Bifunctional Imidazolium-Based Ionic Liquid Decorated UiO-67 Type MOF for Selective CO2 Adsorption and Catalytic Property for CO2 Cycloaddition with Epoxides

Article abstract of DOI:10.1021/acs.inorgchem.6b03169

A bifunctional robust and highly porous imidazolium-based ionic liquid decorated UiO-67 type MOF (UiO-67-IL, 1) was successfully constructed via solvothermal assembly of the imidazolium-based ligand and Zr(IV) ions. It exhibits a highly selective adsorption for CO₂ over CH₄ and N₂. Furthermore, 1 herein can be used as a highly active heterogeneous catalyst for CO₂ cycloaddition with epoxides under atmospheric pressure with or without cocatalyst TBAB (n-Bu₄NBr).

Full text of DOI:10.1021/acs.inorgchem.6b03169

Article
pubs.acs.org/IC
Bifunctional Imidazolium-Based Ionic Liquid Decorated UiO-67 Type
MOF for Selective CO2 Adsorption and Catalytic Property for CO2
Cycloaddition with Epoxides
Luo-Gang Ding, Bing-Jian Yao,* Wei-Ling Jiang, Jiang-Tao Li, Qi-Juan Fu, Yan-An Li, Zhen-Hua Liu,
Jian-Ping Ma, and Yu-Bin Dong*
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong
Normal University, Jinan 250014, P. R. China
S
* Supporting Information
ABSTRACT: A bifunctional robust and highly porous imidazolium-based
ionic liquid decorated UiO-67 type MOF (UiO-67-IL, 1) was successfully
constructed via solvothermal assembly of the imidazolium-based ligand and
Zr(IV) ions. It exhibits a highly selective adsorption for CO2 over CH4 and N2.
Furthermore, 1 herein can be used as a highly active heterogeneous catalyst for
CO2 cycloaddition with epoxides under atmospheric pressure with or without
cocatalyst TBAB (n-Bu4NBr).
Recently, room-temperature ionic liquids (ILs) have
attracted much attention owing to their unique properties
such as nonvolatility, nonflammability, and recyclability.7
Specifically, some ILs, for example, imidazolium-based
polymeric ionic liquids, have been demonstrated to be a
unique species in the CO2 chemical fixation and useful catalysts
for the cycloaddition of CO2 to epoxides.8 In principle, ILs such
as imidazolium-based IL units can be grafted and immobilized
in MOFs via ligand functionalization. In this way, the
advantages of MOFs and ILs would be combined together to
lead to the expected multifunctional porous materials for CO2
selective adsorption and its chemical transformation under mild
conditions.
We recently reported a series of functional NMOFs via
postsynthetic approach.9 Therein, imidazolium-containing
MOFs are demonstrated to be a highly active class of triphase
catalysts for the azidation and thiolation between aqueous/
organic phases.9b The imidazolium decoration yields that were
achieved via postsynthesis functionalization of a Cd(II)-MOF,
however, are only within a range of 60−70%. Different from
our previous case, we report herein a new imidazolium-based
UiO-67 type MOF (UiO-67-IL, 1) via direct ligand
functionalization (Scheme 1). Compared to a postsynthetic
approach, the IL-functionalized yield based on this ligand
modification method is quantitative and the resulting MOF
INTRODUCTION
■
Carbon dioxide (CO2), accumulating in the atmosphere at an
alarming rate, is deemed as the major greenhouse gas causing
global warming over the past several decades.1 In order to
reduce the greenhouse effect, CO2 capture and storage/
sequestration (CCS) and its chemical transformation are
areas of intense current interest.2 For overcoming these two
great challenges, synthetic porous materials are currently used
to adsorb and store CO2.3 On the other hand, catalytic CO2
conversion into value-added industrial products can make waste
CO2 be an abundant, nontoxic, and inexpensive C1 building
block.2a Although recent progress on both CCS and catalytic
CO2 conversion is remarkable,4 bifunctional materials which are
able to incorporate both selective CO2 adsorption and catalytic
conversion are still rarely reported.
It is known that metal−organic frameworks (MOFs) have
high porosity, tunable composition and pore surface, so they
might be promising porous materials for selective gas
adsorption and effective heterogeneous catalysts.5 Recently, a
postsynthetic method was proposed and widely used as a
powerful tool to modify and enhance the functionality of
MOFs.6 However, the postsynthetic yields, especially for the
organic functional group transformation, are usually not that
ideal, which results from the inherent imperfection of the
heterogeneous solid−liquid reactions. Thus, the synthesis of
the desired MOFs based on the premodified ligands could be
an alternative approach to solve this problem.
Received: December 27, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b03169
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Inorganic Chemistry
Article
3.90 (6H, s, −OCH3), 4.74 (2H, q, −NCH2CH), 5.19, 5.33 (2H, d,
−CHCH2), 5.54 (2H, s, −NCH2Ar), 5.89 (1H, m, −CH2CHCH2),
7.63, 8.01 (2H, d, −NCHCHN−), 7.42−7.52, 8.03−8.11 (7H, m,
−Ar), 8.80 (1H, s, −NCHN). FTIR (KBr, cm−1): 3167 (m), 3054 (m)
2956 (m), 2896 (m), 2836 (m), 1712 (s), 1529 (w), 1604 (w), 1556
(w), 1123 (m), 905 (m), 762 (m). ESI-MS: calcd for (C23H23N2O4)+
M+, m/z 391.17; found, m/z 391.16.
Scheme 1. Synthesis of UiO-67-IL (1)
Synthesis of L. C was hydrolyzed by LiOH (10 equiv) in a
methanol/water (3:1, v/v) at room temperature (12 h). Then the pH
value of the reaction solution was adjusted to 2−3 by hydrobromic
acid. The product precipitated from the solution to generate
imidazolium-based ligand L as white crystalline solids in 90% yield.
1H NMR (400 MHz, DMSO-d6), δ (ppm): 4.74 (2H, q, −NCH2CH),
5.19, 5.33 (2H, d, −CHCH2), 5.53 (2H, s, −NCH2Ar), 5.89 (1H, m,
−CH2CHCH2), 7.63, 8.00 (2H, d, −NCHCHN−), 7,42−7.49, 8.02−
8.08 (7H, m, −Ar), 8.82 (s, −NCHN), 13.12 (2H, s, −COOH). FTIR
(KBr, cm−1): 3377 (s), 3160, 3092 (m), 2859 (m), 2776 (m), 1717
(s), 1529 (w), 1604 (m), 1556 (m), 1100 (m), 987 (m), 920 (m), 769
(m). ESI-MS: calcd for C21H19N2O4Br, M+, m/z 442.05; found, m/z
442.04.
Synthesis and Characterization of UiO-67-IL (1). ZrCl4 (28.80
mg, 0.12 mmol) and 50 equiv of acetic acid (0.34 mL) were dissolved
in DMF (5 mL) by using ultrasound for about 10 min. The linker
(43.56 mg, 0.12 mmol) was added to the clear solution in an
equimolar ratio with regard to ZrCl4. The tightly capped flasks were
kept in an oven at 120 °C under static conditions. After 24 h, the
reaction system was cooled to room temperature and the precipitate
was isolated by centrifugation. The solids were suspended in fresh
DMF (10 mL). After standing at room temperature for 5 h, the
suspension was centrifuged and the solvent was decanted off. The
obtained particles were washed with ethanol (10 mL) several times in
the same way as described for washing with DMF. Finally, the solids
were dried under reduced pressure to generate the as synthesized UiO-
67-IL (1) crystals. Yield, 69.2%. IR (KBr, cm−1): 3077 (m), 1604 (m),
1552 (m), 1522 (w), 1424 (s), 1146 (m), 780 (m), 657 (m).
Elemental analysis (%) calcd for C21H19.6Zr1N2O5.3Br1 (desolvated): C
45.32, H 3.54, N 5.04, Zr 16.37, Br 14.39. Found: C 43.87, H 3.58, N
4.98, Zr 16.86, Br 14.24.
Adsorption Measurements. Gas adsorption experiments were
carried out with a Micromeritics ASAP 2020/TriStar 3000 volumetric
gas sorption instrument. Prior to the measurement, the sample was
soaked in ethanol for 48 h to exchange DMF solvent molecules; after
centrifugation and drying, the sample was loaded in a sample tube and
dried under high vacuum at 393 K for 10 h to remove the residual
solvent molecules in the channel. About 200 mg of the dissolved
sample was used for the entire adsorption measurement. The nitrogen
sorption isotherm was collected at 77 K in a liquid nitrogen bath, in
order to study its permanent porosity and robustness. For selective
adsorption evaluation, the gas sorption experiments of CO2, N2, and
CH4 were carried out at 273 K in an ice−water bath, and at 298 K in a
temperature controlled circular bath, respectively.
Isosteric Heat Adsorption. The isosteric heat of adsorption
represents the strength of the interactions between adsorbate
molecules and the adsorbent lattice atoms and can be used as a
measurement of the energetic heterogeneity of a solid surface. The
isosteric heat of adsorption at a given amount can be calculated by the
Clausius−Clapeyron equation as
material thereby presents high-density IL active sites. In
addition, CO2 adsorption and separation selectivity over N2
and CH4 was preliminarily explored on 1. More interestingly,
UiO-67-IL (1) exhibits an excellent catalytic performance for
efficient chemical transformation of CO2 to cyclic carbonates
under atmospheric pressure (1 atm).
EXPERIMENTAL SECTION
Materials and Instrumentations. All the chemicals were
■
obtained from commercial sources (Acros) and used without further
1
purification. H NMR data were collected on a Bruker Avance-400
spectrometer. Chemical shifts are reported in δ relative to TMS.
Infrared spectra were obtained in the 400−4000 cm−1 range using a
Bruker ALPHA FT-IR spectrometer. Elemental analyses were
performed on a PerkinElmer model 2400 analyzer. ICP measurement
was performed on an IRIS Interpid (II) XSP and NU AttoM. HRMS
analysis was carried out on a Bruker maXis UHR-TOF ultrahigh
resolution quadrupole-time-of-flight mass spectrometer. Thermogravi-
metric analyses were carried out on a TA Instruments Q5
simultaneous TGA under flowing nitrogen at a heating rate of 10
°C/min. The XRD pattern was obtained on D8 ADVANCE X-ray
powder diffractometer (XRPD) with Cu Kα radiation (λ = 1.5405 Å).
The scanning electron microscopy (SEM) micrographs were recorded
on a Gemini Zeiss Supra TM scanning electron microscope equipped
with energy-dispersive X-ray detector (EDS).
Synthesis of Intermediate A. 4-Bromo-3-methylbenzoic acid
(5.00 g, 23.40 mmol), 4-carboxyphenylboronic acid (4.80 g, 28.90
mmol), K2CO3 (25.80 g), and tetrakis(triphenylphosphine)palladium
(1.50 g) were added to a toluene/ethanol/water solution (50 mL,
1:1:1, v/v). The reaction was conducted with reflux for 24 h with
magnetic stirring. The pH value of aqueous phase was adjusted to 2.0
by diluted hydrochloric acid (1M), followed by the esterification with
methanol (75 mL) at reflux for 12 h in the presence of concentrated
sulfuric acid (5 mL). After that, the pH of the reactant solution was
adjusted to 7.0 by saturated solution of sodium carbonate. The
product was collected and dried in vacuum to generate A (5.35 g,
1
91%). H NMR (400 MHz, CDCl3), δ (ppm): 2.30 (3H, s, −CH3),
3.95 (6H, s, −OCH3), 7.26−8.12 (7H, m, −Ar). FTIR (KBr, cm−1):
3100 (w), 2950 (s), 2844 (m), 1712 (s), 1602 (s), 1552 (m), 1461
(m), 1108 (m), 762 (m). ESI-MS: calcd for C17H16O4 M+, m/z
284.10, found, m/z 284.11.
Synthesis of B. A mixture of intermediate A (2.54 g, 8.96 mmol),
N-bromosuccinimide (1.83 g, 10.30 mmol), AIBN (0.15 g, 0.90
mmol), and benzene (45 mL) was stirred at 80 °C for 12 h. After
removal of the solvent in vacuum, the crude product was purified by
column chromatography on silica gel using petroleum ether−
dichloromethane (1:1 v/v) as the eluent to generate B (1.45 g,
45%). 1H NMR (400 MHz, DMSO-d6), δ (ppm): 3.91 (6H, s,
−OCH3), 4.69 (2H, s, −ArCH2Br), 7.45−8.30 (7H, m, −Ar). FTIR
(KBr, cm−1): 3100 (w), 2956, 2838 (m), 1720 (s), 1602 (m), 1552
(m), 1461 (m), 1116 (m), 763 (m), 627 (m). ESI-MS: calcd for
C17H15O4Br M+, m/z 362.02; found, m/z 362.17.
⎛
⎜
⎞
⎟
∂ ln P
∂T
Q st = −RT2
⎝
⎠
na
where Qst is the isosteric heat of adsorption (kJ/mol), P is the pressure
(kPa), T is the temperature, R is the gas constant, and na is the
adsorption amount (mmol/g).
Calculations of the Adsorption Selectivity. The selectivity of
the preferential adsorption of component 1 over component 2 in a
binary mixture containing 1 and 2 can be formally defined as
Synthesis of C. A mixture of intermediate B (1.45 g, 4.00 mmol),
1-allylimidazole (0.95 g, 8.80 mmol), and acetonitrile (45 mL) was
stirred at 80 °C for 2 h. After removal of the solvent in vacuum, the
residue was purified by column chromatography on silica gel using
dichloromethane−methanol (20:1, v/v) as the eluent to generate C as
q1/q2
Sads
=
1
P/P
1
2
yellow oil (1.33 g, 85%). H NMR (400 MHz, DMSO-d6), δ (ppm):
B
DOI: 10.1021/acs.inorgchem.6b03169
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a
Scheme 2. Synthesis of Ligand L
a
(i) 4-Carboxyphenylboronic acid, K2(CO3)2, tetrakis(triphenylphosphine)palladium, toluene/ethanol/water solution (1:1:1, v/v), reflux, 24 h. (ii)
MeOH/H2SO4, reflux, 12 h. (iii) N-Bromosuccinimide, AIBN, benzene, 80 °C, 12 h. (iv) 1-Allylimidazole, CH3CN, 80 °C, 2 h. (v) LiOH/MeOH−
H2O (rt, 12 h), HBr.
Figure 1. (a) Simulated PXRD pattern of UiO-67 and measured PRXD pattern of UiO-67-IL (1). (b) SEM image of 1. (c) Dynamic light scattering
(DLS) measurement of 1. (d) TGA trace of UiO-67-IL (1) and its PRXD pattern after heating at 200 °C for 1 h.
Figure 2. Left: Nitrogen sorption isotherms of 1 and UiO-67. Right: Density functional theory pore-size distribution for UiO-67 and 1 determined
from their nitrogen adsorption isotherms at 77 K.
where q1 and q2 are the absolute loadings. In all of the calculations to
be presented below, the calculations of Sads are based on the use of the
ideal adsorbed solution theory (IAST) of Myers and Prausnitz. These
calculations are carried out using the pure-component isotherm fits of
absolute loadings.
Catalytic Cycloaddition of CO2 with Epoxides. The solvent-
free cycloaddition reaction was carried out in a flask (10 mL) with
magnetic stirrer with known amounts of epoxides, catalyst (UiO-67-IL,
1), and cocatalyst (TBAB) charged into the reactor. The reactor was
conducted at a desired temperature, and CO2 was added through a
balloon under 1 atm of pressure. With epichlorohydrin and styrene
oxide as model substrate, the effect of temperature, reaction times, and
amount of catalyst and cocatalyst on the performance were
systematically investigated for the utilization of CO2 for useful high-
value chemicals.
Catalyst Regeneration. After each catalytic cycle, 1 was recovered
by centrifugation, then rinsed with ethanol, and dried in vacuum at 90
°C for the next catalytic run under the same reaction conditions.
Leaching Test. The solid catalyst 1 was separated from the hot
reaction solution right after reaction for 2.5 h. The reaction was
continued with the filtrate in the absence of 1 for an additional 9.5 h.
No further increase in the yield of cyclic carbonate was detected, which
confirms the catalytically active sites for this CO2 cycloaddition of
epichlorohydrin located on 1.
C
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Figure 3. Left: CO2, CH4, and N2 adsorption isotherms of 1 at 273 K. Right: CO2, CH4, and N2 adsorption isotherms of UiO-67-IL at 298 K.
Figure 4. Left: Isosteric heat of adsorption for CO2 at different CO2 loadings. Middle: Pressure-dependent selectivity profiles on 1 calculated by the
IAST method for CO2/N2 (50:50) and CO2/CH4 (50:50) binary mixtures at 273 K. Right: Pressure-dependent selectivity profiles on 1 calculated by
the IAST method for CO2/N2 (50:50) and CO2/CH4 (50:50) binary mixtures at 298 K.
ification, the bromination reaction was carried out with NBS in
benzene to generate B. The imidazolium group was introduced
by the reaction of substituted benzyl bromide B with 1-
allylimidiazole to provide C. Dicarboxylic ligand L was obtained
via hydrolyzation of C by LiOH in MeOH/H2O mixed solvent
system at room temperature in good yield. The corresponding
1H NMR, FTIR, and MS spectra for A, B, C, and L are
provided in the Supporting Information.
RESULTS AND DISCUSSION
■
Ligand Synthesis. As shown in Scheme 2, ligand L with
imidazolium unit was synthesized as white crystalline solids.
The ligand backbone of A was obtained by the palladium-
catalyzed C−C coupling reaction between 4-bromo-3-methyl-
benzoic acid and 4-carboxyphenylboronic acid. After ester-
Table 1. 1-Catalyzed CO2 Cycloaddition with
Epichlorohydrin with or without TBAB (n-Bu4NBr)
a
Synthesis and Characterization of UiO-67-IL (1). UiO-
67-IL (1) was prepared according to the reported method.10 As
indicated in Scheme 1, ZrCl4 and L reacted in DMF under
solvothermal conditions (120 °C, 24 h) to yield a buffy
crystalline powder of UiO-67-IL (1). The powder X-ray
diffraction (PXRD) pattern indicates that 1 is highly crystalline
and its structure is identical to that of pristine UiO-67 (Figure
1).10c So the introduced imidazolium group did not affect the
UiO-67 framework formation under the reaction conditions.
Scanning electronic microscopy (SEM) measurement shows
that 1 was obtained as uniform nanoscale particles (ca. 363
nm), and they showed a regular octahedral shape (Figure 1),
which is further supported by the dynamic light scattering
(DLS) measurement (Figure 1). In addition, thermostability of
1 was examined by thermogravimetric analysis (TGA) and
PXRD measurement. The evacuated sample of 1 was heated at
ca. 200 °C for 1 h; no framework collapse was observed (Figure
1), although this decomposition temperature is significantly
lower than that of its pristine UiO-67 (decomposition at 520
°C).10d The thermostability of 1 is good enough to be the
heterogeneous catalyst for most organic transformations.
To examine the permanent porosity, N2 adsorption property
of 1 was measured at 77 K (Figure 2). It is similar to that of
UiO-67; 1 also exhibits type I isotherms, featuring character-
istics of microporous materials. The N2 absorption amount of 1
was 284 cm3/g, and its Brunauer−Emmett−Teller (BET)
b
c
entry 1(% mol) TBAB (% mol) T (°C) t (h) yield (%)
TOF
0.09
1
0.7
25
25
25
25
50
90
50
50
50
50
90
90
90
90
90
90
48
48
48
72
3
3
21
80
93
77
99
28
53
68
80
75
88
88
81
95
95
2
1.0
1.0
1.0
1.0
1.0
3
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
1.5
1.5
1.5
2.28
1.76
4
5
35.06
45.08
12.75
9.05
6
3
7
3
8
8
9
12
48
3
7.74
10
11
12
13
14
15
16
2.28
34.15
13.66
10.02
18.00
7.92
8
12
3
8
12
5.28
a
b
Reaction conditions: epichlorohydrin, 1 atm of CO2. Calculated
based on the number of active sites in 1. The yields were determined
by H NMR spectra.
c
1
D
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Table 2. Yields of Various Cyclic Carbonates Prepared from the Cycloaddition of CO2 with Corresponding Epoxides Catalyzed
a
by 1 with and without TBAB
a
b
1
Reaction conditions: for catalyst 1, 12 h, 90 °C, CO2 1 atm. For catalyst 1−TBAB, 3 h, 90 °C, CO2 1 atm. The yields were determined by H
NMR spectra (Supporting Information).
273 K under 1 atm, which is similar to the reported CO2
loading amount for UiO-67 (22.9 cm3 g−1 at 298 K, and 48.6
cm3 g−1 at 273 K) under the same conditions.5c,10d Compared
to UiO-67, no decrease in CO2 uptake capacity of 1 was
observed although the involved large-sized imidazolium groups
have led to largely decreased surface area and pore volume in 1
as mentioned above.
Table 3. UiO-67 or UiO-67-TBAB Catalyzed
Epichlorohydrin CO2 Cycloaddition
a
catalyst
(mol %)
cocatalyst
(mol %)
conversion
(%)
entry catalyst
t (h)
TOF
1
2
3
4
5
6
UiO-
67
1.5
1.5
1.5
0.7
0.7
0.7
none
3
<3
<3
8
UiO-
67
none
8
14
3
Differently from CO2 uptake, only a small amount of CH4
(6.0 cm3 g−1) or N2 (1.2 cm3 g−1) uptake was observed based
on 1 at 298 K under 1 atm pressure. As we know, isosteric
analysis of CO2 adsorption isotherms collected at various
temperatures allows an estimation of the coverage-dependent
isosteric heat of adsorption (Qst),10d where the behavior of this
function is determined by the relative magnitudes of the
adsorbent−adsorbate and adsorbate−adsorbate interactions.
On the basis of CO2 adsorption isotherms at 273 and 298 K,
the Qst of CO2 was then calculated by the virial method
(Supporting Information). As shown in Figure 4, the Qst value
for 1 is 25−27 kJ mol−1, being much higher than that of 15.8−
15.9 kJ mol−1 for UiO-67.10d The higher Qst of 1 can be
ascribed to the high density of open imidazolium sites on 1. To
predict the selective CO2 separation performance over N2 and
CH4 at 298 K, the selectivity for 50:50 CO2/CH4 and CO2/N2
binary mixtures was estimated up to 1 atm from the single-
component gas-adsorption isotherms utilizing ideal adsorbed
solution theory (IAST) based on a dual-site Langmuir−
Freundlich (DSLF) simulation. As indicated in Figure 4, the
CO2/CH4 selectivity of 1 calculated for a 50:50 CO2/CH4
mixture is 4.9 at 1 atm and 273 K, and the CO2/N2 selectivity
calculated for a 50:50 CO2/N2 mixture is 33.3 at the same
conditions. The selectivity for CO2/CH4 and CO2/N2,
UiO-
67
none
6.67
22.03
18.22
8.09
UiO-
67
TBAB, 1
TBAB, 1
TBAB, 1
46
82
98
UiO-
67
8
UiO-
67
14
a
Reaction conditions: solvent free, CO2 (1 atm), 90 °C.
surface area was found to be 846 m2/g. These values are
significantly smaller than those of pristine UiO-67 (N2
absorption amount, 710 cm3/g; surface area, 2113 m2/g).
The corresponding pore volume of 1 based on the maximum
nitrogen adsorption is 0.43 cm3/g (Figure 2), which is less than
half compared with its parent UiO-67 (1.08 cm3/g). Such
differences are clearly caused by the introduced imidozalium
groups which inevitably occupy the framework pores’ place.
CO2 Selective Adsorption. Considering high porosity
together with the incorporation of exposed Lewis acid sites and
the high-density imidazolium groups within 1, its adsorption
isotherms on the evacuated 1 for CO2, CH4, and N2 were
examined, respectively. As illustrated in Figure 3, 1 gives a CO2
uptake amount of 22.4 cm3 g−1 at 298 K and 47.7 cm3 g−1 at
E
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Figure 5. Left: Reaction time examination (black line) and leaching test (red line) for CO2 cycloaddition of epichlorohydrin catalyzed by 1. Reaction
conditions: solvent free, CO2 (1 atm), 90 °C, 1 (1.5, mol %). The solid catalyst of 1 was filtrated from the reaction solution after 2.5 h, whereas the
filtrate was transferred to a new vial and reaction was performed under the same conditions for an additional 9.5 h. Middle: Recycling catalytic test.
Yields obtained at 8 h in repeated runs of the CO2 cycloaddition with epichlorohydrin catalyzed by 1 (solvent free, CO2 (1 atm), 90 °C, 1 (1.5, mol
%)). Yield %: run 1, 99%; run 2, 99%; run 3, 97%; run 4, 97%; run 5, 96%. Right: Corresponding PXRD patterns for each catalytic cycle.
Table 4. Summary of the Reported CO2 Cycloaddition of Epichlorohydrin Catalyzed by MOFs and MOF−TBAB Catalytic
Systems
entry
catalyst
catalyst (mol %)
cocatalyst (mol %)
t (h)
P (MPa)
T (°C)
yield (%)
ref
1
USTC-253-TFA
ZIF-8
1
TBAB, 6.5
none
72
4
0.1
0.7
1.2
0.1
0.1
0.1
1
25
100
80
38.2
98.2
89.2
93
12
13
14
15
16
16
17
18
19
20
21
22
2
2.4
1.6
0.3
0.2
0.2
0.49
0.64
1
3
CHB(M)
TBAB, 1.6
TBAB, 2.5
TBAB, 10
TBAB, 10
none
6
4
MOF-5
12
48
48
3
50
5
(Cu4L1)n
25
85
6
HKUST-1
IL-ZIF-90
25
56
7
120
25
94
8
UMCM-1-NH2
Ni-MOF
TBAB, 0.64
TBAB, 3
TBAB, 7
TBAB, 10
none
24
4
1.2
2
55
9
100
25
84
10
11
12
13
14
MMPF-18
Cu3(BTC)2
Meim-UiO-66
UiO-67-IL (1)
UiO-67-IL (1)
0.25
2.74
0.745
0.7
1.5
48
4
0.1
0.7
0.1
0.1
0.1
99
100
120
90
63.8
93
24
3
TBAB, 1
none
99
this work
this work
8
90
95
however, is as high as 7.6 and 381.4 at 1 atm and 298 K,
respectively (Figure 4). On the basis of measured adsorption
data for 1 and UiO-67 collected at 273 and 298 K, there is a
clear affinity for CO2 according to the order 1 > UiO-67, which
clearly resulted from the introduced imidazolium functional
groups.
CO2 Cycloaddition Catalyzed by 1 and 1−TBAB. With
the above information in hand, we set out to study the catalytic
CO2 cycloaddition reactions based on 1. The CO2 cyclo-
addition with epoxides was performed under 1 atm pressure
because 1 has a good affinity toward CO2.
In a typical experiment, CO2 cycloaddition with epichlor-
ohydrin was chosen as a model reaction to optimize the
reaction conditions. As shown in Table 1, the pure 1 or
cocatalyst TBAB showed a low catalytic efficiency for the CO2
cycloaddition with epichlorohydrin at room temperature (Table
1, entries 1 and 2). The catalytic performance of 1 together
with cocatalyst TBAB exhibited a positive correlation with the
increase of reaction temperature and reaction time. For
example, the yields of cycloaddition in the presence of 1 (0.7
mol %) and TBAB (1.0% mol) increased from 80 to 93% with
the reaction time extending from 48 to 72 h at 25 °C (Table 1,
entries 3 and 4). The yield, however, was up to 77% at 50 °C
and 99% at 90 °C only over 3 h (Table 1, entries 5 and 6),
respectively. The catalytic performance exceeded the bench-
mark of other reported poly(ionic liquid) with similar yield
under higher pressure of 1 MPa.11 On the other hand, 1 is also
able to efficiently promote this CO2 cycloaddition in the
absence of cocatalyst TBAB. As indicated in Table 1 (entries
7−10), the maximal yield is 80% at 50 °C over 48 h. When the
reaction was carried out at 90 °C, the yield increased to 88%
within 8 h (Table 1, entries 12 and 13). Notably, a 95% yield of
the carbonate product was obtained at 90 °C in 12 h with 1.5
mol % amount of 1 (Table 1, entries 14−16).
The scope and generality of the present catalytic CO2
cycloaddition can be further extended to other kinds of
epoxides. Under the optimized reaction conditions, cyclo-
addition of CO2 with epibromohydrin gave excellent yields with
or without TBAB (Table 2, entries 1 and 2), which are
comparable with those of epichlorohydrin. For substrate of
styrene oxide, the corresponding cyclic carbonate was obtained
in 51% yield in the presence of only 1 (1.5 mol %, Table 2,
entry 5). However, the yield was significantly improved (up to
98%) by addition of the cocatalyst TBAB (Table 2, entry 6).
Similarly, the reaction of glycidyl phenyl ether with CO2 gave a
moderate yield (52%, Table 2, entry 7) catalyzed by only 1
under the reaction conditions, while the cyclic carbonate was
obtained in 99% yield with the assistance of TBAB (Table 2,
entry 8). Compared to aromatic epoxides, the long chain
substituted epoxide 1,2-epoxydecane, however, gave much
lower yields even in the presence of TBAB as cocatalyst (entries
9 and 10), which might result from the weak interactions
between the substrates and the MOF framework.
F
DOI: 10.1021/acs.inorgchem.6b03169
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
For further confirming catalytic function of 1, a series of
control experiments were carried out. The related results are
summarized in Table 3. As shown in Table 3, almost no
catalytic activity of pure UiO-67 has been observed even when
the reaction was performed at 90 °C for 8 h (yield, <3%, Table
3, entries 1 and 2). The carbonate was obtained only in 8%
yield even after 14 h (Table 3, entry 3). In contrast, 1 with
imidazolium groups gave an excellent yield of 95% under the
same conditions (Table 1). On the other hand, the UiO-67-
TBAB catalytic system led to an incremental yield for
epichlorohydrin CO2 cycloaddition as time went on, and
arrived at a maximal yield of 98% over 14 h (Table 3, entries 4−
6); meanwhile the 1−TBAB system afforded 99% yield within
only 3 h (Table 1). Therefore, the embedded imidazolium
group in 1 did play a key role in this CO2 cyclic addition
reaction.
In order to gain insight into the heterogeneous nature of 1,
the hot leaching test was performed. As shown in Figure 5, no
further reaction occurred without 1 after initiation of the
epichlorohydrin CO2 cycloaddition at 2.5 h, indicating that no
leaching of the catalytically active sites occurs and that 1
features a typical heterogeneous catalyst nature. After that, we
examined the recyclability of 1. After each catalytic run, the
solid catalyst of 1 could be easily collected by centrifugation,
washed with ethanol, dried at 90 °C, and reused in the next run
under the same conditions. Within five catalytic runs, yields of
between 99 and 96% were obtained (Figure 5). The PXRD
patterns of 1 and those after reuse within five catalytic runs
indicate that the structural integrity of 1 was well-preserved
(Figure 5).
Table 4 shows some recent progress of MOF and MOF−
TBAB heterogeneous catalysts for CO2 cycloaddition of
epichlorohydrin. Compared to various reported MOF catalysts
(Table 4), 1 herein exhibits very good catalytic performance for
this CO2 cyclic addition in the form of catalyst as either 1 or a
1−TBAB composite system. So, it could be a new valuable
family member for MOFs to heterogeneous epoxide CO2
cycloaddition catalysts.
In summary, we report a new bifunctional robust and highly
porous imidazolium-based ionic liquid decorated UiO-67 type
MOF (UiO-67-IL, 1) which was prepared by a ligand-
modification approach. It exhibits a highly selective adsorption
for CO2 over CH4 and N2. Furthermore, 1 herein can be used
as a highly active heterogeneous catalyst for CO2 cycloaddition
with epoxides under atmospheric pressure with or without
cocatalyst TBAB (n-Bu4NBr).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from NSFC (Grants
21671122, 21604049, 21475078, and 21271120), 973 Program
(Grant 2013CB933800), Shandong Provincial Natural Science
Foundation (BS2015CL015), China, and A Project of
Shandong Province Higher Educational Science and Technol-
ogy Program (J15LC05). We thank Prof. S. L. Qiu, Dr. T. Ben
(Jilin University), Prof. H. L. Jiang, and Dr. M. Ding
(University of Science and Technology of China) for very
helpful discussion.
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ASSOCIATED CONTENT
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* Supporting Information
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1
Calculation of Qst and H NMR, FTIR, IR, and MS
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AUTHOR INFORMATION
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Corresponding Authors
*E-mail: yaobingjian1986@163.com.
*E-mail: yubindong@sdnu.edu.cn.
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