Microporous and Mesoporous Materials 153 (2012) 294–301

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Microporous and Mesoporous Materials

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Chiral salen Mn(III) immobilized onto sulfoalkyl-modified zincpoly(styrene-phenylvinylphosphonate)-phosphate as effective catalystsfor epoxidation of unfunctionalized olefins

Jing Huang, Xiangkai Fu ⇑, Changwei Wang, Huaizhi Zhang, Qiang MiaoCollege of Chemistry and Chemical Engineering Southwest University, Research Institute of Applied Chemistry Southwest University, The Key Laboratory of AppliedChemistry of Chongqing Municipality, The Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of Education, Chongqing 400715, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 August 2011Received in revised form 19 October 2011Accepted 20 October 2011Available online 28 October 2011

Keywords:Zinc poly(styrene-phenylvinylphosphonate)phosphateAxial baseSalen Mn(III)Heterogeneous catalystsAsymmetric epoxidation

1387-1811/$ - see front matter Crown Copyright � 2doi:10.1016/j.micromeso.2011.10.025

⇑ Corresponding author. Tel.: +86 23 68253704; faxE-mail address: fxk@swu.edu.cn (X. Fu).

The heterogeneous catalysts 3a–3c immobilized chiral salen Mn(III) onto sulfoalkyl-modified zincpoly(styrene-phenylvinylphosphonate)phosphate (ZnPSPPP) were prepared and used for the enantiose-lective epoxidation of unfunctionalized olefins. The catalysts were characterized with FT-IR, UV–vis,XRD, SEM, TEM. All the prepared heterogeneous catalysts exhibited much higher chiral induction thanthe homogeneous Jacobsen’s catalyst did and could be reused six times without significant loss of activityand enantioselectivity. Surprisedly, the conversions and enantiomeric excess (ee) values increased in theabsence of axial base N-methylmorpholine N-oxide (NMO), which were different to what most literaturesreported. Meanwhile, the role of axial ligand NMO in the asymmetric epoxidation of unfunctional olefinwas also discussed here. Furthermore, this novel type of catalyst can also be validly used in large-scalereactions with superior catalytic disposition being maintained at the same level, which possessed thepotentiality for application in industry.

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1. Introduction

Chiral salen Mn(III) complexes are excellent catalysts for theasymmetric epoxidation of unfunctionalized olefins [1–5]. Immo-bilization of Mn(salen) complexes onto solid supports are oftenof greater interest than homogeneous catalysts, since the activematerials prepared are very easy to handle, retrieve, and recyclecompared to their homogeneous counterpart [6]. Various attemptstowards immobilization of metal-organic complexes have beenmade previously [7–9]. Hybrid organic–inorganic materials in gen-eral represent the natural interface between two worlds of chem-istry each with very significant contributions to the field ofmaterials science, and each with characteristic properties that re-sult in distinct advantages and limitations [10].

Since the discovery of the aluminophosphate family of molecu-lar sieves [11], research on metal phosphonates is undergoingrapid expansion because of their potential applications in the areasof sorption and ion exchange, catalysis, and sensors [12–14]. Dueto their unique properties, such as their high thermal stabilityand the variability of organic functional groups that can be intro-duced, special interest centers on the layered phosphonates [15].Our research has, for many years, been concerned with metal

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phosphonate chemistry for catalysts and catalyst supports. Wehave reported the preparation of zirconium phosphate-phospho-nate derivatives ZSPP, ZPS-IPPA, ZPS-PVPA and the application ofsupports in immobilizating chiral salen Mn(III) [16–19]. Most ofthe heterogeneous catalysts exhibited great activities and enanti-oselectivities with O-coordinating axial base for asymmetric epox-idation of unfunctionalized olefins. Generally, the cocatalysts,which include variously substituted pyridines, imidazoles, andN-oxides, play an important role in accelerating the reactionsand sometimes increasing the enantioselectivities. Given the con-siderable usefulness of axial base in epoxidation, various cocata-lysts have been widely used in several oxidation processes[20,21]. However, it was reported that no additives were requiredto achieve high conversions and ee values with a chiral salenMn(III) complex immobilized on phenoxy-modified ZPS-PVPArecently [22]. Respecting the different roles of N-oxides in epoxi-dation and the mechanism of this exceptional phenomenon rarelyreported, we synthesized crystalline support ZnPSPPP under mildconditions completely differentiated to the traditional methodsand immobilized chiral salen Mn(III) complex onto sulfoalkyl-modified ZnPSPPP as effective catalysts for epoxidation of unfunc-tionalized olefins. Moreover, we examined the influence of axialbase on the catalytic performance and the enantioselectivity ofthe catalysts in large-scale reactions with being maintained atthe same level.

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2. Experimental

2.1. Materials and methods

(1R,2R)-1,2-Diaminocyclohexane, a-methylstyrene, indene,n-nonane, N-methylmorpholine N-oxide (NMO) and m-chloroper-benzoic acid (m-CPBA) were purchased from Alfa Aesar. Other com-mercially available chemicals were laboratory-grade reagents fromlocal suppliers.

FT-IR spectra were recorded from KBr pellets using a BrukerRFS100/S spectrophotometer (USA) and diffuse reflectanceUV–vis spectra of the solid samples were recorded in the spectro-photometer with an integrating sphere using BaSO4 as standard.Number- and weight-average molecular weights (Mn and Mw)and polydispersity (Mw/Mn) were estimated by Waters1515 gelpermeation chromatograph (GPC; against polystyrene standards)using THF as an eluent (1.0 mL min�1) at 35 �C. The Mn contentsof the catalysts were determined by a TAS-986G (Pgeneral, China)atomic absorption spectroscopy. Scanning electron microscopy(SEM) analysis was performed on KYKY-EM 3200 (KYKY, China)microscopy. Transmission electron microscopy (TEM) analysiswas obtained on TECNAI10 (PHILIPS, Holland) apparatus. The con-versions (with n-nonane as internal standard) and the ee valueswere analyzed by gas chromatography (GC) with a ShimadzuGC2010 (Japan) instrument equipped using a chiral column(HP19091G-B213, 30 m � 0.32 mm � 0.25 lm) and FID detector,injector 230 �C, detector 230 �C. The column temperature forindene, a-methylstyrene was in the range of 80–180 �C. The homo-generous chiral salen Mn(III) catalyst was synthesized according tothe standard literature procedures [23].

2.1.1. Preparation of styrene-phenylvinyl phosphonic acid copolymer(PS-PVPA)

1-Phenylvinyl phosphonic acid (PVPA) was synthesized accord-ing to literature [24] 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-PVPAcopolymer as literature [19] yield 7.52 g. GPC: Mn = 32083,m = 36, n = 6.8, Mw/Mn = 2.

2.2. Synthesis of the support (Scheme 1)

2.2.1. Preparation of zinc poly(styrene-phenylvinylphosphonate)phosphate (ZnPSPPP)

PS-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 applied in the synthesis of ZnPSPPP according to the litera-ture [25]. IR (KBr): Vmax/cm�1 3059, 3028, 2923 (CH), 1686, 1493,1453, 756, 698 (–C6H5), 1027 (P@O). Calcd.: C, 56.21%; H, 5.02%.Found: C, 55.08%; H, 4.97%.

2.2.2. Preparation of chloromethyl-zinc poly(styrene-phenylvinylphosphonate)phosphate (ZnCMPSPPP)

The anhydrous zinc chloride (4.5 g, 33.4 mmol) was added to asolution of ZnPSPPP (7.5 g, 8.42 mmol) pre-swelled in chloroform(50 mL) at 30 �C for 1 h. Chloromethyl methyl ether (15 mL,0.19 mol) was added drop by drop subsequently and the colorchanged from light gray to claret. The reaction mixture was heatedto 40 �C step by step and kept at 40 �C for 10 h, then allowed to cooldown to room temperature. Triethylamine was added drop by droptill neutralization. Following this, the mixture was filtered, washed

with deionized water (3 � 30 mL), methanol (20 mL) and dried un-der vacuum to obtain ZnCMPSPPP (5.29 g, 76.8%). IR (KBr): 3026,2925 (CH), 2337 (O@P–OH), 1605, 1545, 1512, 1495 (–C6H5),1271 (P@O), 705 (C–Cl) cm�1. Calcd.: C, 45.16%; H, 4.35%. Found:C, 44.48; H, 4.09%.

2.2.3. Synthesis of sulfonic-zinc poly(styrene-phenylvinylphosphonate)phosphate (ZnSPSPPP)

ZnCMPSPPP (1.5 g, 1.07 mmol) was stirred in 30 mL of distilledwater for 0.5 h. Then 2.0 g sulfating or sulfoalkylating reagentsNa2SO3 (aqueous solution, a) HOCH2SO3Na (alcoholic solution, b)or HOCH2CH2SO3Na (alcoholic solution, c) was added. SaturatedNaOH solution was used to adjust pH to about 10. The mixturewas stirred at 80 �C for 24 h and then cooled down to room tem-perature. Then glacial acetic acid was added slowly till neutraliza-tion and the solution was poured into methanol, filtered, washedwith distilled water (3 � 30 mL), methanol (3 � 30 mL) and driedunder vacuum to afford the solids 1.73 g, 1.65 g, 1.69 g, with yields85.0%, 81.3%, 82.2%, respectively. The products were abbreviated as1a, 1b, 1c in turn. 1a Calcd.: C, 37.29%; H, 3.15%. Found: C, 36.16%;H, 3.05%. 1b Calcd.: C, 37.06%; H, 3.41%. Found: C, 36.46%; H, 3.15%.1c Calcd.: C, 38.66%; H, 3.79%. Found: C, 37.15%; H, 3.18%.

2.3. Grafting chiral salen Mn(III) catalyst onto ZnSPSPPP

A solution of ZnSPSPPP 1a–1c (0.5 g) in tetrahydrofuran(40.0 mL) was vigorously stirred for 30 min (Scheme 2). Chiralsalen Mn(III) complex 2 (2 g, 3.15 mmol) was added to the systemin batches. Saturated NaOH solution was used to adjust pH toabout 9, then kept at 70 �C for 24 h, cooled down to room temper-ature. Following this, glacial acetic acid was added drop by drop tillneutralization. Then the solvent was removed by rotary evapora-tion and the residue was washed with dichloromethane(3 � 30 mL), ethanol (3 � 20 mL), respectively, until no Mn couldbe detected by AAS and dried in vacuum with yields 80.2%,83.1%, 81.8%, respectively. The brown catalysts were abbreviatedas 3a-3c in turn. 3a Calcd.: C, 59.98%; H, 6.05%; N, 5.44%. Found:C, 58.16%; H, 5.64%; N, 5.04%. 3b Calcd.: C, 59.17%; H, 6.04%; N,5.26%. Found: C, 59.01%; H, 5.91%; N, 5.01%. 3c Calcd.: C, 59.4%;H, 6.14%; N, 5.18%. Found: C, 59.06%; H, 5.94%; N, 4.96%.

2.4. General procedure for asymmetric epoxidation

The catalytic properties of 3a–3c were studied for olefins incombination with m-CPBA/CH2Cl2 as efficient oxidant system.The Mn content was 0.55, 0.60, 0.62 mmol/g. A typical epoxidationprocess with m-CPBA as oxidant is processed in a solution ofCH2Cl2 (3 mL) containing olefin (0.5 mmol), n-nonane (internalstandard, 90 lL, 0.5 mmol), NMO as axial additive (1.0 mmol, ifnecessary), catalysts (5.0 mol%, based on Mn element) at 0 �C for1 h. The reaction was monitored by gas chromatography. Whenthe conversion was steady, NaOH aq (1 M, 1.5 mL, if necessary)and n-hexane were added to the solution. The organic phase wasconcentrated and purified by flash chromatography. The yieldsand ee values of epoxides were determined by GC.

2.5. Chemical analysis (Na content)

In a white porcelain crucible, a sample of 50 mg ZnPSPPP wasput in it and was heated up to 700 �C for 5 h in Muffle furnace.Due to the high temperature, ZnPSPPP decomposed. Then 20 mLof hydrochloric acid (1:1) was added to the porcelain crucibleand was heated to boiling for 30 min on the electric furnace. Inthe resulting solution, the sodium content was determined by AAS.

Scheme 1. Synthesis of the support.

Scheme 2. Synthetic route of the supported catalyst.

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3. Results and discussion

3.1. Characterizations of the supports and the immobilized catalysts

3.1.1. The content of phosphonic acid in the copolymer PS-PVPAAs described in gel-permeation chromatography (GPC), copoly-

mer PS-PVPA has average molecular weight (Mn) = 32083, Mw/Mn = 2. The content of phosphonic acid in the copolymer couldbe calculated using the following formula:

W104nþ 184

¼ C0V

where W is the mass of the copolymer; n is the number of styrene inthe unit of the copolymer; C0 is the concentration of sodiumhydroxide standard solution and V is the consumed volume ofsodium hydroxide standard solution corresponding to the place ofsudden change of pH in pH–V NaOH titration curve.

Using this formula, it could be inferred that average every seg-ment of the molecule chain –(St)m1–(PVPA)n–(St)m2–(PVPA)n–(St)m3– in the copolymer contained 6.8 organic phosphonates(n = 6.8). Consequently, the result that the copolymer average wascomprised in 36 units (m = 36) could be deduced.

3.1.2. Na content of ZnPSPPPThe sodium content in sample 1c was 1.7%, which were 0.2%

lower than that of theoretical values; this can probably be attrib-uted to the surface-bound or intercalated water leading to the aug-ment of the molecular weight.

3.1.3. FT-IR spectraAs shown in Fig. 1, the strong band at 3400 cm�1 was ascribed

to the –OH, which verified the presence of surface-bound or inter-calated water. The characteristic IR bands of the chiral Mn(III) salencomplexes could be observed in the FT-IR spectra of heterogeneous

Fig. 1. FT-IR spectra of 3a, 3b, 3c and ZnPSPPP.

J. Huang et al. / Microporous and Mesoporous Materials 153 (2012) 294–301 297

catalysts. The bands at 2980, 2933 and 2904 cm�1 were ascribed totert-butyl groups, and the band assigned to C@N stretching vibra-tions was seen at around 1630 cm�1. The characteristic bands ofthe SO3 group were at 1133, 1110 and 616 cm�1. The other bandsat 1485, 1396 and 1371 cm�1 were assigned to the deformationvibrations of C–H bond in alkyl groups. All the results of FT-IR char-acterization confirmed the successful immobilization of the chiralMn(III) salen complex.

3.1.4. UV–vis spectraThe diffuse reflectance UV–vis spectra of the homogeneous cat-

alyst 2 and the immobilized catalysts 3a–3c are given in Fig. 2. Thespectra of 3a–3c showed features similar to that of complex 2 atsome bands. The bands at 259 and 332 nm can be attributed tothe charge transfer transition of the salen ligand. The band at438 nm is due to ligand-to-metal charge transfer transition, andthe bands at 509 nm may be assigned to the d–d transition ofMn(III) salen complex. According to the catalysts 3a–3c, there wereblue shifts from 332, 438, and 509 nm to 321, 420, and 498 nm,respectively, which were ascribed to the interaction between theMn(III) salen complex and the support. The UV–vis spectra furtherconfirmed chiral Mn(III) salen complex 2 was successfully immobi-lized on the support.

Fig. 2. UV–vis spectra of 3a, 3b, 3c and Jacobsen’s catalyst 2.

3.1.5. Powder XRDThe XRD diffractograms of ZnPSPPP, ZnPSPP, ZnP are presented

in Fig. 3(a). As can be seen, XRD patterns of ZnPSPPP displayed abroad 001 peak (the lowest-angle diffraction peak in the pattern),accompanied by other peaks at higher-order 00n peaks at largerangles and lower intensities, such as at 38.2�. XRD patterns ofZnPSPPP at the vicinity of 3.8�, 21.6�, 38.2� showed similar peaksto that of ZnPSPP, which was due to the section of zinc poly(sty-rene-phenylvinylphosphonate) in ZnPSPPP. Meanwhile, XRD pat-terns of ZnPSPPP close to 18.4�, 24.7�, 28.6� displayed nearlyidentical to that of Zn3(PO4)2, which was originated in the part ofthe inorganic zinc phosphate in ZnPSPPP. Therefore, it could be de-duced that ZnPSPPP was not a mixture of zinc poly(styrene-phen-ylvinylphosphonate) and zinc inorganic phosphate but zincpoly(styrene-phenylvinylphosphonate)phosphate hybrid materialsand possessed crystalline structure. In addition, the interlayer dis-tances or d-spacings of zinc planes for crystalline ZnPSPPP can bedetermined from the 00n peaks in the powder XRD pattern (viathe Bragg equation, nk = 2dsinh). Consequently, the conclusioncould be deduced that the interlayer distance of ZnPSPPP(22.75 ± 0.09 Å) were nearly 11 Å broader than that of Zn3(PO4)2

(10.61 Å), providing comfortable microenvironment for variousguest molecules. It was most likely due to the styrene-phenylvinyl-phosphonic acid copolymer chain introduced in ZnPSPPP, makingthe zinc layer stretched and becoming broader.

Fig. 3. (a) XRD diffractogram of (1) zinc poly(styrene-phenylvinylphospho-nate)phosphate (ZnPSPPP); (2) zinc poly(styrene-phenylvinylphosphonate) (ZnPSPP); (3) zinc phosphate (ZnP). (b) XRD diffractograms of the catalysts 3a–3c.

298 J. Huang et al. / Microporous and Mesoporous Materials 153 (2012) 294–301

The XRD diffractograms of heterogeneous catalysts 3a–3c arealso presented in Fig. 3(b). XRD data showed that after the immo-bilization of chiral Mn(III) salen complex 2, the intensities of all ofthe peaks decreased. Meanwhile, the positions of main diffractionpeaks at about 3.8� remained, indicating that the catalysts still re-mained intact after the immobilization.

3.1.6. Analysis of surface morphologyThe SEM of ZnPSPPP was shown in Fig. 4(a). The results revealed

that the smooth morphology anomalous suborbicular segmentswere congregation of proportion of zinc poly(styrene-phen-ylvinylphosphonate) in ZnPSPPP and the areatus structure mainlyaggregation of the parts of zinc phosphate in ZnPSPPP. Meanwhile,support with various caves, holes, pores and channels with differ-ent shapes and sizes existed in every particle. Some microporesand secondary channels which could increase the surface area of

Fig. 4. (a) SEM photograph of the support ZnPSPPP. (b) TEM photograph of thesupport ZnPSPPP.

the catalyst and provide enough chance for substrates to accessto the catalytic active sites are also clear in Fig. 4(a). The ZnPSPPPwas further investigated by TEM. Shown in Fig. 4(b), the structureof the ZnPSPPP was spheroid, its channels, holes and cavums couldbe discerned clearly, and their sizes were about 70–80 nm. TEMalso showed that the average diameter of these secondary channelsamong the layers of the supports were around 50–60 nm. Variouscrystallites with different sizes could form holes, porous, channelsand increase the surface area of the catalysts, providing enoughspace for substrates to access to the catalytic active sites.

The SEM of heterogeneous catalyst 3c was shown in Fig. 5(a).Compared with that of support, SEM of 3c showed that the surfacemorphology was changed after immobilization and displayed manysmall caves and channels with irregular shapes. Simultaneously,surface areas increased (from 4.9 to 32.5 m2/g), which was differentto that of the catalysts immobilized on zirconium phosphate-phosphonate support (surface areas decreased) [16,19]. The TEMphotograph of the heterogeneous catalyst 3c was also shown inFig. 5(b). It was shown that the heterogeneous catalyst possessedmany holes, porous and channels, providing enough active sitesfor catalytic reaction. It may be one of the main factors that theheterogeneous chiral catalysts displayed the excellent catalyticactivities and enantioselectivity.

3.2. Enantioselective epoxidation of nonfunctionalized alkenes

The catalytic results obtained were summarized in Table 1. Allreactions proceeded efficiently and rapidly. The heterogeneous cat-alysts 3a–3c indicated superior conversions and ee values (entries5–7, 13–15). For instance, indene was quantitatively converted

Fig. 5. (a) SEM photograph of the catalyst 3c. (b) TEM photograph of the catalyst 3c.

Table 1Asymmetric epoxidation of alkenes catalyzed by catalysts 3a–3c.a

Entry Substrateb Catalyst Oxidant Conv. (%)c ee (%)d

1 A 2 m-CPBA/NMO 94 542 A 3a m-CPBA/NMO 24 67d

3 A 3b m-CPBA/NMO 4.9 73d

4 A 3c m-CPBA/NMO 5.5 92d

5 A 3a m-CPBA 90 95d

6 A 3b m-CPBA 85 98d

7 A 3c m-CPBA 89 >99d

8 A ZnPS-PPP m-CPBA >99 09 B 2 m-CPBA/NMO 92 6510 B 3a m-CPBA/NMO 15 9.5e

11 B 3b m-CPBA/NMO 4.6 17e

12 B 3c m-CPBA/NMO 5.7 41e

13 B 3a m-CPBA >99 73e

14 B 3b m-CPBA >99 84e

15 B 3c m-CPBA >99 89e

16 B ZnPS-PPP m-CPBA 86.7 0

a Reactions were carried out at 0 �C in CH2Cl2 (3.0 mL) with alkene (0.5 mmol),m-CPBA (1.0 mmol), NMO (340.0 mg, 2.50 mmol, if necessary), nonane (internalstandard, 90.0 lL, 0.5 mmol) and immobilized salen Mn(III) complexes (0.0250mmol, 5.0 mol.%).

b A = Indene, B = a-methyl-styrene.c Conversions were determined by GC with a chiral capillary column (HP19091G-

B233, 30 m � 0.32 mm � 0.25 lm).d Epoxide configuration 1S,2R.e Epoxide configuration S.

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into its epoxide with 89% conversion and >99% ee value catalyzedby 3c, as well as a-methylstyrene with >99% conversion and 89%ee value.

Noticeably, the enantioselectivity increased with the increasingof linkage lengths. Immobilized catalysts gave ee values (from 95%to 99%) for the asymmetric epoxidation of a-methylstyrene (en-tries 5–7) as well as ee values (from 73% to 89%) for the epoxidationof indene (entries 13–15), accompanied with the increase of thelinkage lengths. The results might be due to the active intermedi-ates attacking the substrate more expediently with the increasingof linker. The flexibility of the sulfating or sulfoalkylating linker be-tween the salen Mn(III) and support was capable of tuning thetransitional and rotational freedoms, potentially leading to en-hance catalytic site interactions [26]. In addition, ZnPSPPP playedlittle impacts on the ee values for the asymmetric epoxidation ofa-methylstyrene and indene (entry 8 and 16 in Table 1), just onlyserved as catalyst support.

ZnPSPPP is constructed by hydrophobic segments of polystyreneand hydrophilic zinc phosphate-phosphonate which will form dif-ferent shapes of channels, holes, cavums, micropores, secondarychannels. The increase in enantiomeric excess may be attributedto the microenvironment effect and confinement effect differingfrom either pure zinc poly(styrene-phenylvinylphosphonate)(ZnPSPP) or pure inorganic zinc phosphate (ZnP) supports. These ef-fects are originated in the layered microporous structure of ZnPSPPPand the balance adjustment between the hydrophobic of polysty-rene parts and the hydrophilic of phosphate parts. Generally, allthe organic groups of the layered zinc phosphonate are located onthe surfaces, interlamellar regions and interlayer surfaces, providingthe substrates to be in close proximity to the catalytic sites [22].

In a word, the axial grafting modes and the flexible linkages, aswell as the restriction in the rotation of the radical intermediate,may all contribute to the increased ee values in asymmetric epox-idation [27].

3.3. The effect of NMO on asymmetric epoxidation of unfunctionalizedolefins

Generally, additives are thought to act as axial ligands on thetransition metal catalyst, which help to activate the catalyst either

toward oxidation or toward reaction with the olefin [28]. Additivesto the Mn(III) salen reaction mixture, such as NMO, generally facil-itated faster reaction rates, higher epoxide yields, and improvedenantioselectivity [29]. However, the additive in this text playedsuch different role that the catalytic activities did not increasebut decrease in the presence of NMO with m-CPBA as oxidativesystem. For example, the ee values for the epoxide of a-methylsty-rene typically decreased from >99% to 92%, and the conversion de-creased from 89% to 5.5% (entries 4 vs 7 in Table 1). Similar resultswere obtained for indene (entries 12 vs 15 in Table 1). Our grouphave reported the exceptional phenomenon according to chiralsalen Mn(III) complex immobilized on phenoxy-modified ZPS-PVPA recently [22]. Simultaneously, the similar performance ob-served in the homogeneous catalysts was also reported [30]. Basedon the reports [22,27], and the case in this text, we noticed thatphenoxy-manganese, phosphate-manganese and sulfoalkyl-man-ganese (RO-Mn) salen complexes could efficiently catalyze theasymmetric epoxidation with higher conversion and ee values inthe absence of NMO, whereas amine-manganese could not. Itwas presumed, (1) axial ligands were steric repulsion betweenthe cyclohexanediamine moiety and the axial ligands coming closeto the Mn center, which brought the axial ligands to one asymmet-ric direction and then put down the salicylidene rings [31]. Mn–Oionic bond which may alter the bond length of Mn(V)@O couldmake the active intermediate unstable so that the obtained conver-sion was lower; (2) Jacobsen had presented the evidence that theN-oxides behaved as axial ligands, since the catalyst with astrapped N-oxide ligand was more active than the standard cata-lyst and is no longer affected by added N-oxide ligands [32].According to the unusual phenomenon, it was thought that –ORsections whose structures were similar with N-oxide ligand be-haved as axial ligands. Thus, there was a steric hindrance inducedby –OR sections and the optimal geometric configuration of thereactive intermediate salen Mn(V)@O was altered when N-oxide li-gand was added, accompanied with the decreased chiral recogni-tion of the catalytic system and the lower enantioselectivity. Itwas the steric hindrance that made the olefins approaching salenMn(V)@O difficultly and the decreased conversion was obtained;(3) there was a balance between Mn(V)@O complex and Mn(III)complex when N-oxide additive bound to the coordinatively unsat-urated Mn(III) complex [33]. However, the balance is interferedwhen Mn(III) is bound to the sulfoalkyl-modified support, whichresults in the increasing of Mn(III) concentration and decreasingof Mn(V)@O concentration. Spontaneously, lower conversion is ob-tained. The optimal configuration of Mn(V)@O intermediate is dis-torted. This induces molecules of substrate to get close to theintermediate equally. In other words, the suffered hindrance is soconcordant that the R- and S- products are obtained almostequally, resulting in lower enantioselectivity.

3.4. Reusability of the supported chiral salen Mn(III) catalyst

To study the stability of the immobilized catalysts during theepoxidation of olefins, we applied catalyst 3c in repeated epoxida-tion reactions with indene as a model substrate. At the end of eachcycle, the catalyst was filtered, washed thoroughly with dichloro-methane, dried under vacuum at 70 �C, and then subjected to thenext run with fresh reactants under similar epoxidation conditions.As shown in Table 2, the catalytic activity and enantioselectivityexhibited no obvious decrease after recycled at least six times inthe epoxidation of indene. The favorable reusability was ascribedto the immobilization of salen Mn(III) onto ZnPSPPP, leading to siteisolation and the stability of the Mn(salen) complexes enhanced[27]. The effective separation induced by the chiral Mn(III) salencomplexes immobilized onto ZnPSPPP contributed to the good sta-bility of the heterogeneous chiral Mn(III) salen catalyst in case that

Table 2The recycles of catalyst 3c in the epoxidation of a-methylstyrene.a

Run Catalyst Time (h) Conv. (%) ee (%)b

1 3c 1 99.90 88.802 3c 1 99.78 84.733 3c 1 95.77 85.914 3c 1 91.75 82.755 3c 1 86.71 81.226 3c 1 81.66 78.427 3c 1 76.65 44.578 3c 1 70.63 42.23

a The reaction conditions are the same as entries 10–12 in Table 1.b Epoxide configuration S.

Table 3Large-scale asymmetric epoxidation reaction of a-methylstyrene.a

Entry Time (h) Conv. (%) ee f(%) TOFg �10�4 (s�1)

1b 5 >99 >99 11.112c 5 >99 >99 11.113d 5 >99 >99 11.114e 5 >99 >99 11.11

a Reactions were carried out at �40 �C in CH2Cl2 with a-methylstyrene,n-nonane, m-CPBA, heterogeneous salen Mn(III) catalysts (5 mol.%). The conversionand the ee value were determined by GC with chiral capillary columns HP19091G-B213, 30 m � 0.32 mm � 0.25 lm.

b The usage amounts of reagents were a-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 a-methylstyrene (50 mmol), n-nonane(50 mmol), heterogeneous catalyst 3b (2.5 mmol), m-CPBA (100 mmol), respectively.

d The usage amounts of reagents were a-methylstyrene (50 mmol), n-nonane(50 mmol), heterogeneous catalyst 3b (0.5 mmol), m-CPBA(100 mmol), respectively.

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

f (S)-form.g Turnover frequency (TOF) is calculated by the expression of

[product]/[catalyst] � time (s�1).

300 J. Huang et al. / Microporous and Mesoporous Materials 153 (2012) 294–301

they would dimerize to inactive l-oxo-Mn(IV) species. The de-crease in the enantioselectivity after reused several cycles mightbe caused by micropores and secondary channels partly pluggedunder the epoxidation condition, resulting in decreased chiral rec-ognition of the catalytic system. Meanwhile, the decrease of theyield can be attributed to the decomposition of the chiral Mn(III)salen complex under epoxidation conditions and the loss of thehyperfine granules of the heterogeneous chiral Mn(III) salen cata-lysts (formed in reaction due to stirring). The Mn content of theheterogeneous catalyst 3c is 0.36 mmol/g for the eighth run, com-pared with the total amount (around 0.62 mmol/g). Under the acidreaction conditions, the supported catalyst was partly destroyedand the effect of the linker and the support was also weakened,owing to the fracture of the bond between the linker and the li-gand. Moreover, other effects can be used to explain these results:(i) leaching of metal complexes from the materials or (ii) blockingof the pores and secondary channels either by inactive Mn(IV)-oxospecies believed to be generated during the catalytic mechanism orby some other insoluble degraded product obtained by side reac-tions, which could not be removed from the materials after severalwashing or (iii) collapsing of some of the pillars during the cataly-sis experiments.

3.5. Large-scale asymmetric epoxidation reaction

We further performed different proportions of large-scale asym-metric epoxidation reactions with n-nonane and a-methylstyreneand m-CPBA. The same catalyst loading of 5 mol% as in the experi-mental scale was used. The large-scale experiments can be facilely

carried out using the same procedure as for the experimental scalereactions. As can be seen from the results summarized in Table 3,delightfully, the conversion and enantioselectivity maintained atthe same level for the large-scale reactions under whichever condi-tion that the large scale is 50 times or 100 (Fig. S1) times as much asthe experimental scale.

4. Conclusion

In conclusion, we have synthesized three kinds of supportedcatalysts that the chiral salen Mn(III) complexes was immobilizedonto sulfoalkyl-modified ZnPSPPP, which exhibited much higherchiral induction than that of the homogeneous Jacobsen’s catalystfor asymmetric epoxidation of unfunctionalized olefins. All theheterogeneous catalysts indicated remarkably increased conver-sions and ee values in the absence of axial base NMO and couldbe performed on a large-scale with the catalytic ability being main-tained at the same level, providing potential values for industrialapplication.

Acknowledgments

Authors are grateful to Southwest University of China and theCommittee for Economics of Chongqing Municipality (grant2008-65) for financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2011.10.025.

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