Functionalized mesoporous silica supported copper(ii) and nickel(ii) catalysts for liquid phase oxidation of olefins

Article abstract of DOI:10.1039/c1dt10157a

Highly ordered 2D-hexagonal mesoporous silica has been functionalized with 3-aminopropyltriethoxysilane (3-APTES). This is followed by its condensation with a dialdehyde, 4-methyl-2,6-diformylphenol to produce an immobilized Schiff-base ligand (I). This m

Full text of DOI:10.1039/c1dt10157a

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PAPER
Functionalized mesoporous silica supported copper(II) and nickel(II) catalysts
for liquid phase oxidation of olefins
Mahasweta Nandi,a,b,c Partha Roy,*d Hiroshi Uyamac and Asim Bhaumik*a
Received 28th January 2011, Accepted 23rd August 2011
DOI: 10.1039/c1dt10157a
Highly ordered 2D-hexagonal mesoporous silica has been functionalized with 3-aminopropyltriethoxy-
silane (3-APTES). This is followed by its condensation with a dialdehyde, 4-methyl-2,6-diformylphenol
to produce an immobilized Schiff-base ligand (I). This material is separately treated with methanolic
solution of copper(II) chloride and nickel(II) chloride to obtain copper and nickel anchored mesoporous
materials, designated as Cu-AMM and Ni-AMM, respectively. The materials have been characterized
by Fourier transform infrared (FT-IR) and UV-vis diffuse reflectance (DRS) spectroscopy, powder
X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption–desorption studies
and 13C CP MAS NMR spectroscopy. The metal-grafted mesoporous materials have been used as
catalysts for the efficient and selective epoxidation of alkenes, viz. cyclohexene, trans-stilbene, styrene,
a-methyl styrene, cyclooctene and norbornene to their corresponding epoxides in the presence of
tert-butyl hydroperoxide (TBHP) as the oxidant under mild liquid phase conditions.
the wastage of products arising during the catalyst separation and
Introduction
its disposal. Thus, now-a-days researchers are keener towards the
Epoxidation of olefins is studied most intensively because epoxides
development of effective heterogenized catalytic systems, which
are versatile building blocks for the synthesis of many bioactive
can be reused for several cycles thereby reducing the cost of
molecules and organic fine chemicals.1 Transition metal com-
production in chemical industries. Furthermore, such systems
pounds have been used as catalysts for the epoxidation of olefins
require minimum effort for the separation of catalysts from the
in the past few decades.2 Schiff-base complexes of manganese(III),
substrate, product(s) and oxidant. The simplest approach to
iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) are reported
transform a successful homogeneous catalyst into a heterogeneous
in the literature where they have been employed as active catalysts
catalyst is by supporting the soluble catalyst on the surface of
an insoluble solid.10 The highest level of development in the
surface immobilization strategy is the covalent attachment of the
organic compound or ligand to the solid surface and subsequent
complexation with metal ions.
for the oxidation of alkanes and alkenes; whereas polymer bound
Schiff-base complexes of ruthenium, iron and molybdenum have
shown better catalytic activity towards cyclooctene oxidation.3
Apart from their catalytic roles in epoxidation, inbound copper
and nickel materials also have significant importance in other
organic conversions. Copper complexes have been used as catalysts
in various liquid-phase catalytic reactions4 e.g. catechol oxidation,5
alkane oxidation,6 sulfoxidation,7 epoxidation,8 etc.; and nickel
compounds also play a crucial role in many catalytic conversions.9
The separation of products from the reaction mixture is the
primary drawback associated with homogeneous catalysis, which
makes use of soluble metal complexes. Whereas in heterogeneous
catalysis, the use of insoluble solid catalysts provides the benefits
of easy separation and catalyst recyclability, thereby minimizing
The use of mesoporous organosilica or related nanomaterials
for the synthesis of effective catalysts through anchoring or
grafting of organic groups within the nanochannels has been an
active area of research. There are several reports where copper(II)
has been anchored on to porous silica materials.11 In such
cases, the amine group containing organic moiety, generally 3-
aminopropyltriethoxysilane, has been first anchored on the surface
of mesoporous silica via Si–O–Si bonds, taking advantage of the
surface silanol groups. Then Schiff-base condensation between
the amine group and a suitable aldehyde leads to the forma-
tion of a coordinating ligand in the mesoporous matrix which
can bind covalently with copper(II) ions. A Cu(II)–Schiff base
complex covalently grafted over 3-aminopropyl functionalized
silica surface showed good catalytic activity in allylic oxidation of
olefins in the presence of tert-butyl hydroperoxide.12 Insoluble Ni-
compounds have also been employed as heterogeneous catalysts
for olefin epoxidation.13 Previously we have reported the synthesis
and characterization of Ni(II),14 Cu(II)15 and Mo(VI)16 complexes
aDepartment of Materials Science, Indian Association for the Cultivation
of Science, Jadavpur, Kolkata, 700 032, India. E-mail: msab@iacs.res.in;
Fax: +91-33-2473-2805
bIntegrated Science Education and Research Centre, Visva-Bharati Univer-
sity, Santiniketan, 731 235, India
cDepartment of Applied Chemistry, Graduate School of Engineering, Osaka
University, 2-1 Yamadaoka, Suita, Osaka, 565-0871, Japan
dDepartment of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700
032, India. E-mail: icroy@yahoo.com; Fax: +91-33-2414 6223
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with N- and O-donor Schiff-base ligands and their applications
as catalysts for epoxidation of olefins under heterogeneous condi-
tions. The heterogenized catalysts were prepared by immobilizing
the complexes onto 2D mesoporous silica.
studies were purified and dried by standard procedures before
use.18 Elemental analysis was carried out in a 2400 Series-II CHN
analyzer, Perkin–Elmer, USA. FT-IR spectra were obtained on
a Nicolet MAGNA-IR 750 spectrometer with samples prepared
as KBr pellets. Characterization of the samples by powder X-
Ray diffraction was performed by using a Bruker AXS D8
Advanced SWAX diffractometer, where the small and wide-angle
goniometers are mounted. The X-ray source was Cu-Ka radiation
(a = 0.15406 nm) with an applied voltage and current of 40 kV
and 20#x00A0;mA, respectively. Mesophases of different samples
were analyzed using a JEOL, JEM 2010 transmission electron
microscope at an accelerating voltage of 200 kV. N2 adsorption
measurements were carried out using a Bel Japan Inc. Belsorp-HP
surface area analyzer at 77 K. Pre-treatment of the samples was
undertaken at 473 K for 3 h under high vacuum. A Shimadzu AA-
6300 double beam atomic absorption spectrophotometer (AAS)
was used for determining the percentage loading of Cu and
Ni through wet chemical analysis. UV-visible diffuse reflectance
spectra (DRS) were recorded on a Shimadzu 2401PC UV-
visible spectrophotometer with an integrating sphere attachment
using BaSO4 as the background standard. Absorption spectra
of dispersed samples were recorded on a Shimadzu UV 2100
spectrophotometer. Solid state 13C CP MAS NMR analyses
were conducted on a CHEMAGNETICS 300 MHz CMX 300
spectrometer. Gas chromatography analysis was performed with
an Agilent Technologies 6890 N network GC system equipped
with a fused silica capillary column (30 m ¥ 0.32 mm) and an FID
detector. All experiments were carried out in air and room tem-
perature unless reported otherwise. 4-Methyl-2,6-diformylphenol
was synthesized following a published procedure.19
To design and synthesize effective metal bound mesoporous
silica materials for catalysis, the metal has to bind tightly with
silica inside the cavity at the mesopore surface so that it does
not percolate out into the reaction mixture during reactions. To
obtain such materials the silica must be functionalized in such a
way that sufficient donor sites are available for the metal cation to
bind on its surface, and it can be used as an efficient catalyst. We
have followed this approach in this present work. A heterogenized
N, O donor Schiff-base ligand has been first immobilized on
ordered mesoporous silica and then transition metal ions have
been coordinated to this ligand. We have also applied this strategy
to prepare Pd-catalysts, which has been successfully utilized for
the synthesis of value added fine chemicals via C–C coupling
reactions.17
We report here the synthesis and characterization of two hetero-
genized catalysts, Cu-AMM and Ni-AMM, comprised of Cu(II)
and Ni(II) ions, respectively (Scheme 1). A heterogenized Schiff-
base ligand, I, is produced when mesoporous silica functional-
ized with 3-aminopropyltriethoxysilane (3-APTES) reacts with
4-methyl-2,6-diformylphenol (DFP). Cu(II) and Ni(II) ions are
allowed to react with this Schiff-base anchored ligand to produce
Cu-AMM and Ni-AMM respectively. These materials have been
used as catalysts for the epoxidation of cyclohexene, trans stilbene,
styrene, a-methyl styrene, cyclooctene and norbornene in the
presence of tert-butyl hydroperoxide as the oxidant under liquid
phase conditions.
Syntheses of catalysts Cu-AMM and Ni-AMM
Experimental
Mesoporous silica was synthesized following
a published
Materials and physical methods
procedure.20 It was functionalized by stirring 0.1 g of it with 0.180
g (0.81 mmol) of 3-APTES in chloroform following a published
method.21 Heterogenized Schiff-base ligand, I, was synthesized by
the reaction between 3-APTES functionalized mesoporous silica
with 4-methyl-2,6-diformylphenol (0.082 g, 0.5 mmol) following a
previously reported procedure.20 For the synthesis of Cu-AMM,
Cyclohexene, styrene, a-methyl styrene, trans-stilbene, cy-
clooctene, norbornene, 3-APTES and tetraethyl orthosilicate
(TEOS) were purchased from Aldrich and used without purifi-
cation. Other reagents were purchased from Loba Chemie and
used without further purification. Solvents used for spectroscopic
Scheme 1 Preparation of Cu-AMM and Ni-AMM. Functionalization of mesoporous silica with (a) 3-aminopropyltriethoxysilane (3-APTES) and then
with (b) 4-methyl-2,6-diformylphenol.
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a methanolic solution of copper(II) chloride dihydrate (0.169 g,
1.0 mmol) was added to the above mention yellowish solid at
room temperature and left overnight. After that it was filtered
and washed with hot methanol several times until the filtrate
became colorless and was finally dried under vacuum. Ni-AMM
has been synthesized using nickel(II) chloride hexahydrate (0.236
g, 1.0 mmol) instead of copper(II) chloride following the same
procedure as used for Cu-AMM.
copper(II) and nickel(II) complexes with similar N2O donor
ligands and coordination environment MN2OCl3 (M Cu or
Ni).22
FT-IR spectroscopy
FT-IR spectra of mesoporous silica (MS), heterogenized Schiff-
base ligand (I), Cu-AMM and Ni-AMM are presented in Fig.
1. The FT-IR spectrum of I shows a band in the region of 3000–
2800 cm-1 and 1655–1500 cm-1. The spectrum suggests the presence
of various N–H and C–H vibration bands in the material. These
bands are generated after functionalization of 3-APTES over
MS and were absent in the mesoporous silica. The FT-IR band
Epoxidation reactions
Catalytic reactions were carried out over the Cu- and Ni-grafted
materials in a similar manner. The heterogeneous oxidation
reactions were performed in a magnetically stirred two necked
round-bottomed flask fitted with a condenser and placed in a
temperature controlled oil bath. Typically, 1.0 g of the substrate
was added to 5 mL of acetonitrile (solvent), followed by the
addition of 0.1 g of the catalyst and the mixture was then preheated
to 333 K. The reaction was initiated with the addition of tert-butyl
hydroperoxide (equimolar with respect to the substrate). Aliquots
from reaction mixtures were collected at regular intervals. After
cooling, nitrobenzene was added as the internal standard. The
substrate and product(s) from the reaction mixture were analyzed
by gas chromatography. They were identified by comparison with
known standards.
at 1655 cm-1 for compound I could be attributed to the C
N
stretching frequency and the band at 1530 cm-1 is probably due
to the C C stretching, which comes from the phenyl ring. In
the FT-IR spectrum of 4-methyl-2,6-diformylphenol, the band at
1681 cm-1 appears due to the presence of aldehyde group,20 but
in the spectrum of I, no peak at around 1681 cm-1 is observed
which indicates that both the aldehyde groups are converted to
imine. Cu-AMM and Ni-AMM showed FT-IR bands at 1630 and
1632 cm-1 which indicate the retention of C N moiety in both
the materials. These bands in Cu-AMM and Ni-AMM are shifted
towards lower wave number regions indicating coordination of the
imine-N with the metal ions.23
One blank reaction has been carried out without adding
any catalyst keeping other experimental conditions unchanged.
Another blank reaction has been performed in the presence of I
under similar conditions.
Results and discussion
Syntheses and elemental analyses
The immobilized Schiff-base ligand has been synthesized via con-
densation between the dialdehyde, 4-methyl-2,6-diformylphenol
and the heterogenized amine. We attempted to synthesize efficient
heterogeneous catalysts by immobilization of metal ions over a
silica host via coordination with electron donor sites. For this
purpose, we first synthesized a heterogeneous ligand and then
metal ions were allowed to coordinate with it. Mesoporous silica
was functionalized in such a way that the amine group was
appended with the silica. Then diformyl compound was allowed
to condense with it to form the Schiff-base. Metal ions were then
allowed to bind with this heterogenized ligand. Atomic absorption
spectroscopy has been used to determine the concentrations of
metal ion and chloride. The results show that Cu-AMM and Ni-
AMM contain 0.34% copper and 0.42% nickel respectively. It
has been further revealed from AAS results that the molar ratio
of metal and chloride is 2 : 2.93 for Cu-AMM and 2 : 2.85 for
Ni-AMM. These results have been supported by the elemental
analyses of the materials. Elemental analyses show that the molar
ratios for nitrogen: metal in Cu-AMM and Ni-AMM are ca.
1.05 : 1 and 1.10 : 1 respectively. The molar ratios for carbon: metal
in Cu-AMM and Ni-AMM are ca. 6.92 : 1 and 6.95 : 1 respectively.
All these data support the possible structure of the catalysts as
shown in Scheme 1. A similar structure in the heterogenized
donor system has been proposed for Zn2+ supported function-
alized mesoporous sensor.20 There are reports of m2-Cl bridged
Fig. 1 FT-IR spectra of mesoporous silica MS (a), heterogenized
Schiff-base ligand I (b), Cu-AMM (c) and Ni-AMM (d).
UV-vis spectroscopy
UV-vis diffuse reflectance spectra of Cu-AMM and Ni-AMM are
shown in Fig. 2 (UP). Both the materials showed broad bands
in the region of 210–290 nm. These bands could be attributed
to p → p* transitions of the ligand. Cu-AMM showed a broad
absorption band ranging from 360–470 nm, with a maximum
at ca. 385 nm. This band could appear due to ligand to metal
charge transfer (LMCT). The weak broad band in the range of
670–740 nm could arise due to d–d transition in metal center
(shown in the inset of Fig. 2 (UP)). Cu(II) complexes immobilized
onto mesoporous silica with a similar coordination environment
showed an identical spectral behavior.15 Ni-AMM showed a ligand
to metal charge transfer band in the region of 430–480 nm.22 In
addition to that, it showed a broad band due to d–d transition
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Fig. 3 Small angle XRD patterns of mesoporous silica MS (a), heterog-
enized Schiff-base ligand I (b), Cu-AMM (c) and Ni-AMM (d).
Fig. 3. The mesoporous silica (Fig. 3a) shows three strong
diffractions for (100), (110) and (200) planes and a weak one
for the (210) plane of 2D-hexagonal mesophase,26 indicating the
formation of a highly ordered mesophase. The Schiff-base ligand
I shows a similar diffraction pattern (Fig. 3b), which indicates
that the 2D hexagonal ordering is retained after grafting with 3-
APTES and functionalization with 4-methyl-2,6-diformylphenol.
An overall decrease in intensity is observed, which could be
attributed to the lowering of local order, such as variations in
the wall thickness or reduction of scattering contrast between the
channel wall of the silicate framework and the Schiff-base ligand
present in I.27 The same pattern is restored for Cu-AMM and Ni-
AMM (Fig. 3c and d) but the intensity of the peaks are further
reduced indicating a further decrease in the ordering. There is a
small but gradual decrease in d-spacing from mesoporous silica
to the catalysts. The d100 values calculated from the corresponding
2q values in the XRD patterns for mesoporous silica, Schiff-base
ligand I, Cu-AMM and Ni-AMM are 3.56, 3.53, 3.49 and 3.50 nm
respectively. This gradual decrease of d-spacing could be attributed
to the contraction of unit cells upon surface functionalization and
further coordination of the Schiff-base and metal ions within the
pores of I.
Fig. 2 UP: UV-vis diffuse reflectance spectra of Cu-AMM (a) and
Ni-AMM (b). Inset: UV-vis diffuse reflectance spectra of Cu-AMM (a¢)
and Ni-AMM (b¢) from 600–800 nm. Down: Solution phase UV-vis spectra
of Cu-DFP/3-APTES (a) and Ni-DFP/3-APTES Schiff-base (b) taken in
methanol.
in the region of 700–770 nm. This implies that copper(II) and
nickel(II) ions have been successfully coordinated to the donor
atoms of the heterogenized Schiff-base ligands. In Fig. 2 (down)
we have shown the solution phase UV-visible spectra of Cu and
Ni complexes of DFP/3-APTES Schiff-bases in methanol. Cu-
DFP/3-APTES Schiff-base showed bands at 264 and 388 nm due
to p → p* and LMCT transitions, respectively.24 Whereas the d–d
transition band of copper(II) appeared in the range of 640–750 nm.
Ni-DFP/3-APTES Schiff-base, on the other hand, showed bands
at 276 and 448 nm due to p → p* and LMCT transitions and a
broad band in the region 675–775 nm due to a d–d transition.25
These spectral data support the fact that heterogenized metal
catalysts also retain their LMCT and d–d transition bands and
thus the presence of metal grafted Schiff-base ligand (I) in the
anchored mesoporous materials.
High resolution TEM
The high resolution transmission electron microscopy images of
MS, heterogenized Schiff-base ligand I, Cu-AMM and Ni-AMM
are shown in Fig. 4. It can be clearly seen that all the materials
have a 2D-hexagonal arrangement of pores (different contrast than
that of the pore walls). The hexagonal ordering of the pores are
restored even after grafting the silica framework with 3-APTES
and functionalization with 4-methyl-2,6-diformylphenol as well
as for metal-bound complexes Cu-AMM and Ni-AMM. The
selected-area electron diffraction pattern (SAED) shown in the
inset of Fig. 4a, further confirms the hexagonal ordering of the
mesopores in MS.
Nitrogen adsorption–desorption
Powder X-ray diffraction
Fig. 5 shows the N2 adsorption–desorption isotherms of the
mesoporous materials (MS), Schiff-base ligand I, Cu-AMM
and Ni-AMM. These isotherms could be classified as type IV
characteristic of mesoporous materials.26–28 The mesopores in MS,
The powder X-ray diffraction patterns for the mesoporous
silica, Schiff-base ligand I and metal-anchored functionalized
mesoporous materials, Cu-AMM and Ni-AMM, are shown in
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Fig. 6 Pore size distribution of mesoporous silica MS (a), heterogenized
Schiff-base ligand I (b), Cu-AMM (c) and Ni-AMM (d) determined by
employing BJH method. The values of Y-axis of (a), (b) and (c) are offset
-1
by +0.024, +0.012 and +0.006 cc A g-1, respectively, for clarity.
˚
less 4-methyl-2,6-diformylphenol will be incorporated into the
material. However, unlike PMO most of the Schiff base sites are
located at the surface, which can bind with active metal centers.
From the above results it is clearly seen that surface area and pore
volume gradually decrease with subsequent functionalization with
4-methyl-2,6-diformylphenol and complexation with metal ions.
Such a drastic decrease in these parameters could be attributed to
the loading of an organic moiety and metal binding, which might
occupy the void space inside the mesopores. Thus we can conclude
that the dialdehyde as well as the metal ions are grafted inside the
mesopore cavity.
Fig. 4 HRTEM images of mesoporous silica (MS) (a), heterogenized
Schiff-base ligand I (b); Cu-AMM (c) and Ni-AMM (d). Selected area
electron diffraction (SAED) pattern of MS is shown in the inset of (a).
Solid state 13C MAS NMR studies
13C CP MAS NMR study provides useful information regarding
the presence of organic moiety and its chemical environment
(interaction of metal ions with the donor sites) in the hybrid
frameworks. In Fig. 7 the 13C CP MAS NMR spectra for I, Cu-
AMM and Ni-AMM are shown. The spectrum for the metal-free
heterogeneous Schiff-base ligand containing mesoporous material
I exhibits strong signals at 8.7, 19.2, 21.8, 28.8, 52.7, 117.3, 127.4
and 168.8 ppm, corresponding to different aliphatic and aromatic
Fig. 5 Nitrogen adsorption–desorption isotherms of mesoporous silica
MS (a), heterogenized Schiff-base ligand I (b), Cu-AMM (c) and Ni-AMM
(d). The values of the Y-axis for (b) and (c) are offset by +100 and +50 cc
g-1, respectively, for clarity.
Schiff base ligand I, Cu-AMM and Ni-AMM are uniform in
size, which is indicated by the sharp increase in N2 uptake for
adsorption, observed at P/P0 ca. 0.27–0.46. Pore size distributions
(PSD) of these samples have been estimated by employing BJH
(Barrett–Joyner–Halenda) method (Fig. 6). The PSD patterns
suggest the peak pore diameter for mesoporous silica, I, Cu-AMM
and Ni-AMM to be 3.15, 2.48, 1.98 and 1.95 nm respectively.
These results are in good agreement with the pore diameter
estimated from the respective TEM image analyses (Fig. 4). The
BET (Brunauer–Emmett–Teller) surface area and pore volume of
MS, I, Cu-AMM and Ni-AMM are 941 m2 g-1 and 0.62 cc g-1; 274
m2 g-1 and 0.16 cc g-1; 169 m2 g-1 and 0.09 cc g-1; and 160 m2 g-1
and 0.08 cc g-1, respectively. It is pertinent to mention here that a
comparatively lower amount of 4-methyl-2,6-diformylphenol has
been allowed to react with 3-APTES functionalized silica than
diimine functionalized PMO material.29 So it is expected that
Fig. 7 13C CP MAS NMR of 2D-hexagonal mesoporous silica containing
Schiff-base ligand I (a), Cu-AMM (b) and Ni-AMM (c).
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Table 1 Epoxidation of olefins over Cu-AMM and Ni-AMMa
carbon atoms present in I.29 This result indicates that the Schiff-
base ligand is grafted in I. On the other hand, for Cu-AMM
and Ni-AMM although all these 13C signals are present, little
shifts in the signals are observed. Specially the signal at 168.8 is
shifted downfield to 163.8 and the intensity for the signal at ca.
52.9 has been largely enhanced. These could be attributed to the
covalent interaction of metal atoms (Cu and Ni) with the lone pair
of electrons on the imine-N (corresponding signals coming from
carbon atoms adjacent to these N atoms) present in the Schiff base
ligand. Solution state 13C NMR chemical shifts of the DFP/3-
APTES Schiff-base precursor used for the synthesis of diimine
containing periodic mesoporous organosilica have been reported
previously by our group.29 Comparison of the 13C NMR spectra
of I in solid state with that reported data in solution shows that
both of these materials have similar 13C chemical shifts. Only three
small intensity peaks of the solution state 13C signals at 8.0, 133.3
and 159.5 ppm could not be resolved in these solid state MAS
NMR for Cu-AMM and Ni-AMM due to weak signal intensity.
Nevertheless, this result suggested that both have same carbon
skeleton. Compound I as well as Cu-AMM and Ni-AMM are
insoluble in common organic solvents, which makes it difficult to
carry out 13C NMR in solution.
Cu-AMM
Ni-AMM
Alkenes
Conversionb Selectivity TOFc Conversionb Selectivity TOFc
93.4
89.5
90.0
91.0
88.0 84.2
38.4 75.2
89.5
88.0
60.0
24.1
86.3
84.0
95.0
92.1
64.2 78.2
90.0
86.4
44.1
39.2
55.0 80.1
Catalytic reactions
The epoxidation reactions of cyclohexene, trans-stilbene, styrene,
a-methyl styrene, cyclooctene and norbornene have been carried
out heterogeneously with Cu-AMM and Ni-AMM as catalysts in
the presence of tert-butyl hydroperoxide as the oxidant at 333 K.
In all the cases, it has been found that the respective epoxide is the
major product. The results of the catalytic activities of Cu-AMM
and Ni-AMM are shown in Table 1. All the liquid phase oxidation
reactions of the olefins have been performed using acetonitrile as
the solvent. As seen in Table 1, the selectivity for the epoxides
for all the substrates is quite high after a 24 h reaction time. It is
quite usual in the case of liquid phase partial oxidation reaction
of olefins that the epoxide formed undergoes hydrolysis in the
presence of water, formed as a by-product from the oxidant in
the reaction mixture, which facilitates the epoxide ring opening.
Thus, after a few hours when the conversions reach their respective
maxima, epoxide selectivity goes down gradually with increase in
selectivity for the respective diols.30 Among the substrates studied
herein, cyclohexene shows the highest conversion of 93.4% with
90% selectivity towards the formation of its corresponding epoxide
(cyclohexene oxide) in the presence of Cu-AMM. 2-Cyclohexen-
1-ol and 2-cyclohexen-1-one are the other by-products formed
during the reaction. For all the substrates, Cu-AMM behaves
as a better catalyst compared to nickel bound catalyst, Ni-
AMM (Table 1). Among all the catalytic transformations, Ni-
AMM shows its lowest activity when trans-stilbene is used as
the substrate. In the presence of Cu-AMM and Ni-AMM, trans-
stilbene is converted to 89.5 and 75.2% respectively with high
selectivity for epoxide. Epoxidation of styrene to styrene oxide is of
particular interest from both scientific as well as commercial point
of view. For epoxidation of styrene, the conversions are found to
be 86.3 and 78.2% with excellent selectivity for epoxide formation
in the presence of both Cu-AMM and Ni-AMM, respectively.
Apart from epoxide, benzaldehyde has been detected as the minor
product for these reactions. Our results for styrene epoxidation are
92.2
90.3
93.1
94.0
65.4 83.8
74.1 82.5
92.0
93.1
44.8
51.7
a Solvent: CH3CN; temperature: 333 K; oxidant: tert-butyl hydroperoxide
peroxide; catalyst: Cu-AMM and Ni-AMM. b conversions were measured
after 24 h of the reaction, average of at least three measurements (each
reaction done at least three times under identical conditions). c TOF:
turnover number = moles of substrate converted per mole of metal center
per hour.
better or comparable to the earlier reports.10,31 For the epoxidation
of a-methyl styrene, the conversions are ca. 84.0 and 80.1% with
high selectivity for epoxide formation in the presence of Cu-AMM
and Ni-AMM, respectively.
These catalysts can also effectively epoxidize relatively
bulky cycloalkenes viz. cyclooctene and norbornene. Exo-
epoxynorbornane is the major product in the case of norbornene
epoxidation. Norbornene is converted to 90.3 and 82.5% in the
presence of Cu-AMM and Ni-AMM, respectively, with high
selectivity. The conversion and selectivity over Cu- and Ni-
grafted functionalized mesoporous materials are better than the
previously reported results on their respective metal oxides.32
Further, in our study, high turnover frequencies (TOFs) have been
observed for all the substrates. This could be attributed to the
higher surface area of our Cu-AMM and Ni-AMM catalysts. Thus
we can conclude that our Cu(II) and Ni(II) Schiff-base complexes
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immobilized over functionalized mesoporous silica can be utilized
for selective epoxidation of a wide range of alkenes.14,15,33
point of view, TBHP has been chosen as the oxidant because of its
better reactivity.
Recycling test
Leaching test
We have studied the recycling efficiency of the catalysts i.e. whether
the catalysts can be reused further for several cycles. To test the
catalytic efficiency for further cycles and also to ensure whether
any metal percolates out from the catalyst, one control experiment
has been performed over the catalysts using cyclohexene as the
substrate. Cyclohexene has been chosen as a representative case
for recycling experiments since maximum conversion takes place
for cyclohexene epoxidation when either Cu-AMM or Ni-AMM
is used as the catalyst. After each reaction cycle the catalysts
were recovered by filtration, washed thoroughly with acetonitrile
and then treated with 0.1 M HCl solution in ethanol at 338 K
for 8 h for regeneration and finally dried at 373 K for 2 h.
The catalytic reactions have been carried out following the same
experimental procedure as that with the original catalysts. The
catalytic activities in five consecutive cycles for the epoxidation
of cyclohexene in the presence of Cu-AMM and Ni-AMM have
been plotted in Fig. 8. As seen from the Figure, the catalytic
activity decreases marginally in the successive catalytic cycles.
However, epoxide selectivity remains almost unaltered in these
repetitive reactions, suggesting a high reusability of these Cu/Ni-
grafted functionalized mesoporous materials in liquid phase
partial oxidation catalysis.
Any Cu or Ni species present in solution phase in the reaction mix-
ture can also catalyze the epoxidation reaction of olefins. Thus it
becomes very important to determine whether the catalyst actually
functions in a heterogeneous manner or not. In order to determine
that hot-filtration tests were performed in the epoxidation of
cyclohexene over both the catalysts Cu-AMM and Ni-AMM.
During the catalytic epoxidation reactions, the solid catalysts were
separated from the reaction mixture under hot conditions through
filtration after 4 h. The conversion of cyclohexene after 4 h is ca.
25%. Then the reaction was continued with the filtrate for another
4 h. However, the reaction mixture did not show any further
improvement in the conversion of cyclohexene. On the other
hand, the uninterrupted reactions in the presence of the catalysts
proceeded as usual with further conversion of cyclohexene to
its epoxide. Further, atomic absorption spectroscopy has been
employed to detect/determine the amount of metal that leached
out into the reaction mixture. It has been found that there
is no detectable Cu/Ni in the reaction mixtures. This further
suggested almost no leaching of metal ions takes place during
the course of the reaction from these catalysts and the catalysts
are heterogeneous in nature.
A blank reaction (control reaction) with cyclohexene, in the
absence of any catalyst under similar experimental conditions
has also been performed. A very poor conversion of cyclohexene
(31%) together with epoxide selectivity ca. 29% was observed
after 24 h of reaction. Apart from this, another blank reaction
was performed in the presence of the Schiff base ligand I as
the catalyst under identical conditions where the conversion of
cyclohexene was ca. 35%. These results suggested the catalytic role
played by Cu/Ni-grafted functionalized mesoporous materials
in the epoxidation reactions of alkenes. The reactivity of these
catalysts can be compared with the previous results obtained using
other catalysts, which contain transition metals heterogenized over
silica frameworks. The catalytic systems discussed in this article
are more or less comparable with that of Mn-,39 Mo-,40 Ru-,41 and
V-complexes.42
Fig. 8 Activity of Cu-AMM and Ni-AMM in the repeating cycles. Error
bars are shown for each conversion.
Probable reaction pathways
Porous titanium silicates have been widely used as catalysts for
the epoxidation of olefins using hydrogen peroxide as the oxidant.
During the catalytic reactions, titanium hydroperoxo species is
formed as the active intermediate.30 Copper complexes have been
used as catalysts for the epoxidation of olefins using TBHP or
hydrogen peroxide in homogeneous media and in all the cases
copper–hydroperoxo species have been detected in the reaction
media.15,33 A similar pathway is expected to be functioning also
in this case and accordingly a probable mechanism has been
proposed in Scheme 2. The reaction may proceed via formation
of a copper–peroxo species together with the formation of tert-
butanol, which is detected during the course of the reactions for
all the reactants. When Ni-complexes are used as catalysts in the
epoxidation reactions, the Ni–hydroperoxo species is the active
intermediate in the presence of TBHP.14 Hence, a similar pathway
Effect of different oxidants
Epoxidation reactions have been carried out using different
oxidants like hydrogen peroxide,34 tert-butyl hydroperoxide,35
molecular oxygen,36 sodium hypochlorite,37 iodosylbenzene,38 etc.
The ability to oxidize styrene in the presence of different oxidants
such as hydrogen peroxide, tert-butyl hydroperoxide (TBHP) and
sodium hypochlorite has been examined using Cu-AMM as the
catalyst under identical reaction conditions. The results show 43%,
86% and 37% conversion of styrene using hydrogen peroxide,
TBHP and sodium hypochlorite as the oxidants, respectively. The
yield of the reactions has been measured after 24 h. It is clear from
the results that TBHP serves as the best oxidant, where as other
oxidants show distinctly low activity. Although hydrogen peroxide
would have been the most desired oxidant from an environmental
12516 | Dalton Trans., 2011, 40, 12510–12518
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bound material (representative case).
is expected to be followed for the nickel grafted Ni-AMM sample
in the liquid phase partial oxidation reactions.
Conclusions
We have synthesized two new catalytic systems Cu-AMM and
Ni-AMM, based on functionalized mesoporous silica. The meso-
porous silica surface is at first functionalized with 3-APTES,
and then allowed to undergo a Schiff base condensation reaction
with the dialdehyde, 4-methyl-2,6-diformylphenol. Active copper
or nickel centers are immobilized through the reaction of the
heterogenized ligand supported mesoporous silica with the salts of
Cu(II) or Ni(II). The materials have been thoroughly characterized
by FT-IR and UV-vis diffuse reflectance spectroscopy, XRD, TEM
and N2 adsorption–desorption studies and 13C CP MAS NMR
spectroscopy. Catalytic reactions carried out with Cu-AMM and
Ni-AMM suggest that these materials are highly active catalysts
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Acknowledgements
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for Science Education and Technology, Osaka University for
solid state NMR studies. This work is financially supported
by a Grant-in-Aid for Scientific Research from the Japan So-
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Adaptable and Seamless Technology transfer Program through
target-driven R&D from Japan Science and Technology Agency
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©