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Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

jou rn al h om epa ge: www.elsev ier .com/ locate /molcata

high efficient large-scale asymmetric epoxidation of unfunctionalized olefinsmploying a novel type of chiral salen Mn(III) immobilized onto layeredrystalline aryldiamine modified zincoly(styrene-phenylvinylphosphonate)-phosphate

ing Huanga, Xiangkai Fua,∗, Gang Wanga, Yaqin Geb, Qiang Miaoa

College of Chemistry and Chemical Engineering Southwest University, Research Institute of Applied Chemistry Southwest University, The Key Laboratory of Applied Chemistry ofhongqing Municipality, The Key Laboratory of Eco-environments in Three Gorges Reservoir Region Ministry of Education, Chongqing 400715, ChinaChongqing YiPaiYin Chemival Products Co. Ltd., China

r t i c l e i n f o

rticle history:eceived 21 November 2011eceived in revised form 8 February 2012ccepted 9 February 2012vailable online 17 February 2012

a b s t r a c t

A series of chiral salen Mn(III) catalysts immobilized onto aryldiamine modified ZnPS-PVPA with differentratio of phosphonate/phosphate for asymmetric epoxidation were synthesized and characterized by FT-IR, diffusion reflection UV–Vis, AAS, N2 volumetric adsorption, SEM, TEM, XPS, XRD and TG. The supportedcatalysts displayed superior catalytic activities in the asymmetric epoxidation of �-methylstyrene andindene with m-CPBA and NaIO4 as oxidants. Moreover, the heterogeneous catalysts were relatively stableand could be recycled nine times in the asymmetric epoxidation of �-methylstyrene. Furthermore, this

eywords:hiral Mn(III) saleninc poly(styrene-henylvinylphosphonate)-phosphateryldiamine modifiedeterogeneous catalyst

novel type of catalyst could also be validly used in large-scale reactions with superior catalytic dispositionbeing maintained at the same level, which possessed the potentiality for application in industry.

Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

symmetric epoxidation

. Introduction

Epoxides are important intermediates for the synthesis ofomplex molecules which are used in fine chemical synthesis.symmetric epoxidation of prochiral alkenes presents a powerfultrategy for the synthesis of enantiomerically enriched epoxides.n the past two decades there has been great progress in cat-lytic asymmetric alkene epoxidation. Important contributionsave been made using Sharpless [1] and Jacobsen/Katsuki [2–4]ystems. Chiral salen–Mn(III) complexes (Jacobsen’s catalyst) showigh enantioselectivities in the epoxidation of unfunctionalizedlkenes under homogenous conditions [5]. However, the separa-ion and recycling of these catalysts are still problematic issues6]. In addition, salen transition metal complexes are very expen-ive materials, mainly in their chiral forms [7]. The conventionalay to solve these problems is to immobilize chiral manganese

III) salen complexes onto solid supports. Various approaches formmobilization of chiral manganese (III) salen complexes haveeen described in an excellent review, which includes grafting the

∗ Corresponding author. Tel.: +86 23 68253704; fax: +86 23 68254000.E-mail address: fxk@swu.edu.cn (X. Fu).

381-1169/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rioi:10.1016/j.molcata.2012.02.008

catalyst on a solid inorganic support such as silica or MCM-41,encapsulation into the pores of zeolites, physical entrapment ina polydimethylsiloxane membrane (polymer support), clays, andactivated carbon [8]. The trend to develop reusable salen–metalcomplexes with high efficiency and catalytic stability is increasingfrom the environmental concerns together with economic consid-erations. Accordingly, methodologies for the heterogenization ofhomogeneous salen–metal complexes have emerged. Among them,heterogenization of salen–metal complexes into/onto inorganic orinorganic–organic hybrid supports is one of the promising strate-gies.

In the last decades, our groups have reported a seriesof organic–inorganic hybrid zirconium phosphonate-phosphates Zr(HPO4)2−x(O3P–G)x·nH2O (x = 0–2, G is organicgroups) as various kinds of catalysts or catalyst sup-ports, such as solid acid catalysts zirconium sulfophenylphosphonate-phosphate Zr (HPO4)2−x(O3PC6H4SO3H)x·nH2O,and zirconium [N,N-di(phosphono-methyl)iminodiacetic acid]Zr[(O3PCH2)2N(CH2COOH)]·nH2O and their palladium com-

plexes as catalysts in hydrogenation [9–12]. Apart fromthese facts, we have also focused on the immobilization ofhomogeneous chiral salen Mn(III) complexes, such as the sup-ported chiral salen Mn(III) catalysts on modified zirconium

ghts reserved.

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2

2

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rtsgRf3rrw(g(sipse0aer

J. Huang et al. / Journal of Molecular

ligostyrenylphosphonate-phosphate (ZSPP) and zirconiumoly(styrene-phenylvinylphosphonate)-phosphate (ZPS-PVPA).

t was noteworthy that the heterogeneous catalysts showedigher enantioselectivity than that of the homogeneous chiralatalyst and superior reusability in asymmetric epoxidation ofnfunctionalized olefins under the same conditions [13–16].

Whereas, few investigations were explored in organic polymer-norganic hybrid zinc phosphonate-phosphate used for themmobilization of chiral salen Mn(III). At the same time, somef the heterogeneous catalysts have proved to be successful inhe epoxidation of olefins, which encouraged us to pursue a newunctional group to bind chiral salen Mn(III) onto zinc poly(styrene-henylvinylphosphonate)-phosphate (ZnPS-PVPA) to generate aecyclable asymmetric epoxidation catalyst. Herein, it is still of aca-emic interest and commercial importance to develop efficient andeusable chiral catalyst for the epoxidation of non-functionalizedlefins. In view of this, we documented that a series of new typef layered crystalline organic polymer-inorganic hybrid materialsnPS-PVPA were synthesized and aryldiamine modified ZnPS-PVPAas applied to immobilize the chiral salen Mn(III) complexes

hrough axial coordination. And we also reported the immobilizedatalysts were stable, recoverable, reusable catalysts with supe-ior enantioselectivity, and could be used in large-scale reactionsith the enantioselectivity being maintained at the same level. In

ddition, the question as to whether or not various proportionsf organic phosphonate to inorganic phosphate, various inorganichosphate resource and different linkages contributed to the cat-lytic activities and enantioselectivities were also examined here.

. Experiment

.1. Materials and instruments

(1R,2R)-(−)-1,2-diaminocyclohexane, chloromethyl methylther (toxic compound), �-methylstyrene, n-nonane, N-ethylmorpholine N-oxide (NMO) and m-chloroperbenzoic

cid (m-CPBA) were supplied by Alfa Aesar. Other commerciallyvailable chemicals were laboratory-grade reagents from localuppliers. Chiral salen ligand and chiral homogeneous catalystalen Mn(III) were synthesized according to the standard literaturerocedures [17] and further identified by analysis and comparisonf IR spectra with literature [18].

Fourier transform infrared spectroscopy (FT-IR) spectra wereecorded from KBr pellets using a Bruker RFS100/S spectropho-ometer (USA) and diffuse reflectance UV–Vis spectra of the solidamples were recorded in the spectrophotometer with an inte-rating sphere using BaSO4 as standard. Proton Nuclear Magneticesonance (1H NMR) and Phosphorus-31 (31P NMR) were per-

ormed on AV-300 NMR instrument at ambient temperature at00 and 121 MHz, respectively. All of the chemical shifts wereeported downfield in ppm relative to the hydrogen and phospho-us resonance of TMS and 85% H3PO4, respectively. Number- andeight-average molecular weights (Mn and Mw) and polydispersity

Mw/Mn) were estimated by Waters1515 gel permeation chromato-raph (GPC; against polystyrene standards) using TetrahydrofuranTHF) as an eluent (1.0 mL/min) at 35 ◦C. X-ray photoelectronpectrum (XPS) was recorded on ESCALab250 instrument. Thenterlayer spacings were obtained on DX-1000 automated X-rayower diffractometer (XRD), using Cu K� radiation and internalilicon powder standard with all samples. The patterns were gen-rally measured between 2.00◦ and 80.00◦ with a step size of

.02◦/min and X-ray tube settings of 36 kV and 20 mA. The C, Hnd N elemental analysis was done on an EATM 1112 automaticlemental analyzer instrument (Thermo, USA). Thermogravimet-ic (TG) analyses were performed on a SBTQ600 thermal analyzer

sis A: Chemical 357 (2012) 162– 173 163

(USA) with the heating rate of 20 ◦C/min from 25 to 1000 ◦C underflowing N2 (100 mL/min). The Mn contents of the catalysts weredetermined by a TAS-986G (Pgeneral, China) atomic absorptionspectroscopy (AAS). Scanning electron microscopy (SEM) wereperformed on KYKY-EM 3200 (KYKY, China) micrograph. Trans-mission electron microscopy (TEM) were obtained on a TECNAI10(PHILIPS, Holland) apparatus. Nitrogen adsorption isotherms weremeasured at 77 K on a 3H-2000I (Huihaihong, China) volumet-ric adsorption analyzer with BET method. The racemic epoxideswere prepared by epoxidation of the corresponding olefins by 3-chloroperbenzoic acid in CH2Cl2 and confirmed by NMR (BrukerAV-300), and the gas chromatography (GC) was calibrated withthe samples of n-nonane, olefins and corresponding racemic epox-ides. The conversions (with n-nonane as internal standard) andthe ee values were analyzed by GC with a Shimadzu GC2010(Japan) instrument equipped using a chiral column (HP19 091G-B213, 30 m × 30 m × 0.32 mm × 0.25 �m) and FID detector, injector230 ◦C, detector 230 ◦C. Ultrapure N2 was the carrier gas (rate34 mL/min) with carrier pressure 39.1 kPa and the injection poretemperature was set at 230 ◦C. The column temperature for indene,�-methylstyrene was programmed in the range of 80–180 ◦C.

2.2. Synthesis of the support (Scheme 1)

2.2.1. Synthesis of styrene-phenylvinyl phosphonic acidcopolymer (PS-PVPA)

1-Phenylvinyl phosphonic acid (PVPA) was synthesized accord-ing to literature [19] and its structures were confirmed by 1H NMR,31P NMR and FT-IR. 1H NMR (CDCl3): 6.06 (d, 1H), 6.23 (d, 1H),7.26–7.33 (m, 3H), 7.48 (m, 2H). 31P NMR (CD3OD): 15.9. IR (KBr):2710, 2240, 1500, 1200, 1040, 950, 780, 720, 700 cm−1.

1-Phenylvinyl phosphonic acid (4 g, 21.7 mmol), styrene (20 mL,173.9 mmol), ethyl acetate (150 mL) and benzoyl peroxide (BPO,1.0 g, 4.7 mmol) were used for preparation of PS-PVPA copoly-mer as literature [15] yield 7.52 g. GPC: Mn = 38608, m = 38, n = 8,Mw/Mn = 2.

2.2.2. Synthesis of ZnPS-PVPAPS-PVPA (1.0 g, 1 mmol), sodium dihydrogen phosphate (0.62 g,

4 mmol), zinc acetate (1.1 g, 5 mmol) and Et3N (0.68 g, 6.7 mmol)were used for the synthesis of ZnPS-PVPA according to the literature[20]. IR (KBr): �max/cm−1 3059, 3028, 2923 (CH), 1686, 1493, 1453,756, 698 ( C6H5), 1027 (P O).

2.2.3. Synthesis of chloromethyl-zincpoly(styrene-phenylvinylphosphonate)-phosphate(ZnCMPS-PVPA)

Chloromethyl methyl ether (9.3 mL), anhydrous zinc chloride(3.32 g, 24.34 mmol) and 1a (5.0 g, 3.4 mmol) were mixed in 40 mLchloroform and stirred at 40 ◦C for 10 h. After cooling down,sodium carbonate saturated solution was added to neutralize themixture, and the solvent was evaporated under reduced pres-sure, filtered, washed with deionized water and dried in vacuoto obtain 2a (5.84 g, 90.1%). 2b–2h were synthesized in compli-ance with the similar course. IR (KBr): �max/cm−1 3026, 2925 (CH),2341(O P OH), 1650, 1542, 1510, 1493 ( C6H5), 1267 (P O), 700(C Cl) cm−1.

2.2.4. Synthesis of arylaminomethyl-zincpoly(styrene-phenylvinylphosphonate)-phosphate(ZnAMPS-PVPA)

Proportional amount of aryldiamines (such as m-NH2PhNH2,

p-NH2PhNH2, and benzidine) was blended with 2a (1 g), Na2CO3(1.06 g, 0.01 mol), CuI (0.2 g, 1 mmol) and alcohol 50 mL (the molratio of aryldiamine to chlorine element in ZnAMPS-PVPA is 5:1),and the mixture was stirred and kept at 70 ◦C for 12 h. After the

164 J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173

nthesi

raqwt

2Z

wEAwtro

2M

t

Scheme 1. The sy

eaction, the mixture was neutralized by dilute hydrochloric acidnd then the solvent was vaporized under decompression. Subse-uently, the product 3a was filtered and washed with deionizedater and dried in vacuo. 3b–3h were gained in accordance with

he same course.

.3. Synthesis grafting chiral salen Mn(III) catalyst ontonAMPS-PVPA (Scheme 2)

Chiral salen Mn(III) (4 mmol) in 10 mL of THF was added drop-ise to the solution of 3a (0.5 g) pre-swelled in THF for 30 min and

t3N (5 mmol) with stirring. Then the mixture was refluxed for 10 h.fter cooling down, the solution was neutralized and the solventas evaporated. The dark brown powder 5a was obtained by fil-

ration and washed thoroughly with CH2Cl2 and deionized waterespectively until no Mn could be detected by AAS. 5b–5h werebtained according to the same process.

.4. Synthesis of homogeneous arylamine modified chiral salen

n(III)

The homogeneous chiral salen Mn(III) were prepared accordingo similar procedure to heterogeneous catalysts (in Section 2.3).

Scheme 2. Synthetic rou

s of the supports.

2.5. Asymmetric epoxidation

2.5.1. Using m-CPBA as oxidantThe activity of the prepared catalysts were tested for the epoxi-

dation of unfunctionalized olefins using m-CPBA/NMO as oxidant.A typical epoxidation process was processed in a solution of CH2Cl2(3 mL) containing olefin (1 mmol), n-nonane (internal standard,1 mmol), NMO (5 mmol), catalysts (5.0 mol%) and m-CPBA (2 mmol)at −40 ◦C for 5 h. After reaction, Na2CO3 (2 mL, 1.0 M) and n-hexanewere added to the solution. The reaction was monitored by gaschromatography. The organic phase was concentrated and purifiedby flash chromatography. The yields and ee values of epoxides weredetermined by GC.

2.5.2. Using NaIO4 as oxidantFor NaIO4/imidazole system, the reaction was carried out in

the 2:1 mixture of acetonitrile: water at room temperature andwith alkene (1 mmol), NaIO4 (2 mmol) in the presence of 5 mol%catalysts.

2.6. The reusability of the catalyst

In a typical recirculation, the equal volume of hexane was addedto the mixture after the reaction. Subsequently, the organic phase

te of the catalysts.

Cataly

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mfsqsbpd

2

ivsdpu

2

pthhs

3

3c

3

wt

wihss

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3

l

eral, organic reactions of heterogeneous catalysis were carried outbelow 200 ◦C. Therefore, catalyst 5c2 had adequate thermal stabilityto be applied in heterogeneous catalytic reactions.

J. Huang et al. / Journal of Molecular

as separated, and the catalyst was washed with hexane andeionized water, and dried over vacuum at 60 ◦C. The recoveredried solid catalyst was weighed and reused in the next run. Invery run the same proportion of the substrate-to-catalyst andolvent-to-catalyst was retained.

.7. General procedure for large-scale asymmetric epoxidationeaction

A solution of catalyst 3b (2.5 mmol), n-nonane (50 mmol) and �-ethylstyrene (50 mmol) in CH2Cl2 (150 mL) at −40 ◦C was stirred

or 30 min. Then, m-CPBA (100 mmol) was added to the solutiontep by step. After reaction, Na2CO3 (100 mL, 1.0 M) was added touench the reaction. And the organic layer was dried over sodiumulfate, and the catalyst was precipitated out from the solutiony adding hexane and kept for subsequent use without furtherurification. The conversion and ee values of the epoxide wereetermined by GC.

.8. Characterization of copolymer PS-PVPA

To a solution of PS-PVPA (0.1 g) in 20 mL of THF, sodium hydrox-de standard solution in water was added titrated slowly withigorous magnetic stirring. According to the consumed volume ofodium hydroxide standard solution originated in the place of sud-en change of pH in pH–V NaOH titration curve, the content ofhosphonic acid in the copolymer PS-PVPA could be calculatedsing the formula.

.9. Chemical analysis

In a white porcelain crucible, a sample of 50 mg ZnPS-PVPA wasut in it and was heated up to 700 ◦C for 5 h in Muffle furnace. Dueo the high temperature, ZnPS-PVPA decomposed. Then 20 mL ofydrochloric acid (1:1) was added to the porcelain crucible and waseated to boiling for 30 min on the electric furnace. In the resultingolution, the sodium content was determined by AAS.

. Results and discussion

.1. Characterizations of the supports and the heterogeneoushiral catalysts

.1.1. The content of phosphonic acid in the copolymer PS-PVPAAs described in GPC, copolymer PS-PVPA has average molecular

eight (Mn) = 38608, Mw/Mn = 2. The content of phosphonic acid inhe copolymer could be calculated using the following formula:

W

104n + 184= C0V

here W is the mass of the copolymer; n is the number of styrenen the unit of the copolymer; C0 is the concentration of sodiumydroxide standard solution and V is the consumed volume ofodium hydroxide standard solution corresponding to the place ofudden change of pH in pH–V NaOH titration curve.

According to the data, it could be deduced that there wereverage 8 styrene units (n = 8) between two segments of PVPA inhe copolymer and the copolymer average was comprised in 38VPA units (m = 38) and further there were average 38 segmentsf the molecule chain –(St)n1–(PVPA)–(St)n2–(PVPA)–(St)n3– in theopolymer.

.1.2. Na content of ZnPS-PVPAThe sodium content in sample 1c was 1.7%, which were 0.2%

ower than that of theoretical values; this could probably be

sis A: Chemical 357 (2012) 162– 173 165

attributed to the surface-bound or intercalated water leading tothe augment of the molecular weight.

3.1.3. IR spectroscopy and UV–Vis spectroscopyThe most informative evidence, which confirmed the anchoring

of the chiral salen Mn(III) complex 4 to the aryldiamine modi-fied ZnPS-PVPA (Fig. S1). The azomethene (C N) stretching bandof the complex 4 appeared at 1612 cm−1 (5 in Fig. S1). While forthe supported catalysts this band was also observed at the vicin-ity of 1613 cm−1. All the samples (5a–5h) and the complex 4 hadshown the same band at 1638 cm−1 which was attributed to thevibration of imine group. The stretching vibration at 1030 cm−1

which was assigned to characteristic vibrations of phosphonateand phosphate in the support was obviously weakened due to theelectronic structure changes for the host–guest interaction. Thecommon prominent bands in the spectra of compounds 1a–1f werethree peaks at 1145, 1089, and 986 cm−1, which were attributedto R–PO3

2− phosphonate stretching vibrations. The adsorptions at1201, 1144, and 1077 cm−1 were due to the phosphonate and phos-phate stretching vibrations. Moreover, an additional band around3408 cm−1 was observed for the samples, which was assigned tothe stretching vibration of N H groups.

Diffuse reflectance UV–Vis spectra (Fig. S2) indicated that thespectra of the supported catalysts displayed features similar tothose of the neat chiral salen Mn(III) complex 4. According to thecomplex 4, the bands at 334 nm could be attributed to the chargetransfer transition of salen ligand. The band at 435 nm was dueto the ligand-to-metal charge transfer transition, and the band at510 nm was assigned to the d–d transition of Mn(III) salen sys-tem. While for the heterogenerous catalysts, all the characteristicbands still appeared in their spectras but the immobilized salenMn(III) catalysts exhibited blue shifts from 334, 435 and 510 nm to330, 427 and 503 nm, which indicated that an interaction existedbetween the salen Mn(III) complex and the aryldiamine modifiedZnPS-PVPA.

3.1.4. Thermal gravimetric analysis and powder XRDAs described in the TG curves (Fig. S3), it could be inferred that

according to 5c2, the initial weight loss was 3.38% below 200 ◦C. Itwas ascribed to surface-bound or intercalated water in this stage.In the temperature range 200–850 ◦C, the organic moieties decom-posed. The total weight loss was found to be 69.32%. Obviously,catalyst 5c2 still kept high stability lower than 200 ◦C. In gen-

Fig. 1. XRD of (a) the heterogeneous catalyst 5c2; (b) ZnPS-PVPA.

166 J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173

ore di

ppaipafswiaibt

3

a2wmrwst5wt

a

Fig. 2. The nitrogen adsorption–desorption isotherm and p

As could be seen from Fig. 1, the XRD patterns of ZnPS-PVPA dis-layed a broad 0 0 1 peak (the lowest-angle diffraction peak in theattern), accompanied with other peaks at higher-order 0 0 n peakst larger angles and lower intensities such as at 38.04◦. Therefore,t could be deduced that ZnPS-PVPA could be applied as meso-orous materials. Although the intensities of all peaks decreasedfter immobilization of Mn(III) salen complexes, the reflectionsor ZnPS-PVPA and the catalyst 5c2 indicated that the mesoporoustructure of the parent supports remained intact on modificationith aryldiamine. Simultaneously, the interlayer distance of the

mmobilized catalyst 5c2 (43.3 ± 0.2 A) was nearly twice as muchs that of ZnPS-PVPA (21 ± 1.2 A), owing to the chiral salen Mn(III)ntroduced in ZnPS-PVPA making the zinc layer stretched andecoming broader. The peculiar appearance was entirely differento the most of the results reported [13–15].

.1.5. Nitrogen adsorption–desorption isothermsBased on the desorption isotherm (Fig. 2), BJH analysis gave

broad and non-uniform distribution of pore size (in the range.5–7 nm). Delightedly, the size of solvated Mn (salen)Cl complexas estimated to be 2.05–1.61 nm by MM 2 based on the mini-ized energy [21]. Herein, ZnPS-PVPA 1c could provide enough

oom to accommodate the solvated chiral Mn(III) salen complex asell as that the local environment inside the mesopores and pore

ize of the support did affect the enantioselectivity of the epoxida-ion reaction. Meanwhile, compared with ZnPS-PVPA, the catalystc2 showed similar distribution of pore size (in the scope of 2–8 nm)

hich was in the mesoporous ranges and maintained characteristic

ype V isotherms.The corresponding textural parameters calculated by N2

dsorption–desorption isotherms were presented in Table 1.

Fig. 3. The modes of the organic goup anc

stribution of (a) ZnPS-PVPA; (b) the supported catalyst 5c2.

As described in Table 1, by means of chloromethylation and ary-lamination, an obvious increase in BET surface area was observed(1c vs. 2c vs. 3c2, from 5 to 37 and to 43 m2/g), represented as 3c2in Scheme 1, as well as increase in the pore volume (1c vs. 2c vs.3c2, from 1.3 to 18.82 and to 24.2 × 10−2 cm3/g) and in average porediameter (1c vs. 2c vs. 3c2, from 3.5 to 10.21 and to 11.39 nm). Incontrast with this phenomenon, a decrease in BET surface area (3c2vs. 5c2, from 43 to 32 m2/g), in pore volume (3c2 vs. 5c2, from 24.2to 5.43 × 10−2 cm3/g) and in average pore diameter (3c2 vs. 5c2,from 11.39 to 1.56 nm) was observed upon immobilization of thecomplex 4 onto ZnPS-PVPA modified by arylaminomethyl (3c2). Onthe basis of this, it could be deduced that some chiral salen Mn(III)complexes were immobilized on the external surface of ZnPS-PVPA and other chiral salen Mn(III) complexes were present insidethe nanopores. In other words, there were two forms of immo-bilization of ligand: inner type and outer type, just as shown inFig. 3.

In Fig. 3, it was displayed the structures of chiral salen Mn(III)immobilized onto ZnPS-PVPA with different x values. Obviously,the polystyrenyl groups were located on the external surfaces orbetween the layers of ZnPS-PVPA. If the x values were big like1e (x = 0.5), the room between two polystyrenyl groups wouldbe small, and in order to exclude the higher energy arrange-ment of pendant groups segregating on the interlayer, most ofthe polystyrenyl groups were pushed out and located on theexternal surface of ZnPS-PVPA. In contrast, if x values weresmall like 1c (x = 0.33) and 1b (x = 0.25), the space between two

polystyrenyl groups would be big. And most of the polystyrenylgroups were naturally located between the layers of ZnPS-PVPA.Just as these special configurations gave rise to different catalyticactivities.

hored (a) inner type; (b) outer type.

J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173 167

Table 1Physicochemical characterization data of 1c, 2c, 3c2, 5a, 5c2, 5f and 5h.

Sample Elemental analysis Surface area (m2/g) Pore volum (×10−2 cm3/g) Average pore diameter (nm) Mn content (mmol/g)

Calc. Found

1cC 56.2 55.1 5 1.3 3.5 –H 5.0 4.9N – –

2cC 45.2 44.5 37 18.82 10.21 –H 4.4 4.1N – –

3cC 59.3 58.9 43 24.2 11.39 –H 5.1 4.9N 6.5 6.4

5aC 61.1 59.2 39 8.51 0.6 0.68H 6.6 6.3N 5.8 5.6

5c2

C 61.3 60.3 32 5.43 1.56 0.72H 6.4 6.0N 5.9 5.5

5fC 63.3 62.4 32 10.16 6.46 0.75H 7.3 7.2N 6.3 6.1

41

3

dMt6osds

3

sinttttiob

5hC 63.5 62.4 41 15.H 6.5 6.1N 5.8 5.6

.1.6. X-ray photoelectron spectroscopyThe XPS spectras of the heterogeneous catalyst 5c2 were

escribed in Fig. 4. The neat chiral salen Mn(III) complex exhibitedn 2p3/2 core level peak at a binding energy of 642.1 eV, while

he immobilized salen Mn(III) complex showed a binding energy at42.5 eV and was in consistent with earlier literature data [22]. Thebserved increase of chemical shift of 0.4 eV for the immobilizedalen complex compared with the neat complex attributed to theifferences in the coordination environment of metal Mn inside thepace structure of ZnAMPS-PVPA.

.1.7. The hypothesized layered structure of ZnPS-PVPAIn the hypothesized models deduced for ZnPS-PVPA (Fig. 5),

ome oxygen atoms of the hydroxyl groups or hydroxy sodiumn the segments of the inorganic phosphate groups were coordi-ated with zinc atoms, making the zinc atoms self-assemble inhe same plane, while the other oxygen atoms of the portion ofhe inorganic phosphate groups in the ZnPS-PVPA stretched overhe surface of the zinc layer. On the other side, there were severalypes of organic polymer phosphonate-PO H (opp-PO H ) formed

3 2 3 2n ZnPS-PVPA: opp-PO3H2 (3) was located on the interlayer surfacef one zinc layer, and was connected to other particle of ZnPS-PVPAy polystyrene segment, in other words, opp-PO3H2 (3) and its one

Fig. 4. XPS spectra of the heterogeneous catalyst 5c2.

7.52 0.66

neighboring opp-PO3H2 were located not in same but in differentparticle of ZnPS-PVPA. The same to opp-PO3H2 group (10). Opp-PO3H2 group (2) and (4) were linked each other by polystyrenechain and situated on the interlayer surface of the two adjacent zinclayers respectively. Opp-PO3H2 group (1) was perched on the inter-layer surface of one zinc layer and joined to opp-PO3H2 group (7) bypolystyrene chain which lied on the surface of another contiguouszinc interlayer space. Both opp-PO3H2 group (5) and opp-PO3H2group(6) which were conjunct to each other were located on theinterlayer surface of the same zinc layer, similar to opp-PO3H2group (8) and opp-PO3H2 group (9). In summary, there were at leastthree imaginable structures: (i) two adjacent opp-PO3H2 groupswere situated on the same layer in the same crystalline grain; (ii)two neighboring opp-PO3H2 groups were located on the differentlayers in the same crystalline grain; (iii) two contiguous opp-PO3H2groups in the uniform particle were perched on diverse crystallinegrains. So pores or channels of various sizes and shapes by appro-priate modification of the styrene-phenylvinyl-phosphonic acidcopolymer chain were formed and consequentially gave birth tosignificant impact on the excellent catalytic activity [20].

3.1.8. Analysis of surface morphology

Shown in Fig. 6, SEM images of 1c indicated the diameter of

the particles of the support were in the scope of micron. And 1cwas consisted irregularly of many small and big layered particles

Fig. 5. The hypothesized layered structure of ZnPS-PVPA.

168 J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173

es of 1

aMaiftvp

tdftpmhlJi

3

eChT

3˛

boiatwwcwMlet[mTS

Fig. 6. SEM imag

nd the particles were aggregates of lots of minor crystalline grains.eanwhile, in the supports there were various caves, holes, porous

nd channels. Some micropores and secondary channels wouldncrease the surface area of the catalyst and provide enough chanceor substrates to access to the catalytic active sites. According to 5c2,he SEM took on the amorphous structure which was loose, andarious caves, holes, porous and channels were existed in everyarticles.

The TEM photography of 1c (Fig. 7) manifested the structure ofhe support was spheroid, its channels, holes and cavums could beiscerned clearly, and their sizes were about 70–80 nm. Allowingor the interlayer spacing (2.12 nm) and the average diameter ofhese secondary channels (50–60 nm), it could be deduced that eacharticle consists of stacks of 25–30 layers of 1c, resulting in fewesopores with the average dimensions of 3.5 nm. While for the

eterogeneous catalyst 5c2, the configuration of it was filiform andoose, and its channels, holes and cavums were also existed in it.ust for this, substrates would have more chance to transfer to thenternal catalytic active sites in solution.

.2. Enantioselective epoxidation of unfunctionalized olefins

The catalytic activities of immobilized catalyst 5a–5h for thepoxidation of �-methylstyrene and indene were studied with m-PBA and NaIO4 as oxidative systems. Jacobsen’s catalyst 4 andomogeneous catalyst 6b were examined for comparable purposes.he data obtained were summarized in Table 2.

.2.1. The effect of the x values in the epoxidation of-methylstyrene and indene

As described in Table 2, under the same conditions, the immo-ilized chiral Mn(III) salen catalyst 5e (x = 0.5) displayed ee valuef 37% (entry 11), lower than the ee value of >99% for the catalystmmobilized on 1c. A possible explanation may be that the contentnd the concentration of the organic parts for 1e were much higherhan those of 1c (Fig. 4), thus space between two organic groupsas very small. Therefore, most of the chiral Mn(III) salen complexas immobilized onto the external surface of 3e (Fig. 3) so that 3e

ould not afford confinement effect originated from the nanoporeshich enhanced chiral recognition between the immobilized chiraln(III) salen catalyst and the substrate [23]. In contrast, when cata-

yst was supported between the layers of the materials like 3c2, thenantioselectivity would be enhanced, and Caplan et al. also foundhe confinement effect for the asymmetric epoxidation of styrene

24]. The superior catalytic ability of catalyst 5c2 was attributed

ainly to the proper pore size of the corresponding support 3c2.he evidence was the nitrogen sorption (BET) results of ZnCM-PP in Table 1. The size of the chiral Mn(III) salen complex 4 was

c (1) and 5c2 (2).

estimated as 2.0 nm × 1.6 nm according to the literature [25]. Themesoporous material 3c2 with nanopore size (around 11.39 nm)could provide enough room for the immobilization of the chiralMn(III) salen complex 4 into the channel of the support.

Meanwhile, the conversions varied from 81.5% to 87.2% and theenantioselectivities varied from 13.3% to 14.6% as the x values were0.25 and 0.2 (entries 5 and 4). In contrast, the x values were from 0.4to 1, accompanied with the conversions from 35.5% to 84.1% and theenantioselectivities from 19.3% to 37% (entries 10–13). Notably, theconversions as x = 0.2, 0.25 were higher than that did as x = 0.4, 0.5,0.75, 1; while the enantioselectivities x = 0.2, 0.25 were lower thanthat of the catalysts when x = 0.4, 0.5, 0.75, 1. In other words, enan-tioselectivity was relatively low and conversion was relative highas x value was little; on the contrary, enantioselectivity showedhigh and conversion displayed low when x value was comparativelylarge. The phenomenon was ascribed to the confinement effect ofthe nanopores.

Remarkablely, the supported catalyst 5c2 with 1:2 ratio oforganic phosphonate to inorganic phosphate displayed excellentconversion and enantioselectivity (conv, >99% and ee, >99%), com-pared with the other supported catalyst in Table 2 (entries 4–5and entries 10–13). Meanwhile, heterogeneous catalyst 5 h alsoshowed higher enantioselectivity than that of Jacobsen’s catalyst4 (ee, 87.2% vs. 54%) and a little lower catalytic activity than thatof the supported catalyst 5c2 (conv, 98.5% vs. >99%; ee, 87.2% vs.>99%). It was denoted that NH4

+ in the support also made theenantioselectivity increased and the effect on the catalytic activ-ity was not well as that of the Na+ in the support. On the basisof these results, it could be inferred that both proper ratio oforganic phosphonate and inorganic phosphate and the pertinentinorganic phosphate resource played vital impacts on the catalyticactivity.

The relatively bulkier alkene like indene (entries 16–28) wasalso chosen to test the activities of the supported catalysts 5a–5hfor the asymmetric epoxidation. These reaction results showedthat the enantioselectivity and activity of the immobilized cata-lyst 5b for indene (ee, 89.9% yield 85.9%) (entry 20) were foundto be lower than that of 5c2 (entry 22) and neat homogenous 6b(ee, 92% yield 65%) (entry 18). The reason may be that indenewas too large to accommodate into the micropores and layers ofZnPS-PVPA. Therefore, indene may merely react with a few activesites on the external surface of the ZnPS-PVPA and 5b could notafford confinement effect for indene. Meanwhile, under the sameconditions the yield of the epoxide with 5a (entry 19) was better

than that of 5b (entry 20), but they showed lower ee value. Onepossible explanation for this result was that the amount of chiralsalen Mn(III) catalytic active centers on external surface for 5a wasmuch more than that for 5b. The higher ee values obtained for the

J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173 169

and th

csbslTnac

TAo

n3

Fig. 7. TEM photograph of 1c (A)

atalyst immobilized in the nanopores than that on the externalurface could be really attributed to the enhanced chiral inductiony the confinement effect of the nanopores. When the nanoporeize of the support was tuned to a suitable value, the chiral cata-ysts in the nanopores could give higher ee values in some cases.

hese results strongly suggested that the confinement effect ofanopores was able to enhance the asymmetric induction as longs the pore size was tuned to a suitable value depending on theatalytic reaction system [26].

able 2symmetric epoxidation of �-methylstyrene and indene catalyzed by homogeneous anxidant systems.

Entry Substratec Catalyst Oxidant system Tim

1 A 4 m-CPBA/NMO 5

2 A 6b m-CPBA/NMO 5

3 A 6b m-CPBA 5

4 A 5a m-CPBA 5

5 A 5b m-CPBA 5

6 A 5c1 m-CPBA 5

7 A 5c2 m-CPBA/NMO 5

8 A 5c2 m-CPBA 5

9 A 5c3 m-CPBA 5

10 A 5d m-CPBA 5

11 A 5e m-CPBA 5

12 A 5f m-CPBA 5

13 A 5g m-CPBA 5

14 A 5h m-CPBA/NMO 5

15 A 5h m-CPBA 5

16 B 4 m-CPBA/NMO 1

17 B 6b m-CPBA/NMO 1

18 B 6b m-CPBA 1

19 B 5a m-CPBA 1

20 B 5b m-CPBA 1

21 B 5c1 m-CPBA 1

22 B 5c2 m-CPBA 1

23 B 5c3 m-CPBA 1

24 B 5d m-CPBA 1

25 B 5e m-CPBA 1

26 B 5f m-CPBA 1

27 B 5g m-CPBA 1

28 B 5h m-CPBA 1

29 A 4 NaIO4/Imidazole 7

30 A 6b NaIO4/Imidazole 7

31 A 6b NaIO4 7

32 A 5c2 NaIO4/Imidazole 7

33 A 5c2 NaIO4 7

34 A 5h NaIO4/Imidazole 7

35 A 5h NaIO4 7

a Reactions were carried out in CH2Cl2 (4 mL) with alkene (1 mmol), n-nonane (ineous salen Mn(III) catalysts (5 mol%) and m-CPBA (2 mmol). The conversion and the e0 m × 0.32 mm × 0.25 �m.b Reaction conditions: alkene (1 mmol), NaIO4 (2 mmol), catalyst (0.03 mmol), CH3CN/Hc A = �-methylstyrene, B = indene.d (S)-form.e Turnover frequency (TOF) is calculated by the expression of [product]/[catalyst] × tim

e heterogeneous catalyst 5c2 (B).

In general, the enantioselectivity in the asymmetric catalyticreactions was usually decreased for the immobilized chiral cata-lysts compared to homogeneous counterparts. However, in manycases, the catalysts confined in nanopores showed comparable oreven higher ee values than the homogeneous catalysts did, which

were simply attributed to the confinement effect of the nanopores[27]. However, the detailed insights of the confinement effect werenot well understood. Thomas and co-workers proposed that theconfinement effect of the nanopores could improve the chiral

d heterogeneous catalysts (5a–5h) with m-CPBA/NMOa and NaIO4/imidazoleb as

e (h) T (◦C) Conv% eed TOFe × 10−4 (S−1)

−40 >99 54 11−40 88 86 10−40 64 90.8 7−40 87.2 14.6 10−40 81.5 13.3 9−40 98.6 >99 11−40 6.6 3.1 1−40 >99 >99 11−40 96.8 >99 11−40 73.7 31.2 8−40 76.2 37 8−40 35.5 19.3 4−40 84.1 27.1 9−40 96.7 1.2 11−40 98.5 87.2 11

0 92 65 510 91.2 25.5 510 98.7 83.7 550 91.7 50 510 85.9 89.9 480 80.5 94.5 450 >99 >99 560 86.4 92.3 480 74.2 78 410 73.3 83.7 410 67.9 93.6 380 48.1 95.8 280 96.1 54.6 53

25 >99 >99 825 >99 >99 825 >99 >99 825 >99 >99 825 >99 >99 825 98.6 >99 825 >99 >99 8

ternal standard, 1 mmol), NMO (5 mmol), homogeneous (5 mol%) or heteroge-e value were determined by GC with chiral capillary columns HP19091G-B 213,

2O (10 mL/5 mL).

e (s−1).

1 Cataly

ielaMdth5bgarTtrictw

3˛

vavccdtts9tooMtAmw(

3

hNirrIcfNTcrpatotewt

70 J. Huang et al. / Journal of Molecular

nduction for the asymmetric catalysis in nanopores by strength-ning the interaction between the incoming reactant and the chiraligand, as well as the catalytic metal center in nanopores. Gener-lly, the yields of products were lower for the heterogeneous chiraln(III) salen catalysts than those for the homogeneous catalysts,

ue to the difficulty in diffusion. As for the asymmetric epoxida-ion of the bulkier olefins like �-methylstyrene and indene, theeterogeneous chiral Mn(III) salen catalyst with small x value asc2 (x = 0.33) could obviously have better yields than that of withig x value as 5e (x = 0.5). With decreasing of the ratio of the organicroups the space of two neighboring chiral salen Mn(III) catalyticctive centers would increase, thus 5c2 (x = 0.33) had the biggeroom than that of 5e (x = 0.5) for the diffusion of bulkier olefins.hese results further demonstrated that the confinement effect ofhe nanopores could change the enantioselectivity of the asymmet-ic epoxidation, such that the enantioselectivity could be enhancedn the nanopores with the optimized pore size. For heterogeneoushiral catalysts in nanopores, the pore effect could provide a wayo improve the asymmetric induction by optimizing the nanoporesith suitable pore structures and sizes.

.2.2. The effect of different linkers in the epoxidation of-methylstyrene and indene

As described in Table 2, the catalyst 5c2 showed higher con-ersion and entainselectivity than those of the catalyst 5c1 in thesymmetric epoxidation of indene (conv%, >99 vs. 80.5; ee, >99s. 94.5), owing to the high symmetry of the catalyst 5c2 whichould decreased the steric obstacles. In addition, the supportedatalyst 5c3 also displayed lower activity than the catalyst 5c2id (conv%, 86.4 vs. >99; ee, 92.3 vs. >99), which was ascribed tohe bulkier linker benzidine making the substrates approachinghe catalyst difficultly. Meanwhile, the homogeneous catalyst 6bhowed higher ee value than that of the Jacobsen’s catalyst 4 (ee,0.8 vs 54%), which indicated that the rigid linker was devotedo the increase of ee values. In other words, the steric propertiesf the linkages really played vital impacts on the configurationf the transition state for the asymmetric epoxidation reactions.oreover, the ee values further increased from 90.8% to >99% after

he homogeneous catalyst 6b was immobilized onto ZnPS-PVPA.bove all, both the support ZnPS-PVPA and the rigid linkers in com-on contributed to the increase of ee values. The similar resultsere obtained according to the asymmetric epoxidation of indene

entries 21–23).

.2.3. The effect of axial ligands: NMO and imidazoleSurprisedly, the heterogeneous catalyst 5c2 and 5h displayed

igh ee values and conversions in the absence of the additiveMO which was commonly required to improve the catalytic activ-

ty (in Table 2). Practically, adding the axial ligand NMO to oureaction mixture did not improve the asymmetric induction butesult in a dramatically reduced enantioselectivity and reactivity.n this text, the ee values for the epoxides of �-methylstyrene typi-ally increased from 3.1% to >99% and the conversions increasedrom 6.6% to >99% (entries 8 vs. 7) without the addition ofMO. This stood in contrast to the most literatures reported [28].he exceptional phenomenon originated in chiral salen Mn(III)omplex immobilized on phenoxy-modified ZPS-PVPA had beeneported by our group recently [16]. It was ascribed to organicolymer–inorganic hybrid support ZPS-PVPA and the phenoxidexial coordinating group. Whereas, this unusual phenomenon inhis text was induced by another factors. At first, the structuresf the immobilized catalysts similar with N-oxide ligand leaded

o the unusual phenomenon. Simultaneously, additives were gen-rally regarded as axial ligands on the transition metal catalyst,hich made for activating the catalyst either toward oxidation or

oward reactivity with the olefin. Thus, there was a steric hindrance

sis A: Chemical 357 (2012) 162– 173

when N-oxide ligand was added and the optimal geometric config-urations of the reactive intermediates salen Mn(V) O and or theirtransition states were altered. It was steric hindrance that madeolefins approach salen Mn(V) O difficultly and the lower ee valueswere obtained.

On the other hand, the heterogeneous catalysts 5a–5h werealso applied in the epoxidation �-methylstyrene in the 2:1 mix-ture of acetonitrile: water with NaIO4 as oxidative system. Asdescribed in Table 2, the supported catalysts 5a–5h showed com-parable catalytic activities to those of the homogeneous catalyst6 and Jacobsen’s catalyst 4. The conversions and enantioselectiv-ities of the catalysts 5a–5h all exceeded 95%, even >99%. It couldbe deduced that the heterogeneous catalysts 5a–5h possessed effi-cient catalytic abilities whether the axial ligand imidazole existedor not. That was to say, the axial ligand imidazole made subtleimpact on the catalytic activities that the conversion and enantios-electivity increased a little in the presence of imidazole (entry 34vs. 35: conv%, from 98.6 to >99; ee%, >99 vs. >99).

In general, the additives NMO and imidazole, which were usedto improve epoxidation yields and enantioselection, binded to theMn(III) center prior to the epoxidation reaction, as evidenced by thealteration of the Mn(III) parallel mode EPR signal [29]. Additives tothe Mn(III) salen reaction mixture, such as NMO and imidazole,generally facilitated faster reaction rates, higher epoxide yields,and improved enantioselectivity. However, the additives in thistext played such different roles that the catalytic activities didnot increase but decrease with the addition of NMO in m-CPBA asoxidative system or slightly increased in the presence of imidazolewith NaIO4 as oxidative system. Furthermore, the additives wereexpensive commonly and the superior catalytic activities were stillobtained in the absence of them. From the commercial viewpoint,the heterogeneous catalyst 5c2 had the potential application inindustry.

3.3. The confinement effect originated in the x values ofZnPS-PVPA

As described in Table 2, the effect of the x value (ZnPS-PVPA)on the confinement effect of the heterogeneous chiral Mn(III) salencatalysts played vital impacts on the results of the enantioselectiveepoxidation of unfunctionalized alkenes.

ZnPS-PVPA could be formed by coprecipitation of the zinc inthe presence of styrene-phenylvinylphosphonic acid copolymer(H2O3P-G) and phosphate (H2O3P-OH), leading to the formationof a porous zinc polystyrene-phenylvinyl phosphonate-phosphatehybrid material. Because in the ZnPS-PVPA hybrid material, residueor side chain for OPP-groups of P-G and inorganic phosphate P-OH are sufficiently different in size, obviously OPP-groups of P-Gare quite bulky and inorganic phosphate P-OH are relatively small.These hybrid materials usually contain a random distribution of theorganic groups such that all layers have identical stoichiometry. Insuch systems, the interlayer spacing (d-space) is a function of pen-dant group stoichiometry, and has a generally linear dependenceon component mole fraction. Intermediate x values result in inter-mediate steric constraints, resulting in d values between the twoorganic groups [20].

Then the models of Zn (NaPO4)1−x[PS-PVPA]x·yH2O series, viz.where x = 0.2, 0.25, 0.33, 0.4, 0.5, 0.75 and 1 were deduced. Aswe know, each zinc atom is bonded to six oxygen atoms, andevery three of these oxygen atoms are bonded to one phospho-rus atom. As a result, the layers of the ZnPS-PVPA are formed. Alayer of ZnPS-PVPA 1e (x = 0.5) presents that in order to exclude

the higher-energy arrangement of pendant groups segregating onthe interlayer, there is always a P-G (B) groups insert between twoP-OH (A) or reverse. That is to say, the ideal model can be simplydenoted as “ABABAB. . .”, and the scheme of the ideal cross section

J. Huang et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 162– 173 171

(abfassesattomcd

bsagaPrmamacapwpacabac

yield (88%) and enantioselectivity (86.1%). The effective separationthe chiral Mn(III) salen complexes by the solid support ZnPS-PVPA contributed to the good stability of the heterogeneous chiral

Table 3The recycles of catalyst 5c2 in the asymmetric epoxidation of �-methylstyrene.a

Run Time (h) Conversion (%) ee (%)b TOFc × 10−4 (S−1)

1 5 >99 >99 112 5 >99 >99 113 5 >99 >99 114 5 >99 >99 115 5 97.8 >99 116 5 96 >99 117 5 92 97 108 5 90 93 109 5 88 86.1 10

10 5 85.2 78.9 911 5 80.1 65.4 912 5 73 29.4 8

a Reactions were carried out at −40 ◦C in CH2Cl2 (2 mL) with �-methylstyrene(1 mmol), n-nonane (internal standard, 1 mmol), m-CPBA (0.38 mmol), het-erogeneous salen Mn(III) catalysts (5 mol%). The conversion and the ee

Fig. 8. The cross section of the x = 0.25, x = 0.33 and x = 0.5.

Fig. 8) indicates that two bulky P-G (B) and four P-OH (A) groupsre located around one P-G group, sterically directed energy allowsulky P-G groups to get out of the layers. Similarly, the ideal modelor ZnPS-PVPA 1c (x = 0.33) can also be denoted as “AABAABAAB. . .”,nd the scheme of the ideal cross section (Fig. 8) suggests that sixmall P-OH (A) groups are located around one P-G group. The d-pace is bigger than that of ZnPS-PVPA 1e (x = 0.5), and it can offernough space for some smaller substrates coordination to the activeites. Meanwhile, confinement effect generated for steric hindrancemong the substrates and active sites. For ZnPS-PVPA 1b (x = 0.25)he ideal model can also be denoted as “BAAABAAABAA. . .”, andhe scheme of the ideal cross section (Fig. 8) suggests that the spacef two neighboring P-G groups is much larger than the other twoaterials. Although there is big room for catalytic reaction, the

onfinement effect and the content of the active sites always areecreased [20].

Qualitatively, the changes in the d spacing with composition cane rationalized in terms of interrelated factors that pertain to theteric interactions of the bulkier styrene-phenylvinyl-phosphoniccid copolymer groups and the conformations of the organicroups. The styrene-phenylvinylphosphonic acid copolymer chainsre located on the external surfaces or between the layers of ZnPS-VPA. If the x values are big, like ZnPS-PVPA 1e (x = 0.5) or more, theoom between two styrene-phenylvinylphosphonic acid copoly-er groups will be small, and in order to exclude the higher-energy

rrangement of pendant groups segregating on the interlayer theost of the styrene-phenylvinylphosphonic acid copolymer groups

re pushed out and located on the external surface of ZnPS-PVPA. Inontrast, if x values are relatively small like ZnPS-PVPA 1c (x = 0.33)nd ZnPS-PVPA 1b (x = 0.25) or less the space between two styrene-henylvinylphosphonic acid copolymer groups will be big whichas in good according with the results obtained by XRD. And moreart of the styrene-phenylvinylphosphonic acid copolymer groupsre naturally located between the layers of ZnPS-PVPA. In con-lusion, the frameworks of ZnPS-PVPA can be easily designed andssembled to generate pores or channels of various sizes and shapes

y appropriate modification of the styrene-phenylvinylphosphoniccid copolymer chain (Fig. 9). The porous hosted materials affectatalytic performance due to a cooperative interaction among

Fig. 9. The hypothesized structure of the heterogeneous catalyst.

the nanoporous solid, immobilizing linker, and Mn–salen com-plex. Mesoporous materials are the most applicable supports forthe immobilization of Mn–salen complexes [30]. Just as this spe-cial structures of ZnPS-PVPA contributed to the excellent catalyticeffect.

3.4. The reusability of the catalyst

To assess the long-term stability and reusability of the supportedchiral salen Mn(III) catalysts, �-methylstyrene was used as a modesubstrate, and recycling experiments were carried out with thecatalyst 5c2. At the end of the each reaction, the catalyst was sep-arated by adding hexane, washed with deionized water and driedcarefully before using it in the next run. Above 95% recycle of thecatalyst was achieved in every run. The recovered dried solid cat-alyst was weighed and reused in the next run. In every run thesame ratio of the substrate-to-catalyst and solvent-to-catalyst waskept. The filtrates were collected for determination of Mn leach-ing. After using of catalyst 5c2 for 12 consecutive times, the resultswere listed in Table 3. Obviously, the yield and the enantioselectiv-ity decreased slightly after recycling for nine times and still gave

value were determined by GC with chiral capillary columns HP19091G-B213,30 m × 0.32 mm × 0.25 �m.

b Same as in Table 2.c Same as in Table 2.

172 J. Huang et al. / Journal of Molecular Cataly

M�teurc50

Sacctbfnddro

3

mam

TL

me3

(

(t

(t

(

Fig. 10. The theoretic changing progress of catalyst 5c2 in acid solution.

n(III) salen catalyst in case that they would dimerize to inactive-oxo-Mn(IV) species. The decrease of the yield could be attributed

o the decomposition of the chiral Mn(III) salen complex underpoxidation conditions [31] and the loss of the hyperfine gran-les of the heterogeneous chiral Mn(III) salen catalysts (formed ineaction due to stirring). When the heterogeneous catalyst recy-led for 9 times, the Mn content of the heterogeneous catalystc2 was 0.46 mmol/g, compared with the total amount (around.72 mmol/g).

The nature of the recovered catalyst 5c2 was followed by IR (Fig.4). The result indicated that characteristic bands of the catalystt 2954, 2864 and 1630 cm−1 disappeared or weaken after recy-ling 10 times. These revealed that the active sites of salen Mn(III)omplex and the ZnPS-PVPA support under acid reaction condi-ions were partly destroyed (Fig. 10). Moreover, other effects coulde used to explain these results: (i) leaching of metal complexesrom the materials or (ii) blocking of the pores and secondary chan-els either by inactive Mn(IV)-oxo species believed to be generateduring the catalytic mechanism [32] or by some other insolubleegraded product obtained by side reactions, which could not beemoved from the materials after several washing or (iii) collapsingf some of the pillars during the catalysis experiments.

.5. Large-scale asymmetric epoxidation reaction

We further performed different proportions of large-scale asym-

etric epoxidation reactions with n-nonane and �-methylstyrene

nd m-CPBA. The same catalyst loading of 5 mol% as in the experi-ental scale was used. The large-scale experiments can be facilely

able 4arge-scale asymmetric epoxidation reaction of �-methylstyrene.a

Entry Time (h) Conversion (%) eef (%) TOFg × 10−4 (S−1)

1b 5 >99 >99 112c 5 >99 >99 113d 5 >99 >99 114e 5 >99 >99 11

a Reactions were carried out at −40 ◦C in CH2Cl2 with �-methylstyrene, n-nonane,-CPBA, heterogeneous salen Mn(III) catalysts (5 mol%). The conversion and the

e value were determined by GC with chiral capillary columns HP19091G-B213,0 m × 0.32 mm × 0.25 �m.b The usage amounts of reagents were �-methylstyrene (1 mmol), n-nonane

1 mmol), heterogeneous catalyst 3b (0.05 mmol), m-CPBA (2 mmol), respectively.c The usage amounts of reagents were �-methylstyrene (50 mmol), n-nonane

50 mmol), heterogeneous catalyst 3b (2.5 mmol), m-CPBA (100 mmol), respec-ively.

d The usage amounts of reagents were �-methylstyrene (50 mmol), n-nonane50 mmol), heterogeneous catalyst 3b (0.5 mmol), m-CPBA (100 mmol), respec-ively.

e The usage amounts of reagents were �-methylstyrene (100 mmol), n-nonane100 mmol), heterogeneous catalyst 3b (5 mmol), m-CPBA(200 mmol), respectively.

f Same as in Table 2.g Same as in Table 2.

[[

[[[

[

[

sis A: Chemical 357 (2012) 162– 173

carried out using the same procedure as for the experimental scalereactions. As can be seen from the results summarized in Table 4,delightfully, the conversion and enantioselectivity maintained atthe same level for the large-scale reactions under whichever con-dition that the large scale is 50 times or 100 (Fig. S5) times as muchas the experimental scale.

4. Conclusions

In summary, novel types of organic polymer-inorganic hybridmaterial layered crystalline ZnPS-PVPA with different content ofthe organic group (x) and different inorganic phosphate have beensynthesized and applied as catalyst supports. Subsequently, thecatalysts immobilized the chiral salen Mn(III) complex onto aryl-diamine modified ZnPS-PVPA through axial coordination were alsoprepared. The heterogeneous chiral Mn(III) salen catalysts exhib-ited comparable or even higher enantioselectivities than those ofhomogeneous catalysts for the asymmetric epoxidation of sev-eral unfunctional olefins in the absence of imidazole and NMO. Inaddition, the influences of x values, linkages and the additives onthe catalytic activities of the heterogeneous Mn(III) salen catalystswere explored at length. Furthermore, the supported chiral Mn(III)salen catalysts are relatively stable and can be recycled nine timesin the asymmetric epoxidation of �-methylstyrene. Remarkably,this prepared heterogeneous chiral Mn(III) salen catalysts catalyzedasymmetric epoxidation reaction can be performed on a large-scalewith the catalytic ability being maintained at the same level. Ina word, chiral salen Mn(III) anchored on ZnPS-PVPA are stable,effective and promising catalysts and may be provided with thepotentiality for industry.

Acknowledgements

This work was financially supported by National Ministry ofScience and Technology Innovation Fund for High-tech Smalland Medium Enterprise Technology (No. 09C26215112399) andNational Ministry of Human Resources and Social Security Start-up Support Projects for Students Returned to Business, Office ofHuman Resources and Social Security Issued 2009 (143).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molcata.2012.02.008.

References

[1] T. Katsuki, K.B. Sharpless, J. Am. Chem. Soc. 102 (1980) 5974.[2] T. Katsuki, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, 2nd edn., Wiley-

VCH, New York, 2000, p. 287 (Chapter 6B).[3] E.N. Jacobsen, M.H. Wu, in: E.N. Jacobsen, A. Pfaltz, H. Yamamoto (Eds.), Com-

prehensive Asymmetric Catalysis, Springer, New York, 1999, p. 649 (Chapter18.2).

[4] T. Katsuki, J. Mol. Catal. A: Chem. 113 (1996) 87, and references cited therein.[5] E.N. Jacobsen, W. Zhang, A.R. Muci, J.R. Ecker, L. Deng, J. Am. Chem. Soc. 113

(1991) 7063.[6] P.M. Morn, G. Hutching, Chem. Soc. Rev. 33 (2004) 108.[7] Q.H. Xia, H.Q. Ge, C.P. Ye, Z.M. Liu, K.X. Su, Chem. Rev. 105 (2005) 1603.[8] C. Baleizao, H. Garcı′a, Chem. Rev. 106 (2006) 3987.[9] X.K. Fu, W. Kang, S.Y. Wen, C.Y. Deng, Chinese J. Appl. Chem. 12 (1995) 47.10] R.Q. Zeng, X.K. Fu, C.B. Gong, Y. Sui, J. Mater. Chem. 41 (2006) 4771.11] R.Q. Zeng, X.K. Fu, C.B. Gong, Y. Sui, X.B. Ma, X.B. Yang, J. Mol. Catal. A: Chem.

229 (2005) 1.12] X.B. Ma, X.K. Fu, J. Mol. Catal. A: Chem. 208 (2004) 129.13] R.F. Bai, X.K. Fu, H.B. Bao, W.S. Ren, Catal. Commun. 9 (2008) 1588.14] W.S. Ren, X.K. Fu, H.B. Bao, R.F. Bai, P.P. Ding, B.L. Sui, Catal. Commun. 10 (2009)

788.15] B.W. Gong, X.K. Fu, J.X. Chen, Y.D. Li, X.C. Zou, X.B. Tu, P.P. Ding, L.P. Ma, J. Catal.

262 (2009) 9.16] X.C. Zou, X.K. Fu, Y.D. Li, X.B. Tu, S.D. Fu, Y.F. Luo, X.J. Wu, Adv. Synth. Catal. 352

(2010) 163.

Cataly

[

[

[[[[

[[

[[[[[

123 (2001) 5710.[30] H.D. Zhang, S. Xiang, C. Li, Chem. Commun. 9 (2005) 1209.

J. Huang et al. / Journal of Molecular

17] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobson, J. Am. Chem. Soc. 112 (1990)2801.

18] M. Palncki, P.J. Pospisil, W. Zhang, E.N. Jacobsen, J. Am. Chem. Soc. 116 (1994)9333.

19] B.W. Gong, X.K. Fu, H.S. Shen, L.H. Ma, Y.J. Xia, Fine Chem. 25 (2008) 603.20] J. Huang, X.K. Fu, G. Wang, C. Li, X.Y. Hu, Dalton Trans. 40 (2011) 3631.21] E.M.M. Garrigle, D.G. Gilheany, Chem. Rev. 105 (2005) 1563.

22] A.R. Silva, J.L. Figueiredo, C. Freire, B.D. Castro, Microporous Mesoporous Mater.

68 (2004) 83.23] H.D. Zhang, Y.M. Zhang, C. Li, J. Catal. 238 (2006) 369.24] N.A. Caplan, F.E. Hancock, P.P.C. Bulman, G.J. Hutchings, Angew. Chem. Int. Ed.

43 (2004) 1685.

[

[

sis A: Chemical 357 (2012) 162– 173 173

25] H. Zhang, C. Li, Tetrahedron 62 (2006) 6640.26] C. Li, H.D. Zhang, D.M. Jiang, Q.H. Yang, Chem. Commun. 6 (2007) 547.27] S. Xiang, Y.L. Zhang, Q. Xin, C. Li, Chem. Commun. 22 (2002) 2696.28] C. Li, Catal. Rev. 46 (2004) 419.29] K.A. Campbell, M.R. Lashley, J.K. Wyatt, M.H. Nantz, R.D. Britt, J. Am. Chem. Soc.

31] C.E. Song, E.J. Roh, B.M. Yu, D.Y. Chi, S.C. Kim, K.J. Lee, Chem. Commun. 7 (2000)615.

32] P.M. Morn, G.J. Hutchings, Chem. Soc. Rev. 33 (2004) 108.