Continuous process for singlet oxygenation of hydrophobic substrates in microemulsion using a pervaporation membrane

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

Continuous process for singlet oxygenation of hydrophobic substrates in microemulsion using a pervaporation membrane Laurent Caron a,∗ , Véronique Nardello a , José Mugge b , Erik Hoving b , Paul L. Alsters b , Jean-Marie Aubry a 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 (1 O2 , 1
g ) is able to oxidize a great deal of hydrophobic substrates from molybdate-catalyzed hydrogen peroxide decomposition, provided a suitable reaction medium such as a microemulsion system is used. However, high substrate concentrations or poorly reactive organics require large amounts of H2 O2 that generate high amounts of water and thus destabilize the system. We report results obtained on combining dark singlet oxygenation of hydrophobic substrates in microemulsions with a pervaporation membrane process. To avoid composition alterations after addition of H2 O2 during the peroxidation, the reaction mixture circulates through a ceramic membrane module that enables a partial and selective dewatering of the microemulsion. Optimization phase diagrams of sodium molybdate/water/alcohol/anionic surfactant/organic solvent have been elaborated to maximize the catalyst concentration and therefore the reaction rate. The membrane selectivity towards the mixture constituents has been investigated showing that a high retention is observed for the catalyst, for organic solvents and hydrophobic substrates, but not for n-propanol (cosurfactant) and water. The efficiency of such a process 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

1. Introduction Microemulsions are thermodynamically stable mixtures of organic compound(s) (oil), water, surfactant(s), and, in most cases, cosurfactant(s) [1]. These colloidal dispersions consist of droplets of oil-in-water (O/W), water-in-oil (W/O), or bicontinuous microdomains ranging from about 10–100 nm in diameter, which results in a transparent medium. The thermodynamic stability of such systems is achieved as a result of a surfactant/cosurfactant system, generally a short chain-length alcohol that decreases interfacial tension down to ultralow values. * Corresponding author.

E-mail address: [email protected] (L. Caron). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.156

Over the past decade, much research has been devoted to microemulsions as reaction media [2,3] and their industrial applications are steadily increasing [4,5], e.g., polymerization [6,7], enzymatic [8], and oxidation [9,10] reactions. Given their specific microstructure, microemulsions are also suitable media for the “dark,” i.e., nonphotochemical, peroxidation of organic substrates with singlet oxygen, 1 O2 (1
g ), generated by molybdate-catalyzed hydrogen peroxide disproportionation, MoO2− 4

2H2 O2 −−−−→ 2H2 O + 1 O2 (100%). pH 9–12

(1)

Indeed, the average droplet size in microemulsions is smaller than the mean travel distance of 1 O2 , which is about 200 nm in water (Fig. 1) [10,11]. Thus, 1 O2 , which is formed in the aqueous droplet, can diffuse freely in the

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sion as reaction medium is, however, rather new and original, since, so far, the main applications of membrane technology to (micro)emulsion treatment concern the destabilization of microemulsions [18–20]. In the present paper, we report the combination of a pervaporation process with the dark singlet oxygenation of organic hydrophobic substrates. The choice of the membrane and the formulation of the microemulsion system are discussed first. The membrane selectivity towards the microemulsion components is then investigated and permeate fluxes are measured to check the proper working of the device, the ultimate goal of the study being the selective removal of water from the reaction mixture under oxidizing conditions. The efficiency of the process is finally illustrated with the peroxidation of a poorly reactive substrate. Fig. 1. Schematic representation of the oxidation of hydrophobic substrates by the hydrophilic chemical source of singlet oxygen, viz., H2 O2 /MoO2− 4 , in a water-in-oil microemulsion.

organic phase and react with the organic substrate before deactivation. Therefore, these media are particularly appropriate to the peroxidation of highly hydrophobic substrates, such as aromatic compounds, olefins, or dienes, by the system H2 O2 /MoO2− 4 as a hydrophilic chemical source of 1 O [10,12]. 2 However, such systems present two main drawbacks. First, the addition of hydrogen peroxide during the reaction results in an increase of the water proportion, which modifies the composition of the microemulsion and gradually causes decreasing performance and low reactor yield (kg product per m3 reactor). Such a drawback limits the use of the microemulsion either to highly reactive substrates or to relatively low concentrations of substrate. Second, the use of surfactant-containing reaction mixtures hampers facile recovery of the desired products. Actually, at the end of the oxidation process, the reaction medium is relatively complex, since it is made up of more than six constituents, namely water, oil, surfactant, cosurfactant, catalyst, and oxidation product(s). Hence, isolation of the products requires either the addition of an extra substance, which is not desirable in many cases [13,14], the inactivation or destruction of the surfactant [15], or a tedious treatment of the microemulsion itself [10,12]. Based on these considerations, a semibatch oxidation process in which water is continuously and selectively removed from the system to maintain the initial composition of the microemulsion appears particularly attractive from an industrial point of view. The use of a membrane process to this end is an ideal solution, since the membrane area can be adapted to the desired water-removal flux, whereas the membrane does not interfere with the reaction itself. There has been increasing interest in recent years in the industrial use of pervaporation membrane separation techniques [16,17] thanks to new membranes with improved chemical inertness and selectivity in the separation of water from mixtures. The combination of a pervaporation process with a microemul-

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. Hydrogen peroxide 50% (17.5 M) was purchased from Prolabo. Sodium laurate was prepared as a 1 mol kg−1 aqueous solution by adding lauric acid (Aldrich) to aqueous sodium hydroxide. 2.2. Procedures In 5-ml SVL tubes the appropriate amounts of oil, cosurfactant, water, catalyst, and aqueous sodium laurate were mixed. The mixtures were maintained at a constant temperature (25 ± 0.1 ◦C) and allowed to stabilize. 2.3. Instrumentation UV/visible spectrophotometry analyses were carried out with a Varian Cary 50 spectrometer. High-performance liquid chromatography (HPLC) analyses were performed on a Waters 600 chromatograph equipped with a Novapak C18 (4-µm) column and a prefilter. Solvents were HPLC grade CH3 OH and Milli-Q water. For detection and quantitation, a Waters 490E multiwavelength UV detector was used. Gas chromatography (GC) analyses were performed on a Agilent 6890 N chromatograph equipped with an apolar HP-1 (60 m × 0.32 mm − 0.25 µm) column. Pervaporation experiments were conducted in a laboratory-scale unit: the experimental setup consists of a vessel, a recirculation pump, a membrane unit, condensers, and a vacuum pump, as illustrated in Fig. 2. The liquid is circulated in cross-flow over the outside of the tubular membrane (Sulzer, type SMS), which contains an active SiO2 layer on top of the ceramic (γ -Al2 O3 ) support material. On the inside of the tubular membrane, a vacuum is applied by using

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

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

3. Results and discussion

Fig. 3. Schematic drawing of the membrane interface.

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

3.1. Choice of the membrane 3.2. Choice of the microemulsion system Membrane technology is more and more often used as a separation technology in modern processes [21,22]. This can be explained by the high selectivity, robustness, and ease of operation, combined with the low energy consumption of the separation process and the decreasing membrane prices over the past two decades. Many membrane processes are available based on different separation principles, such as microfiltration, ultrafiltration, nanofiltration, gas separation, osmosis, reverse osmosis, and electrodialysis. A large number of (organic or inorganic) membrane materials can be used, depending on the intended applications. For our application we chose a robust hydrophilic ceramic membrane since peroxidation of organic substrates in the microemulsion system involves harsh, strongly oxidizing reaction conditions. The presence of organic solvents and oxidizing species such as hydrogen peroxide, singlet oxygen and peroxomolybdates excludes the use of polymeric materials. Ceramic membranes, which form the main class of inorganic membranes, are suitable for this application given their chemical and thermal stability, and their resistance to a broad range of pH conditions. The investigated device is a Sulzer [23] Pervap SMS (silica membrane system), consisting of a hollow γ -Al2 O3 cylinder with a SiO2 outer layer (Fig. 3). The microemulsified reaction mixture circulates on the feed side of this ceramic porous (different levels mesa, macro and micro) material, while on the downstream side, a vacuum is applied to remove the permeate (ideally pure water) as a vapor (P = 6–7 mbar). We chose to focus on inorganic pervaporation membranes rather than inorganic reverse osmosis membranes because the latter are not yet commercially available. In addition, pervaporation offers the advantage over reverse osmosis that the

Several efficient microemulsions have been described for the peroxidation of hydrophobic substrates by 1 O2 [10,12]. For an industrial process, the components of the microemulsion have to be inexpensive and nontoxic. With regard to these requirements, the best formulation was found to be based on sodium laurate (LS) as a cheap surfactant, npropanol as the cosurfactant, and toluene as a good and chlorine-free organic solvent. On the other hand, the addition of the catalyst Na2 MoO4 · 2H2 O to the oil/water/surfactant/alcohol mixture modifies the microemulsion and different systems may appear according to the amounts of sodium molybdate and surfactant + alcohol. In order to define the best composition for the microemulsion, phase diagrams, also called optimization diagrams [1], have been elaborated. They plot the (surfactant + cosurfactant) proportion versus the catalyst concentration in the aqueous phase. Such diagrams typically exhibit gammashaped boundaries corresponding to an interface curvature inversion induced by the interaction between the catalyst and the surfactant, resulting in a decrease of the repulsion between the surfactant polar heads. Thus, scans performed as a function of the MoO2− 4 concentration enable detection of transitions between mono- (Winsor IV), bi- (Winsor I and II), and triphasic (Winsor III) microemulsion systems, depending on the interface curvature and on the surfactant amount. All the scans were carried out with both the alcoholto-surfactant ratio and the salted-water-to-oil ratio kept constant (equal to 1 w/w). Fig. 4 shows the boundaries delimiting the different types of Winsor obtained by scanning the system toluene/water/LS/PrOH with increasing concentrations of sodium molybdate.

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Table 1 Retention factors R towards sodium molybdate for various feed concentrations at 25 ◦ C CF a (mmol L−1 )

CP after 2 h (mmol L−1 )

CP after 3 h (mmol L−1 )

αcatalyst/water

R

100 500

0.09 0.17

0.11 0.14

900 3600

0.9990 0.9997

a C , feed concentration. F

Fig. 4. Optimization diagram for the toluene/water/LS/n-PrOH/Na2 MoO4 system at 25 ◦ C (n-PrOH/LS = 50/50 (w/w) and toluene/(water + catalyst) = 50/50 (w/w)). (") Composition of the system used for the peroxidation of β-pinene. (The region with low concentrations of surfactant + cosurfactant could not be explored because the phase separation was too slow.)

On the basis of this binary diagram, the choice of the composition of the microemulsion was first made among the three polyphasic systems to decrease the amount of surfactant. Indeed, Winsor I, II, and III systems may be obtained with lower amounts of surfactant than the monophasic system Winsor IV, usually used for the present application [10]. On a second time, with a view to set up a continuous process, the Winsor I system, in which an O/W microemulsion is in equilibrium with an excess of oil, was preferred to the two other polyphasic systems (i.e., Winsor II and III), since the presence of an oil phase allows a straightforward recovery of the lipophilic oxidation products by continuous extraction from the microemulsion into the oil phase. Even though Winsor I systems are obtained with lower amounts of catalyst compared to Winsor II and III types, they can still be obtained both with relatively high amounts of sodium molybdate (≈1 mol kg−1 ) and with low amounts of (surfactant + cosurfactant) thanks to the inclination of the gamma. Thus, the catalyst concentration in Winsor I is high enough to achieve sufficiently high reaction rates. Finally, by comparing the generation of singlet oxygen in the three polyphasic microemulsion systems, it appeared that, in the presence of an excess of water phase (as in Winsor II and III), most of the 1 O2 is not only generated in the aqueous excess phase, but also deactivated, as there is no available substrate in that phase. This results in a loss of efficiency for the peroxidation process, which takes place exclusively in the microemulsion phase. Based on all these considerations, Winsor I systems appeared to be the most efficient ones for continuous peroxidation of hydrophobic substrates with chemically generated 1 O2 , provided that the composition of the Winsor I is close to the boundary Winsor I/Winsor III to keep a high rate of phase separation. This biphasic system is not less complex than the previously used one-phase microemulsion [10] but enables a straightforward recovery of the peroxidation products in the organic excess phase which is both catalyst and surfactantfree.

Thus, the chosen microemulsion for the peroxidation continuous process was constituted of 4% (w/w) of LS, 4% (w/w) of n-PrOH, 46% (w/w) of toluene, and 46% (w/w) of aqueous sodium molybdate (0.8 M). Such a formulation corresponds to a Winsor I system, namely an O/W microemulsion in equilibrium with an excess of toluene and some n-propanol, which arises from the cosurfactant partitioning between the microemulsion phase and the oily excess phase. This composition was chosen by taking into account the rate of phase separation which becomes dramatically low in the poor surfactant part of the diagram due to cosurfactant partitioning [24]. 3.3. Selectivity of the membrane toward the components of the microemulsion In order to study the membrane behavior and especially its selectivity toward the microemulsion, aqueous solutions of alcohol, surfactant, and catalyst have been tested separately, as well as biphasic toluene/water mixtures. For each of these solutions, after a circulation time of 3 h through the membrane, the collected permeates have been thawed and submitted to analysis. First, aqueous solutions of catalyst have been allowed to circulate through the ceramic membrane module, and analysis have been performed on the permeate by UV spectroscopy at 245 nm (ε = 2000 L mol−1 cm−1 ) to determine the sodium molybdate concentration. The average membrane retention R and selectivity αA/B (towards A compared to B) in Table 1 are calculated according to R=1− αA/B =

CP , CR

(yA /yB ) , (xA /xB )

(2)

(3)

where CP and CR are, respectively, the permeate and retentate concentrations, whereas y and x are the mole fraction in retentate and in permeate [25]. Permeates fluxes J are calculated using the formula [4] m , (4) At where m is the mass of permeate, A the membrane surface area, and t the permeation time. Similar experiments performed with biphasic toluene/ water mixtures are summarized in Table 2. J=

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Table 2 Membrane selectivity towards water for toluene/water mixtures at 25 ◦ C Vtoluene /Vwater in the feed

Pervaporation time (h)

Permeate flux (kg m−2 h−1 )

[Toluene]permeate (g L−1 )

αtoluene/water

R

50/50 67/33 90/10

3.5 3.5 2.0

3.4 3.0 3.2

0.05 0.03 0.02

17,000 58,000 390,000

0.99990 0.99995 0.99997

Table 3 Membrane selectivity toward short-chain alcohols Alcohol

Number of phases

[Alcohol]permeate (mol kg−1 )

αalcohol/water

R

n-Propanol n-Butanol n-Pentanol

1 1 2

0.58 ± 0.03 0.26 ± 0.01 0.075 ± 0.01

<1 2 7

<0 0.48 0.85

(A = alcohol, B = water) at 25 ◦ C. (Alcohol feed concentration: 0.5 mol L−1 .)

of 1 O2 , the triperoxomolybdate MoO(O2)2− 3 , which is the active 1 O2 generating species [26]. Permeate fluxes are not disrupted by hydrogen peroxide additions and are even slightly increased because of a temperature increase in the reactor despite the cooling system. Moreover, analysis of the permeate reveals no reduction of the membrane performance as a result of membrane degradation under the oxidizing conditions. 3.4. Influence of the temperature

High retentions have been measured for both catalyst and organic solvent, which confirms the membrane selectivity towards water. Sodium laurate (M = 222 g mol−1 ) should exhibit the same behavior, but no permeate could be collected for surfactant aqueous solutions because of foam formation in the reactor, which causes bad circulation conditions and membrane wetting. On the other hand, high permeation has been recorded for n-propanol in aqueous solutions (Table 3). Although the comparison with n-butanol and n-pentanol shows significant improvement with regard to retention, the membrane performances are rather low for short-chain alcohols given their hydrophilicity and their low molecular volumes. For the present work, n-propanol was preferred to longer-chain alcohols despite its low membrane selectivity. Indeed, faster rates of phase separation have been observed for this alcohol and evaporation under vacuum of the organic excess phase, which contains some cosurfactant, is easier, without alteration of the oxidation products. Additional experiments have been performed under operating conditions (Table 4) of oxidation, with aqueous sodium molybdate solutions and with a Winsor IV type microemulsion. During the peroxidation process, several different types of peroxomolybdates intermediates are formed when hydrogen peroxide is added. The addition of H2 O2 is performed in several batches in order to maintain the ratio H2 O2 /MoO2− 4 lower than 3.5 and to form preferentially the main precursor

Temperature strongly increases the H2 O2 decomposition rate by molybdate catalyst. Indeed, a 5 ◦ C increase in temperature approximately doubles the 1 O2 production rate. Membrane performance is also very temperature-sensitive (J roughly doubles every 10 ◦ C): this dependence can be expressed by an Arrhenius-type relationship [27,28] (Eq. (5)) where EA is the preexponential factor, Ep the activation energy of permeation and T the operating temperature: J = EA exp(−Ep /RT ).

(5)

In order to measure membrane performances and to check this relationship in the case of our system, pure water, toluene/water mixtures and microemulsions have been pervaporated at different temperatures as shown on Figs. 5 and 6. J increases with T for the three systems but lower fluxes are measured for the Winsor I microemulsion medium as 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 preparative (1 M) peroxidation of organic substrates using a Winsor I microemulsion system as a reaction medium. The investigated system is described below (Table 5) and indicated by the dot in Fig. 4.

Table 4 Membrane selectivity towards the catalyst in aqueous molybdate solutions and in a monophasic microemulsion Feed

Feeda [H2 O2 ] (mol L−1 )

J (kg m−2 h−1 )

[Na2 MoO4 ]permeate (mol L−1 )

αcatalyst/water

R

[Na2 MoO4 ] 0.1 M

0.5 2.5 0 0.75

2.6 ± 0.1 2.8 ± 0.3 1.6 ± 0.1 1.7 ± 0.1

<10−3 <10−3 <10−3 <10−3

>100 >100 >150 >150

>0.990 >0.990 >0.993 >0.993

Winsor IVb [Na2 MoO4 ] 0.15 M

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.

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

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

Fig. 6. Evolution of the permeate flux with temperature for a Winsor I (toluene/water/LS/PrOH/Na2 MoO4 ) microemulsion and for the corresponding toluene/water mixture.

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

n-PrOH Water Na2 MoO4 ·2H2 O Sodium laurate Toluene

Oil phase (g L−1 )

Microemulsion phase (g L−1 )

25 – – – (q.s. 800 mL)

54 (q.s. 200 mL) 166 72 126

The operating temperature was maintained at 25 ◦ C (higher temperatures imply higher H2 O2 decomposition rates, higher amounts of added water, and thus higher membrane surfaces in order to achieve sufficiently high fluxes). The reaction medium consisted of one volume of microemulsion and four volumes of oil to facilitate the recovery of the oxidation products by a simple phase separation of the oil phase from the microemulsion layer and subsequent evaporation of the oil phase at the end of the reaction. The average catalyst concentration in the system was equal to 137.2 mmol L−1 and the H2 O2 decomposition rate was assessed as 36.9 mmol min−1 L−1 (2.2 mol h−1 L−1 ).

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 about 1 kg m−2 h−1 (according to Fig. 6), with a membrane surface equal to 58 cm2 . To keep the reaction medium composition constant and thus to avoid the decrease of performance noticed when large amounts of hydrogen peroxide are added, the membrane surface must be adapted so that the water formed through the addition of H2 O2 corresponds exactly to the water removed by the pervaporation process. Two cases have to be considered: As the reaction medium is not affected by low amounts of water, reactive substrates can be oxidized without the use of a membrane. For less reactive substrates or higher substrate concentrations, more hydrogen peroxide is required to obtain reasonable yields and pervaporation stages are necessary to remove the added water. The Foote reactivity index β of some investigated substrates, namely the minimum concentration of substrate required so that the interaction with 1 O2 becomes preponderant over the deactivation by the reaction medium are given in Table 6 (this notion is only valid in a “kinetically” homogeneous medium). Given their high reactivity, α-terpinene, 1, and β-citronellol, 2, do not require dewatering stages on the membrane, even for preparative concentrations up to 1 mol L−1 . Thus, our choice fell on β-pinene, 3, which that could not be oxidized in such a system up to now. As pervaporation is the limiting step of the process in this experiment (on industrial scale, the design should be adapted), we chose to separate oxidation and pervaporation stages: H2 O2 was added in batches until no additional conversion could be obtained. According to Fig. 7, as water is added to the biphasic microemulsion, dilution makes the microreactors inefficient given increasing losses of singlet oxygen. Thus, the obtained milky emulsion circulated through the membrane and permeates were collected until the elimination of a sufficient amount of water. Additional amounts of hydrogen peroxide enabled to increase the substrate conversion confirming the usefulness of the dewatering process, even though permeate fluxes were rather low, which implies quite long pervaporation times (limited by the experimental facility of 58 cm2 ). Additional pervaporation stages are required to reach higher yields. Ideally, water should be removed continuously to keep the microemulsion composition constant which was not possible given the available laboratory membrane.

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

is considerably simplified by the use of biphasic reaction medium: the desired products are collected in an organic phase that is in equilibrium with a microemulsion phase where the oxidation takes place. No tedious treatment of the microemulsion phase is necessary since the oxidation products are continuously extracted from it thanks to the organic excess phase of the Winsor I system. To avoid the destabilization of the system which may occur when the cosurfactant is co-pervaporated with water, composition adjustments may be necessary, by adding n-propanol at the rate as it disappears through the membrane. Finally, the process is industrially and economically attractive as the components of the microemulsion system are inexpensive and environmentally harmless, and, moreover, the microemulsion reaction medium can be recycled for additional peroxidations by simple phase separation of the microemulsion phase from the organic phase.

Acknowledgment The authors are grateful that this work was financially supported by DSM Pharma Chemicals.

References

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

The dilution of the reaction medium as hydrogen peroxide is added gives rise to two problems: • a decrease of 1 O2 average lifetime (τ
(water) = 4 µs  τ
(oil)); • a decrease of the probability that 1 O2 meets oil droplets in its range of action (Fig. 8). The combination of both these effects is responsible for the reaction medium performance decrease during the peroxidation.

4. Summary and conclusion We have demonstrated an improved process for the peroxidation of even poorly reactive organic compounds at a preparative scale. Indeed, the addition of large amounts of hydrogen peroxide—and thus of large amounts of water— is not a limitation any more, since the generated or added water, which otherwise induces phase transitions and decreasing performance, is removed continuously from the system. Moreover, the recovery of the oxidation products

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