CO2 promoted synthesis of unsymmetrical organic carbonate using switchable agents based on DBU and alcohols

Article abstract of DOI:10.1039/c8nj01638k

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for the transesterification of dimethyl carbonate (DMC) with various alcohols and amines, which afforded unsymmetrical organic carbonate and carbamate. It was observed that the transesterification was accelerated under pressurized CO₂ in this work. The activity is very high and the best result (89% conversion with 98% selectivity to unsymmetrical carbonate) was obtained for the DBU/alcohol/DMC/CO₂ system. The addition of CO₂ to DBU/ethanol generated the DBU cation salt, [DBUH][OCOOCH₂CH₃], which dissociated more favorably under increasing reaction temperature even under pressurized CO₂. The salt could also help to activate DMC by H-bond interaction. The reaction system can be extended easily for the catalytic synthesis of carbamates from amines and DMC. After the reaction, the salt was separated from the reaction mixture and DBU can be recovered by the feasible thermal decomposition, offering a straightforward strategy for the recycling of DBU. On the basis of these results, a plausible mechanism involving the role of both DBU and CO₂ has been proposed.

Full text of DOI:10.1039/c8nj01638k

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CO promoted synthesis of unsymmetrical
2
organic carbonate using switchable agents based
Cite this:DOI: 10.1039/c8nj01638k
on DBU and alcohols†
Qingwen Gu, Jian Fang, Zichen Xu, Wenxiu Ni, Kang Kong and Zhenshan Hou
*
1
,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for the transesterification
of dimethyl carbonate (DMC) with various alcohols and amines, which afforded unsymmetrical organic
carbonate and carbamate. It was observed that the transesterification was accelerated under pressurized CO
in this work. The activity is very high and the best result (89% conversion with 98% selectivity to unsymmetrical
carbonate) was obtained for the DBU/alcohol/DMC/CO system. The addition of CO to DBU/ethanol
generated the DBU cation salt, [DBUH][OCOOCH CH ], which dissociated more favorably under increasing
reaction temperature even under pressurized CO . The salt could also help to activate DMC by H-bond
2
2
2
2
3
Received 7th April 2018,
Accepted 26th June 2018
2
interaction. The reaction system can be extended easily for the catalytic synthesis of carbamates from amines
and DMC. After the reaction, the salt was separated from the reaction mixture and DBU can be recovered by
the feasible thermal decomposition, offering a straightforward strategy for the recycling of DBU. On the basis
DOI: 10.1039/c8nj01638k
rsc.li/njc
2
of these results, a plausible mechanism involving the role of both DBU and CO has been proposed.
which has drawbacks such as low productivity of EMC and easy
Introduction
10,11
production of by-products.
Except for the above two methods,
The development of environmentally benign chemicals and EMC can also be prepared by the transesterification of dimethyl
1
synthetic methods is a challenging field in green chemistry.
carbonate (DMC) with diethyl carbonate (DEC) or DMC with
As important green organic intermediates, unsymmetrical and ethanol. The transesterification of DMC with DEC can be
1
2
13
2 4
symmetric organic carbonates play essential roles in the chemical catalyzed using MgAl O composites, carbon-supported MgO,
2,3
14
industry. For example, ethyl methyl carbonate (EMC) as the metal–organic frameworks (MOFs), zeolitic imidazole frame-
1
5,16
17
simplest unsymmetrical carbonate, is widely applied in methylation, works (ZIF-8, ZIF-67),
carbonate phosphonium salts etc.
ethylization and methoxycarbonylation reactions. Moreover, it can However, this route suffers from low production efficiency,
act as an established solvent, as a useful protecting group of alcohols poor yields of EMC and poor atom economy. For the trans-
4
5
and phenols, and as a co-solvent in a non-aqueous electrolyte of esterification of DMC with ethanol to synthesize EMC, hetero-
6,7
lithium ion cells.
geneous catalysts such as molecular sieves (4 Å) and TiO
In general, the synthesis of EMC is divided into three nanofibers were discovered to be effective.
categories. A traditional way to produce EMC is the esterifica- geneous catalysts, metal complex Co (CO) showed appreciable
2 8
2
1
8,19
Among homo-
8
tion of methyl chloroformate with alcohol catalyzed by base catalytic activity in the reaction of DMC with primary alcohols
9
catalysts, but this route is widely considered not environmentally and gave alkyl methyl ethers with high selectivity and high
20
benign since the reagents used are highly toxic. Another method is yields (95–98%). Unfortunately, the high reaction temperature
the oxidative carbonylation of CO, O , methanol and ethanol, (180 1C) and the sensitivity of Co (CO) to air limited the practical
application of the system. In another study, stoichiometric
Brønsted acid catalysts p-toluenesulfonic acid or H SO and
Lewis acid catalysts AlCl or FeCl have been used in the
2
2
8
2
4
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,
School of Chemistry and Molecular Engineering, East China University of Science
and Technology, Shanghai 200237, People’s Republic of China.
E-mail: houzhenshan@ecust.edu.cn
3
3
reaction of alcohols or phenols with DMC, but a long reaction
time could be necessary to afford good conversion and
2
1
†
Electronic supplementary information (ESI) available: The pK
b
values of different
selectivity. Thus, more research of the transesterification of
DMC with alcohols has been focused on base catalysts. For
example, both 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) can be applied as
homogeneous basic catalysts for the production of alkyl/aliphatic
1
bases; the observation of phase behavior; H NMR spectra of the equimolar mixture
13
of DBU/methanol and DBU/ethanol before and after bubbling CO
2
;
C NMR
spectrum of [DBUH][O(CO)OCH CH ]; the conductivity of DBU, ethanol and their
2
3
1
13
mixture; H NMR, C NMR spectra, FT-IR data and ESI-MS data of all isolated
products. See DOI: 10.1039/c8nj01638k
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1
3
1
methyl carbonates and polycarbonates from alcohols/diols and 100 MHz C NMR). High temperature H NMR spectra were
5,22
DMC.
MCM-41-TBD and polystyrene-supported DBU have been recorded on a Bruker AVANCE 500 MHz instrument (500 MHz
1
established to solve the difficult catalyst-product separation of
H NMR). FT-IR spectra were recorded at room temperature on a
homogeneous basic catalysts like TBD/DBU in reaction mixtures, Magna 550 (Nicolet, USA) FT-IR spectrometer. A spectrum of dry
but the immobilization of a guanidine/amidine base onto an KBr was also recorded as the background. Mass spectra were
inorganic siliceous mesoporous support by a covalent bond is recorded on a Xevo G2 TOF spectrometer in the electrospray
23–25
normally needed.
Moreover, the transesterification reactions positive ion mode (ESI†). The phase behavior was directly observed
catalyzed by them commonly need higher reaction temperatures during the whole reaction process by using a high pressure
and longer reaction times (Table S1, ESI†). An alternative and more autoclave equipped with transparent glasses on opposite sides
efficient separation method is to take advantage of the switchable (Fig. S1, ESI†). The conductivity of the liquid was measured using
2
6
property of the CO /superbase system. It had been reported that a Leici DDS-307 conductivity meter according to the previously
2
33,34
TBD was separated readily from a homogeneous mixture by reported procedure.
bubbling CO into the reaction mixture at the end of the aldol
condensation reaction. TBD can be reused for the consecutive Transesterification reaction between DMC and ethanol
2
27
catalytic recycles. Moreover, the switchable CO
2
/superbase system
could work even in the presence of alcohols. The existence of CO
can help the separation of the superbase in a simple way in the
alcohol/CO system by forming a [base-H][O(CO)OR] salt when
bubbling acid gas (CO ) through an equimolar mixture of a super-
The transesterification reaction of dimethyl carbonate (DMC)
and ethanol to ethyl methyl carbonate (EMC) was used as a
model reaction to evaluate the catalytic performance. The
catalyst DBU (5 mmol), ethanol (5 mmol), DMC (15 mmol)
and toluene (3 mL) were added into a Teflon bush, and then the
bush was put into a 50 mL stainless steel autoclave (a working
capacity of approximately 40 mL) and sealed. The autoclave was
28
2
2
2
29–31
base and an alcohol.
Furthermore, the ionic product can be
converted back to the superbase and the alcohol by vacuumizing,
heating or bubbling N through the ionic compound.
2
In this work, we have performed the transesterification of
DMC with ethanol by using DBU as a catalyst. Most interestingly,
2
we found that the introduction of CO into DBU/ethanol/DMC at
the beginning of the reaction enhanced the catalytic activity
significantly when the reaction temperature was above 90 1C.
Moreover, this demonstrated that DBU/ethanol/CO can act as a
switchable catalyst, which existed in the form of solid and had
poor solubility in nonpolar solvents after the reaction, and thus
32
2
purged with dry CO three times to remove the air, and charged
with CO to the desired pressure. Finally, the reactor was heated in
2
a temperature-controlled heating jacket to the desired tempera-
ture. After the reaction, the reactor was immediately cooled in an
ice-water bath, and CO was slowly released while extra anhydrous
2
toluene at 10 1C was employed to absorb the trace amounts of
DMC and the products entrained by CO . After depressurization,
2
2
the anhydrous toluene in the cold trap was added to the reactor to
wash off the absorbed substances on the wall, and then the
mixture in the reactor was transferred to a 25 mL vessel.
Anhydrous ethyl ether was added to the reaction mixture to
promote catalyst separation. The white solid was separated by
standing and filtration, and the filtrate was analyzed by GC and
GC-MS analysis. The white solid ([DBUH][O(CO)OR]) was further
washed with ethyl ether two times and dried under vacuum at
2
allowed the recycling of DBU under pressurized CO . The present
approach has the advantages of excellent catalytic activity, easy
separation and benign to the environment.
Experimental section
Chemical materials
2
50 1C to remove the alcohol and CO . The left DBU can be reused
directly in the next reaction. Various carbonates and carbamates
were synthesized by the transesterification reaction of DMC with
corresponding alcohols and amines according to a procedure similar
to that mentioned above. The reaction products were analyzed by gas
chromatography on a Shimadzu GC-2014 apparatus equipped with a
KB-50 column (30 m ꢀ 0.32 mm ꢀ 0.50 mm) and an FID detector.
GC-MS analyses were conducted on an Agilent-6890/GC-5973 MS
instrument with a HP-5 column (30 m ꢀ 0.25 mm ꢀ 0.25 mm). The
products were also isolated by silica gel column chromatography
All chemicals were analytical grade and commercially available. The
organic base, 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU), was pur-
chased from Aladdin Bio-Chem Technology Co., Ltd, and directly
used as received. Dimethyl carbonate (DMC) was purchased from
Adamas Reagent Co., Ltd, and was also used without further
purification. Ethanol and toluene were both purchased from
Shanghai Titan Scientific Co., Ltd, and dried by using standard
methods. All other kinds of alcohols and amines such as
n-propanol, i-propanol, n-butanol, t-butanol, n-hexanol, benzyl
alcohol, ethylene glycol, furfuryl alcohol, 4-pyridinemethanol,
n-propylamine, i-propylamine and n-butylamine were purchased
from Sinopharm Chemical Reagent Co., Ltd and were also used
1
and analyzed by H NMR spectroscopy (as shown in the ESI†).
Results and discussion
without further purification. CO (499.95%) and N (499.999%)
2 2
were supplied by Shanghai Weichuang Gas Factory.
Catalytic performance
The organic superbase can effectively catalyze transesterifica-
tion between the alcohol and dimethyl carbonate (Scheme 1).
Characterization
1
13
Room temperature H NMR and C NMR spectra were recorded As a typical example, the reaction of DMC with ethanol could
1
35
on a Bruker AVANCE 400 MHz instrument (400 MHz H NMR, afford two products; EMC and diethyl carbonate (DEC).
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DBU can form an ionic product in the presence of alcohol
and CO , which could favor DBU separation. Normally, DBU
2
could be separated by CO2 bubbling into the solution after
Scheme 1 Synthesis of unsymmetrical organic carbonate from DMC and
alcohol.
reaction. However, the effect of CO on the transesterification
2
reaction has rarely been considered. In this work, we observed
that the addition of CO
important impact on the reaction. Initially, the effects of
reaction conditions (temperature, CO pressure and reaction
2
to the reaction system produced an
2
time) on the catalytic activity were investigated by using DBU as
a catalyst. As shown in Fig. 2a, the reaction temperature had a
significant effect on the reaction. With an increase in the
temperature from 60 1C to 100 1C, the conversion of ethanol
increased apparently from 9% to 89%, and the conversion
increased slowly to 91% at 110 1C. The higher temperature
might result in the decomposition of DBU. Consequently, it was
suggested that 100 1C was the appropriate temperature for the
transesterification reaction. The influence of the reaction time
is shown in Fig. 2b. Under the conditions of 100 1C and
1.0 MPa, the conversion of ethanol had a gradual increase
from 1 h to 6 h and then remained almost unchanged with high
selectivity from 6 to 8 h, which could be assigned to the reaction
Fig. 1 Conversions of ethanol and selectivities to DEC over different base equilibrium.
catalysts. Reaction conditions: DBU 0.76 mL (5 mmol), ethanol 0.29 mL
2
Sequentially, the impact of CO pressure on the transesterifica-
tion reaction of DMC was examined and depicted in Fig. 2c. It is
noteworthy that the conversion of ethanol was increased obviously
(
5 mmol), DMC 1.26 mL (15 mmol), toluene 3 mL 90 1C, 6 h.
2
Firstly, different basic catalysts were evaluated for the trans- as the CO pressure was increased to 1.0 MPa. However, the
esterification of DMC with ethanol to synthesize EMC, and conversion of ethanol had a slight decrease while selectivity showed
the results were summarized in Fig. 1. It was observed that no obvious changes at a higher pressure range. The enhancement
triethylamine (Et
CO only exhibited low activity although these bases afforded to the formation of an intermediate product during the reaction.
high selectivity to EMC. When DBU and TBD were used as the [DBUH][O(CO)OCH CH ] is normally considered as an inter-
base catalysts, the conversion of ethanol was 77.2% and 92.8%, mediate resulting from DBU, ethanol and CO (Scheme 2). The
respectively. However, DBU afforded much higher selectivity to initial increase in the conversion of ethanol could possibly be
EMC (98.7%) while TBD could only afford moderate selectivity explained due to the fact that the introduction of CO can result
to EMC (o90%). The catalytic activity of these various basic in consumption of ethanol to form [DBUH][O(CO)OCH CH ],
3
2
N), 1,1,3,3-tetramethylguanidine (TMG) and of the catalytic activity by increasing CO pressure might be due
K
2
3
2
3
2
2
2
3
catalysts was found to be in the following order: TBD 4 KOH 4 which could act as a catalyst precursor and facilitate the formation
DBU 4 K CO 4 TMG E Et N. Interestingly, the catalytic of EMC. However, as CO pressure was higher than 1.0 MPa, more
activity order was partly consistent with the basicity order of CO
2
3
3
2
2
dissolved in toluene, which could capture too much DBU to
that is, TBD 4 KOH 4 form the catalyst precursor [DBUH][O(CO)OCH CH ] with ethanol,
. Expect for TMG and K CO resulting in a more difficult dissociation of the precursor into the
value of the basic catalysts, the higher active species (see the following discussion). This might be also
the catalytic activity achieved. It indicated that the stronger related to a dilution effect whereby the excess CO reduced the
basicity of the catalysts enables the reaction within a certain concentration of DMC in the vicinity of the catalyst.
basic range. In order to identify further the effect of CO on the catalytic
On the basis of the results above, one of the advantages of transesterification between ethanol and DMC, the conversion/time
using DBU as the catalyst is that DBU shows excellent catalytic curves of the transesterification under different CO pressures were
performance and could be recycled in the form of an ionic plotted and shown in Fig. 3. It was indicated that the conversions of
compound because the ionic compound has poor solubility in ethanol under CO pressures of 0.5 MPa, 1.0 MPa and 1.8 MPa were
weak polar solvents, such as toluene. It should be noted that all higher than that without CO , and the highest activity was
although KOH exhibited high conversion (88%) and selectivity obtained under a CO pressure of 1.0 MPa. This result revealed that
90%), the separation is difficult and large amounts of residues the introduction of CO indeed facilitated the transesterification
3
6,37
different bases (Table S2, ESI†),
DBU 4 TMG 4 Et N 4 K CO
the smaller the pK
2
3
3
2
3
2
3
,
b
2
2
2
2
2
2
(
2
3
8
are generated after the reaction. Moreover, DBU is also much reaction of DMC in the DBU/ethanol system.
cheaper than TBD. Hence, among all these basic catalysts, DBU
Next, the dependence of ethanol conversions on the reaction
was chosen as a catalyst in the following investigation owing to temperature has been examined in the presence or absence of
2
its excellent catalytic performance, low cost and potential CO , respectively. As shown in Fig. 4, the catalytic activity
recyclability. increased with the reaction temperature either without CO
2
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Fig. 2 Effects of reaction parameters on ethanol conversion for transesterification. Reaction conditions: DBU 0.76 mL (5 mmol), ethanol 0.29 mL
5 mmol), DMC 1.26 mL (15 mmol); toluene 3 mL. (a) Time 5 h, CO pressure 1.0 MPa, (b) temperature 100 1C, CO pressure 1.0 MPa, (c) temperature
00 1C, time 8 h. The selectivity to EMC was always close to 98%.
(
2
2
1
or with CO
CO had a negative effect on the transesterification reaction
between DMC and ethanol when the reaction temperature was
lower than 80 1C. In contrast, CO facilitated the reaction
2
in the reaction system. However, the addition of
2
2
Scheme 2 Reversible reaction between CO
2
and DBU in ethanol.
significantly when the reaction temperature was over 90 1C,
as compared with that without CO2 (Fig. 4). These results
demonstrated that as the reaction temperature was increased
2
under pressurized CO , the transesterification reaction might
Fig. 3 Ethanol conversion as a function of reaction time under different
CO pressures for the transesterification reaction. Reaction conditions:
DBU 0.76 mL (5 mmol), ethanol 0.29 mL (5 mmol), DMC 1.26 mL (15 mmol),
toluene 3 mL, 100 1C. (a) Without CO , (b) under 0.5 MPa CO , (c) under
.0 MPa CO , (d) under 1.8 MPa CO . The selectivity to EMC was always
close to 98%.
2
Fig. 4 Conversions of ethanol at different temperatures in the presence
or absence of CO . Reaction conditions: time 4 h, DBU 0.76 mL (5 mmol),
2
2
2
1
2
2
ethanol 0.29 mL (5 mmol), DMC 1.26 mL (15 mmol), toluene 3 mL. The
selectivity to EMC was always close to 99%.
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follow a different approach resulting from the mutual-interaction
among CO , DBU and ethanol, which might be dependent on the
2
reaction temperature.
Phase behavior
The mutual-interaction among CO
reflected directly by observing visually the phase behavior under
pressurized CO . It was observed that the white solid formed
immediately at room temperature even if CO was bubbled into
2
, DBU and ethanol can be
2
2
the mixture of ethanol and DBU dissolved in toluene. To clarify
further the phase behavior of the reaction mixture during the whole
reaction process, a high-pressure visual autoclave equipped with
39
transparent glasses was employed (Fig. S1, ESI†). Fig. S2 (ESI†)
shows the visual observations of phase behavior as the temperature
ranged from 25 1C to 90 1C. The reaction mixture includes three
2
phases of gas (CO ), liquid (toluene and substrates), and a white
solid at low temperatures (o90 1C) while two phases of gas and
liquid at high temperatures (Z90 1C).
As shown in Fig. S2 (ESI†), DBU, DMC, ethanol and toluene
were miscible and the solution is transparent at room temperature
(Fig. S2a, ESI†). After the steel reactor was purged three times with
0
.1 MPa CO and then charged with CO up to 0.5 MPa at 25 1C, the
2
2
white solid formed at the bottom of the glass tube and a liquid
phase co-existed (Fig. S2b, ESI†). The white solid formed is believed
to be [DBUH][O(CO)OCH CH ] (for identification, see the next
2 3
paragraph), which had a limited solubility in the presence of a
large of amount of toluene. It can be seen that the ionic product
was accumulated mostly at the bottom of the liquid phase when
the temperature was up to 50–70 1C and the CO pressure remained
2
at 0.5 MPa (Fig. S2c and d, ESI†). However, when the temperature
was increased to 90 1C, the white solid almost disappeared and the
homogeneous liquid phase became transparent (Fig. S2e, ESI†). As
the CO
consisting of vapor and a transparent liquid can be still observed
Fig. S2e–h, ESI†). Nevertheless, after the reaction mixture was
2
pressure reached up to 3.0 MPa at 90 1C, a biphasic system
(
cooled to 25 1C and depressurized, the white solid formed again
and was accumulated again at the bottom of the liner (Fig. S2i,
ESI†). At the same time, the liquid level decreased due to the
release of CO
2
. From the visual observation under reaction
Fig. 5 After different reaction times, the as-obtained white solid was
analyzed using the FT-IR spectra (top) and H NMR spectra (bottom).
conditions, it is very clear that the present transesterification reaction
1
occurred under the gas–liquid biphasic condition. The results Reaction conditions: temperature 100 1C, CO
2
pressure 1.0 MPa, 5 mmol
DMC, 5 mmol DBU, 5 mmol ethanol, 3 mL toluene were added, respec-
tively. (a) Reaction 3 h, (b) reaction 5 h, (c) reaction 8 h, (d) the isolated
white solid was dried at 60 1C for 1 h, (e) DBU. The chemical shift appeared
at d = 3.79 was attributed to the proton from residual DMC.
2 3
indicated that the ionic compound ([DBUH][O(CO)OCH CH ]) was
formed preferentially at a lower temperature, while it could
undergo a structure transformation under higher temperatures
and thus displayed good solubility.
Transformation of DBU in the course of transesterification reaction
The existence of the C–O–C group and the (OQC–O–)–R group
In order to identify the structure of the white solid, as observed confirmed the anion moiety of the ionic compound. The N–H
ꢁ
1
ꢁ1
previously (Fig. S2, ESI†), the white solid was isolated carefully stretching of the separated solids at 3117 cm and 3240 cm
4
0
at different reaction times and then characterized by using the indicated the DBU protonation. The weak absorption at
FT-IR spectra and H NMR spectra. As shown in Fig. 5a–c (top), 1590 cm is associated to the n(CQN) group of the DBU ring.
two bands at 1107 cm and 1207 cm could be due to the In addition, the bands at 2859 cm and 2935 cm were due to
symmetric and unsymmetrical C–O–C stretching vibrations. the C–H stretching vibration in the ring and from the alcohol.
There also appears a strong absorption at 1647 cm and a The FT-IR spectra of all the separated solids gave a broad
weak peak at 1324 cm which belong to CQO and C–O of the spectral response at B3382 cm due to the trace water in the
1
ꢁ1
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
4
1
ꢁ
1
ꢁ
1
ꢁ1
3
0
(OQC–O–)–R group stretching absorption bands, respectively.
thin KBr disk prepared under an air atmosphere.
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For the sake of comparison, the white solid was isolated, and
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1
H NMR analysis clearly showed that CO
2
, DBU and ethanol
2 3
then further dried at 50 1C under vacuum to obtain a colorless can form the ionic product [DBUH][O(CO)OCH CH ], which is
1
liquid, which was then characterized by FT-IR and H NMR as further converted into [DBUH][O(CO)OCH ] as the transesterifica-
3
well (Fig. 5d, top). The as-obtained liquid showed a peak at tion reaction proceeds. [DBUH][O(CO)OCH ] can be separated and
3
ꢁ
1
1
613 cm , which is attributed to the CQN ring stretching dried by simple drying, allowing for the effective recycling of DBU.
vibration of the DBU ring. Meanwhile, two peaks that appeared Considering that the reaction of DBU, ethanol and CO to
at 2925 cm and 2850 cm correspond to the C–H stretching produce [DBUH][O(CO)OCH CH ] is an exothermic reaction, the
vibration of the DBU ring as well (Fig. 5d and e, top). Actually, formation of the intermediate species [DBUH][O(CO)OCH CH
2
ꢁ
1
ꢁ1
2
3
2
3
]
both pure DBU (Fig. 5e) and the as-obtained liquid showed was not favorable under increasing temperature conditions. As
1
very similar vibrations in the FT-IR spectra, which means that shown in Fig. 6 (top), the H NMR spectra of the reaction mixture
[DBUH][O(CO)OR] generated DBU owing to thermal decom- were recorded over the temperature range of 25–55 1C. Equimolar
position, alcohol and CO . The latter two contents are removed DBU and ethanol was added dropwise to an NMR tube under a N2
2
while the DBU was left, providing an attractive approach for the atmosphere at room temperature. CO was bubbled in the solution
2
recycling of DBU.
for 30 min at a required test temperature, and then the tube was
1
The H NMR spectra of the isolated white solids are also capped and sealed with Teflon tape and reserved in a heat
shown in Fig. 5a–c (bottom). Obviously, the H NMR signals of preservation cup. It can be seen that the protons H3,5,10 and
1
all hydrogen protons on the DBU ring shifted to high field as the H2,9,11 moved to high field and even H6 close to the functional
reaction time extended, even the H6 proton shifted more obviously, group of N–H showed more obvious tendency with increasing
from 2.61 to 2.50 (Fig. 5a–c), which might be related to the formation temperature, revealing the possible changes of the DBU cation
0
of more [DBUH][O(CO)OCH ] (the equation shown in Fig. 5). This salts. The peaks at d = 1.25 assigned to the overlay of H14 and H14
3
was consistent with the results shown in Fig. S3 and S4 (ESI†), the
H2–6, H9–11 protons from [DBUH][O(CO)OCH
at higher field (Fig. S3, ESI†), as compared with the protons from
DBUH][O(CO)OCH CH ] (Fig. S4, ESI†). Compared to the spectra of
3
] indeed gave signals
[
2
3
pure DBU (Fig. 5e, bottom), the ionic products showed a chemical
shift towards lower field, and the hydrogen protons (H2,6,9,11) that
were close to the functional group of N–H showed a larger shift
(Fig. 5a–c vs. Fig. 5e). The two new peaks observed at d = 3.99 and
d = 3.58 were assigned to the H13 proton and the H15 proton in
+
+
[
2 3 3
O(CO)OCH CH ] and [O(CO)OCH ] , respectively when the
reaction time was 3 h. It was found that the ratio of H13/H15
gradually decreased with prolonged reaction time from 3 h to
5
8
h, and even the peak of the H13 proton was hardly observed after
3
h. This means that the amount of [DBUH][O(CO)OCH CH ]
2
in the white precipitate decreased while the amount of [DBUH]-
O(CO)OCH ] gradually increased with the reaction time. This
[
3
phenomenon was consistent with the fact that the consuming
ethanol reacted with DMC to afford EMC and the by-product
1
3
MeOH. In addition, the C NMR spectra of the white solid as
observed in the phase behavior have been measured when CO
was purged into the tube (Fig. S2, ESI†). As shown in Fig. S5
ESI†), three peaks observed at d = 159.71, d = 60.44 and
d = 15.81 were assigned to C12, C13 and C14 in the anion moiety
2
(
4
2
of the ionic compound, respectively. This result proved that the
white solid was a DBU-based salt, which resulted from the
2
reaction of DBU, CO , and ethanol, being consistent with that
1
of the H NMR spectra.
The other new peak that appeared at d = 1.21–1.25 could be
attributed to the H14 proton of the –CH in the anion part of
3
the ionic compound (Fig. 5). Along with all other peaks, the
new peaks demonstrated the formation of the ionic product
(
[DBUH][O(CO)OCH ]). Moreover, once the isolated solid was
3
1
Fig. 6 H NMR spectra of the mixture of DBU and ethanol after CO
bubbled through the solution at different temperatures (a) 25 1C, (b) 35 1C,
c) 45 1C, (d) 55 1C using internal CDCl as a chemical shift reference (top).
2
was
dried under vacuum, the resulting liquid showed a very similar
1
H NMR spectrum to that of DBU, which was highly consistent
(
3
+
1
with the FT-IR data and provided further verification on the
The molar ratio of DBU/[DBUH] was determined from the H NMR
recyclability of DBU. Thus, the combination of FT-IR and spectra and found to change with temperature (bottom).
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+
in the –CH
3
of ethanol and [O(CO)OCH
2
CH
3
] were indistin-
of uncon-
0
guishable. However, the H13 and H13 in the –CH
verted ethanol and [O(CO)OCH CH ] displayed a totally
2
+
2
3
different resonance signal and appeared at d = 3.67 and
d = 3.97, respectively. It could be clearly seen that the intensity
of the peak at d = 3.97 decreased while the intensity of the peak
at d = 3.67 increased when the test temperature was increased,
which revealed that the amount of [DBUH][O(CO)OCH CH ]
2 3
decreased while the amount of ethanol increased when the
temperature was increased from 25 1C to 55 1C. Accordingly, the
+
1
molar ratio of DBU to [DBUH] was derived from the H NMR
peak area, and the effect of temperature on the ratio of DBU/
+
[
DBUH] is shown in Fig. 6 (bottom). We could see that the ratio
+
of DBU/[DBUH] increased with an increase in the temperature
and the improvement of the amount of DBU was enormous
when the temperature was increased from 45 1C to 55 1C.
As previously reported by Xie et al. as well as Phan et al.,
the decomposition onset temperature of a DBU-based ionic
Fig. 7 Conductivity of the DBU/ethanol/CO
pressure at 40 1C. The conductivity was measured with a mixture of DBU
5.5 mL) and ethanol (6.1 mL).
2 2
system as a function of CO
(
4
0,43
compound is around 50–60 1C.
Thus, the concentration
decrease of [DBUH][O(CO)OCH CH ] with increasing tempera- Reaction mechanism
2
3
ture was attributed to the reverse reaction caused by the thermal
decomposition of the ionic compound. Although the molar ratio
of DBU/[DBUH] at the reaction temperature (90–100 1C) and a high
pressure (1.0 MPa) cannot be determined from NMR measure-
ment, more dissociation of this ionic compound into DBU was
highly believable. The results were also in good agreement with
44
On the basis of the previous report, the reaction catalyzed by
DBU in the absence of CO2 could proceed according to the
following mechanism:
+
DBU + DMC " [DBU-CO(O)CH
3
][OCH
OH " EMC + CH OH + DBU
3
3
]
the phase behavior shown in Fig. S2 (ESI†), in which the ionic [DBU-CO(O)CH
][OCH
] + CH
CH
3
3
2
3
product was not soluble and existed as a white solid at low
temperature in toluene, while DBU and ethanol originated from
This mechanism is catalytic and is likely to be active because
the dissociation of the DBU salt were miscible with toluene and there will always be some free DBU resulting from the decom-
thus remained in an almost homogeneous phase with increasing position of the DBU-based salts to promote this reaction at
temperature.
Considering the formation of ionic compounds under pres- results, the addition of CO
surized CO
, the conductivity of the mixed DBU/ethanol system implied that CO might have played a crucial role in the
slightly high temperatures. However, according to our present
2
can facilitate the reaction, which
2
2
was subsequently measured under different CO2 pressures. reaction as follows:
A mixture of DBU (5.5 mL) and ethanol (6.1 mL) was added
to the steel reactor for conductivity measurement. Excess
3 2 2 2 3
DBU + CH CH OH + CO " [DBUH][O(CO)OCH CH ]
ethanol (nethanol/nDBU = 3) was added to increase the solubility
of [DBUH][O(CO)OCH CH ], which allowed for a homogeneous
2 3
[
DBUH][O(CO)OCH CH ] + DMC " EMC + [DBUH][OCH ] + CO
2
3
3
2
phase and to ensure the accuracy of the test. As shown in Fig. 7,
the conductivity was recorded after the temperature reached
[DBUH][OCH
3
] + CO
2
" [DBUH][O(CO)OCH
3
]
equilibrium for 2 h at 40 1C. The conductivity of the mixture of In this case, the role of DBU is the activation of CO
DBU and ethanol is very low (90.8 ms cm ). However, after the generate an alternative pathway of EMC generation via a
2
that might
ꢁ
1
steel reactor was purged three times with 0.1 MPa CO , the [DBUH][O(CO)OCH CH ] intermediate as described. To identify
2
2
3
ionic compound [DBUH][O(CO)OCH CH ] formed in ethanol the reaction mechanism more clearly, the reaction kinetics
2
3
ꢁ
1
exhibited a high conductivity of ca. 2040 ms cm . Then, the was then investigated for the transesterification reaction with
conductivity of DBU/ethanol/CO gradually increased with an the initial rate method. The relationship between the reaction
increase in the CO pressure and maximized at 3620 ms cm
under the condition of 0.9 MPa CO . The results indicated the (ESI†). The study showed 1.32-order plots for the concentration
2
ꢁ
1
2
0
rate (R ) and the concentrations of DBU is shown in Fig. S6
2
ionic change of the system and revealed the formation of the of DBU (0.50–1.25 M). This implied that the reaction inter-
salt [DBUH][O(CO)OCH CH ]. However, the conductivity of mediate step could need more than one molecule of DBU to
2
3
DBU/ethanol/CO2 showed a slight decrease when the CO2 complete the catalytic cycle. Nevertheless, the first catalytic
pressure continued to increase to 1.1 MPa or a higher pressure. mechanism cannot be excluded, which likely results in the
It could possibly be explained that more CO
in ethanol with increasing pressure, resulting in the reduced [DBUH][O(CO)OCH
polarity of the solution as well as the produced diluting effect. which displayed the combined effect of the catalytic and
2
can be dissolved fractional reaction order. The free DBU and DBU-based salt
2
CH ] coexisted in the reaction mixture,
3
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the only factor determining the activity and selectivity of the
51
transesterification reaction. It should be noted that the high
dissociation of the DBU salt was necessary in order to generate
the rich intermediate species [DBU(CO)OCH ] from free DBU. This
3
was the reason why CO can promote the reaction only under a
2
higher reaction temperature. The synergistic activation of DMC by
+
free DBU and a small amount of [DBUH] would be more favorable
for the nucleophilic attack by ethanol to produce DEC. An unprece-
dented reaction pathway towards acyclic organic carbonate has been
5
reported when [Nb(OEt) ] coupledwith 4-dimethylamino-pyridine
(
DMAP) was used as the catalyst system, where [Nb(OEt) ] functioned
5
as a Lewis acid and as a source of a nucleophilic anion. In this
case, two molecules of DMAP are necessary to activate a
hemicarbonate intermediate, facilitating the intramolecular
nucleophilic attack of the ethoxide group to the CQO bond
5
2
to afford linear carbonates. In our experimental observations,
+
no Lewis acid existed but weak Brønsted acid sites [DBUH]
could play a similar role in promoting the nucleophilic attack of
the ethoxide group to the CQO bond to afford linear carbo-
Scheme 3 Proposed reaction mechanism for the transesterification nates as shown in Scheme 3. Afterwards, the intermediate
reaction between DMC and ethanol under pressurized CO
2
.
species DBUH-OMe was recovered at the same time. Next, the
cooled reaction mixture containing DBU, MeOH and CO was
easily transformed into a large amount of [DBUH][O(CO)OCH
stoichiometric pathways, resulting in the enhanced activity and also a small amount of [DBUH][O(CO)OCH CH ] at the end
under pressurized CO of the reaction. Finally, the white solid was dried under vacuum
2
3
]
2
3
2
.
With respect to the results above, a reasonable reaction and the recovery of DBU was achieved accordingly.
mechanism is proposed as shown in Scheme 3. Initially,
[
DBUH][O(CO)OCH CH ] was formed easily from DBU, ethanol
2 3
Substrate scope and reusability
2
and CO at room temperature. However, when the temperature
was increased to 90–100 1C, accompanied by the dissociation of The above results demonstrated that DBU, promoted with
the DBU salt and the release of CO
, the intermediate species CO was a highly efficient homogeneous catalyst for the trans-
2
2
DBUH-OEt generated. However, the dissociation reaction of esterification reaction of DMC and ethanol in toluene. Thus, a
the DBU salt was reversible and this means that the salt series of alcohols were tested to synthesize the corresponding
2 3
[DBUH][O(CO)OCH CH
] and DBUH-OEt might coexist in the carbonates under optimized reaction conditions to check the
present reaction system. Moreover, as shown in Table S3 (ESI†), scope of the substrates over the present catalytic system and
the conductivity of the mixture of DBU and ethanol (the molar the results are summarized in Table 1. It could be found that
ꢁ
1
ratio of DBU/ethanol is 1 : 3) was 77.9 ms cm , which was much the system was applicable to the transesterification reaction of
ꢁ
1
higher than those of both ethanol (4.14 ms cm ) and DBU DMC with various terminal alcohols (Table 1, entries 1–4),
ꢁ
1
(
2.85 ms cm ). The sharp rise in the conductivity of the DBU/ indicating that a variety of aliphatic alcohols and aromatic
ethanol mixture could be explained by the production of the alcohols produced the corresponding carbonic esters with high
intermediate species, in which DBU had a tendency to accept a conversions and selectivities within the first 10 h. Ethylene
proton from ethanol, resulting in the activation of ethanol as a glycol could also react with DMC to produce 1,3-dioxolan-2-one
2
9
nucleophile.
Similar results were reported for reactions with a conversion of 47.2%. Moreover, the present catalytic
4
5–47
between DBU and propanol/benzyl alcohol/glycerol.
system can also be extended to the synthesis of carbamates
In the previously reported reaction mechanism of transes- from amines and DMC, although terminal amines need a
terification, DBU acts as a nucleophile towards a variety of longer reaction time to produce the corresponding carbamates
electrophiles, such as DMC, and activate them by generating (Table 1, entries 7–9), when compared with alcohols. The
the cationic intermediate N-alkoxycarbonyl DBU derivative reactions performed with isopropanol and t-butanol (Table 1,
[
DBU(CO)OCH
]. Thereafter, it underwent a nucleophilic attack entries 10 and 11) showed no product formation during a
3
4
4,48
18
by an alcohol to afford unsymmetrical carbonate (path 1).
reaction time of 24 h, likely due to the steric hindrance.
However, in the present system, a small amount of the DBU-derived When heterocyclic compounds were used as substrates, furfuryl
+
cation [DBUH] could further activate the cationic intermediate alcohol offered 46.6% conversion but 4-pyridinemethanol gave
[
DBU(CO)OCH
] by hydrogen bonding interactions (path 2). The no reactivity (Table 1, entries 6 and 12). All products were
3
strong hydrogen bonding interactions have been identified to play isolated by silica gel column chromatography method and
1
13
a crucial role in activating an ester bond, leading to the formation their purity (498%) was confirmed by H NMR and C NMR
41,49,50
of the final products,
and meantime the basicity could not be (Fig. S7–S24, ESI†).
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a
Table 1 Transesterification reaction between DMC and alcohol/amine in the presence of pressurized carbon dioxide
b
Entry
1
Substrates
Products
T (h)
Con. (%)
Sel. (%)
97.0
10
85.5(28.5)
2
3
10
10
83.1(27.7)
79.7(25.6)
96.2
99.0
4
10
65.5(21.8)
99.0
5
6
14
22
47.2(15.7)
46.6(15.5)
97.8
99.0
7
8
9
13
13
13
91.8(30.6)
61.2(20.4)
74.0(24.7)
92.5
92.3
99.0
1
1
0
1
24
24
0
0
0
0
1
2
24
0
0
a
b
Reaction conditions: DMC 1.26 mL (15 mmol), alcohols or amines (5 mmol), DBU 0.76 mL (5 mmol), 100 1C, 1.0 MPa CO
2
.
The conversion was calculated
using alcohols and amines. The values in parenthesis refer to DMC conversion.
In the next step, the reusability of the catalyst DBU was
examined under the optimal conditions. After the reaction, the
catalyst existed in the form of [DBUH][O(CO)OR]. Anhydrous
ethyl ether was added to the reaction mixture to help decrease
the solubility of [DBUH][O(CO)OR] in the solvent. Then the
catalyst was recovered by simple standing and filtration, fol-
lowed by washing with ethyl ether three times and then dried
under vacuum at 50 1C. Subsequently, the recovered DBU can
be reused for the next recycle. The reusability of the DBU
catalyst is shown in Fig. 8. It indicated that the catalyst could
be reusable for at least five times with high product selectivity,
although the conversion of ethanol had a slight decrease
possibly due to the loss of a trace amount of DBU in the
consecutive recycles. It was found that the separation efficiency
of the DBU catalyst is 80.2% after five runs, which might be the
reason for the loss of catalytic activity.
Fig. 8 Recyclability of DBU. Reaction conditions: DBU 0.76 mL (5 mmol),
ethanol 0.29 mL (5 mmol), DMC 1.26 mL (15 mmol), toluene 3 mL, 100 1C,
1.0 MPa CO , 8 h.
2
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Conclusions
10 B. Xu, R. J. Madix and C. M. Friend, J. Am. Chem. Soc., 2011,
133, 20378–20383.
In summary, DBU has been demonstrated as an effective
catalyst for the synthesis of various unsymmetrical carbonates
using DMC and corresponding alcohols. A high conversion of
alcohols and the excellent selectivity of unsymmetrical carbo-
nate can be achieved under the optimal reaction conditions. It
was found that the reaction can be facilitated in the presence of
1
1
1
1
1
1 J. Guilera, R. Bringu ´e , E. Ram ´ı rez, M. Iborra and J. Tejero,
Appl. Catal., A, 2012, 413–414, 21–29.
2 J. Wang, L. Han, S. Wang, J. Zhang and Y. Yang, Catal. Lett.,
2014, 144, 1602–1608.
3 G. Zhao, J. Shi, G. Liu, Y. Liu, Z. Wang, W. Zhang and M. Jia,
J. Mol. Catal. A: Chem., 2010, 327, 32–37.
4 Y. Zhou, J. Song, S. Liang, S. Hu, H. Liu, T. Jiang and B. Han,
J. Mol. Catal. A: Chem., 2009, 308, 68–72.
5 X. Zhou, H. P. Zhang, G. Y. Wang, Z. G. Yao, Y. R.
Tang and S. S. Zheng, J. Mol. Catal. A: Chem., 2013, 366,
1
.0 MPa of CO through the synergistic activation of DMC by
2
+
DBU and the cation [DBUH] formation under pressurized CO ,
2
as compared with that without CO
2
. Furthermore, the DBU/
alcohol/CO system existed in the solid form after the reaction
2
in toluene, which helps DBU to be easily separated and reused
with high activity and selectivity. The present catalytic system
can also be extended for the efficient synthesis of carbamates
from DMC and amines. Based on the catalytic performance and
characterization, we proposed a reasonable mechanism depict-
4
3–47.
6 L. Yang, L. Yu, M. Sun and C. Gao, Catal. Commun., 2014,
4, 86–90.
1
1
1
1
2
2
2
2
2
2
2
2
2
5
7 M. Selva, A. Caretto, M. No `e and A. Perosa, Org. Biomol.
Chem., 2014, 12, 4143–4155.
8 S.-H. Pyo and R. Hatti-Kaul, Adv. Synth. Catal., 2016, 358,
ing the role of both DBU and CO for this reaction. The results
2
of this work provided an example of the applications of a
switchable DBU/alcohol/CO system as a metal and halogen
2
ion-free, recyclable and inexpensive catalytic system in organic
synthesis and catalysis.
8
34–839.
9 X. Zhang, X. Ke, Z. Zheng, H. Liu and H. Zhu, Appl. Catal., B,
014, 150–151, 330–337.
2
0 R. I. Khusnutdinov, N. A. Shchadneva and Y. Y. Mayakova,
Russ. J. Org. Chem., 2014, 50, 790–795.
1 S. Jin, A. J. Hunt, J. H. Clark and C. R. McElroy, Green Chem.,
Conflicts of interest
2
016, 18, 5839–5844.
2 R. Bernini, E. Mincione, F. Crisante, M. Barontini, G. Fabrizi
and P. Gentili, Tetrahedron Lett., 2007, 48, 7000–7003.
3 S. Carloni, D. E. D. Vos, P. A. Jacobs, R. Maggi, G. Sartori and
R. Sartorio, J. Catal., 2002, 205, 199–204.
4 E. Quaranta, A. Angelini, M. Carafa, A. Dibenedetto and
V. Mele, ACS Catal., 2013, 4, 195–202.
5 Y. V. S. Rao, D. E. D. Vos and P. A. Jacobs, Angew. Chem., Int.
Ed., 1997, 36, 2661–2663.
6 F. S. Pereira, E. R. deAzevedo, E. F. d. Silva, T. J. Bonagamba
and A. E. Job, Tetrahedron, 2008, 64, 10097–10106.
7 I. Cota, F. Medina, J. E. Sueiras and D. Tichit, Tetrahedron
Lett., 2011, 52, 385–387.
8 X. Cao, H. Xie, Z. Wu, H. Shen and B. Jing, ChemCatChem,
2012, 4, 1272–1278.
29 Z. Liu, P. Hu, X. Meng, R. Zhang, H. Yue, C. Xu and Y. Hu,
Chem. Eng. Sci., 2014, 108, 176–182.
30 H. Zheng, X. Cao, K. Du, J. Xu and P. Zhang, Green Chem.,
2014, 16, 3142–3148.
There are no conflicts to declare.
Acknowledgements
The authors are grateful for support from the National Natural
Science Foundation of China (21373082, 21773061) and the
innovation Program of Shanghai Municipal Education Com-
mission (15ZZ031), and the Fundamental Research Funds for
the Central Universities.
References
1
2
3
4
5
6
7
8
9
G. Fiorani, A. Perosa and M. Selva, Green Chem., 2018, 20,
88–322.
Y. Zhou, Q. Jin, Z. Gao, H. Guo, H. Zhang and X. Zhou, RSC
Adv., 2014, 4, 7013–7018.
P. Wang, S. Liu, X. Ma, Y. He, A. S. Alshammari and Y. Deng,
RSC Adv., 2015, 5, 25849–25856.
O. Kreye, L. C. Over, T. Nitsche, R. Z. Lange and M. A. R.
Meier, Tetrahedron, 2015, 71, 293–300.
2
31 P. Munshi, A. D. Main, J. C. Linehan, C.-C. Tai and
P. G. Jessop, J. Am. Chem. Soc., 2002, 124, 7963–7971.
32 I. Anugwom, P. M ¨a ki-Arvela, P. Virtanen, P. Damlin,
R. Sj o¨ holm and J.-P. Mikkola, RSC Adv., 2011, 1, 452–457.
33 J. Zhang, C. Yang, Z. Hou, B. Han, T. Jiang, X. Li, G. Zhao,
Y. Li, Z. Liu, D. Zhao and Y. Kou, New J. Chem., 2003, 27,
333–336.
H. Mutlu, J. Ruiz, S. C. Solleder and M. A. R. Meier, Green
Chem., 2012, 14, 1728–1735.
D. S. Hall, J. Self and J. R. Dahn, J. Phys. Chem. C, 2015, 119,
22322–22330.
E. Knipping, C. Aucher, G. Guirado and L. Aubouy, New 34 S. Ren, Y. Hou, W. Wu, Q. Liu, Y. Xiao and X. Chen, J. Phys.
J. Chem., 2018, 42, 4693–4699. Chem. B, 2010, 114, 2175–2179.
A.-A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 35 T. Keller, J. Holtbruegge, A. Niesbach and A. G o´ rak, Ind. Eng.
51–976. Chem. Res., 2011, 50, 11073–11086.
P. Tundo, F. Trotta, G. Moraglio and F. Ligorati, Ind. Eng. Chem. 36 K. Kaupmees, A. Trummal and I. Leito, Croat. Chem. Acta,
9
Res., 1988, 27, 1565–1571.
2014, 87, 385–395.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
3
3
3
4
4
4
4
4
4
7 K. Popov, L. H. J. Lajunen, A. Popov, M. Hannu-Kuure and 46 Y. Miao, N. Stanley, A. Favrelle, T. Bousquet, M. Bria,
A. Vendilo, Inorg. Chem. Commun., 2002, 5, 223–225.
8 I. Cota, R. Chimentao, J. Sueiras and F. Medina, Catal.
Commun., 2008, 9, 2090–2094.
9 R. Zhang, L. Guo, J. Chen, H. Gan, B. Song, W. Zhu, L. Hua and
Z. Hou, ACS Sustainable Chem. Eng., 2014, 2, 1147–1154.
A. Mortreux and P. Zinck, J. Polym. Sci., Part A: Polym.
Chem., 2015, 53, 659–664.
47 M. K. Munshi, S. M. Gade, M. V. Mane, D. Mishra, S. Pal,
K. Vanka, V. H. Rane and A. A. Kelkar, J. Mol. Catal. A:
Chem., 2014, 391, 144–149.
0 Y. Xie, R. Parnas, B. Liang, Y. Liu, C. Tao and H. Lu, Chin. 48 W.-C. Shieh, S. Dell and O. Repic, J. Org. Chem., 2002, 67,
J. Chem. Eng., 2015, 23, 1728–1732. 2188–2191.
1 M. K. Munshi, P. S. Biradar, S. M. Gade, V. H. Rane and 49 K. Li, Y. Yan, J. Zhao, J. Lei, X. Jia, S. H. Mushrif and Y. Yang,
A. A. Kelkar, RSC Adv., 2014, 4, 17124–17128. Phys. Chem. Chem. Phys., 2016, 18, 32723–32734.
2 Z.-Z. Yang, L.-N. He, Y.-N. Zhao, B. Li and B. Yu, Energy 50 F. S. H. Simanjuntak, J. S. Choia, G. Lee, H. J. Lee, S. D. Lee,
Environ. Sci., 2011, 4, 3971–3975.
3 L. Phan, D. Chiu, D. J. Heldebrant and H. Huttenhower, Ind.
Eng. Chem. Res., 2008, 47, 539–545.
M. Cheong, H. S. Kim and H. Leea, Appl. Catal., B, 2015, 165,
642–650.
51 M. K. Munshi, S. M. Gade, V. H. Rane and A. A. Kelkar, RSC
Adv., 2014, 4, 32127–32133.
4 M. Carafa, E. Mesto and E. Quaranta, Eur. J. Org. Chem.,
2
011, 2458–2465.
5 Y. Wang, Q. Han and H. Wen, Mol. Simul., 2013, 39,
22–827.
52 B. Dutta, J. Sofack-Kreutzer, A. A. Ghani, V. D’Elia, J. D. A.
Pelletier, M. Cokoja, F. E. K u¨ hn and J.-M. Basset, Catal. Sci.
Technol., 2014, 4, 1534–1538.
8
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