A study on the heterobinuclear site of Co(salen)/TiO2-SiO 2 catalysts in the oxidative carbonylation of aniline

Article abstract of DOI:10.1002/aoc.1577

Titania-silica immobilized Co(salen) complexes containing the heterobinuclear site were prepared by the sol-gel method for the catalytic synthesis of methyl N-phenylcarbamate (MPC) by the oxidative carbonylation of aniline. It was found that the Ti: Si mole ratio had an important effect on the catalytic performance of Co(salen) complexes. When the Ti: Si ratio was 0.1, titania-silica supported Co(salophen) showed the best catalytic activity. Under the reaction conditions, Co(salophen)/TS-0.1, 0.5 g, aniline 11 mmol, methanol 25 ml, KI 2.2 mmol, CO:O₂ 9:1, total pressure 6 MPa, 150 C, 3 h, the conversion of aniline and the selectivity of MPC were 60.7 and 88.1%, respectively. The XRD studies showed that titania was highly dispersed in the silica matrix. Co(salophen)/TS-0.1 was reused five times with no significant loss of the activity, and no Co leaching was observed in the reaction. Copyright

Full text of DOI:10.1002/aoc.1577

Full Paper
Received: 16 July 2009
Revised: 25 September 2009
Accepted: 29 September 2009
Published online in Wiley Interscience: 26 November 2009
(www.interscience.com) DOI 10.1002/aoc.1577
A study on the heterobinuclear site
of Co(salen)/TiO2 –SiO2 catalysts
in the oxidative carbonylation of aniline
Fu-Ming Meia, Li-Juan Chena and Guang-Xing Li∗ a,b
Titania-silica immobilized Co(salen) complexes containing the heterobinuclear site were prepared by the sol–gel method for
the catalytic synthesis of methyl N-phenylcarbamate (MPC) by the oxidative carbonylation of aniline. It was found that the
Ti : Si mole ratio had an important effect on the catalytic performance of Co(salen) complexes. When the Ti : Si ratio was 0.1,
titania-silica supported Co(salophen) showed the best catalytic activity. Under the reacti◦on conditions, Co(salophen)/TS-0.1,
0.5 g, aniline 11 mmol, methanol 25 ml, KI 2.2 mmol, CO : O2 9 : 1, total pressure 6 MPa, 150 C, 3 h, the conversion of aniline and
the selectivity of MPC were 60.7 and 88.1%, respectively. The XRD studies showed that titania was highly dispersed in the silica
matrix. Co(salophen)/TS-0.1 was reused five times with no significant loss of the activity, and no Co leaching was observed in
c
the reaction. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Keywords: Co(salen); oxidative carbonylation; sol–gel process; aniline; methyl N-phenylcarbamate
Introduction
prompted us to develop efficient and recoverable heterogeneous
catalysts by immobilizing Co(salen) complexes on different inor-
ganic supports. In a previous paper,[15] we prepared a series of
heterogeneouscatalystsbya‘shipinbottle’method. TheCo(salen)
complexes were encapsulated in the cages of zeolite Y, and the im-
mobilized catalysts showed good activity and reusability with little
Co leaching. In the continuity, Co(salen) and Co(salophen) were
covalently bonded to silica matrix via a sol–gel process. These
heterogeneous catalysts demonstrated highly catalytic activities
for the oxidative carbonylation of aniline to MPC.[16]
Titania–silica mixed oxide is an interesting support material,
which combines the advantages of both TiO2 (active support)
and SiO2 (high thermal stability and mechanical strength). The
interaction of TiO2 with SiO2 will generate a new kind of double
active site during the preparation of this mixed oxide. It is well
knownthatthetitania–silicacompositemolecularsievesareakind
of highly effective catalysts for many oxidation reactions.[17–19] In
this paper, we found that the covalent attachment of Co(salen)
complexes on titania-silica prepared by the sol–gel method
produced the heterobinuclear site in these immobilized Co(salen)
complexes, andtheyshowedhighcatalyticactivityinthesynthesis
of MPC by oxidative carbonylation of aniline.
Carbamates are important compounds having extensive applica-
tion in the fields of chemical industry, pharmaceutics and agricul-
ture. They are also intermediates to synthesize isocyanates, which
are important raw materials in the production of polyurethane.
The traditional production of carbamates is the phosgene process,
which uses hazardous phosgene as a raw material and causes
serious problems. Therefore, in the last two decades, the de-
velopment of environmentally benign processes has attracted
significant interest.[1–4] Among these processes, the synthesis of
carbamates from amines by oxidative carbonylation is promising.
Then, the carbamates can be thermally cracked to highly selec-
tively convert to isocyanates. These reactions can be depicted by
the following two equations:
RNH2 + RꢁOH + CO + 1/2 O2 −−−→ RNHCOO Rꢁ + H2O (1)
ꢀ
RNHCOO Rꢁ−−−→ RNCO + RꢁOH
(2)
Most reported catalyst systems for reaction (1) are noble metal
salt or complex catalysts. Although these noble catalysts show
high activity, they are not economically feasible for the practical
synthesis of carbamates. Therefore, there is a need to develop a
low cost and highly active catalyst for the reaction.
Metal salen complexes are effective catalysts for the epox-
idation of unfunctionized olefins using dioxygen or peroxide
as oxidant.[5–9] Under mild conditions, these macro-cyclic com-
plexes can be reversibly reacted with oxygen to form O2-binding
adduct.[10,11] Inoxidativecarbonylation,thecapabilityofactivating
O2 is advantageous to increase the solubility of O2 in the reaction
medium. In our previous exploration of homogeneous Co(salen)
catalysts in carbonylation of aniline to N,Nꢁ-diphenylurea (DPU)
or methyl N-phenylcarbamate (MPC), we found that they were
active and selective.[12–14] However, the drawbacks of the homo-
geneous catalysts, such as difficulty of separation and recovery,
∗
Correspondence to: Guang-Xing Li, School of Chemistry and Chemical
Engineering, Huazhong University of Science and Technology, Wuhan 430074,
People’s Republic of China. E-mail: ligxabc@163.com
a
SchoolofChemistryandChemicalEngineering,HuazhongUniversityofScience
and Technology, Wuhan 430074, People’s Republic of China
b
Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong
University of Science and Technology, Wuhan, 430074, People’s Republic of
China
c
Appl. Organometal. Chem. 2010, 24, 86–91
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

Heterobinuclear site of Co(salen)/TiO2 –SiO2 catalysts
Experimental
of 5a is described in detail as follows. Firstly, TEOS (10 g, 48 mmol)
and dilute HCl (4.4 ml, 1 M HCl) were slowly added to a solution
containing complex 4a prepared in situ in absolute methanol. The
formed brown solution was stirred at room temperature for 3 h,
then TTBO, the amount of which was varied with the Ti : Si ratio, in
5 ml of absolute methanol was added drop-wise, and the solution
was stirred at room temperature for 24 h. Secondly, a solution of
tetrabutylammonium chloride (1.2 g, 4.4 mmol) in 15 ml absolute
methanol and five drops of concentrated NH4OH were added, the
mixture was stirred for 0.5 h and then placed in a thermostatic
water bath at 30 ◦C without stirring to form a gel in 3 h. The formed
wet gel was aged for 96 h at the same temperature, washed with
wateranddriedinair.Thedrygelwasgroundandsoxhletextracted
with methanol and diethyl ether mixture for 48 h. The solid was
then dried at 110 ◦C under vacuum for 24 h to obtain a xerogel.
The series of different xerogels 5a (5b) prepared with different
Ti : Si ratios are simplified as 5a (b):TS-X, where X represents the
value of the Ti : Si ratio. Xerogels 6a and 6b were prepared by the
same procedure described above, but no titanium tetrabutoxide
was added.
Catalyst Preparation
Two homogeneous catalysts, Co(salen) [salen, N,Nꢁ-
bis-(salicyldiene)–ethylene–diamine] and Co(salophen) [sa-
lophen, N,Nꢁ-bis-(salicyldiene)-o-phenylenediamine], were syn-
thesized according to the literature.[20] Their structures were
characterized by FT-IR, 1HNMR and elemental analysis.
The covalent attachment of Co(salen) complexes onto TiO2-
SiO2 by the sol–gel method is shown in Scheme 1. In this synthesis
route, in order to perfectly retain the geometry of the immobilized
Co(salen) complexes, the Co(salen) complexes were first prepared,
and then were modified with the trimethoxysilane group to
form the precursors. This methodology is reported to be better
than another approach in which the Co2+ complexation was
performed after the introduction of the trimethoxysilane group
on the ligand.[21]
Synthesis of 4-Hydroxy-1,3-benzenedicarboxaldehyde (1)
The pure TiO2 –SiO2 xerogel was prepared according to the
following procedure. TEOS (10 g, 48 mmol) in absolute methanol
(15 ml) was first hydrolyzed with dilute HCl (4.4 ml, 0.1 M) at room
temperature for 1 h, then TTBO (1.63 g, 4.8 mmol) in methanol
(5 ml) was added drop-wise under stirring, and the mixture was
heated to 30 ◦C and allowed to age at 30 ◦C for 96 h to form a gel.
The wet gel was dried in air to remove water and solvent, followed
by calcinating at 550 ◦C for 2 h, and then ground to powder.
4-Hydroxy-1,3-benzenedicarboxaldehyde (1) was synthesized
from 5-chloro-methyl salicylaldehyde by the Sommelet reaction.
Firstly, 5-chloromethyl salicylaldehyde was synthesized from sal-
icylaldehyde by chloromethylation according to the procedure
described in the literature.[22] Then 5-chloromethyl salicylalde-
hyde (6.1 g, 35.8 mmol) was treated with hexamethenetetramine
(6.5 g, 46.4 mmol) in 36 ml 50% CH3COOH aqueous solution un-
der stirring, the reaction mixture was refluxed for 1 h, and 16 ml
of concentrated hydrochloric acid was added. Finally, the yellow
solution was refluxed for another 5 min, and cooled using an ice
bath to give 1 as light yellow needle crystals (3.3 g, 60% yield).
1HNMR (CDCl3, 400 MHz): δ (ppm) 7.13 (d, 1H), 7.91 (d, 1H), 8.17 (d,
1H), 10.11 (s, 1H), 10.36 (s, 1H), 11.47 (s, 1H).
Determination of the Catalyst Characteristics
The FTIR spectra of samples in KBr pellet were recorded on a Bruker
Equinox 55 FTIR spectrophotometer in the range 4000–400 cm−1
.
The diffuse reflectance UV–vis was measured on a Shimadzu UV-
2550 PC UV–vis spectrophotometer in the range 800–200 nm
with barium sulfate as a reference. Thermogravimetric analysis of
materials was conducted on an automatic derivatograph Perkin
Elemer TGA-7 instrument at a heating rate of 10 ◦C per minute
under an Ar atmosphere. The C, H, N elemental analysis of the
homogeneous catalysts was carried out on a Vario EL III element
analyzer. The Co content of the supported catalysts was measured
by atomic absorption spectroscopy using a Perkin Elmer AA-300
atomic absorption spectrophotometer. The titanium loading was
determined by colorimetric method, the samples were dissolved
in hot concentrated sulfuric acid followed by treatment with
H2O2. The XRD patterns were collected with a Philips X-Pert
Pro X–ray Diffraction instrument, in the 2θ range 4–50◦ at a
scan rate of 13 deg/min. XPS was performed in a VG multilab
2000 spectrometer, using Mg-Al Kα X-ray source with the passing
energy flow of 100 eV. The samples were degassed in a chamber at
5 × 10−10 mbar at room temperature. C1s transition was adjusted
at 284.6 eV.
Synthesis of bis-silylated Co(salen) complexes (4a, 4b)
The formyl modified salen ligands (2a, 2b) and corresponding Co
(II) complexes (3a, 3b) were synthesized by simple condensation
and complexation. The general procedure is as follows: two
equivalent moles of compound 1 and one equivalent mole of
diamine were dissolved in methanol, and the mixture was refluxed
at 323 K for 40 min to form the ligand. The complexation of
ligand with Co2+ in methanol under an inert atmosphere gave the
homogeneous Co(salen) complexes (3a, 3b).
The synthesis of 4a or 4b was performed under the same
procedure; the detail of the preparation of 4a is given as an
example. A solution of 3a (0.96 g, 2.5 mmol) in 25 ml methanol
was mixed with 3-aminopropyltrimethoxysilane (AMPTSi) (1.1 g,
5 mmol) in 15 ml methanol under N2 protection; the mixture was
heated and refluxed for 12 h. After evaporation of a small amount
of solvent, the mixture containing the bis-silylated product 4a was
not separated and used directly in the following step.
Oxidative Carbonylation of Aniline to MPC
Preparation of xerogels [5a (b), 6a (b)]
Thehydrolysisandpoly-condensationofthebis-silylatedCo(salen)
complexes (4a, 4b) with tetraethyl orthosilicate (TEOS) and
titaniumtetrabutoxide(TTBO)toformtheheterogeneouscatalysts
(5a, 5b) were conducted in a same way. Because of the different
hydrolysis rate of TEOS and TTBO, the pre-hydrolysis of TEOS and
theuseofanacidasacatalystinhydrolysiseffectivelypromotedthe
homogeneity of the titanium in the silica matrix. The preparation
The catalytic reactions were carried out in a 100 ml polytetraflu-
oroethylene tube in a stainless steel autoclave equipped with a
mechanical stirrer and an automatic temperature controller. Het-
erogeneous catalysts (0.5 g), KI (0.365 g) as promoter and aniline
(1.023 g, 11 mmol) in anhydrous methanol (25 ml) were charged
into the reactor, and the reactor was pressurized with a mixture
of CO and O2 to the total pressure of 6 MPa, CO : O2 was 9 : 1. The
c
Appl. Organometal. Chem. 2010, 24, 86–91
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/aoc

F.-M. Mei, L.-J. Chen and G.-X. Li
N
Co
O
N
N
N
CHO
OH
Co salt
OHC
CHO
O
CHO
NH2
H2N
OHC
OH HO
2a, 2b
OHC
3a, 3b
1
AMPTSi
MeOH
N
N
Co
O
O
N
N
a: - CH2CH2-
b: - C6H4-
MeO Si
MeO
4a, 4b
Si
OMe
OMe
OMe
MeO
N
N
Co
O
O
N
N
O
Co
N
N
O
N
N
Si
O
O
O
O
O
Si
O
O
Si
O
Si
O
O
O
O
O
Ti
O
O
O
Ti
O
O
O
O
O
O
n
n
6a, 6b
5a, 5b
Scheme 1. Preparation of Co-salen complexes anchored on SiO2 or TiO2 –SiO2 support.
autoclave was heated to 150 ◦C and held at that temperature for
Table 1. Spectroscopic characteristics of different samples
3 h. After the reaction, the reactor was cooled to room tempera-
ture and degassed. The heterogeneous catalyst was recycled by
filtration followed by being thoroughly washed with methanol
and dried under reduced pressure. The quantitative analysis of
liquid products was carried out using an Agilent 1790 gas chro-
matograph equipped with an HP-5 (30 m × 0.32 mm × 0.25 µm)
capillary column and flame ionization detector.
Samples
UV–vis/λd−d (nm)
IR/νC N (cm−1
)
2a (2b)
–
1633 (1615)
1616 (1603)
1619 (1608)
1621 (1609)
1622 (1611)
3a (3b)
412 (426)
415 (427)
413 (423)
418 (424)
4a (4b)
5a (5b)/TS-0.1
6a (6b)
Results and Discussion
XRD Analysis
Spectroscopic Characteristics
XRD analysis was performed to study the morphology of the
samples. Figure 1 shows the XRD patterns of the pure support
TS-0.1, fresh Co(salophen)/TS-0.1 and used Co(salophen)/TS-0.1.
It can be seen from Fig. 1 that the XRD patterns of the three
samples had a wide hump in the range of 2θ from 15 to 30◦,
typical of amorphous silica. In the pure titania–silica mixed oxide
and hybrid xerogels containing Co(salen) complexes prepared by
sol–gel method, amorphous TiO2 was homogeneously dispersed
onsilica.InusedCo(salophen)/TS-0.1,TiO2 whichwasincorporated
in silica matrix was dominantly amorphous but the aggregation of
TiO2 in small crystals of anatase TiO2 was observed (reflection at
2θ = 25.3, 18.1, 36.0◦).[25]
The IR and UV–vis spectra for free ligands (2a, 2b) and materials
containingtheCo(salen)complexes[3–6a(b)]arelistedinTable 1.
The characteristic bands of the salen imine functionality in the
range of 1650–1600 cm−1 were observed in the IR spectra of all
the samples. The complexation of free ligand with Co (II) cation
caused a decrease in the ν(C
value [comparison of 2a (2b)
N)
with 3a (3b)]. In the IR spectra of 5a (5b)/TS-0.1, in addition to
the bands at 1060–1180 cm−1 attributed to –Si–O–Si–stretching
vibration, a band at 930–952 cm−1 attributed to –Si–O–Ti–was
observed and the intensity of the band was affected by the
addition of titanium.[23] A broad weak absorption at about 400 nm
corresponding to d–d charge transition of ligand and central Co2+
was observed in the UV–vis spectra of the bis-silylated modified
Co-salen complexes [4a (b)] and hybrid xerogels [5–6a (b)], which
were similar to that of Co(salen) described in the literature.[24] In
addition, intense π –π∗ and n–π∗ ligand charge transfer bands in
the range 220–350 nm were present in all cases. A small shift in
ν(C N) vibration in the IR spectra and small changes in absorption
in the UV–vis spectra from the hybrid xerogels to the bis-silylated
modified Co(salen) complexes [4a (b)] were caused by the change
in the geometrical structure of the materials.
XPS Study
The XPS spectra for Si2p are shown in Fig. 2a. The peak of Si2p
was located at 104.1 eV for Co(salophen)/SiO2, while it decreased
to 103.2 eV for Co(salophen)/TS-0.1. This fact demonstrates that
the chemical environment of the silica underwent some variation
as the Ti was introduced into the system. In Co(salophen)/SiO2,
the silica existed in Si–O–Si linkages, while a portion of the
Si–O–Si linkages was replaced by hetero-Si–O–Ti linkages in
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 86–91

Heterobinuclear site of Co(salen)/TiO2 –SiO2 catalysts
3000
2500
2000
1500
1000
500
∗
∗
∗ TiO2
∗
5b/TS-0.1 (used)
5b/TS-0.1 (fresh)
pure TS-0.1
0
10
15
20
25
30
35
40
2θ/O
Figure 1. X-ray diffraction spectra of pure TiO2-SiO2 support and Co (salophen)/TS-0.1.
531.9EV
104.14 eV
(b)
(a)
531.2eV
103.2 eV
Co(salophen)/TS-0.1
Co(salophen)/SiO2
Co(salophen)/TS-0.1
Co(salophen)/SiO2
110
105
100
95 528
530
532
534
BE (eV)
BE/eV
Figure 2. XPS Si2p (a) and O1s (b) of Co(salophen)/SiO2 and Co(salophen)/TS-0.1.
Co(salophen)/TS-0.1.TheformationofSi–O–Tilinkagesdecreased
the electron density of Si species resulting from the substitution
of Si atoms by the less electronegative and more polarizable Ti
atoms, and caused the reduction of binding energy of Si 2p.[26] In
comparison with the XPS value of the O1s in Co(salophen)/SiO2,
it was somewhat smaller for Co(salophen)/TS-0.1 (Fig. 2b). This
result also indicated that a portion of Ti–O bondages were formed
in Co(salophen)/TS-0.1.
Thermogravimetric Analysis
The thermal stability of the catalysts was measured by TG and DTA
as shown in Fig. 4. The fresh and used Co(salophen)/TS-0.1 had
similar TGA curves, and both had similar weight loss associated
with the decomposition of the complex. Based on these facts, we
concluded that no leaching of active complex happened during
recycling of the catalyst. This result was also confirmed by the Co
content of the hybrid catalyst determined by AAS. The data show
that the fresh Co(salophen)/TS-0.1 contained 1.67 wt% Co, and
the Co content for runs 2–5 was 1.67, 1.66, 1.66 and 1.66 wt%,
respectively (in Table 3). The Co content was almost unchanged.
The thermal stability of the recovered Co(salophen)/TS-0.1 after
5 runs was somewhat lower than the fresh sample. The decompo-
sition temperature of the fresh Co(salophen)/TS-0.1 was 495 ◦C,
while it was lowered to 460 ◦C for the used Co(salophen)/TS-0.1.
The TG and DTA data show that the hybrid catalysts had adequate
thermal stability under the reaction conditions.
Figure 3 shows the XPS spectra of O1s region and its curve
fitting results for fresh and used Co(salophen)/TS-0.1. In the
fresh Co(salophen)/TS-0.1, Ti interacted with Si to form Ti–O–Si
heterolinkages, while in the used Co(salophen)/TS-0.1, in the
region of O1s, a peak attributed to Ti–O–Ti was observed,
which indicates that a portion of the Ti–O–Si linkages was
destroyed under the reaction conditions, and these Ti species
were aggregated to form fine anatase TiO2. This fact is consistent
with the results of XRD analysis. However, in the experiments, it
was found that the morphological change of Ti species for the
second used catalyst had a minor effect on the catalytic activity
(shown in Table 3).
Effect of Ti : Si Ratio on Catalytic Activity
Figure 5 shows the effect of the Ti : Si mole ratio in the catalysts
on the catalytic activity of Co(salophen)/TS. The introduction of Ti
into the silica matrix led to a significant enhancement in aniline
c
Appl. Organometal. Chem. 2010, 24, 86–91
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/aoc

F.-M. Mei, L.-J. Chen and G.-X. Li
(a)
(b)
Si-O-Si
Ti-O-Si
Si-O-Si
Ti-O-Si
Ti-O-Ti
520
525
530
535
520
525
530
535
Binding Energy (eV)
Binding Energy (eV)
Figure 3. XPS spectra of O1s region for fresh (a) and used (b) Co(salophen)/TS-0.1.
0.3
100
90
80
70
60
50
40
Co(salophen)/TS-0.1 (fresh)
Co(salophen)/TS-0.1 (used)
0.2
TGA
DTA
0.1
0.0
-0.1
-0.2
-0.3
495°C
460°C
100
200
300
400
500
600
700
Temperature (°C)
Figure 4. TGA and DTA of fresh and used Co(salophen)/TS-0.1.
conversion (from ca 30% to ca 60%), and the content of Ti in
the catalyst had an important effect on the catalytic activity.
When the Ti : Si ratio was changed from 0.05 to 0.1, the catalytic
activity of Co(salophen)/TS increased. When more Ti was added
[Co(salophen)/TS-0.2],decreasesinMPCyieldsandselectivitywere
observed, causedbythesidereactionderivedfromthemoreacidic
active centers of TiO2 –SiO2 mixed oxide. It can be concluded that
the Ti : Si ratio 0.1 is the most suitable one.
SiO2 support. These results show that the heterobinuclear site in
titania–silicaimmobilizedCo(salen)complexeswasformed, which
could be Co species (Co-salen complex) and Ti species (TiO2). They
had a synergetic effect on the oxidative carbonylation. For the
heterogeneous catalysts, the selectivity of MPC was obviously
higher than that for the homogeneous analogs, although the
conversion of aniline was lower. The increase in MPC selectivity
could be explained by the isolation effect of the catalytic site
in the heterogeneous catalytic system, which means that the
complex is not prone to degradation or dimerization as is shown
in homogeneous form.[27]
The Catalytic Activity of the Heterogeneous Catalysts
The reusability of the heterogeneous catalysts was also tested.
The data are listed in Table 3. The heterogeneous catalyst was
filtered after each run and washed with methanol, then added to
fresh substrate and solvent for the next run. After each run, a tiny
portion of the catalyst (<1%) was taken out and was analyzed by
AAS to determine the Co concentration in the catalyst. As is shown
forthefirstrunandsecondruninTable 3, theCocontentremained
constant, but a slight decrease in the conversion of aniline and
yield of MPC was observed. According to the results of XRD and
XPS, a small portion of amorphous TiO2 was aggregated to form
The catalytic results of the heterogeneous Co-salen/TS and their
homogeneous analogs in oxidative carbonylation of aniline are
shown in Table 2. When the reaction was carried out on pure
SiO2, no MPC was found, while the TiO2 –SiO2 mixed oxide (TS-0.1)
shows a certain catalytic activity with conversion of 31.2% and a
yield of 10.9%. This fact indicates that the incorporated Ti species
in TiO2 –SiO2 mixed oxide can afford a kind of catalytic center
for oxidative carbonylation of aniline to carbamate. It is also seen
in Table 2 that the Co–salen complexes anchored on TiO2 –SiO2
support had higher activities relative to those anchored on pure
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 86–91

Heterobinuclear site of Co(salen)/TiO2 –SiO2 catalysts
100
Table 3. Oxidative carbonylation of aniline catalyzed with fresh or
used Co(salophen)/TS-0.1
convAN YMPC SMPC
Run
Co mol (%)
ConvAN (%)
YMPC (%)
SelectMPC (%)
80
60
40
20
1st
1.67
1.67
1.66
1.66
1.66
60.7
55.7
53.8
53.6
53.3
53.5
48.8
47.0
46.8
46.5
88.1
87.6
87.3
87.2
87.2
2nd
3rd
4th
5th
Reaction conditions: Co(salophen)/TS-0.1 0.5 g, aniline 1.023 g
(11 mmol), methanol 25 ml, KI 0.365 g (2.2 mmol), CO 5.4 MPa, O2
0.6 MPa, 150 ◦C, 3 h.
0
Acknowledgments
Co(salophen) Co(salophen) Co(salophen) Co(salophen) Co(salophen)
/SiO2
/TS-0.05
/TS-0.07
/TS-0.1
/TS-0.2
The authors acknowledge the financial support by the Research
Foundation for the Returned Overseas Chinese Scholars, Ministry
of Education of China; and the Research Foundation of Natural
Science of Hubei Province, China. We also thank the Analytical and
Testing Center, Huazhong University of Science and Technology
for the spectroscopic analyses.
Figure 5. Effect of Si/Ti ratio on the catalytic activity of heterogeneous
catalysts. Reaction conditions: catalysts 0.5 g, aniline 1.023 g (11 mmol),
methanol 25 ml, KI 0.365 g (2.2 mmol), CO 5.4 MPa, O2 0.6 MPa, 150 ◦C, 3 h.
Y: yield; S: selectivity.
Table 2. Oxidative carbonylation of aniline catalyzed by Co(salen)
complexes
References
Co mol
(%)a
ConvAN
(%)
YMPC
(%)
SelectMPC
(%)
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Catalyst
Co(salen)
2.00
2.00
–
1.68b
1.70b
1.66b
1.67b
83.6
90.2
31.2
31.4
30.8
59.3
60.7
60.1
65.7
10.9
26.7
25.7
52.3
53.5
71.9
72.8
34.9
85.0
83.6
88.2
88.1
Co(salophen)
TS-0.1
Co(salen)/SiO2
Co(salophen)/SiO2
Co(salen)/TS-0.1
Co(salophen)/TS-0.1
a Based on aniline (AN); b determined by AAS.
Reaction conditions: catalysts 0.5 g, aniline 1.023 g (11 mmol),
methanol 25 ml, KI 0.365 g (2.2 mmol), CO 5.4 MPa, O2 0.6 MPa,
150 ◦C, 3 h.
fine anatase TiO2 particles after reaction. Thus, we deduced that
the morphological variation of Co(salophen)/TS-0.1 probably led
to the slight decrease in catalytic activity of the catalyst. However,
from the second run to the fifth run, very similar conversion of
aniline and yield of MPC were obtained for the used catalyst,
and more importantly, no Co leaching was observed based on
AAS in the reaction mixture. These facts demonstrate that the
heterogeneous catalysts are effective and recoverable.
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Conclusions
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Co(salen) complexes supported on titania-silica support were
preparedbythesol–gelmethod.Theheterobinuclearsitesinthese
heterogeneous catalysts were formed and they were effective
in oxidative carbonylation of aniline, and more importantly,
they were stable under the reaction conditions, and recoverable
without loss of their activities. The application of heterogeneous
Co(salen) catalysts in carbonylation of aromatic amines can
provide a novel and economically feasible catalytic system for
an environmentally benign process of synthesizing carbamates.
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c
Appl. Organometal. Chem. 2010, 24, 86–91
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