Mesoporous silicas functionalized with aminopropyl via co-condensation: Effective supports for chiral Mn(III) salen complex

Microporous and Mesoporous Materials 142 (2011) 214–220

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Mesoporous silicas functionalized with aminopropyl via co-condensation: Effective supports for chiral Mn(III) salen complex Lan-Lan Lou a, Shu Jiang a, Kai Yu b, Zhicheng Gu a, Runan Ji a, Yanling Dong a, Shuangxi Liu a,⇑ a Institute of New Catalytic Materials Science and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, PR China b College of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China

a r t i c l e

i n f o

Article history: Received 24 September 2010 Received in revised form 11 November 2010 Accepted 1 December 2010 Available online 6 December 2010 Keywords: Co-condensation Mesoporous hybrid Chiral Mn(III) salen complex Asymmetric epoxidation Pore size

a b s t r a c t The 3-aminopropyl-functionalized mesoporous materials with various amounts of 3-aminopropyl groups and nanopore sizes were synthesized through direct co-condensation of tetraethoxysilane with 3-aminopropyltriethoxysilane (APTES) using alkyltrimethylammonium bromide as template and applied to immobilize chiral Mn(III) salen complex. All the mesoporous hybrids and heterogeneous catalysts had ordered hexagonal mesostructure with uniform pore size distributions. The obtained heterogeneous catalysts exhibited comparable conversion and enantiomeric excess to those of the homogeneous counterpart for the asymmetric epoxidation of olefins even with a very low catalyst dosage (0.6 mol%), which were thrice more active than the catalysts supported on the 3-aminopropyl graft-modified mesoporous silicas. The improved catalytic efficiency was mainly attributed to the homogeneous distribution of chiral Mn(III) salen complexes on the surface of co-condensation modified supports. Both the APTES dosage and nanopore size of mesoporous hybrid material were found to have great influences on catalytic performance, and the catalyst with suitable APTES content and larger pore size showed higher catalytic activity and enantioselectivity. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Since 1992, mesoporous siliceous materials with large specific surface area, ordered pore structure, tunable pore diameter and well-defined pore size distribution have received noticeable attention, especially with regard to potential applications in the areas of adsorption, separation, catalysis, biotechnology, and chromatography [1–6]. Incorporating organic components into mesoporous silicas to create organic–inorganic hybrid materials provides the possibility to combine the numerous functional variations with the advantages of inorganic matrix. This is highly significant in the development of multifunctional materials and is particularly applicable to heterogeneous catalysis [6]. In general, the organic functionalization can be achieved either by a grafting method or by a co-condensation process. The grafting method is relatively simple and convenient, and a variety of organofunctional groups can be introduced into mesoporous silicas in this way. However, the organic silanes would react preferentially at the external surface and near the pore openings, leading to a nonhomogeneous distribution of the organic groups in the mesopores. Compared with grafting, co-condensation of silane and organosilane could provide organically functionalized mesoporous ⇑ Corresponding author. Tel./fax: +86 22 23509005. E-mail address: [email protected] (S. Liu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.002

materials with more homogeneous distribution of organic units [6–9], which was mainly attributed to the fact that the organic functionalities were direct components of the matrix. At the same time, pore blocking could be effectively avoided in these hybrid materials. Chiral Mn(III) salen complexes have proven activity and enantioselectivity for the asymmetric epoxidation of olefins under homogeneous [10–12] and heterogeneous [13–21] conditions, which are highly significant in the synthesis of chiral intermediates in the pharmaceutical and agrochemical fields. Over the last decade, the heterogenization of chiral Mn(III) salen complexes within inorganic matrixes has received much attention [22–37] due to efficient product purification and easy catalyst recovery. Several different strategies have been adopted to immobilize chiral Mn(III) salen complexes on silica-based mesoporous materials. The nanopores of mesoporous silica immobilized with chiral catalysts are typical ‘‘nanoreactors’’ [28]. Some recent research studies [27,28,32–35] indicated that improved catalytic performance could be obtained through finely adjusting the microenvironment of the nanoreactor. In most cases, the organosilanes acting as linkage groups between mesoporous material and the chiral Mn(III) salen complex were anchored onto the silica surface by covalent grafting, which would in turn lead to a nonhomogeneous distribution of the nanoreactors on the supports. An obviously high catalyst/substrate

L.-L. Lou et al. / Microporous and Mesoporous Materials 142 (2011) 214–220

ratio was often demanded over the heterogeneous Mn(III) salen catalysts based on such organically functionalized mesoporous materials, resulting in a decrease in catalytic efficiency. In this work, a series of 3-aminopropyl-modified mesoporous materials were synthesized by co-condensation and employed to immobilize chiral Mn(III) salen complex. The as-synthesized catalysts exhibited high conversion and enantiomeric excess (ee), as expected, in the asymmetric epoxidation of olefins even at a relatively low catalyst dosage. Compared with the catalysts based on graft-modified mesoporous hybrids, the present catalysts were thrice more active. The enhanced catalytic efficiency was mainly due to the more homogeneous distribution and thus more effective utilization of the nanoreactors in the co-condensation modified mesoporous silicas. In addition, the effect of the size of nanoreactors on catalytic performance was studied by fine-tuning of the pore size of mesoporous supports. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS; AR), ammonia (25 wt.%; AR), and styrene were provided by Jiangtian Chemical Technology Co., Ltd. in Tianjin. Alkyltrimethylammonium bromide CnH2n+1(CH3)3–NBr (CnTAB for brevity, n = 14, 16, and 18; AR), was obtained from Jintan Huadong Chemical Research Institute in Jiangsu. a-Methylstyrene and N-methylmorpholine-N-oxide (NMO) were provided by Aldrich. Indene was purchased from Fluka. 3-Aminopropyltriethoxysilane (APTES) and m-chloroperbenzoic acid (m-CPBA) were supplied by Acros Organics. cis/trans-1,2-Diaminocyclohexane and 2-tert-butylphenol were purchased from Alfa Aesar. (1R,2R)-( )Diaminocyclohexane was resolved from the technical grade isomers mixture in >99% ee by the reported procedure [38]. 3tert-Butyl-5-chloromethyl-2-hydroxybenzaldehyde was synthesized from 2-tert-butylphenol as described previously [35]. All of the solvents used in the present study were purified before use. 2.2. Characterization 1 H NMR spectra were recorded using a Varian Mercury Vx-300 spectrometer at 300 MHz. ICP-AES results were obtained on an ICP-9000(N+M) spectrometer (TJA Co.). The elemental analysis of C, H and N was carried out on a Perkin–Elmer 240C analyzer. Powder X-ray diffraction (XRD) patterns were collected on an Rigaku D/max-2500 diffractometer with CuKa radiation at 40 kV and 100 mA. N2 adsorption–desorption analysis was performed at 77 K on a Micromeritics TriStar 3000 apparatus. The surface area was estimated using the BET equation, and the pore size distribution was determined by the BJH model. Transmission electron microscopy (TEM) measurements were carried out on a Philips Tecnai T20ST electron microscope operating at 200 kV. FT-IR spectra were recorded on a Bruker Vector 22 spectrometer using KBr pellets in the 400–4000 cm 1 region. Solid-state 29Si CP/MAS NMR experiments were performed on a Varian InfinityPlus-400 spectrometer equipped with a 7.5 mm probe (MAS was set to 4.5 kHz). The products of the epoxidation reaction were determined by GC with a chiral capillary column (RESTEK RTBetaDEXse, 30 m  0.25 mm  0.25 lm), using a Fuli 9790II gas chromatograph equipped with a flame ionization detector with the column temperature programmed in the range of 333–423 K.

2.3. Synthesis of chiral Mn(III) salen complex C The monotartarate salt of (1R,2R)-( )-diaminocyclohexane (9 mmol) and 3-tert-butyl-5-chloromethyl-2-hydroxybenzalde-

215

hyde (18 mmol) were refluxed for 0.5 h in absolute ethanol (65 mL) under stirring, then K2CO3 (18 mmol) was added. After refluxing for another 2.5 h, deionized water (18 mL) was added. The mixture was kept at 277 K for 12 h, and the yellow precipitated ligand L was collected by filtration. 1H NMR (CDCl3, 300 MHz): d (ppm) 1.19 (t, J = 7.2 Hz, 6H), 1.40 (s, 18H), 1.43–1.95 (m, 8H), 3.33 (m, 2H), 3.45 (q, J = 7.2 Hz, 4H), 4.33 (s, 4H), 6.98 (d, J = 2.1 Hz, 2H), 7.21 (d, J = 2.1 Hz, 2H), 8.26 (s, 2H), 13.86 (s, 2H). The chiral ligand L (5 mmol) and Mn(OAc)2
4H2O (15 mmol) were refluxed in absolute ethanol (100 mL) for 0.5 h. Then LiCl (15 mmol) was added and the mixture was refluxed for an additional 2.5 h. Then the solvent was completely evaporated under reduced pressure and the residue was washed with deionized water (3  25 mL) and brine (2  20 mL), and then recrystallized from dichloromethane-hexane to give chiral Mn(III) salen complex C as a brown solid. 2.4. Preparation of 3-aminopropyl-modified mesoporous materials A series of 3-aminopropyl-modified MCM-41-type mesoporous materials with different APTES amounts and pore sizes were synthesized via co-condensation between TEOS and APTES using CnTAB (n = 14, 16, and 18) as template. A typical synthesis procedure was carried out as follows. The template of CnTAB was dissolved in warm deionized water and ammonia was added to this solution under vigorous stirring. The required quantities of TEOS and APTES were then added dropwise into the solution. After stirring for an additional 0.5 h at room temperature, a gel with a molar composition of 1.0Si:0.12CTAB:8.0NH4OH:114H2O was obtained. TEOS and APTES were used as the source of Si in various molar ratios of 90:10, 95:5 and 97.5:2.5. The resulting gel was transferred into a Teflon-lined autoclave for crystallization at 383 K for 72 h. The solid product was recovered by filtration, washed with deionized water, dried in air, and then Soxhlet-extracted with an acidic ethanol solution for 72 h to remove the template. The as-synthesized MCM-41-type mesoporous materials were marked as CnM41(z) (n = 14, 16, and 18; z = 10, 5, and 2.5), where n and z(%) represent the alkyl chain length of the template and the molar ratio of APTES to total Si, respectively. 2.5. Heterogenization of chiral Mn(III) salen complex C The synthesis process is shown in Scheme 1. The obtained mesoporous material CnM41(z) (1.0 g) was added to a solution of chiral Mn(III) salen complex C (0.12 g) in toluene (50 mL). The mixture was refluxed under vigorous stirring for 24 h. After being cooled to room temperature, the brown solid product was collected by filtration, washed with dichloromethane, and then Soxhlet-extracted with acetone for 24 h. The as-synthesized heterogeneous chiral Mn(III) salen catalysts were marked as CnM41(z)–C (n = 14, 16, and 18; z = 10, 5, and 2.5). The contents of Mn(III) salen in these catalysts, determined by ICP-AES based on Mn element, were 0.034, 0.034, 0.070, 0.040, and 0.050 mmol g 1 for C16M41(2.5)– C, C16M41(5)–C, C16M41(10)–C, C14M41(5)–C, and C18M41(5)–C, respectively. 2.6. Asymmetric epoxidation of olefins Enantioselective epoxidation reactions were carried out using the catalyst (0.006 mmol, 0.6 mol%, based on Mn element) with styrene, a-methylstyrene and indene as substrates (1 mmol) in dichloromethane (10 mL) in the presence of m-CPBA (2 mmol) as oxidant and NMO (5 mmol) as axial base at 273 K for 8 h (heterogeneous) or 2 h (homogeneous). Once the reaction was complete, the immobilized catalysts were separated by filtration, and the filtrate was washed with 1 N NaOH (10 mL) and brine (10 mL), and

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H

H N Cl N Mn O O NH2 t-Bu t-Bu

NH2 H

H OH

OH O

Si O

O

N

+

EtOH2C

N Mn

toluene

O

EtOH2C

O Cl

Support

t-Bu

CnM41(z)

CH2OEt

OH

reflux

CH2OEt

OH

t-Bu

Si

O

O

O

C

Support

CnM41(z)-C Scheme 1. The synthesis process of heterogeneous catalyst CnM41(z)–C.

then dried over MgSO4. The conversions and ee values were determined by GC, with toluene (40 lL) as an internal standard.

4

Q

3. Results and discussion

Q

3

3.1. Characterization of materials

3.1.2. Solid-state 29Si CP/MAS NMR spectrometry Fig. 1 shows the 29Si CP/MAS NMR spectrum of 3-aminopropylmodified mesoporous material C16M41(10). As shown in Fig. 1, the resonance peaks assigned to Q4 [(OSi)4Si] and Q3 [(OSi)3Si(OR)] (R = H and Et) silicons could be observed at d = 113 and 104 ppm, respectively. Besides these peaks, the sample exhibited two more resonance peaks at d = 56 and 71 ppm representing silicon atoms in positions T2 [CSi(OSi)2(OR)] (R = H, Et) and T3 [CSi(OSi)3], respectively [39]. The presence of peaks T2 and T3 confirmed the successful incorporation of organic groups into the silica matrix. It also can be noticed from Fig. 1 that most of APTES covalently bonded with silica matrix through two or three Si–O–Si bonds. 3.1.3. Powder XRD Fig. 2 shows the powder XRD patterns of modified mesoporous materials C16M41(z) and the corresponding heterogeneous catalysts C16M41(z)–C. The materials C16M41(z) exhibited three clear XRD peaks assigned to reflections at (1 0 0), (1 1 0), and (2 0 0), which are typical for the formation of well-arranged 2-D hexagonal mesostructures. Moreover, with the increase in the loading of organic functional groups, the relative intensities of the XRD peaks decreased and the (1 0 0) peak became broader, which indicated that the existence of organic groups could disturb the long-range

Table 1 The contents of element N and Si in the mesoporous materials C16M41(z). Sample

N (wt.%)

Si (wt.%)

Molar ratio (N/Si) (%)

C16M41(2.5) C16M41(5) C16M41(10)

0.53 0.66 1.78

54.35 58.27 49.94

2.0 2.3 7.1

T

100

50

0

2

T

3

-50

-100

-150

-200

-250

ppm Fig. 1.

29

Si CP/MAS NMR spectrum of C16M41(10).

(100)

Intensity (a.u.)

3.1.1. Chemical analysis The contents of element N and Si in the mesoporous materials C16M41(z) were determined by elemental analysis and ICP-AES, respectively. The results, given in Table 1, show that the N/Si molar ratio, which represents the molar ratio of organosilane to total Si in the materials C16M41(z), increased with the increasing of initial ratio of APTES to TEOS. The material C16M41(10) possesses a markedly higher loading of organic functional groups, while C16M41(5) has a slightly higher organic group content than C16M41(2.5).

(110) (200)

(a) (b) (c) (d) (e) (f)

2

4

6

8

10

2Theta (deg.) Fig. 2. Powder XRD patterns of (a) C16M41(2.5), (b) C16M41(2.5)–C, (c) C16M41(5), (d) C16M41(5)–C, (e) C16M41(10), and (f) C16M41(10)–C.

mesoporous ordering. After the immobilization of chiral Mn(III) salen complex C, a slight decrease in the intensities of the XRD peaks was observed. However, the well-resolved reflections (1 0 0), (1 1 0), and (2 0 0) for C16M41(z)–C indicated that the ordering of mesoporous hexagonal channels was still maintained after immobilization. Fig. 3 describes the XRD patterns of C14M41(5), C16M41(5), and C18M41(5). It could be found that all the three materials possessed ordered hexagonal mesostructures. Moreover, with the decrease in alkyl chain length of the template, the XRD peak at (1 0 0) was shifted to higher 2h value, which signified that the pore size of mesoporous materials decreased gradually.

L.-L. Lou et al. / Microporous and Mesoporous Materials 142 (2011) 214–220

channels with uniform pore size can be clearly observed. This demonstrated that the materials C16M41(z) synthesized via cocondensation had a well ordered hexagonal mesostructures and that the support framework long range order was maintained after the immobilization of chiral Mn(III) salen complex C. The TEM result is in good agreement with that acquired from XRD and N2 sorption analysis discussed above.

Intensity (a.u.)

(100)

(110) (200)

(a) (b) (c)

2

4

6

8

10

2Theta (deg.) Fig. 3. Powder XRD patterns of (a) C14M41(5), (b) C16M41(5), and (c) C18M41(5).

3.1.4. Nitrogen sorption All the 3-aminopropyl-modified materials CnM41(z) and heterogeneous catalysts CnM41(z)–C were characterized by N2 sorption. The materials CnM41(z) showed type IV isotherms according to the IUPAC [40], with hysteresis loops characteristic of mesoporous solids. The corresponding textural parameters calculated by N2 adsorption–desorption isotherms are listed in Table 2. A gradual decrease in BET surface area could be observed for 3-aminopropyl-modified materials with increasing APTES dosage. It could also be noticed that with the increase in alkyl chain length of the template, the pore diameter of CnM41(5) increased. The results were consistent with that of XRD characterization. Moreover, it can be seen from Table 2, with the immobilization of chiral Mn(III) salen complex C, a slight decrease in BET surface area, pore volume, and pore diameter was observed compared with the supports CnM41(z). This indicated that the complex C was located mainly into the nanopores of the supports. The immobilization of complex C on C16M41(10) led to a relatively obvious change in textural parameters, which may be due to the higher loading of Mn(III) salen complex in C16M41(10)–C. Fig. 4 shows the low-temperature N2 adsorption–desorption isotherms and pore size distributions of the heterogeneous catalysts C16M41(z)–C and C18M41(5)–C. It could be seen that all the heterogeneous catalysts exhibited characteristic type IV isotherms with well-defined sharp inflections and uniform pore sizes in the mesopore ranges. This indicated that the ordered mesoporous structure was preserved in the immobilized catalysts. 3.1.5. TEM TEM micrographs of the heterogeneous catalyst C16M41(5)–C are shown in Fig. 5. The regular hexagonal arrangement of the

Table 2 The structure parameters of parent supports and heterogeneous catalysts. Sample C16M41(2.5) C16M41(2.5)–C C16M41(5) C16M41(5)–C C16M41(10) C16M41(10)–C C14M41(5) C14M41(5)–C C18M41(5) C18M41(5)–C

217

SBET (m2 g 786 752 606 601 412 377 515 514 540 523

1

)

Pore volume (cm3 g 1)

Pore diameter (nm)

0.74 0.68 0.58 0.58 0.34 0.29 0.37 0.34 0.62 0.57

2.47 2.41 2.43 2.42 2.72 2.13 1.88 1.88 2.74 2.70

3.1.6. FT-IR spectroscopy The FT-IR spectra of modified mesoporous materials CnM41(z) and heterogeneous catalysts CnM41(z)–C exhibited specific bands at around 1080, 800, and 460 cm 1 assigned to characteristic vibrations of the Si–O–Si framework and a broad band near 3430 cm 1 for the adsorbed water. Fig. 6 shows the representative FT-IR spectra in the scan range of 1250–3200 cm 1 for aminopropyl-modified mesoporous material C16M41(10) and heterogeneous catalyst C16M41(10)–C. The bands at 2960, 2922, and 2858 cm 1 could be attributed to C–H stretching vibrations of alkyl groups, and the IR absorption at 1470 cm 1 was due to scissor bending vibrations of the –CH2–CH2–CH2– groups. The IR bands appeared at 1416, 1389, and 1340 cm 1 could be assigned to C–H deformation vibrations of alkyl groups. Furthermore, with the immobilization of Mn(III) salen complex C, a new characteristic IR band appeared at 1540 cm 1, which was attributed to the stretching vibrations of azomethine groups (H–C@N). The results of FT-IR characterization confirmed the successful modification of aminopropyl groups and immobilization of chiral Mn(III) salen complex C. 3.2. Catalytic performance of the heterogeneous catalysts The epoxidation of olefins was carried out over CnM41(z)–C using m-CPBA/NMO as the oxidant system. For comparison, the chiral Mn(III) salen pre-catalyst C was also investigated. It was found that the substrate was epoxidized by m-CPBA to racemic product when catalyst and NMO were absent and that no observable epoxidation reaction occurred in the presence of m-CPBA and NMO but without catalyst. The asymmetric epoxidation of styrene catalyzed by C16M41(5)–C was investigated with different substrate/catalyst molar ratios, and the results are summarized in Fig. 7. An increase in styrene conversion and product ee value could be observed when the substrate/catalyst ratio reduced. Excellent catalytic activity (91% conversion) and enantioselectivity (42% ee value) that are comparable to homogeneous results were achieved as the substrate/catalyst ratio was 167 (0.6 mol% catalyst). This is noteworthy that the amount of heterogeneous catalyst was much lower than the reported work [34–37]. The comparison experiments showed that the MCM-41 immobilized chiral Mn(III) salen catalyst synthesized via a post-grafting process provided comparable activity and enantioselectivity to the homogeneous counterpart at a dosage of more than 2 mol% under otherwise the same reaction conditions. Thus notably high turnover frequency (TOF, 19.0 h 1) was provided by the present catalyst C16M41(5)–C. This can be attributed mainly to the more homogeneous distribution of active sites in catalyst C16M41(5)–C, which was synthesized through a co-condensation procedure. The active sites could be more readily accessed and effectively utilized, which would lead to improved catalytic performance. Control experiment with C16M41(5) showed no styrene conversion (entry 4 in Table 3), indicating that the aminopropyl-modified mesoporous material was catalytically inactive for olefin epoxidation. In the heterogeneous catalyst, APTES acts as a covalent linker to anchor the chiral Mn(III) salen complex C on the surface of mesoporous supports. To investigate the effect of APTES dosage on the catalytic performance of heterogeneous catalysts, the mesoporous materials C16M41(z) synthesized with different APTES dosages

L.-L. Lou et al. / Microporous and Mesoporous Materials 142 (2011) 214–220

C16M41(2.5)-C Volume Adsorbed (cm /g STP)

400

400

3

1.2

0.8

3

dV/dD (cm /g·nm)

300

200

0.4

0.0

100

0

5

10

15

20

Adsorption Desorption

C16M41(5)-C

300

200

0.8

3

Adsorption Desorption

3

Volume Adsorbed (cm /g STP)

500

dV/dD (cm /g·nm)

218

0.4

0.0

100

0

Pore Diameter (nm)

0.0

0.2

0.4

0.6

0.8

5

10

15

20

Pore Diameter (nm)

1.0

0.0

0.2

Relative Pressure (P/P0)

0.4

0.6

0.8

1.0

Relative Pressure (P/P 0)

(B)

(A) 250

450

150

3

dV/dD (cm /g·nm)

0.4

100

0.2

0.0

50

0

5

10

15

C18M41(5)-C

300

0.6

3

3

200

Adsorption Desorption

dV/dD (cm /g·nm)

Volume Adsorbed (cm /g STP)

C16M41(10)-C

3

Volume Adsorbed (cm /g STP)

Adsorption Desorption

150

20

0.3

0.0 0

Pore Diameter (nm)

0.0

0.2

0.4

0.6

0.8

5

10

15

20

Pore Diameter (nm)

1.0

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P 0)

Relative Pressure (P/P0)

(D)

(C)

Fig. 4. N2 adsorption–desorption isotherms and pore diameter distribution profiles (insert) of (A) C16M41(2.5)–C, (B) C16M41(5)–C, (C) C16M41(10)–C, and (D) C18M41(5)–C.

Fig. 5. TEM micrographs of C16M41(5)–C.

were applied to immobilize the Mn(III) salen complex C. The assynthesized catalysts C16M41(z)–C were evaluated in the asymmetric epoxidation of olefins, and the results are listed in Table 3. It could be found that these heterogeneous catalysts exhibited rather good catalytic activities and enantioselectivities for the epoxidation of styrene, a-methylstyrene and indene. The catalysts C16M41(5)–C and C16M41(2.5)–C provided relatively higher catalytic performances. Especially, the catalyst C16M41(5)–C showed comparable conversions and ee values to the homoge-

neous counterpart, which indicated that the suitable APTES dosage was beneficial for obtaining higher catalytic performance. The catalyst C16M41(10)–C exhibited the lowest activities and enantioselectivities among these heterogeneous catalysts. This may be explained by the fact that the increased diffusional resistance with smaller pores, which was due to the relatively higher loading of APTES and Mn(III) salen complex in C16M41(10)–C, made some of the active sites in the channels of the immobilized catalyst inaccessible.

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45

1416 2858

90

2960 (a)

1470

1389 1340

35 70

30

60

2922

25

50

3200

3000

2800

1600

1500

1400

1300

ee (%)

1540

(b)

Conversion (%)

Transmittance (%)

40 80

120

160

-1

200

240

S/C

Wavenumber (cm ) Fig. 6. FT-IR spectra of (a) C16M41(10) and (b) C16M41(10)–C.

Fig. 7. Effect of substrate/catalyst molar ratio on conversion and ee value of styrene epoxidation catalyzed by C16M41(5)–C.

The mesoporous materials CnM41(5) with different pore sizes were applied to immobilize the Mn(III) salen complex C. The obtained catalysts CnM41(5)–C were evaluated in the asymmetric epoxidation of styrene, a-methylstyrene and indene, and the effect of the fine-tuning of pore size on the catalytic performance was studied. The results, given in Table 4, show that the conversions and ee values increased obviously with gentle increasing nanopore sizes in the mesoporous supports. The immobilized catalysts C16M41(5)–C and C18M41(5)–C exhibited comparable catalytic activities and enantioselectivities to the homogeneous counterpart. And the catalyst C14M41(5)–C with the smallest pore size showed obviously lower catalytic performance (entries 1, 6 and 9), possibly due to its relatively higher diffusional resistance. Among these immobilized catalysts, the catalyst C18M41(5)–C presented the highest catalytic activities and enantioselectivities (entries 5, 8 and 11); for example, 100% conversion for styrene and a-methylstyrene was obtained. This could be attributed mainly to this catalyst’s larger pore size and easier material transmission. In summary, the catalytic performance of these immobi-

lized catalysts was closely related to the pore sizes; the catalyst with larger pore size showed better activity and enantioselectivity, and the catalyst possessing smaller pore size gave lower conversions and ee values. To study the stability of the heterogeneous catalyst during the epoxidation reaction, the catalyst was separated by filtration after the reaction was complete, and the filtrate was detected by ICP-AES. The results showed that only a slight Mn leaching (<1%) was found for all the immobilized catalysts, indicating that the Mn(III) salen complex C was strongly bonded to the organofunctionalized mesoporous supports. To further confirm that the observed catalysis is truly heterogeneous, the catalyst C16M41(5)–C was filtrated from the reaction system of styrene epoxidation after 0.5 h (ca. 41% conversion) and the filtrate was allowed to continuously react under the same reaction temperature. Little further reaction was observed, and both the conversion and ee value changed marginally after another 7.5 h reaction (entries 3 and 4 in Table 4). Thus it can be proposed that the epoxidation reaction is predominantly catalyzed by the heterogeneous catalyst.

Table 3 Asymmetric epoxidation of olefins catalyzed by homogeneous catalyst C and heterogeneous catalysts C16M41(z)–Ca.

R1

R2 +

R3

NMO/m-CPBA

R4

Catalyst CH2Cl2, 273 K

2 R1 O R

R3

* *

R4

Entry

Catalyst

Substrate

Conv. (%)b

ee (%)c

Config.

TOF (h

1 2 3 4 5 6 7 8 9 10 11 12 13

C C16M41(2.5)–C C16M41(5)–C C16M41(5) C16M41(10)–C C C16M41(2.5)–C C16M41(5)–C C16M41(10)–C C C16M41(2.5)–C C16M41(5)–C C16M41(10)–C

Styrene

100 83 91 73 100 88 95 80 99 98 98 97

49 38 42 34 42 30 31 31 91 76 86 73

R R R R R R R R 1R,2S 1R,2S 1R,2S 1R,2S

– 17.3 19.0 15.2 – 18.3 19.8 16.7 – 20.4 20.4 20.2

a-Methylstyrene

Indene

1

)

a Reactions were performed in CH2Cl2 (10 mL) with catalyst (0.006 mmol), substrate (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K for 2 h (homogeneous) or 8 h (heterogeneous). b Conversion % determined by GC with chiral column using toluene as internal standard. c Ee % determined by GC with RESTEK RT-BetaDEXse chiral column.

220

L.-L. Lou et al. / Microporous and Mesoporous Materials 142 (2011) 214–220

Table 4 Asymmetric epoxidation of olefins catalyzed by heterogeneous catalysts with different pore sizesa.

a b c

Entry

Catalyst

Substrate

Conv. (%)

ee (%)

Config.

1 2 3b 4c 5 6 7 8 9 10 11

C14M41(5)–C C16M41(5)–C C16M41(5)–C C16M41(5)–C C18M41(5)–C C14M41(5)–C C16M41(5)–C C18M41(5)–C C14M41(5)–C C16M41(5)–C C18M41(5)–C

Styrene

63 91 41 43 100 63 95 100 90 98 98

18 42 31 31 44 22 31 33 76 86 90

R R R R R R R R 1R,2S 1R,2S 1R,2S

a-Methylstyrene

Indene

The reaction conditions were the same as those in Table 3. The reaction time was 0.5 h. The reaction time was 8 h and the catalyst was removed after 0.5 h.

4. Conclusion The chiral Mn(III) salen complex was immobilized on the MCM41-type mesoporous materials functionalized by aminopropyl groups through a co-condensation procedure. The obtained heterogeneous catalysts were active and enantioselective for the epoxidation of styrene, a-methylstyrene, and indene. In particular, these catalysts exhibited excellent catalytic performance even with a relatively lower catalyst amount (0.6 mol%), thus providing notably high TOF values for olefin epoxidation, which is due in part to the homogeneous distribution of the active centers. The suitable APTES dosage was beneficial for obtaining higher catalytic performance, and the catalyst C16M41(5)–C showed the highest catalytic activity and enantioselectivity among the catalysts C16M41(z)–C. The nanopore sizes had a considerable effect on the catalytic performance of the immobilized catalysts. The catalyst C18M41(5)–C with larger pore size gave the best conversions and ee values for the epoxidation of olefins. Acknowledgments

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This work was supported by the National Natural Science Foundation of China (Grant No. 20773069), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 200800551017), the Fundamental Research Funds for the Central Universities, and MOE (IRT-0927).

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