Direct synthesis of diphenyl carbonate from phenol and carbon dioxide over Ti-salen-based catalysts

Article abstract of DOI:10.1166/jnn.2015.11272

Various metal-salen catalysts were prepared for use in the direct synthesis of diphenyl carbonate (DPC) from phenol and carbon dioxide. We found that metal-salen complexes containing titanium as central metal species retained suitable Lewis acid property for the reaction. It was revealed that the catalytic activity of Ti-salen complexes could be controlled by introducing appropriate substituents into salen ligand. Insertion of phosphonium salts into para-position of aromatic aldehyde of salen ligand enhanced solubility of the catalyst in the methanol-phenol solution, and tert-butyl substituent in the salen ligand induced selective formation of DPC due to steric effect. In addition, introduction of various bridging groups into salen ligand caused change in electronic property of central metal atom. Among the catalysts tested, Ti-(t-butyl)salphen(PPh₃) Cl showed the best catalytic performance at 100 °C and 60 bar. The catalytic system utilizing Ti-(t-butyl)salphen(PPh₃) Cl catalyst was then optimized by conducting the reaction at various reaction temperatures and pressures.

Full text of DOI:10.1166/jnn.2015.11272

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 8353–8358, 2015
Copyright © 2015 American Scientific Publishers
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Printed in the United States of America
Direct Synthesis of Diphenyl Carbonate from Phenol and
Carbon Dioxide Over Ti-Salen-Based Catalysts
Ki Hyuk Kang1, Jin Oh Jun1, Seung Ju Han1, Kihyeok Kwon2, O-Sung Kwon2,
Boknam Jang2, and In Kyu Song1ꢀ∗
1School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University,
Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
2Samsung Cheil Industries Inc., Gocheon-dong, Uiwang 437-711, South Korea
Various metal-salen catalysts were prepared for use in the direct synthesis of diphenyl carbonate
(DPC) from phenol and carbon dioxide. We found that metal-salen complexes containing titanium as
central metal species retained suitable Lewis acid property for the reaction. It was revealed that the
catalytic activity of Ti-salen complexes could be controlled by introducing appropriate substituents
into salen ligand. Insertion of phosphonium salts into para-position of aromatic aldehyde of salen
ligand enhanced solubility of the catalyst in the methanol-phenol solution, and tert-butyl substituent
in the salen ligand induced selective formation of DPC due to steric effect. In addition, introduction of
various bridging groups into salen ligand caused change in electronic property of central metal atom.
Among the catalysts tested, Ti-(t-butyl)salphen(PPh ꢀCl showed the best catalytic performance at
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3
100 ꢀC and 60 bar. The catalytic system utilizing Ti-(t-butyl)salphen(PPh ꢀCl catalyst was then
IP: 191.243.58.187 On: Fri, 29 Jan 2016 12:32:10
3
Copyright: American Scientific Publishers
optimized by conducting the reaction at various reaction temperatures and pressures.
Keywords: Diphenyl Carbonate, Phenol, Carbon Dioxide, Lewis Acid, Ti-Salen Catalyst.
1. INTRODUCTION
complex has been recognized as an efficient catalyst,
because Lewis acid property of the complex can be eas-
ily modified for the desired reaction via functionaliza-
tion of salen ligand.10ꢀ11 However, the catalytic activity of
the reported catalysts is still extremely low. In addition,
the previous catalytic systems mostly require high cata-
lyst/reactant ratio. Therefore, a systematic research on the
metal-salen catalyst for the direct synthesis of DPC from
phenol and carbon dioxide would be worthwhile.
Figure 1 shows the possible mechanism for the direct
synthesis of DPC from phenol and carbon dioxide over
metal-salen catalyst.7–9 First, phenol is activated to phe-
noxide species by the central metal atom of the catalyst.
Insertion of carbon dioxide into phenoxide species then
generates phenyl carbonate-like intermediate, which is
transformed into salicylic acid-like intermediate via chem-
ical rearrangement. DPC and phenyl salycilate (PS) are
finally produced through transesterification of correspond-
ing intermediate with methanol, followed by the reaction
with phenoxide species.
Diphenyl carbonate (DPC) is an important chemical
widely used in various industries. DPC has been used as
a carbonylating agent and a phenylating agent in synthetic
chemistry.1–3 Furthermore, DPC has attracted recent atten-
tion as an eco-friendly raw material for polycarbonate,
because polycarbonate has been conventionally produced
using highly toxic phosgene.3
Commercial routes for DPC synthesis typically include
oxidative carbonylation of phenol and transesterification
of dimethyl carbonate with phenol.3 However, the for-
mer mainly uses a corrosive reactant gas such as carbon
monoxide,4 while the latter requires multi-step synthesis
processes and additional separation processes.5 In order to
overcome these drawbacks, direct synthesis of DPC from
phenol and carbon dioxide has attracted much attention as
a promising process.6–9
Several homogeneous catalysts with Lewis acid property
have been employed for the direct synthesis of DPC from
phenol and carbon dioxide.6–9 Among them, metal-salen
In this work, various metal-salen catalysts were prepared
for use in the direct synthesis of DPC from phenol and
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 10
1533-4880/2015/15/8353/006
doi:10.1166/jnn.2015.11272
8353

Direct Synthesis of Diphenyl Carbonate from Phenol and Carbon Dioxide Over Ti-Salen-Based Catalysts
Kang et al.
OH
O
C
O
C
O
C
O
O
O
O
M(salen)
O
OCH3
+ CH3OH
Transesterfication
+ CO2
Methyl phenyl
carbonate
Diphenyl carbonate
(DPC)
Phenyl carbonate-like
intermediate
OH
O
C
OH
O
C
OH
O
C
O
M(salen)
O
OCH3
+ CH3OH
Transesterfication
Salicylic acid-like
intermediate
Methyl salycilate
Phenyl salycilate
(PS)
Figure 1. Mechanism for the direct synthesis of DPC from phenol and carbon dioxide over metal-salen catalyst.
carbon dioxide. The metal-salen complexes were modified
by introducing appropriate substituents into salen ligand.
XPS analyses were conducted to elucidate the effect of
bridging groups on the electronic properties of the cata-
lysts. Optimum reaction conditions were then investigated
by conducting the reaction at various reaction temperatures
and pressures.
as reported in the literature.13 Ethylenediamine, (1R,2R)-
(−)-1,2-diaminocyclohexane, o-phenylenediamine, and
(1R,2R)-(+)-1,2-diphenylethylenediamine were used as
bridging groups to obtain H2-(t-butyl)salen(PPh3ꢁCl, H2-
(t-butyl)cysalen(PPh3ꢁCl,
H2-(t-butyl)salphen(PPh3ꢁCl,
and H2-(t-butyl)saldiphen(PPh3ꢁCl, respectively. 3 mmol
of salen ligand and 3 mmol of Ti(OiPr)4 were dissolved
in 50 ml of CH2Cl2 with vigorous stirring at room tem-
perature for 3 h under an anaerobic atmosphere, and
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2. EXPERIMENTAL DETAILS
the mixture was then dried in vacuum oven. The pre-
IP: 191.243.58.187 On: Fri, 29 Jan 2016 12:32:10
2.1. Preparation of M-Salen(PPh3)Cl (M = Ti, Zr, and
Copyright: American Scientific Publishers
pared catalysts were denoted as Ti-(t-butyl)salen(PPh3ꢁCl,
Ti-(t-butyl)cysalen(PPh3ꢁCl, Ti-(t-butyl)salphen(PPh3ꢁCl,
and Ti-(t-butyl)saldiphen(PPh3ꢁCl, respectively. Ti-(t-
butyl)salen(PPh3ꢁCl: 1H NMR (400 MHz, CDCl3, 23.5 ꢀC)
ꢂ = 1ꢃ19 (s, 14H), 1.92 (s, 18H), 3.98 (s, 4H), 5.39
(d, J = 13ꢃ4 Hz, 4H), 7.02–7.67 (m, 34H), 8.01 (s, 2H);
Co) Complexes
H2-salen(PPh3ꢁCl and H2-salen ligands were prepared
according to the standard procedures described in the
literatures.10ꢀ12 3 mmol of H2-salen ligand and 3 mmol of
metal precursor (Ti(OiPr)4, Zr(OPr)4, and Co(CH3CO2ꢁ2ꢁ
were dissolved in 30 ml of CH2Cl2 with vigorous stir-
ring at room temperature for 3 h under an anaero-
bic atmosphere, and the mixture was then dried in
vacuum oven. The prepared catalysts were denoted as
M-salen(PPh3ꢁCl (M = Ti, Zr, and Co). Ti-salen was also
prepared using salen ligand and Ti(OiPr)4 by the similar
1
Ti-(t-butyl)cysalen(PPh3ꢁCl: H NMR (400 MHz, CDCl3,
ꢀ
24.7 C) ꢂ = 1ꢃ17 (s, 14H), 1.81 (s, 18H), 4.00 (m, 2H),
5.53 (d, J = 13ꢃ4 Hz, 4H), 7.24–7.71 (m, 42H), 8.02
1
(s, 2H); Ti-(t-butyl)salphen(PPh3ꢁCl: H NMR (400 MHz,
ꢀ
CDCl3, 24.8 C) ꢂ = 1ꢃ18 (s, 14H), 2.47 (s, 18H), 5.42
1
(d, J = 13ꢃ8 Hz 4H), 6.77–7.62 (m, 38H), 8.43 (s, 2H);
methods descꢀribed above. Ti-salen: H NMR (400 MHz,
Ti-(t-butyl)saldiphen(PPh3ꢁCl: 1H NMR (400 MHz,
CDCl3, 24.5 C) ꢂ = 1ꢃ12 (s, 14H), 4.38 (s, 4H), 7.16–
ꢀ
1
CDCl3, 24.8 C) ꢂ = 1ꢃ17 (s, 14H), 2.01 (s, 18H), 3.99
7.78 (m, 8H), 8.06 (s, 2H); Ti-salen(PPh3ꢁCl: H NMR
(400 MHz, CDCl3, 21.4 ꢀC) ꢂ = 1ꢃ24 (s, 14H), 3.47
(s, 4H), 5.41 (d, J = 8ꢃ0 Hz, 4H), 6.94–7.78 (m, 36H),
8.07 (s, 2H); Zr-salen(PPh3ꢁCl: 1H NMR (400 MHz,
CDCl3, 22.7 ꢀC) ꢂ = 0ꢃ91 (s, 14H), 3.58 (s, 4H),
5.57 (d, J = 8ꢃ2 Hz, 4H), 7.52–7.71 (m, 38H); Co-
(d, J = 6ꢃ9 Hz, 2H), 5.36 (d, J = 15ꢃ1 Hz, 4H), 6.68–7.64
(m, 44H), 8.04 (s, 2H).
2.3. Characterization
Molecular structures of metal-salen catalysts were
examined by 1H-NMR (nuclear magnetic resonance)
measurements using a 400 MHz NMR spectrometer
(JeolJNM-LA400 with LFG, JEOL). X-ray photoelectron
spectroscopy (XPS) analyses were conducted to measure
binding energies of Ti 2p3/2 and N 1s for the catalysts
using Mg/Al source (AXIS-HSi, KRATOS). All the spec-
tra were calibrated using C 1s peak (284.5 eV) as a
reference.
1
ꢀ
salen(PPh3ꢁCl: H NMR (400 MHz, CDCl3, 22.7 C) ꢂ =
2ꢃ23 (s, 3H), 4.45 (s, 4H), 6.03 (d, J = 8ꢃ2 Hz, 4H), 7.45–
7.66 (m, 36H), 9.65 (s, 2H).
2.2. Preparation of Ti-Salen Complexes Containing
Various Bridging Groups
H2-salen(PPh3ꢁCl ligand containing tert-butyl substituent
in ortho-position of aromatic aldehyde was synthesized
8354
J. Nanosci. Nanotechnol. 15, 8353–8358, 2015

Kang et al.
Direct Synthesis of Diphenyl Carbonate from Phenol and Carbon Dioxide Over Ti-Salen-Based Catalysts
N
N
N
O
N
O
Ti
M
Z
O
O
PrOi
OiPr
–ClPh3+P
P+Ph3Cl–
Ti-salen
Ti-salen(PPh3)Cl: M = Ti; Z = (OiPr)2
Zr-salen(PPh3)Cl: M = Zr; Z = (OiPr)2
Co-salen(PPh3)Cl: M = Co; Z = OAc
N
O
N
O
N
N
Ti
Ti
O
O
PrOi OiPr
PrOi
OiPr
–ClPh3+P
P+Ph3Cl–
–ClPh3+P
P+Ph3Cl–
Ti-(t-butyl)salen(PPh3)Cl
Ti-(t-butyl)cysalen(PPh3)Cl
N
O
N
O
N
N
Ti
Ti
O
O
PrOi
OiPr
PrOi OiPr
P+Ph3Cl–
–ClPh3+P
–ClPh3+P
P+Ph3Cl–
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Copyright: American Scientific Publishers
Ti-(t-butyl)salphen(PPh3)Cl
Ti-(t-butyl)saldiphen(PPh3)Cl
Figure 2. Metal-salen catalysts prepared in the work.
2.4. Direct Synthesis of DPC from Phenol and
Carbon Dioxide
systematically prepared various metal-salen catalysts as
shown in Figure 2. Successful formation of the catalysts
was confirmed by 1H-NMR analyses. Catalytic perfor-
mance of M-salen(PPh3ꢁCl (M = Ti, Zr, and Co) and
Ti-saꢀlen in the direct synthesis of DPC performed at
100 C and 60 bar for 8 h is listed in Table I (entries
1–4). In the reaction, DPC and PS were produced with-
out any by-products. Ti-salen(PPh3ꢁCl exhibited a bet-
ter catalytic performance than Ti-salen in the reaction
Direct synthesis of DPC from phenol and carbon dioxide
was carried out in a teflon-lined stainless steel autoclave
reactor (100 ml). Methanol (0.37 mol), phenol (0.11 mol),
catalyst (2.1 mmol), and K2CO3 (1.4 mmol) were charged
into the reactor. The reactor was purged with carbon diox-
ide to remove air, and then it was pressurized up to 30
bar using carbon dioxide. After heating the reactor to the
ꢀ
reaction temperature (70–110 C) with constant stirring
(600 rpm), the autoclave was additionally pressurized up
to reaction pressure (50–90 bar) using carbon dioxide. Cat-
alytic reaction was conducted for 8 h. After the reaction,
the reactor was cooled to room temperature and depressur-
ized. Reaction products were sampled and analyzed with
a gas chromatograph (HP 5890 II) equipped with a flame
ionization detector (FID).
Table I. Catalytic performance of various metal-salen complexes in the
direct synthesis of DPC from phenol and carbon dioxide.a
Amount of DPC Amount PS
Entry
Catalyst
(mmol)
(mmol)
1
2
3
4
5
6
7
8
Ti-salen
Ti-salen(PPh3 ꢁCl
Zr-salen(PPh3ꢁCl
Co-salen(PPh3 ꢁCl
0.24
2.01
0.01
0
2.32
2.21
2.88
2.25
0.16
1.16
0.01
0.01
1.00
1.05
1.18
1.03
Ti-(t-butyl)salen(PPh3 ꢁCl
Ti-(t-butyl)cysalen(PPh3 ꢁCl
Ti-(t-butyl)salphen(PPh3 ꢁCl
Ti-(t-butyl)saldiphen(PPh3 ꢁCl
3. RESULTS AND DISCUSSION
3.1. Catalytic Performance of M-Salen(PPh3)Cl
(M = Ti, Zr, and Co) Complexes
Notes: aReaction conditions: methanol (0.37 mol), phenol (0.11 mol), catalyst (2.1
mmol), K2CO3 (1.4 mmol), 100 ꢀC, 60 bar, 8 h.
For functionalization of metal-salen catalysts for direct
synthesis of DPC from phenol and carbon dioxide, we
J. Nanosci. Nanotechnol. 15, 8353–8358, 2015
8355

Direct Synthesis of Diphenyl Carbonate from Phenol and Carbon Dioxide Over Ti-Salen-Based Catalysts
Kang et al.
(entries 1 and 2). This was because introduction of phos-
phonium salts into para-position of aromatic aldehyde of
salen ligand enhanced solubility of the catalyst in the
methanol-phenol solution due to its hydrophobic and ionic
properties.12 It should be noted that the catalytic activ-
ity was significantly changed with respect to central metal
species of the complexes. The amount of DPC decreased
in the order of Ti-salen(PPh3ꢁCl > Zr-salen(PPh3ꢁCl > Co-
salen(PPh3ꢁCl. From these results, it was found that metal-
salen complex containing central titanium atom in the
phosphonium-substituted salen ligand served as an effi-
cient Lewis acid catalyst for the direct synthesis of DPC
from phenol and carbon dioxide.
activity of Ti-salen complexes was changed depending
on the identity of bridging group in the salen ligand.
The amount of DPC decreased in the order of Ti-(t-
butyl)salphen(PPh3ꢁCl > Ti-(t-butyl)salen(PPh3ꢁCl > Ti-
(t-butyl)saldiphen(PPh3ꢁCl > Ti-(t-butyl)cysalen(PPh3ꢁCl.
According to the literatures,16ꢀ17 introduction of various
bridging groups into salen ligand causes change in elec-
tronic property of central metal atom. Thus, it is expected
that Lewis acid strength of central titanium atom as elec-
tron acceptor was favorably tuned by the change of bridg-
ing group in the salen ligand.
3.3. Electronic Properties of Ti-Salen Complexes
Containing Various Bridging Groups
3.2. Catalytic Performance of Ti-Salen Complexes
Containing Various Bridging Groups
In order to elucidate the effect of bridging groups on the
electronic properties of Ti-salen complexes, XPS analy-
ses were conducted. Figure 3 showed the XPS spectra
of central titanium atom (Ti 2p3/2ꢁ and nitrogen atom
(N 1s) of imine group in the catalysts. All the cata-
lysts exhibited binding energies of Ti 2p3/2 at 457.3–
457.7 eV, which were in good agreement with bind-
ing energies of Ti 2p3/2 for organotitanium complexes.18
Interestingly, binding energy of Ti 2p3/2 for Ti-(t-
butyl)salphen(PPh3ꢁCl was significantly higher than that
for Ti-(t-butyl)salen(PPh3ꢁCl, indicating the increment of
Lewis acid strength of central titanium atom. This was
attributed to the conjugation effect caused by double
On the basis of above results, we attempted to mod-
ify ortho-position in aromatic aldehyde and bridging
group of Ti-salen(PPh3ꢁCl as shown in Figure 2. Cat-
alytic performance of Ti-salen complexes containing var-
ious bridging groups in the direct synthesis of DPC
from phenol and carbon dioxide is summarized in
Table I (entries 5–8). It is interesting to note that
the amount of DPC over Ti-(t-butyl)salen(PPh3ꢁCl was
higher than that over Ti-salen(PPh3ꢁCl, while the amount
of PS over Ti-(t-butyl)salen(PPh3ꢁCl was lower than
that over Ti-salen(PPh ꢁCl (entries 2 and 5). Insertion
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3
bond-bridging of phenyl moiety in the rigid salphen lig-
of tert-butyl substituent into saIlPen: 1l9ig1a.n2d43i.s58k.n1o8w7nOtno: Fri, 29 Jan 2016 12:32:10
17
14ꢀ15
Copyright: American Scientific Publishers
and, leading to electron-deficient imine. This was well
demonstrated by observing the increased binding energy
of N 1s of Ti-(t-butyl)salphen(PPh3ꢁCl as presented in
Figure 3. In the case of Ti-(t-butyl)cysalen(PPh3ꢁCl and
Ti-(t-butyl)saldiphen(PPh3ꢁCl, on the other hand, binding
cause steric effect in the catalytic reaction.
There-
fore, tert-butyl substituent in the salen ligand might
induce steric hindrance for rearrangement of intermedi-
ates, leading to selective formation of phenyl carbonate-
like intermediate. It should be noted that the catalytic
398.72 eV
457.72 eV
Ti-(t-butyl)salphen(PPh3)Cl
Ti-(t-butyl)salphen(PPh3)Cl
398.60 eV
457.35 eV
Ti-(t-butyl)salen(PPh3)Cl
Ti-(t-butyl)salen(PPh3)Cl
398.53 eV
457.33 eV
Ti-(t-butyl)saldiphen(PPh3)Cl
Ti-(t-butyl)saldiphen(PPh3)Cl
398.53 eV
457.32 eV
Ti-(t-butyl)cysalen(PPh3)Cl
Ti-(t-butyl)cysalen(PPh3)Cl
454
456
458
460 396
397
398
399
400
Binding energy (eV) of Ti 2p3/2
Figure 3. XPS spectra of Ti 2p3/2 and N 1s for Ti-salen complexes containing various bridging groups.
J. Nanosci. Nanotechnol. 15, 8353–8358, 2015
Binding energy (eV) of N 1s
8356

Kang et al.
Direct Synthesis of Diphenyl Carbonate from Phenol and Carbon Dioxide Over Ti-Salen-Based Catalysts
5
energies of Ti2p3/2 and N 1s slightly decreased compared
to Ti-(t-butyl)salphen(PPh3ꢁCl due to their single bond-
bridging and non-planar conformation in the salen lig-
and. Binding energies of central titanium atom decreased
in the order of Ti-(t-butyl)salphen(PPh3ꢁCl > Ti-(t-
butyl)salen(PPh3ꢁCl > Ti-(t-butyl)saldiphen(PPh3ꢁCl > Ti-
(t-butyl)cysalen(PPh3ꢁCl. This trend was well consistent
with the trend of catalytic performance of the complexes,
indicating that Lewis acid strength of the central titanium
atom served as a crucial factor in the reaction. Among the
catalysts tested, Ti-(t-butyl)salphen(PPh3ꢁCl with the high-
est Lewis acid strength was found to be the most efficient
catalyst in the direct synthesis of DPC.
DPC
PS
4
3
2
1
0
50
60
70
80
90
3.4. Effect of Reaction Conditions on the
Reaction pressure (bar)
Catalytic Performance
Figure 5. Effect of reaction pressure on the catalytic performance in
the direct synthesis of DPC from phenol and carbon dioxide over Ti-(t-
butyl)salphen(PPh3 ꢁCl at 90 ꢀC for 8 h.
The dependence of reaction conditions was investigated
to optimize the catalytic system for the direct synthe-
sis of DPC from phenol and carbon dioxide over Ti-(t-
butyl)salphen(PPh3ꢁCl. The effect of reaction temperature
on the direct synthesis of DPC is shown in Figure 4. It was
found that the amount of DPC exhibited a volcano-shaped
curve with respect to reaction temperature and showed
maximum value at 90 ꢀC. On the other hand, the amount of
PS monotonically increased with increasing reaction tem-
perature, indicating that lower reaction temperature was
required for more selective formation of phenyl carbonate-
sure to 90 bar did not affect the amount of DPC due to
dilution effect by excessive concentration of supercritical
carbon dioxide in the liquid phase.19 Tꢀhe largest amount
of DPC was obtained at 80 bar and 90 C.
4. CONCLUSIONS
Direct synthesis of DPC from phenol and carbon diox-
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ide over various metal-salen catalysts was carried out in
like intermediate in the rearrangement reaction.
IP: 191.243.58.187 On: Fri, 29 Jan 2016 12:32:10
this work. It was revealed that the catalytic activity of Ti-
salen complexes could be favorably tuned by the insertion
of appropriate substituents into para-, ortho-, and bridg-
ing positions of salen ligand. Among the catalysts tested,
Ti-(t-butyl)salphen(PPh3ꢁCl with the highest Lewis acid
strength was found to be the most efficient catalyst in
the direct synthesis of DPC. The catalytic activity of Ti-
(t-butyl)salphen(PPh3ꢁCl could be enhanced by changing
reaction temperature and pressure.
Copyright: American Scientific Publishers
Figure 5 shows the effect of reaction pressure on the
direct synthesis of DPC at 90 C. It was observed that the
ꢀ
amount of DPC increased with increasing reaction pressure
up to 80 bar. In particular, the amount of DPC significantly
increased in the range of 70–80 bar due to phase transition
of carbon dioxide to supercritical state, which enhanced
solubility of carbon dioxide in the methanol-phenol mix-
ture solution. However, further increase of reaction pres-
4
Acknowledgments: This work was supported by Sam-
DPC
sung Cheil Industries Incorporated.
PS
3
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Received: 23 June 2014. Accepted: 8 December 2014.
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8358
J. Nanosci. Nanotechnol. 15, 8353–8358, 2015