CO2 adsorption and catalytic application of Co-MOF-74 synthesized by microwave heating

Article abstract of DOI:10.1016/j.cattod.2011.08.019

High-quality Co-MOF-74 crystals were successfully synthesized in 1 h by microwave heating (Co-MOF-74(M)). The XRD pattern and textural properties of Co-MOF-74(M) including the BET surface area (1314 m² g^-1) were virtually identical t

Full text of DOI:10.1016/j.cattod.2011.08.019

Catalysis Today 185 (2012) 35–40
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
CO2 adsorption and catalytic application of Co-MOF-74 synthesized by
microwave heating
Hye-Young Choa, Da-Ae Yanga, Jun Kima, Soon-Yong Jeongb, Wha-Seung Ahna,∗
a Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
b Applied Chemistry and Engineering Division, KRICT, Yuseong 305-600, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2011
Received in revised form 1 August 2011
Accepted 9 August 2011
Available online 30 August 2011
High-quality Co-MOF-74 crystals were successfully synthesized in 1 h by microwave heating (Co-MOF-
74(M)). The XRD pattern and textural properties of Co-MOF-74(M) including the BET surface area
(1314 m2 g−1) were virtually identical to those of a sample synthesized in 24 h by the solvothermal
method (Co-MOF-74(S), 1327 m2 g−1). Average particle size of the former (ca. 50 ␮m long and 8 ␮m
wide) was, however, significantly smaller than that of the latter (ca. 300 ␮m long and 70 ␮m wide).
The H2O adsorption capacities of the crystals at 25 ◦C were 466 and 605 mg g−1 for Co-MOF-74(M) and
Co-MOF-74(S), respectively. The adsorption isotherms of Co-MOF-74(M) for CO2 and N2 showed a high
CO2 adsorption capacity (288 mg g−1) and excellent selectivity over N2 (>25:1) at 25 ◦C. Eight consecu-
tive adsorption–desorption cycles established that there was no deterioration in the adsorption capacity,
which showed reversible adsorbent regeneration at 100 ◦C under He flow for a total duration of 1100 min.
Co-MOF-74(M) also demonstrated excellent catalytic performance in cycloaddition of CO2 to styrene
oxide under relatively mild reaction conditions (2.0 MPa, 100 ◦C) with close to 100% selectivity to car-
bonate confirmed by GC–MS, 1H NMR, and FT-IR. Styrene oxide conversion increased with CO2 pressure
and reaction temperature. No appreciable effect of catalyst particle size was detected, and Co-MOF-74(M)
could be reused 3 times without loss of catalytic activity and with no structural deterioration.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Co-MOF-74
Microwave heating
CO2 adsorption
CO2 cycloaddition
Styrene oxide
1. Introduction
sis of cyclic carbonates via the metal-catalyzed coupling of CO2
and epoxides [10,11], which are valuable as monomers, aprotic
There has been a rapid growth in research activities world-
wide for CO2 capture, storage, and utilization (CSU), spurred by
increasing awareness of the link between CO2 accumulation in the
atmosphere and global warming [1]. The most common method
for CO2 capture is gas absorption, monoethanol amine (MEA) being
the most widely used solvent. However, the amine based sys-
tems for CO2 removal suffer from a high energy requirement for
regeneration as well as corrosion problems [2]. Thus, alternative
CO2 processes for CO2 capture via adsorption on solid media need
consideration. For this purpose, zeolites [3], activated carbons [4],
hydrotalcite-like compounds [5], and metal oxides [6] have been
investigated, and metal organic frameworks (MOFs) are currently
receiving wide attention as an adsorbent material for future appli-
cation [7].
polar solvents, pharmaceutical/fine chemical intermediates, and
for many biomedical applications [12,13]. Various catalysts have
been developed for the cycloaddition of CO2, but the effective cat-
alysts are generally homogeneous and require a co-catalyst. To
overcome the separation problem, several heterogeneous catalysts
have also been suggested [14–19], but most of them suffer the
inherent problem of lower activity [19]. It remains to be seen if
a heterogeneous catalyst that is efficient under mild reaction con-
ditions can be proposed.
Metal-organic frameworks (MOFs) are made of metal ions or
clusters interconnected by organic ligands [20]. They have great
potential as adsorbents/catalysts due to their extremely large
surface area, well-ordered porous structures, and diverse means
available for functionalization. Various applications, including gas
storage, separation, magnetism, and catalysis have been devel-
oped, including CO2 adsorption and heterogeneous catalysts for
fine chemicals [21–23].
CO2 is an attractive C1 building block in organic synthesis [8].
However, due to the inert nature of CO2, efficient catalytic chem-
ical fixation remains a significant synthetic challenge [9]. One of
the most promising methodologies in this area is the synthe-
The crystal structure of the MOF-74 in Scheme 1 is built around
a 1-D honeycomb motif with pores of 1.1–1.2 nm diameter and
helical chains of edge-condensed metal–oxygen coordination octa-
hedrals located at the intersections of the honeycomb, in which
the metal is square–pyramidally coordinated [24]. One of the six
∗
Corresponding author. Tel.: +82 32 860 7466; fax: +82 32 872 0959.
E-mail address: whasahn@inha.ac.kr (W.-S. Ahn).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.08.019

36
H.-Y. Cho et al. / Catalysis Today 185 (2012) 35–40
calculated by the BET method. Morphological features were exam-
ined by SEM (Hitachi S-4200). The thermal stability of the samples
was evaluated using a thermogravimetric analyzer (TGA, SCINCO
thermal gravimeter S-1000); 10 mg of each sample was heated at
10 ◦C min−1 to 600 ◦C under air flow (30 mL min−1).
2.3. Water vapor adsorption measurement
Water vapor adsorption measurement was conducted using a
BELSORP-Max (BEL, JAPAN) at 25 ◦C (P0 = 3.169 kPa). Water vapor
was developed by vaporizing water under ultra-high vacuum at
40 ◦C, which was purified through soaking procedures by repeat-
edly freezing the water using liquid nitrogen and consequently
melting the water and evacuating bubbles of other dissolved
matters. Prior to the water vapor adsorption test, samples were
evacuated through the same pretreatment procedure as for the N2
adsorption.
Scheme 1. The structure of Co-MOF-74 used in this work (carbon atoms, gray; oxy-
gen atoms, red; cobalt atoms, yellow). (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of the article.)
oxygen atoms coordinated to the metal comes from a solvent water
molecule, and the MOF-74 structure develops coordinatively unsat-
urated (open) metal sites in Lewis acidity upon removal of the
solvent molecules attached to the metal sites by heating in vacuum.
Co-MOF-74 is one of the MOF-74 series, which can be prepared by
replacing the typical zinc atoms with cobalt in the MOF-74 struc-
ture [25,26].
In this work, Co-MOF-74 samples were synthesized by the
alternative microwave-eating method, and their physicochemical
properties were compared against those of a sample prepared by
the conventional solvothermal method. The Co-MOF-74 samples
were then examined for their adsorption properties for CO2 and
water vapor. Finally, their potential application as a heterogeneous
catalyst for high-pressure cycloaddition reaction of CO2 to styrene
oxide was investigated for the first time. It will be very meaning-
ful for CSU work if Co-MOF-74 is proven effective both as a carbon
dioxide adsorbent and as a catalyst for its chemical fixation.
2.4. CO2 and N2 adsorption measurement
CO2 and N2 sorption isotherms under static conditions were
obtained by means of a BELsorp(II)-mini (BEL, Japan) at 0 ◦C and
25 ◦C using activated solid samples. The experimental adsorp-
tion data were fitted to the Langmuir–Freundlich equation,
and the heat of adsorption was then calculated by applying
the Clausius–Clapeyron equation. CO2 desorption and adsorp-
tion/desorption cyclic experiments were conducted using a TGA
unit connected to a gas flow panel. Ultra-high purity He was used as
a purge gas in the initial activation and desorption experiments, and
adsorption was carried out using ultra-high purity CO2 (99.999%). A
feed flow rate of 30 mL min−1 to the sample chamber was controlled
using an MFC.
2.5. Cycloaddition of CO2 to styrene oxide
2. Experimental
Styrene oxide (5 mmol) and Co-MOF-74 (20 mg) were mixed in
chlorobenzene (30 mL) and put into a 100 mL stainless steel high-
pressure reactor. Prior to reaction, the catalyst Co-MOF-74 was
dried at 100 ◦C for 1 h under vacuum and then immediately trans-
ferred to the reactor. The temperature of the reactor was increased
and maintained at 100 ◦C, and the reactor was pressurized with CO2
(99.999%) up to 2.0 MPa. The reaction vessel was kept connected to
a CO2 source throughout the reaction via a one-way check valve to
maintain the pressure at the desired level. The reaction mixture was
stirred (200 rpm) under the pressurized conditions for a period of
0.5–4 h. When the reaction was completed, the reactor was quickly
cooled down in cold water, and then pressure was released slowly.
After catalyst separation by centrifugation, conversion, selectivity,
and product yield were determined by GC (Agilent 7890, HP-5 col-
umn; 30 m, 0.320 mm, 0.25 ␮m). The cyclic carbonate was isolated
by removing chlorobenzene solvent at 80 ◦C under vacuum, and the
products were washed with dichloromethane 3 times in an N2 envi-
ronment. The structure of the product was determined by GC–MS
(Varian 1200L with 3800GC), 1H NMR (Varian Inova 400, 400 MHz
using CDCl3 as solvent), and FT-IR (Nicolet iS10 FT-IR spectrome-
ter). The separated catalyst was washed with acetone 3 times and
dried at 100 ◦C for 2 h under vacuum for reuse.
2.1. Microwave synthesis of Co-MOF-74
First,
0.148 mmol
of
cobalt
nitrate
hexahydrate
(Co(NO3)2·6H2O, Sigma–Aldrich) and 0.044 mmol of 2,5-dihydroxy
benzenedicarboxylic acid (H4DHBDC, Sigma–Aldrich) were dis-
solved in 18 mL of a mixed solvent (DMF:ethanol:water = 1:1:1
(v/v/v)) and put into a 35 mL tube, which was sealed with a
rubber septum and placed in a microwave oven (Discover S-class,
CEM, Maximum power of 300 W). The resulting mixture was
heated to 130 ◦C, held for 1 h, and then cooled to room tem-
perature. For comparison, Co-MOF-74 was also synthesized by
the conventional solvothermal method following a procedure
reported in the literature [26]; synthesis details are described
in Supporting information (SI). The red-orange crystals were
separated by centrifugation. After washing three times with
N,Nꢀ-dimethylformamide (DMF, Sigma–Aldrich), they were placed
in methanol, which was decanted and replenished four times for
two days. The solvent was removed under vacuum at 250 ◦C for 5 h,
yielding a dark-purple porous material. The activated materials
were stored in a desiccator under vacuum.
2.2. Characterization
3. Results and discussion
XRD patterns of the Co-MOF-74 samples were obtained on
a Rigaku diffractometer using CuK␣ (ꢀ = 1.54 A) radiation. N2
3.1. Microwave synthesis of Co-MOF-74
˚
adsorption/desorption isotherms were obtained on an ASAP-2020
(Micromeretics, USA) sorptometer at liquid nitrogen temperature.
The samples were degassed at 250 ◦C for 5 h (3 × 10−3 torr) prior
to adsorption. The specific surface areas of the samples were
Fig. 1 compares the XRD pattern (a), N2 adsorption-desorption
isotherm (b), and thermal stability (c) of the Co-MOF-74 sam-
ple synthesized by microwave heating at optimum condition
(hereafter, Co-MOF-74(M)) with those prepared by solvothermal

H.-Y. Cho et al. / Catalysis Today 185 (2012) 35–40
37
Table 1
Textural properties of Co-MOF-74(S) and Co-MOF-74 (M).
Samples
SBET (m2 g−1
)
VPore (cm3 g−1
)
SExt (m2 g−1
)
SMicro (m2 g−1
)
Product yield (%)
Co-MOF-74(S)
Co-MOF-74(M)
1327
1314
0.52
0.51
9
20
1318
1294
68
76
Table 2
Textural properties of Co-MOF-74(M) synthesized at different conditions.
Conditions
SBET (m2 g−1
)
VPore (cm3 g−1
)
Product yield (%)
Temperature (◦C)
Power (W)
Time (min)
100
120
180
180
120
150
180
180
180
180
210
210
210
60
60
60
60
30
40
60
70
30
40
60
1318
1319
1402
1411
1193
1155
1314
1273
123
0.49
0.51
0.71
0.69
0.49
0.47
0.51
0.51
0.01
0.65
0.50
7
45
58
63
69
75
76
74
84
72
74
130
1270
1321
Fig. 1. (A) Powder XRD, (B) N2 sorption isotherms at −196 ◦C, and (C) TGA profiles of Co-MOF-74(S) and Co-MOF-74(M) samples.
Fig. 2. SEM images of (A) Co-MOF-74(S) and (B) Co-MOF-74 (M).

38
H.-Y. Cho et al. / Catalysis Today 185 (2012) 35–40
method (hereafter, Co-MOF-74(S)). The XRD patterns of Co-MOF-
74(M) and Co-MOF-74(S) (Fig. 1a) were very close to each other
and were in good agreement with the pattern reported by Diet-
zel et al. [25]. Both samples exhibited type I isotherms in N2
adsorption isotherms at −196 ◦C with no hysteresis, and the
BET surface area of Co-MOF-74(M) was 1314 m2 g−1 with a pore
volume of 0.51 cm3 g−1, whereas the corresponding values for
Co-MOF-74(S) were 1327 m2 g−1 and 0.52 cm3 g−1, respectively
(Table 1). The XRD patterns of Co-MOF-74(M) samples prepared
under various microwave synthesis conditions (synthesis temper-
ature, microwave irradiation power level, and synthesis time) are
shown in Fig. S1, and the corresponding textural properties of Co-
MOF-74(M) under various synthesis conditions are summarized
in Table 2. According to the results, the optimized Co-MOF-74(M)
was synthesized at 130 ◦C with 180 W microwave power level for
1 h, which resulted in the highest surface area with a reasonably
high product yield (ca. 76% based on ligand). Co-MOF-74(S), on the
other hand, resulted in a somewhat lower crystal yield (68%). Fig. 1c
shows TGA profiles of Co-MOF-74(S) and Co-MOF-74(M). The first
weight loss up to 150 ◦C can be attributed to the solvent used for
synthesis (H2O and ethanol) and washing (methanol) removal from
the cavities, whereas the second weight loss around 200 ◦C is due
to the removal of guest molecules from open metal sites on Co-
MOF-74. The TGA pattern comprising two-step weight reduction
is a unique feature of porous MOF materials having open metal
sites [25]. Co-MOF-74(S) showed the final weight loss in a temper-
ature range from 280 ◦C to 300 ◦C due to structural collapse, and
the final collapse for Co-MOF-74(M) occurred at a slightly higher
temperature.
Fig. 2 shows SEM images of Co-MOF-74(S) and Co-MOF-74(M).
Both samples were made of single-phase hexagonal column-
structured particles with long aspect ratios but Co-MOF-74(S) (ca.
300 ␮m long and 70 ␮m wide) was significantly larger than Co-
MOF-74(M) (ca. 50 ␮m long and 8 ␮m wide). This difference in
particle size also resulted in a larger external surface area for
Co-MOF-74(M) (Table 1). Microwave synthesis has been widely
applied in inorganic materials synthesis due to the benefits of rapid
synthesis kinetics, uniform particle morphology, and phase purity
[27]. An applied oscillating electric field in the microwave approach
is coupled with the permanent dipole moment of the molecules in
the synthesis medium, inducing molecular rotations – resulting in
rapid heating of the liquid phase [28]. In microwave synthesis, the
average particle size of the product is typically smaller than that
of solvothermal synthesis because the short reaction time limits
particle growth, while nucleation is not hindered.
Fig. 3. Water vapor adsorption–desorption isotherms at 25 ◦C.
reversibly through a single step. This presumably reflects the fact
that Co-MOF-74 is composed of only one type of pore in uniform
1-D hexagonal channels, and the pores did not cause hysteresis
toward water vapor. The isotherms quickly rising at the low pres-
sure region (P/P0 < 0.1) indicate the hydrophilic character of open
metal sites (Lewis acidic sites) in Co-MOF-74, and well lined-up
oxygen sites between organic ligands and metal complex enhance
binding water molecules via hydrogen bonding. Water adsorp-
tion subsequently becomes saturated as the P/P0 increases, which
reflects the hydrophobic charater of Co-MOF-74 due to benzene
rings that comprise the micropores [29]. Zeolite 13X and activated
carbons are well known for their water vapor adsorption properties
[30,31], and the water adsorption properties of commercial zeolite
13X (Sigma–Aldrich) and activated carbon (G-80, Darco) were also
plotted for comparison. Co-MOF-74 clearly demonstrated a higher
affinity toward water vapor than zeolite 13X and activated car-
bon, largely due to the regularly arranged oxygens connected to
benzene rings and the open metal sites showing strong Lewis acid-
ity. Interestingly, the water adsorption capacity of Co-MOF-74(S),
605 mg g−1, was significantly higher than that of Co-MOF-74(M),
466 mg g−1. While it was reported previously that even same mate-
rials could possess different surface characteristics if they are
prepared via different routes [32,33], it is also plausible that the
particle size differences of the two samples had also an influ-
ence over the water sorption behaviour; longer 1-D channels in
Co-MOF-74(S) could accommodate extra water molecules via inter-
molecular attraction. Stability of MOF-74 in steam was reported
to be superior to those of other metal organic framework (MOF)
structures [34].
3.2. Water vapor adsorption measurement
Water adsorption isotherms of Co-MOF-74(S) and Co-MOF-
74(M) are shown in Fig. 3. Both of them adsorbed water vapor
Fig. 4. (A) CO2 and N2 sorption isotherms of Co-MOF-74(M) at 0 ◦C and 25 ◦C and (B) the corresponding heat of adsorption.

H.-Y. Cho et al. / Catalysis Today 185 (2012) 35–40
39
Table 3
Effect of reaction conditions in catalytic CO2 coupling reaction of styrene oxide.
Catalyst
Pressure (MPa)
Temperature (◦C)
Time (h)
Conversion (%)
Co-MOF-74(M)
Co-MOF-74(M)
Co-MOF-74(M)
Co-MOF-74(M) 1st recycle
Co-MOF-74(M) 2nd recycle
Co-MOF-74(M)
2.0
2.0
2.0
2.0
2.0
1.0
2.0
60
80
4
4
4
4
4
4
4
38
78
96
95
95
49
0
100
100
100
100
100
Without catalyst
Reaction conditions: chlorobenzene 30 mL, styrene oxide 5 mmol, and catalyst 20 mg.
Table 4
Comparison with other catalytic systems on cycloaddition reaction of CO2 to styrene oxide.
Catalyst
co-Catalyst
Ratioa
Pressure (MPa)
Temperature (◦C)
Time (h)
Yield (%)
Ref.
Co-complexb
MOF-5
Zn3[Co(CN)6]2
Ion-exchanged resin
n-Bu4NBr supported SiO2
Mg–Al oxidef
DMAPc
1:1(:2)
24:1(:3)
60:1(:2)
12:1
60:1
1:1
2.0
0.1
0.4
8.0
8.0
0.5
2.0
120
50
3
15
6
12
8
85.8
92.0
97.0
95.2
97.0
90.0
96.0
[15]
[19]
[14]
[16]
[17]
[18]
Present work
n-Bu4NBrd
n-Bu4NCle
140
100
150
120
100
–
–
–
–
15
4
Co-MOF-74
30:1
a
Ratio = substrate:catalyst (:co-Catalyst).
b
c
d
e
f
Co-complex, cobalt-tetraamidomacrocyclic complex.
DMAP, 4-dimethylamonopyridine.
n-Bu4NBr, tetrabutylammonium bromide.
n-Bu4NCl, tetrabutylammonium chloride.
DMF was used as solvent.
Fig. 5. CO2 adsorption–desorption recycling test of Co-MOF-74(M) using high-
purity CO2.
Fig. 6. Effect of reaction time on the catalytic activity of Co-MOF-74(S) and Co-MOF-
3.3. CO2 and N2 adsorption measurement
74(M) in cycloaddition of CO2 to styrene oxide at 100 ◦C and 2.0 MPa.
Adsorption and desorption equilibrium isotherms of CO2 and
N2 on Co-MOF-74(M) at 0 ◦C and 25 ◦C are plotted in Fig. 4a. Virtu-
ally identical isotherms obtained for Co-MOF-74(S) are not shown.
The steepness of the slope in the low-pressure region of the CO2
adsorption isotherm indicates strong adsorption of CO2 onto Co-
MOF-74(M), while that of the N2 isotherms is quite linear and
exhibits low uptake, signaling a weak interaction between N2 and
the adsorbent. Up to 288 mg g−1 of CO2 uptake was obtained at
25 ◦C and 1 bar, which is significantly higher than the adsorption
capacities of other inorganic adsorbents including the amine-
functionalized ones [35]. The N2 uptake at 25 ◦C, on the other hand,
was only 1.7 mg g−1 at 1 bar. As a result, the selectivity of CO2 over
sites become saturated, and the adsorbate–adsorbent interaction
becomes governed mainly by dispersion [36,37].
Stability of the adsorbent is another important parameter
to be considered. Fig. 5 shows the CO2 adsorption–desorption
recycling performance of Co-MOF-74(M) in eight consecu-
tive adsorption–desorption runs; close CO2 adsorption capacity
(210 mg g−1) to the value measured by the static equilibrium
adsorption isotherm was measured. The adsorbed CO2 at 25 ◦C
could be completely desorbed at 100 ◦C; 92%, 95%, and 97% of CO2
was found to be desorbed at 25 ◦C, 50 ◦C, and 75 ◦C, respectively. No
deterioration in adsorption capacity was detected, and excellent
regeneration and adsorption–desorption stability were observed
for a total duration of 1100 min.
N2 in Co-MOF-74(M) was very high (CO2:N2 = 25:1); CO2 was more
strongly adsorbed than N2 because of the significantly stronger
quadruple moment of CO2 in comparison to N2.
Fig. 4b shows the corresponding heats of CO2 adsorption over
coverage, which decreases sharply with the loading amount (from
ca. 51 to 29.5 kJ mol−1); at the low surface coverage, CO2 molecules
are preferably adsorbed onto the most energetic adsorption sites
of open metal Lewis acid character. As loading increases, these
3.4. Cycloaddition of CO2 to styrene oxide
The catalytic property of Co-MOF-74 was tested for the cycload-
dition reaction of CO2 to styrene oxide. As shown in Fig. 6, styrene
oxide conversion steadily increased from 41% to 96% as the reaction

40
H.-Y. Cho et al. / Catalysis Today 185 (2012) 35–40
time elapsed from 0.5 to 4 h at 100 ◦C and 2.0 MPa CO2 pressure
condition over Co-MOF-74 in chlorobenzene. A single product of
4-phenyl-1,3-dioxolan-2-one was obtained, as confirmed by FT-
IR and 1H NMR in Figs. S2 and S3, respectively [14]. It was also
established that both Co-MOF-74(M) and Co-MOF-74(S), despite
significantly different particle sizes, produced very similar con-
version profiles, implying that negligible pore diffusion affects the
reaction rates under the given set of reaction conditions.
As shown in Table 3, styrene oxide conversion increased from
38% to 96% as the temperature increased from 60 ◦C to 100 ◦C, and
the conversion decreased from 96% to 49% as the CO2 pressure was
reduced from 2.0 MPa to 1.0 MPa. Apparently, the concentration of
CO2 dissolved in chlorobenzene increased at higher pressure, which
in turn increased the reaction rates in the liquid phase. To assess
the stability of Co-MOF-74(M) during the liquid phase reaction, a
catalyst recycling test was conducted at 100 ◦C. At the end of reac-
tion, the catalyst was recovered by filtration, washed with acetone,
activated at 100 ◦C under vacuum for 2 h to remove the adsorbed
species, and then reused. As shown in Table 3, styrene oxide con-
versions for 3 repeated runs produced virtually constant values.
Powder XRD patterns of the fresh catalyst and those after the sec-
ond and third runs were also measured (Fig. S4). These showed
almost identical diffraction patterns with the same intensity and
indicated that the structural integrity of Co-MOF-74 was main-
tained during the reaction. The reaction did not occur without a
catalyst.
Previous reports on the synthesis of cyclic carbonate from CO2
and epoxide suggested that concurrent presence of both Lewis base
sites and Lewis acid sites are desirable for the reaction [10]; the
former activate CO2, which then attacks styrene oxide adsorbed on
the latter. We believe that the six oxygen atoms (five oxygen atoms
from the ligand and one from H2O used as solvent) located around
cobalt atoms function as a base, whereas open metal cobalt is well
known to function as a Lewis acid. We compared our results with
the activity of several other catalyst systems reported in the liter-
ature for the same reaction in Table 4. We found that Co-MOF-74
is indeed very promising as a catalyst for cycloaddition reaction in
two respects; a co-catalyst, such as quaternary ammonium salt is
not needed, and the reaction proceeds under relatively mild reac-
tion conditions of lower pressure and temperature than others.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cattod.2011.08.019.
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Acknowledgements
This work was supported by the Carbon Dioxide Reduction &
Sequestration R&D Center (CDRS) in Korea and by the KRICT OASIS
Project from Korea Research Institute of Chemical Technology.