New Chiral Catalytic Membranes Created by Coupling UV

Jun 25, 2012 - For the HFM-salens in type of segments, on which only about 2% of mono-CH2Cl–salen–Co(III) used in homogeneous catalysis was ...
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New Chiral Catalytic Membranes Created by Coupling UVPhotografting with Covalent Immobilization of Salen−Co(III) for Hydrolytic Kinetic Resolution of Racemic Epichlorohydrin Zhi-Ping Zhao,* Mei-Sheng Li, Jia-Yin Zhang, Hui-Na Li, Peng-Peng Zhu, and Wen-Fang Liu School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China ABSTRACT: New chiral catalytic functional membranes (HFM-salens) were obtained successfully by covalent immobilizing aminated mono-CH2Cl−salen−Co(III) catalysts onto polyethylene (PE) hollow fiber membranes (HFMs) that had been, in advance, modified by UV-induced graft polymerization of acrylic acid. The catalytic properties of HFM-salens were investigated by carrying out hydrolytic kinetic resolution experiments of racemic epichlorohydrin. For homogeneous mono-CH2Cl−salen− Co(III) catalyst, the enantioselectivity (ee) and yield (Y) of S-epichlorohydrin were 86.67% and 57.26%, respectively. For the HFM-salens in type of segments, on which only about 2% of mono-CH2Cl−salen−Co(III) used in homogeneous catalysis was immobilized, an ee of 5.45% and a Y of 8.23% were achieved. Moreover, the ee and Y values reached 44.78% and 47.01%, respectively, using only eight HFM-salens packed in a catalytic membrane reactor (CMR), and the ee and Y values can be further improved by increasing packing density of fibers in the CMR. As an environment-friendly technology, immobilization of catalysts on membranes makes it easy to reuse the catalysts and reduce the amount of catalysts significantly. The results offer great potential for these modified membranes in the CMR for further research.

1. INTRODUCTION The chiral salen−Co(III) catalyst is one of the most important ligands for homogeneous asymmetric catalytic reactions and asymmetric synthesis.1,2 Because of its high catalytic activity and wide range of applications, the chiral salen−Co(III) catalyst shows a good industrial application prospect.3−6 However, for homogeneous catalysis process, separation and recycling of the catalysts often prove difficult. Recently, the development of environment-friendly technologies has prompted much research in heterogeneous catalysis and in particular the heterogenization of known active homogeneous catalysts for oxidation.7 In theory, the potential engineering benefits of heterogenization include facilitation of catalyst separation from reagents and reaction products, simplification of methods for catalyst recycling, ruggedness, and the possible adaptation of the immobilized catalyst to continuous-flow processes. Therefore, to immobilize the homogeneous catalysts, various supports and strategies have been employed. The supports, such as ionic liquids,8 mesoporous materials,9,10 activated carbon, 1 1 zeolite, 1 2 polymer, 7 clay compounds, 1 3 polydimethylsiloxane(PDMS) membranes,14 silica particles, and so on,15 have been investigated. The strategies include coordination polymerization method,15 mesoporous molecular sieve method,16 chemical binding,8,17 sol−gel,18 etc. Among these different heterogenization routes, encapsulation by polymers seems advantageous because in addition to stabilizing and protecting the particles, polymers offer unique possibilities for modifying both the environment around catalytic sites and the access to these sites.19 Thus, the entrapping of catalysts in polymeric membranes offers a new possibility for catalyst design. Mac Leod et al.20−22 studied the encapsulation of Mn(salen) complexes in PDMS-based membranes. These membrane materials offer several advantages concerning their affinity for reagents being the major property. The membrane © 2012 American Chemical Society

not only controls the access of both the substrate and the oxidant to the active site but also improves the contact between the reactants and the catalyst. Caselli et al.23 embedded salen− Co(II) complexes into polymeric flat membranes and designed a new flat catalytic membrane reactor for cyclopropanation reaction. The results showed that the interaction between the polymeric material and the organic ligand is well matched. It is intuitive to understand that the development of this innovative catalytic method should improve the synthesis of useful fine chemicals as cyclopropanes using environmentally benign technologies. Obviously, to obtain more efficient catalysts, there are still a lot of improvements to be made for the catalytic membrane reactor, such as membrane types, membrane materials, reactor structures, and operation conditions. Among the different existing polymeric membranes, polyethylene (PE) hollow fiber membranes (HFMs) give the main advantage of having a very high-volume area, which allows the manufacture of modules with a large filter surface but with a reduced volume.24 Moreover, HFMs properties can be tailored by surface modification carried out through different techniques such as electronic beam, UV photografting, plasma, treatment with ozone, and chemical oxidation.25 Furthermore, these methods may help to obtain various types of grafted polymeric chains. Among them, UV photografting has obtained much interest of researchers for its low operation cost, weak penetration of absorbed UV light, and the required mild reaction conditions, which does not affect the bulk polymer.26−28 Meanwhile, this kind of grafting is very stable due to the covalent bond existing between the grafted polymer Received: Revised: Accepted: Published: 9531

May 8, 2012 June 20, 2012 June 25, 2012 June 25, 2012 dx.doi.org/10.1021/ie3011935 | Ind. Eng. Chem. Res. 2012, 51, 9531−9539

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resolution (HKR) of racemic epichlorohydrin.29 In the monoCH2Cl−salen−Co(III) catalyst, it is well known that the active −CH2Cl group can react with ethylenediamine easily. If the NH2-containing groups are chemically introduced into the catalysts, they will be suitable and stable to react with −COOH groups. In this work, the mono-CH2Cl−salen−Co(III) catalyst was further aminated with ethylenediamine first. Then, the aminated mono-CH2Cl−salen−Co(III) was immobilized by chemical binding onto PE HFMs that were, in advance, modified by UV-induced graft polymerization of acrylic acid (AA). The new catalytically active membranes (HFM-salens) were applied in the HKR of racemic epichlorohydrin in a catalytic membrane reactor (CMR). For comparison, these heterogeneous membrane catalysts in type of small segments and homogeneous catalysts were also evaluated in the HKR of racemic epichlorohydrin.

and the membrane. It can also be useful for adding chemical groups (−COOH, −NH2, −CHO, −OH etc) at the surface of the membrane to provide the anchorage sites and facilitate the immobilization of another functional compounds. Thus, to generate functionalized PE membranes (COOH-containing groups) suitable for the immobilization of chiral salen−Co(III) complexes, the UV photografting technique has been employed in this work. However, in the traditional chiral salen−Co(III) catalysts (Figure 1),6 in which the 3,5-di-tert-butyls are inactive, there are no suitable functional groups used for covalent immobilization with the functionalized PE membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. PE HFMs were supplied by ICCAS (Institute of Chemistry, Chinese Academy of Sciences (CAS)). Unless otherwise noted, all chemicals were analytical reagent (AR). Benzophenone (BP), AA (without removing the inhibitor), methyl isobutyl ketone, and epichlorohydrin were purchased from Tianjin Bodi Chemical Holding Co., Ltd.; MgSO4 from Beijing East Longshun Chemistry Syntheses Technique Center; tetrahydrofuran, acetone, ethanol, toluene, ethylene diamine, potassium carbonate, methylene chloride,

Figure 1. Structure of traditional chiral salen−Co(III)(OAC).

Recently, we have synthesized a new salen catalyst (Scheme 1), (R,R)-N-(3-tert-5-chloromethylsalicylaldehyde)-N-(3,5-ditert-butyl salicyladehyde)-1,2-cyclohexane-diamine cobalt acetic acid monohydrate (mono-CH2Cl−salen−Co(III)), which has a mono-chloromethyl group and was successfully used in a homogeneous catalytic reaction of the hydrolytic kinetic

Scheme 1. Synthesis Route of Chiral Mono-CH2Cl−Salen−Co(III) Catalysts

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sodium chloride, sodium bicarbonate, and N, N-dimethylformamide (DMF) from Beijing Chemical Plant; and 1-(3dimethylamino propyl)-3-ethyl carbodiimide hydrochloride (EDC.HCl, 99%) from Shanghai Covalent Chemical Co., Ltd. 2.2. Amination of Chiral Mono-CH2Cl−Salen−Co(III) Catalysts. The syntheses route of mono-CH2Cl−salen− Co(III), which has mono-chloromethyl group, was reported in our previous work.29 In order to introduce the −NH2 group into it, the mono-CH2Cl−salen−Co(III) was functionally modified with ethylenediamine in toluene solvent. In addition to providing amino groups, the ethylenediamine could play an ″arm″ role between membrane and catalyst, and this may provide the immobilized catalyst a necessary flexibility. 2.3. Modification of PE Membranes by UV-Initiated Graft Polymerization. To graft the −COOH groups onto the original PE HFMs, the experimental method was as follows. First, the HFMs were washed twice with deionized (DI) water for 5 min, and then dried in a vacuum oven until constant weight and weighed using an analytical balance with 10−5 precision. Second, the quantified PE HFMs were immersed in 30 mL of acetone solution containing photoinitiator (BP). Then, it was installed in the UV photografting pilot to generate free radicals by irradiating with a high-pressure mercury lamp for 7 min and stirred at room temperature for 5 min. The same process was repeated once. Third, the quantitative activated PE HFMs were immersed in 30 mL of 1.9 M AA solution using water−ethanol mixture as the solvent. The solution was purged by nitrogen gas to remove dissolved air in solution and stirred for 10 min. The graft polymerization was carried out by irradiating the PE HFMs with the high-pressure mercury lamp for a certain time. Then, the solution was removed from the UV photografting pilot and heated to 40 °C; the reaction was maintained for a certain time. A mild reaction temperature of 40 °C was selected so as to avoid the homopolymerization of AA monomers. After the graft polymerization, to remove any residual monomer and polymer, the PE HFMs grafted with AA (HFM-AAs) were washed in acetone, DMF, acetone, and DI water in order. Then, the PE HFM-AAs were dried in a vacuum oven at room temperature for 48 h and weighed. The grafting yield (GY) was determined by the following m − m0 GY = 1 × 100% m0

Scheme 2. Immobilization Route of Aminated Chiral Salen− Co(III) Catalyst on the HFM-AAs

2.5. Characterization Methods. Scanning Electron Microscopy (SEM) analysis was done on a SM-740/F scanning electron microscope. Prior to SEM analysis, the membrane specimens were carefully taken from the middle of the elements (lengthwise) using a pair of pincers and sputter coated with gold. Information about the presence of specific functional groups on the PE membrane surface was obtained by Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) (Nicolet 380). X-ray Photoelectron Spectroscopy (XPS) analyses were performed on a PHI ESCA 5300 equipped with a monochromatic Al Kα radiation (1253.6 eV) at an operating power of 300 W. The measurements have been performed at a takeoff angle (TOA) fixed at 45° (measured with respect to the sample surface). For calibration, the carbon C1s peak was used (284.8 eV). 2.6. HKR Experiments of Racemic Epichlorohydrin. The main and side reactions are shown in Scheme 3. The Scheme 3. HKR Reactions of Racemic Epichlorohydrin

where m1 and m0 are the weights of the grafted and original PE HFMs, respectively. 2.4. Immobilization of Chiral Salen−Co(III) catalysts. The immobilization route of aminated chiral salen−Co(III) is showed in Scheme 2. The modified PE HFMs (0.7734 g), i.e., HFM-AAs, were dipped into an EDC-HCL solution of DMF and stirred at a certain temperature for 0.5 h. Then, the aminated chiral monoCH2Cl−salen−Co(III) solution of DMF was added under stirring. A few hours later, the membranes immobilized with chiral salen−Co(III) catalysts were washed in acetone, NaHCO3 and DI water in order and then were dried in a vacuum oven until constant weight. The immobilization degree (ID) was determined by the following w − w0 ID = 1 × 100% w0

performance of HFM-salens was investigated using a CMR with recycle of permeate (Figure 2). Eight or two PE HFM-salens with the length of 15 cm were assembled in the CMR. A mixed solution of 2 mL racemic epichlorohydrin, 0.4 mL DI water, and 4 mL DMF was used as feed solution. The reaction solution flew toward and across the catalytic membranes. Then, an effective contact between the reactants and the immobilized catalysts was obtained. The temperature was maintained at 20

where w1 and w0 are the weights of the catalyst-immobilized AA-grafted PE HFMs (HFM-salens) and HFM-AAs, respectively. 9533

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Figure 2. Catalytic membrane reactor (CMR) for the HKR of racemic epichlorohydrin.

°C. The samples were taken periodically and analyzed by GC (TECHCOMP 7890II), using a GAMMA DEX 225 instrument equipped with a 30 m × 0.25 mm ×0.25 μm film thickness silica capillary columns. For the HFM-salens in type of small segments and homogeneous catalysts, the experiments were carried out in a jacketed batch reactor equipped with a stirrer at 20 °C. In a typical experiment, the reactor was loaded with 10 mL racemic epichlorohydrin and water, which molar ratio relative to the racemic epichlorohydrin is 0.655 and 0.350 g catalytic membrane cut into small segments.

3. RESULTS AND DISCUSSION 3.1. UV-Initiated Graft of PE HFMs. 3.1.1. SEM and XPS Analysis. Figure 3 shows the SEM images of surfaces of the original membrane and HFM-AAs. Membrane pores were progressively filled with polymerized AA with increase in grafting time, i.e., in the GY. Most of the pores at the surface were filled when the GY approached to 9.31% as indicated in Figure 3 (b). As the grafting reaction proceeded, a denser grafted layer was formed on the membrane surface when the GY approached 12.95% as shown in Figure 3 (c). Figure 4 shows the XPS survey scan spectra of original PE membranes and HFM-AAs with GY of 9.31%. It is shown that the C1s peak decreased a little, while the O1s peak relatively increased. The oxygen content increased from 8.28% of the original PE membrane to 13.71% of the HFM-AAs. Correspondingly, the percent ratio of O/C increased from 0.0903% to 0.1589%. Thus, it can be confirmed that the carboxylic acid groups (−COOH) were introduced onto the original PE membrane. 3.1.2. Grafting Yield of AA. Figure 5 shows the effect of monomer concentrations on the GY. The concentration of photoinitiator (BP) was 0.24 mol/L, which was optimized in previous work.29 The preirradiation and reaction time of the graft polymerization were 4 and 30 min, respectively. Evidently, the GY, depending on monomer concentration, was almost linear under comparatively low concentrations, but this enhancement of GY was not provided by monomer concentration beyond a certain monomer concentration. This could be partly attributed to the limited number of active free radicals on the membrane surface.

Figure 3. SEM micrographs of the surface of (a) original PE membrane and (b,c) HFM-AAs with grafting yield of 9.31% and 12.95%, respectively.

Figure 4. XPS survey scan spectra of original PE HFMs (a) and HFMAAs (b).

On the other hand, the substantial amount of polymer grafted onto the membrane substrate may inhibit the diffusion 9534

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3.2. Characterization of the Catalytic Functional Membranes. 3.2.1. ATR-FTIR Analysis of the Aminated Mono-CH2Cl−Salen−Co(III). To introduce the −NH2 group into the catalyst, the mono-CH2Cl−salen−Co(III), which was synthesized in our previous work,29 was functionally modified with ethylenediamine in toluene solvent. Figure 7 shows the

Figure 5. Effect of monomer concentration on the grafting yield of AA.

of monomer into the PE for further grafting. Also, the effect of UV preirradiation time on the AA GY at various reaction times was studied (Figure 6). The GY increased in proportion to UV

Figure 7. ATR-FTIR spectra of the aminated mono-CH2Cl−salen− Co(III).

ATR-FTIR spectra of the aminated mono-CH2Cl−salen− Co(III) measured in the ATR mode. In the aminated monoCH2Cl−salen−Co(III), IR peaks corresponding to C−NH2 stretching were observed at 3190 and 3095 cm−1, respectively. These peaks indicate that −NH2 groups were successfully grafted to the original catalysts. In addition to providing amino groups, the ethylenediamine could play an ″arm″ role between membrane and ligand, and this may provide the immobilized catalyst a necessary flexibility. 3.2.2. XPS Analysis of the Catalytic Functional Membranes. The covalent immobilization was carried out at 0 °C for 30 h, and an ID of 9.12% was obtained. Figure 8 shows the XPS survey scan spectra of the HFM-AAs and HFM-salens. The most obvious change between HFM-AAs and HFM-salens is the exhibitions of N and Co atoms, which are contained in Figure 6. Effect of UV preirradiation time on the AA grafting yield at various reaction times.

preirradiation time up to about 4.5−5.0 min and then increased slowly at same reaction time. It is known that the free radicals created by UV irradiation in solid polymers may be increased with irradiation time.30 Subsequently, there would be more surface free radicals for grafting reaction. On the other hand, at the same preirradiation time, the GY of PE HFMs was found to increase rapidly with reaction time. For example, the GY increased from 13.60% to 18.22% by increasing the reaction time from 30 to 70 min under the preirradiation time of 5.5 min. These could be attributable to the gradual increase in polymer grafting chain with increasing reaction time at every active site. This result is in reasonable agreement with the result for SEM analysis in Figure 3. As the GY increased, more internal and surface pores of PE membranes were filled with more grafted monomers. Thus, to save abundant pores for immobilization of the chiral salen−Co(III) catalysts on the inner pore wall, the GY of 9.31% was selected in the present work.

Figure 8. XPS survey scan spectra of HFM-AAs (a) and HFM-salens (b). 9535

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Table 1. Atomic Percent Concentration and Ratio of HFM-AAs and HFM-salens sample

C

N

O

Co

N/C

O/C

Co/C

HFM-AAs HFM-salens

86.29 84.57

− 4.38

13.71 10.65

− 0.40

− 0.0518

0.1589 0.1259

− 0.00473

of 284.8 eV represents the peak of the hydrocarbon. The peak at the binding energy of 289.0 eV corresponds to the binding energy of the −CO and OC−O bonds. HFM-salens show four new peaks at the binding energy of 286.1, 288.2, 288.0, and 292.6 eV, different than those shown in HFM-AAs. These peaks correspond to each C−N, OC−N−H, CN, and −C−C*-(aromatic ring) bond. From the percent peak area of HFM-AAs and HFM-salens calculated from C1s core level spectra (Table 2), it is shown that the amounts of −CO and −COO groups decreased from 11.11% (HFM-AAs) to 4.05% (HFM-salens) after immobilization. Relatively, in HFM-salens, the amounts of new C−N, OC−N−H, CN, and −C− C*− (aromatic ring) groups increased. This result is consistent with the result depicted above. Thus, it can be concluded that the aminated mono-CH2Cl−salen−Co(III) catalysts were successfully immobilized on the AA-grafted PE HFMs surface. 3.3. Catalytic Properties. The new catalytic membranes, HFM-salens, were evaluated in the HKR experiments of racemic epichlorohydrin in a CMR at 20 °C. For comparison, the HFM-salens in type of small segments and homogeneous chiral salen−Co(III) catalysts were also investigated under the same reaction conditions. The results (Figure 10) show that the homogeneous catalyst had high catalytic activity in the HKR experiments of racemic epichlorohydrin. The enantioselectivity (ee) and yield (Y) values of (S)-epichlorohydrin increased with increasing catalyst content from 0.5% to 0.7% (molar ratio of catalyst to racemic epichlorohydrin). The ee values increased rapidly with reaction time until about 45 h and then leveled off, while the Y values increased first with reaction time until about 30−40 h and then decreased slightly with increasing reaction time. When the catalyst content was 0.7%, a highest ee value of 86.67% was achieved at about 50 h, and a highest Y value of 57.26% was obtained at about 40 h. HFM-salens with ID of 9.12% and 8.79% were used in the experiments, and the weight of HFM-salens segments was 0.350 g. It was then calculated that the contents of catalyst, which was immobilized on the sample membranes, were only about 2% of the content of mono-CH2Cl−salen−Co(III), which was used in homogeneous catalysis process, in which the catalyst of 0.7% (molar ratio of homogeneous catalyst to racemic epichlorohydrin) was employed. Figure 11 shows the effect of reaction time on ee and Y values for the two kinds of catalysts. As shown, the ee and Y values of (S)-epichlorohydrin increased with an increase in ID from 8.79% to 9.12%. Different from the homogeneous process, the ee and Y values of (S)epichlorohydrin increased slowly with increasing reaction time during the experimental time of 72 h. There was no highest value for Y. It was also found that for the HFM-salens with ID of 9.12% an ee of 5.45% and Y of 8.23% were achieved. The new heterogeneous membrane catalysts exhibited a HKR property for the racemic epichlorohydrin. However, there was insufficient catalysis due to the low ID and inadequate contacts between the reactants and the immobilized catalysts. Then, the ID was further improved, and a new catalytic membrane reactor (CMR) was designed to increase catalytic properties. After immobilization, eight or two PE HFM-salens with the length of 15 cm were assembled in the CMR for the HKR of

the membrane HFM-salens, suggesting the aminated monoCH2Cl−salen−Co(III) catalysts were introduced on the AAgrafted PE membrane surface. Table 1 shows quantitative atomic percent concentration and ratio of the samples. The obvious change between HFM-AAs and HFM-salens is that oxygen content decreased from 13.71% in HFM-AAs to 10.65% in HFM-salens. Obviously, the covalent grafting caused a decrease in the percent ratio of O/ C from 0.1589% to 0.1259%. This was due to the condensation during covalent immobilization reaction between the −OH groups on the HFM-AAs and the −NH2 groups in the aminated mono-CH2Cl−salen−Co(III). Similarly, as shown in C1s spectra of the samples (Figure 9), a peak at binding energy

Figure 9. C1s spectra of HFM-AAs (a) and HFM-salens (b). 9536

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Table 2. Percent Peak Area of XPS C1s Core Level Spectra of HFM-AAs and HFM-salens sample

−C−C−,−CH (284.8ev)

OC−O,C−O (289.0ev)

C−N (286.1ev)

OC−N−H (288.2ev)

CN (288.0ev)

−C−C*− (aromatic ring) (292.6ev)

HFM-AAs HFM-salens

88.89 84.01

11.11 4.05

− 1.42

− 4.05

− 1.42

− −

Figure 10. HKR experiments of racemic epichlorohydrin using homogeneous mono-CH2Cl−salen−Co(III) catalyst: (a) ee values, (b) Y values.

Figure 11. HKR experiments of racemic epichlorohydrin using heterogeneous membrane catalysts: (a) ee values, (b) Y values.

racemic epichlorohydrin immediately. The ID of HFM-salens was 12.02%. It was then calculated that the total catalysts used in the CMR were about 3.40% of that used in homogeneous catalysis process for two fibers. For the two fibers, the results (Figure 12) show that the ee and Y values of (S)epichlorohydrin increased slowly with the reaction time. The ee and Y values can reach 15.40% and 22.40%, respectively, when the reaction time was prolonged to 72 h. The CMR exhibited a higher catalytic property compared with the type of segments. These may be attributed to an effective contact between the reactants and the immobilized catalysts in the CMR. In addition, a higher ID, which was optimized, was also an important factor. Moreover, as shown in Figure 13 and Table 3, the ee and Y values increased greatly with an increase in the number of fibers. For the eight fibers, the results show that the ee and Y values of (S)-epichlorohydrin increased

slowly with the reaction time. The ee and Y values can reach 37.67% and 44.00%, respectively, when the reaction time was prolonged to 72 h, and a 44.78% ee value and a 47.01% Y value were obtained without using an internal standard substance. It is inferred that the internal standard substance may affect the ee and Y values because of its dilution. For the earlier PDMS membrane-supported catalysts, the complexes (salen−Mn) were occluded in a PDMS flat membrane, and the activity of the catalytic membranes was tested in the epoxidation of olefins. The highest ee value was 52% for the epoxidation of styrene, and only 18% for the epoxidation of trans-βmethylstyrene.14 Note, importantly, the ee and Y values can be improved by increasing the packing density of PE HFMsalens in this work. The results offer great potential for these modified membranes in the CMR for further research. 9537

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membrane outer surface and inner pore wall. The novel heterogeneous membrane catalysts were efficiently used for the HKR of racemic epichlorohydrin. For the HFM-salens in type of segments, on which only about 2% of mono-CH2Cl−salen− Co(III) used in homogeneous catalysis was immobilized, an ee of 5.45% and a Y of 8.23% were achieved. Moreover, the ee and Y values reached 44.78% and 47.01%, respectively, in the CMR with only eight PE HFM-salens. Meanwhile, as an environment-friendly technology, immobilization of catalysts on membranes makes it easy to reuse the catalysts and reduce the amount of catalysts significantly. However, the ee and Y values were not more than 50% due to the nonuniform structures of PE HFMs and the low packing density. Importantly, the ee and Y values can be improved by increasing packing density of fibers in the CMR. The results offer great potential for these modified membranes in the CMR for further research. Above all, the feasibility of HFM-salens application in continuous CMR is proven in our results. Nevertheless, due to few detailed reports on the HKR reactions of racemic epichlorohydrin with using the similar HFM-salens CMR, the catalysis comparison to other studies cannot be given in this work.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-68911032. Fax: +86-10-68911032. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No.20676015, No.20976012, No.20806009) and the Research Fund for the Doctoral Program of Higher Education of China (No.20091101110035) for their financial support.

Figure 12. HKR experiments of racemic epichlorohydrin using heterogeneous membrane catalysts in the CMR: (a) ee values, (b) Y values.

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Table 3. Catalytic Results for Different Catalyst Types catalyst type homogeneous chiral salen−Co(III) catalysts HFM-salens in type of small segments HFM-salens packed in a CMRb

catalyst contenta (%)

number of HFMsalens

ID (%)

Y (%)

ee (%)

0.70

−

−

57.26

86.67

0.015

−

9.12

8.23

5.45

0.12 0.48 0.48

2 8 8

12.02 12.02 12.02

22.40 44.00 47.01c

15.40 37.67 44.78c

REFERENCES

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a

Molar ratio of chiral salen−Co(III) catalysts to racemic epichlorohydrin. bThe length of HFM-salens is 15 cm. cWithout using internal standard substance.

4. CONCLUSIONS The heterogenization of mono-CH2Cl−salen−Co(III) catalysts on the surface of PE-based polymeric membranes were successfully achieved. First, PE HFMs were modified by UVinduced graft polymerization of AA. The polar functional groups (−COOH) were introduced on the original PE membrane. Second, the aminated mono-CH2Cl−salen−Co(III) catalysts were immobilized on the AA-grafted PE 9538

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dx.doi.org/10.1021/ie3011935 | Ind. Eng. Chem. Res. 2012, 51, 9531−9539