Solvent-free oxidation of alcohols by hydrogen peroxide over chromium Schiff base complexes immobilized on MCM-41

Article abstract of DOI:10.1007/s11243-009-9316-7

A series of chromium(III) Schiff base complexes immobilized on MCM-41 were prepared and characterized by various physicochemical and spectroscopic methods. The complexes were used for the selective oxidation of alcohols by 30% hydrogen peroxide without any organic solvent, phase transfer catalyst or additive. The immobilized complexes proved to be effective catalysts and generally exhibited much higher catalytic performance than their corresponding homogeneous analogs. The catalytic performance of the immobilized complexes was also found to be closely related to the Schiff base ligands used. Under the optimal reaction conditions, secondary alcohols, cyclic alcohols and benzyl alcohol were prevailingly oxidized to their corresponding ketones or aldehydes. Springer Science+Business Media B.V. 2009.

Full text of DOI:10.1007/s11243-009-9316-7

Transition Met Chem (2010) 35:213–220
DOI 10.1007/s11243-009-9316-7
Solvent-free oxidation of alcohols by hydrogen peroxide over
chromium Schiff base complexes immobilized on MCM-41
•
•
•
Xiaoli Wang Gongde Wu Wei Wei
Yuhan Sun
Received: 17 August 2009 / Accepted: 20 November 2009 / Published online: 11 December 2009
Ó Springer Science+Business Media B.V. 2009
Abstract A series of chromium(III) Schiff base com-
plexes immobilized on MCM-41 were prepared and char-
acterized by various physicochemical and spectroscopic
methods. The complexes were used for the selective oxi-
dation of alcohols by 30% hydrogen peroxide without any
organic solvent, phase transfer catalyst or additive. The
immobilized complexes proved to be effective catalysts
and generally exhibited much higher catalytic performance
than their corresponding homogeneous analogs. The cata-
lytic performance of the immobilized complexes was also
found to be closely related to the Schiff base ligands used.
Under the optimal reaction conditions, secondary alcohols,
cyclic alcohols and benzyl alcohol were prevailingly oxi-
dized to their corresponding ketones or aldehydes.
Therefore, clean, inexpensive oxidants such as molecular
oxygen or hydrogen peroxide have increasingly been
adopted in alternative routes. However, the addition of
dioxygen to hydrocarbons generally requires high activation
energies due to the high energy needed to break the O–O
double bond and the spin mismatch between the ground state
of O2 (triplet) and organic reductants (singlet) [3–5]. This
greatly enhances the potential of hydrogen peroxide, espe-
cially at low concentrations as an oxidant. Nevertheless,
hydrogen peroxide can be an ideal waste-avoiding oxidant
only when it is used in a controlled manner without the need
for organic solvents and other toxic compounds. Therefore,
in recent years, much work has been done to develop sol-
vent-free oxidation systems with hydrogen peroxide.
Catalytic oxidation using transition metal Schiff base
complexes is attractive, in view of their accessible syn-
thesis and versatile coordination structures, and various
examples of such transition metal complexes have been
reported [6–9]. Compared to homogeneous complexes,
heterogeneous systems have the inherent advantages of
easy separation and better handling properties, so the het-
erogenization of known homogeneous catalysts has
attracted much attention [10–15]. A popular approach is to
immobilize the homogeneous catalyst on the support
through a surface-bound tether containing a functional
group. This methodology has been employed successfully
in the functionalization of various traditional types of silica
supports [16–18]. More recently, MCM-41 materials have
been used extensively as ideal inorganic supports due to
their high surface area and sharply distributed pore
dimensions in the mesoporous ranges, which allow easy
access of reactants to the active sites and also functional-
ization of bulky catalytic sites within the pores [13, 19, 20].
In the present investigation, a series of conventional
chromium(III) Schiff base complexes were immobilized on
Introduction
Selective oxidation of alcohols to their corresponding
aldehydes and ketones is a key transformation in organic
synthesis [1]. Traditional oxidation methods have generally
involved the use of stoichiometric amounts of inorganic
oxidants such as chromium chloride and potassium per-
manganate, and generated large quantities of waste [2].
X. Wang (&) Á G. Wu (&)
Department of Environment and Technology, Nanjing Institute
of Technology, Nanjing 211167, People’s Republic of China
e-mail: wangxiaoli212@njit.edu.cn; wangxiaoli212@126.com
G. Wu
e-mail: wugongde@njit.edu.cn
W. Wei Á Y. Sun
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan 030001,
People’s Republic of China
123

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Transition Met Chem (2010) 35:213–220
MCM-41, and the resulting immobilized complexes were
used for the selective oxidation of alcohols by 30%
hydrogen peroxide. The role of different Schiff base
ligands in the catalytic performance of the immobilized
catalysts has been explored.
O
N
N
R
O
N
O
N
O
Si
Cr
R
O
O
NH2
Cr
Cl
O
1
2
Experimental
Scheme 1 Homogeneous chromium (III) Schiff base complexes and
their immobilized analogs were prepared as described in ‘‘Experimen-
tal’’ section. L1: R = –CH2CH2–; L2: R = –CH2CHNHCHCH2–;
3-aminopropyltriethoxysilane (APTES), salicylaldehyde,
ethylenediamine, diethyltriamine, 1,2-diamino-cyclohex-
ane (chair conformation), o-phenylenediamine and bina-
phthyldiamine were obtained from Aldrich, and other
chemicals were obtained from the Shanghai Chemical
Reagent Company. All raw materials were analytical
reagents and used as received.
L3: R =
; L4: R =
; L5: R =
and filtered through a fine-porous filter paper. The collected
powder was washed overnight in a Soxhlet extractor using
absolute ethanol and acetonitrile with an ethanol/acetoni-
trile molar ratio of 1:1 to remove free ligands and homo-
geneous complexes adsorbed on the external surface of
support. The solid sample was further stirred in 0.01 mol/L
NaCl solution for 24 h to remove the remaining uncoor-
dinated metal ions. Finally, the mixture was filtered, and
the obtained solid was dried in air at 80 °C for 10 h. By
changing the diamine, a series of homogeneous chromium
Schiff base complexes (abbreviated as 1-L1–L5) and their
corresponding immobilized analogs (abbreviated as 2-L1–
L5) were prepared, and their structures are depicted in
Scheme 1. For comparison, the homogeneous complex 1-L1
was directly immobilized on MCM-41 without a linker, and
the resulting sample was denoted as 1-L1-MCM-41. The
synthesis procedure of 1-L1-MCM-41 was similar to that of
immobilized complex 2-L1, except MCM-41 was used as
support in place of APTES-MCM-41.
Preparation of the catalyst
The synthetic procedure for pure siliceous MCM-41 (Si-
MCM-41) has been described previously [21]. 3-Amino-
propyltriethoxysilane-modified MCM-41 (APTES-MCM-
41) was prepared according to a known procedure [22].
In order to prevent decomposition of the complexes
during immobilization, as reported in the literature [23], the
homogeneous Schiff base complexes were prepared first,
by the condensation of salicylaldehyde and the appropriate
diamines in 2:1 molar ratio. In a typical synthesis, diamine
(10 mmol) was suspended in absolute ethanol (60 mL). A
solution of salicaldehyde (20 mmol) in absolute ethanol
(60 mL) was added dropwise with vigorous stirring to the
suspension. The resulting mixture was refluxed for 3 h,
then neutralized with the appropriate amount of NaOH to
adjust the pH close to 7, prior to the addition of solid
CrCl3Á6H2O (12 mmol). After refluxing for another 6 h, the
mixture was cooled and filtered under reduced pressure.
The collected solid was washed with absolute ethanol and
dried in air, then recrystallized from absolute ethanol.
The homogeneous complexes were bound to MCM-41
by one axially coordinating propyl chain spacer, which
allowed the maximum conformational mobility of the
active complexes. Since the chromium centers in the
homogeneous complexes were either five or six coordinate
with a substitutionally labile water molecule, immobiliza-
tion of the homogeneous complexes onto APTES-MCM-41
could be readily achieved through coordination of the
chromium to a terminal NH2 group of the surface-bound
tether via simple addition or ligand substitution reactions
[16]. Typically, to a suspension of freshly dried APTES-
MCM-41 (2 g) in dry toluene (50 mL), a solution of the as-
prepared homogeneous complex (8 mmol) in dry toluene
(50 mL) was added. The mixture was vigorously stirred
under reflux for 6 h. The resulting suspension was cooled
Characterization procedures
Powder X-ray diffraction (XRD) experiments were per-
formed at room temperature on a Rigaku D Max III VC
˚
instrument with Ni-filtered Cu Ka radiation (k = 1.5404 A)
at 40 kV and 30 mA, in the 2h range of 1–9° at a scan rate
of 1°/min. The specific surface area, total pore volume and
average pore diameter were measured by N2 adsorption–
desorption methods using a Micromeritics ASAP-2000
instrument (Norcross, GA). The samples were outgassed at
80 °C and 10-4 Pa overnight, and then the adsorption–
desorption isotherms were recorded by passing nitrogen
into the sample, which was kept under liquid nitrogen.
Carbon and nitrogen contents were determined using a
Vario EL analyzer with a relative error of less than 0.1%.
The chromium contents in the samples were measured by
inductively coupled plasma (ICP) emission spectroscopy
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Transition Met Chem (2010) 35:213–220
215
(Perkin-Elmer ICP OPTIMA-3000). FTIR spectra of all
samples were recorded in KBr disks at room temperature on
a Shimadzu (model 8201 PC) spectrophotometer. UV–vis
spectra were recorded on a Shimadzu (model 2501 PC)
spectrophotometer using optical-grade BaSO4 powder as
reference.
Catalytic performance
The oxidation of alcohols was carried out in a 100-mL
Teflon-lined and magnetically stirred autoclave. Typically,
a mixture of substrate and catalyst in the required molar
ratio was stirred for about 10 min, then the corresponding
amount of H2O2 was added. The autoclave was heated to
the desired temperature within 5 min. After running the
reaction for the desired time, the products were filtered and
analyzed using an Agilent GC-6890N gas chromatograph
with a capillary 30-m HP-5 column and FID detector.
Fig. 1 FTIR spectra of immobilized complex samples
the different ligands used, assigned to the C=N stretching
vibration of the imine groups. In addition, the two or three
bands in the low frequency region (450–650 cm-1) indi-
cated the successful coordination of phenolic oxygen and
azomethine nitrogen to the transition metal [24–26]. In the
FTIR spectra of the immobilized complexes, apart from the
bands in the overlapping region of the silica backbone, the
other bands of the homogeneous complexes were all
clearly observed though there were some marginal shifts in
the positions of these bands due to the immobilization. This
indicates that the quadridentate coordination structures of
the homogeneous complexes all survived the immobiliza-
tion procedure.
Results and discussion
Characterization of the catalysts
Chemical analysis revealed that a small amount of carbon
and nitrogen was present in the parent MCM-41 due to the
remnant surfactant. Much increased carbon and nitrogen
contents as well as a C/N molar ratio of 3.06 (the expected
value is 3) were observed in APTES-MCM-41, confirming
its successful preparation. The elemental contents in all
five homogeneous complexes were quite comparable with
the calculated values, indicating that the as-prepared
homogeneous complexes held the expected elemental
composition. Simultaneously, the N/Cr molar ratios of
all the immobilized complexes were well consistent with
the expected values; moreover, there was no significant
difference in the elemental composition of MCM-41 and
1-L1-MCM-41. This indicated that the homogeneous
complexes had been successfully immobilized on MCM-41
through the linker and that the amount of ligands and
homogeneous complexes adsorbed on the surface of the
support was negligible due to the Soxhlet extraction step.
Furthermore, the deviation of the observed N/Cr molar
ratio between the homogeneous chromium complexes and
their immobilized analogs provided further supporting
evidence that the immobilization was achieved through
coordination of chromium to a terminal NH2 group of the
surface-bound tether.
The UV–vis spectra of the homogeneous complexes all
displayed one characteristic peak at about 380–450 nm
typical of the metal–ligand band and one broad peak at
about 570–610 nm associated with d-d transitions [27, 28].
The UV–vis spectra of all the immobilized complexes were
quite similar to those of their corresponding homogeneous
complexes (see Fig. 2), further confirming that the immo-
bilized chromium Schiff base complexes were successfully
prepared.
The mesostructures of the immobilized materials were
confirmed by the powder X-ray diffraction and N2
adsorption–desorption experiments. The powder XRD
pattern of fresh Si-MCM-41 showed a very intense
reflection (d100) and two additional much weaker reflec-
tions (d110, d200) (see Fig. 3), which could be indexed to
hexagonal lattice. In the powder XRD patterns of the
immobilized complexes, the intense reflections (d100) were
still present, suggesting that the hexagonal pore arrange-
ment of their parent material was intact [19, 20]. Simul-
taneously, some loss of intensity of this peak (d100) was
FTIR spectra of the resulting immobilized complexes
are shown in Fig. 1, and the diagnostic bands are listed in
Table 1. The successful preparation of homogeneous
complexes could be confirmed by the presence of the
characteristic bands at 1,600–1,620 cm-1, depending on
123

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Transition Met Chem (2010) 35:213–220
Spectroscopic data
Table 1 Elemental compositions and spectroscopic data for the complex samples
Samples
Found (calcd.) %
C
N
Cr
C/Na
N/Cra
FTIR (cm-1
)
kmax (nm)
MCM-41
APTES-MCM-41
1-L1-MCM-41
1-L1
2.0
0.1
0
23.8
–
–
–
–
–
5.9
2.3
0
3.1 (3.0)
–
–
2.1
0.1
0
21.9
–
–
–
54.3 (54.3)
60.0 (60.0)
58.9 (58.9)
59.8 (59.8)
70.6 (70.6)
2.5
7.9 (7.9)
11.7 (11.7)
6.9 (6.9)
7.0 (7.0)
4.9 (4.9)
1.4
14.7 (14.7)
14.4 (14.4)
12.8 (12.8)
13.0 (13.0)
9.0 (9.0)
1.7
2.0 (2.0)
3.0 (3.0)
2.0 (2.0)
2.0 (2.0)
2.0 (2.0)
3.0 (3.0)
4.0 (4.0)
3.0 (3.0)
3.0 (3.0)
3.0 (3.0)
1,602, 585, 465
1,620, 612, 522
1,620, 602, 535
1,610, 539, 462
1,607, 608, 545
1,602, 585, 465
1,620, 612, 522
1,620, 602, 535
1,610
390, 600
390, 609
416, 606
446, 575
380, 595
390, 600
390, 610
417, 606
447, 578
380, 595
1-L2
–
1-L3
–
1-L4
–
1-L5
–
2-L1
6.4
5.3
7.5
7.7
12.3
2-L2
1.9
1.7
1.6
2-L3
3.1
1.5
1.8
2-L4
3.0
1.4
1.7
2-L5
4.3
1.2
1.5
1,609, 609, 545
a
Molar ratio
Fig. 2 UV–vis spectra of immobilized complex samples
Fig. 3 XRD patterns of MCM-41 and immobilized complex samples
observed in comparison with MCM-41, and the peaks
(d110) and (d200) were absent, which might be attributed to
a decrease in local order, such as variations in the wall
thickness as previously mentioned by Lim et al. [29].
The N2 adsorption–desorption isotherms of 2-L1, 2-L2,
2-L3 and 2-L4 displayed type IV isotherms with clear
hysteresis loops associated with capillary condensation (see
Fig. 4), indicating the mesoporous character of the samples
[30]. The reduction in surface areas and pore sizes of the
immobilized complexes in comparison with their parent
MCM-41 could be due to the attaching to the walls of
Si-MCM-41 with the organic moieties [31] (see Table 2).
2-L5 showed type IV/I isotherms with an ill-defined cap-
illary condensation region and a sharp decrease in BET
surface and pore volume, which might be attributed to the
blocking of the pore originating from the replacement of
surface silanol groups by much more bulky organic groups
immobilized onto the inner wall of mesoporous MCM-41.
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Transition Met Chem (2010) 35:213–220
217
reactant and product around active sites, as discussed in our
published articles in detail [23]. In the presence of 2-L5,
though the selectivity to benzaldehyde was slightly
enhanced, the benzyl alcohol conversion was lower than
for the corresponding homogeneous analog 1-L5, which
can be explained as mainly due to the diffusion limitation
under the heterogeneous conditions and is in good agree-
ment with the result of N2 sorption.
The ligand structure was also, as usual, an important
factor in determining the activity and selectivity of
immobilized catalyst (see Table 3). In 2-L1 and 2-L2, the
bridge groups were all alkyl groups; however, 2-L2 showed
higher catalytic performance due to the additional N atom
with isolated electron pair increasing the donation capacity
of the ligand. The bridge groups in 2-L4 and 2-L5 were
phenyl and binaphthyl groups, respectively, which con-
tributed to the formation of the p-extended coordination
structures. This could facilitate the oxidation and so afford
the best catalytic performance of 2-L4. 2-L5 exhibited
much lower catalytic performance, probably because the
introduction of support resulted in a diffusion limitation.
The worst catalytic performance, over 2-L3, could be due
to the higher steric encumbrance around the chromium
center, originating from the chair conformation of the
bridge group. Consequently, the catalytic performance
(benzaldehyde yield) observed over the immobilized
complexes with different Schiff base ligands was in the
following order: 2-L4 [ 2-L2 [ 2-L1 [ 2-L5 [ 2-L3.
The reaction was extended to different alcohols, and
moderate to high catalytic performances were obtained
over the representative catalyst 2-L4 (see Table 4). As
expected, aliphatic secondary alcohols, cyclic alcohols and
benzylic alcohol were all oxidized in good yields, and the
major products were the corresponding ketones or alde-
hydes. For primary alcohol oxidations, more substrate and
longer reaction times were needed to offset the slow
reaction rate, and the major product was the carboxylic
acid. Generally, the alcohols were first oxidized to the
corresponding aldehydes and ketones when H2O2 was used
as oxidant. Ketones were obtained as the final oxidation
products while aldehydes could be over oxidized into their
corresponding acids. Therefore, much higher yields of
ketones were obtained as the oxidation products of sec-
ondary and cyclic alcohols in the present investigation.
Moreover, the intermediate product of primary alcohol (1-
hexylaldehyde) was found to be more easily over oxidized
to the carboxylic acid (1-hexanoic acid) than was benzylic
alcohol (benzaldehyde), which might be due to different
reaction mechanisms.
Fig. 4 N2 adsorption–desorption isotherms of MCM-41 and immo-
bilized complex samples: the solid symbols: the adsorption isotherms;
the open symbols: the desorption isotherms
Table 2 Structural parameters for MCM-41 and the immobilized
complex samples
Samples
BET surface Average pore Density of Cr complex
sizea (nm)
area (m2/g)
(910-7 mol/m2)
MCM-41 1,050
2.76
2.28
2.25
2.30
2.25
2.16
–
2-L1
2-L2
2-L3
2-L4
2-L5
a
695
692
700
695
685
4.70
4.45
4.95
4.70
4.21
Calculated by the BJH method from the desorption isotherm
Catalytic performance
The oxidation of a model compound, namely benzyl
alcohol, to benzaldehyde, was first examined without any
organic solvent, phase transfer catalyst or additive. As
expected, only three products, namely, benzaldehyde,
benzoic acid and benzyl benzoate were detected. Other
products, if any, present as minor constituents could not be
detected by the gas chromatograph under the conditions
used herein. The resulting data in Tables 3, 4 were the
average at least three runs.
The results in Table 3 show that no significant amount
of benzaldehyde was formed in the absence of catalyst.
When the reaction was performed over the immobilized
complexes except 2-L5, on the whole, much higher cata-
lytic performance was obtained than over their corre-
sponding homogeneous complexes. Such obviously
enhanced catalytic performance could, in part, be attributed
to the ability of the support to highly disperse the active
complexes and/or to control the local concentrations of the
A further study on the catalytic reusability of the rep-
resentative immobilized complex 2-L4 was also carried
out, and the results are also listed in Table 4. After the first
catalytic run, the heterogeneous catalyst was separated
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Transition Met Chem (2010) 35:213–220
Table 3 Catalytic performance of the different immobilized complexes in benzyl alcohol oxidation
Samples
Benzyl alcohol
conversiona (%)
Selectivityb (%)
Benzaldehyde
yieldc (%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
Blankd
1-L1
1-L2
1-L3
1-L4
1-L5
2-L1
2-L2
2-L3
2-L4
2-L5
3.0
12.2
15.0
8.7
100
0
0
3.0
8.0
65.4
70.2
59.3
75.6
73.5
98.2
100
23.5
19.8
22.6
18.2
21.4
1.3
11.1
10.0
18.1
6.2
5.1
0.4
0
10.5
5.2
20.5
25.3
42.5
43.2
24.7
45.2
23.0
15.5
18.6
41.7
43.2
17.4
45.2
17.7
0
70.5
100
19.1
0
10.4
0
77.0
16.2
6.8
Reaction conditions: benzyl alcohol 50 mmol, 30% H2O2 125 mmol, heterogenized catalysts 0.25 g or homogeneous catalysts 1.25 mmol,
50 °C, 4 h
a
Conversion = (moles of BzOH reacted/moles of BzOH in the feed) 9 100
b
Selectivity = (moles of BzOH converted to the products/moles of BzOH reacted) 9 100
c
Yield = (moles of BzOH converted to the products/moles of BzOH in the feed) 9 100
d
Blank means that the oxidation was performed in the absence of catalyst
Table 4 Catalytic performance of the representative immobilized complex in the oxidation of alcohols
Substrates
Reaction conditions
Recycle runs
Conversiona (mol%)
Major product
Yieldb
(mol%)
Benzyl alcohol: 0.05 mol; H2O2: 0.125 mol;
Cat.c: 0.25 g; 50 °C; 4 h.
1
2
3
4
45
44
43
41
45
44
42
38
OH
OH
CHO
2-Hexanol: 0.05 mol; H2O2: 0.0625 mol;
Cat.: 0.2 g; 80 °C; 3 h.
1
2
3
4
92
91
86
80
89
88
82
74
O
O
O
Cyclovaleranol: 0.05 mol; H2O2: 0.0625 mol;
Cat.: 0.25 g; 80 °C; 3 h.
1
2
3
4
99
98
95
91
99
98
93
87
OH
1-Hexanol: 0.1 mol; H2O2: 0.125 mol;
Cat.: 0.2 g; 80 °C; 10 h.
1
2
3
4
69
67
63
59
65
62
57
53
OH
OH
Cyclohexanol: 0.05 mol; H2O2: 0.0625 mol;
Cat.: 0.2 g; 80 °C; 6 h.
1
2
3
4
97
96
92
89
97
94
88
82
O
OH
a
Conversion = (moles of substrate reacted/moles of substrate in the feed) 9 100
b
c
Yield = (moles of substrate converted to the products/moles of substrate in the feed) 9 100
Catalyst used here is the representative immobilized complex 2-L4
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Transition Met Chem (2010) 35:213–220
219
immobilized complexes. Similarly, the recovered catalysts
were analyzed by FTIR and XRD, and no significant
change was observed (see Figs. 5 and 6). Therefore, the
immobilized complexes were stable in the selective oxi-
dation of alcohols under the mild conditions used herein.
Conclusion
A series of immobilized chromium(III) Schiff base com-
plexes were successfully prepared and exhibited promising
catalytic performance toward the selective oxidation of
alcohols by 30% H2O2 in the absence of organic solvent,
phase transfer catalyst or additive. The significant differ-
ences in the catalytic performance of the immobilized
complexes could be attributed to their different ligand
structures, and the immobilized complex 2-L4 with
p-extended coordination structure was found to exhibit the
best catalytic performance. Under the optimal reaction
conditions, the major products of secondary alcohols, cyclic
alcohols and benzyl alcohol were ketones and aldehydes,
while primary alcohols gave carboxylic acids.
Fig. 5 FTIR spectra of 2-L4: a as prepared, b used four times
Acknowledgments The authors acknowledge the financial support
from National Science Technology Foundation of China (No.
2006BAC2A08) and NJIT Scientific Research Foundation for Intro-
duce Talents (KXJ 08033 and KXJ 08034).
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