Synthesis and characterization of bis[2-(1H-Benzimidazol-2-yl)benzoato] nickel(II), and its use for preparation of dimethyl carbonate from methanol and CO2

Article abstract of DOI:10.1007/s11164-013-1030-6

A new complex, bis[2-(1H-benzimidazol-2-yl)benzoato]nickel(II) (NiL ₂), has been synthesized and characterized. It was used as catalyst for synthesis of dimethyl carbonate (DMC) from methanol and CO₂ in the presence of dicyclohexyl c

Full text of DOI:10.1007/s11164-013-1030-6

Res Chem Intermed
DOI 10.1007/s11164-013-1030-6
Synthesis and characterization of bis[2-(1H-
benzimidazol-2-yl)benzoato]nickel(II), and its use
for preparation of dimethyl carbonate from methanol
and CO2
•
Yanning Shi Xilang Jin Yechen Hu Shuli Wang
•
•
•
•
Jianli Li Zhen Shi
Received: 12 November 2012 / Accepted: 7 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract
A
new complex, bis[2-(1H-benzimidazol-2-yl)benzoato]nickel(II)
(NiL2), has been synthesized and characterized. It was used as catalyst for synthesis
of dimethyl carbonate (DMC) from methanol and CO2 in the presence of dicyclo-
hexyl carbodiimide (DCC) as promoter. The effects of temperature, reaction time,
and amount of catalyst on the reaction, and the reusability of the catalyst, were
investigated. A catalytic mechanism is proposed. The results revealed that
NiL2 ? DCC had excellent catalytic activity, because this catalytic system pro-
moted facile formation of DMC, with acceptable yield, in the presence of a small
amount of NiL2 and under moderate conditions (80 °C, 1.0 MPa). Moreover, the
catalyst has been consecutively used five times without significant loss of activity.
Keywords Dimethyl carbonate ꢀ Methanol ꢀ NiL2 ꢀ DCC
Introduction
Dimethyl carbonate (DMC) is an important compound in so-called green chemistry.
DMC is very important as an environmentally benign synthetic reagent because it
works as a nontoxic substitute for phosgene, dimethyl sulfate, and methyl halides in
methylation and carbonylation processes [1]. DMC can also be used as an
environmentally friendly fuel additive and as a polar solvent [2, 3]. Currently,
several processes have been proposed for industrial synthesis of DMC, including the
methanolysis of phosgene [4], oxidative carbonylation of methanol [5], and
Electronic supplementary material The online version of this article (doi:10.1007/s11164-013-1030-6)
contains supplementary material, which is available to authorized users.
Y. Shi ꢀ X. Jin ꢀ Y. Hu ꢀ S. Wang ꢀ J. Li (&) ꢀ Z. Shi
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education,
College of Chemistry and Materials Science, Northwest University, Xi’an 710069, Shaanxi, China
e-mail: lijianli@nwu.edu.cn
123

Y. Shi et al.
H
N
Fig. 1 The proposed structure
of NiL2
N
O
O
O
O
Ni
N
N
H
transesterification [6]. However, these processes have disadvantages. In contrast,
synthesis of DMC from methanol and CO2 (Eq. 1) is an interesting alternative
synthetic method from the perspectives of both atom economy and risk reduction
[7].
O
ð1Þ
2 MeOH
+
CO2
+
H2O
MeO OMe
This method of synthesis of DMC is appealing environmentally but poses
significant challenges, because the reaction is thermodynamically quite unfavorable.
To circumvent this problem, several promoters, for example dimethoxypropane
(DMP) [8], acetals [9], and molecular sieves [10], have been used as dehydrating
reagents for this process to remove water and thus facilitate formation of DMC.
Recently dicyclohexyl carbodiimide (DCC) has been found to be a very active
promoter under milder conditions [11]. The effectiveness of this promoter is
attributed to conversion of carbodiimide into the relevant urea (Eq. 2). This also
makes the overall reaction thermodynamically more favorable. A variety of
catalysts have been used for direct synthesis of DMC, including organometallic
compounds [12], ZrO2 [13], heteropolyacid [14], and supported Cu–Ni bimetal
catalysts [15].
O
O
ð2Þ
+ CO2
+
CyN
C
NCy
+
CyNHCNHCy
2 MeOH
MeO OMe
It has been reported that Ni(OAc)2 has good catalytic activity in CO2-based
carboxylation of methanol [16]. DMC can be produced with high yield and
selectivity when the synthesis is conducted under near supercritical conditions. This
rigorous method is difficult to control, however. In the course of studying the
synthesis of DMC from CH3OH and CO2, we found that NiL2 (Fig. 1) was an
effective catalyst for formation of DMC in the presence of DCC (as promoter). The
catalyst promoted facile formation of DMC at 80 °C and moderate pressure of CO2
(1.0 MPa) with high yield.
In this paper, we describe the synthesis and characterization of NiL2, which has
proved to be an excellent catalyst in the one-step synthesis of DMC. The effect on
the process of temperature, reaction time, and amount of catalyst, and the reusability
of the catalyst, have been investigated. A catalytic mechanism has been proposed.
123

Characterization of bis[2-(1H-benzimidazol-2-yl)benzoato]nickel(II)
Experimental
Reagents and characterization techniques
o-Phthalic acid, o-phenylenediamine, PPA, DCC, NaOH, Ni(OAc)2ꢀ4H2O,
NiCl2ꢀ6H2O, NiSO4ꢀ7H2O, Ni(NO3)2ꢀ6H2O, and methanol were all analytical-
reagent or guaranteed-reagent-grade. CO2 was 99.999 % pure.
Elemental analysis (C, H, and N) was performed with a Vario EL-III CHNOS
instrument. FT-IR spectra in the range 4,000–400 cm-1 were recorded on a Bruker
1
Equinox-550 spectrophotometer, using the KBr pellet technique. H and 13C NMR
spectra were obtained at room temperature by use of a Varian Inova 400 MHz
apparatus and tetramethylsilane (TMS) for reference. Molecular weights were
measured by electrospray ionization mass spectrometry (ESI-MS).
Catalyst preparation
The catalyst was prepared by reaction of Ni(OAc)2ꢀ4H2O with 2-(1H-Benzimidazol-
2-yl)benzoic acid. The ligand was synthesized by polyphosphoric acid (PPA)-
catalyzed condensation of o-phthalic acid with o-phenylenediamine [17]
(Scheme 1).
Synthesis of 2-(1H-benzimidazol-2-yl)benzoic acid (HL)
o-Phenylenediamine (10.8 g, 0.1 mol) and o-phthalic acid (16.6 g, 0.1 mol) were
mixed with a sufficient quantity of PPA to give a stirrable paste. The mixture was
heated slowly to 250 °C, and the resulting solution was stirred at 250 °C for 4 h, left
to cool to approximately 100 °C, and poured into a large volume of rapidly stirred
water. The mixture was then cooled on ice and made alkaline by the addition of
50 % sodium hydroxide solution. The alkaline slurry was isolated by filtration,
dried, and recrystallized from suitable solvents. Yield: 71 %. MP: 278 °C. FT-IR:
m, cm-1, 3355, 2739, 1625, 1538, 1490, 1448, 1319, 1283, 1152, 1117, 1001, 920,
1
860, 829, 786, 749, 707, 648, 523, 479. H NMR (400 MHz, DMSO-d6) d 8.22
(s, 1H), 7.89 (s, 1H), 7.65 (m, 3H), 7.50–7.25 (m, 3H). 13C NMR (101 MHz,
DMSO-d6) d 158.77, 144.53,.144.37, 140.87, 140.70, 135.83, 133.70, 124.19.
H
N
HOOC
N
N
NH2
NH2
COOH
COOH
O
PPA
Ni(OAc)2.4H2O
NaOH /CH3OH
O
O
Ni
+
O
N
H
N
HL
N
H
NiL2
Scheme 1 Preparation of the catalyst NiL2
123

Y. Shi et al.
Synthesis of Ni complex (NiL2)
HL (0.476 g, 2.0 mmol) and NaOH (0.080 g, 2.0 mmol) were dissolved in a
sufficient amount of methanol, then Ni(OAc)2ꢀ4H2O (0.249 g, 1.0 mmol) in 20 ml
methanol was added to this solution with stirring. The product began to precipitate
from the solution. After 1 h at room temperature, the solid was isolated by filtration,
washed with methanol, and air-dried. The yield was 82 %. Anal calcd. for
C28H18N4NiO4 %: C 63.08, H 3.40, N 10.51; found C 62.95, H 3.32, N 10.63.
HRMS (ESI): calcd. for C28H18N4NiO4: 533.0754 (M ? H); found: 533.0748.
FT-IR: m, cm-1, 3423, 1619, 1561, 1493, 1435, 1380, 1324, 1285, 1229, 1140, 1092,
1004, 880, 810, 746, 650, 588, 481, 435, 417.
DMC synthesis
High-pressure reactions were conducted in a 250-ml stainless steel autoclave
equipped with a mechanical stirrer. The reaction temperature was monitored by
means of an internal thermocouple. General procedure for synthesis of DMC from
methanol and CO2 [18]: methanol (32.00 g, 1 mol), NiL2 (0.53 g, 1.0 mmol), and
DCC (2.06 g, 10 mmol) were placed in the autoclave, which was sealed and flushed
with CO2 three times, then pressurized to 1.0 MPa. The mixture was stirred at the
desired temperature. After a reaction time of 6 h, the reactor was cooled to room
temperature, and pressure was gently released. Reaction products were separated
and quantitatively analyzed by GC (capillary column) with flame ionization
detection (FID), with n-butyl alcohol as internal standard. All the volatile products
were also characterized with GC–MS using a Shimadzu GC-2010 gas chromato-
graph connected to a GCMS-QP 2010 Plus mass spectrometer.
Results and discussion
Structural characterization of the catalyst
The IR spectrum of HL contains a strong mN–H stretching band at 3,355 cm-1. A
broad absorption band at 3,200–2,500 cm-1 is attributed to intramolecular hydrogen
bonding. Bands at 1625, 920, and 749 cm-1 were attributed to mC=O, dOH, and the
1
four adjacent H atoms in the phenyl ring, respectively. H NMR and 13C NMR
spectra of HL were also consistent with the structure.
The mOH, mC=O, and dOH absorption bands are not observed in the IR spectrum of
NiL2, indicating Ni–O bond formation. There is a shift of the C=N vibration to
lower frequencies (1,619 cm-1), which is ascribed to N–metal coordination. The
presence of bands at 435 and 481 cm-1 for NiL2, ascribed to mNi–O and mNi–N
vibrations, respectively, are conclusive evidence of bonding of the oxygen and
nitrogen atoms to the nickel ion. HRMS data of for NiL2 correspond perfectly to the
expected [M ? H]? m/z value. From these results and similar complexes reported in
literature [19], it can be concluded that the HL is coordinated to the metal ion via the
123

Characterization of bis[2-(1H-benzimidazol-2-yl)benzoato]nickel(II)
Table 1 Catalytic activity of NiL2
Entry
Catalysta
Promoter
DMC yieldb (%)
1
2
3
4
5
6
7
Ni(OAc)2
NiL2
None
None
DCC
DCC
DCC
DCC
DCC
0
0
Ni(OAc)2
NiCl2
11
10
9
NiSO4
Ni(NO3)2
NiL2
10
34
Reaction conditions: CH3OH (1.0 mol), CO2 (1.0 MPa), catalyst (1 mmol), DCC (10 mmol), 70 °C, 6 h
a
Ni(OAc)2, NiCl2, NiSO4, and Ni(NO3)2 were obtained from Ni(OAc)2ꢀ4H2O, NiCl2ꢀ6H2O,
NiSO4ꢀ7H2O, Ni(NO3)2ꢀ6H2O, respectively
Based on DCC
b
phenolate oxygen and the imidazole nitrogen atoms; the proposed structure of the
complex is depicted in Fig. 1.
Catalytic activity of NiL2
The catalytic activity of NiL2 was tested in the synthesis of DMC from methanol
and CO2. Obviously, as shown in Table 1, NiL2 alone cannot drive the reaction
under these conditions. The same was observed for Ni(OAc)2. This is because the
reaction is thermodynamically quite unfavorable. To identify the optimum catalyst
for the reaction promoted by DCC, several nickel salts were examined, including
Ni(OAc)2, NiCl2, NiSO4, and Ni(NO3)2. However, they had much lower activity
(entries 3–6). In contrast, remarkable enhancement of catalytic activity was
observed for NiL2 (entry 7).
Effect of reaction temperature on formation of DMC
The effect of reaction temperature on the formation of DMC was investigated in the
range 50 to 100 °C. The results, presented in Table 2, show that a lower reaction
temperature is more favorable for DMC formation, and maximum yield of DMC
was observed at 80 °C. This result is in agreement with thermodynamic calculation
and with those reported in literature [16, 20].
Effect of reaction time on the formation of DMC
Results from study of the effect of reaction time on the yield of DMC are listed in
Table 3. Experiments showed that the yield of DMC rapidly increased up to 3 h.
This result indicates that NiL2–DCC enables very effective formation of DMC.
When the reaction time was longer than 3 h, the yield of DMC decreased. This is
because of decomposition of DCC and reversal of the equilibrium.
123

Y. Shi et al.
Table 2 Effect of temperature on the formation of DMC
Entry
Temp. (°C)
DMC yielda (%)
1
2
3
4
5
6
50
60
18
26
34
47
40
33
70
80
90
100
Reaction conditions: CH3OH (1.0 mol), CO2 (1.0 MPa), NiL2 (1 mmol), DCC (10 mmol), 6 h
a
Based on DCC
Table 3 Effect of reaction time on the formation of DMC
Entry
Reaction time (h)
DMC yielda (%)
1
2
3
4
5
6
2
3
42
58
47
32
31
21
6
9
12
24
Reaction conditions: CH3OH (1.0 mol), CO2 (1.0 MPa), NiL2 (1 mmol), DCC (10 mmol), 80 °C
a
Based on DCC
Effect of amount of catalyst on the formation of DMC
Table 4 summarizes the effect of the amount of catalyst on the synthesis of DMC. It
was found that use of 0.5 mmol NiL2 resulted in maximum yield of DMC. When the
amount of catalyst was increased beyond 0.5 mmol, the yield of DMC remained
stable. This indicates that 0.5 mmol catalyst was sufficient to obtain DMC in good
yield under the conditions specified.
Reusability of the catalyst in the formation of DMC
The reusability of catalysts is one of the important benefits of large-scale operations
and commercial applications. Therefore, we investigated the reusability of NL2 in
the synthesis of DMC from methanol and CO2. Significant loss of the catalytic
activity was not observed even after five runs (Table 5). The catalyst can be
recovered by simple filtration, washing, and drying at 60 °C.
123

Characterization of bis[2-(1H-benzimidazol-2-yl)benzoato]nickel(II)
Table 4 Effect of amount of catalyst on the formation of DMC
Entry
Amount of catalyst (mmol)
DMC yielda (%)
1
2
3
4
5
0.25
0.5
1
27
61
58
60
59
2
4
Reaction conditions: CH3OH (1.0 mol), CO2 (1.0 MPa), DCC (10 mmol), 80 °C, 3 h
a
Based on DCC
Table 5 Reusability of the catalyst in the formation of DMC
Entry
Run no.
DMC yielda (%)
1
2
3
4
5
1
2
3
4
5
61
60
59
58
58
Reaction conditions: CH3OH (1.0 mol), CO2 (1.0 MPa), NiL2 (0.5 mmol), DCC (10 mmol), 80 °C, 3 h
a
Based on DCC
O
O
O
CH3OH
[Ni]
1
step A
step D
CH3
Cy
O
N
H
δ+
CyHN
O
δ
δ
CyHN
NHCy
[Ni] :OCH3
O
O
CH3
6
O
H
2
δ-
4
[Ni]
H2O
step B
step C
CO2
δ-
[Ni]
δ+
O
O
O
C
H
CyN
C NCy
CH3
O
CH3
CyHN
3
N
CH3OH
Cy
5
Scheme 2 The proposed catalytic reaction mechanism
Proposed reaction mechanism
On the basis of the experimental results and the literature [11, 16], we proposed a
plausible catalytic cycle (Scheme 2). In step A, [Ni] 1 is regarded as the catalytic
123

Y. Shi et al.
species for activation of methanol. Step B is insertion of CO2 into an Ni–O bond
with formation of 3. At the same time, addition of methanol to the cumulene forms
o-methylisourea 5 that may further interact with 3 to form 4. The last step is
conversion of 4 into DMC and urea 6 with regeneration of the catalyst. The urea 6
can also be reconverted into DCC [21].
Conclusions
In summary, we have developed a new catalytic system for formation of DMC from
methanol and CO2. The experimental results show that NiL2–DCC had excellent
catalytic activity, because the catalytic system, containing a small amount of NiL2,
promoted facile formation of DMC under moderate conditions (80 °C, 1.0 MPa)
with an acceptable yield. The NiL2 could be easily synthesized, recovered simply,
and consecutively reused. The reaction described in this article may further be
developed for application to the synthesis of DMC.
Acknowledgments This work was supported by the National Natural Science Foundation of China (nos
20972124, 21272184), the Shaanxi Science and Technology Co-ordination Innovation Engineering
Project (No. 2011K12-77), the Special Science Research Foundation of Education Committee in Shaanxi
Province (No. 12JK0584), and the Xi’an City Science and Technology Project (No. CXY1123-1).
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