Synthesis of quinazoline-2,4(1H,3H)-dione from carbon dioxide and 2-aminobenzonitrile using mesoporous smectites incorporating alkali hydroxide

Article abstract of DOI:10.1039/c3cy00977g

A series of magnesium containing mesoporous smectites has been prepared with and without incorporation of alkali hydroxide (NaOH, KOH or LiOH) and employed for the reaction of CO₂ with aminobenzonitrile to produce quinazoline-2,4(1H,3H)-dione. The effects of the quantity and kind of the incorporated alkali atoms on the catalytic properties of the smectites were investigated. Characterization of the smectites has shown that the incorporation of alkali atoms reduces their surface area and total pore volume but enhances the amount and strength of their basic sites. The product yield increases with the amount of alkali atoms incorporated. The incorporation of Li was less effective than that of Na and K for the enhancement of the yield. It has been suggested that weak and/or moderate base sites are responsible for the reaction. The active sites should be alkali hydroxide particles existing between the smectite layers for the alkali incorporated smectites, while for the un-incorporated smectite, the active sites should be the Mg atoms and/or the neighboring O atoms. The Na incorporated smectite was deactivated by repeated catalyst recycling, while such deactivation was not observed with the un-incorporated smectite. The reason for the deactivation was discussed in connection with the structures of the active sites and the actions of the reaction intermediate. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00977g

Catalysis
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Synthesis of quinazoline-2,4(1H,3H)-dione from
carbon dioxide and 2-aminobenzonitrile using
mesoporous smectites incorporating
alkali hydroxide
Cite this: Catal. Sci. Technol., 2014,
4, 1563
*
Shin-ichiro Fujita, Masahiro Tanaka and Masahiko Arai
A
series of magnesium containing mesoporous smectites has been prepared with and without
incorporation of alkali hydroxide (NaOH, KOH or LiOH) and employed for the reaction of CO2 with
aminobenzonitrile to produce quinazoline-2,4(1H,3H)-dione. The effects of the quantity and kind of the
incorporated alkali atoms on the catalytic properties of the smectites were investigated. Characterization
of the smectites has shown that the incorporation of alkali atoms reduces their surface area and total
pore volume but enhances the amount and strength of their basic sites. The product yield increases with
the amount of alkali atoms incorporated. The incorporation of Li was less effective than that of Na and K
for the enhancement of the yield. It has been suggested that weak and/or moderate base sites are
responsible for the reaction. The active sites should be alkali hydroxide particles existing between the
smectite layers for the alkali incorporated smectites, while for the un-incorporated smectite, the
active sites should be the Mg atoms and/or the neighboring O atoms. The Na incorporated smectite
was deactivated by repeated catalyst recycling, while such deactivation was not observed with the
un-incorporated smectite. The reason for the deactivation was discussed in connection with the
structures of the active sites and the actions of the reaction intermediate.
Received 27th November 2013,
Accepted 24th December 2013
DOI: 10.1039/c3cy00977g
www.rsc.org/catalysis
(poly-amidine),13 and tetramethylguanidine (TMG),14 an inor-
ganic base Cs2CO3,15 ionic liquids 1-butyl-3-methylimidazolium
Introduction
CO2 is a cheap, abundant natural carbon source and one of
the important C1 building blocks, and it can sometimes replace
toxic chemicals such as phosgene, isocyanates or carbon mon-
oxide, leading to greener processes.1–3 However, because of its
inert nature, developing efficient catalytic processes for the uti-
lization of CO2 for organic synthesis still remains as a challeng-
ing task. Recently, efforts were made for the reaction of CO2
with aminobenzonitriles (1) to produce rather complex mole-
cules of quinazoline-2,4(1H,3H)-diones (2) (Scheme 1), which
are a class of important intermediates in the pharmaceutical
industry that function as a key building block for the synthesis
of several useful and valuable medicines. For this reaction,
CO2 can replace toxic reagents such as phosgene, potassium
cyanate, and chrolosufonyl isocyanate, which have been
employed for the conventional synthesis of 2.4–8 Several
research groups reported various catalyst systems for the syn-
thesis of 2 by reacting CO2 with 1. Those include organic
bases 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),9–12 N-methyl-
tetrahydropyrimidine (MTHP),13 polymer-supported amidine
hydroxide ([Bmim]OH)16 and 1-butyl-3-methylimidazolium
acetate ([Bmim]Ac),17 a solid base catalyst MgO/ZrO2,18 and a
monomeric tungstate [(n-C4H9)4N+)]2[WO4] (TBA2[WO4]).19
Recently, Han et al. have reported that the reaction of CO2
with 1 can proceed in water without any catalyst.20 Although
this reaction system is interesting, it requires higher reaction
temperatures and a longer reaction time to obtain 2 with
reasonable yields.
Synthesized smectites are a new kind of mesoporous
materials.21–23 Smectite is one of the layered clay minerals in
which one layer consists of one octahedral sheet sandwiched
by two tetrahedral sheets. The octahedral sheet contains divalent
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido
University, Sapporo 060-8628, Japan. E-mail: marai@eng.hokudai.ac.jp
Scheme 1 Synthesis of quinazoline-2,4(1H,3H)-dione 2 from CO2 and
aminobenzonitrile 1.
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or trivalent cations such as Mg2+, Zn2+, Fe3+ and Al3+ surrounded
by six oxygen atoms, and the tetrahedral sheet contains Si4+
cations surrounded by four oxygen atoms. Those divalent or tri-
valent metal cations make the acidic and basic properties of the
smectites tunable.21–29 The tri-layers are negatively charged and
are held together by electrostatic interaction with exchangeable
cations in the interlayer region. Additionally, incorporation of
alkali atoms into smectites is also possible.27,30 As the first step
of the synthesis of smectites, precursor materials are prepared
by mixing aqueous solutions of raw materials (water glass and
divalent or trivalent metal salt). Then, the slurries of the pre-
cursors and water are treated hydrothermally. By adding alkali
compounds into the slurry before the hydrothermal treat-
ment, alkali atoms can be introduced into smectites. Our
group has reported that the incorporation of alkali atoms into
the smectites makes them good catalysts for such base cata-
lyzed reactions as transesterification of ethyl acetate with
methanol, Knoevenagel condensation of benzaldehyde with
ethyl cyanoacetate, and some other reactions relevant to
chemical fixation of CO2 to organic carbonate compounds.31–34
These observations led us to the use of alkali incorporated
basic smectites for the synthesis of 2.
that had a slightly larger average pore diameter. As already
discussed,34 the decrease in the BET surface area and the total
pore volume can be explained by the presence of alkali
hydroxide particles between the layers of smectite structure
on the basis of a model of “smectite-intercalated smectite”
proposed by Torii et al.17,18
The reaction of 1 and CO2 was carried out using those
smectite catalysts prepared. Fig. 1 plots the yield of product 2
against the amount of alkali atoms incorporated. The lowest
yield was obtained with SM and it increased with an increase
in the amount of Na incorporated. Similar enhancement in
the yield was also observed with SM–K and SM–Li; however,
Li incorporation was less effective for the enhancement in
the yield compared to Na. This may partly be explained by
the formation of lithium silicate,34 which is considered to
be catalytically inactive. Such alkali silicate compounds were
not observed to form for all SM–Na and SM–K samples.
As shown in Table 1, the BET surface area and the total pore
volume tended to decrease with the increasing amount of
alkali atoms incorporated. Those textural properties cannot
explain the differences in the catalytic activity among the
smectites used. Their basic properties should be responsible
for those differences.
The basic properties of synthesized smectite catalysts were
examined by TPD of adsorbed CO2. Fig. 2 shows the TPD pro-
files obtained. SM showed small CO2 desorption peaks
around 90 °C and 180 °C and all the SM–Na catalysts showed
the ones around 100 °C and 250 °C. With increasing Na con-
tent, the intensity of the peak at 250 °C increased and tended
to level off at higher Na contents. In addition to these two
peaks, three peaks appear above 600 °C over SM–Na samples
containing large amounts of Na and they grow with its con-
tent. CO2 TPD profiles obtained with SM–K and SM–Li were
slightly different from those over SM–Na. SM–K and SM–Li
showed only one broad peak above 600 °C and the latter cata-
lyst also showed a peak at 480 °C,34 which is not observed
over the other catalysts. Thus, the peak intensity and temper-
ature are different among the catalysts, meaning that the
strength and the number of base sites vary with the composi-
tion of the catalysts. The peak around 100 °C was ascribable
to both the desorption of CO2 physisorbed and of CO2
adsorbed on weak base sites; however, the contribution of
In the present study, a series of magnesium containing
smectites into which various amounts of alkali hydroxide were
incorporated were prepared and used for this reaction. The
effects of the quantity and kind of the incorporated alkali
atoms on the catalytic properties of the smectites were investi-
gated. The reaction results were discussed in connection with
the basic properties of the catalysts that were determined by
temperature programmed desorption of adsorbed CO2.
Results
Influence of catalyst composition on textural properties
and catalytic activity
Table 1 lists the chemical compositions determined by an
X-ray fluorescence method (XRF) and BET surface areas, total
pore volumes, and average pore diameters of the smectite
catalysts prepared. Both the BET surface area and the total
pore volume tended to decrease with the increasing amount of
alkali atoms incorporated and the average pore diameter did
not change with the content of alkali atoms, except for SM–Li
Table 1 Composition and textural properties of smectite catalysts prepareda
Number of atoms per unit cell
SBET
Vpore
dav
Catalyst
Si
Mg
Na
K
Li
(m2 g−1
)
(cm3 g−1
)
(nm)
a
b
c
d
e
f
SM
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
4.60
5.67
5.88
5.89
6.09
6.13
6.09
5.41
0.06
0.84
1.51
1.80
2.25
2.45
0.47
0.47
0
0
0
0
0
0
1.7
0
0
0
0
0
0
0
0
2.56
463
695
392
379
173
200
190
118
0.433
0.440
0.242
0.233
0.107
0.124
0.126
0.124
2.53
2.46
2.46
2.47
2.48
2.52
2.65
4.22
SM–Na-1
SM–Na-2
SM–Na-3
SM–Na-4
SM–Na-5
SM–K
g
h
SM–Li
a
SBET, Vpore and dav denote BET surface area, total pore volume, and average pore diameter, respectively.
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Table 2 Amounts of base sites over smectite catalysts
Amount of base site (μmol g−1
)
Number of alkali
Catalyst
atoms per unit cell Weak
Medium
Strong
a
b
c
d
e
f
SM
0.09
3.1
18.0
41.0
50.7
44.0
34.8
59.5
25.6
0.3
37
0
9
17
SM–Na-1 0.84
SM–Na-2 1.51
SM–Na-3 1.80
SM–Na-4 2.25
SM–Na-5 2.64
123
157
165
199
178
149
36
100
223
78
g
h
SM–K
SM–Li
2.17a
3.03b
109
a
b
Sum of Na and K. Sum of Na and Li.
Fig. 1 Influence of the amount of alkali atoms incorporated on the
activity of smectite catalysts for the synthesis of 2 from CO2 and 1. (●),
SM; (○), SM–Na; (□), SM–K; (■), SM–Li. Reaction conditions: catalyst,
0.4 g; 1, 8.5 mmol; CO2, 4 MPa (at r. t.); DMF, 10 cm3; 130 °C; time, 6 h.
Fig. 3 Relationship between the activity for the synthesis of 2 and the
amount of (A) weak and (B) moderate base sites of the smectite
catalysts. (●), SM; (○), SM–Na; (□), SM–K; (■), SM–Li.
moderate base sites present on the smectite catalysts are respon-
sible for the reaction, and the presence of the strong base
sites is not required for the reaction.
Fig. 2 CO2 TPD profiles over smectite samples synthesized. The letters
given denote the same samples as given in Table 1.
Influence of reaction conditions and catalyst recycling
physisorbed CO2 to this peak would be small, because
SM showed a smaller peak than SM–Na catalysts despite the
former catalyst having a larger surface area than the latter
ones (except for SM–Na-1), as listed in Table 1. So, the peaks
around 100 °C were assigned to the desorption of CO2
adsorbed on weak base sites.
The SM–Na-4 catalyst was selected and used for several reac-
tion experiments by varying the reaction time or the CO2
pressure. Fig. 4 illustrates the variation of the yield of 2 with
the reaction time. It is seen that an excellent yield above 80%
Although conclusive definition was difficult, the peaks
seen between 200 and 600 °C were tentatively assigned to the
desorption of CO2 adsorbed on moderate base sites, while
the peaks above 600 °C were attributed to that of CO2
adsorbed on strong base sites. Table 2 lists the amounts of
these three types of base sites estimated from the peak areas
in the TPD runs. Those structures will be discussed later.
As shown above, the catalytic activity and the number of
base sites of smectite increased with the content of alkali atoms.
So, the relationship between the catalytic activity observed and
the number of basic sites estimated from the peak area in the
TPD runs was examined. The activity correlated well with the
amount of either weak or moderate base sites, as shown in
Fig. 3. No good correlation was seen with the amounts of strong
base sites. These findings strongly suggest that the weak and/or
Fig. 4 Influence of reaction time on the synthesis of 2 using SM–Na-4.
Reaction conditions: catalyst, 0.4 g; 1, 8.5 mmol; DMF, 10 cm3; 130 °C.
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was achieved within 9 h but the yield increased marginally in
a longer reaction time of 16 h.
The influence of CO2 pressure on the reaction was studied
(Fig. 5). It was found that the CO2 pressure had little effect
on the product yield in a CO2 pressure range between 1 and
4 MPa. Even at a lower pressure of 0.5 MPa, the decline in
the yield was small. These results are similar to those obtained
with DBU and Cs2CO3,11,15 but different from those obtained
with [Bmim]OH and MgO/ZrO2.16,18 With the former two cata-
lysts, the product yield did not significantly change at CO2
pressures above 1 MPa. With the latter two catalysts, the prod-
uct yield significantly decreased with lowering the CO2 pres-
sure below 3 MPa. Thus, the dependence of the overall
reaction rate on the CO2 pressure is different depending on
the catalysts employed.
Fig. 6 Recyclability study of (●) SM and (○) SM–Na-4 on the synthesis
of 2. Reaction time was 16 h and 3 h for SM and for SM–Na-4, respec-
tively. The other reaction conditions were the same as those for Fig. 1.
Easy separation and recycling is one of the merits of hetero-
geneous catalysts. So, the recycling tests were conducted with
SM and SM–Na-4 to study the stability of the catalysts. In each
recycling experiment, the catalyst was separated by filtration
after a reaction run, washed with ethanol, dried in air, and
then used for the next run. The catalyst activity was evaluated
on the basis of the produced amount of 2 per weight of the cat-
alyst, because a certain portion of the catalyst was lost during
the recycling process. The results of the recyclability study are
illustrated in Fig. 6. The SM catalyst could be recycled three
times without loss in the activity. On the other hand, the activ-
ity of SM–Na-4 changed during the catalyst recycling experi-
ment. It slightly increased after the first recycling and then
gradually decreased after the catalyst recycling is repeated.
To elucidate the reason for the deactivation, the number of
Na atoms in SM–Na-4 recycled five times was measured by
XRF. It was found that the number of Na atoms contained in
the recycled catalyst was reduced to below one third of that in
the fresh one. Furthermore, recycling did not significantly
change the textural properties of the catalyst. These results sug-
gest that the leaching of Na species occurred in the course of
the reaction, causing the deactivation. The reaction runs were
also carried out using SM–Na-4 samples pretreated at 130 °C
for 16 h (corresponding to five-times recycling) in pure DMF
under pressurized CO2 or in a DMF solution of 1 in the absence
of CO2. It was found that no deactivation occurred after these
pretreatments, revealing the requirement of the co-presence of
the reactants 1 and CO2 for the catalyst deactivation.
The catalytic activity of the leached Na species was exam-
ined. After the reaction was carried out with SM–Na-4 for 3 h,
it was filtered out from the reaction mixture. Then, the fil-
trate was further subjected to the reaction for a long time of
19 h. The yield of 2 produced was estimated to be 40%,
revealing that the activity of the leached Na was about one
fifth of the activity of SM–Na-4, which could produce 2 with a
yield of about 30% during the 3 h reaction.
Discussion
As shown in Table 2, the incorporation of alkali atoms into
SM significantly increased the amounts of base sites. Among
SM–Na samples, the amount of weak base sites showed a max-
imum at a Na content of 1.8 atoms per unit cell, but the differ-
ence due to the Na content was small. On the other hand, the
amounts of moderately and strongly basic sites varied in
wider ranges with the Na content than the weak base sites.
The amount of moderately basic sites leveled off at a certain
Na content around 1.8 atoms per unit cell, and from this con-
tent level, the amount of strongly basic site started to increase
appreciably. In our previous work,29 the features of base sites
were discussed and the weak, moderate, and strong base sites
were assigned to the surfaces of the smectite layers free from
alkali atoms, alkali hydroxide particles having weak or no
interactions with the layers, and the ones strongly interacting
with the layers, respectively. However, comparison of SM and
SM–Na-1 in Table 2 clearly showed that the amount of weak
base sites also significantly increased even with a small
amount of Na incorporation. If the weak base sites are free
from Na atoms, such a change could not be observed. So, the
assignments for the weak and moderate base sites were
reconsidered and modified. Here, the weak base sites were
Fig. 5 Influence of CO2 pressure on the synthesis of 2 using SM–Na-4.
Reaction conditions: catalyst, 0.4 g; 1, 8.5 mmol; DMF, 10 cm3; 130 °C;
time, 6 h.
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assigned to both the surfaces of the smectite layers free from
alkali atoms and alkali hydroxide particles not interacting at
all with the smectite layers, while the moderate ones were
assigned to the particles weakly interacting with them. Taking
into account the difference in the amount of the weak base
sites between SM and SM–Na-1, the contribution of the alkali
free surfaces of the layers to this type of base site would be
small over SM–Na-1. As shown in Fig. 3, the yield of 2 was well
correlated to the amounts of both the weak and moderate
base sites. It can be said that alkali hydroxide particles have a
crucial role for the synthesis of 2.
Our group has reported that, over alkali-incorporated smec-
tite catalysts, moderate base sites are responsible for the trans-
esterification reactions of ethyl acetate and propylene carbonate
with methanol, while both moderate and strong base sites are
responsible for the cycloaddition of CO2 to propylene oxide and
the one-pot synthesis of dimethyl carbonate from CO2, metha-
nol and propylene oxide.31,32,34 Thus, the types of base sites
contributing to the title reaction are different from the ones
contributing to those reactions previously studied.
Scheme 2 illustrates the reaction mechanism for the title
reaction previously proposed for several different base
catalysts.11–17 The base catalysts promote the reaction between
the amino group of 1 and CO2 to produce a carbamate species,
which is finally converted to the product 2 through the nucleo-
philic cyclization and the rearrangement by way of the isocya-
nate intermediate. Usually, the reaction of CO2 with the amino
group directly attached to a phenyl ring, for example, the
amino group of aniline, is more difficult than the one attached
to an alkyl chain. The promotional effect of the base on this
difficult reaction step would in turn result in the strong interac-
tion between the base and the resultant carbamate species.
In the cases of alkali incorporated smectites, alkali hydroxide
particles would interact strongly with the intermediate carba-
mate. As described above, SM–Na-4 was deactivated with a
decrease in the Na content of the catalyst after repeated catalyst
recycling and the deactivation was not observed after the pre-
treatments in the absence of either reactant 1 or the other reac-
tant, CO2. The strong interaction of the alkali particles with the
carbamate would cause the dissolution of the alkali species
into the solvent, resulting in the catalyst deactivation. It should
be noted that the alkali incorporated smectite could be
recycled without loss in the activity for the transesterification
of ethylene carbonate with methanol.33 Thus, the recyclability
of the alkali incorporated smectite depends on the reaction for
which it is employed.
The catalyst deactivation due to recycling was not observed
with SM (Fig. 5), which contains exchangeable Na cations
compensating the electric charge of the smectite layers but
not alkali hydroxide particles. Because MgO could catalyze the
title reaction,18 the active sites of SM would be Mg atoms and/
or the neighboring O atoms on the surfaces of the smectite
layers. The exchangeable Na cations involved in SM might
have very low activity, if any.
On the basis of the aforementioned discussion, the active
sites over SM–Na for the title reaction can be illustrated as
shown in Fig. 7. There are two types of active sites. The first
and second sites involve Mg atoms in the octahedral sheet
and NaOH particles existing on the surface of the tri-layer,
respectively. A certain fraction of the NaOH particles weakly
interact with the layer. The activity of SM–Na depends on
the number of the second sites. Naturally, the active sites of
SM are the first type.
Table 3 compares the present catalyst of SM–Na-4 with
those in the literature for the synthesis of 2 from 1 and CO2.
The organic bases, Cs2CO3, and [Bmim]Ac could be used at
lower temperatures and/or lower CO2 pressures (entries 1–5
and 7). One of the reasons for this would be their homogeneous
nature. TBA2[WO4] was also more active than SM–Na-4 (entry 9).
Considering the differences in the reaction conditions, the
activity of SM–Na-4 is comparable with those of [Bmim]OH
and MgO/ZrO2 (entries 6 and 8). It should again be noted that
[Bmim]OH and MgO/ZrO2 gave lower yields of 2 at lower CO2
pressures, while such a decrease in the yield by lowering CO2
pressure was small for the reaction with our SM–Na-4 catalyst
(Fig. 5).
As shown above, the incorporation of alkali hydroxide into
smectite greatly enhanced the catalytic activity for the title
reaction. This was attributed to the formation of alkali hydrox-
ide particles between the smectite layers. Unfortunately, the
Scheme 2 A reaction mechanism proposed so far for the quinazoline-2,4(1H,3H)-dione synthesis from CO2 and benzonitrile.11–16
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that of Na and K for the enhancement of the yield. It has
been suggested that weak and/or moderate base sites involv-
ing alkali hydroxide particles existing between the smectite
layers are responsible for the reaction. The Na incorporated
smectite is deactivated by the repeated catalyst recycling
because of the dissolution of the Na species into DMF. Such
a deactivation is not observed with SM. It has been proposed
that there is a strong interaction between NaOH particles and
the reaction intermediate species of carbamate, resulting in
the catalyst deactivation.
Experimental
Synthesis of smectite catalysts
A
smectite catalyst was synthesized by a hydrothermal
method in which water glass (SiO2 28%, Na2O 9%) was used
as the starting material. Water glass (12.9 g) was dissolved in
30 cm3 of water. To this solution, 22.5 cm3 of a 2 N NaOH
solution and 30 cm3 of a solution containing magnesium
chloride hydrate were consecutively added under stirring. The
resulting solution was kept under stirring at ambient temper-
ature for 15 h. The Si–Mg precipitates that formed were sepa-
rated by filtration and washed with water 4 times. A slurry was
prepared from the Si–Mg precipitates and 20 cm3 of water.
Then, the slurry was transferred into an autoclave and heated
at 200 °C under autogaseous water vapor for 3 h. The product
obtained was filtered and dried at 100 °C for 15 h. This cata-
lyst is designated as SM herein.
Several smectites incorporated with alkali hydroxides were
also prepared in a similar manner in which some amount of
sodium, potassium or lithium hydroxide was added to the
slurry of Si–Mg precipitates before the hydrothermal treat-
ment. The procedures for these alkali incorporated smectites
were described elsewhere.27 These catalysts are abbreviated
as SM–Na-n (n = 1–5), SM–K, and SM–Li. The chemical com-
positions of the smectite catalysts prepared were determined
by an X-ray fluorescence method and listed in Table 1.
Fig. 7 Active sites over SM–Na catalysts.
catalytic activity was decreased by the repeated use because of
the dissolution of the active alkali species into the solvent.
The stability of the catalyst may further be improved by opti-
mizing preparation conditions and incorporating different
metal cations into the octahedral sheet and/or the interlayer.
Those aspects are open for further interesting work. A few
groups employed supercritical CO2, water, or an ionic liquid
as a green solvent for the title reaction.11,16,17,20 The use of the
smectite catalysts in those media is also interesting and worth
investigating in the future.
Conclusions
In the present study, a series of magnesium containing meso-
porous smectites (SM, M–Na, SM–K, SM–Li) has been pre-
pared with and without incorporation of alkali hydroxide and
employed for the synthesis of 2. The incorporation of alkali
atoms reduces their surface area and total pore volume but
enhances the amount and strength of their basic sites. The
product yield increases with the amount of alkali atoms
incorporated. The incorporation of Li is less effective than
Characterization of catalysts
X-ray powder diffraction (XRD) patterns of the smectite sam-
ples prepared were recorded on a JEOL JDX-8030 diffractometer
using Ni filtered Cu-Kα radiation. The samples were scanned
Table 3 Comparison of the present catalyst of SM–Na-4 with the earlier reported ones for the synthesis of 2 from 1 and CO2
Entry
Catalyst
1/Catalyst
Pressure (MPa)
Temperature/time (°C/h)
Yield (%)
Reference
1
2
3
4
5
6
7
8
9
10
DBU
MTHP
10 mmol/1 mmol
10 mmol/3 mmol
0.33 mmol/0.33 mmol
5 mmol/0.1 mmol
20 mmol/5 mmol
20 mmol/5 mmol
5 mmol/5 mmol
20 mmol/0.24 g
10
80/4
91
71
61
82
94
90
92
92
90
67
12
13
13
14
15
16
17
18
0.1
0.1
10
100/24
100/24
120/4
100/4
120/18
90/10
130/12
100/24
130/6
Poly-amidine
TMGa
Cs2CO3
1.3b
3b
[Bmim]OH
[Bmim]Ac
MgO/ZrO2
TBA2[WO4]
SM–Na-4
0.1
3.7b
2
1 mmol/0.02 mmol
8.5 mmol/0.4 g
19
4b
Present study
a
Tetramethylguanidine. b Initial pressure at room temperature.
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in the range of 2–70° with a step of 0.02°. BET surface area, total
pore volume, and pore size distribution were obtained from
nitrogen adsorption–desorption isotherms at −196 °C using a
Nova 1200 automatic physisorption instrument (Quantachrome
Corporation). The samples were degassed at 100 °C under
vacuum for 30 min prior to the measurements.
The basic properties of the catalysts were measured by tem-
perature programmed desorption (TPD) of adsorbed CO2
using a conventional flow reactor. After a stream of a 20 vol.%
CO2–He mixture was passed over 0.2 g of a catalyst sample at
room temperature for 20 min, gases in the reactor were
flushed with a helium stream. Then, the temperature was
ramped at a heating rate of 10 K min−1 up to 900 °C. The efflu-
ent from the reactor was analyzed by mass spectrometry.
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Catalytic reaction
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The synthesis of 2 was carried out in a 50 cm3 autoclave. In a
typical run, the reactor was charged with the catalyst (0.4 g),
1 (8.5 mmol) and DMF (10 cm3) and purged with CO2 several
times, and then liquid CO2 was further injected into the reac-
tor up to 4 MPa under stirring. The reaction mixture was
stirred at 130 °C for 6 h. After the reaction, the catalyst was
filtered off, washed with ethanol, and dried under vacuum.
The filtrate was evaporated in vacuum to remove the solvent
DMF and the un-reacted 1. The crude product of 2 was
washed with diethyl ether and dried under vacuum.
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