Structure-control in assembly of nanosheets: Novel chiral catalysts for the asymmetric epoxidation of α-methylstyrene without axial base

Article abstract of DOI:10.1016/j.catcom.2012.08.028

Heterogeneous chiral catalysts with different structure, surface character and location of active species were assembled by salen Mn(III) complex and α-Zr(HPO₄)₂ nanosheets. The bumpy edge-accumulated surfaces of nanosheets lead the

Full text of DOI:10.1016/j.catcom.2012.08.028

Catalysis Communications 28 (2012) 111–115
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Structure-control in assembly of nanosheets: Novel chiral catalysts for the
asymmetric epoxidation of α-methylstyrene without axial base
a
a
a
a
a
b
Chenhui Hu , Lihong Zhang , Junfeng Zhang , Liyuan Cheng , Zheng Zhai , Meng Li ,
b
a,
Jing Chen , Wenhua Hou ⁎
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
a
b
Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2012
Received in revised form 20 August 2012
Accepted 25 August 2012
Available online 30 August 2012
Heterogeneous chiral catalysts with different structure, surface character and location of active species were
assembled by salen Mn(III) complex and α-Zr(HPO ) nanosheets. The bumpy edge-accumulated surfaces of
4 2
nanosheets lead the catalyst to have a high ee value without axial base in the asymmetric epoxidation of
α-methylstyrene.
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Chiral salen Mn(III) complex
α-Zr(HPO
4 2
)
Nanosheets
α-Methylstyrene
Epoxidation
1
. Introduction
disorderly piled-up structure in which the basal plane is relatively large
and smooth (Fig. 1C). These two different surfaces could provide differ-
ent environments for guest molecules.
The heterogenization of chiral salen Mn(III) complexes (CSM) has
attracted particular attention and many types of salen Mn(III) catalysts
have been designed and supported on various carriers [1]. Nevertheless,
the resulted heterogeneous catalysts have often shown a decrease in
activity and enantioselectivity. Recently, the ionic salen Mn(III) com-
plexes provide a new way for their heterogenization on inorganic
carriers via a simple ion exchange reaction. Layered inorganic materials
such as anionic clays and cationic Zn–Al layered double hydroxides
were used to incorporate salen complexes [2,3]. Unfortunately, these
layered inorganic materials all suffer a relatively lower ion exchange ca-
In the work, α-ZrP nanosheets (ZrPNS) were obtained by exfoliating
α-ZrP with tetrabutylammonium hydroxide (TBAOH) [6b], and could
be further edge-modified with α-naphtyl phosphonic acid (α-NPA),
resulting in another version of ZrPNS noted as ZrPNS-edge (Fig. 1D).
The cationic CSM, namely Jacobsen's catalyst [7], was prepared and
heterogenized with ZrPNS and ZrPNS-edge respectively through self-
assembly. Two heterogeneous chiral catalysts with different structure,
surface character and location of CSM were prepared by controlling
the loading amount of CSM. The resulted heterogeneous chiral catalysts
were evaluated in the asymmetric epoxidation of α-methylstyrene. A
detailed characterization and discussion about the different catalytic
behaviors was provided.
4 2 2
pacity. By comparison, layered α-Zr(HPO ) ·H O (α-ZrP), which is fa-
mous for its huge ion-exchange capacity (600 mmol/100 g), ease of
dimensional and surface functionality control, and facile process of in-
tercalation/exfoliation, has been proved to be an extremely efficient
support [4].
2. Experimental
Nanosheets, a novel class of advanced nanomaterials which can be
obtained by the exfoliation of layered compounds, exhibit extraordi-
nary physicochemical properties and are often used as modules of var-
ious nanomaterials [5]. The resulted nanosheets have two large basal
planes (~μm) and thin edges (~1 nm) that may result in two distinct
atomic environments (Fig. 1A) [6]. Besides, the exfoliated nanosheets
can be restacked in two different modes. One is orderly-stacked layered
structure along z axis with bumpy surface (Fig. 1B). The other is a rather
2.1. Catalyst preparation
α-ZrP was prepared by HF approach, as described by Xu et al. [8].
The uniform and stable translucent colloidal suspension of α-ZrP
−
3
nanosheets (ZrPNS) was prepared by first adding 4.96×10
mol of
−
3
TBAOH to 6.64×10
mol of α-ZrP in 1000 mL of deionized water
and then the resulted solution was stirred for 72 h at ambient tem-
perature [6b], followed by centrifugation at 9000 rpm.
Another version of nanosheets was prepared as follows:
⁎
Corresponding author. Tel.: +86 25 83686001; fax: +86 25 83317761.
E-mail address: whou@nju.edu.cn (W. Hou).
−
4
−4
8.4×10
mol of TBAOH was first added into 4.64×10
mol of
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.08.028

112
C. Hu et al. / Catalysis Communications 28 (2012) 111–115
Fig. 1. The schematic structure of (A) a single α-ZrP nanosheet, (B) bumpy edge-accumulated surface of orderly-restacked nanosheets along z axis, (C) smooth basal plane in disorderly
piled-up structure, (D) the edge modification of α-ZrP nanosheet with α-naphthyl group.
α-ZrP in 100 mL of deionized water at room temperature and then
After reaction, the heterogeneous catalysts were separated by cen-
trifugation, and the filtrate was washed with 5 mL of 1 mol/L NaOH
−
4
4
.8×10
mol of α-NPA was added and stirred for 2 h to allow the
exchange of phosphates. After that, the suspended colloid was
restacked with 0.1 mol/L HCl, washed with deionized water and
dried at 333 K. The resulted solid was exfoliated with TBAOH again,
followed by centrifugation at 9000 rpm to obtain the colloidal sus-
pension of ZrPNS-edge.
2 4
and 5 mL of brine, and then dried over anhydrous Na SO . For the ho-
mogeneous catalyst, the solvent was removed and the product was
separated and collected by a short column of silica gel. Conversions
and ee values were determined by GC using the internal standard
method with a chiral β-cyclodextrin capillary column (RESTEK RT-
BetaDEXse, 30 m×0.25 mm×0.25 μm).
Two catalysts were prepared by using two versions of nanosheets
and changing the loading amount of CSM. Concretely, ZrPNS (2 mg/mL)
−
6
was combined with 8.92×10
mol of CSM to obtain catalyst noted as
3. Results and discussion
−
5
CSM/ZrPNS. ZrPNS-edge (1.7 mg/mL) was mixed with 2.52×10
mol
of CSM and the resulted catalyst was noted as CSM/ZrPNS-edge. (Figure
S1 in Supporting Information).
Because α-ZrP can be easily hydrolyzed with TBAOH at edges, an ad-
dition of the appropriate amount of α-naphthyl phosphonic acid
(α-NPA) to the suspension of ZrPNS enables the edge-modification of
nanosheets with organic groups. XRD, IR, UV, SEM and elemental anal-
yses results confirmed that the organic groups were successfully an-
chored at the edges of ZrPNS (Figure S2 in Supporting Information).
The XRD patterns of the resulted catalysts are shown in Fig. 2
2
.2. Catalyst characterization
XRD patterns were obtained on a Philips X'Pert X-ray diffractometer
with monochromatized Cu Kα radiation. FT-IR spectra were recorded on
a Nicolet 6700 spectrometer with a resolution of 2 cm and 64 scans in
the range of 400–4000 cm . The size distribution and morphology
were analyzed by transmission electron microscopy (TEM JEOL JEM-
(
inset: photos of catalysts). The as-prepared α-ZrP exhibits all charac-
−
1
teristic reflections which are well matched with literature [6a], indi-
cating a highly ordered lamellar structure. By comparison, CSM/
ZrPNS exhibits a new diffraction peak at 2θ=6.4° (d002 =13.8 Å)
−
1
2
00CX, operated at an accelerating voltage of 200 kV) and scanning
electron microscopy (SEM JEOL JEM-6300F). The BET surface area was
measured at 77 K by using a Micromeritics ASAP 3020 volumetric ad-
sorption analyzer and the pore-size distributions were obtained using
the BJH model. The manganese content in catalysts was determined by
atomic absorption spectroscopy (AAS) using a Varia AA240Duo spec-
trometer. Elemental analyses of C, H, and N were carried out on a
Vario EL III elemental analyzer. UV–vis reflectance spectroscopic mea-
surements were performed on a Shimadzu UV-3600 spectrophotometer
4
by dispersing sample on BaSO .
2
.3. Catalyst test
Enantioselective epoxidation reactions were performed in 5 mL
of CH Cl at 273 K using heterogeneous and homogeneous cata-
2
2
lysts (0.01 mmol, based on Mn element) with α-methylstyrene
as substrate (0.5 mmol), toluene (40 μL) as the internal standard,
N-methylmorpholine-N-oxide (NMO) as an axial base, and
m-chloroperoxybenzoic acid (m-CPBA, 1 mmol, added in five equal
portions) as an oxidant.
Fig. 2. XRD patterns of (A) α-ZrP, (B) CSM/ZrPNS, and (C) CSM/ZrPNS-edge.

C. Hu et al. / Catalysis Communications 28 (2012) 111–115
113
and a large decrease of peak intensity. Considering the size of salen
Table 1
Some physico-chemical properties of α-ZrP and two heterogeneous chiral catalysts.
Mn(III) (20.5 Å×16.1 Å) [9], the thickness of α-ZrP layer (ca. 10 Å)
+
and the diameter of TBA ions (ca.10 Å) [6a], it can be suggested
Sample
Mn loading
mmol/g
S
Pore volume Pore size Microanalysis
BET
2
+
3
that the exchange of cationic CSM with TBA ions mainly occurs at
m /g cm /g
nm
(%)
the edges of ZrPNS where there is a relatively higher electron density
C
H
/
N
/
2
−
+
due to the partial hydrolysis of HPO
are resided between nanosheets through a single-layer arrangement
6b]. For CSM/ZrPNS-edge, the characteristic (002) diffraction peak
4
, and the remained TBA ions
α-ZrP
CSM/ZrPNS
CSM/ZrPNS-edge 1.25
/
3.5 0.015
20.5 0.043
91.9 0.145
/
3.4
3.6
/
9.0
0.30
4.0 0.5
[
11.6 4.4 0.2
is no longer observed. Since the edges of nanosheets are modified
by naphthyl groups, CSM can only be loaded on the basal planes of
+
nanosheets to replace TBA ions therein, preventing nanosheets
CSM is mainly loaded on the basal planes of ZrPNS-edge, shows a
much lower ee value of 39 % in 9 h (Entry 5), although it has the much
higher content of Mn, surface area and pore volume (Table 1). These re-
sults suggest that the catalytic performance is closely related with the
intrinsic structure of catalyst rather than the loading amount of CSM
and the unique surface character of α-ZrP nanosheets is essential to
the ee value. The thin and bumpy edge-accumulated surface plays a dif-
ferent role from large and smooth basal plane.
With the addition of NMO, both catalysts afford a surprisingly low
conversion, ee value and TOF, especially a greatly decreased ee value
for CSM/ZrPNS. It indicates that the costly NMO is unfavorable to the
enhancement of catalytic performance, especially enantioselectivity.
Furthermore, to exclude the possibility that the catalytic perfor-
mances were caused by different fractions of catalyst species leached
+
from reassembling with TBA ions and resulting in an irregularly
stacking of ZrPNS-edge.
As can be seen in Fig. 3, individual nanosheets with smooth basal
planes can be obtained after exfoliation. In addition, ZrPNS-edge
shows a morphology with bumpy edges while ZrPNS flat edges. The
resulted catalysts exhibit different morphologies due to the location
of CSM and the restacking ways of nanosheets. CSM/ZrPNS exhibits
a relatively more ordered layered structure while CSM/ZrPNS-edge
presents a rather disordered piled-up structure, being consistent
with XRD results.
Furthermore, IR results also confirm the successful loading of CSM
on ZrPNS and ZrPNS-edge, respectively (Figures S3). The contents of
Mn in two catalysts are determined as 0.30 and 1.25 mmol/g, respec-
tively. Both two heterogeneous catalysts exhibit a characteristic type
2 2
to the solution, the solvent CH Cl was changed to hexane in which
IV N
2
adsorption-desorption isotherm and a uniform pore diameter
CSM is insoluble and the ee value was found to be increased from
78 % to 87 %.
(
~3.5 nm) (Figure S4). The resulted mesoporous structure can be at-
tributed to the irregular and loosely restacking of nanosheets. Never-
theless, due to the highly disordered arrangement of nanosheets,
CSM/ZrPNS-edge has a much higher surface area and pore volume
than CSM/ZrPNS (Table 1). The above catalyst characterizations con-
firm that CSM is successfully immobilized on α-ZrP nanosheets and
the two heterogeneous asymmetric catalysts have different structure,
surface character and location of CSM.
Table 2 lists the catalytic performance of two chiral catalysts.
According to the previous literature, the same total amount of active
species (~2 mol% of olefins, based on the Mn content) was used for ho-
mogeneous and heterogeneous catalysis to exclude the possibility that
the different catalytic performances were caused by the various densi-
ties of active species on the support. Interestingly, without NMO, CSM/
ZrPNS in which CSM is mainly loaded at the edge of ZrPNS, shows the
highest ee value of 78 % at a conversion of 100 % in 6 h (Entry 2). Even
when the amount of catalyst is decreased 3 times, an ee value of 31 %
is still reached (Entry 3). By comparion, CSM/ZrPNS-edge in which
It is generally believed that the ee value is mainly related to the at-
tack of substrate oriented to active intermediate salen Mn(V)=O for
asymmetric epoxidation of olefins. As shown in Fig. 4A, Jacobsen et al.
believed that there were four different approaches (a, b, c and d)
while Hosoya et al. considered that the two benzene rings of salen
Mn(V)=O were folded, one up and one down during the reaction,
and the substrate got close to salen Mn(V)=O through approach e
[10,11]. Recently, Zou et al. reported that the attack oriented might
be approach a and the stereo-effect of position 3 and 3′ also played
an important role [12]. In addition, it was stated that NMO could
bring the metal closer to the substrate, leading to the enhancement
of enantioselectivity [13].
Based on our results, it can be suggested that approach a is the opti-
mum attack oriented and the catalytic performance is mainly associated
with the surface character. Concretely, for CSM/ZrPNS, the active center
is more accessible to reactants since CSM is mainly exposed at the edges
of ZrPNS. Besides, the thin edges of nanosheets may act as the role of
NMO and the bumpy edge-accumulated surface will greatly restrict
other attack directions, leading to a high conversion, ee value and TOF
via approach a (Fig. 4B). When NMO is added, the surroundings of the
active center appear too crowded and approach a is greatly obstructed,
resulting in a great decrease in conversion, ee value and TOF.
Table 2
Asymmetric epoxidation of α-methylstyrene catalyzed by CSM and two chiral heteregeneous
a
catalysts .
Entry Catalyst
Add.
t (h) Con (%) ee (%)b TOF (10−3 s−1)c
1
2
3
4
5
6
7
CSM
NMO
–
1
6
6
6
9
9
9
100
100
100
21
100
22
50
78
31
13
39
23
15
13.89
2.31
6.95
0.49
1.55
0.34
1.02
CSM/ZrPNS
CSM/ZrPNSd
CSM/ZrPNS
–
NMO
–
CSM/ZrPNS-edge
CSM/ZrPNS-edge
CSM/ZrPNS-edge
NMO
NMO
e
100
a
Reaction conditions: α-methylstyrene (0.5 mmol), m-CPBA (1 mmol), catalyst (2 mol%,
based on Mn content), NMO (2.5 mmol), CH
2
Cl
2
(5 mL). Reaction temperature, 273 K.
b
R-configuration.
TOF=[product]/([catalyst]×time).
CSM/ZrPNS (0.67 mmol%, 10 mg).
c
d
Fig. 3. TEM images of ZrPNS (A) and ZrPNS-edge (B), and SEM mages of CSM/ZrPNS (C) and
CSM/ZrPNS-edge (D).
e
NMO (0.25 mmol).

114
C. Hu et al. / Catalysis Communications 28 (2012) 111–115
Fig. 4. (A) The attack-oriented for substrate to the active intermediate salen Mn(V)=O, (B) CSM on the thin edge. (C) CSM on the basal plane.
By comparison, for CSM/ZrPNS-edge, CSM is mainly loaded on the
provide a new idea for the design and fabrication of novel heteroge-
neous chiral catalysts with potential applications.
basal plane of α-ZrP nanosheets since the edges of nanosheets are
modified with organic groups. The large and smooth basal plane of
α-ZrP nanosheets could act as the role of NMO, but it may also restrict
all attack directions to some extent, resulting in a much lower ee
value and TOF (Fig. 4C).
After adding NMO, the active centers are greatly obstructed by
both the basal plane and NMO, leading to a decrease in conversion,
ee value and TOF. When the amount of NMO is decreased 10 times,
the conversion and TOF are greatly increased. Nevertheless, the ee
value is decreased (Entry 7). The reason may be that a small amount
of NMO is not enough to fundamently change the attack approach.
This system is also suitable to other substrates such as styrene.
Table S1 lists the preliminary recycling results of CSM/ZrPNS. When
the used catalyst was recycled by the post-treatment and re-used for
the second, third and fourth times, the corresponding ee values were
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (Grant Nos. 21073084 and 20773065), Natural Science
Foundation of Jiangsu Province (Grant No. BK2011438), and 973 Pro-
ject (Grant No. 2009CB623504). We aslo thank Prof. Limin Zheng for
providing α-naphthyl phosphonic acid.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.08.028.
3
1 %, 35 % and 33 %, respectively, being quite stable although much
lower than the original value of 78 %. Owing to the reversible assembly
and the exfoliation/flocculation properties of nanosheets, it is expected
that CSM and nanosheets in the used catalyst can be reassembled into
the original structure by controlling the molar ratio of TBAOH to
nanosheets, and thus the ee value might be recovered. The correspond-
ing work is currently under way in our lab.
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