Functionalized dicyandiamide-formaldehyde polymers as efficient heterogeneous catalysts for conversion of CO2 into organic carbonates

Article abstract of DOI:10.1039/c3gc42331j

The application of the dicyandiamide-formaldehyde polymer (DFP), which is a kind of organic polymer flocculant, as a supporter for conversion of CO ₂ into organic carbonates was studied in this work, and four kinds of dicyandiamide-formaldehyde polymers were synthesized and employed as efficient single component, metal-free, multifunctional, polyquaternium catalysts for cycloaddition of CO₂ with epoxides. The results indicated that the ammonium bromide modified dicyandiamide-formaldehyde polymer (ABMDFP) was the optimal catalyst among the four polymers and was observed to be suitable for various substrates. A possible mechanism of ring-opening of epoxides assisted by the hydroxyl and activation of CO₂ induced by amines was proposed. In addition, the catalyst could be easily recovered and reused six times with only a slight loss of catalytic activity. Furthermore, the catalyst exhibited good activity in the synthesis of dimethyl carbonate (DMC) through a transesterification reaction. The functional organic polymers in this study were proved to be efficient heterogeneous catalysts for the cycloaddition reaction of CO₂, which was applicable to the development of fixed-bed continuous flow reactors. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42331j

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Functionalized dicyandiamide–formaldehyde
polymers as efficient heterogeneous catalysts for
conversion of CO2 into organic carbonates†
Cite this: Green Chem., 2014, 16,
2771
Xiang-Lei Meng,a,b Yi Nie,*a,b Jian Sun,b Wei-Guo Cheng,b Jin-Quan Wang,b
Hong-Yan Heb and Suo-Jiang Zhang*b
The application of the dicyandiamide–formaldehyde polymer (DFP), which is a kind of organic polymer
flocculant, as a supporter for conversion of CO2 into organic carbonates was studied in this work, and
four kinds of dicyandiamide–formaldehyde polymers were synthesized and employed as efficient single
component, metal-free, multifunctional, polyquaternium catalysts for cycloaddition of CO2 with epoxides.
The results indicated that the ammonium bromide modified dicyandiamide–formaldehyde polymer
(ABMDFP) was the optimal catalyst among the four polymers and was observed to be suitable for various
substrates. A possible mechanism of ring-opening of epoxides assisted by the hydroxyl and activation of
CO2 induced by amines was proposed. In addition, the catalyst could be easily recovered and reused six
times with only a slight loss of catalytic activity. Furthermore, the catalyst exhibited good activity in the
synthesis of dimethyl carbonate (DMC) through a transesterification reaction. The functional organic
polymers in this study were proved to be efficient heterogeneous catalysts for the cycloaddition reaction
of CO2, which was applicable to the development of fixed-bed continuous flow reactors.
Received 12th November 2013,
Accepted 26th February 2014
DOI: 10.1039/c3gc42331j
www.rsc.org/greenchem
1. Introduction
Carbon dioxide (CO2) is considered to be the main component
of the greenhouse gases, which is also a nontoxic, abundant,
natural, inexpensive and renewable C1 building block. There-
fore, the efficient transformation of CO2 into useful organic
compounds is of growing interest for resource utilization
and sustainable development.1 One of the typical routes for
efficiently utilizing CO2 is the synthesis of five-membered
cyclic carbonates via the cycloaddition of CO2 with epoxides
(Scheme 1, step 1),2–4 because cyclic carbonates can be
extensively used as intermediates in the production of pharma-
ceuticals and fine chemicals.5–7
In the past few decades, numerous catalysts have been de-
veloped for the production of cyclic carbonates, including metal
oxides,8 alkali metal salts,9 quaternary onium salts,10 ionic
liquids,11 transition metal complexes12 and functional organic
compounds and polymers.13 Although fixation of CO2 with
Scheme 1 Synthesis of cyclic carbonates (step 1) and the transesterifi-
cation process for DMC production (step 2).
epoxides into cyclic carbonates has been researched exten-
sively, the design of efficient, inexpensive, highly selective,
stable, metal-free and reusable catalysts for this reaction is still
a challenge.
Recently, functional organic polymers have received increas-
ing attention in the field of catalysis due to the macromolecu-
lar architecture and possibility of creating new properties and
functions.14 In this respect, He and co-workers reported that a
quaternary ammonium salt-functionalized chitosan is an
efficient and recyclable catalyst for the synthesis of cyclic car-
bonates from CO2 and epoxides.15 They also prepared polyethy-
lene glycol (PEG)-supported quaternary ammonium salt as a
recyclable catalyst for the reaction.16 The PEG6000-supported
catalyst was dissolved during the reaction and solidified by
aCollege of Chemistry and Chemical Engineering, Qufu Normal University,
Shandong, 273165, China. E-mail: ynie@home.ipe.ac.cn
bBeijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green
Process and Engineering, State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing,
100190, China. E-mail: sjzhang@home.ipe.ac.cn
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3gc42331j
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cooling after the reaction, which attained a homogeneous reac- 3b), and investigated the activity of this class of catalysts for
tion and heterogeneous separation. Park and co-workers the cycloaddition of CO2 with epoxides. All of these kinds of
immobilized imidazolium ionic liquids onto cellulose and pro- catalysts have shown an excellent impact on promoting the
posed that the –COOH groups on the support material could reaction. Furthermore, the catalysts can be easily recovered by
stabilize the cyclic carbonate formed and promote the reac- filtration and reused at least six times with only a slight loss of
tion.17 North and co-workers reported polystyrene-supported catalytic activity. Due to the structure of many amines within
one-component, bimetallic aluminium (salen) complexes, the molecule, the catalytic activity for DMC synthesis through
using the ammonium group as a convenient linker for the syn- a transesterification reaction was also studied (Scheme 1, step
thesis of cyclic carbonates at room temperature and atmos- 2), and the catalyst 2b that exhibited high activity, 90% yield of
pheric pressure.18 Han and co-workers prepared a highly cross- DMC together with 99% selectivity was obtained.
linked polymer-supported ionic liquid as a heterogeneous catalyst
for the cycloaddition reaction,13a in which 3-butyl-1-vinylimidazo-
lium chloride was covalently anchored onto the divinylben-
zene-cross-linked polymer matrix. Current studies showed that
2. Results and discussion
2.1 Activities of various catalysts
hydrogen bond donors (HBD) could activate the ring-opening
of epoxides and enhance the reaction for fixation of CO2 with
epoxides.19–22 In this respect, we also synthesized hydroxyl
functionalized polymers as active and insoluble catalysts for
fixation of CO2.13e,f,21d,f Nevertheless, in most of these func-
tional organic polymers, the synthetic method is complex as
well as time-consuming. In addition, unsatisfactory activities
and harsh reaction conditions are still disadvantages that need
to be improved. Therefore the design of inexpensive, simply
synthesised, efficient, highly selective and reusable catalysts
still remains to be studied toward effective CO2 conversion.
The dicyandiamide–formaldehyde polymer (DFP) (Fig. 1, a),
which is well-known as a kind of organic polymer flocculant,23
is a potential catalyst for the fixation of CO2, not only because
it has abundant amino and hydroxyl groups that can activate
the CO2 and epoxides but also because of the possibility of
creating new functions via the addition of a modifier, such as
ammonium chloride, aluminum chloride, acrylamide, etc.,
especially ammonium chloride modified dicyandiamide–
formaldehyde polymer (ACMDFP) (Fig. 1, 1b). ACMDFP has
functional groups of HBD and amino-groups, as well as several
halogen anions within the molecule, which have been proved
much helpful for promoting catalytic activity. Moreover, the
relative simpleness of their synthesis method and the inexpen-
sive raw materials make this class of catalysts very interesting
particularly for the fixation of CO2. Herein we synthesized
ammonium bromide modified dicyandiamide–formaldehyde
polymers (ABMDFP) (Fig. 1, 2b) and ammonium iodide modi-
fied dicyandiamide–formaldehyde polymers (AIMDFP) (Fig. 1,
After the four kinds of catalysts were achieved, their activities
were investigated via the cycloaddition reaction of propylene
oxide (PO) with CO2 to synthesize propylene carbonate (PC),
and the corresponding results were displayed in Table 1.
As shown in Table 1, all of the catalysts exhibited high
selectivity (entries 1–4), which demonstrated negligible cre-
ation of by-products in the cycloaddition reactions. It was
found that 2b showed much higher activity than 1b (entry 3 vs. 2).
However, 2b exhibited a lower PC yield than 3b (entry 3 vs. 4). For
comparison, we investigate the catalytic activity of dicyandiamide
(DCD), NH4X and hydroxyl-functionalized poly(ionic liquids)
(PILs). The results revealed that the activity of DCD was very
low (entry 5), and the activity of NH4X was in the order of NH4I
> NH4Br > NH4Cl (entries 6, 7, 8), which was in accord with
reports that the activities of anions increased in the order of I−
> Br− > Cl− with the same cation.24 Catalyst 2b could show
96% PC yield, which was much higher than that of the NH4Br
(entry 3 vs. 7). The better catalytic activity of 2b was most likely
related to the synergistic catalysis effected by amino and
Table 1 Catalysts screening for the synthesis of propylene carbonatea
PO Conv.b
(%)
PC Select.b
(%)
Entry
Catalyst
1c
2c
3c
4c
5
DFP
58
67
96
99
25
48
85
92
90
94
99
99
99
99
99
99
99
99
99
99
ACMDFP
ABMDFP
AIMDFP
DCD
6
7
NH4Cl
NH4Br
NH4I
8
9d
10d
PILse
PILsf
a Reaction conditions: PO (14.3 mmol), catalyst (2.0 mol%), CO2
pressure (2.0 MPa), reaction time (3.5 h) and temperature (130 °C).
b Conversion and selectivity were determined by GC using biphenyl as
the internal standard. c Catalyst (1.5 mol% calculated by monomers).
d Catalyst (1.5 mol% calculated by ILs). Every experiment was repeated
3
times. DCD =
dicyandiamide. e PILs were synthesized by co-
polymerization of 1-vinyl-3-carboxyethylimidazolium bromide with the
cross-linker divinylbenzene.13f f PILs were synthesized by co-
polymerization of 1-vinyl-3-hydroxyethyl-imidazolium bromide with
the vinylpyridine.13e
Fig. 1 Structures of the DFP (a), ACMDFP (1b), ABMDFP (2b) and
AIMDFP (3b).
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hydroxyl groups in 2b. We studied the performances of the cat-
alysts with and without the halide anion (entries 2, 3, 4 vs. 1),
and the results showed that the activity of the catalyst with the
halide anion (entries 2, 3, 4) was much higher than that
without the halide anion (entry 1). The heterogeneous catalyst
2b, which could be easily prepared by using ammonium
bromide, dicyandiamide and methanol, was observed to have
higher activity. Besides, the yield was better than the PILs
(entries 9, 10) we prepared in our previous work.13e,f Compared
with 3b, the starting materials to prepare 2b were relatively
inexpensive and more stable. Therefore, 2b was chosen as the
optimal catalyst for further investigation.
Fig. 3 Effect of pressure. Reaction conditions: PO (14.3 mmol), catalyst
(1.5 mol%), reaction time (4 h), temperature (130 °C).
2.2 Effect of reaction conditions
The reaction conditions such as the influence of temperature,
CO2 pressure, reaction time and dosage of catalysts on the
cycloaddition reactions of CO2 with PO were investigated to
find the most appropriate and efficient reaction conditions.
The effect of temperature on the PC yield is shown in Fig. 2. It
could be observed that the yield of PC increased sharply from
20% to 98% with the increase of temperature from 100 °C to
130 °C. Then the PC yield stayed almost constant when temp-
erature increased further.
However, a too high temperature would lead to reduction of
PC yield, probably due to side reactions such as the isomeriza-
tion to acetone.25 To our delight, the temperature did not
appear to reduce the selectivity of PC, since there was no sig-
nificant change in selectivity with increasing temperature.
Therefore, the optimal temperature for this reaction was
chosen as 130 °C.
The influence of CO2 pressure on the yield of PC was inves-
tigated at 130 °C for 4 h with 2b as the catalyst (Fig. 3). It
could be obviously seen that the reaction pressure showed a
great effect on PC yield. With the rise of CO2 pressure in the
low-pressure region (0.5–1.5 MPa), the PC yield increased
rapidly, but CO2 pressure showed little effect on the PC yield
in the range of 1.5–2.5 MPa, and the PC yield decreased
obviously when the CO2 pressure was above 2.5 MPa. A similar
influence of CO2 pressure on catalytic activity had been
observed in other catalytic systems in the literature.26–28
According to these reports, it could be explained that PC was
in its liquid form under the adopted reaction conditions. The
concentration of CO2 in the liquid phase rose with increasing
pressure when the reaction was carried out in the low-pressure
region, which promoted the reaction. But a too high CO2
pressure would decrease PC yield because of the lower concen-
tration of PO in the liquid phase since PO is also a reactant.29
So 2.0 MPa was chosen as the most suitable pressure.
The dependence of PC yield and selectivity on reaction time
was also investigated under identical reaction conditions. As
shown in Fig. 4, the yield of PC increased rapidly in the initial
stage and stayed almost steady after 4 h. Meanwhile, nearly all
the PO could be converted within 4 h. Conclusively, a reaction
time of 4 h was the optimal choice for the synthesis of PC.
Additionally, the effect of the catalyst amount on the reac-
tion was investigated under the same reaction conditions
(Fig. 5). The results indicated that the catalyst amount had a
significant impact on the reactions. It could be found that the
increase of the catalyst amount resulted in a moderate increase
in PC yield under a low catalyst amount level (0–1.5 mol%).
Thereafter, further increasing the amount of the catalyst
exhibited little influence on PC yield, which indicated an
optimal catalyst amount of 1.5 mol%.
Fig. 2 Effect of temperature. Reaction conditions: PO (14.3 mmol),
Fig. 4 Effect of reaction time. Reaction conditions: PO (14.3 mmol),
catalyst (1.5 mol%), CO2 pressure (2.0 MPa), reaction time (4 h).
catalyst (1.5 mol%), CO2 pressure (2.0 MPa), temperature (130 °C).
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Fig. 5 Effect of catalyst amount. Reaction conditions: PO (14.3 mmol),
CO2 pressure (2.0 MPa), reaction time (4 h), temperature (130 °C).
Fig. 7 FT-IR spectra of the fresh and the six times reused ABMDFP.
2.3 Recycling of the catalyst
Recycle experiments were also conducted to examine the stabi-
lity of the 2b catalyst for the reaction at 130 °C, 2.0 MPa for
4 h. During the cycle experiments, the catalysts were easily
recovered by simple filtration, and then used for the next cycle.
The results displayed in Fig. 6 show that the 2b catalyst could
be reused for six runs with only a slight depression in its
catalytic activity, while the selectivity of the product remained
at 99%.
The FT-IR spectra of 2b which had been used six times are
shown in Fig. 7. And there was no obvious change of the two
FT-IR curves which initially proved that the structure of the
catalyst was stable after six times reuse. SEM images of the
catalyst reused six times are also shown in Fig. 8, and the mor-
phology of 2b hardly changed after being used six times. In
addition, the thermogravimetric curve of 2b is exhibited in
Fig. 9. It was found that the 2b is still thermally stable up to
270 °C after it had been used in the reaction, which is much
higher than the temperature used for the cycloaddition of CO2.
Both the SEM and the thermogravimetric curve further proved
the high stability of the catalyst structure, while the stability of
the catalyst structure also provided evidence for the stability of
the catalyst activity.
Fig. 8 SEM images of the fresh (left) and the six times reused (right)
ABMDFP.
Fig. 9 The thermogram of the catalysts (fresh and reused six times).
2.4 Catalytic activity of various epoxides with CO2
In order to test the adaptability to different epoxides, a battery
of epoxide substrates were examined under the optimized reac-
tion conditions for the synthesis of corresponding cyclic car-
bonates in the presence of 2b, and the results are summarized
in Table 2. The results indicated that the 2b is suitable for a
variety of terminal epoxides with high yield and selectivity of
the corresponding cyclic carbonates. The catalyst activity
declined with the increase of carbon chain length (entries
Fig. 6 Reuse of catalyst ABMDFP. Reaction conditions: PO (14.3 mmol),
catalyst (1.5 mol%), CO2 pressure (2.0 MPa), temperature (130 °C), reac-
tion time (4 h).
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a
Table 2 Catalytic activity of the reaction of various epoxides with CO2
Entry
1
Epoxides
Carbonates
Time (h)
4
Yieldb (%)
99
2
3
4
4
98
98
91
4
4
Scheme 2 The proposed mechanism for the cycloaddition reaction.
Firstly, the hydrogen atom of the hydroxyl group led to the
C–O bond polarized by hydrogen bond interaction, which
made the ring opening of epoxide easier. In the meantime, the
nucleophilic attack of the halide anion on the less sterically
hindered carbon atom of the epoxide resulted in the ring
opening easily and the epoxide transformed into a ring-
opened intermediate (step 1). Subsequently, the amines of the
catalyst coordinated with CO2 to form the carbamate salt
(step 2). Then, nucleophilic attack of the intermediate on the
carbamate salt produced the alkyl carbonate (step 3). Finally,
the cyclic carbonate was obtained by intramolecular ring-
closure and the catalyst was regenerated (step 4). Additionally,
the interactions of 2b, epoxide and CO2 were simulated by
density functional theory (DFT) study. As shown in Scheme 3,
5
6
4
5
97
99
7
5
99
a Reaction conditions: epoxides (14.3 mmol), catalyst (1.5 mol%), CO2
pressure (2.0 MPa), temperature (130 °C). b Determined by GC.
1–4). Of all the epoxides surveyed, the activity of ethylene oxide
(entry 1) was the highest, and 99% yield of ethylene carbonate
(EC) was achieved in 4 h.
2.5 A possible mechanism for fixation of CO2 into cyclic
carbonates
Based on the reports21d,f,30,31 and the results above, we pro-
posed a possible mechanism for chemical fixation reaction of
Scheme 3 The interaction of ABMDFP, epoxide and CO2 calculated by
CO2 into cyclic carbonate catalyzed by ABMDFP (Scheme 2). the DFT study.
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Table 3 Catalysts screening for the synthesis of DMCa
necked flask with 4 mL of methanol, and the mixture was
heated at reflux for 20 min in an oil bath at 60 °C, then 0.60 g
NH4Br were added to the flask and heated for 3 h at 85 °C.
After the reaction, the mixture was cooled down to room temp-
erature. The top phase was poured out and the residue was
washed three times with ethyl acetate and then dried at 60 °C
for 12 h under vacuum. Subsequently, the solid polymer was
obtained. Following a similar procedure, the catalysts 1b and
3b were synthesized by using ammonium chloride and
ammonium iodide instead of ammonium bromide respect-
ively. The DFP was synthesized without any modifier.
Entry
Catalyst
Conv.b (%)
DMC select.b (%)
1
2c
3
4
5
6
ABMDFP
ABMDFP
KOH
K2CO3
NaOH
90
78
80
88
87
90
99
99
99
99
99
99
MeONa
a Reaction conditions: EC (12.5 mmol), CH3OH (4.0 g, 125 mmol),
catalyst (2.5 mol%, ABMDFP was calculated by monomers),
temperature (150 °C), reaction time (6.0 h). b Determined by GC. c PC
(12.5 mmol).
3.3 Characterization of the catalysts
The catalysts were characterized by thermogravimetric analysis
(TGA, PERKIN-ELMER7 Series Thermal Analysis System) at a
heating rate of 10 °C min−1. The morphology of the polymer
was observed using a scanning electron microscope (JEOL JSM
6700F). Fourier transform infrared (FT-IR) spectra were
recorded on a Thermo Nicolet 380 spectrophotometer with
anhydrous KBr as standard (Thermo Electron Co.). 13C NMR
spectra were recorded on a Bruker Avance-III 400M Solid-State
NMR Spectrometer.
FT-IR spectroscopic studies were carried out on DFP, 1b, 2b
and 3b (Fig. 10). All of the samples were dried under vacuum at
60 °C for 24 h before being detected. The catalyst DFP, 1b, 2b
and 3b showed a typical strong peak related to –OH stretching
at about 3500–3000 cm−1. DFP displayed a characteristic bands
of CuN centered at 2172 cm−1 peaks, while the other catalysts
did not, which demonstrated a complete reaction with NH4X.
The 13C NMR spectra of the four catalysts were also studied.
As shown in Fig. 11, the NMR signal at 165 ppm in DFP was
attributed to the CuN, and the signal apparently disappeared
in 1b, 2b and 3b, which proved the success of their synthesis.
the hydrogen atom of 2b was coordinated with the oxygen of
epoxide through a hydrogen bond leading to the length of the
C–O bond increased from 1.434 Å to 1.459 Å, which made the
ring-opening much easier. The DFT study also indicated that
the bond angle of CO2 was changed to 179 °C from a linear
molecule after interaction with amines, which could activate
the CO2 in some degree.
2.6 Catalytic activity towards the transesterification process
for DMC production
The synthesis of DMC through transesterification reaction cata-
lysed by ABMDFP with EC and PC as substrates was also
studied due to the special structure of many amines within the
molecule. As shown in Table 3, the 2b catalyst exhibited high
activity and selectivity towards transesterification of EC or PC
with methanol to the DMC. Furthermore, the results also
demonstrated that the activity of PC was much lower than EC
under the same conditions, presumably owing to its steric
hindrance (entry 1 vs. 2). By comparison, several conventional
catalysts were also investigated under the same reaction
conditions. It was found that the catalytic activity of ABMDFP
was similar to the conventional catalysts (entry 1 vs. 4, 5, 6) or
even better (entry 1 vs. 3). According to the published
reports,32 and the results in this study, the catalytic activity of
the catalysts for the transesterification process could be attri-
buted to the activation of alcohol by amines.
3.4 Procedure of the cycloaddition reaction of CO2 with
propylene oxide
The coupling reaction of carbon dioxide and epoxide was
carried out in a 25 mL stainless-steel reactor equipped with a
3. Experimental
3.1 Chemical reagents
Carbon dioxide was supplied by Beijing Analytical Instrument
Factory with a purity of 99.95%. Ammonium iodide, 1,2-epoxy-
octane and cyclohexene oxide were purchased from Alfa
Aesar-A Johnson Matthey Company. Other reagents and chemi-
cals (analytic grade) were bought from Sinopharm Chemical
Reagent Co., Ltd, and were used without further purification.
3.2 Synthesis of dicyandiamide–formaldehyde polymers
We synthesized four kinds of dicyandiamide–formaldehyde
polymers. In a typical reaction, NH4Br (1.00 g) and dicyandi-
amide (1.12 g) were poured in sequence into a 25 mL three- Fig. 10 The FT-IR spectra of DFP, ACMDFP, ABMDFP and AIMDFP.
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through transesterification reaction was also studied, and a
catalyst that exhibited high activity, 90% yield of DMC together
with 99% selectivity was obtained. In conclusion, this class
of functional organic polymers was inexpensive, easy to
synthesize and recycle, and proved to be environmentally
efficient catalysts for the cycloaddition reaction of CO2.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (21076113, 21003129 and 21006112).
Notes and references
Fig. 11 The 13C NMR spectra of DFP, ACMDFP, ABMDFP and AIMDFP.
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were charged into the reactor at room temperature, then CO2
was added at a suitable pressure from a reservoir tank to main-
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methanol
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4. Conclusions
A
series of dicyandiamide–formaldehyde polymers were
synthesized and used as catalysts for conversion of CO2 into
organic carbonates. The ABMDFP was the best catalyst of
all the polymers, which was applicable to various epoxides.
Furthermore, the catalyst could be easily recovered by filtration
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