Effect of pore size on the performance of mesoporous material supported chiral Mn(III) salen complex for the epoxidation of unfunctionalized olefins

Article abstract of DOI:10.1016/j.jcat.2007.09.009

A series of mesoporous MCM-41 and MCM-48 materials with different pore sizes were synthesized and used as supports to immobilize chiral Mn(III) salen complex. The heterogeneous catalysts were characterized by XRD, FT-IR, DR UV-vis, and N₂ sorpt

Full text of DOI:10.1016/j.jcat.2007.09.009

Journal of Catalysis 252 (2007) 312–320
www.elsevier.com/locate/jcat
Effect of pore size on the performance of mesoporous material supported
chiral Mn(III) salen complex for the epoxidation of unfunctionalized olefins
Kai Yu a,b, Zhicheng Gu a,b, Runan Ji a,b, Lan-Lan Lou a,b, Fei Ding a,b, Cui Zhang a,b,
Shuangxi Liu a,b,∗
a
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, PR China
b
State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, PR China
Received 26 July 2007; revised 13 September 2007; accepted 13 September 2007
Available online 23 October 2007
Abstract
A series of mesoporous MCM-41 and MCM-48 materials with different pore sizes were synthesized and used as supports to immobilize chiral
Mn(III) salen complex. The heterogeneous catalysts were characterized by XRD, FT-IR, DR UV–vis, and N sorption, and the results indicated the
2
successful immobilization of chiral Mn(III) salen complex. The confinement effect of pore size on the catalytic performance of the heterogeneous
catalysts was studied for the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as an oxidant. It was found that
the conversions and enantiomeric excess (ee) values were closely correlated with the pore sizes of parent supports. The catalysts immobilized on
the large-pore mesoporous supports exhibited higher conversions, and for the catalysts immobilized on MCM-41 materials, the ee values improved
with increasing pore size. However, for the MCM-48 material-supported catalysts, the compatible pore size of the support with the substrate was
beneficial for obtaining higher enantioselectivity in olefin epoxidation.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Confinement effect; Pore size; Mesoporous materials; Chiral Mn(III) salen complex; Immobilization; Asymmetric epoxidation; Unfunctionalized olefins
1. Introduction
the advantages of heterogeneous catalysis systems, including
easy catalyst/product separation and simple catalyst recycling.
Many kinds of mesoporous materials with large surface ar-
eas and controllable pore sizes of 2–50 nm were used as ma-
trixes to immobilize chiral Mn(III) salen complexes, which al-
lows easy diffusion of reactants to the active sites located on
the inner surface. Moreover, these mesoporous materials can
be easily organically functionalized due to the abundant silanol
groups on their surface. Over the last decade, several differ-
ent strategies have been adopted to immobilize chiral Mn(III)
salen complexes on mesoporous materials. Zhang et al. [13,14]
reported a new method of axially immobilizing chiral Mn(III)
salen complexes through a phenyl sulfonic group onto the sur-
face of mesoporous supports, which led to an improved enan-
tiomeric excess (ee) over that of the free complex. Kureshy
et al. successfully immobilized chiral Mn(III) salen complexes
on pyridine N-oxide-modified mesoporous supports through
axial coordination [15] and on aminopropyl-modified meso-
porous supports through covalent grafting [16]. An effective
approach based on supported ionic liquid was reported by Lou
The asymmetric epoxidation of unfunctionalized olefins is
an important reaction for synthesizing a variety of valuable chi-
ral building blocks that can be easily transformed into other
useful chiral compounds through regioselective ring-opening
reactions [1], and thus is widely used in the synthesis of phar-
maceuticals and agrochemicals. Chiral Mn(III) salen complexes
have demonstrated activity and selectivity for the enantioselec-
tive epoxidation of unfunctionalized olefins under both homo-
geneous [2,3] and heterogeneous [4–11] conditions. However,
in general, the expensive catalysts are quite difficult to separate
and recycle in homogeneous catalytic systems. In recent years,
the heterogenization of chiral Mn(III) salen complexes within
inorganic matrixes has received much attention [12–24] due to
*
Corresponding author. Fax: +86 22 23509005.
E-mail address: sxliu@nankai.edu.cn (S. Liu).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.09.009

K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
313
et al. [17,18] for the immobilization of chiral Mn(III) salen
complexes on mesoporous materials, and excellent catalytic
performance was obtained for the immobilized catalysts. The
chiral Mn(III) salen complexes were immobilized on MCM-41
[19] and MCM-48 [20,21] through a multistep grafting method.
The ion-exchange method was reported to immobilize chiral
Mn(III) salen complexes on Al-MCM-41 [22–24].
as the carrier gas with the injection port temperature set at
523 K. The column temperature for styrene, α-methylstyrene,
and indene was programmed in the range of 333–423 K, and
for 1-phenylcyclohexene, the temperature was in the range of
353–423 K. For each of the epoxidation reactions, the con-
version and ee value were determined by averaging the values
from two parallel measurements, of which the results should
not differ by more than 0.5%, otherwise additional measure-
In most cases, the heterogenized chiral Mn(III) salen cata-
lysts led to lower activity and enantioselectivity compared with
the homogeneous counterparts for asymmetric epoxidation, and
only a few heterogeneous catalysts have exhibited higher ee
values than the homogeneous ones. The improvement of enan-
tioselectivity was mainly attributed to the confinement effect
of supports in these literatures [12–17,20], which was also pro-
posed for other heterogeneous asymmetric reactions by Thomas
et al. [25] and Caplan et al. [26]. However, the effect of pore size
on the catalytic performance of mesoporous material-supported
chiral Mn(III) salen complex has seldom been systemically in-
vestigated for the heterogeneous asymmetric epoxidation. In
this work, a series of mesoporous MCM-41 and MCM-48 ma-
terials with different pore sizes were synthesized and used as
supports to immobilize chiral Mn(III) salen complex. The as-
synthesized catalysts were used in the epoxidation of unfunc-
tionalized olefins with the combination of m-chloroperbenzoic
acid (m-CPBA) and N-methylmorpholine-N-oxide (NMO) as
the oxidant system, which could achieve excellent catalytic per-
formance rapidly. The effect of the nanopores of the supports
on activity and enantioselectivity of the heterogeneous catalysts
was studied.
1
ments on the same sample need to be done. H and 13C {1H}
NMR spectra were recorded at 300 and 75 MHz, respectively,
using a Varian Mercury Vx-300 spectrometer. Elemental analy-
sis was conducted with a Perkin–Elmer 240C analyzer. Mass
spectra were performed on a VG ZAB-HS (FAB) spectrometer.
The content of Mn in the heterogeneous catalysts was deter-
mined by inductively coupled plasma–atomic emission spec-
trometry (ICP-AES) on an ICP-9000 (N + M) spectrometer
(TJA Co.), after the samples were calcined and dissolved in
hydrogen fluoride. Powder X-ray diffraction (XRD) patterns
of the samples were obtained on a R/max-2500 diffractometer
with CuKα radiation (40 kV and 100 mA) in a scan range of
0.6◦ < 2θ < 80◦. FT-IR spectra were recorded on KBr pellets
in a BRUKER VECTOR 22 spectrometer in the region of 400–
4000 cm−1. Diffuse-reflectance UV–vis (DR UV–vis) spectra
were obtained on a Shimadzu UV-2550 UV–vis spectropho-
tometer in the range of 220–800 nm. N2 adsorption–desorption
analysis was carried out at 77 K on a Micromeritics TriStar
3000 apparatus. The analytical data were processed by the BET
equation for surface areas and by the BJH model for pore size
distributions.
2. Experimental
2.2. Synthesis of siliceous mesoporous materials
2.1. General
2.2.1. Synthesis of MCM-41 with different pore sizes
A series of highly ordered hexagonal siliceous MCM-41
with different pore sizes were synthesized according to the liter-
ature method [30,31]. The alkylammonium salts CnTAB (n =
12, 14, 16, 18) with different alkyl chain lengths and TEOS
were used as template and silica source, respectively. A gel
(molar) composition of 1TEOS/0.12CnTAB/8NH4OH/114H2O
was used. In a typical synthesis, the template of CnTAB was
dissolved in warm deionized water, and the ammonia was added
to this solution under vigorous stirring. The required quantity of
TEOS was then added dropwise into the solution. After 0.5 h of
stirring at room temperature, the resulting gel was allowed to
crystallize at 383 K for 52 h in a Teflon-lined autoclave. The
solid product was recovered by filtration, washed with deion-
ized water, dried at room temperature, and calcined at 823 K in
air for 6 h to remove the template. According to the differences
in the alkyl chain length of the template, the as-synthesized ma-
terials were marked as nMCM-41 (n = 12, 14, 16, 18).
Tetraethyl orthosilicate (TEOS; AR), alkyltrimethylammo-
nium bromide CnH2n+1(CH3)3–NBr (CnTAB for brevity,
n = 12, 14, 16, 18; AR), ammonia (25 wt%; AR), and sodium
hydroxide (NaOH; AR) were used to prepare siliceous meso-
porous materials. 1-Phenylcyclohexene, NMO and α-methyl-
styrene were provided by Aldrich. Indene was obtained from
Fluka. 3-Aminopropyltriethoxysilane and m-CPBA were pur-
chased from Acros Organics. Cis/trans-1,2-diaminocyclohex-
ane and 2-tert-butylphenol were supplied by Alfa Aesar. 3-tert-
Butyl-2-hydroxybenzaldehyde was synthesized from 2-tert-
butylphenol as described previously [27]. (1R, 2R)-(-)-1,2-
Diaminocyclohexane was resolved from the technical grade
isomers mixture in >99% enantiomeric excess by the reported
procedure [28]. All of the solvents used in the present study
were purified before use.
Racemic epoxides were synthesized by the epoxidation of
corresponding olefins with m-CPBA in CHCl3 at 273 K [29]
and detected by gas chromatography (GC). The conversions
(with toluene as internal standard) and ee values of epoxidation
reaction were determined by GC with a chiral β-cyclodextrin
capillary column (RESTEK RT-BetaDEXse, 30 m×0.25 mm×
0.25 µm), using a Rock GC7800 gas chromatograph equipped
with a flame ionization detector. Ultrapure nitrogen was used
2.2.2. Synthesis of MCM-48 with different pore sizes
The alkylammonium salts CnTAB (n = 12, 14, 16) with
different alkyl chain lengths were used as templates to syn-
thesize the siliceous MCM-48 with different pore sizes. When
CnTAB (n = 14, 16) was used as template, a typical synthe-
sis procedure was carried out as follows [30,31]. The template

314
K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
of CnTAB was dissolved in warm deionized water, and to this
solution the required quantity of TEOS and the aqueous so-
lution of NaOH were added orderly under vigorous stirring.
After stirring for another 2 h, a gel with a molar composition of
1TEOS/0.46CnTAB/0.41NaOH/52.95H2O was obtained. The
resulting gel was allowed to crystallize at 383 K for 72 h in
a Teflon-lined autoclave. The solid product was recovered by
filtration, washed with deionized water, and dried in air at room
temperature overnight. The template was removed by calcina-
tion at 823 K in air for 6 h. According to the difference of the
chain length of the template, the as-synthesized materials were
marked as nMCM-48 (n = 14, 16).
NMR (75 MHz in CDCl3): δ (ppm) 24.5, 29.6, 33.3, 35.0, 65.6,
72.6, 72.9, 118.4, 127.6, 129.7, 137.4, 160.2, 165.7.
Ligand C (2.7 g, 5 mmol) and Mn(OAc)2·4H2O (3.7 g,
15 mmol) were dissolved in absolute ethanol (100 ml), and
the resulting mixture was refluxed for 0.5 h. Then LiCl (0.6 g,
15 mmol) was added, and the mixture was refluxed for an ad-
ditional 2.5 h under exposure to air. After the reaction mix-
ture was cooled to room temperature, the solvent was com-
pletely removed under vacuum. The residue was dissolved in
dichloromethane (100 ml) and washed with deionized water
(3×25 ml) and brine (2×20 ml). After drying over MgSO4, the
solution was evaporated under vacuum, and the residue was re-
crystallized from dichloromethane-hexane to give 1 as a brown
solid. Elemental analysis calcd (%) for C30H38N2O2MnCl3: C,
58.12; H, 6.18; N, 4.52; found: C, 58.42; H, 6.22; N, 4.50;
FAB-MS m/z: calcd (C30H38N2O2MnCl3) 618.1, found 618.0;
FT-IR (KBr): 3009, 2947, 2865, 1610, 1543, 1438, 1388, 1343,
1312, 1266, 1235, 1204, 1166, 1090, 829, 782, 564 cm−1; DR
UV–vis: 286, 325, 438, 510 nm.
When CnTAB (n = 12) was used as template, the synthesis
procedure was carried out as follows. The template of C12TAB
was dissolved in warm deionized water, and the NaOH aque-
ous solution was added to this solution under stirring. Then the
required quantity of TEOS was added dropwise into the solu-
tion. After stirring for 0.5 h at room temperature, a gel with a
molar composition of 1TEOS/1.10CnTAB/0.46NaOH/112H2O
was obtained. The gel was then transferred into a Teflon-lined
autoclave for crystallization at 393 K for 168 h. The solid prod-
uct was recovered by filtration, washed with deionized water,
dried at room temperature, and calcined at 823 K in air for 6 h to
remove the template. The as-synthesized material was marked
as nMCM-48 (n = 12).
2.4. Heterogenization of chiral Mn(III) salen complex 1
The synthesis process is shown as Scheme 1. A suspension
of 3-aminopropyltriethoxysilane (0.6 ml) and calcined siliceous
mesoporous support (1.0 g) in 100 ml of toluene was stirred un-
der refluxing for 8 h. The resulting solid was filtered, washed
thoroughly with ethanol and diethyl ether, and then dried at
313 K under vacuum for 12 h. The dried 3-aminopropylsilyl-
functionalized material (1.0 g) was added to a solution of com-
plex 1 (0.12 g) in 50 ml of toluene. The mixture was refluxed
under stirring for 24 h. The brown solid products were collected
by filtration, washed with diethyl ether, and Soxhlet-extracted
with dichloromethane for 24 h. According to the difference
of the supports, the heterogeneous catalysts were marked as
nM48-1 (n = 12, 14, 16) and nM41-1 (n = 12, 14, 16, 18). The
content of aminopropyl groups in their modified supports was
ca. 2.0 mmol/g as determined by elemental analysis, and the
loading of complex 1 in the heterogeneous catalysts was in the
range of 0.11–0.13 mmol/g based on Mn element analysis by
ICP-AES.
2.3. Synthesis of the homogeneous chiral Mn(III) salen
complex 1
The chiral Mn(III) salen complex 1 was obtained through
the synthesis sequence given in Scheme 1. A mixture of 3-
tert-butyl-2-hydroxybenzaldehyde (A) (2.7 g, 15.2 mmol),
paraformaldehyde (1.0 g, 33.3 mmol), and tetrabutylammo-
nium bromide (0.47 g, 1.46 mmol) in 11 ml of concentrated hy-
drochloric acid was stirred vigorously at 313 K for 3 days [32].
The reaction mixture was repeatedly extracted with diethyl
ether (3 × 15 ml), and the organic phase was washed with 5%
NaHCO3 (2 × 10 ml) and brine (2 × 10 ml), then dried over
MgSO4. Evaporation of the solvent under vacuum afforded
3-tert-butyl-5-chloromethyl-2-hydroxybenzaldehyde (B) as a
1
yellow crystalline solid (3.4 g, 99% yield). H NMR (CDCl3,
300 MHz): δ (ppm) 1.43 (s, 9H), 4.59 (s, 2H), 7.44 (d, 1H),
7.53 (d, 1H), 9.87 (s, 1H), 11.87 (s, 1H).
2.5. Asymmetric epoxidation of unfunctionalized olefins
The monotartarate salt of (1R, 2R)-(-)-1,2-diaminocyclo-
hexane (1.6 g, 6 mmol) and compound B (2.7 g, 12 mmol) were
refluxed for 3 h with K2CO3 (1.7 g, 12.3 mmol) in absolute
ethanol (45 ml) under stirring. The reaction mixture was cooled
to room temperature, and 12.5 ml of deionized water was added.
The mixture was kept at 273–277 K for 12 h, after which the
yellow precipitate was collected by filtration. The crude solid
was redissolved in dichloromethane (50 ml) and washed with
deionized water (2×15 ml) and brine (10 ml). After drying over
MgSO4, the solvent was removed under vacuum, after which
the chiral ligand C was isolated as a yellow creamy solid (2.7
To quantitatively compare the catalytic performance, the
amount of heterogeneous catalysts was normalized based on the
same amount of Mn(III) salen complex. Enantioselective epox-
idation reactions were carried out using catalysts 1, nM48-1
and nM41-1 (0.02 mmol, 2 mol%, based on Mn element) with
styrene, α-methylstyrene, indene, and 1-phenylcyclohexene as
substrates (1 mmol) and m-CPBA (2 mmol) as an oxidant in
10 ml of dichloromethane containing toluene (40 µl) as an in-
ternal standard and NMO (5 mmol) as an axial base at 273 K.
Once the reaction was complete, the supported catalysts were
separated by filtration, and the filtrate was washed with 1 N
NaOH (10 ml) and brine (10 ml), then dried over MgSO4. The
conversions and ee values were determined by GC, with toluene
as an internal standard. The filtrate was detected by ICP-AES,
1
g, 85% yield). H NMR (CDCl3, 300 MHz): δ (ppm) 1.40 (s,
18H), 1.45–1.99 (m, 8H), 3.30–3.33 (m, 2H), 4.32 (s, 4H), 6.97
(d, 2H), 7.20 (d, 2H), 8.27 (s, 2H), 13.87 (bs, 2H); 13C {1H}

K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
315
Scheme 1. The synthesis of chiral Mn(III) salen complex 1 and heterogeneous catalysts nM41-1 and nM48-1.
and only a slight Mn leaching (<1.5%) was found. Moreover,
it showed no further increase in the conversion while fresh re-
actant was being added to the filtrate.
thesized nMCM-41 and nMCM-48 materials have uniform pore
sizes and that their pore sizes regularly decrease with decreas-
ing alkyl chain length of the templates.
3. Results and discussion
3.2. Characterization of the heterogeneous catalysts
3.1. Characterization of the supports
3.2.1. Powder X-ray diffraction
The powder XRD patterns of the calcined supports
(16MCM-41 and 16MCM-48), the 3-aminopropyltriethoxysil-
ane-modified supports (AM-16MCM-41 and AM-16MCM-
48), and the heterogeneous catalysts (16M41-1 and 16M48-
1) are shown in Fig. 3. Although a slight decrease in the
relative intensities of the XRD reflections can be seen, the
well-resolved reflections (110) and (200) for AM-16MCM-
41 and reflection (220) for AM-16MCM-48 indicate that the
mesoporous structure of the parent supports remained intact
on modification with 3-aminopropyltriethoxysilane. After the
immobilization of chiral Mn(III) salen complex 1, the in-
tensities of all peaks decreased. However, the XRD pattern
of immobilized catalyst showed a similar periodic structure
to that of the corresponding support. These findings demon-
strate that 3-aminopropyltriethoxysilane modification and chi-
ral Mn(III) salen complex 1 immobilization had little ef-
fect on the long-range mesoporous ordering of parent sup-
ports.
The synthesized mesoporous materials nMCM-41 and
nMCM-48 were characterized by XRD and N2 sorption. Fig. 1
shows the powder XRD patterns of calcined nMCM-41 and
nMCM-48. The mesoporous materials nMCM-41 exhibited
three XRD peaks corresponding to (100), (110), and (200) re-
flections, which can be indexed to a two-dimensional hexagonal
lattice (Fig. 1A). The XRD patterns of nMCM-48 (Fig. 1B)
showed a very intense peak assigned to reflection at (211) along
with a shoulder peak at reflection (220) and the sextet patterns
observed in the region of 3◦–6◦ that could be attributed to re-
flections (321), (400), (420), (332), (422), and (431), which are
typical for cubic cells. Moreover, with the decreased alkyl chain
length of the templates, the XRD peak at (100) in Fig. 1A and
the XRD peak at (211) in Fig. 1B were shifted to higher 2θ
values.
Fig. 2 shows the pore size distributions of calcined nMCM-
41 and nMCM-48 materials. It can be seen that all of the syn-

316
K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
(A)
(A)
(B)
Fig. 2. (A) Pore diameter distributions of calcined nMCM-41, (a) n = 12,
(b) n = 14, (c) n = 16, (d) n = 18. (B) Pore diameter distributions of calcined
nMCM-48, (a) n = 12, (b) n = 14, (c) n = 16.
(B)
IR spectra confirmed the successful modification of the meso-
porous support with 3-aminopropyltriethoxysilane and immo-
bilization of chiral Mn(III) salen complex 1.
Fig. 1. (A) Powder XRD patterns of calcined nMCM-41, (a) n = 12, (b) n = 14,
(c) n = 16, (d) n = 18. (B) Powder XRD patterns of calcined nMCM-48,
(a) n = 12, (b) n = 14, (c) n = 16.
3.2.3. UV–vis spectroscopy
The diffuse-reflectance UV–vis spectra of chiral Mn(III)
salen complex 1, the mesoporous supports (18MCM-41 and
14MCM-48), and the heterogeneous catalysts (18M41-1 and
14M48-1) are shown in Fig. 5. The spectra of heterogeneous
catalysts 18M41-1 and 14M48-1 showed features similar to
those of complex 1, with no obvious absorption in the spec-
tra of 14MCM-48 and 18MCM-41. In the spectrum of chiral
Mn(III) salen complex 1, the bands at 286 and 325 nm could
be attributed to the charge transfer transition of salen ligand,
the band at 438 nm was due to ligand-to-metal charge transfer
transition, and the band at 510 nm may be assigned to the d–d
transition of Mn(III) salen complex. After immobilization of
Mn(III) salen complex on mesoporous materials, all of the char-
acteristic bands appeared in the spectra. But a blue shift from
286 and 438 nm to 260 and 408 nm, respectively, indicated the
presence of interaction between Mn(III) salen complex and the
support. The DR UV–vis spectra further confirmed the immo-
bilization of chiral Mn(III) salen complex 1 on the supports.
3.2.2. FT-IR spectroscopy
Fig. 4 shows the representative FT-IR spectra in the scan
range of 1300–3100 cm−1 for the calcined support 16MCM-
48, the 3-aminopropyltriethoxysilane-modified support AM-
16MCM-48, and the heterogeneous catalyst 16M48-1. The in-
tensities of the bands near 3000 cm−1, which were due to C–H
stretching vibrations of alkyl groups, increased with the modifi-
cation of 3-aminopropyltriethoxysilane and the immobilization
of chiral Mn(III) salen complex 1. In the spectrum of AM-
16MCM-48, the IR bands at 1560 and 1470 cm−1 could be
assigned to N–H deformation vibrations of amido groups and
scissor bending vibrations of the –CH2–CH2–CH2–groups, re-
spectively. Moreover, the bands at 1420 and 1389 cm−1 were
attributed to C–H deformation vibrations of alkyl groups. On
immobilization of Mn(III) salen complex 1, two new character-
istic IR bands appeared at 1540 and 1450 cm−1, which were due
to the stretching vibrations of azomethine groups (H–C=N) and
the deformation vibrations of C–H bond, respectively. The FT-

K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
317
(A)
Fig. 5. Diffuse-reflectance UV–vis spectra of (a) Mn(III) salen complex 1,
(b) 14M48-1, (c) 18M41-1, (d) calcined 14MCM-48, (e) calcined 18MCM-41.
Table 1
The structure parameters of parent supports and heterogeneous catalysts
Sample
S
Pore volume
Pore size
(Å)
BET
2
−1
3
−1
(m g
)
(cm g
)
18MCM-41
18M41-1
16MCM-41
16M41-1
14MCM-41
14M41-1
12MCM-41
12M41-1
16MCM-48
16M48-1
14MCM-48
14M48-1
12MCM-48
12M48-1
1112
577
941
556
841
477
873
691
1200
991
1201
667
1257
546
1.11
0.45
0.82
0.39
0.71
0.29
0.59
0.32
1.11
0.56
0.80
0.29
0.82
0.25
31.7
21.2
27.9
18.8
21.9
18.6
18.9
17.1
26.0
19.9
23.2
19.0
18.6
17.0
(B)
Fig. 3. (A) Powder XRD patterns of (a) calcined 16MCM-41, (b) AM-16MCM-
41, (c) 16M41-1. (B) Powder XRD patterns of (a) calcined 16MCM-48,
(b) AM-16MCM-48, (c) 16M48-1.
3.2.4. Nitrogen sorption
All of the parent supports and the heterogeneous catalysts
were characterized by N2 sorption. The corresponding textural
parameters calculated by N2 adsorption–desorption isotherms
are presented in Table 1. Compared with parent supports, a large
decrease in BET surface area, pore volume, and pore size was
observed on immobilization of chiral Mn(III) salen complex
1 onto mesoporous materials. This evidences that the chiral
Mn(III) salen complexes are located mainly on the inner sur-
faces of the supports. Fig. 6 shows the low-temperature N2
adsorption–desorption isotherms and the pore size distributions
of the heterogeneous catalysts 18M41-1 and 16M48-1. It can
be seen that the immobilized catalysts maintained characteristic
type IV isotherms [33] and uniform pore size in the mesopore
ranges.
3.3. Heterogeneous asymmetric epoxidation
The heterogeneous catalysts were evaluated in the epoxida-
tion of styrene, α-methylstyrene, indene, and 1-phenylcyclo-
hexene in CH2Cl2 at 273 K for 8 h using m-CPBA/NMO as
Fig. 4. FT-IR spectra of (a) calcined 16MCM-48, (b) AM-16MCM-48, (c)
16M48-1.

318
K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
Table 2
Asymmetric epoxidation of styrene and α-methylstyrene catalyzed by homo-
a
geneous complex 1 and heterogeneous catalysts
Entry
Catalyst
Substrate
Time
(h)
Conversion
Ee
(%)
c
b
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
Styrene
2
8
8
8
8
8
8
8
2
8
8
8
8
8
8
8
>99
>99
97.2
79.6
72.2
>99
>99
47.6
>99
>99
95.7
80.1
67.7
>99
>99
29.0
49.0 (R)
49.0 (R)
48.0 (R)
36.0 (R)
28.8 (R)
47.7 (R)
49.0 (R)
15.3 (R)
41.8 (R)
39.5 (R)
38.5 (R)
32.9 (R)
29.4 (R)
37.2 (R)
40.4 (R)
25.1 (R)
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
1
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
α-Methylstyrene
(A)
a
Reactions were performed in CH Cl (10 ml) with catalyst (0.02 mmol),
2
2
substrate (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K.
b
Conversion (%) determined by GC with chiral column using toluene as in-
ternal standard.
c
Ee (%) determined by GC with RESTEK RT-BetaDEXse chiral column.
styrene and α-methylstyrene epoxidation (entries 6, 7, 14, and
15). This may be due to the confinement effect of the nanopores
of 14MCM-48, which was more compatible with the molecular
size of styrene and α-methylstyrene.
As shown in Table 2, the heterogeneous catalysts 14M41-1,
12M41-1, and 12M48-1 demonstrated lower activity and enan-
tioselectivity (entries 4, 5, 8, 12, 13, and 16) compared with
the homogeneous catalyst 1 and the other four kinds of het-
erogeneous catalysts, due mainly to the small pore sizes of the
corresponding supports. The results of N2 sorption (Table 1)
indicate that the BET surface area, pore volume, and pore di-
ameter were obviously decreased on immobilization of chiral
Mn(III) salen complex 1 onto the parent supports 14MCM-41,
12MCM-41, and 12MCM-48, indicating that the chiral Mn(III)
salen complex 1 was present inside the channels of these sup-
ports. However, the blockage of pore channels by the Mn(III)
salen complex could occur. Moreover, the relatively small pore
size of the obtained catalysts would increase the diffusional
resistance of reactants to the active sites located on the in-
ner surfaces of the supports. Thus, these catalysts exhibited
decreased performance. In particular, the lowest catalytic ac-
tivities and enantionselectivities (e.g., 47.6% conversion and
15.3% ee value for styrene epoxidation) were provided by the
catalyst immobilized on 12MCM-48, which had the smallest
pore diameter among the parent supports.
(B)
Fig. 6. N adsorption–desorption isotherms and pore diameter distribution pro-
2
files (insert) of (A) 18M41-1, (B) 16M48-1.
the oxidant. The blank experiments showed that the asymmet-
ric epoxidation hardly occurred in the absence of chiral Mn(III)
salen complex. For comparison, the chiral Mn(III) salen cata-
lyst 1 was also investigated under the same reaction conditions,
but shortening the reaction time to 2 h.
Table 2 gives the results of the asymmetric epoxidation
of styrene and α-methylstyrene catalyzed by homogeneous
and heterogeneous catalysts. The catalysts 18M41-1, 16M41-
1, 16M48-1, and 14M48-1 were active and enantioselective
for the epoxidation of styrene and α-methylstyrene, and con-
versions and ee values comparable to those of homogeneous
catalyst 1 were obtained (entries 2, 3, 6, 7, 10, 11, 14, and 15),
which was attributed mainly to the large pore sizes of the corre-
sponding supports. The size of the Mn(III) salen complex 1 was
estimated as 2.0 × 1.6 nm according to the literature [14,34].
The mesoporous materials 18MCM-41, 16MCM-41, 16MCM-
48, and 14MCM-48 had pore sizes >2.3 nm, sufficiently large
for the immobilization of the chiral Mn(III) salen complex 1
into the channels of supports. The catalyst 18M41-1 showed
higher conversions and ee values for the epoxidation of styrene
and α-methylstyrene compared with those of 16M41-1 (entries
2, 3, 10, and 11). However, compared with 16M48-1, catalyst
14M48-1 exhibited lower conversions and higher ee values for
In summary, the catalytic performance of heterogeneous cat-
alysts was closely correlated with the pore sizes of the parent
supports for the epoxidation of styrene and α-methylstyrene.
For the catalysts nM41-1, the activity and enantioselectivity
were improved with the increasing pore sizes of the parent sup-
ports. The catalyst 18M41-1 exhibited the best catalytic perfor-
mance (e.g., >99% conversion and 49.0% ee value for styrene
epoxidation). For the catalysts nM48-1, with an increase in pore

K. Yu et al. / Journal of Catalysis 252 (2007) 312–320
319
Table 3
4. Conclusion
Asymmetric epoxidation of indene and 1-phenylcyclohexene catalyzed by ho-
mogeneous complex 1 and heterogeneous catalysts
a
The chiral Mn(III) salen complex was immobilized onto
a series of mesoporous materials with different pore sizes.
The as-synthesized catalysts were active and enantioselective
for the epoxidation of styrene, α-methylstyrene, indene, and
1-phenylcyclohexene. The effects of pore size on the activ-
ity and enantioselectivity of the heterogeneous catalysts were
studied. For the catalysts immobilized on nMCM-41 materi-
als, the catalytic activity and enantioselectivity were improved
with the increase of pore sizes of parent supports. The hetero-
geneous catalyst 18M41-1 gave the highest conversions and ee
values for the epoxidation of olefins. And for the nMCM-48
material-supported catalysts, the large pore size was benefi-
cial to obtain higher catalytic activity, and the compatible pore
size may have been responsible for the increased enantioselec-
tivity in the olefin epoxidation. The catalyst 14M48-1 exhib-
ited the best enantioselectivity for the epoxidation of styrene,
α-methylstyrene, and indene. In particular, a higher ee value
than the homogeneous ee (93.3 vs 91.0%) was achieved for
indene epoxidation. However, the highest ee value for the epox-
idation of 1-phenylcyclohexene was provided by the catalyst
16M48-1.
Entry Catalyst
Substrate
Time Conversion
b
Ee
(%)
c
(h)
(%)
1
2
3
4
5
6
7
8
9
1
Indene
2
8
8
8
8
8
8
8
2
99.0
91.0 (1R, 2S)
87.8 (1R, 2S)
87.0 (1R, 2S)
73.3 (1R, 2S)
62.5 (1R, 2S)
89.1 (1R, 2S)
93.3 (1R, 2S)
50.4 (1R, 2S)
83.9 (1S, 2S)
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
1
96.9
96.7
78.8
77.3
97.5
97.3
52.4
1-Phenylcyclo-
hexene
>99
10
11
12
13
14
15
16
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
8
8
8
8
8
8
8
93.2
79.8
37.6
23.2
98.9
73.8
23.0
78.7 (1S, 2S)
60.7 (1S, 2S)
38.3 (1S, 2S)
34.6 (1S, 2S)
79.6 (1S, 2S)
70.6 (1S, 2S)
16.0 (1S, 2S)
a
Reactions were performed in CH Cl (10 ml) with catalyst (0.02 mmol),
2
2
substrate (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K.
b
Conversion (%) determined by GC with chiral column using toluene as in-
ternal standard.
c
Ee (%) determined by GC with RESTEK RT-BetaDEXse chiral column.
Acknowledgments
size, the conversions increased from 47.6 to >99% for styrene
epoxidation and from 29.0 to >99% for α-methylstyrene epox-
idation. However, the compatible pore size may be responsible
for the improved ee values, and the catalyst 14M48-1 exhib-
ited the highest ee value for the epoxidation of styrene and
α-methylstyrene.
This work was supported by the National Natural Science
Foundation of China (grants 20773069 and 29973016) and
the National Key Technologies R&D Program of China (grant
2006BAC02A12).
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Table 3 gives the results of the asymmetric epoxidation of in-
dene and 1-phenylcyclohexene catalyzed by homogeneous and
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the size of 1-phenylcyclohexene and exhibited the best enan-
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