Comparison of Ta–MCM-41 and Ti–MCM-41 as catalysts for the enantioselective epoxidation of styrene with TBHP

Article abstract of DOI:10.1016/j.crci.2017.02.008

Chiral Ti–MCM-41 and Ta–MCM-41 catalysts have been prepared by grafting of Ti(OⁱPr)₄ and Ta(OEt)₅ and the modification with R-(+)-diethyl L-tartrate or R-(+)-diisopropyl L-tartrate. In general, the solid catalysts are more active and selective than their homogeneous counterparts in the epoxidation of styrene with tert-butyl hydroperoxide. The enantioselectivities depend on both the nature of the chiral ligand and the calcination temperature of the support, as it is supposed this controls the type of surface species that are formed. The best result of 71% ee is obtained with DIPT–Ta–MCM₅₅₀ and is the first example of the use of a Ta catalyst for the enantioselective epoxidation of unfunctionalized alkenes. Nonetheless, the recovered Ta catalysts are less active and selective.

Full text of DOI:10.1016/j.crci.2017.02.008

C. R. Chimie xxx (2017) 1e6
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Full paper/M eꢀ moire
Comparison of TaeMCM-41 and TieMCM-41 as catalysts for
the enantioselective epoxidation of styrene with TBHP
Marwa Fadhli a, **, Ilyes Khedher , Jos eꢀ M. Fraile
a
b, *
a
Universit ꢀe de Tunis El-Manar, Facult ꢀe des sciences de Tunis, Laboratoire de chimie des mat ꢀe riaux et catalyse, 2092 Tunis, Tunisia
Instituto de Síntesis Química y Cat aꢀ lisis Homog ꢀe nea (ISQCH), Facultad de Ciencias, C.S.I.C.-Universidad de Zaragoza, 50009 Zaragoza,
b
Spain
a r t i c l e i n f o
a b s t r a c t
i
Article history:
Chiral TieMCM-41 and TaeMCM-41 catalysts have been prepared by grafting of Ti(O Pr)4
and Ta(OEt) -tartrate or R-(þ)-diisopropyl
and the modification with R-(þ)-diethyl
Received 19 January 2017
Accepted 27 February 2017
Available online xxxx
5
L
L-
tartrate. In general, the solid catalysts are more active and selective than their homoge-
neous counterparts in the epoxidation of styrene with tert-butyl hydroperoxide. The
enantioselectivities depend on both the nature of the chiral ligand and the calcination
temperature of the support, as it is supposed this controls the type of surface species that
are formed. The best result of 71% ee is obtained with DIPTeTaeMCM550 and is the first
example of the use of a Ta catalyst for the enantioselective epoxidation of unfunctionalized
alkenes. Nonetheless, the recovered Ta catalysts are less active and selective.
Keywords:
Asymmetric catalysis
Supported catalysts
Tantalum
Titanium
©
2017 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Mesoporous materials
Epoxidation
1. Introduction
and selectivity with the practical advantages that are
associated with heterogeneous catalysts [7,8]. Chiral metal
Enantiomerically pure epoxides are very useful syn-
esalen complexes have been successfully immobilized on
different solid supports using a variety of immobilization
methods [9], but epoxidation is usually more effective with
cis-disubstituted alkenes and with oxidants that are
considered to be environmentally unfriendly (mCPBA,
PhIO). The immobilization of Tietartrate complexes has
been investigated to a lesser degree with a lower number of
successful examples [10]. Polytartrates [11,12] and tar-
tramides supported on inorganic siliceous materials [13]
thetic intermediates in the synthesis of different biologi-
cally active compounds [1]. Classically, the two most
important methods for the enantioselective epoxidation of
alkenes include the use of homogeneous chiral catalysts
such as Tietartrates, which are used for the epoxidation of
allylic alcohols with alkyl hydroperoxides [2], and Mn
esalen catalysts, which are used for the epoxidation of
unfunctionalized alkenes with different oxidants [3]. More
recently, the use of hydrogen peroxide as an oxidant has
been extensively studied using different Mn, Fe, and Ti
complexes [4e6].
and Tietartrate grafted on silicas [14] are relevant exam-
i
ples. The later were prepared by grafting of Ti(O Pr)
4
on a
silica support [15] and then modified with a tartaric acid
derivative [16]. In the same way that unmodified Tiesilica
is able to catalyze the epoxidation of unfunctionalized al-
kenes with alkyl hydroperoxides [17], it has been shown
that the modified solids lead to moderate enantiose-
lectivity in the epoxidation of styrene [18,19].
The immobilization of homogeneous catalysts on solid
supports could feasibly yield the advantages of high activity
*
Corresponding author.
*
* Corresponding author.
E-mail addresses: marwafadhli@hotmail.fr (M. Fadhli), jmfraile@
Tantalum can be grafted on silica in the same way as Ti,
and it has been shown that those solids are able to activate
unizar.es (J.M. Fraile).
http://dx.doi.org/10.1016/j.crci.2017.02.008
1631-0748/© 2017 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
selective epoxidation of styrene with TBHP, Comptes Rendus Chimie (2017), http://dx.doi.org/10.1016/j.crci.2017.02.008

2
M. Fadhli et al. / C. R. Chimie xxx (2017) 1e6
H
2
O
2
in the epoxidation of alkenes [20e25]. These kinds of
patterns. After the grafting of the metal centers, these
values are even lower, with a more significant effect in the
case of Ta species. Nonetheless, the variations in site den-
sity are not significant, with values from 0.24 to 0.31 of Ti/
supported catalysts are also able to decompose alkyl hy-
droperoxides [26], and after being modified with chiral
tartrates, they showed an excellent performance in the
enantioselective epoxidation of allylic alcohols [27e29].
We recently described how tartrate modified TaeMCM-41
catalyzed the enantioselective oxidation of methyl phenyl
sulfide with both H O and alkyl hydroperoxides, with
2 2
moderate to low enantiomeric excesses [30].
In view of the parallel behavior of supported Ti and Ta in
other reactions, we decided to compare the performance of
these metals grafted on MCM-41 in the epoxidation of
styrene with tert-butyl hydroperoxides (TBHPs) and the
2
2
nm and from 0.27 to 0.32 of Ta/nm .
Carbon analyses (Table 1) of the unmodified catalysts
are all of a similar value considering the isopropoxide/Ti
ratios are found in the range of 1.7e2.4 and the ethoxide/Ta
ratios in the range of 2.7e3.5. Although a mixture of surface
species would be expected, the bipodal one appears to be
the most significant contributor for both Ti and Ta catalysts.
The treatment with chiral tartrates always increases the
carbon content of the solids, although it is difficult to es-
timate the true content of tartrate. Assuming the correct
formation of the species shown in Scheme 1, the C/Ti ratios
would be in the range of 8e11 for DET and 10e13 for DIPT,
and the experimental values (Table 1) are in the ranges of
9.5e12.9 and 11.1e13.6, respectively. For Ta, the C/Ta ratios
would be 8e12 for DET and 10e14 for DIPT, and the
experimental values are 8.5e10.6 and 9.0e13.7, respec-
tively. The values less than the expected range may be
attributed to the presence of sites without tartrate, because
of intrapore diffusional problems, and the values beyond
the expected one may be because of the complexation of
tartrates without forming a chelate.
effect of the modification with R-(þ)-diethyl
DET) and R-(þ)-diisopropyl -tartrate (DIPT) in an enan-
tioselective variant of this reaction.
L-tartrate
(
L
2
. Experimental section
Ta catalysts were prepared by grafting Ta(OEt)
5
on
MCM-41, which had been pretreated at different temper-
atures (550, 650, and 750 C), and then modified with DET
or DIPT as described in a previous article [30]. Ti catalysts
were prepared in a similar way using Ti(O Pr)
in a previous article [19].
ꢀ
i
4
as described
All the catalysts were characterized by powder small
angle X-ray diffraction, N adsorptionedesorption iso-
This possibility was explored by solid state 13C NMR
(Fig. 1). All the samples show two broad signals corre-
sponding to the carboxylate group, at 184 and 175 ppm (as
a shoulder for the Ti catalysts), in agreement with the
presence of both coordinated and uncoordinated carbox-
ylate groups, respectively [31]. In the same way, two signals
appear in the methine (OOCeCHOH) zone, at around
75 ppm for free hydroxyls and at 85e90 ppm corre-
sponding to the Ti-alkoxide or Ta-alkoxide [32,33]. These
spectra seem to indicate the presence of chelated and
unchelated tartrates on the active sites [19,30].
2
therms at 77 K, metal analysis by inductively coupled
plasma emission spectroscopy, organic analysis, and solid-
state NMR, as described elsewhere [19,30]. The epoxidation
of styrene (10 mmol) with TBHP (6 mmol, 5.5 solution in
ꢀ
decane) was carried out in acetonitrile (10 mL) at 70 C
under an inert atmosphere with 0.015 mmol catalyst, as
described elsewhere [19]. The catalysts were filtered,
thoroughly washed with dichloromethane, dried under
vacuum, and reused under the same conditions.
3
. Results and discussion
3.2. Epoxidation of styrene
3
.1. Characterization of the catalysts
All the catalysts were tested in the epoxidation of sty-
rene with TBHP (Scheme 2), using an amount of oxidant
below the stoichiometric one (0.6 equivalents) with respect
to styrene, in an attempt to minimize the overoxidation of
the products [34]. In fact, phenyl acetaldehyde and benz-
aldehyde were detected and identified in all the reactions,
together with minor amounts of other products such as
tert-butyl benzoate or 1-phenylethane-1,2-diol. The sty-
rene conversion and the styrene oxide yield (Table 2) were
calculated with respect to the added amount of oxidant.
Acetonitrile was chosen as solvent, given its good general
performance in epoxidation reactions with TBHP [35].
The grafting of the Ti and Ta precursors on the surface of
MCM-41 may give rise to three types of species, monop-
odal, bipodal, and tripodal (Scheme 1). Monopodal species
allow the chelation with the tartrate and leave at least one
remaining alkoxy group, which is able to be substituted by
the hydroperoxide oxidant. The same is true for the Ta-
bipodal species, except that there is no remaining alkoxy
group in the Ti-bipodal and Ta-tripodal species, and the
activation of the hydroperoxide might be carried out by the
cleavage of one SieOeM bond (Scheme 1), keeping the
chelate necessary for a higher enantioselectivity.
5
First of all, the catalytic performance of Ta(OEt) and the
Table 1 gathers the results obtained from the elemental
and textural analyses of the catalysts. The amount of graf-
two tartrate complexes was tested in solution. As can be
seen in Table 2, the three homogeneous catalysts are not
very active, as shown by the low conversion after 24 h, and
activity is lower than the analogous Ti catalyst, with the
ꢁ
1
ted titanium and tantalum is always around 0.4 mmol g
,
indicating that a complete grafting of the precursor has
occurred. The increase in the calcination temperature
produces a decrease in surface area, pore volume, and
mean pore diameter values, but the hexagonal structure of
MCM-41 is preserved, as shown by the XRD diffraction
5
only exception being DIPTeTa(OEt) . The selectivity to
styrene oxide is always very low, comparable with that
obtained with the Ti catalysts. Interestingly, the Taetartrate
complexes led to a moderate enantioselectivity of 61% ee
Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
selective epoxidation of styrene with TBHP, Comptes Rendus Chimie (2017), http://dx.doi.org/10.1016/j.crci.2017.02.008

M. Fadhli et al. / C. R. Chimie xxx (2017) 1e6
3
Scheme 1. Possible tartrate-modified surface Ti and Ta species on MCM-41 and activation of an alkyl hydroperoxide.
Table 1
Textural and structural properties and elemental analysis of the catalysts.
2
3
Sample
S
BET (m /g)
V
p
(cm /g)
D
p
(nm)
M (mmol/g)
C/M
Ti
Ta
Ti
Ta
Ti
Ta
Ti
Ta
Ti
Ta
MCM550
MCM650
MCM750
M-MCM550
M-MCM650
M-MCM750
DETeM-MCM550
DETeM-MCM650
DETeM-MCM750
DIPTeM-MCM550
DIPTeM-MCM650
DIPTeM-MCM750
1001
945
925
966
921
814
804
820
730
928
802
788
0.76
0.75
0.64
0.75
0.61
0.57
0.65
0.55
0.52
0.65
0.53
0.49
3.03
2.87
2.75
2.99
2.66
2.35
2.56
2.62
2.34
2.80
2.64
2.04
e
e
e
e
e
e
876
784
822
842
743
782
732
804
797
0.65
0.63
0.49
0.60
0.54
0.46
0.51
0.53
0.44
3.01
2.52
2.54
2.45
2.60
2.03
2.32
2.20
1.99
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.39
0.41
0.39
0.38
0.40
0.38
0.38
0.40
0.38
5.2
7.3
6.9
9.5
11.2
12.9
11.1
13.6
11.3
7.0
5.5
7.1
9.6
10.6
8.5
13.7
9.8
9.0
S
BET ¼ surface area; V
p
¼ pore volume; D ¼ mean pore diameter (determined by the BJH [BarretteJoynereHalenda] method).
p
with DET, lower than the 81% ee obtained with Ti, and a 75%
ee with DIPT, thus improving on the 50% ee reached with Ti.
The grafting of Ta on MCM-41 significantly improves the
catalytic activity, as seen in the conversions after 24 h,
although not in the productivity at longer reaction times.
The best result in catalytic activity was obtained with Ta
eMCM550 and it decreases when increasing the calcination
temperature. Conversely, the selectivity to styrene epoxide
was not improved in a similar way as it happened in the
case of Ti, illustrating that the Ta sites remain highly active
in the epoxide rearrangement and in the overoxidation
reactions, in the same way as the soluble catalysts.
in agreement with a ligand accelerated catalytic reaction, as
described for other closely related epoxidation reactions
[36]. In the case of DET, the most active solids with both Ti
and Ta are those prepared with MCM550 and MCM650
.
These solids are also the most selective to styrene oxide by
Ti, with values of around 45% after 24 h, but the selectivity
is lower with Ta, as it happened with the unmodified solids.
The best result is 34% with DETeTaeMCM650. In the case of
DIPT, the activity of the three solids is similar, thus
improving the results obtained with Ti in the solids
ꢀ
calcined at 650 and 750 C. Conversely, the selectivity to
styrene oxide is very different in the three solids, ranging
from 53% after 24 h with DIPTeTaeMCM650 to only 11%
with DIPTeTaeMCM750. The behavior with respect to
calcination temperature is then completely different to that
The most significant changes happen when the solids
are treated with the chiral tartrates, DET and DIPT. In most
cases, the epoxidation of styrene is faster. This behavior is
Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
selective epoxidation of styrene with TBHP, Comptes Rendus Chimie (2017), http://dx.doi.org/10.1016/j.crci.2017.02.008

4
M. Fadhli et al. / C. R. Chimie xxx (2017) 1e6
the enantioselectivity and the rest of the catalytic results
activity and selectivity to epoxide). The effect of calcina-
(
tion temperature is more evident in Ta solids treated with
DIPT, as enantioselectivity decreases in the order
MCM550 > MCM650 > MCM750. It is remarkable that the best
result, 71% ee obtained with DIPTeTaeMCM550, is the best
one described for this reaction with this kind of solids. In
trying to rationalize this performance, the larger pore size
of the support must be able to accommodate the Taetar-
trate chelate complex and may be one of the key parame-
ters for this success.
Most of the heterogeneous catalysts described for sty-
rene epoxidation are based on Mnesalen complexes,
immobilized on different supports, and through different
strategies. Many parameters including the oxidant used for
epoxidation affect the final results of the reaction. Enan-
tioselectivities with NaClO typically range between 25%
and 52% ee with the Mnesalen catalyst covalently bonded
to mesoporous materials [37], whereas it is lower (40% ee)
with graphene oxide grafted catalysts [38], and a 78% ee
can be reached with anionic Mnesalen complexes
exchanged on ZneLa double hydroxides, albeit with only
Fig. 1. CP-MAS 13C NMR spectra: (a) DIPTeTaeMCM750 and (b) DIPTeTi
eMCM750
.
25% conversion [39]. Results reported with m-chlor-
operbenzoic acid are also moderate, with values of 13e56%
ee for catalysts immobilized on poly(styrene-
phenylvinylphosphonate)-phosphate through anchored
phenoxide groups as axial ligands [40], similar to 16e41%
ee found with covalent-bonded complexes [37]. Only the
use of iodosylbenzene allows reaching very high enantio-
selectivities (99% ee) with anionic exchanged complexes
Scheme 2. Epoxidation of styrene with TBHP and side reactions.
observed for the Ti solids. Although the DIPTeTi materials
lead to similar enantioselectivities with very different cat-
alytic activities, the DIPTeTa catalysts show similar activ-
ities with very different enantioselectivities.
Regarding enantioselectivity, there is no clear trend and
this must arise to an uncontrolled factor in the process of
complex formation. The typical results with DET are similar
with both Ti and Ta, leading to values in the range of 54
e58% ee, although in each case one of the solids, DETeTi
eMCM750 and DETeTaeMCM650, do not reach those re-
sults. Moreover, there is no apparent relationship between
[39]. Thus, our results are comparable to those reported in
the literature, using easily prepared Ti and Ta catalysts with
commercially available chiral ligands and a more environ-
mentally friendly oxidant.
Given the similar enantioselective results obtained with
both Ti and Ta, it can be assumed that both types of cata-
lysts follow the same mechanism. Although the Sharpless
epoxidation with Tietartrate complexes takes place
through a dimeric species [32], the site isolation obtained
on the solid supports precludes these kinds of centers on
Table 2
a
Epoxidation of styrene with TBHP catalyzed by Ti and Ta catalysts.
Entry
Catalyst
Conversion (%)b
Styrene oxide selectivity
% eec
Ti
b
(%)
Ti
Ta
Ti
Ta
Ta
d
1
2
3
4
5
6
7
8
9
M(OR)
DETeM(OR)
n
56 (79)
52 (75)
16 (40)
36 (64)
38 (58)
23 (44)
68 (79)
62 (74)
27 (54)
59 (64)
19 (73)
26 (52)
40 (52)
26 (42)
23 (59)
58 (57)
37 (38)
29 (37)
59 (69)
73 (86)
17 (28)
43 (51)
45 (60)
41 (51)
5 (9)
6 (9)
11 (9)
16 (15)
4 (6)
4 (9)
e
e
d
n
80
50
e
61
75
e
d
DIPTeM(OR)
M-MCM550
M-MCM650
M-MCM750
DETeM-MCM550
DETeM-MCM650
DETeM-MCM750
DIPTeM-MCM550
DIPTeM-MCM650
DIPTeM-MCM750
n
28 (22)
32 (26)
29 (25)
44 (36)
45 (36)
23 (32)
46 (26)
48 (34)
42 (47)
14 (23)
18 (19)
4 (8)
24 (23)
34 (33)
23 (21)
24 (23)
53 (42)
11 (11)
e
e
e
e
55
56
38
62
38
45
54
26
58
71
49
37
1
1
1
0
1
2
a
b
c
ꢀ
Reaction conditions: styrene (10 mmol), TBHP (6 mmol, 5.5 M in decane), acetonitrile (10 mL), catalyst (0.015 mmol of Ta), 70 C, inert atmosphere.
Determined by GC. Values after 24 h and in parenthesis values after 7 days.
Determined by HPLC with a Chiralpack OD-H column.
d
i
Ti(O Pr)
4
or Ta(Oet)
5
.
Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
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M. Fadhli et al. / C. R. Chimie xxx (2017) 1e6
5
the solids and monomeric species are more likely to be
involved (Scheme 3). The monopodal Ti center (a) should
activate TBHP by the substitution of the remaining iso-
propoxide group (see Scheme 1) and additional coordina-
tion of the distal oxygen atom. The proximal oxygen is the
most electrophilic one and hence is transferred to the
alkene in a spiro-like transition state [41] (front view in
Scheme 3b). In contrast to the epoxidation of allylic alco-
hols, the alkene is not coordinated to the metal center, and
the oxygen transfer can be compared to the carbene
transfer as in a cyclopropanation reaction [42]. The result-
ing Ti species would be similar to the starting one but with
a tert-butoxy group instead of an isopropoxy one. In the
case of Ta, the mechanism would be the same, but Ta would
start in a five-coordinated state, expanded to six with the
coordination of the distal oxygen of TBHP. In fact, this al-
lows the successful reaction with bipodal Ta species
Fig. 2. Variation of enantioselectivity with reuse in some of the catalysts.
the complexes is the enantioselectivity, which is directly
related to the presence of metaletartrate species on the
solid. As can be seen in Fig. 2, the drop in enantioselectivity
is much more important in the case of Ta catalysts, indi-
cating that the Taetartrate complexes are less stable than
the Tietartrate. This drop in enantioselectivity is also
accompanied with a decrease in the catalytic activity and
the selectivity to epoxide, which can be attributed to the
loss of the ligand acceleration effect. The tartrate was not
(Scheme 3c), whereas in the case of Ti at least one of the
CeOeTi bonds (to tartrate) or SieOeTi (to silica support)
would be broken. In all cases, as there is no other apparent
interaction, the enantioselectivity must be controlled by
steric effects, first of all in the orientation of the tert-butyl
group, probably in the less hindered quadrant of the com-
plex (see Scheme 3b) [41], and then in the approach of the
styrene. The variations in enantioselectivity may be
attributed to the partial formation of nonchelated com-
plexes with tartrate in an uncontrolled manner, as indi-
cated by the NMR spectra, making it difficult to draw in-
depth conclusions about the steric effects.
1
detected in the solution by H NMR, either because of the
low concentration or because it remains retained on the
silica surface of the solid support. The lack of Ta leaching
even with more coordinating reagents and products, as in
the sulfide oxidation with hydrogen peroxide [29], seems to
confirm the hypothesis of the loss of ligand by decom-
plexation as the main deactivation mechanism.
Catalyst reusability is of major importance for hetero-
geneous catalysts and is often cited as an advantage over
homogeneous catalysts. Some of the catalysts were recov-
ered by filtration at the end of the reactions, then thor-
oughly washed with dichloromethane, and dried in
ꢀ
4. Conclusions
vacuum at 60 C for 12 h before reuse under the same
conditions. The lack of activity in the liquid phase obtained
by filtration experiments demonstrated that the solid cat-
alysts were heterogeneous in nature. Nonetheless, the re-
sults in the reuse experiments showed a significant
deactivation, more important in the case of Ta catalysts. The
most important result to compare the relative stability of
Ti-MCM-41 and TaeMCM-41, prepared by grafting of
i
Ti(O Pr)
4
and Ta(OEt)
5
on MCM-41, can be modified with
chiral tartrates, leading to similar surface species with both
metals. All the solids are active in the epoxidation of sty-
rene with TBHP and produce improved activity and selec-
tivity to styrene oxide, by comparison to the precursors and
the analogous complexes in a solution. Moderate enantio-
selectivities are obtained, with values ranging between 26%
and 71% ee depending on the metal, the nature of the chiral
ligand, and the calcination temperature of the support.
Interestingly, the best result is obtained with DIPTeTa
eMCM550, and this is thus the first example of use of a Ta
catalyst for the enantioselective epoxidation of a non-
functionalized alkene. Nonetheless, the Ta catalysts seem to
be less recoverable than Ti ones, probably because of a
reduced stability of the tartrate complex. The way forward
from this point will be the control of the species (monop-
odal, bipodal, or tripodal) formed on the surface, and the
proper complexation with tartrate together with the search
for ligands that can form more stable Ta complexes.
Acknowledgments
Funding: This work was supported by the Spanish
Ministerio de Economía y Competitividad (grant CTQ2014-
Scheme 3. Proposed mechanism for styrene epoxidation with Ti and Ta
catalysts.
Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
selective epoxidation of styrene with TBHP, Comptes Rendus Chimie (2017), http://dx.doi.org/10.1016/j.crci.2017.02.008

6
M. Fadhli et al. / C. R. Chimie xxx (2017) 1e6
5
2367-R) and the Diputaci oꢀ n General de Arag oꢀ n (E11
Group cofinanced by the European Regional Development
Funds).
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(
[18] Z. Fu, D. Yin, Q. Xie, W. Zhao, A. Lv, D. Yin, Y. Xu, L. Zhang, J. Mol.
Catal. A 208 (2004) 159e166.
M.F. thanks the Tunisian “Minist eꢁ re de l’Enseignement
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Please cite this article in press as: M. Fadhli, et al., Comparison of TaeMCM-41 and TieMCM-41 as catalysts for the enantio-
selective epoxidation of styrene with TBHP, Comptes Rendus Chimie (2017), http://dx.doi.org/10.1016/j.crci.2017.02.008