Enhancements of dimethyl carbonate synthesis from methanol and carbon dioxide: The in situ hydrolysis of 2-cyanopyridine and crystal face effect of ceria

Article abstract of DOI:10.1016/j.cclet.2015.05.005

This paper describes the effect of the in situ hydrolysis of 2-cyanopyridine and its derivatives on the synthesis of dimethyl carbonate (DMC) from CO₂ and methanol over CeO₂. 2-Cyanopyridine, with the highest electronic charge number of the carbon in the cyanogroup, is the most effective agent to accelerate the desired reaction by a decrease of water. CeO₂ (1 1 0) planes are active for the hydrolysis of 2-cyanopyridine, further enhancing the DMC formation by in situ removal of water effectively. The DMC yield is improved drastically up to 378.5 mmol g cat^-1 from 12.8 mmol g cat^-1 with the in situ hydrolysis of 2-cyanopyridine over rod-CeO₂ (1 1 0) catalyst.

Full text of DOI:10.1016/j.cclet.2015.05.005

Chinese Chemical Letters 26 (2015) 1096–1100
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Enhancements of dimethyl carbonate synthesis from methanol and
carbon dioxide: The in situ hydrolysis of 2-cyanopyridine and crystal
face effect of ceria
Sheng-Ping Wang *, Jing-Jie Zhou, Shu-Yang Zhao, Yu-Jun Zhao, Xin-Bin Ma
Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
This paper describes the effect of the in situ hydrolysis of 2-cyanopyridine and its derivatives on the
Received 2 April 2015
synthesis of dimethyl carbonate (DMC) from CO
highest electronic charge number of the carbon in the cyanogroup, is the most effective agent to
accelerate the desired reaction by a decrease of water. CeO (1 1 0) planes are active for the hydrolysis of
-cyanopyridine, further enhancing the DMC formation by in situ removal of water effectively. The DMC
2 2
and methanol over CeO . 2-Cyanopyridine, with the
Received in revised form 14 April 2015
Accepted 27 April 2015
Available online 15 May 2015
2
2
ꢀ
1
ꢀ1
yield is improved drastically up to 378.5 mmol g cat from 12.8 mmol g cat with the in situ hydrolysis
of 2-cyanopyridine over rod-CeO (1 1 0) catalyst.
Keywords:
2
Dimethyl carbonate
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
CeO
Hydrolysis
-Cyanopyridine
Crystal face
2
Published by Elsevier B.V. All rights reserved.
2
1
. Introduction
the reaction system, but it is extremely difficult for physical drying
agents to absorb water from the mixture of methanol and CO
high temperature [18]. Therefore, a variety of chemical dehydrated
reagents, such as butylene oxide [19], CH I [20] and acetonitrile
[21], have been exploited for in situ removal of water, which has
the advantage of working even far above 100 8C. Compared with
other dehydration agents, 2-cyanopyridine is easy to hydrolyze
and regenerate by dehydration very conveniently, especially, the
2
at
Dimethyl carbonate (DMC) has attracted much attention in the
chemical field as an environmentally benign and biodegradable
chemical in recent years [1–4]. The applications of DMC have
covered a variety of fields [5,6]. As reported previously, several
reaction routes have been presented for DMC formation [7,8]. The
3
2
direct synthesis of DMC from methanol and CO is highly favorable
since such a method is environmentally benign by nature [9]. A
great many catalysts have been employed in the direct synthesis of
carbonic ester over recent decades, such as Cu based catalysts
hydrolysis of 2-cyanopyridine which also can be catalyzed by CeO
2
[22]. This is more favorable for a co-existed reaction system using
the same catalyst for DMC formation. Therefore, the DMC yield
improved effectively using 2-cyanopyridine as the dehydrated
[
10,11], cerium-based catalysts [12–14], and zirconia-based
catalysts [1,15]. Among them, nanoceria has shown the favorable
catalytic performance for the reaction of methanol and CO [16].
2 2
agent over CeO catalyst [23,24]. CeO possesses three crystal faces
2
(1 1 0), (1 1 1), and (1 0 0). The theoretical simulations have
indicated that the different crystal faces exhibit different proper-
ties, further influencing the catalytic activity [25]. Our preliminary
However, the DMC yield is far from satisfactory because of the
thermodynamic limitation of the direct synthesis of DMC reaction.
In situ removal of water coproduced in the reaction system is an
important strategy to effectively improve the yield of DMC. Several
studies have shown that rod-CeO
face showed more favorable catalytic performance than cube-CeO
(1 0 0) and octahedron-CeO (1 1 1) for the direct synthesis of DMC
[26]. It is therefore necessary to study the effect of the crystal face
of CeO on the hydrolysis of 2-cyanopyridine, and its further
2
with the most (1 1 0) crystal
2
dehydration agents have been applied to remove H
reaction system [17]. It is convenient to add physical drying agents
to decrease H O because there is no other product coproduced in
2
O from the
2
2
2
influence on DMC formation.
On the other hand, the hydrolysis performance of the
derivatives of 2-cyanopyridine is likely to be different because
the derivatives of 2-cyanopyridine with a similar structure possess
*
Corresponding author.
E-mail address: spwang@tju.edu.cn (S.-P. Wang).
http://dx.doi.org/10.1016/j.cclet.2015.05.005
001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
1

S.-P. Wang et al. / Chinese Chemical Letters 26 (2015) 1096–1100
1097
Table 1
The yield of DMC with different dehydration agents.
Dehydration agent
2-Cyano pyridine
350.7
2-Cyano furan
231.9
2-Cyano-5-fluorpyridine
333.8
5-Chloro-2-cyanopyridine
163.2
5-Bromo-2-cyanopyridine
59.4
ꢀ
1
DMC/mmolg cat
Reaction conditions: Amorphous CeO
2
catalyst mcat = 0.1 g, Vmethanol = 15 mL, PCO ¼ 5 MPa, t = 3 h, ndehydration = 50 mmol, T = 140 8C.
2
a different electron charge of the carbon in the cyanogroup. Thus,
it is essential to evaluate the influence of the derivatives of
was purged with CO
2
. After that, the autoclave was pressured with
CO to 5 MPa and heated to 140 8C for 3 h with mechanical stirring.
2
2
-cyanopyridine with the different substituents on DMC forma-
tion. This paper reports the effective synthesis of DMC from
methanol and CO with the co-existed hydrolysis reaction of
-cyanopyridine and its derivatives over CeO catalysts. The
electronic charge number of the carbon in the cyano group of the
-cyanopyridine and its derivatives with different substitute
atoms and heterocyclic ring atoms were evaluated through DFT
calculations. The influence of morphologies of CeO with different
crystal faces (1 1 0), (1 0 0) and (1 1 1) on the hydrolysis of the
-cyanopyridine was carried out to determine the active crystal
face for the hydrolysis reaction and co-existed formation of DMC.
2.4. The DFT calculation of the 2-cyanopyridine and its derivatives
2
2
2
The stable structure of the 2-cyanopyridine and its derivatives
was optimized by the DFT/B3LYP/DNP. All the atoms in the stable
structure were further conducted by a population analysis.
2
2
2.5. The hydrolysis of 2-cyanopydine over CeO
morphology
2
with a different
2
In a typical procedure for the hydrolysis of 2-cyanopyridine to
-picolinamide, 0.1 g 2-cyanopyridine, 3 g H O, and 0.03 g CeO
2
2
2
with different morphologies were added into the autoclave. The
2
. Experimental
resulting mixture was vigorously stirred at 140 8C. After the
reaction, the mixture was extracted with CHCl
. Results and discussion
In the absence of 2-cyanopyridine, the yield of DMC is only
3
three times.
2
.1. Catalyst preparation
3
Amorphous CeO
2
was prepared by calcinating the Ce(N-
O
3
)
3
ꢁ6H O at 600 8C for 3 h. This kind of catalyst was used to
2
ꢀ1
optimize the dehydration agents.
CeO catalysts with the different morphologies were prepared
by the hydrothermal method. For the octahedron-CeO , 4 mmol
Ce(NO O and 0.04 mmol Na PO O were dissolved in
distilled water, respectively with 80 mL of distilled water. After
being stirred at room temperature for 1 h, the solution was put into
the 100 mL Teflon-lined stainless autoclave. Subsequently, the
autoclave was transferred to the drying oven and heated for 12 h at
8.6 mmol g cat in the direct synthesis reaction from methanol and
CO over amorphous CeO catalyst. It is noticeable that the maximal
production of the DMC reached 350.7 mmol g cat
2
2
2
ꢀ
1
2
with the
3
)
3
ꢁ6H
2
3
4
ꢁ12H
2
addition of 50 mmol 2-cyanopyridine. It is a much higher yield
than that obtained in the presence of 2,2-dimethoxypropaneby
2 2
using CeO -ZrO as a catalyst, which reported that the DMC yield
ꢀ1
ꢀ1
increased from 1.6 mmol g cat to 14.0 mmol g cat [27]. There-
fore,itisquiteeffectiveforthepromotionofDMCformationbyusing
1
60 8C. The precipitates were obtained by filtration, washed with
ethanol and deionized water several times, and then dried at 80 8C
for 12 h and calcined at 600 8C for 5 h to get octahedron CeO
A hydrothermal process was adopted to obtain rod-CeO
cube-CeO . 480 mmol NaOH and 4 mmol Ce(NO O were
2
the in situ hydrolysis of 2-cyanopyridine over a CeO catalyst.
2
.
3.1. Effect of a different dehydration agent
2
and
2
3
)
3
ꢁ6H
2
Table 1 illustrates the dependence of DMC formation in the
direct synthesis reaction of DMC over amorphous CeO with
different dehydration agent. As Table 1 shows, the highest yield of
dissolved in 10 and 70 mL of deionized water, respectively. After
being stirred at room temperature for 30 min, the purple slurry
was transferred into a 100 mL Teflon-lined stainless autoclave and
2
ꢀ1
DMC, 350.7 mmol g cat , was obtained with the addition of 2-
cyanopyridine.
2 2
heated at 90 8C (170 8C) for 24 h to obtain rod-CeO (cube-CeO ).
After the hydrothermal treatment, the precipitates were separated
by filtration, washed with deionized water and ethanol several
times. After drying at 80 8C for 24 h, the products were calcined at
According to the reaction path reported previously (shown in
2
Fig. 1) [22], the nitrile hydration by CeO starts with the
6
00 8C for 5 h.
2.2. Catalyst characterization
2
The morphology micrographs of the CeO samples were taken
by both a JEM-2100F transmission electron microscope (TEM) and
a high resolution transmission electron microscopy (HRTEM) by
2
using an accelerating voltage of 200 kV. The CeO samples were
characterized by a scan electron microscope (SEM) conducted
under high vacuum on a Hitachi S-4800.
2.3. The synthesis of DMC from CH
3 2
OH and CO with 2-cyanopyridine
and its derivatives
The reaction was carried out in a stainless steel autoclave
reactor with an inner volume of 100 mL. In a typical procedure,
1
5 mL CH
3
OH, 50 mmol 2-cyanopyridine or its derivatives and
0
.1 g catalyst were transferred into the autoclave before the reactor
Fig. 1. The mechanism of 2-cyanopyridine hydrolysis [22].

1
098
S.-P. Wang et al. / Chinese Chemical Letters 26 (2015) 1096–1100
2-cyanopyridine. So, the carbon in the cyanogroup of the 2-
cyanopyridine has a more positive charge number than its
derivatives because of the electron-withdrawing effect of the
nitrogen in the ring. With the addition of the 2-cyanopyridine,
ꢀ
1
the yield of DMC is 350.7 mmol g cat . Comparatively, 2-cyano-5-
fluorpyridine, 5-chloro-2-cyanopyridine, and 5-bromo-2-cyanopyr-
ꢀ1
idine yielded only 333.8, 163.2 and 59.4 mmol g cat of DMC,
respectively, which suggests that a substitute group at the 5-
position relative to the nitrogen atom is not favorable for the DMC
formation. So, the hydration activity of cyano group is a positive
correlation with the electronic charge number of the carbon in the
cyanogroup. The 2-cyanopyridine, which had the highest electronic
charge number of the carbon in the CN and the most negative charge
in the nitrogen, was suitable for dehydration and DMC formation.
As to 2-cyanopyridine and 2-cyanofuran, as shown in Fig. 2, the
oxygenof2-cyanofuranhasamore negativechargenumberthanthe
nitrogen of 2-cyanopyridine, indicating its more strong adsorption
Fig. 2. Electronic charge of C atom and N atom of the different dehydration agents.
hydrogen; : carbon; : nitrogen; : oxygen; : fluorine; : chlorine;
bromine. (a) 2-Cyano-5-fluorpyridine; (b) 5-chloro-2-cyanopyridine; (c) 5-
bromo-2-cyanopyridine; (d) 2-cyanopyridine; (e) 2-cyanofuran.
:
:
n+
ability on Ce . However, the charge number of the carbon atom in
the cyanogroup of 2-cyanofuran is less than that in 2-cyanopyridine,
d
ꢀ
leading to the weak adsorbing ability of OH . With the addition
of 2-cyanopyridine and 2-cyanofuran, the yield of DMC was
350.7 mmol g cat and231.9 mmol g cat , respectively.Itdemon-
strates that the hydrolysis capacity of the cyanogroup with
heteroatom N located adjacent to the a carbon in the CN group is
superior to the cyanogroup with heteroatom O.
d
+
d
ꢀ
ꢀ1
ꢀ1
dissociation of H
and CeO change to the nitrile–CeO
This nitrile–CeO complex will undergo an addition of H to a
nitrile carbon atom to give the amide, accompanied by a
2
O on CeO
2
to give H and OH (Step 1). Nitrile
2
2
adsorption complex (Step 2).
d
ꢀ
2
regeneration of the adsorption site on CeO
the rate-determining step.
2
(Step 3). Step 3 is
3.2. The active crystal face for the 2-cyanopyridine hydrolysis
From the reaction route of Step 3, the nitrogen (or oxygen) in
n+
the ring is adsorbed on the Ce . The more negative charge (which
3.2.1. Morphological and structural characterization
indicates the more Lewis basicity) of the nitrogen (or oxygen)
atom, the more easy it is to hydrolyze. On the other hand, the OH
Morphologies and crystal sizes of the three ceria catalysts were
characterized by SEM, TEM, and HRTEM, as shown in Figs. 3–5.
d
ꢀ
attacks the carbon in the cyanogroup. The more positive charge of
the carbon is a benefit to hydrolyzation. Thus, it can be seen that
the electronic charge of the carbon atom in the CN and the nitrogen
2
Fig. 3a is the SEM image of the octahedron CeO . The octahedrons
possessed perfect morphology and mostly perfect crystallinity
with smooth surfaces and clear-cut edges and corners. It is
noticeable that the predominately crystal faces of the octahedron
(or oxygen) atom in the ring are vital to the hydrolysis of the
cyanogroup. Fig. 2 shows the DFT calculation results of a carbon
charge in the CN and nitrogen charge in the ring of 2-cyanopyridine
and its derivatives. From the data we can see that the 2-
cyanopyridine possessed the most negative charge of the nitrogen
and most positive charge of the carbon in the cyanogroup.
Comparatively, for its derivatives having an electron-withdrawing
group of F, Cl, and Br, the electronic charge number of the carbon
was 0.260, 0.241, and 0.198, respectively, which is lower than that
of 2-cyanopyridine. Although the substitution of hydrogen of 2-
cyanopyridine by the F, Cl, Br is favorable for the enhancement of
the positive charge number of the carbon in the cyanogroup, the
electronic charge number of the carbon in cyanogroup is also
affected by the nitrogen atoms in the ring of the 2-cyanopyridine
and its derivatives, which has the opposite effect on the carbon.
As shown in Fig. 2, the electronic charge number of the nitrogen in
the ring of 2-cyano-5-fluorpyridine, 5-chloro-2-cyanopyridine,
and 5-bromo-2-cyanopyridine was less negative than that of
CeO
interplanar spacing of 0.311 nm calculated from the FFT pattern
(Fig. 3c). From the SEM image of uniform cube-CeO exhibited in
Fig. 4a, the edge length of the cube-CeO was about 50 nm. The
interplanar spacing was calculated to be 0.263 nm of the mainly
exposed planes which was corresponding to (1 0 0) planes of CeO
showing that the cube-CeO was comprised by six (1 0 0) planes
(Fig. 4c). This could be clearly seen in the staggered distributed
rod-CeO from Fig. 5a. Nano-rods had a diameter of 4 nm and a
2
(HRTEM image in Fig. 3c) was (1 1 1) planes with an
2
2
2
,
2
2
length ranging from 60 nm to 200 nm (Fig. 5a). The TEM images
revealed that the nano-rods exposed a large amount of (1 1 0) with
small (1 1 1) planes (Fig. 5c).
3.2.2. Catalytic activities of the different morphological CeO
Catalytic tests in DMC synthesis from CO and methanol were
performed over ceria catalysts with the different morphologies of
octahedron-CeO , rod-CeO and cube-CeO , respectively. The results
2
catalysts
2
2
2
2
2
Fig. 3. Images of octahedron-CeO particles: (a) SEM, (b) TEM, (c) HRTEM.

S.-P. Wang et al. / Chinese Chemical Letters 26 (2015) 1096–1100
1099
2
Fig. 4. Images of cube-CeO particles: (a) SEM, (b) TEM, (c) HRTEM.
2
Fig. 5. Images of rod-CeO particles: (a) SEM, (b) TEM, (c) HRTEM.
are presented in Table 2. The three samples exhibited different
catalytic activities, in the order of rod-CeO > octahedron-
CeO > cube-CeO . Rod-CeO catalysts exhibited the best catalytic
performance, 12.8 mmol g cat of DMC, nearly 8.5 times of the
DMC yield over cube-CeO
smallest activity for the 2-cyanopyridine hydration among the
three ceria catalysts with different morphologies. The (1 1 0)
crystal face manifested better activity than (1 1 1) and (1 0 0) for
DMC formation co-existing with 2-cyanopyridine hydration. Thus,
it can be inferred that (1 1 0) plane is an active factor for hydrolysis
of 2-cyanopyridine, further accelerating the formation of DMC.
To further verify the favorable catalytic performance of rod-
2
2
2
2
ꢀ
1
2
.
The results of the addition of 2-cyanopyridine to the DMC direct
synthesis system under the same condition with a different
morphological catalyst were also displayed in Table 2. It could be
clearly seen that the DMC yield increased surprisingly up to
CeO
idine hydration, the hydrolysis of the 2-cyanopyridine catalyzed by
CeO with different morphologies was conducted, respectively,
and the results were shown in Table 3. It is worth noting that rod-
CeO with a crystal face of (1 1 0) exhibited remarkable catalytic
performance with the conversion of 2-cyanopyridine reaching
93.7%, in comparison to the conversion of 6.7% and 21.4% with CeO
2
exposing the most (1 1 0) crystal faces for the 2-cyanopyr-
ꢀ1
ꢀ1
3
78.5 mmol g cat
from 12.8 mmol g cat , showing that rod-
2
CeO
2
which exposed the most (1 1 0) crystal face, exhibited
outstanding catalytic performance in the hydrolysis of 2-cyano-
pyridine, further speeding up the DMC formation significantly.
2
ꢀ1
Comparatively, the least DMC yield of 9.6 mmol g cat from CO
and methanol was obtained over cube-CeO with the (1 0 0) crystal
2
2
2
(1 0 0) and (1 1 1). Therefore, (1 1 0) is the most active crystal face
for 2-cyanopyridine hydrolysis. The much higher DMC yield was
obtained because of the effective removal of water by the in situ
face in the presence of 2-cyanopyridine, meaning that it had the
hydrolysis of 2-cyanopyridine over rod-CeO
2
exposing the most
Table 2
(
1 1 0) crystal face in the co-existed reaction system.
2
The DMC yield with the different morphologies of CeO .
ꢀ
1
Catalyst
DMC/mmol gcat
4. Conclusions
Without 2-cyanopyridine
With 2-cyanopyridine
The synthesis of dimethyl carbonate from carbon dioxide and
methanol was drastically enhanced by the in situ hydrolysis of 2-
Rod-CeO
2
12.8
3.0
378.5
65.5
9.6
Octahedron-CeO
Cube-CeO
2
2
cyanopyridine over the CeO catalyst. The hydrolysis properties of
2
1.5
cyanogroup of 2-cyanopyridine and its derivatives were related to
the electronic charge number of carbon in the cyanogroup. 2-
Cyanopyridine with the highest carbon electronic charge in
cyanogroup was the most preferable dehydrant, correspondingly,
resulting in the highest DMC yield. The DMC yield obtained by the
Reaction conditions: Vmethanol = 15 mL, PCO ¼ 5 MPa, t = 3 h, ndehydration = 50 mmol,
2
mcat = 0.1 g, T = 140 8C.
Table 3
different morphological CeO
face of (1 1 0) exhibited better activity than cube-CeO
octahedron-CeO (1 1 1). (1 1 0) planes were active factors for the
2
showed that rod-CeO
2
with a crystal
2
The conversion of 2-cyanopyridine with the different morphologies of CeO .
2
(1 0 0) and
Catalyst
Rod-CeO
Conversion of 2-cyanopyridine/%
2
hydrolysis of 2-cyanopyridine, further enhancing the DMC
2
93.7
21.4
6.7
Octahedron-CeO
Cube-CeO
2
formation by in situ removing water. The maximum DMC yield
ꢀ1
2
reached up to 378.5 mmol g cat
2
over the rod-CeO catalyst,
Reaction conditions: t = 3 h,
¼ 3 g.
m2-cyanopyridine = 0.1 g,
mcat = 0.03 g, T = 140 8C,
which was a 30-fold increase in comparison to that obtained
without any water removal.
m
2
H O

1
100
S.-P. Wang et al. / Chinese Chemical Letters 26 (2015) 1096–1100
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