Pillared cobalt-amino acid framework catalysis for styrene carbonate synthesis from CO2 and epoxide by metal-sulfonate-halide synergism

Article abstract of DOI:10.1002/cctc.201300756

The sulfonate anion is proposed as a remarkable partaker in catalyzing epoxide-CO₂ cycloaddition for cyclic carbonate synthesis. The role is illustrated by the concerted action of a sulfonate-rich cobalt-amino acid framework catalyst [{Co(4,4′-bipy)(L-cys)(H₂O)}×H ₂O]_n (2 D-CCB) and a quaternary ammonium bromide co-catalyst in synthesizing styrene carbonate (SC) at a turnover number of 228. SC yield at atmospheric pressure is presumed to result from the activation of CO₂ by the sulfonate group. The involvement of SO₃ ^- anions as basic sites in 2 D-CCB is ascertained from the initial rate (r₀) for catalyzing Knoevenagel condensation reactions and by using CO₂ temperature programmed desorption. Microwave pulses are used for synthesizing 2 D-CCB at a rate that is 288-fold faster than conventionally employed solvothermal methods. Unambiguous evidence for the pulsating role-play of sulfonate groups in 2 D-CCB is perceived by comparing the activity of an analogous metal organic framework (3 D-CCB) in which the sulfonate oxyanions are jammed by coordination with cobalt. 2 D-CCB is analyzed for heterogeneity, and reused four times. Copyright

Full text of DOI:10.1002/cctc.201300756

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DOI: 10.1002/cctc.201300756
Pillared Cobalt–Amino Acid Framework Catalysis for
Styrene Carbonate Synthesis from CO and Epoxide by
2
Metal–Sulfonate–Halide Synergism
Amal Cherian Kathalikkattil, Roshith Roshan, Jose Tharun, Han-Geul Soek, Hyeong-Seok Ryu,
[a]
and Dae-Won Park*
The sulfonate anion is proposed as a remarkable partaker in
venagel condensation reactions and by using CO temperature
2
catalyzing epoxide–CO cycloaddition for cyclic carbonate syn-
programmed desorption. Microwave pulses are used for syn-
thesizing 2D-CCB at a rate that is 288-fold faster than conven-
tionally employed solvothermal methods. Unambiguous evi-
dence for the pulsating role-play of sulfonate groups in 2D-
CCB is perceived by comparing the activity of an analogous
metal organic framework (3D-CCB) in which the sulfonate oxy-
anions are jammed by coordination with cobalt. 2D-CCB is an-
alyzed for heterogeneity, and reused four times.
2
thesis. The role is illustrated by the concerted action of a sulfo-
nate-rich cobalt–amino acid framework catalyst [{Co(4,4’-
bipy)(l-cys)(H O)}·H O] (2D-CCB) and a quaternary ammonium
2
2
n
bromide co-catalyst in synthesizing styrene carbonate (SC) at
a turnover number of 228. SC yield at atmospheric pressure is
presumed to result from the activation of CO by the sulfonate
2
À
group. The involvement of SO3 anions as basic sites in 2D-
CCB is ascertained from the initial rate (r ) for catalyzing Knoe-
0
Introduction
[
3]
Increasing concerns over global warming have prompted re-
searchers to develop strategies that could minimize the emis-
sion of carbon dioxide from industries and power plants as
a byproduct. The quests for green technologies that utilize this
greenhouse gas as a C1 feedstock for chemical manufacture
would apparently triumph over the sequestration routes that
inflict sewage liabilities. Recent years have witnessed the emer-
gence of versatile catalysts that promote the transformation of
at appreciable yields. Nevertheless, the harsh conditions are
circumvented to some extent by immobilizing the ionic liquids
or similarly active homogeneous species over heterogeneous
materials such as silica, polymers, or biopolymers. More recent-
ly, metal–organic frameworks (MOFs)—an emerging class of
porous, crystalline materials—are explored as materials that
[4]
are capable of meeting specific needs; this includes catalysis,
by a precise control of the coordination geometries and by ju-
diciously choosing organic spacers in their construction. Thus,
their customizability with a meticulous modulation of the con-
stituent organic struts and/or metal-containing clusters con-
tributes to the overwhelming interest in engaging MOFs or co-
ordination polymers (CPs) as exquisite materials for versatile
CO to products such as dimethyl carbonate, N,N’-disubstituted
2
ureas, cyclic carbonates, cyclic urethanes, formic acid, and so
[
1]
forth. Amongst such processes, the cycloaddition of epoxides
and CO to produce cyclic carbonates is an atomically-econom-
2
ical reaction. Cyclic carbonates find applications as solvents,
electrolytes in lithium-ion batteries, and as intermediates in
the synthesis of ethylene glycol, acyclic carbonates, pharma-
[5]
applications.
A few preeminent transition-metal-based porous di/tri-car-
boxylate bridged MOFs, zeolitic imidazolate MOFs (ZIFs) and so
[
2]
ceuticals, and polymers.
Catalysts such as metal oxides, metal complexes, organome-
tallic compounds, and ionic liquids are engaged as successful
forth, with high CO -adsorption capacities have been recently
2
employed as catalysts for epoxide–CO2 cycloadditions; this
relies on the efficacy of various inherent functional groups
such as hydroxyl or amine groups, or by post-synthetic modifi-
candidates in the cycloaddition of CO and epoxides. Whereas
2
most of them often fall short in fulfilling a viable post-catalytic
recovery method; many easily recyclable ones often require
more intense reaction conditions in achieving cyclic carbonates
[
1h, 6]
cation (PSM) techniques.
In our attempt to extend the
study towards the influence of various functional groups of
MOFs in catalysis, we noticed that the role of sulfonate anions
[7]
remained scarcely explored. However, a few sulfonic acids
have been explored as Lewis acid sites in several other catalyt-
[a] A. C. Kathalikkattil, R. Roshan, Dr. J. Tharun, H.-G. Soek, H.-S. Ryu,
Prof. D.-W. Park
School of Chemical and Biomolecular Engineering
Pusan National University
Busan 609-735 (South Korea)
Fax: (+82)51-512-8563
[8]
ic processes. To the best of our knowledge, the role of the
sulfonate anion has seldom been studied or employed in any
cycloaddition reactions, which involve epoxides and CO , de-
2
spite its efficiency as a leaving group. Other than ring-opening
anions, the activation of epoxides toward ring opening is im-
portant for an efficient cycloaddition process. The literature
E-mail: dwpark@pusan.ac.kr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300756.
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suggests that a number of cobalt complexes have efficiently
performed this function in the production of cyclic carbonates
crowave-mediated methods as a viable pathway for reducing
the duration of catalyst syntheses and increasing catalytic effi-
ciencies encouraged us to examine microwave energy irradia-
[
9]
and polycarbonates from epoxides and CO2. This prompted
us to combine both of these aspects for establishing an effi-
cient catalytic system. Owing to the practicality in achieving re-
active functional groups by a prudential alteration of its build-
ing blocks, a MOF class of material is selected as an appropri-
[11]
tion for 2D-CCB synthesis. Previous reports on the micro-
wave-assisted synthesis of MOFs by using high-boiling solvents
such as N,N’-dimethylformamide (DMF) were performed at
a controlled temperature below the boiling point. As a result,
it takes a prolonged synthesis time, which extends even up to
ate candidate for encompassing both cobalt and reactive sulfo-
À
[6k]
nate (SO ) functional groups as synergistic participants for
an hour in some instances. Unlike MOFs that are synthesized
3
catalyzing cyclic carbonate synthesis. The a-amino acid l-cyste-
ic acid in its deprotonated state is identified as a suitable
spacer with a sulfonate source. A 2D cobalt–cysteate coordina-
in high-boiling solvents, the comparatively low boiling point of
a water–methanol (1:1) mixture opens up the possibility of in-
trinsic temperature control wherein synthesis of 2D-CCB can
be facilitated at a constant microwave power. Gratifyingly,
a short time span of 10 min (at 100 W power) is enough for
the synthesis of 2D-CCB(M), in the microwave (M=micro-
wave).
[
10]
tion polymer (2D-CCB) (Figure 1) is employed as the catalyst
Structural aspects and nature of the sulfonate anions
2
D-CCB [{Co(4,4’-bipy)(l-cys)(H O)}·H O] exists as a 2D coordi-
2 2 n
nation polymer with a brick-wall topology. 2D-CCB contains
À
SO3 groups that are oriented freely between adjacent rectan-
gular grids, which are stacked by H-bonding interactions along
the third dimension, forming the network (Figure 1). To study
the variations in catalytic activity that occur as a result of the
Figure 1. Mercury diagram depicting parallel 2D rectangular grids in 2D-
CCB (metal center and reactive functional groups shown in the ball-and-
stick model).
nature of the SO groups—from anionic species (reactive func-
3
tional groups) to a frozen state (coordinative functional
groups), we chose a 3D framework, which was reported earlier
[12]
by Huang et al.
[{Co (4,4’-bpy) (l-cys) (H O) }·3H O] (3D-
2 2 2 2 2 2 n
À
2
4
for demonstrating the role of SO3 groups in the cycloaddition
CCB) is a 3D MOF with (4 .8 )-intermediate value theorem top-
ology, derived from the same constituent moieties of 2D-CCB
namely, l-cysteic acid, the 4,4’-bipyridyl group, and cobalt. De-
spite their similarities and that the cobalt centers have distort-
ed octahedral geometries; 3D-CCB has the oxyanion of the sul-
fonate group coordinated to cobalt (Scheme 1). Variations in
their structures can be ascribed to the differences in synthesis
time, pH, temperature, and nature of the counter anions (of
the metal salt).
of styrene oxide (SO) and CO in the presence of a tetra-n-bu-
2
tylammonium bromide (TBAB) co-catalyst. Microwave power is
investigated as an energy-efficient tool for reducing the dura-
tion required for the synthesis of 2D-CCB to a more practical
timeframe. A clearer perception about the contribution of the
sulfonate moiety of 2D-CCB in affecting the cycloaddition of
SO and CO is sought by comparing its catalytic activity with
2
an analogous MOF (3D-CCB) with a varied coordination state
from the same building blocks that lack any sulfonate
oxyanions.
Characterization
To confirm the crystallinity, structural integrity, and bulk homo-
geneity of the microwave-synthesized catalyst 2D-CCB(M), its
Results and Discussion
Microwave-assisted synthesis of the catalyst
In synthesizing 2D-CCB by a solvothermal route, the need for
considerable amounts of energy along with prolonged reac-
tion durations: more than two days at the expense of an ele-
vated temperature of 1408C, stands as a major obstruction.
However, a direct-mixing method or slow evaporation as alter-
native techniques may generally result in a significant fall in
phase purity of 2D-CCB; this particularly owes to the insoluble
nature of the basic cobalt carbonate precursor in the synthesis
medium. The use of microwave energy addresses the above
concerns by promoting efficient molecular collisions with sig-
nificant reductions in time and energy by potentially offering
a more rapid synthetic alternative. Our previous reports on mi-
Scheme 1. A representation of variations in the binding of l-cysteic acid
with cobalt in 2D-CCB (left) and 3D-CCB (right). Sulfonate exists as an anion
in the former, whereas it exists in a coordinated state to cobalt in the latter.
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Figure 4. TGA analyses of 2D-CCB(S) and 2D-CCB(M).
Figure 2. XRD patterns of 2D-CCB(S) and 2D-CCB(M) in comparison to the
single crystal simulated pattern.
ples, which further corroborates the similarities in the thermal
stabilities of 2D-CCB(S) and 2D-CCB(M).
X-ray diffraction pattern (PXRD) is compared with that of the
solvothermally prepared catalyst 2D-CCB(S), in which S=solvo-
thermal, and its simulated X-ray diffraction pattern from the
Physical examinations of the catalysts’ textural features are
conducted by using field emission scanning electron microsco-
py (FESEM). As shown in Figure 5, the catalyst prepared in
10 min by the microwave-assisted method appears as an ag-
gregation, in comparison to the large single crystals obtained
by the solvothermal method (Figure 5a–d). However, further
magnification reveals that 2D-CCB(M) is comprised of tiny crys-
tals (Figure S1, Supporting Information).
[
10]
single crystal data analysis (Figure 2). The similarities in their
patterns, particularly, the absence of any additional peaks with
respect to the simulated pattern confirm that the bulk of 2D-
CCB(S) and 2D-CCB(M) samples purely belong to the 2D-CCB
phase.
Similarly, the FTIR bands of 2D-CCB(S) and 2D-CCB(M) estab-
lish the solvothermal and microwave catalysts as chemically
identical (Figure 3). The FTIR peaks associated with the sulfo-
À1
[10,13]
nate group are observed in the 1200–1100 cm region,
which corresponds to the SÀO stretching frequencies of the
sulfonate group.
Figure 5. FESEM images of the solvothermally synthesized (top) and micro-
wave synthesized (bottom) 2D-CCB samples. Images in the left side are of
the order 100 mm, whereas those in the right side are their respective mag-
nifications in 20 mm scale.
Figure 3. FTIR spectra of 2D-CCB(S) and 2D-CCB(M).
Thermogravimetric analyses (TGA) are conducted after com-
pletely drying the samples in vacuo; therefore, their degrada-
tion curves do not reflect the loss of lattice water molecules
2D-CCB is a multifunctional catalyst that consists of a cobalt
metal center, a sulfonate anion, and several lone pairs of elec-
trons, which belong to O- and N-atoms of the framework. CO2-
(Figure 4). Hence, the first major weight loss is observed within
and NH -temperature programmed desorption (TPD) is per-
3
180–2308C for both samples, which corresponds to the loss of
formed with 2D-CCB(M) for examining the acid–base charac-
coordinated water molecules. 2D-CCB begins to undergo
teristics and binding tendencies of CO by basic sites (includ-
2
framework degradation at approximately 3008C for both sam-
ing sulfonate sites). Figure S2 (Supporting Information) depicts
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the CO -TPD profile and Figure S3 the NH -TPD pro-
2
3
file. Table S1 (Supporting Information) accounts the
weak/medium/strong acid–base sites of 2D-CCB in
À1
[6j]
mmolg . Kim et al. have observed by using TPD
that lone pair electrons in metal coordinated O- and
N-atoms of MOFs can act as basic sites. MOFs such as
Cu–BTC
(BTC=benzene-1,3,5-tricarboxylic
acid),
MOF-5, UIO-66, Mg-MOF-74, MIL-101 and so forth,
À1
possess basic sites that range from 0–36.4 mmolg
in concentration (CO -TPD). Though a direct compari-
2
son is not possible, we follow a similar pretreatment
and program of CO -TPD for the 2D-CCB(M) catalyst
2
À1
and observe 85.1 mmolg
of basic sites
À1
À
(
21.2 mmolg weak, and 63.9 mmolg strong basic
sites), which implies there are a higher number of
basic sites that can also be attributed to sulfonate
Figure 6. Comparison of acid–base sites by the initial rate (r
venagel condensation (base-catalyzed) reaction and acetalization (acid-catalyzed) reac-
value
for the Knoevenagel condensation, which is attributed to the basicity contributed by the
reactive sulfonate groups in 2D-CCB.
0
) estimation from the Knoe-
anions. NH -TPD shows that 2D-CCB(M) possesses tion by using 3D-CCB and 2D-CCB(S) as catalysts. 2D-CCB(S) has an exceptional r
0
3
À1
a total acid site concentration of 133 mmolg .
For a detailed understanding with experimental
evidence on the acid and base sites available in the
materials, we engaged the catalyst for acetalization
(
acid site determination) and Knoevenagel condensation reac-
Table 1. Influence of various catalysts in the cycloaddition of SO and
CO .
2
[
a]
tions (base site determination). The initial reaction rates (r ) are
0
estimated for the Knoevenagel condensation (base-site cata-
lyzed) reaction between malononitrile and benzaldehyde, and
the acid-site catalyzed acetalization between benzaldehyde
and ethanol (Figure 6). Reactions are performed by using 2D-
CCB and 3D-CCB on a comparative manner to unveil the role
of the sulfonate groups. The acetalization reactions with 3D-
CCB(S) and 2D-CCB(S) proceed at approximately the same
[b]
Entry
Catalyst
none
Conversion of SO
[%]
Selectivity to SC
[%]
[
c]
[c]
1
2
3
4
5
6
7
0
0
2D-CCB(S)
18.5
39.2
89.5
91.1
10.9
65.9
65.2
99.9
99.9
99.9
55.2
96.5
TBAB
2D-CCB(S)/TBAB
2D-CCB(M)/TBAB
3D-CCB
3
3
À1 À1
rates, with r values at 50.2ꢁ10 and 44.9ꢁ10 molmin g ,
3D-CCB/TBAB
0
respectively.
[
a] Reaction conditions: Styrene oxide=25 mmol (2.85 mL at 258C),
Thus, the acidic sites present in both catalysts can be as-
sumed to be similar in number, which correspond to an ap-
proximately equivalent number of cobalt centers. Similarly, the
0
.1 MPa PCO2, 12 h, 1008C 600 rpm. [b] Catalyst mol%=0.4 [c] Determined
by GC.
rate determination of the 3D-CCB catalyst yields an r value of
0
3
À1 À1
4
9.8ꢁ10 molmin
g
for the Knoevenagel condensation,
version and over 99.9% selectivity (entries 2 and 3). Interest-
ingly, a 1+1>2 sort of outcome (89.5% conversion, 99.9% se-
lectivity) is achieved with a 2D-CCB(S)/TBAB system, which in-
dicates a synergistic catalytic operation (entry 4). On the other
hand, the microwave-synthesized counterpart 2D-CCB(M)/
TBAB shows 91.1% conversion with 99.9% SC selectivity
(entry 5), which accentuates the competency of microwave
energy not merely as a rapid catalyst synthesis technique, but
also as an efficient route that produces catalysts that maintain
the qualities of conventional catalysts.
which reflects the number of basic sites. Even though anionic
species are absent in 3D-CCB, it is likely that heteroatoms con-
taining lone pairs of electrons may contribute to the observed
[6k]
reactivity; similar to an observation by Cho et al.
for
a cobalt-MOF surrounded with oxygen atoms. Surprisingly, for
2
1
D-CCB, a threefold increase in the initial rate (169ꢁ
3
À1 À1
0 molmin g ) in comparison with 3D-CCB is observed for
the Knoevenagel condensation. This enhanced reaction rate,
which is attributable to sulfonate oxyanions, demonstrates that
the reactive sulfonate functional groups are capable of ena-
bling high catalytic activities in base-catalyzed reactions.
Based on these observations, a rationalized mechanism for
the cycloaddition of SO and CO2 that involves the cobalt
center and sulfonate groups of 2D-CCB in association with the
bromide anions of TBAB is proposed herein (Scheme 2). The
cycloaddition reaction commences with epoxide ring activa-
tion by the cobalt center of 2D-CCB, as in the case of a typical
metal-center-catalyzed cycloaddition, whereby the epoxide
Cycloaddition of SO and CO2
The cycloaddition reaction of styrene oxide (SO) and CO is
2
performed at 0.1 MPa and 1008C for 12 h with 0.4 mol% of
catalyst (Table 1). In the absence of a catalyst, no styrene car-
bonate (SC) is yielded (entry 1). Cycloaddition with 2D-CCB(S)
alone gives SO conversion of 18.5% with 65.2% selectivity for
SC, whereas cycloaddition with TBAB alone shows 39.0% con-
[9]
ring is rendered susceptible to ring opening. Subsequent ep-
oxide ring opening is achieved by the attack of the bromide
anion of TBAB on the least hindered b-carbon atom of the ep-
[11]
oxide ring. Previous reports of epoxide-CO cycloaddition re-
2
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2
Scheme 3. Cycloaddition of styrene oxide and CO catalyzed by 3D-CCB/
Scheme 2. Proposed mechanism for the cycloaddition of styrene oxide and
TBAB, which proceeds in the absence of sulfonate anions.
2
CO catalyzed by 2D-CCB/TBAB.
actions suggest that a contemporaneous presence of acid and
for carbonate complex formation analogous to intermediate-
b (Scheme 2) does not prevail in this mechanism. Instead, in-
termediate-a’ (Scheme 3) attempts a cycloaddition reaction di-
[
6j]
base sites are desirable for a successful catalysis. However,
D-CCB facilitates the cycloaddition reaction in a more syner-
2
gistic manner than two-component acid–base catalysis. The
oxyanion of the sulfonate moiety of 2D-CCB attacks the
rectly with the CO molecule to form intermediate-b’. There-
2
fore, this step may proceed at a much slower rate than the ad-
dition to the carbonate complex step that is mentioned in 2D-
CCB. This phenomenon is evidenced by the reduced SO con-
versions with 3D-CCB than 2D-CCB (Table 1, entries 4 and 7).
The reduced SO conversion with 3D-CCB could serve as the
experimental evidence for the involvement of sulfonate oxyan-
ions of 2D-CCB in providing efficient SO conversions because
the final ring-closure step is similar to that of the 2D-CCB cata-
lyzed reaction.
carbon atom of CO , which is considered as one of the most
2
oxidized states of carbon that induces its inertness. This attack
results in the wreckage of its inertness, thereby forming a car-
bonate complex (intermediate-a); fortifying the cycloaddition
reaction. In the subsequent step, the oxyanion of the ring-
opened intermediate (intermediate-b) attacks the carbon of
the carbonate complex, thus detaching the SO ÀCO linkage
2
3
À
back to the SO3 anion; wherein a third intermediate (inter-
mediate-c) is formed. This step is also favored because of the
The effects of varying catalyst concentration are examined
for different ratios of 2D-CCB(M)/TBAB in the range 0.25–
0.1 mmol of catalyst/co-catalyst with 25 mmol of SC (0.1–
À
better leaving ability of the SO group compared to nucleo-
3
À
À [14]
philes such as Br or Cl . Finally, ring closure takes place
À
with the elimination of a Br ion to generate SC, thus liberat-
0.4 mol% catalysts) at 1008C and a CO pressure of 0.1 MPa for
2
ing TBAB and regenerating 2D-CCB, thenceforth moving to
the next cycle of cycloaddition.
12 h (Table 2). A steady increment in the SO yield is observed
from 0.1 to 0.4 mol% for 2D-CCB(M) with a constant co-cata-
To gather further experimental evidence for the synergistic
role of sulfonate groups involved in 2D-CCB, the analogous
catalyst 3D-CCB with no reactive sulfonate group is employed
as the catalyst under similar conditions. With the total conver-
sion of SO by using 3D-CCB/TBAB (Table 1, entry 7) slightly ex-
ceeding the summation of its individual active centers (en-
tries 3 and 6), a cooperative mechanism may be inferred to
exist in this reaction as well. However, the 65.9% SO conver-
sion is far less prominent against the conversion of 91.9% SO
with 2D-CCB/TBAB under similar conditions, which sheds some
light on the role that is performed by the sulfonate group. A
plausible mechanism is illustrated in Scheme 3. Activation of
the epoxide ring of SO followed by epoxide ring opening by
the bromide anion takes place in a manner similar to the afore-
mentioned mechanism of 2D-CCB/TBAB; however, the chance
Table 2. Effect of 2D-CCB(M)/TBAB ratio in the yield of styrene carbona-
te.
[a]
Entry
2D-CCB(M)
mol%]
TBAB
[mol%]
Ratio of
2D-CCB(M)/TBAB
Yield of SC
[%]
[
1
2
3
4
5
6
7
0.1
0.2
0.3
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.3
0.2
0.1
1:4
1:2
3:4
1:1
4:3
2:1
4:1
21.5
48.7
79.4
91.1
82.3
51.9
24.7
[
a] Reaction conditions: Styrene oxide=25 mmol (2.85 mL at 258C),
0.1 MPa PCO2, 12 h, 1008C.
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lyst concentration of 0.4 mol% TBAB; for which the maximum
conversion is observed at 0.4 mol% 2D-CCB (entries 1–4). Simi-
larly, studies that use 0.1–0.4 mol% TBAB with 0.4 mol% of
2
D-CCB(M) follow a similar trend; they show maximum conver-
sion at 0.4 mol% TBAB (entries 4–8). Thus, 0.4 mol% of 2D-
CCB(M) and TBAB (entry 4) are identified as the optimal cata-
lyst ratios under the employed conditions (91.1% conversion
and 99.9% selectivity).
The effect of reaction duration on the 2D-CCB(M)-catalyzed
cycloaddition of SO and CO is depicted in Figure 7. SO conver-
2
sion increases with increasing reaction duration, in the range
of 3 to 12 h, in which the highest conversion is obtained at
12 h. Prolonged reactions beyond 12 h do not give an appreci-
able increment in SC yields, which establishes the optimum re-
action time as 12 h, whereby 91.9% SO conversion and 99.9%
SC selectivity are obtained.
2 2
Figure 9. Effect of CO pressure on the reactivity of SO and CO by using
2D-CCB(M) in (a) 12 h and (b) 4 h, respectively at 1008C and 0.4 mol% of
catalyst.
91.9% SC yield is attained with an atmospheric pressure of
CO . This may be attributed to the easy formation of carbonate
2
complex (intermediate-b) between the sulfonate group of 2D-
CCB and CO , thereby leading to facile cycloaddition. Further
2
increase in CO pressure up to 2 MPa apparently favors an in-
2
creased SC formation, but the rise is insignificant. The effect of
CO pressure on the SC yield is also studied under shorter reac-
2
tion durations (4 h). Although moderate SC formation is ob-
served under atmospheric pressure of CO , it is observed that
2
SO conversion increases as a higher CO pressure is applied
2
(
up to 2 MPa).
A comparison of the catalytic performances of various MOFs
reported to catalyze the SO-CO2 cycloaddition is shown in
Table 3. For a sensible comparison, cycloadditions performed
with 0.4/0.4 mol% of 2D-CCB(M)/TBAB at 1008C under the
conditions of (a) atmospheric pressure of CO2 for 12 h and
Figure 7. Effect of reaction time on the reactivity of SO and CO
2
by using
2
D-CCB(M) as the catalyst at 0.1 MPa, 1008C, and 0.4 mol% of catalyst.
Similarly, with the reaction temperature, the catalytic activity
increases progressively from 40–1008C, whereby an SC yield of
1.9% is reached in 12 h at 0.1 MPa pressure of CO (Figure 8);
(b) 2 MPa CO for 4 h are taken into account to match closely
2
with the reported conditions. Based on the conversion per
equivalent of metal ion, the former conditions furnish a turn-
over number (TON) of 228, whereas the latter provide a TON
of 213. Turnover numbers are calculated for the reported cata-
9
2
no noticeable increase in activity is found up to 1408C.
The effect of CO pressure (0 to 2.0 MPa) on the yield of SC
2
is depicted in Figure 9. For a 12 h long reaction at 1008C,
lysts in the cycloaddition of SO and CO based on their respec-
2
tive formula weights available from the crystallographic infor-
mation file from the Cambridge Structural Database. Even
though a direct comparison with the reported catalysts is im-
practical, we attempt a comparison of the catalysts at the clos-
est reaction conditions. Initially, the results of SO cycloaddition
at mild pressures of CO (0.1–1.0 MPa, entries 1–5) are com-
2
pared to those of 2D-CCB/TBAB operated at atmospheric pres-
sures (entry 6). MOF-5, affords a SC yield of 92% at 508C under
atmospheric pressures (entry 1), however this proceeded under
a catalyst concentration of 2.5 mol%, which is much higher
than the 0.4 mol% concentration with 2D-CCB(M). Moreover,
even at a prolonged reaction duration of 15 h, MOF-5 gives
a TON of 37. In comparison to MOF-5, the ZIF-8 catalyst gives
a yield of 55% (entry 2) at 0.7 MPa CO , which presents an ap-
2
preciable TON of 79. MIL-68-In-NH , Cr-MIL-101 and Co-MOF-74
2
(
entries 3–5) give TONs less than the 2D-CCB/TBAB system
Figure 8. Effect of reaction temperature on the reactivity of SO and CO
12 h) by using 0.4 mol% of 2D-CCB(M) at 0.1 MPa pressure of CO
2
(
2
.
though they use higher catalyst mol%, CO2 pressures (0.7–
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a distinctively reduced conver-
sion is observed in the fourth
and fifth runs (87.7 and 85% SO
conversions, respectively).
Table 3. Comparison of the catalytic activities of 2D-CCB catalyst with previously reported MOF catalysts at
the most matching conditions in styrene oxide–CO
2
cycloaddition. Entries 1–6 compare lesser CO
2
pressures,
whereas 7–10 compare those at moderately higher pressures.
[
b]
Entry Catalyst
Catalyst
Co-catalyst [mmol]
or solvent [mL]
T
CO
2
Pressure
T
SC Yield TON
Ref
FTIR analysis was conducted
on the recycled 2D-CCB(M) cata-
lyst after each cycle until the
end of the fifth run. The catalyst
maintains its chemical integrity
throughout the recycling pro-
cess, which is determined by
the similarity of the peaks from
the first run until the fifth
[
a]
[mol%]
[8C] [MPa]
[h] [%]
1
2
3
4
5
6
7
8
9
1
MOF-5
ZIF-8
MIL-68(In)NH
Cr-MIL-101
Co-MOF-74
2D-CCB
2.5
0.7
9.1
1.2
1.6
0.4
2.1
0.56
0.75
0.4
TBAB (2.5 mmol)
–
–
50 0.1
100 0.7
150 0.8
70 0.8
15 92
37
79
8
28
31
228
46
143
115
213
[3d]
[6d]
[6l]
[6e]
[6k]
this work
[6j]
[6f]
[6m]
this work
5
8
55
74
2
TBAB (1.7 mmol)
24 33
49
12 91
ClC
TBAB (0.4 mmol)
ClC (30 mL)
6
H
5
(30 mL)
100
1
4
100 0.1
UIO-66-NH
2
6
H
5
100
80
80
2
2
2
2
4
4
4
4
96
80
86
85
Ni(salphen)
Ni(saldpen)
2D-CCB
TBAB (2.0 mmol)
TBAB (2.0 mmol)
TBAB (0.4 mmol)
0
100
(
Figure 11).
[
a] mol% of catalyst/cocatalyst to epoxide. MOF mol% calculated per equivalent of metal ion (formula weight
The recycled 2D-CCB(M) cata-
n+
as in crystallographic information, CIF). [b] moles of SO converted per mole of M . ClC
solvent; TBAB=tetrabutylammonium bromide co-catalyst.
6
H
5
=chlorobenzene as
lysts were also analyzed by
PXRD technique (Figure 12). The
major characteristic peaks of 2D-
CCB(M) remain intact, which im-
1
MPa) or solvents. Comparing the catalysts that operate at
plies that the catalyst maintains its structure throughout the
process. An increase in noise is observed, which can be indica-
tive of a slight decrease in crystallinity; however, a closer in-
spection of the XRD patterns of the recycled catalysts after the
higher CO pressures (2 MPa, entries 7–10), namely UIO-66-NH ,
2
2
Ni(salphen) and Ni(saldpen) MOFs, 2D-CCB(M)/TBAB achieves
higher TONs, which demonstrates the competency of this cata-
lyst system among reported MOFs.
Catalyst recycling
MOFs such as ZIF-8 and Cu–BTC (BTC=benzene-1,3,5-tricar-
[
6d,i]
boxylic acid) have been reported
to pose recyclability prob-
lems in the cycloaddition of epoxide and CO because of local
2
structural disorder and/or active site/pore blocking by residual
carbonaceous deposits. We performed catalyst recycling stud-
ies for 2D-CCB under an atmospheric pressure of CO at 1008C
2
for 12 h (Figure 10). It is observed that SC selectivity (99.9%) in
the first run is maintained throughout the first four cycles,
whereas only a slight decrease in SO conversion is encoun-
tered in the first three runs (91.1>90.2>89.5%). However,
Figure 11. FTIR spectra of the recycled 2D-CCB(M) catalyst.
Figure 10. Catalyst recycling with 0.4 mol% 2D-CCB(M)/0.4 mol% TBAB at
1
008C, 0.1 MPa CO
2
, and 12 h (99.9% SC selectivity is maintained).
Figure 12. XRD patterns of the recycled 2D-CCB(M) catalysts.
ChemCatChem 2014, 6, 284 – 292 290
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third and fourth runs reveals the presence of an additional
peak, which appears as a shoulder on the right side of the
drich. CO
tions without further purification.
2
of 99.999% purity was used for the cycloaddition reac-
2
q=20.8 peak. This may be presumed as the blocking of cata-
lytically active sites, and a possible explanation for the de-
crease in catalytic activities during the fourth and fifth cycload-
dition runs.
Synthesis of the catalyst
Solvothermal synthesis (2D-CCB): [{Co(l-cys)(4,4’-bpy) (H O)}·H O]
n
2
2
was synthesized solvothermally in a similar manner to a previous
We next evaluated the heterogeneity of 2D-CCB by using in-
ductively coupled plasma optical emission spectrometry (ICP-
OES) analysis of the filtrate for any metal leaching. The ICP-OES
analysis reveals that only a miniscule amount of cobalt—as
low as 0.2 ppm—is present in the filtrate, in comparison to
[9]
report by the author. Bipyridine (0.624 g, 4 mmol) dissolved in
methanol (30 mL) was added drop wise to an aqueous dispersion
(30 mL) of L-cysteic acid (0.677 g, 4 mmol) and basic cobalt carbon-
ate (0.476 g, 2 mmol), and stirred for 15 min. The contents were
transferred to a Teflon-lined autoclave (100 mL), sealed, and heated
gradually to 1408C. After 50 h, the oven was regressively cooled to
room temperature whereby light pink block-like crystals of 2D-
3
0–50 ppm metal concentrations reported with MOF
[
6e]
catalysts.
CCB(S) were obtained. FTIR (KBr): n˜ =3580–3167 (s), 1609(s), 1580
À1
(
(
m), 1419 (m), 1211(s), 1119 (m), 1046 (s), 817 (s), 723 (m), 570 cm
w). The catalyst was activated by immersing it in dry dichlorome-
Conclusions
thane for 3 d with regular replenishment, followed by vacuum-
drying at 708C for 12 h.
The sulfonate-functionalized 2D rectangular grid framework
[
{Co(4,4’-bipy)(l-cys)(H O)}·H O] , denoted as 2D-CCB, is dem-
Microwave-assisted synthesis: 2D-CCB was synthesized under mi-
crowave irradiation in a Pyrex glass reactor tube (40 mL) fitted
inside a multimode microwave reactor (KMIC-2 KW), which had
a continuously adjustable power source (range 0–2 kW) with a 3-
stub tuner, operating at a frequency of 2.450 GHz. l-Cysteic acid
2
2
n
onstrated as the first example of a sulfonate-functionalized
MOF catalyst that participates in the cycloaddition of epoxides
and CO . The synergistic catalytic action of the oxyanionic sul-
2
fonate moiety and the cobalt center in association with the
bromide anions of TBAB provides a TON of 228 in the cycload-
(0.169 g, 1 mmol) was dissolved in water (10 mL, doubly distilled).
Cobalt(II) carbonate hydroxide (0.119 g, 0.5 mmol) was suspended
in this solution; 4,4’-bipyridine (0.156 g, 1 mmol) in methanol
dition of SO and CO even at atmospheric pressure, which also
2
yields SC at a high selectivity (99.9%). The oxyanion of the sul-
fonate group, which is proposed to form a carbonate complex
(
10 mL) was added dropwise and stirred. The contents were trans-
ferred to the prepared Pyrex glass tube, sealed, and irradiated by
microwaves at 100 W for 10 min. The reaction mixture was then al-
lowed to cool gradually to room temperature. Pink crystals of 2D-
CCB, denoted as 2D-CCB(M), were obtained in >75% yield upon
cooling. FTIR (KBr): n˜ =3578–3165 (s), 1607 (s), 1578 (m), 1418 (m),
1
with CO , facilitates the reaction even under an atmospheric
2
pressure of CO , which further accelerates the reaction owing
2
to the better leaving group ability of the sulfonate anion. An
energy-efficient and time-saving synthesis of 2D-CCB is accom-
À1
207 (s), 1118 (m), 1046(s), 817(s), 723(m), 572 cm (w). Activation
plished by microwave irradiation (100 W) in 10 min, which is
of the catalyst was accomplished by a procedure similar to that
used for 2D-CCB(S).
th
1
/288 of the time required for conventional solvothermal syn-
thesis, while maintaining its structural and catalytic qualities.
The role of the sulfonate group in catalysis is experimentally
demonstrated by comparing its catalytic activity with a similar
Synthesis of 3D-CCB: The six-day long solvothermal synthesis of
[
{Co (l-cys) (4,4’-bipy) (H O) }·3H O] (3D-CCB) reported by Huang
2 2 2 2 2 2 n
[12]
et al. was repeated with a slight modification as follows: a mix-
À
MOF 3D-CCB, which is deprived of SO3 groups. The acid–base
ture of l-cysteic acid monohydrate (0.374 g, 2 mmol) and KOH
sites were compared by performing Knoevenagel condensation
and acetalization reactions. The microwave-synthesized 2D-
CCB(M) catalyst is successfully reused up to four times without
degradation of the covalent/coordinate linkage of the catalyst
framework. The potential of sulfonate-functionalized MOFs can
be extended to other processes also. Further explorations in
the coordination modes, geometry, and functional group of
MOFs in controlling the activity of MOF catalysts for cycloaddi-
tion reactions are in progress.
(
0.224 g, 4 mmol) in water (20 mL) was added dropwise to an
aqueous solution (20 mL) of Co(NO ) ·6H O (0.582 g, 2 mmol). 4,4’-
3
2
2
Bipyridine (0.312 g, 2 mmol) in methanol (20 mL) was added slowly
to this mixture and stirred for 30 min, transferred and sealed in
a Teflon-lined autoclave (100 mL), and heated at 1008C for 6 d.
Crystals of 3D-CCB(S) were obtained after slowly cooling the solu-
tion to room temperature. FTIR (KBr): n˜ =3430 (s), 1655 (s), 1642
(
6
s), 1428 (m), 1227 (m), 1193 (m), 1039 (s), 852 (w), 723 (w),
À1
05 cm (w).
Acknowledgements
Experimental Section
This study was supported by the Korean Ministry of Education
through the National Research Foundation (2012-001507), Global
Frontier program and the Brain Korea 21 Project. The authors are
grateful to KBSI for analysis.
Reagents and methods
l-Cysteic acid monohydrate (>99.0%) and 4,4’-dipyridyl (bipy;
9
8%) and styrene oxide (SO) were purchased from Aldrich, Korea
and used as received. Cobalt(II) carbonate hydroxide was pur-
chased from Junsei Chemical Co., Ltd. l-Cysteic acid was pur-
chased from Tokyo Chemical Industry Co., Ltd. Doubly distilled
water was used for the catalyst synthesis. Acetone, methanol, and
dichloromethane (anhydrous,>99.8%), were purchased from Al-
Keywords: amino acids · cobalt · carbon dioxide fixation ·
metal–organic frameworks · microwave chemistry
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