Ni(salphen)-based metal-organic framework for the synthesis of cyclic carbonates by cycloaddition of CO2 to epoxides

Article abstract of DOI:10.1039/c2ra22550f

A well-defined homogeneous molecular catalyst Ni(salphen) was introduced as a "metalloligand" in a MOF, providing an efficient and recyclable heterogeneous catalyst for the synthesis of cyclic carbonates by the cycloaddition of CO₂ to epoxides

Full text of DOI:10.1039/c2ra22550f

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Ni(salphen)-based metal-organic framework for the
synthesis of cyclic carbonates by cycloaddition of CO2 to
epoxides†
Yanwei Ren, Yanchao Shi, Junxian Chen, Shaorong Yang, Chaorong Qi and Huanfeng Jiang*
A well-defined homogeneous molecular catalyst Ni(salphen) was introduced as “metalloligand” in MOF,
providing an efficient and recyclable heterogeneous catalyst for the synthesis of cyclic carbonates by
cycloaddition of CO2 to epoxides under relatively mild conditions.
Introduction
Although carbon dioxide (CO2) is a nontoxic, non-flammable and abundant C1 building block, the scope of the
synthetic applications of CO2 is limited because of its inert nature. The development of efficient synthetic methods
for CO2 fixation is therefore an important and challenging subject.1 Among them, cyclic carbonates derived from
the coupling reactions of CO2 with epoxides are promising target molecules since it can be widely used for various
purposes, such as electrolytes in lithium-ion batteries, raw materials for polycarbonate, and polar aprotic solvents.2
Various homogeneous metal complexes catalysts, such as M(salen) complexes3 have been developed for the
cycloaddition of CO2 and epoxides. Although these homogeneous catalysts usually exhibit high activity and
selectivity at mild temperature in the presence of co-catalysts, their practical application remain limited because of
catalyst instability and difficulty in catalyst/product separation. Immobilization of homogeneous catalysts can
facilitate its recovery and reuse and therefore is of considerable interest to academia and industry.4 According to
this strategy, various heterogeneous catalysts have been developed for coupling reaction CO2 with epoxides, such
as functional polymers,5 ion-exchange resins,6 and quaternary ammonium or phosphonium supported on SiO2.7
Recently, based on the great potential as adsorbents/catalysts due to extremely large surface area and well-ordered
porous structure, several metal-organic frameworks (MOFs) heterogeneous catalysts have been studied for CO2
fixation.8 However, these studies rely on the intrinsic catalytic activity (e.g., weak Lewis acidity) of the
metal-connecting points. A more rational strategy than such an “opportunistic” approach is to introduce
well-defined homogeneous molecular catalysts (or precatalyst) into the MOF structures.9 Until now, a few MOFs
based on M(salen) ligands with an additional functional group such as carboxylates,10 p-benzoic acid groups,11 and
p-pyridyl groups12 in the para or meta position to the OH group have been studied as new generation of
heterogeneous catalysts that are capable of catalyzing more advanced and value-added reactions. Surprisingly, no
MOF catalyst employing this synthetic strategy for CO2 fixation has been reported so far. Herein we report a new
dicarboxyl-functionalized nickel salphen complex Ni-H2L (Scheme 1) as bridging metalloligand incorporating into
a 3D MOF as self-supported heterogeneous catalyst for coupling reaction CO2 with epoxides in the presence of
quaternary ammonium salts. To our knowledge, it will be the first occasion to demonstrate that a M(salen)-based
MOF material can be efficient both as a CO2 adsorbent and as a catalyst for its chemical fixation.
Results and discussion
The Ni(salphen) ligand Ni-H2L was synthesized by
a Schiff base condensation reaction between
o-phenylenediamine and (E)-3-(3-tert-butyl-5-fomyl-4-hydroxyphenyl)acrylic acid and then metalated with
Ni(OAc)2·4H2O. Reaction of CdCl2 and with Ni-H2L in dimethylformamide (DMF)/H2O at 80 ℃ for 96 h afford
brown block single crystals of [Cd2(Ni-L)2(H2O)4]·3DMF (1) in good yield. The product was stable in air and
insoluble in water and common organic solvents and was formulated on the basis of elemental analysis, IR
spectroscopy, and thermogravimetric analysis (TGA). Phase purity of the bulk sample was established by
comparison of their observed and simulated powder X-ray diffraction (PXRD) patterns (Supporting information,
Fig S1).
Single-crystal X-ray diffraction study showed that 1 adopts a 3D nanoporous framework and crystallizes in the
triclinic space group P-1 with two Ni-L units, two Cd ions and four coordination H2O molecules in the asymmetric
unit. Basic building block, dinuclear Cd2 clusters C-I and C-II (Fig 1a) with a C2 axis passing through two bridging
oxygen atoms, clustered by two bidentate and two tridentate carboxylate groups of four Ni-L units. In the two Cd2
clusters, one of the independent Cd ions is coordinated by five oxygen atoms from three carboxylate groups, and
two coordination water molecules (with trans-position in C-I, cis-position in C-II). Ni-L units exhibit two
coordination modes including bis-bidentate chelating mode and bis-tridentate chelating-bridging carboxylate
groups (Fig S2). Each Ni ion is coordinated in a nearly square-planar geometry with two nitrogen atoms and two
oxygen atoms form the L ligand (Ni-Oavg = 1.839 Å, Ni-Navg = 1.850 Å). Each Cd2 cluster in 1 is thus linked by
1

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four Ni-L ligands, and each Ni-L ligands is linked to two Cd2 cluster to generate a porous 2D network along a-axis
(Fig S3). All thick lamellar networks are stacked together via supermolecular interaction to form a 3D nanoporous
framework with 1D ellipse-shape channels cross section of ~ 1.0×1.4 nm along b-axis, which are filled with DMF
molecules (Fig 1b). Therefore, the channel surfaces are uniformly stacked with Ni-L units with coordinatively
unsaturated Ni2+ ions, and the Ni-Ni distance of adjacent Ni-L layers is about 5.128 Å, which are accessible to
guest molecules and available for cooperative double-Lewis acidic activation. PLATON calculation13 reveals that
1 has 41.3% of the total volume available for guest inclusion. TGA revealed that the coordination water and DMF
molecules could be readily removed in the temperature range 30-120 K, and the framework are stable up to 150 K
(Fig S4). After a sample of 1 was ground and heated at 100 K for 5 h under vacuum to obtain activated 1, PXRD
of the resultant powder showed a broad diffraction pattern similar to that of the pristine sample. This result
indicates that the nanoporous framework was maintained after removal of solvent molecules. To determine the
permanent porosity of 1, 77 K N2 adsorption was carried out on the evacuated framework (Fig S5). A type I
isotherm was observed, indicating that 1 is microporous, and the apparent surface area was calculated using the
Langmuir method to be 358m2/g (245 m2/g BET). The CO2 adsorption measurement of the activated 1 showed that
up to 25 cm3 g-1 of CO2 uptake was obtained at 273 K and 1.0 atm (Fig S6), a moderate adsorption capacity of
lower-pressure CO2 as compared to reported MOFs,14 indicating its potential application for CO2 capture and
conversion.
The moderate stability and uptake for CO2 of the Ni(salphen)- based MOF 1 prompted us to explore its
utilization as a heterogeneous catalyst for the synthesis of cyclic carbonates by cycloaddition of CO2 to epoxides,
since M(salen) complexes are promising catalysts for CO2 fixation reaction.3a The activity of various catalysts was
tested at 80 ℃ and 2 MPa using the reaction of propylene oxide (PO) and CO2 to produce propylene carbonate
(PC). As shown in Table 1, the catalytic performance of activated 1 depends strongly on the ammonium salts used,
and highest activity was observed using NBu4Br (entries 1-4). Theoretically, iodide is the best promoter in
accordance to the increasing nucleophilicity, but in the presence of microporous MOF the diffusion of the large
iodide might be hampered, thus explaining the slightly reduced activity compared to the bromides. When the
catalyst was not used, the yield of PC was very low (12%), indicating Lewis acid activation of PO was necessary
for the reaction (entry 5). To answer the question whether the reaction was catalyzed by Ni-L units or the
coordinatively unsaturated Cd active sites exposed in the 1D channel of activated 1, we performed the tests
reaction under standard conditions using Ni-H2L and [Cd(bpdc)]n (H2bpdc = 4,4’-biphenyldicarboxylic acid),15
respectively as the catalysts (entries 6 and 7, the molar amounts of Ni-L and Cd were the same as in 1). Results
revealed the yield of PC was all higher than no catalyst test, but significantly lower than 1, clearly indicating that
synergistic and/or additive activation of two kinds of Lewis acid in 1 occurs during the reaction process. Another
reason for the higher activity of this MOF catalyst is that it is a porous material, which is favourable for the
increasing of CO2 uptake, and facilitates the access of reactants to the active sites and product diffusion through
open channels. At the same time, we found the amount of NBu4Br had a great influence on the reaction. Increasing
the amount of NBu4Br from 1 mol% to 3 mol% could enhance the yield of PC significantly (entries 6, 8 and 9).
The PC yield was also increased with an increasing reaction temperature or time (entries 10-12), but higher
temperature reactions were not investigated, because this catalyst is unstable more than 150 K. The results of the
recycling experiments showed no obvious decrease in activity of the catalyst after being used three times (entries
13 and 14). Furthermore, the solid catalyst recovered from the catalytic reaction exhibited the same PXRD as the
pristine solid of MOF 1 (Fig S1), unambiguously supporting the stability of the MOF framework during the
catalytic reactions. The good recyclability may be due to that this catalyst was synthesized by the solvothermal
method and therefore resisted moderate temperature and pressure. This catalytic capability of the self-supported
Ni(salphen) MOF catalyst can compare to that of other MOF based heterogenerous catalysts reported in the
literatures for the cycloaddition reaction of CO2 to epoxide (Table 2).
Table 3 summarized the coupling reactions of CO2 with different epoxides catalyzed by 1/ NBu4Br system, and
the data show that the catalytic system could convert epoxides 2a-2d to the corresponding cyclic carbonates
effectively under the same conditions. The conversion for 2e is lower partly because it has bulky space size,
reducing its diffusion rate in the 1D open channel of solid catalyst. This result further suggests that the catalytic
reaction took place in the channel not on the catalyst surface. Indeed, when extended reaction time to 24 h, the
yield reach to 82 %.
Based on above analysis, a plausible Lewis acidic activation mechanism is proposed for this CO2 fixation
reaction, which is similar to that of the homogenous catalytic system with Co(salen)X complexes as catalysts and
quaternary ammonium salts as co-catalysts.16 First of all, the porosity and polar nature of 1 favors the CO2 uptake
and further increases the local concentration of CO2 in the channel. The coupling reaction is initiated through the
coordination of the coordinatively unsaturated Ni2+ (or Cd2+) in 1 to the oxygen atom of the epoxide, and this step
can activate the epoxy ring. Secondly, the Br- generated from NBu4Br attacks the less-hindered carbon atom of the
coordinated epoxides, followed by the ring opening step. Then, the oxygen anion of the opened epoxy ring
interacts with CO2, forming an alkylcarbonate anion, which is converted into the corresponding cyclic carbonate
through the ring closing step. Simultaneously, the catalyst is regenerated.
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Conclusions
In summary, we introduced a well-defined homogeneous Ni(salphen) catalyst as “metalloligand” in a porous
MOF. The Ni(salphen) units and coordinatively unsaturated Cd active sites accessible via the open MOF channels
were utilized to generate an efficient heterogeneous catalyst for the coupling reactions of CO2 with epoxides under
relatively mild conditions. The MOF catalyst features a high local density of coorperative layer Ni(salphen) motifs,
exhibiting improved catalytic performance relative to the monomeric homogeneous catalyst. This solid catalyst
can be easily recycled and reused without any apparent loss of catalytic activity after being used three times. This
work established a new strategy in the rational design of effective self-supported MOF catalysts for CO2 absorption
and in situ fixation based on functional metallosalens or metalloprophyrins.
Acknowledgements
We are grateful to the National Natural Science Foundation of China (Nos. 20932002 and 21172076), the
National Basic Research Program of China (973 Program) (No. 2011CB808600), the Natural Science Foundation
of Guangdong Province, China (No. 10351064101000000), and the Fundamental Research Funds for the Central
Universities (No. 2011ZM0044) for the financial support.
Notes and references
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China. Fax:
+86-20-22236337. Email: jianghf@scut.edu.cn
† Electronic supplementary information (ESI) available: Synthesis and characterization of Ni-H2L ligand and MOF 1, X-ray
crystallography, PXRD data, TGA data, nitrogen and carbon dioxide adsorption isotherms, and catalytic reaction. CCDC:
900477.
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N
O
N
O
Ni
HOOC
COOH
Scheme 1. Schematic representation of Ni-H2L.
(a)
C-I
C-II
(b)
Fig.1 (a) Building block in 1 and (b) view of 3D porous structure of 1 along the b-axis.
Table 1. Reaction of CO2 and PO used by different catalysts and ammonium salts at various conditionsa
Entry
1
Catalyst
Ammonium salt
None
Yield [%]g
0
1
2
NBu4Cl
NBu4Br
NBu4I
26
80
72
12
38
25
56
75
85
82
1
3
1
4
1
5
None
NBu4Br
NBu4Br
NBu4Br
NBu4Br
NBu4Br
NBu4Br
NBu4Br
6
Ni-H2L
7
[Cd(bpdc)]n
8b
9c
10d
11e
1
1
1
1
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12f
NBu4Br
NBu4Br
NBu4Br
86
80
78
1
1
1
13(2nd)
14(3rd)
a Typical reaction conditions: PO (10 mmol), ammonium salts (3 mol %), catalyst (0.05 g), CO2 pressure (2 MPa), reaction
temperature (80 ℃), reaction time (4 h). b1 mol % NBu4Br was used. c2 mol % NBu4Br was used. dReaction temperature is
e
f
g
100 ℃. Reaction time is 10 h. Reaction temperature is 100 ℃ and reaction time is 24 h. The yields were determined by
GC with an internal standard.
Table 2. Comparison with other MOF based heterogeneous catalytic systems on cycloaddition reaction of CO2 to epoxide.
Catalyst
Co-catalyst
Pressure (MPa)
Time (h) Yield (%) Ref.
Temperature (K)
MIXMOF-5a
MOF-5
NEt4Br
3.0
1.0
2.0
0.7
0.7
2.0
2.0
2.0
140
50
3
4
4
4
4
4
4
5
63.0b
97.6b
80.0b
52.0c
73.1c
96.0e
95.0e
84.0e
8a
NBu4Br
8b
Ni(salphen)-MOF
ZIF-8
NBu4Br
80
Present work
-
-
-
-
-
80
8e
8e
8c
8d
8f
ZIF-8-fd
80
Co-MOF-74
Mg-MOF-74
F-IRMOF-3e
100
100
140
aMIXMOF-5 represent mixed-linker MOF-5 with general formula Zn4O(BDC)x(ABDC)3-x, in which BDC is
benzene-1,4-dicarboxylate, ABDC is 2-aminobenzene-1,4-dicarboxylate. bEpoxide in this cycloaddtion reaction is
propylene oxide. cEpoxide in this cycloaddtion reaction is chloropropylene oxide. dZIF-8-f represent the grafting of
ethylene diamine in the ZIF-8 framework. eepoxide in cycloaddtion reaction is styrene oxide.
Table 3. Various carbonates synthesis catalyzed by 1 in the presence of NBu4Bra
O
O
1/NBu4Br
O
O
pCO2 = 2 MPa
80 K ,
R
R
2
3
Entry
R
Time/h
Yield(%)
2
1
2
3
4
5
6
2a
2b
2c
2d
2e
2e
Me
4
80
84
81
76
55
82
CH2Cl
Ph
4
4
CH2Ph
4
CH2OPh
4
CH2OPh
24
a reaction conditions: see Table 1.
5