CO2 activation and promotional effect in the oxidation of cyclic olefins over mesoporous carbon nitrides

Article abstract of DOI:10.1039/c0gc00951b

Mesoporous carbon nitrides (MCN) were prepared by a nano-casting method using mesoporous silica as a template with different carbon and nitrogen sources like melamine only (MS-MCN), urea-formaldehyde (UF-MCN) and melamine-glyoxal (MG-MCN). These mesoporous carbon nitride materials possess nitrogen moieties which behave like a CO₂-philic surface facilitating oxidation of cyclic olefins by molecular oxygen in the co-presence of CO₂ below supercritical conditions. The co-presence of CO₂ augmented the conversions of cyclic olefins at low pressures of CO₂, depicting a promotional effect. Approaches towards quantification of promotional effects and insights into the promotional aspects have been studied.

Full text of DOI:10.1039/c0gc00951b

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Volume 13 | Number 6 | June 2011 | Pages 1365–1560
Themed issue: Green Solvents for Synthesis
ISSN 1463-9262
COVER ARTICLE
Park et al.
CO2 activation and promotional
effect in the oxidation of cyclic
olefins over mesoporous carbon
nitrides
1463-9262(2011)13:6;1-T

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PAPER
CO2 activation and promotional effect in the oxidation of cyclic olefins over
mesoporous carbon nitrides†‡
Mohd Bismillah Ansari, Byung-Hoon Min, Yong-Hwan Mo and Sang-Eon Park*
Received 25th December 2010, Accepted 9th February 2011
DOI: 10.1039/c0gc00951b
Mesoporous carbon nitrides (MCN) were prepared by a nano-casting method using mesoporous
silica as a template with different carbon and nitrogen sources like melamine only (MS-MCN),
urea-formaldehyde (UF-MCN) and melamine-glyoxal (MG-MCN). These mesoporous carbon
nitride materials possess nitrogen moieties which behave like a CO2-philic surface facilitating
oxidation of cyclic olefins by molecular oxygen in the co-presence of CO2 below supercritical
conditions. The co-presence of CO2 augmented the conversions of cyclic olefins at low pressures of
CO2, depicting a promotional effect. Approaches towards quantification of promotional effects
and insights into the promotional aspects have been studied.
operating in dense phase CO2 for oxidation reactions12 and
Introduction
hydrogenation19 reactions in homogeneous media.20
Carbon dioxide chemistry has become a more attractive area
The alternative way for utilization of carbon dioxide currently
of research not only because of environmental concerns but
evokes great interest as an oxygen source. In our earlier studies
also due to its potential utilization as an alternative and
we have reported the soft oxidant behavior of CO2 in the
economical feedstock.1–4 The utilization of CO2 as a chemical
oxidative dehydrogenation of ethylbenzene, toluene and xylene
feedstock is limited and requires large amounts of energy
in both gaseous and liquid-phase, respectively.21–22 The role
and/or hydrogen for the abstraction of oxygen. Recently, CO2
of CO2 in these reactions provides several merits such as
was utilized as a dense phase under so-called supercritical
abstraction of hydrogen from alkanes, alkenes, alcohols and
conditions as an activity enhancer, a green alternative for
alkyl aromatics through catalytic activation of carbon dioxide
organic solvents 5–11 and as a miscibility enhancer with gaseous
to form CO and an oxygen species, acceleration of reaction
reactants like oxygen12 and H2.13 Dense phase CO2 systems have
rate, enhancement of product selectivity, alleviation of chemical
drawbacks of low reaction rates, high operating pressures and
equilibrium and suppression of total oxidation.21 We also
inadequate solubilities.14 However, Jessop et al. have resolved
the solubility problem through a switchable system based on
reported the promotional effect of CO2 in oxidation of alkyl
aromatics with O2 as oxidant using a homogeneous Co/Mn/Ni,
1,8-diazabicyclo-[5.4.0]-undec-7-ene which facilitated organic
Br catalyst.23 The co-presence of CO2 with O2 contributed to
synthesis and eliminate separation steps by tuning polarities
enhance the reaction rates, higher conversion and selectivities as
in the presence of carbon dioxide.15 Another system which
well. This new system operates under the subcritical pressures of
elminates the solublity problem is based on a carbon dioxide
CO2 and is coined as a carbon dioxide promoted systems (CPS)
expanded liquid (CXL) which utilizes partial replacement of
dense phase CO2 with organic solvents.16 This system mainly
focus on behavioral aspects of the expansion of solvents with
CO2. Leitner et al.17 and Beckmann et al.18 have designed certain
instead of CO2 expanded system (CXL) which can be easily
discerned by the low pressure modus operandi and catalytic CO2
activation. This inspired us to develop a heterogenous catalytic
system which would be able to activate carbon dioxide especially
CO2-phillic polymers which readily dissolve in carbon dioxide
in an oxidation reaction.
and are considered as new green supports for catalytic systems
As a strategy carbon nitrides were chosen to play a role like
a CO2-phillic surface24 and as an organocatalyst25–26 because
of high nitrogen contents similar to CO2-activating organic
Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemical
molecules such as amines, N-heterocyclics, DBU, guanidine,
melamine and poryphyrins which were mainly used in carbon
dioxide activation for the preparation of cyclic carbonates.24,27–29
We were intrigued by the report of Antonietti et al. on carbon
nitride mediated CO2 activation, inwhichCO2 serve as an oxygen
source for oxidation of benzene to phenol. In order to judge the
Fusion Technology, Department of Chemistry, Inha University, Incheon,
402-751, Korea. E-mail: separk@inha.ac.kr; Fax: +82 32 872 8670
† This paper was published as part of the themed issue of contributions
from the Green Solvents – Alternative Fluids in Science and Application
conference held in Berchtesgaden, October 2010.
‡ Electronic supplementary information (ESI) available. See DOI:
10.1039/c0gc00951b
1416 | Green Chem., 2011, 13, 1416–1421
This journal is
The Royal Society of Chemistry 2011
©

ability of our materials towards CO2 activation, we have tested
the insertion of carbon dioxide into propylene oxide and also
the oxidation of benzene to phenol whereby CO2 served as an
oxygen source.
160 ◦C for 5 h. The pore size distribution (PSD) was calculated
using the Barrett–Joyner–Halenda (BJH) approach. The specific
surface areas were calculated using Brunauer–Emmett–Teller
(BET) method and the pore parameters were determined
from desorption branches by BJH methods. Raman spectra
was recorded on HR 800, Horiba/Jobin-Yvon were employed
to analyze the chemical structure of the catalyst. Electron
spectroscopy for chemical analysis (ESCA) was recorded on a
Sigma Probe ThermoVG, U. K. monochromatic Al–Ka 12 kV,
100 W, 400 micrometre, for a wide scan with pass energy of 50 eV
and a step size of 1.0 eV, whereas for a narrow scan with pass
energy of 20 eV and a step size of 0.1 eV a Avantage ThermoVG
was used. Elemental analysis was carried out on an Elemental
analyzer (EA1112, Thermo). Electron energy loss spectroscopy
(EELS) was measured by a TEM instrument (JEM 2100F,
JEOL). Product analysis was done by gas chromatography
(Agilent 6890 N, HP-5 capillary column, FID detector) and
further confirmed by GC-MS (Agilent Technologies, GC6890N,
MS5975).
The catalytic behaviour of carbonaceous materials have
been studied towards oxidation and oxidative dehydrogenation
reactions and it is presumed that surface oxygen functionalities
serve as active sites for oxidation.25,30–31 Recently Kakimoto et al.
have reported a carbon based metal-free catalytic system for
oxidation of alcohols using molecular oxygen.31 In here we are
reporting the role of ‘N’ containing carbonaceous materials
especially carbon nitrides for carbon dioxide activation and
also towards oxidation of cyclic olefins by molecular oxygen in
synergism with CO2 (Scheme 1). The promotional effect of the
co-presence of CO2 with molecular oxygen has been studied and
an insight towards the promotional aspects has been presented.
Catalytic reaction
Carbon dioxide activation. The reaction between propylene
oxide and carbon dioxide was carried out in DMF. In a typical
reaction procedure a 100 ml steel autoclave was charged with
5 ml of DMF, 20 mg of catalyst and 1.5 ml of propylene oxide
at 80 psi of carbon dioxide. The reaction mixture was heated to
373 K with constant stirring at 300 rpm for a period of 10 h. The
aliquots were subjected to GC and GC-MS analysis.
Scheme 1 Carbon dioxide activation and oxidation of cyclic olefins.
The reaction of benzene to phenol conversion was carried out
in a 100 ml stainless steel autoclave fitted with a teflon mantel,
10 ml benzene, 200 mg triethylamine as co-reactant and 100 mg
catalyst was placed in the autoclave. The autoclave was then
flushed three times with CO2, pressurized with CO2 (80 psi), and
then heated to 423 K with constant stirring at 300 rpm. The
reaction mixture was neutralized with 1 ml HCl and injected
directly into the GC–MS (Agilent Technologies, GC6890N, and
MS5975).
Experimental
Materials and methods
Glyoxal, urea, formaldehyde, propylene oxide, benzene,
dimethylacetylene-1,2-dicarboxylate, tetramethyl thiourea, pal-
ladium chloride and all the cyclic olefins were purchased from
Aldrich and used without further purification; melamine was
purchased from TCI. All the solvents used were of analytical
grade and used without further purification. Mesoporous car-
bon nitride (MCN) materials were synthesized via a hard tem-
plating route by using disk-shaped 2D hexagonal mesoporous
silica. The template mesoporous silica, was synthesize by our
earlier reported procedure.32 Three kinds of mesoporous carbon
nitrides were prepared by varying the carbon and nitrogen
sources and named accordingly; melamine (MS-MCN), urea-
formaldehyde (UF-MCN) and melamine-glyoxal (MG-MCN).
The appropriate amount of nitrogen and carbon precursor was
added to the template in order to polymerize over it to form a
porous nanocomposite. The porous nanocomposites obtained
were dried and then carbonized at 550◦ in a nitrogen flow
followed by treatment with 3% HF solution in order to remove
the silica.
Oxidation of cyclic olefins. All oxidation experiments were
performed in a 100 ml autoclave equipped with magnetic stirring
and an automatic temperature controller. A typical procedure
for the oxidation of cyclic olefins was as follows: cyclic olefin
(5 mmol), 10 ml of DMF and 20 mg catalyst was placed in
the autoclave fitted with a Teflon mantle. The autoclave was
flushed with dioxygen several times followed by adjustment of
the pressure ratios 80 psi with appropriate O2/N2 or O2/CO2
ratios. Under stirring at 300 rpm the autoclave was heated to
373 K and maintained for 10 h. After the reaction, the autoclave
was cooled, and the reaction products were analyzed by GC and
identified by GC-MS
Characterization
Recyclability. The catalyst was filtered after the reaction and
washed several times with water to remove the DMF followed by
2–3 times washing with ethanol. The catalyst was dried overnight
in a vacuum oven at 150 ◦C and reused for the subsequent cycle
in the oxidation of cyclohexene.
Nitrogen adsorption and desorption measurements including
surface areas and pore volumes were measured at liquid nitrogen
temperature using a Micromeritics porosimeter (model ASAP-
2020). Prior to the measurement, the samples were degassed at
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Green Chem., 2011, 13, 1416–1421 | 1417
©

Table 1 Cycloaddition of CO2 to propylene oxide over carbon nitride
Analysis of gaseous mixture after reaction
IR spectra of gases. The gaseous mixture after the oxidation
reaction was analyzed by a specially designed setup as shown in
Figure S1 (Supplementary Information‡). Initially the IR cell is
evacuated followed by connecting with an autoclave equipped
with an intermediate valve for controlling the pressure and a
moisture trap. The gas is slowly released into the IR cell and
analyzed by infrared spectroscopy.
Conversion
of 1 (%)b
Selectivity
for 2 (%)b
Entry
Catalysta
Pauson–Khand. Pauson–Khand reaction was conducted
with a small modification of the procedure reported by Chen
et al.33 for confirmation of CO formation. The remaining
gaseous mixture after oxidation was transferred to another au-
toclave which was charged with 0.5 mmol of dimethylacetylene-
1,2-dicarboxylate, 0.5 mmol n-hexene in 10 ml acetonitrile and
temperature to 50 ◦C.
1
2
3
MS-MCN
UF-MCN
MG-MCN
34
28
25
90
92
98
a Reaction conditions: 20 mg catalyst, 10 ml DMF, temperature 373 K,
80 psi CO2, 10 h. b Estimated by GC.
Carbon dioxide activation over MCNs
Results and discussion
Initially catalytic activities of MCNs were tested for CO2 activa-
tion in carbonation of propylene oxide to propylene carbonate
as a model system. The formation of propylene carbonate in the
presence of CO2 inferred the existence of functionality in MCNs
for CO2 activation (Table 1).
In order to further confirm the role of MCNs the conversion of
benzene to phenol in the presence of CO2 was also investigated.
In this reaction carbon dioxide serve as an oxygen source as
reported by Antonietti et al.24 All the MCNs showed pretty
good activities for the formation of cyclic carbonate and phenol
(Table 2). These results are closely related with nitrogen contents
of MCNs which serve as Lewis base sites.
Spectral and textural properties of MCNs
The elemental analysis of MCN materials MS-MCN, UF-MCN
and MG-MCN showed the occurrence of C, H, and N with C : N
ratios of 0.6, 2.5, and 5.3, respectively (Table S2‡). In addition,
the existence of nitrogen, carbon and oxygen functionalities
were also inferred by elemental analysis which were further
verified by XPS (X-ray photoelectron spectroscopy) spectra
of the MCN materials (Figure S3‡). The O1s binding energy
revealed various kinds of oxygen functionalities: C O at 532.2
eV C–O (533.3 eV), O–H (533.7 eV) and quinine type C O at
531 eV. The C1s binding energy of MS-MCN shows mainly one
carbon species with a binding energy of 288.2 eV, corresponding
to a b-C–N C coordination, whereas the sp2 C–C was observed
at 284 eV, the sp2 C–N at 285.9 eV, and the sp2 C O at 287.4
eV.34 In the N1s spectrum the main signal shows occurrence of
C–N–C groups (398.7 eV) and tertiary nitrogen N–(C)3 groups
(400.1 eV). Deconvolution of the N1s XPS signals also reveals a
weak additional signal at 402.7 eV, indicative of amino functions
carrying hydrogen (C–N–H).34–35 The EELS spectra (Figure
S4‡) of MCNs revealed the presence of ionization edges at ca.
284 and 401 eV corresponding to the C–K and N–K shells,
respectively. The C and N regions show sharply defined p* and
s* fine structural features, which could be assigned to the sp2
hybridization (usually observed in “graphitic” structures).26 The
Raman spectra of MCN materials (Figure S5‡) consist of D and
G bands in different proportions. The G band lies in the range of
1560–1600 cm-1 and indicates E2g in-plane stretching motions
of pairs or chains of C-sp2 bonded atoms. The D band around
1350 cm-1, relates to A1g breathing mode of six fold aromatic
rings and only becomes active in the presence of disorder.36–38
All the characterization results coincide with those reported
earlier.26
Oxidation of cyclic olefins over MCNs
The catalytic behaviors of MCNs were studied towards oxida-
tion, due to the presence of surface oxygen on the MCNs as
observed by elemental analysis, further more spectral analysis
by XPS inferred the occurrence of quinone type oxygen which
may serve as active sites for oxidation similar to carbonaceous
materials.25,30 However the exact reaction mechanism is uncer-
tain at this stage and further studies are in progress. Oxidation
of cyclohexene was carried out with molecular oxygen in the
absence and/or presence of CO2 to see the synergetic effect for
conversions and selectivities. The investigation on the influence
of CO2 in the oxidation of cyclohexene by varying gas ratios in
both O2/N2 and O2/CO2 were performed (Table 3).
As shown in (Table 3), at a ratio 0.066 of O2 (entry 1) no
conversion of cyclohexene was observed, this might be due to
low partial pressure of O2, which was insufficient to achieve
the product. A further increase in the O2 ratio increases the
product formation (Table 3, entries 2–5). However, gas ratios
higher than 0.333 merely increase the conversion of cyclohexene
(Table 3, entries 4–5). Hence, the catalytic activity towards other
cylic olefins was tested at ratio of 0.333 only.
The porous properties of the MCN materials were confirmed
by the nitrogen sorption analysis. The N2 sorption shows type
IV isotherms with H1-type hysterisis loops (Figure S6‡) which
were typical for mesoporous materials having cylindrical type
mesostructures.32 The BET surface areas of materials were
around 150–500 cm3 g-1 and pore size distributions were in range
of 3–5 nm.
In all the cases augmented conversions of cyclic olefins
were observed in O2/CO2 compared to O2/N2 (Table 3). This
promotional effect due to the co-exsistence of CO2 with oxygen
was studied quantitatively and had been calculated as a per-
centage difference in the conversion of substrate in O2/CO2 and
O2/N2; represented as DC (%) by following equation (eqn (1)):
1418 | Green Chem., 2011, 13, 1416–1421
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Table 2 Catalytic activities of MCNs for phenol production from benzene via carbon dioxide activation
Entry
Catalysta
Conversion of 3 (%)b
Selectivity of 4 (%)b
Selectivity of 5 (%)b
1
2
3
MS-MCN
UF-MCN
MG-MCN
30.0
29.9
20.7
71
89
86
29
11
14
a Reaction conditions: 200 mg catalyst, 10 ml DMF, temperature 423 K, 80 psi CO2, 10 h. b Estimated by GC.
Table 3 The influence of CO2 on oxidation of cyclohexene
Entry
Gas ratio (Psi)
Conversion (%) in O2/CO2
Conversion (%) in O2/N2
DC (%)a
1
2
3
4
5
0.066
0.142
0.230
0.333
0.454
0
0
9
18
24
24
0
16
25
33
34
28.0
16.3.
15.7
15.7
Reaction conditions: 20 mg MS-MCN, 10 ml DMF, temperature 373 K, pressure 80 psi, time 10 h; Estimated by GC analysis.a Conversion (%) of
cyclic olefin.
remain almost same (Figure 1). There is a marginal decrease
in conversion of each cycle, and this may be due to the physical
loss while handling.
⎛
⎞
⎟
⎟
⎟
⎟
⎛
⎞
⎟
⎟
⎟
⎟
⎜
⎜
C
− C
⎜
⎜
O
O
O
O
⎜
⎜
2
2
⎜
⎜
⎝
CO ⎠
⎝
N ⎠
2
2
(1)
DC(%)=
⎛
⎜
⎞
⎟
⎟
⎟
⎟
⎛
⎜
⎞
⎟
⎟
⎟
⎟
C
+ C
⎜
⎜
⎜
⎜
2
2
⎜
⎜
⎝
CO ⎠
⎝
N ⎠
2
2
where,
O2
O2
CO2
= Conversionin
, CO2 = Conversionin
CO2
N2
CO
N
2
2
The oxidation reactions of various cyclic olefins such as
cyclopentene, cyclohexene, cyclooctene and cyclododecene were
also investigated over MCNs to observe the promotional effect
of CO2 (Table 4). All the catalysts showed higher conversion in
the co-presence of CO2 and positive DC (%) values were gained
for all the substrates, indicating the enhancement of conversions
due to the co-presence of CO2. The selectivities for products 4,
5 and 6 in O2/CO2 (Table 4) were different with those observed
in O2/N2; the epoxide selectivity in O2/CO2 is relatively higher
in all cases compared with O2/N2.
Fig. 1 Recyclability of MCNs in the oxidation of cyclohexene.
The higher conversions were observed in the 5 and 6 mem-
bered ring olefins (Table 4 entries 1–12) compared with the 8
and 12-membered ring olefins (Table 4 entries 13–24). The low
conversion in the case of the large membered rings might be
due to diffusion limitation or hampering inside the pores of the
catalyst reducing the accessibility of active sites.
Role of CO2 and promotional aspects
In order to analyze the influence of carbon dioxide in oxidation
we carried experiments to analyze the gaseous mixture from
the autoclave. The IR spectra of the gaseous mixture after
the reaction depicted the existence of additional bands around
2174 cm-1 and 2115 cm-1 (Figure S7c‡) which may correspond to
a gaseous carbon monoxide doublet which is well documented.39
Whereas no such absorption bands were observed before the
reaction (Figure S7a‡). In order to confirm the formation of
carbon monoxide from CO2 during the reaction, IR spectra of
Recyclability of catalyst
The recyclability of all the catalyst were tested for oxidation
of cyclohexene up to three cycles, and no significant changes
in the catalytic activity were observed and the conversions
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Green Chem., 2011, 13, 1416–1421 | 1419
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Table 4 Oxidation of cyclic olefins and the promotional effect of CO2
Selectivity (%)a
7
Entry
n
1
1
1
2
2
2
4
4
4
8
8
8
Cat.
MS
UF
MG
MS
UF
MG
MS
UF
MG
MS
UF
MG
Gases
Conversion of 6 (%)
8
9
DC (%)
12.6
11.6
11.6
15.5
14.2
17.2
27.0
36.0
26.0
30.0
30.0
28.5
1
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
OC
O
40
31
35
28
24
19
33
24
24
18
17
12
21
12
15
7
12
7
17
9
13
7
9
37
30
38
28
40
32
30
25
38
30
40
31
>99
>99
94
24
22
24
23
27
25
21
16
23
21
27
22
—
—
—
—
—
—
—
—
—
—
—
—
29
40
38
42
32
40
49
53
39
44
33
40
—
—
—
—
—
—
—
—
—
—
—
—
2
3
4
5
6
7
8
98
9
>99
>99
>99
>99
87
78
>99
>99
10
11
12
5
Reaction conditions: 20 mg catalyst, 10 ml DMF, temperature 373 K, pressure 80 psi, gas ratio 0.333, time 10 h; Product analyzed by GC and
GC-MS.a Trace amount of diol was observed.
a synthetic mixture of 5% CO in helium was recorded which
revealed similar a absorption pattern, conforming the exsistence
of CO (Figure S7b‡) in the gaseous mixture after the reaction.
It is presumed that the mode of operation responsible for
the promotional effect is due to activation of carbon dioxide
by formation of carbamate as reported earlier, the carbon
dioxide not only serve as oxygen source but also produce
carbon monoxide which is in consonance with an earlier report
on carbon nitride systems.24 The further confirmation of CO
formation was also investigated similar to Antonietti et al. by
the Pauson–Khand reaction. The formation of Pauson–Khand
product further confirms the formation of CO (Scheme S8‡).
dioxide in the oxidation system revealed a promotional effect
with higher conversions of cyclic olefins. An approach towards
quantitative estimation of the promotional effect has been
made to understand the synergistic effect of carbon dioxide
in the oxidation system. The insight into promotional aspects
revealed that the carbon dioxide is acting as a source of oxygen
which is inferred by formation of carbon monoxide and surface
carbamate.
Acknowledgements
Authors thank for financial support National Research Foun-
dation of Korea (NRF) grant funded by the Korea government
(MEST) (2009-0083073) (No. 2010-0001956).
Conclusions
The mesoporous carbon nitride materials have been successfully
synthesized by a hard templating method. The synthesized
MCNs show catalytic activity towards activation of carbon
dioxide which was evidenced by cycloaddition of CO2 to
propylene oxide and conversion of benzene to phenol. The
MCN materials show catalytic activity towards the oxidation
of cyclic olefins by molecular oxygen. The presence of carbon
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