Benzyl Alcohol Oxidation to Benzaldehyde in Multiphase Membrane

(3, 11, 12) Many oxidations of this type are carried out using stoichiometric ... (19) The molar ratio for alcohol:H2O2:catalyst was 10:20.2:1. ... Fr...
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Organic Process Research & Development 2008, 12, 982–988

Benzyl Alcohol Oxidation to Benzaldehyde in Multiphase Membrane Reactor M. G. Buonomenna* and E. Drioli ITM-CNR c/o UniVersity of Calabria, Via P.Bucci 87030, Rende (CS) 87036, Italy

Abstract: Alcohol oxidations are typically performed with stoichiometric reagents that generate hazardous waste and are usually run in halogenated solvents. A biphasic water-alcohol system with a hydrophobic PVDF membrane at the interface, operating in a flat membrane reactor, is a new solution for the selective oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH). BzOH can be oxided selectively to benzaldehyde with hydrogen peroxide as oxidizing agent and ammomium molybdate as catalyst. The obtained results showed that, by operating in counter-current mode with an organicphase flow rate of 1.7 × 103 mL/h and an aqueous-phase flow rate of 10 × 103 mL/h using composite membrane M1, a conversion of BzOH to BzH of 5.13% with a selectivity of 97.6% has been obtained. The conversion increased up to 47.9% with a selectivity of 98.4% by keeping the two phases in contact using symmetric membrane M4. Compared with conventional reactors, the obtained results affirmed that the use of a membrane contactor plays a crucial role, similar to that of phase-transfer catalyst without the need for organic solvents. The use of hydrogen peroxide oxidant and the absence of promoters and other additives make the reaction interesting from both an economic and environmental point of view.

Introduction High energy prices have placed a premium on efficient processes that operate at lower reaction severity and with reduced separation demands. The global economy also forces manufacturers to continually improve their processes to maintain competitiveness. Frequently, these gains are obtained by changing the process flowsheet, making compromises between capital improvements and operating cost savings. In other cases, efficiency gains can be realized without significant capital investment by redesigning the reaction system.1 Process intensification (PI) is the innovative design strategy aiming to improve manufacturing and processing by decreasing the equipment-size/productivity ratio, energy consumption and waste production by using innovative technical solutions. Membrane processes meet the requirements of PI because they have the potential to replace conventional energy-intensive techniques, to ac* Corresponding author. E-mail: [email protected].

(1) Nehelsen, J.; Mukherjee, M.; Porcelli, R. V. Chem. Eng. Prog. 2007, 103, 31. 982 • Vol. 12, No. 5, 2008 / Organic Process Research & Development Published on Web 08/23/2008

Figure 1. Schematization of the used membrane reactor.

complish the selective and efficient transport of specific components, and to improve the performance of reactive processes.2 The oxidation of primary alcohols to aldehydes is a fundamentally important laboratory and commercial procedure.3-10 Aldehydes are valuable both as intermediates and as high-value components for the perfume industry. 3,11,12 Many oxidations of this type are carried out using stoichiometric oxygen donors such as chromate or permanganate, but these reagents are expensive and have serious toxicity issues associated with them. 3,11,13-16 The increasing demand for environmental-conscious chemical processes has impelled many researchers to explore truly efficient oxidation methods using aqueous H2O2, an ideal oxidant in this context: hydrogen peroxide is by far the least environmentally demanding among the peroxides; in fact, water is the only byproduct of these reactions. (2) Charpentier, J. C. Ind. Eng. Chem. Res. 2007, 46, 3465. (3) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (4) Beller, M.; Bolm, C. Transition Metals for Organic Synthesis; Verlag GmbH & Co. KGaA: Weinheim, Germany, 2004. (5) ten Brink, G.; Arends, I. W. C. E; Sheldon, R. A. Science 2000, 287, 1636. (6) Vazylyev, M.; Sloboda-Rozner, D.; Haimov, A.; Maayan, G.; Neumann, R. Top. Catal. 2005, 34, 93. (7) Pagliaro, M.; Campestrini, S.; Ciriminna, R. Chem. Soc. ReV. 2005, 34, 837. (8) Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 26, 10657. (9) Marko′, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch, C. J. Science 1996, 274, 2044. (10) Schultz, M. J.; Adler, R. S.; Zierkiewicz, W.; Privalov, T.; Sigman, M. S. J. Am. Chem. Soc. 2005, 127, 8499. (11) Pillai, U. R.; Sahle-Demessie, E. Appl. Catal., A 2003, 245, 103. (12) Hudlicky, M. Oxidations in Organic Chemistry; American Chemical Society: Washington, DC, 1990. (13) Griffith, W. P.; Joliffe, J. M. Stud. Surf. Sci. Catal. 1991, 66, 395. (14) Cainelli, G.; Cardillo, G. Chromium Oxidants in Organic Chemistry; Springer: Berlin, 1984. (15) Lee, D. G.; Spitzer, U. A. J. Org. Chem. 1970, 35, 3589. (16) Menger, F. M.; Lee, C. Tetrahedron Lett. 1981, 22, 1655. 10.1021/op800129k CCC: $40.75  2008 American Chemical Society

Table 1. Characteristics of membranes used as membrane contactors R (deg) membrane code

type

material

M1

composite

M2

integrally asymmetric integrally PVDF asymmetric symmetric PVDF

M3 M4

fluoropolymer (top layer); pp (bottom layer) PVDF

top bottom δ rp (µm) (µm) layer layer

Table 4. Selectivity to BzH, BzOH conversion in co-current and counter-current mode (organic phase flow rate: 1.7 × 103 mL/h; aqueous phase flow rate: 10 × 103; membrane M1)

238

0.2

90 ( 2 140 ( 3

circulation mode

67

0.2

110 ( 2 123 ( 2

co-current counter-current

57

0.07

90 ( 2 119 ( 2

30

0.9

120 ( 2 122 ( 3

Scheme 1

M1 M2 M3 M4

BzOH conversion H2O2 to organic-phase aqueous-phase conversion BzH flow rate flow rate to selectivity (%)b (mL/h) × 103 (mL/h) × 103 to BzH (%)a BzHc 5.7 7.5 10

5.7 7.5 10

94.3 91.8 90.5

2.86 3.86 6.3

2.87 3.88 6.33

a Selectivity to BzH ) [mmol BzH/(mmol BzH + mmol BzA)] × 100. b BzOH conversion to BzH ) (mmol BzH/mmol BzOH initial) × 100. c H2O2 conversion to BzH ) (mmol BzH/mmol H2O2 initial) × 100.

Table 3. Selectivity, BzOH conversion at different flow rates of organic phase (membrane M1, co-current mode) organic-phase aqueous-phase flow rate flow rate (mL/h) × 103 (mL/h) × 103 1.7 3.2

10 10

selectivity BzOH H2O2 to conversion conversion BzH to to (%) BzH (%) BzH (%) 96 91.5

4.44 4.24

4.46 4.26

Sheldon and co-workers5 obtained good yields from a biphasic system for the catalytic conversion of primary and secondary alcohols using a water-soluble palladium(II) bathophenanthroline complex. However, because the reaction rate is largely determined by the solubility of the alcohols, the reaction rate gradually decreases for higher alcohols. Among the oldest, but very efficient catalysts which have been well studied7-10 and which are still under active investigation are the derivatives of some transition metal ions, i.e., V, Mo, and W, in their highest oxidation states. Anionic peroxomolybdenum complexes, for example,

BzOH conversion to BzH (%)

H2O2 conversion to BzH (%)

96 97.6

4.44 5.13

4.46 5.16

Table 5. Selectivity, BzOH conversion using different membranes (organic-phase flow rate: 1.7 × 103 mL/h; aqueous phase flow rate: 10 × 103 mL/h; counter-current mode)

membrane

Table 2. Selectivity, BzOH conversion at different flow rates of organic and aqueous phases (membrane M1, co-current mode)

selectivity to BzH (%)

selectivity to BzH (%)

BzOH conversion to BzH (%)

H2O2 conversion to BzH (%)

97.6 98.5 98.8 98.4

5.13 33.0 2.10 47.9

5.16 33.16 2.11 48.1

rendered soluble in chlorinated solvents by the presence of a lipophilic countercation, allowed obtaining in a monophasic system quantitative yields of aldehydes from primary alcohols.17 Alternative procedures consisted of a two-phase oxidation method with hydrogen peroxide under phase-transfer conditions: a neutral lipophilic ligand acts as extracting agent. For example, the commercial ammonium molybdate (NH4)6Mo7O24 · 4H2O in combination with a phase-transfer catalyst, (n-C4H9)4NCl, and a base (K2CO3) catalyzed the selective oxidation of secondary alcohols with H2O2 in THF solvent with a molar ratio of 10:40:1 alcohol:H2O2: catalyst. The more hindered alcohol moiety was selectively oxidized in the presence of a less hindered one with a conversion of 90% in 6 days.20 Na2WO4 · 2H2O and Na2MoO4 · 2H2O were used as catalysts, and a lipophilic tetraalkylammonium compound was used as an extracting agent in the oxidation of primary and secondary alcohols in 1,2 dichloroethane by Bortolini et al.19 The molar ratio for alcohol:H2O2:catalyst was 10:20.2:1. In our early work,21 we reported a novel method for selective oxidation of benzyl alcohol to benzaldehyde. A polymeric microporous membrane acts as a barrier to “keep in contact” the organic phase containing benzyl alcohol and the aqueous phase with hydrogen peroxide. The membrane’s role in this system was to improve the contact between the two different reactive phases, the catalyst being physically separated from the membrane, thus realizing an inert membrane reactor.21-23 In this work, we studied the oxidation of benzyl alcohol to benzaldehyde in a multiphase membrane reactor, with (17) Bortolini, O.; Campestrini, S.; Di Furia, F.; Modena, G. J. Org. Chem. 1987, 52, 5467. (18) Trost, B. M.; Masuyama, Y. Tetrahedron Lett. 1984, 25, 173. (19) Bortolini, O.; Conte, V.; Di Furia, F.; Modena, G. J. Org. Chem. 1986, 51, 2661. (20) Buonomenna, M. G.; Drioli, E. Appl. Catal., B 2007; http://dx.doi.org/ 10.1016/j.apcatb.2007.10.003. (21) Julbe, A.; Farrusseng, D.; Guizard, C. J. Membr. Sci. 2001, 181, 3. (22) Miachon, S.; Dalmon, J. A. Top. Catal. 2004, 29, 59. (23) Ozdemir, S. S.; Buonomenna, M. G.; Drioli, E. Appl. Catal., A 2006, 307, 167. Vol. 12, No. 5, 2008 / Organic Process Research & Development

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Figure 2. (a) BzH concentration in the organic phase and (b) BzH (open symbols) and BzA (closed symbols) concentration in the aqueous phase vs time at different flow rates of organic and aqueous phases (membrane M1, co-current mode).

Figure 3. (a) BzH concentration in the organic phase and (b) BzH (open symbols) and BzA (closed symbols) concentration in the aqueous phase vs time at different flow rates of organic phase (membrane M1, co-current mode).

Figure 4. (a) BzH concentration in the organic phase and (b) BzH (open symbols) and BzA (closed symbols) concentration in the aqueous phase vs time in co-current and counter-current mode (organic phase flow rate: 1.7 × 103 mL/h; aqueous-phase flow rate: 10 × 103; membrane M1).

recirculation of both solutions, i.e. organic and aqueous phases at the two membrane interfaces. This system showed a high selectivity to benzaldehyde (>98%), due to benzaldehyde permeation across the membrane to the organic phase where it takes shelter from over-oxidation. The effect of parameters such as axial flow velocity or current mode (co-current or counter-current) 984

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type of membrane was investigated. In particular, different membranes both commercial and prepared by means of phase inversion based on hydrophobic polymers (PVDF) were used as membrane contactors. The selected and tested membranes are characterized by different wetting properties and pore size to study their influence on the reaction conversion and extraction capability by the membrane in

Figure 5. EDX analysis of the selective top layer of commercial membrane, M1.

the organic compartment of the desired product, benzaldehyde. The catalyst used was ammonium molybdate, (NH4)6Mo7O24 · 4H2O. In the literature, this catalyst has been reported to be effective in the oxidation of alcohols with H2O2 in combination with a phase-transfer catalyst in chlorinated solvents. From both economic and environmental viewpoints, the selective system proposed in this study, working in the absence of organic solvents and other types of additives, has as a consequence the reduction of toxic waste and byproduct, making the process ecologically more acceptable. Experimental Section Chemicals. BzOH (MW ) 108.14 g/mol, purity 99.99%) from Sigma-Aldrich was used both as reagent and as solvent. BzH (MW ) 106.12 g/mol, purity 99.99%) and BzA (MW ) 122.12 g/mol, purity 99.99%) from SigmaAldrich were used for analytical calibrations. Ammonium molybdate tetrahydrate, (NH4)6Mo7O24 · 4H2O (MW ) 1235.86 g/mol, purity 99.98%), from Sigma-Aldrich were employed as catalysts. H2O2 (30 wt % solution in water) from Sigma-Aldrich was the oxidant. For the preparation of polymeric membranes, PVDF Solef 6020 was supplied from Solvay Inc. N,N-dimethylacetamide (DMA, reagent grade) and acetone, used as solvents, were purchased from Fluka; water, used for the coagulation bath, was double distilled. Apparatus. Experimental tests were carried out at 333 K in the reactor made with a two-compartment cell to separate the organic and the aqueous phases. The system was in a flat sheet configuration with a membrane surface area of 4.91 × 10-4 m2. The volume of the aqueous and organic solutions in each compartment of the experimental apparatus was 37 cm3. The aqueous and organic solutions were continuously recirculated on both membrane surfaces using two pumps at fixed flow rates (Figure 1). The reaction progress was monitored by periodically sampling the organic and aqueous phases. Four different membranes based on PVDF were used in the experiments. In Table 1 some important characteristics such as thickness (δ), pore radius (rp), and water contact angle (R) of the sides of the two membranes have been reported. M1 is a commercial

membrane supplied by Alfa Laval (formerly DSS), Sweden. M2-M4 have been prepared by means of the phase inversion technique.23 PVDF was dissolved at 25 °C in the solvent DMA at 12 wt % (M2) and 20 wt % (M3). The solutions were magnetically stirred for at least one day to guarantee complete dissolution of the polymer. The solutions were cast uniformly onto a glass substrate by means of a hand-casting knife (BRAIVE Instruments) with a knife gap set at 250 µm and immersed in a coagulation bath of water after an exposure time to air of 30 s. A different procedure was used for the preparation of M4. The polymer (10 wt %) was dissolved at 25 °C in DMA (24 wt %) and acetone (64 wt %). The solution was magnetically stirred for at one day to guarantee complete dissolution of the polymer. The solution was cast onto a glass substrate by means of a hand-casting knife (BRAIVE Instruments) with a knife gap set at 250 µm and exposed to air for 3 h. Then the cast film was immersed in a coagulation bath of water. After complete coagulation, the membranes (M2-M4) were transferred into a pure water bath, which was refreshed frequently, for at least 24 h to remove the traces of solvent. The membranes were stored in a deionized water bath until tested for water permeability. For scanning electron microscopy (SEM), permporometry, and contact angle and thickness measurements, the membrane samples were dried at 60 °C under vacuum. The morphologies of the dried membranes (at 60 °C overnight) were examined using SEM, Cambridge, Stereoscan 360, at 20 kV. For cross-sectional analysis the membrane samples were freeze fractured in liquid nitrogen. All samples were sputter-coated with gold before analysis. Membrane pore dimensions were determined by means of a capillary flow porometer CFP 1500 AEXL (Porous Materials Inc. PMI, Ithaca, New York, U.S.A.). Contact angles of water droplets on the membrane surfaces were measured by sessile drop method using CAM 200 contact angle meter (KSV Instruments Ltd., Helsinki, Finland). Oxidation Reaction. In a typical experiment, the membrane was located between the cell compartments with the top surface at the aqueous-phase side. BzOH (200 mmol) was in one compartment of the two-cell compartment and in the other compartment were loaded 15 mL of water, 4 mmol of catalyst, and 199 mmol of H2O2. Obtained results are reported as selectivity to BzH, BzOH conversion to BzH, a reaction time of 4 h. Analysis. The analysis of the organic and aqueous phases was carried by GLC using a 6890 network GC system of Agilent on a HP-5 (30 m × 0.320 mm × 0.25 µm) column. Results and Discussion The BzOH oxidation to BzH occurs by the following reaction: (NH4)6Mo7O24·4H2O

C6H5CH2OH + H2O2 f C6H5CHO + 2H2O Using a hydrophobic membrane24 three steps happen: (i) BzOH permeation across the membrane from the Vol. 12, No. 5, 2008 / Organic Process Research & Development

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organic phase to the membrane interface aqueous side; (ii) interfacial oxidation reaction; (iii) BzH permeation across the membrane to the organic phase where it takes shelter from overoxidation (Scheme 1). Due to the “compartmentalization” of the two reagent reactions (i.e., the catalytically active species and the substrate), the mass-transfer processes involved in bringing the reactants together and product oxygenates diffusing out off the membrane may have a strong influence over the complete catalytic performance (Scheme 1). For an optimal performance, resistances to mass transport should be minimized, and reactions should be carried out in a reaction-limited regime. The aim of the reported tests was to compare the system performance by changing the flow rates in the two compartments, operating in co-current and counter-current modes, and using the commercial membrane M1 (Table 1) membrane interface, where the oxidation reaction takes place, testing all membranes reported in Table 1. Effect of Organic- and Aqueous-Phase Flow Rates. The effect of organic- and aqueous-phase flow rates on the progress of oxidation of BzOH was studied in a flow-rate range of (5.7-10) × 103 mL/h. Experimental results are reported in Table 2 and Figure 2a,b. It is observed in these experiments that BzOH conversion to BzH increased as flow rate was raised (from 2.86% to 6.3%) with a loss of reaction selectivity from 94.3% to 90.5%, operating at 5.7 × 103 mL/h and 10 × 103 mL/h, respectively. In fact, increasing the liquid flow rate will enhance the mass transfer. However, in practice, the liquid flow velocity cannot be increased indefinitely. At higher liquid velocity, in fact, the liquid has a greater potential to wet the membrane, thus reducing the mass-transfer rate.26 Specifically, the effect on the reaction progress operating at different flow rates is clearly visible from the plots of BzH and BzA Vs time in the two reaction phases, i.e. organic (Figure 2a) and aqueous (Figure 2b). In the organic phase, the production of BzH proceeded for 60 min with a similar trend operating both at 7.5 × 103 mL/h and 10 × 103 mL/h (∼13.9 g/L), whilst a higher concentration of product was achieved during the time at 10 × 103 mL/h (19 g/L at 240 min). Analysis of the aqueous phase for detecting BzH and the overoxidation product, i.e. BzA, was carried out to examine the effect of different flow rates on the extraction capability of the membrane from the aqueous compartment to the organic one. It is noteworthy that at 10 × 103 mL/h, in the aqueous phase, the highest concentration of BzH produced during the reaction was detected, determining an overall conversion to the desired product of 6.3%, with a selectivity of 90.5% (Table 2). Therefore, the next experiments were carried out in order to facilitate BzH permeation across the membrane to the organic phase where it takes shelter from overoxidation, in this way increasing selectivity. First, the flow rate in the aqueous phase was kept constant at 10 × 103 mL/h, whilst different flow rates for the organic phase (3.2 × 103 mL/h and 1.7 × 103 mL/h) lower than that of the aqueous phase were used to avoid a breakthrough effect in the aqueous compartment, resulting in 986

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the mixing of the two phases. The experimental results are reported in Table 3 and Figure 3a,b. Interestingly, selectivity and conversion increased when operating at a lower organic-phase flow rate. In particular, we observed a conversion of 4.44% with a selectivity of 96% at 1.7 × 103 mL/h and 10 × 103 mL/h flow rates of organic and aqueous phase, respectively. In the experiments reported above, using the same flow rates for both organic and aqueous phases and increasing them from 5.7 × 103 to 10 × 103 (Table 2), 94.3% of selectivity was observed with a conversion of 2.86%. From data reported in Figure 3b, higher concentrations of BzH and BzA during the time in the aqueous phase operating at 3.2 × 103 mL/h have been detected compared to those observed at 1.7 × 103 mL/h. Therefore, for the next experiments this value for the organic-phase flow rate has been selected. Effect of Current Mode. The mode of current (co-current or counter-current) was another variable explored in this work to find the best operating conditions to increase selectivity and conversion to BzH. In Table 4 and Figure 4a,b the results obtained while operating at flow rates of 1.7 × 103 and 10 × 103 mL/h, respectively, for organic and aqueous phases in co-current and counter-current modes have been reported. Increases of selectivity (from 96% to 97.6%) and conversion (from 4.44% to 5.13%) have been obtained, operating in counter-current mode compared to operating in co-current mode. From Figure 4a,b it is evident that the increase of BzH concentration in organic phase vs time is higher in countermode than that in co-current mode. This result is confirmed by the analyses of the aqueous phase: BzH concentration in the aqueous compartment is slightly lower when operating in the counter-current mode with respect to results when operating in the co-current mode. Effect of the Membrane Characteristics. The influence of the hydrophobic/hydrophilic character of the porous membrane material was investigated. The aim of these tests was to compare the system performance by changing the membrane interface where the oxidation reaction takes place (Scheme 1). Three asymmetric membranes (M2-M4) with different thicknesses, pore size, and porosity (Table 1), but all prepared using PVDF, were used and compared with the commercial membrane, M1. M1 is a commercial composite membrane with the skin layer based on a fluoropolymer and the sublayer of PP. In Figure 5, the EDX analysis of membrane top surface of M1 is shown. In Figure 6a, SEM analysis of the membrane cross sections and of the top surface for the four membranes M1-M4 is shown. M2 and M3 are both asymmetric membranes but with some differences: microchannels and macrovoids are visible near the top layer on the sponge sublayer of M2; M3 showed macrocavities directly from the top layer. M4 is a symmetric membrane, characterized by a nodular morphology. All the membranes surfaces are characterized by the presence of pores: the difference concerns their sizes as shown in Table 1 and Figure 6b. Analyzing the selective side (top layer) of the three membranes (Figure 6b), M1 and M2 have pores of size of 0.2 µm,

Figure 6. (a) SEM analysis of cross sections of PVDF membranes (Table 1); for M1, cross section of selective layer based on PVDF is shown. (b) SEM analysis of the top layer of PVDF membranes (Table 1).

M3 has the smallest pores (0.07 µm). Due to the different preparation conditions which include the presence of a volatile solvent added to that hydroscopic,26 membrane M4 has the biggest pores of 0.9 µm.

An important difference that should be correlated with the reaction progress concerns the contact angle to water for the two membrane sides of M1-M4 (Table 1). (24) Mulder, M. Basic Principles of Membrane Technology; Kluwer: Dordrecht, 1991. Vol. 12, No. 5, 2008 / Organic Process Research & Development

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Figure 7. BzH concentration in the organic phase vs time, testing four different membranes as membrane contactors, M1-M4. (Organic-phase flow rate: 1.7 × 103 mL/h; aqueousphase flow rate: 10 × 103 mL/h; counter-current mode).

M4 has almost the same hydrophobic character for the two sides (top and bottom), with an R value of ∼120°. However, the top and bottom layers of M1, M2, and M3 membranes have different properties; for these membranes, the top surface is more hydrophilic (115°, for all). These different characteristics of hydrophobicity depended on the preparation conditions that are diverse for membranes M1 and M2 compared to those for M3 as reported in the literature.28 During the reaction tests, the membranes were located in the membrane reactor with the top surface from the aqueous phase side, i.e. with the more hydrophobic membrane side in contact with the organic phase. The results obtained have been reported in Table 5 and Figure 7. On the basis of these results, it is evident that the type of membrane used at the interface between the two reaction phases (25) Drioli, E.; Criscuoli, A. Curcio, E. Membrane Contactors: Fundamentals, Applications and Potentialities; Elsevier: Amsterdam, 2006; Chapter 10, Mass Transfer with Chemical Reaction. (26) Malek, A.; Li, K.; Teo, W. K. Ind. Eng. Chem. Res. 1997, 36, 784. (27) Buonomenna, M. G.; Macchi, P.; Davoli, M.; Drioli, E. Eur. Polym. J. 2007, 43, 1557. (28) Gugliuzza, A.; Ricca, F.; Drioli, E. Desalination 2006, 200, 26. (29) Van der Bruggen, B.; Schaep, J.; Wilms, D.; Vandecasteele, C. J. Membr. Sci. 1999, 156, 29.

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affected the formation of BzH. In particular, the best system performance was observed by using membrane M4: conversion of 47.9% with selectivity to BzH of 98.4% has been obtained. Membrane M3 showed the worst results in terms of conversion (2.11%) with a selectivity of 98.8%. It is noteworthy that selectivity using all the four membranes is >97%, showing that the operation conditions selected (organic and aqueous flow rates, current mode) favoured the extraction of BzH (formed at the membrane aqueous interface) in the organic compartment. From Figure 7, similar trends for membranes M2 and M4 can be observed, whilst for both M1 and M3, formation of BzH reached a plateau after 120 min. At 240 min, concentrations of BzH of 48 g/L and 21.3 g/L were observed for these membranes, respectively. The factor that has been taken in account to rationalize the different reactivities observed, changing the membrane, was the hydrophobic character [highly hydrophobic (>120°)] of the membrane side at the interface with aqueous phase, being the side in contact with organic phase (bottom layer) for all the membranes. In the case of M1 and M3 membranes with a more hydrophilic top layer (in contact with the aqueous phase), the active catalytic species (i.e., the formed peroxocomplexes) should permeate across the membrane, increasing the transport resistance and lowering the driving force to transport of the lipophilic substrate. However, across membranes M4 and M2, more hydrophobic (i.e., more organophilic) from both the sides, the preferential direction of transport is that from the organic phase to the aqueous phase, keeping in contact the BzOH at the interphase membrane-aqueous phase with the active catalytic species. Therefore, the observed trend for the performance in terms of conversion (Table 5) i.e. M4 > M2 > M1 > M3, reflected the different hydrophobicities of the membrane top layers (Table 1). Received for review June 3, 2008. OP800129K