by u

480 L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485

type

d as

easof

ess arh asn,umbe

ce-s iningandgenma-fivene tois aist-

onesa,ide,ure

nescauser-

at th

mi-

d for

mul-d to

bet,and

esrd-t

mi-dia-ntn inma-

aturelystlsionrmednIs,

ctanthol-on-

it-thetra-

Fig. 2. Experimental setup for the silica membrane system.

condensers (C) and a vacuum pump from Vacuubrand,CVC2 (vacuum controller MZ2C).

3. Results and discussion

3.1. Choice of the membrane

Membrane technology is more and more often usea separation technology in modern processes[21,22]. Thiscan be explained by the high selectivity, robustness, andof operation, combined with the low energy consumptionthe separation process and the decreasing membrane pricover the past two decades. Many membrane processeavailable based on different separation principles, sucmicrofiltration, ultrafiltration, nanofiltration, gas separatioosmosis, reverse osmosis, and electrodialysis. A large nber of (organic or inorganic) membrane materials canused, depending on the intended applications.

For our application we chose a robust hydrophilicramic membrane since peroxidation of organic substratethe microemulsion system involves harsh, strongly oxidizreaction conditions. The presence of organic solventsoxidizing species such as hydrogen peroxide, singlet oxyand peroxomolybdates excludes the use of polymericterials. Ceramic membranes, which form the main class oinorganic membranes, are suitable for this application gtheir chemical and thermal stability, and their resistanca broad range of pH conditions. The investigated deviceSulzer[23] Pervap SMS (silica membrane system), consing of a hollowγ -Al2O3 cylinder with a SiO2 outer layer(Fig. 3). The microemulsified reaction mixture circulatesthe feed side of this ceramic porous (different levels mmacro and micro) material, while on the downstream sa vacuum is applied to remove the permeate (ideally pwater) as a vapor (P = 6–7 mbar).

We chose to focus on inorganic pervaporation membrarather than inorganic reverse osmosis membranes bethe latter are not yet commercially available. In addition, pvaporation offers the advantage over reverse osmosis th

e

e

-

e

e

Fig. 3. Schematic drawing of the membrane interface.

separation of water from lower alcohols (present in thecroemulsion as cosurfactant) is much better.

3.2. Choice of the microemulsion system

Several efficient microemulsions have been describethe peroxidation of hydrophobic substrates by1O2 [10,12].For an industrial process, the components of the microesion have to be inexpensive and nontoxic. With regarthese requirements, the best formulation was found tobased on sodium laurate (LS) as a cheap surfactann-propanol as the cosurfactant, and toluene as a goodchlorine-free organic solvent.

On the other hand, the addition of the catalyst Na2MoO4·2H2O to the oil/water/surfactant/alcohol mixture modifithe microemulsion and different systems may appear accoing to the amounts of sodium molybdate and surfactan+alcohol. In order to define the best composition for thecroemulsion, phase diagrams, also called optimizationgrams[1], have been elaborated. They plot the (surfacta+cosurfactant) proportion versus the catalyst concentratiothe aqueous phase. Such diagrams typically exhibit gamshaped boundaries corresponding to an interface curvinversion induced by the interaction between the cataand the surfactant, resulting in a decrease of the repubetween the surfactant polar heads. Thus, scans perfoas a function of the MoO2−

4 concentration enable detectioof transitions between mono- (Winsor IV), bi- (Winsorand II), and triphasic (Winsor III) microemulsion systemdepending on the interface curvature and on the surfaamount. All the scans were carried out with both the alcoto-surfactant ratio and the salted-water-to-oil ratio kept cstant (equal to 1 w/w).Fig. 4shows the boundaries deliming the different types of Winsor obtained by scanningsystem toluene/water/LS/PrOH with increasing concentions of sodium molybdate.

L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485 481

eofsepa-

theng

f surb-ha-pli-a/W

asin-s a

ntoinedndgh

i-n inc-let

s, ithas

atesultichsedd toof

these

n the

thetant

tra-

on-%f

cor-ul-

meinghase.ratethearti-

s

iallyonsepa-hthed and

wedandpec-

em-

en-

e

ne/

Fig. 4. Optimization diagram for the toluene/water/LS/n-PrOH/Na2MoO4system at 25◦C (n-PrOH/LS= 50/50 (w/w) and toluene/(water+ catal-yst) = 50/50 (w/w)). (") Composition of the system used for thperoxidation of β-pinene. (The region with low concentrationssurfactant+ cosurfactant could not be explored because the phaseration was too slow.)

On the basis of this binary diagram, the choice ofcomposition of the microemulsion was first made amothe three polyphasic systems to decrease the amount ofactant. Indeed, Winsor I, II, and III systems may be otained with lower amounts of surfactant than the monopsic system Winsor IV, usually used for the present apcation [10]. On a second time, with a view to set upcontinuous process, the Winsor I system, in which an Omicroemulsion is in equilibrium with an excess of oil, wpreferred to the two other polyphasic systems (i.e., Wsor II and III), since the presence of an oil phase allowstraightforward recovery ofthe lipophilic oxidation prod-ucts by continuous extraction from the microemulsion ithe oil phase. Even though Winsor I systems are obtawith lower amounts of catalyst compared to Winsor II aIII types, they can still be obtained both with relatively hiamounts of sodium molybdate (≈1 molkg−1) and with lowamounts of (surfactant+ cosurfactant) thanks to the inclnation of the gamma. Thus, the catalyst concentratioWinsor I is high enough to achieve sufficiently high reation rates. Finally, by comparing the generation of singoxygen in the three polyphasic microemulsion systemappeared that, in the presence of an excess of water p(as in Winsor II and III), most of the1O2 is not only gen-erated in the aqueous excess phase, but also deactivas there is no available substrate in that phase. This rein a loss of efficiency for the peroxidation process, whtakes place exclusively in the microemulsion phase. Baon all these considerations, Winsor I systems appearebe the most efficient ones for continuous peroxidationhydrophobic substrates with chemically generated1O2, pro-vided that the composition of the Winsor I is close toboundary Winsor I/Winsor III to keep a high rate of phaseparation. This biphasic system is not less complex thapreviously used one-phase microemulsion[10] but enables astraightforward recovery of the peroxidation products inorganic excess phase which is both catalyst and surfacfree.

-

e

d,s

-

Table 1Retention factorsR towards sodium molybdate for various feed concentions at 25◦C

CFa

(mmol L−1)CP after 2 h(mmol L−1)

CP after 3 h(mmol L−1)

αcatalyst/water R

100 0.09 0.11 900 0.9990500 0.17 0.14 3600 0.9997

a CF, feed concentration.

Thus, the chosen microemulsion for the peroxidation ctinuous process was constituted of 4% (w/w) of LS, 4(w/w) of n-PrOH, 46% (w/w) of toluene, and 46% (w/w) oaqueous sodium molybdate (0.8 M). Such a formulationresponds to a Winsor I system, namely an O/W microemsion in equilibrium with an excess of toluene and son-propanol, which arises from the cosurfactant partitionbetween the microemulsion phase and the oily excess pThis composition was chosen by taking into account theof phase separation which becomes dramatically low inpoor surfactant part of the diagram due to cosurfactant ptioning [24].

3.3. Selectivity of the membrane toward the componentof the microemulsion

In order to study the membrane behavior and especits selectivity toward the microemulsion, aqueous solutiof alcohol, surfactant, and catalyst have been tested srately, as well as biphasic toluene/water mixtures. For eacof these solutions, after a circulation time of 3 h throughmembrane, the collected permeates have been thawesubmitted to analysis.

First, aqueous solutions of catalyst have been alloto circulate through the ceramic membrane module,analysis have been performed on the permeate by UV stroscopy at 245 nm (ε = 2000 Lmol−1 cm−1) to determinethe sodium molybdate concentration. The average mbrane retentionR and selectivityαA/B (towardsA comparedto B) in Table 1are calculated according to

(2)R = 1− CP

CR,

(3)αA/B = (yA/yB)

(xA/xB),

whereCP andCR are, respectively, the permeate and rettate concentrations, whereasy andx are the mole fraction inretentate and in permeate[25].

Permeates fluxesJ are calculated using the formula[4]

(4)J = m

At,

wherem is the mass of permeate,A the membrane surfacarea, andt the permeation time.

Similar experiments performed with biphasic toluewater mixtures are summarized inTable 2.

482 L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485

Table 2Membrane selectivity towards water for toluene/water mixtures at 25◦C

Vtoluene/Vwaterin the feed

Pervaporationtime (h)

Permeate flux(kg m−2 h−1)

[Toluene]permeate

(g L−1)

αtoluene/water R

50/50 3.5 3.4 0.05 17,000 0.9999067/33 3.5 3.0 0.03 58,000 0.9999590/10 2.0 3.2 0.02 390,000 0.99997

:

t andvity

col-for

i-

rded

-aneivenorinstercohohaselter-

per-mul-pesdro-

sor

xide-

tem.n ofegra-

-.itivex-

-

heckter,per-

erium-

par-in-in-

Table 3Membrane selectivity toward short-chain alcohols

Alcohol Numberof phases

[Alcohol]permeate

(mol kg−1)

αalcohol/water R

n-Propanol 1 0.58± 0.03 <1 <0n-Butanol 1 0.26± 0.01 2 0.48n-Pentanol 2 0.075± 0.01 7 0.85

(A = alcohol, B = water) at 25◦C. (Alcohol feed concentration0.5 mol L−1.)

High retentions have been measured for both catalysorganic solvent, which confirms the membrane selectitowards water. Sodium laurate (M = 222 gmol−1) shouldexhibit the same behavior, but no permeate could belected for surfactant aqueous solutions because of foammation in the reactor, which causes bad circulation condtions and membrane wetting.

On the other hand, high permeation has been recofor n-propanol in aqueous solutions (Table 3). Although thecomparison withn-butanol andn-pentanol shows significant improvement with regard to retention, the membrperformances are rather low for short-chain alcohols gtheir hydrophilicity and their low molecular volumes. Fthe present work,n-propanol was preferred to longer-chaalcohols despite its low membrane selectivity. Indeed, farates of phase separation have been observed for this aland evaporation under vacuum of the organic excess pwhich contains some cosurfactant, is easier, without aation of the oxidation products.

Additional experiments have been performed under oating conditions (Table 4) of oxidation, with aqueous sodiumolybdate solutions and with a Winsor IV type microemsion. During the peroxidation process, several different tyof peroxomolybdates intermediates are formed when hygen peroxide is added. The addition of H2O2 is performed inseveral batches in order to maintain the ratio H2O2/MoO2−

4lower than 3.5 and to form preferentially the main precur

-

l,

of 1O2, the triperoxomolybdate MoO(O2)2−3 , which is the

active1O2 generating species[26].Permeate fluxes are not disrupted by hydrogen pero

additions and are even slightly increased because of a temperature increase in the reactor despite the cooling sysMoreover, analysis of the permeate reveals no reductiothe membrane performance as a result of membrane ddation under the oxidizing conditions.

3.4. Influence of the temperature

Temperature strongly increases the H2O2 decompositionrate by molybdate catalyst. Indeed, a 5◦C increase in temperature approximately doubles the1O2 production rateMembrane performance is also very temperature-sens(J roughly doubles every 10◦C): this dependence can be epressed by an Arrhenius-type relationship[27,28] (Eq. (5))whereEA is the preexponential factor,Ep the activation energy of permeation andT the operating temperature:

(5)J = EA exp(−Ep/RT ).

In order to measure membrane performances and to cthis relationship in the case of our system, pure watoluene/water mixtures and microemulsions have beenvaporated at different temperatures as shown onFigs. 5and 6. J increases withT for the three systems but lowfluxes are measured for the Winsor I microemulsion medas well as a break in the curve from 55◦C on account of viscosity modification.

3.5. Continuous peroxidation of organic substrates

The process described above was applied to the preative (1 M) peroxidation of organic substrates using a Wsor I microemulsion system as a reaction medium. Thevestigated system is described below (Table 5) and indicatedby the dot inFig. 4.

Table 4Membrane selectivity towards the catalyst in aqueousmolybdate solutions and in a monophasic microemulsion

Feed Feeda [H2O2](mol L−1)

J

(kg m−2 h−1)[Na2MoO4]permeate

(mol L−1)

αcatalyst/water R

[Na2MoO4] 0.1 M 0.5 2.6± 0.1 <10−3 >100 >0.9902.5 2.8± 0.3 <10−3 >100 >0.990

Winsor IVb 0 1.6± 0.1 <10−3 >150 >0.993[Na2MoO4] 0.15 M 0.75 1.7± 0.1 <10−3 >150 >0.993

a Batch additions every 15 min for 3 h.b 37.5 wt% toluene, 12.5 wt%n-propanol, 12.5 wt% LS, 37.5 wt% water+ catalyst.

L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485 483

er.

or I-

nic

gherighmi-eryn ofuentTheal tos-

outeone no-ded,ater

y

iumateslesseeldsadded

ter-iven-

m-

tion(onhose

ob-ictineder-

t-firm-ughngofto

on-tantem-

Fig. 5. Evolution of the permeation flux with temperature for pure wat

Fig. 6. Evolution of the permeate flux with temperature for a Wins(toluene/water/LS/PrOH/Na2MoO4) microemulsion and for the corresponding toluene/water mixture.

Table 5Composition of the Winsor I system used for the peroxidation of orgasubstrates (1 mol L−1) at 25◦C

Oil phase(g L−1)

Microemulsion phase(g L−1)

n-PrOH 25 54Water – (q.s. 200 mL)Na2MoO4·2H2O – 166Sodium laurate – 72Toluene (q.s. 800 mL) 126

The operating temperature was maintained at 25◦C(higher temperatures imply higher H2O2 decompositionrates, higher amounts of added water, and thus himembrane surfaces in order to achieve sufficiently hfluxes). The reaction medium consisted of one volume ofcroemulsion and four volumes of oil to facilitate the recovof the oxidation products by a simple phase separatiothe oil phase from the microemulsion layer and subseqevaporation of the oil phase at the end of the reaction.average catalyst concentration in the system was equ137.2 mmol L−1 and the H2O2 decomposition rate was asessed as 36.9 mmol min−1 L−1 (2.2 mol h−1 L−1).

Table 6β values and reactivity constants for some typical substrates[29]

Substrate β (mmol L−1) log(kr + kq)

1 4 8.0

2 160 5.9

3 ≈1000 ≈5.3

The average permeate (water) flux was fixed to ab1 kg m−2 h−1 (according toFig. 6), with a membrane surfacequal to 58 cm2. To keep the reaction medium compositiconstant and thus to avoid the decrease of performancticed when large amounts of hydrogen peroxide are adthe membrane surface must be adapted so that the wformed through the addition of H2O2 corresponds exactlto the water removed by the pervaporation process.

Two cases have to be considered: As the reaction medis not affected by low amounts of water, reactive substrcan be oxidized without the use of a membrane. Forreactive substrates or higher substrate concentrations, morhydrogen peroxide is required to obtain reasonable yiand pervaporation stages are necessary to remove thewater. The Foote reactivity indexβ of some investigatedsubstrates, namely the minimum concentration of substrarequired so that the interaction with1O2 becomes prepondeant over the deactivation by the reaction medium are gin Table 6(this notion is only valid in a “kinetically” homogeneous medium).

Given their high reactivity,α-terpinene,1, andβ-citro-nellol, 2, do not require dewatering stages on the mebrane, even for preparative concentrations up to 1 mol L−1.Thus, our choice fell onβ-pinene,3, which that could notbe oxidized in such a system up to now. As pervaporais the limiting step of the process in this experimentindustrial scale, the design should be adapted), we cto separate oxidation and pervaporation stages: H2O2 wasadded in batches until no additional conversion could betained. According toFig. 7, as water is added to the biphasmicroemulsion, dilution makesthe microreactors inefficiengiven increasing losses of singlet oxygen. Thus, the obtamilky emulsion circulated through the membrane and pmeates were collected until the elimination of a sufficienamount of water. Additional amounts of hydrogen peroxide enabled to increase the substrate conversion coning the usefulness of the dewatering process, even thopermeate fluxes were rather low, which implies quite lopervaporation times (limited by the experimental facility58 cm2). Additional pervaporation stages are requiredreach higher yields. Ideally, water should be removed ctinuously to keep the microemulsion composition conswhich was not possible given the available laboratory mbrane.

484 L. Caron et al. / Journal of Colloid and Interface Science 282 (2005) 478–485

nn.

e de-

ox-

forr-

per-t a

ts ofr—dedde-thects

tionanicse

f therod-

anictabi-fac-entsitis in-s ofen-

tionim-the

lly

c-

h-

e-

41..

r-

ng.

99

.ura,

00)

ems,

Fig. 7. β-pinene (1 mol L−1) conversion in a Winsor I microemulsiosystem and total volume increase versus singlet oxygen concentratio(T = 25◦C; Vmicroemulsion= 20%;Voil excess phase= 80%).

Fig. 8. Schematic representation of the reaction mixture performanccrease due to dilution.

The dilution of the reaction medium as hydrogen peride is added gives rise to two problems:

• a decrease of1O2 average lifetime (τ� (water)= 4 µs�τ� (oil));

• a decrease of the probability that1O2 meets oil dropletsin its range of action (Fig. 8).

The combination of both these effects is responsiblethe reaction medium performance decrease during the peoxidation.

4. Summary and conclusion

We have demonstrated an improved process for theoxidation of even poorly reactive organic compounds apreparative scale. Indeed, the addition of large amounhydrogen peroxide—and thus of large amounts of wateis not a limitation any more, since the generated or adwater, which otherwise induces phase transitions andcreasing performance, is removed continuously fromsystem. Moreover, the recovery of the oxidation produ

is considerably simplified by the use of biphasic reacmedium: the desired products are collected in an orgphase that is in equilibrium with a microemulsion phawhere the oxidation takes place. No tedious treatment omicroemulsion phase is necessary since the oxidation pucts are continuously extracted from it thanks to the orgexcess phase of the Winsor I system. To avoid the deslization of the system which may occur when the cosurtant is co-pervaporated with water, composition adjustmmay be necessary, by addingn-propanol at the rate asdisappears through the membrane. Finally, the processdustrially and economically attractive as the componentthe microemulsion system are inexpensive and environmtally harmless, and, moreover, the microemulsion reacmedium can be recycled for additional peroxidations by sple phase separation of the microemulsion phase fromorganic phase.

Acknowledgment

The authors are grateful that this work was financiasupported by DSM Pharma Chemicals.

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