Synthesis, characterization and catalytic activity of Mn(III)- and Co(II)-salen complexes immobilized mesoporous alumina

Article abstract of DOI:10.1016/j.molcata.2005.07.005

Mn(III) and Co(II)-schiff base complexes were immobilized over mesoporous alumina through the reaction of mesoporous alumina functionalized 3-aminopropyl triethoxy silane (3-APTES) and salicylic aldehyde via schiff base condensation. The surface propertie

Full text of DOI:10.1016/j.molcata.2005.07.005

Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
Synthesis, characterization and catalytic activity of Mn(III)- and
Co(II)-salen complexes immobilized mesoporous alumina
∗
V.D. Chaube, S. Shylesh, A.P. Singh
Inorganic and Catalysis Division, National Chemical Laboratory, Pune 411008, India
Received 6 May 2005; received in revised form 24 June 2005; accepted 1 July 2005
Available online 10 August 2005
Abstract
Mn(III) and Co(II)–schiff base complexes were immobilized over mesoporous alumina through the reaction of mesoporous alumina func-
tionalized 3-aminopropyl triethoxy silane (3-APTES) and salicylic aldehyde via schiff base condensation. The surface properties of the func-
tionalized catalysts were analyzed by a series of characterization techniques like elemental analysis, PXRD, FTIR, N adsorption–desorption,
2
TG-DTG, DR UV–vis, XPS, etc. PXRD and adsorption–desorption analysis shows that the mesostructure of alumina remains intact after
various modifications, while spectral technique show the successful anchoring of the neat complexes inside the porous alumina support.
The catalytic activity of the functionalized metal-salen complexes examined in the liquid phase oxidation of styrene and cyclohexene shows
that the functionalized salen complexes are more active and selective than the corresponding neat metal complexes. Further, the catalyst
(
Mn-S-NH
which shows that the immobilized metal-salen complexes are stable under the present reaction conditions.
2005 Elsevier B.V. All rights reserved.
2
-MA) was recycled three times in the oxidation of styrene and no major change in the conversion and selectivity is observed,
©
Keywords: Mesoporous alumina; Immobilization; Schiff base complexes; Oxidation
1
. Introduction
cess by the suitable anchoring of various organic pendant
groups [4]. Thus, the synthesis of various organic–inorganic
hybrid mesoporous materials emerges as a potential tool to
anchor various homogenous metal salts/complexes and it is
interesting to probe the stability of such materials during var-
ious modification processes.
Heterogenization of homogenous catalysts has been an
interesting area of research in an academic and in an indus-
trial point of view, as this method can provide an ideal
way for combining the advantages of homogenous catalysts
and simultaneously avoiding its disadvantages like handling
and reusability [5,6]. Mn/Co (salen) complexes have been
reported as efficient catalysts for the epoxidation of various
olefins [7–9]. The increasing interest towards this reaction
brought some authors to develop the heterogenous form of
Mn(III)/Co(II)-salen catalysts and to date, three kinds of
approaches have been generally adopted [10–16]. First, Mn-
salen complexes were supported on polymers for a better
reusability of the materials. For instance, Janssen et al. [17]
have synthesized a dimeric form of (salen) Mn(III) ligand
After the discovery of silica based ordered mesoporous
materials of M41S family by Mobil researchers [1], much
attention had been focused in the synthesis of non-siliceous
mesoporous materials due to its potential applications in
the field of separation science, nano science, catalysis, etc.
Huo et al. [2] and Sayari and Liu [3] demonstrated the syn-
thesis of a variety of non-siliceous mesoporous materials,
among them, only a few exhibit better stability and increased
mesostructural ordering after the removal of the structure
directors which limits its use as a catalyst or catalyst support.
Hence, the synthesis of non-siliceous mesoporous materials
with better stability remains a challenge because compared
to its oxide form, the mesoporous materials possess high sur-
face areas and variable pore sizes and hence can be finely
tuned for specific applications like selective adsorption pro-
∗
Corresponding author. Tel.: +91 20 2589 3761; fax: +91 20 2589 3761.
E-mail address: apsingh@cata.ncl.res.in (A.P. Singh).
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.07.005

8
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V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
and retained this complex in the cross-linked polymer mem-
brane and used as a catalyst for epoxidation reactions, while
Mintolo et al. [18] synthesized the polymer-bound salen
Mn(III) complex by copolymerization of salen complex with
styrene and divinylbenzene. Secondly, the encapsulation of
salen complex was performed by using the so-called, ‘ship in
a bottle’ technique. Sabater et al. [19] synthesized Mn(III)-
salen complex of simple structure inside the supercage of
Y-zeolites and showed that the catalytic activity was simi-
lar to that of chloride complexes in the homogenous phase.
Alternatively, Mn(III)-salen ligands were immobilized by
an ion exchange mechanism like the embedding of selec-
tive homogenous Mn(III) cationic salen complexes into the
pores of mesoporous MCM-41 materials [20]. Hence, novel
routes for the anchoring of these metal complexes over vari-
ous supports having good stability and open channels rely as
a potential way to extend their applicability and reusability.
The present work deals with the synthesis and characteri-
zation of metal (salen) complexes immobilized mesoporous
alumina, performed by the chemical anchoring of the com-
plex to the support surface via suitable modifications. The
heterogenity of the developed materials were verified in the
epoxidation reaction of various olefins and the performance
was compared with the homogenous counterparts. To the best
of our knowledge, this is the first report detailing the immo-
bilization of salen-ligated metals via condensation procedure
over mesoporous alumina.
(99.5%) was added slowly to the gel mixture. The mix-
ture was aged for 24 h at room temperature and then heated
under static conditions at 110 C in a glass jar for 2 days.
◦
The solid material obtained was then filtered, washed with
◦
ethanol and dried at 100 C for 4–5 h. Finally, the material
◦
was calcined at a temperature of 450 C with a tempera-
ture ramp of 1 C/min from room temperature to the final
◦
temperature. The calcination atmosphere was nitrogen dur-
◦
ing the earlier stages (<200 C) and air at final tempera-
tures.
2.1.2. Preparation of 3-APTES functionalized
mesoporous alumina (NH -MA)
2
To a suspension of 10 g of calcined mesoporous alumina
in 50 ml dry toluene, 2.68 g of 3-aminopropyl triethoxy silane
was added slowly and heated to reflux with continuous stir-
ringfor8 hundernitrogenatmospheres(Scheme1). Thepow-
dery sample containing amino groups was filtered, washed
with acetone and then soxhlet extracted using a solution mix-
ture of diethyl ether and dichloromethane (1:1) for 24 h and
dried under vacuum. Elemental analysis shows that 0.89% of
nitrogen gets introduced into the mesoporous support, which
indicates that ∼52% of 3-APTES was immobilized per gram
of mesoporous alumina.
2.1.3. Preparation of neat Co(II) and Mn(III) complexes
Cobalt and manganese (salen) precursors were prepared
according to the following procedure. The appropriate metal
(
cobalt and manganese) acetate hydrate (6.0 mmol) in ethanol
(10 ml) was added slowly to a solution of salicyl aldehyde
2.94 g, 24.0 mmol) in ethanol (40 ml). The solutions were
2
. Experimental
(
2
.1. Synthesis
stirred at room temperature for 24 h and their subsequent con-
centration leads to precipitation of the corresponding metal
(salen) precursor (Scheme 2). The solid product obtained was
washed with copious amounts of chloroform and dried in vac-
The materials used for the synthesis were aluminium iso-
propoxide (Aldrich), lauric acid (Loba Chemie), 1-propanol
Merck), 3-aminopropyl triethoxy silane (3-APTES, Lan-
◦
(
uum at 100 C for 24 h. Elemental analysis data obtained after
caster), salicyl aldehyde (Merck), cobalt(II) acetate and man-
ganese(III) acetate (Aldrich).
purification are as follows; theoretical value (actual value) of
C (%) 56.56 (58.18), H (%) 3.36 (4.11) for Mn complex,
while Co complex shows C (%) 58.18 (56.49) and H (%)
3.32 (3.01).
2
.1.1. Preparation of mesoporous alumina (MA)
Non-siliceous mesoporous alumina was synthesized
according to the following procedure at a temperature of
2.1.4. Preparation of salen Co(II) and salen Mn(III)
complexes immobilized on modified mesoporous alumina
(Co/Mn-S-NH2-MA).
◦
1
10 C using carboxylic acid (lauric acid) as surfactant
[21]. The composition of gel mixture; Al-isopropoxide:lauric
acid:1-propanolis1:0.03:26. Typically, analuminumhydrox-
ide suspension was prepared by the hydrolysis of 43.8 g of
Al-isopropoxide with 10.3 g of deionised water in 275 g of
The synthesized neat complex, viz. (bis-salicyl alde-
hyde) M, has been chemically anchored to the aminopropyl-
modified alumina support, using a procedure as shown in
Scheme 3 [22]. The material was then soxhlet extracted, using
1
-propanol (99%). After stirring for 1 h, 10.8 g of lauric acid
Scheme 1.

V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
81
Scheme 2.
Scheme 3.
a mixture of CHCl3–EtOH mixture for 12 h to remove the
unreacted organic residue from the mixture.
full width at half maximum of the 4f7/2 core level of the
gold surface. The error in all the binding energy (BE) values
was within ± 0.1 eV. The correction in binding energy was
performed by using the C1s peak of carbon at 284.9 eV, as
reference.
2
.2. Characterization
Analysis of the organic material incorporated on the solid
mesoporous material was carried out by using EA1108 Ele-
mental Analyzer (Carlo Erba Instruments). X-ray diffraction
measurements were carried out on a Rigaku Miniflex diffrac-
2.3. Catalytic measurements
In a typical reaction, TBHP (0.45 g, 5 mmol) was added
to a solution of styrene (0.52 g, 5 mmol) in acetonitrile (6 g)
containing 0.2 g of catalyst (pre-activated at 100 C for 2 h).
◦
tometer using Cu K␣ radiation at a scan rate of 3 /min from
◦
◦
◦
1
.5 to 50 . FTIR spectra of the solid samples were taken
−
1
◦
in the range of 4000–400 cm on a Shimadzu FTIR 8201
instrument by diffuse reflectance scanning disc technique.
The specific surface area, total pore volume and average
pore diameter were measured by N2 adsorption–desorption
method using NOVA 1200 (Quanta chrome). The samples
The reaction mixture was magnetically stirred at 60 C in a
silicone oil bath. Samples of the reaction mixture were taken
periodically and analyzed by gas-chromatograph (HP 6890)
equipped with a flame ionization detector (FID) and a cap-
illary column (5 ␮m thick cross-linked methyl silicone gum,
0.2 m × 50 m). The products were further identified by GC-
MS analysis (Shimadzu 2000 A).
◦
were activated at 200 C for 3 h under vacuum and then
the adsorption–desorption was conducted by passing nitro-
gen into the sample, which was kept under liquid nitrogen.
Pore size distribution (PSD) was obtained by applying the
BJH pore analysis applied to the adsorption branch of the
nitrogen adsorption–desorption isotherm. Thermogravimet-
ric and differential thermogravimetric (TG-DTA) analysis
of the neat and immobilized Co-salen complexes and Mn-
salen complexes was recorded on a Rheometric Scientific
3. Results and discussion
Fig. 1 depicts the XRD patterns of as-synthesized,
calcined, aminopropyl-modified and complex functional-
ized mesoporous alumina. The diffraction patterns of as-
synthesized and calcined forms of mesoporous alumina
shows a broad reflection at a 2θ value of 2.6, assigned to
the (1 0 0) reflection of the hexagonal lattice. Even though
the XRD patterns are devoid from any higher order reflec-
tions, the intensity of the characteristic d1 0 0 peak of calcined
mesoporous alumina is quite pronounced andis similarto ear-
lier reports over non-siliceous mesoporous materials [23,24].
The absence of higher order reflections indicates that the
pore walls are amorphous or there is a lack of correlation
between the structures of adjacent pores. Further, after mod-
(
STA 1500) analyzer. Diffuse reflectance UV–vis spectra
were recorded in the range 200–600 nm with a Shimadzu UV-
101 PC spectrometer equipped with a diffuse reflectance
2
attachment, using BaSO4 as reference. XPS spectra were
recorded on a VG micortech multilab-ESCA 3000 spectrom-
eter equipped with a twin anode of Al and Mg. All the
measurements are made on powder samples using Mg K␣
X-ray at room temperature. Base pressure in the analysis
−
10
chamber was 4 × 10
Torr. The overall energy resolution
of the instrument is better than 0.7 eV, determined from the

8
2
V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
ing of the hydrogen bonded internal alumina groups. How-
ever, after amine functionalization, new bands appeared at
−
1
2
939 and 2879 cm which are assigned to the asymmetric
and symmetric stretching vibrations of CH2 groups and thus
the presence of these bands on NH2-MA sample shows that
amino propyl groups get anchored on the inner wall surface
of mesoporous alumina [25,26]. Further, the presence of band
−
1
at 3300 cm in the NH2-MA sample, assigned to the N H
stretching vibrations of the propyl amino groups, confirms
the successful functionalization of 3-APTES groups on the
alumina surface (Fig. 2C). After complex immobilization, the
band due to N H vibration gets disappeared with the forma-
−
1
tion of a new band at 1630 cm , which is the characteristic
stretching vibration of C N band [27,28]. These two results
clearly indicate the anchoring of Co and Mn complexes over
the amino groups of modified mesoporous alumina surface,
as shown in Scheme 3. Further, the immobilized complex
materials show the characteristic peaks of the neat com-
plexes and thereby confirm that the complex species gets
anchored on the pendant organic groups of the mesoporous
support.
Fig. 1. XRD patterns of: (a) as-synthesized mesoporous alumina; (b) cal-
cined mesoporous alumina; (c) aminopropyl-modified mesoporous alumina;
(d) Co-salen immobilized mesoporous alumina.
ifications, the intensity of the characteristic d1 0 0 reflection
gets decreased while the mesostructure remains intact. These
results show that the anchoring of the functionalized moieties
It is well known that the introduction of homogenous cat-
alysts or metals on porous supports shows a decrease in its
specific surface area and pore volume. The support, meso-
(3-APTES and further anchoring of salen complexes), inside
2
−1
the pore channels of mesoporous alumina, had not damaged
the overall structure of the support material.
porous alumina, shows a high surface area of 450 m g
−
1
and pore volume of 0.4212 ccg ; after metal complex func-
2
−1
The IR spectra of as-synthesized, calcined, 3-APTES-
modified and Co(II)- and Mn(III)-salen immobilized meso-
tionalization, the surface area gets reduced to 223 m g for
2
−1
Co-S-NH -MA and 234 m g for Mn-S-NH -MA. Thus,
2
2
−
1
porous alumina in the 4000–400 cm range are presented in
Fig. 2A and B. The as-synthesized samples exhibit bands at
the decrease in mesoporous volume (∼52%) and surface
area (∼48%) after metal immobilization is indicative of the
grafting of complex inside the channels of mesoporous alu-
mina and a detailed list of surface area, pore volume and
pore diameter of support and modified samples are given in
Table 1. It is clear from table that even though silylation pro-
cedures changed the textural properties of the mesoporous
−
1
2
930 and 2870 cm corresponding to C H vibrations of the
surfactant molecules, but in the case of calcined samples, the
peaks disappeared due to removal of the template molecules.
In detail, the spectra of calcined alumina show peaks in the
−
1
range of 3600–3200 cm , attributed to the hydroxyl stretch-
Fig. 2. FTIR spectra of: (A) Co-salen containing mesoporous alumina and (B) Mn-salen containing mesoporous alumina, where (a) as-synthesized mesoporous
alumina; (b) calcined mesoporous alumina; (c) aminopropyl-modified mesoporous alumina; (d) metal-salen immobilized mesoporous alumina; (e) neat metal
−
1
complex, while (C) shows the zoom over 2700–3700 cm region of: (a) calcined mesoporous alumina; (b) aminopropyl-modified mesoporous alumina; (c)
metal-salen immobilized mesoporous alumina.

V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
83
Table 1
Textural properties of mesoporous alumina and functionalized samples
2
−1
−1
Pore volume (ml g )
Sample
d1 0 0 (nm)
SBET (m g
)
Pore diameter (nm)
MA
NH2-MA
Mn-S-NH2-MA
Co-S-NH2-MA
3.17
3.23
3.20
3.33
450
359
234
223
0.4212
0.2311
0.1821
0.1710
4.43
n.d.
3.76
3.89
n.d., not determined.
material, the decrease is more prominent after complex
immobilization and are due to the presence of bulkier organic
moieties inside the pore channel of the support. These results
reveal that the anchoring of complex groups had occurred
inside the pore channels of mesoporous alumina. Further
the adsorption–desorption isotherms of mesoporous alumina
and metal complex immobilized mesoporous alumina are of
Type IV, according to the IUPAC classification and its steep
condensation behaviour indicates the existence of uniformly
sized mesopores (Fig. 3). In detail, the alumina support shows
an inflection in the P/Po ∼ 0.5 range, while after complex
immobilization, the P/Po value changed to a lower value of
attributes to the decomposition of template molecules from
the pore channels. A complete decomposition of the sur-
factant groups below 400 C indicates that the calcination
◦
procedure opted for the removal of structure directors was
optimum (see Section 2). In accordance with that, the DTG
spectrum of calcined alumina shows only a sharp weight loss
◦
below ∼100 C, without any weight losses at higher tem-
perature regions. On the contrary, the TG/DTG spectrum of
◦
NH2-MA shows two steps of weight loss, one below ∼100 C
◦
and the other in the region of 340–380 C. As mentioned, the
former weight loss is due to the desorption of physisorbed
water, while the second weight loss is attributed to the loss
of organo propyl amino fragments [26]. The DTA results fur-
ther confirm the above results that a strong exothermic peak
∼
0.45, indicative of some sort of structural damage to the
material after modifications and are consistent with the XRD
results. Moreover, the BJH pore size distribution analysis
◦
is observed around ∼360 C for the decomposition of the
shows that the material posses uniformly sized mesopores
˚
propyl groups from the alumina surface. The decomposition
profiles of homogenous metal complexes get completed at
(
see inset of Fig. 3) centered at ca. ∼44 A (Fig. 3A) for alu-
◦
˚
mina support and ca. ∼38 A (Fig. 3B) for the metal complex
∼468 and ∼423 C for Co- and Mn-salen complexes, respec-
functionalized samples.
tively, with the residues amounting to cobalt and manganese
oxides. Interestingly, after immobilization, the final decom-
Thermal analysis had been used to monitor the decompo-
sition profiles of the neat as well as anchored metal complexes
and the results obtained are depicted in Fig. 4. As-synthesized
mesoporous alumina shows two steps of weight loss, when
◦
positiontemperaturesgetsincreasedupto∼626and∼619 C
for Co-S-NH2-MA and Mn-S-NH2-MA complexes, respec-
tively, due to the mutual stabilization of propyl amino groups
and the complexes and are indirect proofs for the entrapment
of complex moieties inside the pore channels of mesoporous
alumina [29].
◦
heated under airflow, one at ∼100 C and the other at
◦
∼
400 C. The former weight loss is usually attributed to the
loss of physisorbed water molecules, while the latter loss
Fig. 3. N2 adsorption–desorption isotherms and pore size distributions (inset) of: (A) calcined mesoporous alumina and (B) Co-salen immobilized mesoporous
alumina.

8
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V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
Fig. 4. Thermogravimetric (A) and differential thermogravimetric (B) results of (a) as-synthesized mesoporous alumina; (b) calcined mesoporous alumina;
c) aminopropyl-modified alumina; (d) Mn immobilized mesoporous alumina; (e) Co immobilized mesoporous alumina; (f) neat Mn complex; (g) neat Co
complex.
(
The UV–vis electronic spectra of mesoporous alumina
MA), 3-APTES-modified mesoporous alumina (NH2-MA),
to the ligand to metal charge transfer transition bands, akin
to related metal (salen) compounds described in the litera-
ture [29–31]. After immobilization, for Co (salen) sample,
the two distinct bands observed in the neat complex gets
overlapped to form a broad band centered around ∼425 nm,
whereas for Mn (salen) sample, the spectrum exhibits multi-
ple bands at ∼280, ∼320 and ∼430 nm. Even though a proper
(
Co(II)/Mn(III)-salen immobilized mesoporous alumina (Co-
S-NH2-MA and Mn-S-NH2-MA) and homogenous adducts
were presented in Fig. 5A and B. The UV–vis spectra of neat
Co complex exhibits two broad bands at ∼280 and ∼390 nm,
while Mn complex exhibits a broad band at ∼415 nm ascribed
Fig. 5. DR UV–vis spectra of: (A) Co-salen containing mesoporous alumina and (B) Mn-salen containing mesoporous alumina, where (a) calcined mesoporous
alumina; (b) aminopropyl-modified mesoporous alumina; (c) metal-salen immobilized mesoporous alumina; (d) neat metal complex.

V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
85
comparison between the neat and immobilized complex is
inappropriate, due to the different coordination spheres, as
an indirect evidence it can be concluded that the occur-
rence of the band at higher wave numbers and its splitting
may be due to the different environment and donor sites of
the heterogenized metal complexes than the neat complexes.
These results are complementary to the data obtained from IR
spectra that the immobilization of Co(II)/Mn(III) complexes
occurred through schiff base condensation with the amino
propyl groups.
increased chemical shift of ∼1 eV for the immobilized salen
complex than the neat complex attributes to the differences
in the coordination environment of metal inside the confined
pore channels of mesoporous alumina than under neat condi-
tions. Similarly, cobalt complex also shows an experimental
trend as in Mn complexes and matches well with the reported
binding energy values of Co 2p3/2 peak, viz. 779.1 eV for neat
cobalt complex and 779.9 eV for immobilized Co-salen com-
plex [35].
X-ray photoelectron spectroscopy (XPS) is a powerful
technique used to investigate the electronic properties of the
species formed on the surface. As the electronic environ-
ment, e.g. oxidation state and/or spin multiplicity influences
the binding energy of the core electrons of the metal, XPS is
extensively used to attain detailed information about the state
of metal species on the surfaces. Fig. 6A and B shows the XPS
spectra obtained for pure homogenous adduct and Co(II)- and
Mn(III)-salen immobilized mesoporous alumina. Briggs and
Seah [32] proposed that the intensity ratio and the energy dif-
ference between the two signals obtained for electron ejected
3.1. Catalytic reactions
Structural characterization results show that the metal
complex is firmly held inside the pore channels of meso-
porous alumina and hence the present materials are applied
in the liquid phase epoxidation reaction of styrene and cyclo-
hexene. The results obtained from the epoxidation reaction
of neat as well as immobilized complexes are presented in
Tables 2 and 3 and the kinetic profiles for the activity of neat
and immobilized complexes under both reactions are further
shown in Fig. 7. From Table 2 and Fig. 7, it is clear that
mesoporous alumina functionalized metal complexes show
an enhanced activity (calculated in terms of turn over num-
ber, TON)andselectivitytowardsthedesiredepoxideproduct
than the neat metal complexes. Since the support alumina
had a high surface area and moderate pore size, we believe
that the confined environment of the metal complex avoids
undesirable side reactions like the over oxidation process, as
from the p1 and p levels can be used as a tool to investigate
the spin multiplicity and therefore to the electronic proper-
ties of the cobalt and manganese compounds [33]. The neat
manganese complex exhibits Mn 2p3/2 core level peak at a
binding energy of ∼642.5 eV, while the immobilized Mn-
salen complex shows a binding energy at ∼643.3 eV and are
in accordance with earlier literature data [34]. The observed
/2
3/2
Table 2
Oxidation of styrene by neat and immobilized metal-salen catalysts
Catalyst
M (%)a
Conversion (%)
Selectivity (%)
Epoxide
TONd
Benz.b
Othersc
MA
–
6.3
29.5
51.4
38.4
55.3
3.9
11.8
27.2
9.8
95.2
79
70.9
71.6
67.9
0.87
9.2
1.9
18.6
3.9
–
Neat Mn (salen)
Mn-S-NH2-MA
Neat Co (salen)
Co-S-NH2-MA
0.69
0.57
0.87
0.63
22.4
47.6
24.6
50.0
28.2
◦
Reaction conditions: styrene, 0.52 g; TBHP (70%), 0.45 g; CH3CN, 6 g; T, 60 C; t, 12 h; catalyst amount, 0.2 g.
a
M, metal content determined from ICP analysis.
Benz., benzaldehyde.
Others, mainly phenyl acetaldehyde and benzoic acid.
b
c
d
TON, turn over number, moles of substrate converted per mole of metal.
Table 3
Oxidation of cyclohexene by neat and immobilized metal-salen catalysts
Catalyst
M (%)a
Conversion (%)
Selectivity (%)
Epoxide
TONc
Oneb
Others
MA
–
5.7
26.7
48.5
40.4
56.3
12.6
13.5
19.5
20.2
27.6
47.1
70.5
69.8
65.2
58.8
40.3
16
10.7
14.6
13.6
25.2
25.2
56.8
33.0
63.3
Neat Mn (salen)
Mn-S-NH2-MA
Neat Co (salen)
Co-S-NH2-MA
0.69
0.57
0.87
0.63
◦
Reaction conditions: cyclohexene, 0.41 g; TBHP (70%), 0.45 g; CH3CN, 6 g; T, 60 C; t, 12 h; catalyst amount: 0.2 g.
a
M, metal content determined from ICP analysis.
One, cyclohexenone.
b
c
TON, turn over number, moles of substrate converted per mole of metal.

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V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
Fig. 6. XPS spectra of: (A) Mn-salen containing mesoporous alumina and (B) Co-salen containing mesoporous alumina, where (a) neat metal complex and (b)
immobilized metal complex.
observed with the neat complexes, leading to better activ-
ity as well as selectivity to epoxides [25,31]. However, as
mentioned earlier, it is inappropriate to compare the cat-
alytic results under neat as well the heterogenized conditions,
since the coordination sphere of both complexes is different.
Among olefins, styrene conversion is higher than cyclohex-
ene, which may be due to the comparatively easier side
chain oxidation than the reactions in the aromatic ring. Inter-
estingly, even though benzaldehyde was the major product
obtained during styrene epoxidation, kinetic studies revealed
that the formation of phenyl acetaldehyde, an isomerized
product of styrene epoxide, is not formed over immobilized
catalysts while under neat catalysts, a significant amount of
its formation is noted. Hence, it is reasonable to assume that
the reaction proceeds under different mechanisms over the
homogenous complexes and heterogenous catalysts. Further,
in order to ascertain whether the activity of the immobilized
catalysts arise from true heterogenous catalysts, the stabil-
ity of metal complexes were probed by performing typical
leaching studies. For that, the catalyst was separated from
the reaction mixture after a definite period of time (viz. 2 h)
and the hot filtrate was probed for further reactions. Interest-
ingly, it was found that the conversion rate essentially gets
terminated after the removal of the catalyst sample, which
confirms that the present anchored metal complexes are sta-
ble in the reaction medium and are not prone for leaching.
Fig. 7. Reaction kinetic profiles of neat and immobilized Co- and Mn-salen complex in the oxidation reaction of: (A) cyclohexene and (B) styrene, where (ꢀ)
neat Mn complex; (᭹) immobilized Mn complex; (ꢁ) neat Co complex; (ꢂ) immobilized Co complex.

V.D. Chaube et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 79–87
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87
Hence, considering the increased stability of the materials, it
[
[
[
[
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is reasonable that between the two complex species shown in
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the reaction of styrene without significant differences in the
conversion (51.4% changed to 46.4%) and selectivity (27.2%
changed to 25.1%) after third run and thereby, relies as novel
heterogenous catalysts for various (ep)oxidation reactions.
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4
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The immobilization of manganese and cobalt complexes
[
[
over mesoporous alumina, modified previously by 3-APTES,
has been achieved by schiff base condensation reaction. Dif-
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selected epoxidation reactions. Moreover, these new catalysts
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We thank our reviewers for constructive suggestions. The
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Mr. Patel for XPS results and Ms. V. Samuel for XRD results.
V.D.C and S.S thanks CSIR, India for senior research fellow-
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