A Series of Metal-Organic Frameworks for Selective CO2 Capture and Catalytic Oxidative Carboxylation of Olefins

Article abstract of DOI:10.1021/acs.inorgchem.8b02293

Three new lanthanide-based metal-organic frameworks (Ln-MOFs), namely MOF-590, -591, and -592 constructed from a tetratopic linker, benzoimidephenanthroline tetracarboxylic acid (H₄BIPA-TC), were synthesized under solvothermal conditions and fully characterized. All of the new MOFs exhibit three-dimensional frameworks, which adopt unprecedented topologies in MOF field. Gas adsorption measurements of MOF-591 and -592 revealed good adsorption of CO₂ (low pressure, at room temperature) and moderate CO₂ selectivity over N₂ and CH₄. Consequently, breakthrough experiments illustrated the separation of CO₂ from binary mixture of CO₂ and N₂ with the use of MOF-592. Accordingly, MOF-592 revealed the selective CO₂ capture effectively without any loss in performance after three cycles. Moreover, MOF-590, -591, and -592 showed to be catalytically active in the oxidative carboxylation of styrene and CO₂ for a one-pot synthesis of styrene carbonate under mild conditions (1 atm CO₂, 80 °C, and without solvent). Among the new materials, MOF-590 revealed a remarkable efficiency with exceptional conversion (96%), selectivity (95%), and yield (91%).

Full text of DOI:10.1021/acs.inorgchem.8b02293

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Series of Metal−Organic Frameworks for Selective CO2 Capture
and Catalytic Oxidative Carboxylation of Olefins
‡
‡
†
,§
Huong T. D. Nguyen,†,‡ Y. B. N. Tran, Hung N. Nguyen, Tranh C. Nguyen, Felipe Gandara,*
́
and Phuong T. K. Nguyen*,†,‡
†Faculty of Chemistry, University of Science, Vietnam National University − Ho Chi Minh City, Ho Chi Minh City 721337,
Vietnam
‡Center for Innovative Materials and Architectures (INOMAR), Vietnam National University − Ho Chi Minh City (VNU-HCM),
Ho Chi Minh City 721337, Vietnam
§Departamento de Nuevas Arquitecturas en Química de Materiales, Materials Science Factory, Instituto de Ciencia de Materiales de
́
Madrid (ICMM-CSIC), Sor Juana Ines de la Cruz 3, Cantoblanco 28049, Madrid, Spain
S
* Supporting Information
ABSTRACT: Three new lanthanide-based metal−organic frame-
works (Ln-MOFs), namely MOF-590, -591, and -592 constructed
from a tetratopic linker, benzoimidephenanthroline tetracarbox-
ylic acid (H4BIPA-TC), were synthesized under solvothermal
conditions and fully characterized. All of the new MOFs exhibit
three-dimensional frameworks, which adopt unprecedented
topologies in MOF field. Gas adsorption measurements of
MOF-591 and -592 revealed good adsorption of CO2 (low
pressure, at room temperature) and moderate CO2 selectivity
over N2 and CH4. Consequently, breakthrough experiments
illustrated the separation of CO2 from binary mixture of CO2 and
N2 with the use of MOF-592. Accordingly, MOF-592 revealed
the selective CO2 capture effectively without any loss in performance after three cycles. Moreover, MOF-590, -591, and -592
showed to be catalytically active in the oxidative carboxylation of styrene and CO2 for a one-pot synthesis of styrene carbonate
under mild conditions (1 atm CO2, 80 °C, and without solvent). Among the new materials, MOF-590 revealed a remarkable
efficiency with exceptional conversion (96%), selectivity (95%), and yield (91%).
candidates for the applications of gas capture6−9 and
INTRODUCTION
■
heterogeneous acid-catalyzed reactions.10−13
Metal−organic frameworks (MOFs) are crystalline materials
The selective adsorption of CO2 and subsequent trans-
that are formed by the linkage of inorganic and organic units
formation into fine chemicals is receiving great interest due to
via strong bonds resulting in porous structures.1 Based on the
these application’s environmental implications.14,15 Numerous
principles of reticular chemistry, MOFs are designed by the
appropriate choice of inorganic and organic building units,
studies have been devoted to the development of MOFs for
reversible CO2 adsorption based on the modification of their
with well-defined geometrical features, to produce structures
structural features and the presence of specific chemical
with targeted topologies.1,2 For those MOFs constructed from
lanthanide-based inorganic clusters linked by multicarboxylic
acid linkers, an extensive number of different three-dimen-
sional (3D) networks have been reported.3 This is primarily
due to the high variability in coordination environment of
lanthanide cations. The use of Ln-based building units in the
construction of new MOFs is attractive though because Ln
cations typically have a high density of coordinated solvent
ligands, which, when removed, can produce unsaturated metal
sites that serve as gas binding or Lewis acidic sites.3 Moreover,
hydroxyl ligands derived from water molecules binding to
metal clusters have been exploited for their Lewis and/or
Brønsted acidity in catalytic reactions.4,5 Therefore, lanthanide-
based metal−organic frameworks (Ln-MOFs) are promising
functionalities.16−20 In this sense, the synthesis of cyclic
carbonate from chemical fixation of CO2 is an interesting
approach to use CO2 as an abundant and nontoxic chemical
feedstock.21 There are extensive studies focused on the
employment of new catalysts in the formation of cyclic
carbonates from cycloaddition of CO2.5,22−27 Furthermore, the
direct synthesis of cyclic carbonate from olefins, so-called one-
pot “oxidative carboxylation”, is an attractive and convenient
approach because the transformation utilizes immediately the
olefin without the requirement of isolating and purifying an
epoxide substrate.28 However, low catalytic efficiency in the
Received: August 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02293
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Inorganic Chemistry
Article
determine the products of the catalytic reaction. To determine the
conversion, selectivity, and yield of catalytic reactions, GC analyses
were carried out using an Agilent GC System 123−0132 equipped
with a flame ionization detector (FID) and a capillary DB-1 ms
column (30 m × 320 μm × 0.25 μm) with the use of biphenyl as an
internal standard.
formation of cyclic carbonates and the appearance of side
products obtained during the reaction limit its applicability,
and only a few efficient catalytic systems and MOF catalysts
have been exploited for this transformation up to now.22,29−31
Therefore, it is still necessary to develop heterogeneous
catalysts suitable for the efficient synthesis of cyclic carbonates
through the oxidative carboxylation reaction.
X-ray Diffraction Analysis. Single-crystal X-ray diffraction
(SCXRD) analysis for MOF-590 and -592 was collected at 100 K
on a Bruker four circle κ-diffractometer equipped with a Cu
INCOATED microsource, operated at 30 W power (45 kV, 0.60
mA) to generate Cu Kα radiation (λ = 1.54178 Å), with a Bruker
VÅNTEC 500 area detector (MICROGAP technology). Single-
crystal X-ray diffraction data for MOF-591 was performed on a Bruker
D8 Venture diffractometer using monochromatic micro focus Cu Kα
(λ = 1.54178 Å) radiation source, operated at 50 W (50 kV, 1.0 mA)
and equipped with a PHOTON 100 CMOS detector (100 K). The
Bruker APEX3 program35 was used for analysis of the raw data
collection, and then SAINT36 was applied for data reduction. An
absorption correction was conducted using SADABS37 routines. The
crystal structures were solved by intrinsic phasing (SHELXT) and
refined by full-matrix least-squares on F2 (SHELXL-2014).38 The
solvent masking procedure in Olex239 program package was used to
remove residual electron density from highly disordered solvent
molecules, which were located in the pores of the structures. Powder
XRD (PXRD) patterns were measured using a Bruker D8 Advance
diffractometer with Ni-filtered Cu Kα (λ = 1.54178 Å) radiation
operated at 40 kV, 40 mA (1,600 W). The system was outfitted with
an antiscattering shield to avoid incident diffuse radiation from hitting
the detector. MOF samples were mounted on zero background
holders and then well-flatted by a spatula. PXRD measurements were
conducted with the 2θ range from 3 to 50°, a step size of 0.02°, and a
fixed count time of 1 s per step.
Synthesis of MOF-590, [Nd2(BIPA-TC)1.5]·8H2O. One mL of a
solution of neodymium(III) nitrate hexahydrate (0.04 M, in H2O)
and 1 mL of a 0.04 M solution of H4BIPA-TC dissolved in
dimethylacetamide (DMAc) were inserted into a Pyrex tube (o.d. ×
i.d. = 10 mm × 12 mm). This was followed by the addition of
deionized water (4 mL) and glacial acetic acid (0.1 mL). Then, the
tube was quickly sealed, sonicated for 30 min, and heated at 120 °C
for 3 days to get yellow rectangular-shaped crystals. The as-
synthesized MOF-590 crystals were then thoroughly washed with
DMAc (3 × 10 mL per day, for 3 days) and subsequently immersed in
anhydrous ethanol (EtOH, 4 × 10 mL per day, for 3 days). To obtain
guest-free material, the ethanol-exchanged sample was evacuated
under reduced pressure at room temperature for 18 h, followed by
Herein, we report the synthesis and characterization of three
new Ln-MOFs prepared with a tetratopic linker, benzoimide-
phenanthroline tetracarboxylic acid (H4BIPA-TC).32 This
linker possesses a planar, π-acidic naphthalene core with
diimide functionality, which favors π−π stacking interactions.33
The resulting new MOFs, [Nd2(BIPA-TC)1.5]·8H2O, [Eu-
(H2BIPA-TC)(BIPA-TC)0.5·8H2O][NH2(CH3)2], and [Tb-
(H2BIPA-TC)(BIPA-TC)0.5·6H2O][NH2(CH3)2] (denoted
MOF-590, -591, and -592, respectively) exhibit 3D architec-
tures with unprecedented topologies in MOF chemistry.34
MOF-592 was proved effective as a potential candidate for the
separation of CO2 from a gas mixture of CO2 and N2. In
addition, MOF-590 was demonstrated to have remarkable
catalytic activity on the oxidative carboxylation of styrene and
CO2 in the formation of styrene carbonate under mild
conditions (0.18 mol % of MOF catalyst, 1 atm of CO2, 80
°C, 10 h), compared to other reported MOFs. Furthermore,
MOF-590 was shown to be a robust catalyst and exhibited
heterogeneous nature with no significant loss of catalytic
performance over five consecutive cycles.
EXPERIMENTAL SECTION
■
Materials and General Procedures. The general procedures,
starting materials, and synthesis of benzoimidephenanthroline
tetracarboxylic acid (H4BIPA-TC) linker can be found in the
Supporting Information (SI), Sections S1 and S2. All chemicals for
linker, MOF synthesis, and the catalytic reactions were purchased and
used without purification. For comparison studies, commercial MOFs
[HKUST-1 (Basolite C300), MOF-177 (Basolite Z377), ZIF-8
(Basolite Z1200), and Al-MIL-53 (Basolite A100)] were acquired
and re-activated to obtain guest-free materials prior to conduct any
measurement. Mg-MOF-74, UiO-67-bpydc, and Nd-BDC were
prepared according to procedures previously reported (SI, Section
S1).22
Elemental microanalyses (EA) were performed using a LECO
CHNS-932 Analyzer. Fourier transform infrared (FT-IR) spectra
were obtained on a Bruker Vertex 70 with samples being well-
dispersed in KBr pellets and the output signals are depicted as vs, very
strong; s, strong; m, medium; sh, shoulder; w, weak; vw, very weak; or
br, broad. Thermal gravimetric analysis (TGA) curves were measured
on a TA Q500 thermal analysis system with the sample held in a
platinum pan under a continuous flow of air. Low-pressure N2, CO2,
and CH4 adsorption isotherms were collected on a Micromeritics
3Flex with the use of He to estimate the dead space. A liquid N2 bath
was used for measurements at 77 K, and a circulating bath of ethylene
glycol/water (1/1, v/v) was applied for measurements at 273, 283,
and 298 K. Breakthrough measurements were performed using a L&C
Science and Technology PSA-300-LC Analyzer with the bed column
dimensions of 14 × 0.635 cm (l. × i.d). A ThermoStar GSD320 mass
spectrometer was equipped with the system for monitoring of CO2,
N2, H2O, and O2 from the gaseous effluent goes through the sample
bed. Ultrahigh-purity grade N2, CH4, and He gases (99.999% purity)
and high-purity grade CO2 (99.995%) were used for all sorption
experiments. 1H and 13C nuclear magnetic resonance spectra (1H
NMR, 13C NMR) were recorded on a Bruker Advance II 500 MHz
spectrometer. Gas chromatography (GC) analyses were performed on
an Agilent GC System 19091s-433 equipped with a mass selective
detector (Agilent 5973N) (GC-MS) using a capillary HP-5MS 5%
phenyl methyl silox column (30 m × 250 μm × 0.25 μm) to
heating at 60 °C for an extra 24 h. E.A.: Calcd for Nd2C45H31N3O26
=
[Nd2(BIPA-TC)1.5]·8H2O: C, 41.00; H, 2.37; and N, 3.19%. Found:
C, 41.26; H, 2.84; and N, 3.52%. FT-IR (KBr, 4000−400 cm−1): 3430
(s, br), 1715 (s), 1678 (vs), 1620 (s), 1548 (s), 1451 (m), 1382 (m),
1349 (vs, sh), 1249 (s, sh), 1194 (w), 1119 (w).
Synthesis of MOF-591, [Eu(H2BIPA-TC)(BIPA-TC)0.5·8H2O]-
[NH2(CH3)2]. 0.6 mL of a 0.043 M solution of europium(III) nitrate
pentahydrate dissolved in deionized water and 15 mg of H4BIPA-TC
(0.025 mmol) were placed in a 4 mL vial. This was followed by the
addition of DMF (1 mL), deionized water (0.2 mL), and glacial acetic
acid (0.25 mL). The vial was sealed and sonicated, then heated to 100
°C for 24 h to produce yellow rectangular-shaped crystals. The as-
synthesized MOF-591 crystals were then thoroughly washed with
DMF (3 × 10 mL per day, for 3 days) and subsequently immersed in
anhydrous EtOH (4 × 10 mL per day for 3 days). To obtain the
guest-free material, the ethanol-exchanged sample was evacuated
under reduced pressure at room temperature for 18 h before heating
at 50 °C for an additional 24 h. E.A.: Calcd for EuC47H41N4O26
=
[Eu(H2BIPA-TC)(BIPA-TC)0.5·8H2O][NH2(CH3)2]: C, 45.90; H,
3.36; and N, 4.56%. Found: C, 45.83; H, 3.39; and N, 4.44%. FT-IR
(KBr, 4000−400 cm−1): 3428 (s, br), 1713 (s), 1676 (vs), 1615 (s),
1577 (s), 1451 (m), 1389 (m), 1349 (vs, sh), 1295 (s, sh), 1196 (w),
1120 (w).
Synthesis of MOF-592, [Tb(H2BIPA-TC)(BIPA-TC)0.5·6H2O]-
[NH2(CH3)2]. 0.6 mL of a 0.08 M solution of terbium(III) nitrate
B
DOI: 10.1021/acs.inorgchem.8b02293
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Inorganic Chemistry
Article
hydrate dissolved in deionized water and 15 mg of H4BIPA-TC
(0.025 mmol) were placed in a 4 mL vial. Following, DMF (1 mL),
deionized water (0.2 mL), and glacial acetic acid (0.2 mL) were
added to the vial. The vial was sealed and sonicated, then heated to
100 °C for 24 h to get yellow crystals with rectangular shape. The as-
synthesized MOF-592 crystals were then thoroughly washed with
DMF (3 × 10 mL per day, for 3 days) and subsequently immersed in
anhydrous EtOH (4 × 10 mL per day, for 3 days). The ethanol-
exchanged sample was evacuated under reduced pressure at room
temperature for 18 h before heating at 50 °C for 24 h. E.A.: Calcd for
Table 1. Crystal Data and Structure Refinement for MOF-
590, MOF-591, MOF-592
MOF-590
MOF-591
MOF-592
empirical
formula
C45H15N3Nd2O26
C47H31EuN4O21
C53H28N6O22Tb
formula
weight
1302.08
1139.72
1259.73
(g mol−1
)
crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
triclinic
P-1
triclinic
P-1
triclinic
P-1
TbC47H37N4O24
=
[Tb(H2BIPA-TC)(BIPA-TC)0.5·6H2O]-
[NH2(CH3)2]: C, 47.01; H, 3.11; and N, 4.67%. Found: C, 47.28;
H, 3.21; and N, 4.60%. FT-IR (KBr, 4000−400 cm−1): 3425 (s, br),
1712 (s), 1677 (vs), 1615 (s), 1578 (s), 1450 (m), 1384 (m), 1349
(vs, sh), 1252 (s, sh), 1199 (w), 1120 (w).
10.2605(9)
12.9996(15)
19.8518(17)
88.684(6)
82.492(5)
73.636(5)
2518.6(4)
2
10.6709(12)
18.1669(19)
18.767(2)
109.774(2)
102.201(2)
97.897(2)
3258.8(6)
2
10.6566(3)
18.1300(5)
19.5006(5)
65.601(2)
74.964(2)
81.317(2)
3309.53(17)
2
One-Pot Oxidative Carboxylation of Styrene and CO2. In a
model experiment, the catalytic reaction was conducted using styrene
(3.9 mmol), activated MOF catalyst (0.18 mol % ratio based on the
molecular weight, [Nd2(BIPA-TC)1.5]·8H2O), [Eu(H2BIPA-TC)-
(BIPA-TC)0.5·8H2O][NH2(CH3)2], [Tb(H2BIPA-TC)(BIPA-
TC)0.5·6H2O][NH2(CH3)2] for MOF-590, -591, and -592), anhy-
drous tert-butyl hydroperoxide (TBHP in decane, 7.4 mmol), and
nBu4NBr (0.31 mmol, 8 mol %) in a 25 mL Schlenk tube. The
mixture reaction was evacuated thoroughly to remove gas impurities,
then purged three times with CO2 and kept at a constant pressure
(∼1 atm) with the use of a balloon filled with CO2. The reaction
mixture was stirred at 80 °C for 10 h and monitored by analyzing with
GC regularly taken aliquots. After completion of reaction, the mixture
was cooled down in an ice bath, and the unreacted CO2 was purged.
The MOF catalyst was removed by centrifugation, and an aliquot of
the reaction was analyzed by GC-FID using biphenyl as the internal
standard to determine the catalytic conversion, selectivity, and yield of
reaction. For the recycling experiment, the recovered catalyst was
rinsed with ethyl acetate (3 × 3 mL) before heating at 50 °C under
vacuum for 24 h and then reused for the next cycles.
Z
ρcalc(g/cm3)
1.717
1.162
1.264
μ(Cu Kα)
16.365
7.446
5.874
(mm−1
)
Rint
R1
wR2
0.1018
0.0603
0.1575
0.2496
0.0871
0.2231
0.1031
0.0566
0.1585
a
b
a
b
2
R1 = ∑∥F0| − |Fc∥/∑|F0|. wR2 = [∑w(F0 − Fc2)2/w(F0)2]1/2 for
3514 reflections satisfying I > 2σ(I) (MOF-590), for 3428 reflections
satisfying I > 2σ(I) (MOF-591), for 8672 reflections satisfying I >
2σ(I) (MOF-592).
RESULTS AND DISCUSSION
■
Crystal Structures of MOF-590 to -592. The ToposPro
(v.5.3) software program40 was employed to simplify the
structures of MOF-590 to -592 and to classify the respective
underlying net. The tetratopic linker was considered as two
triangular nodes.41 The details of structural elucidation are
provided in the SI, Section S4.
MOF-590. The single-crystal structure of MOF- 590 was
solved in the triclinic P-1 space group (no. 2) with lattice
parameters a = 10.260 Å, b = 12.999 Å, c = 19.851 Å, α =
88.684(6)°, β = 82.492(5)°, and γ = 73.636(5)° (Table 1). Its
asymmetric unit contains two crystallographically independent
Nd(III) ions, one and half BIPA-TC4− ligands, and eight
coordinated water molecules. In MOF-590’s structure, the first
Nd atom, Nd1, is bound to six carboxylate-O atoms from four
BIPA-TC ligands and three additional water ligands. The
corresponding simplified secondary building unit (SBU)
adopts a tetrahedral geometry. The second Nd center, Nd2,
forms a dimeric SBU, with two bridging and two chelating
carboxylate groups. Five water ligands complete the coordina-
tion sphere of each one of the Nd atoms. The simplified SBU
can be viewed as a parallelogram, with four points of extensions
(Figure 1A). Interestingly, the overall framework topology of
MOF-590 is a new net, denoted nkp, with a point symbol of (4
× 102)2(4.6.84)2(4.62)2(42.62.82)(62.8)2 due to the linkage of
two distinct types of 4-c inorganic SBUs and three distinct
types of 3-c nodes upon simplifying the organic linkers (Figure
1B,C and SI, Section S4, Figure S7). According to PLATON
calculations,42 MOF-590 has a ∼15.1% void space in fully
desolvated form, and the pore volume is 0.085 cm3 g−1.
Figure 1. Single-crystal structure of MOF-590. (A) Connection of
BIPA-TC and tetrahedral-shaped [Nd(-COO)4(H2O)3]− and paral-
lelogram-shaped [Nd2(-COO)4(H2O)10]2+ SBUs result in (B) MOF-
590. (C) The structure of MOF-590 exhibits the new nkp topology.
Color scheme: Nd, blue and orange polyhedra; C, black; O, red; N,
green; all H atoms are omitted for clarity.
MOF-591 and MOF-592. SCXRD analyses revealed that
MOF-591 and -592 are isostructural frameworks, with identical
C
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connections and conformations of Eu and Tb, respectively. We
found that the diffracting quality of the MOF-591 crystals was
lower than that of the MOF-592 (SI, Section S3). Thus, we
describe here in detail the crystal structure of MOF-592 as a
representative example. Accordingly, MOF-592 crystallizes in
the triclinic P-1 space group (no. 2) with lattice parameters of
a = 10.657 Å, b = 18.130 Å, c = 19.501 Å, α = 65.601(2)°, β =
74.964(2)°, and γ = 81.317(2)° (Table 1). Its asymmetric unit
contains one crystallographically independent Tb(III) ion, one
partially deprotonated H2BIPA-TC2− ligand, a half fully
deprotonated BIPA-TC4− ligand, one coordinated water
molecule, two coordinated DMF molecules, one dimethylam-
monium (DMA+) cation, and one water molecule. The Tb
cluster in MOF-592 framework is found to be a dinuclear
[Tb2(COO)8(DMF)4(H2O)2]2− SBU (Figure 2A). In partic-
of 8-c inorganic SBUs and three different kinds of 3-c nodes
derived from the organic linkers (Figure 2B,C, and SI, Section
S4, Figure S8). According to PLATON,42 the void space of
MOF-592 in its fully desolvated form is 47.6%, and the pore
volume is 0.43 cm3 g−1.
Structural Characterization, Including Porosity, and
CO2 Adsorption Properties. The synthesis of bulk MOF-
590 to -592 was confirmed by PXRD analysis, whereas the
diffraction patterns of as-synthesized samples were in agree-
ment with those simulated from the single-crystal data (SI,
Section S5, Figures S9−S11). The solvent-exchanged MOF-
590 to -592 were activated under vacuum, first at room
temperature for 16 h, and then heating at 50 °C for 24 h. The
PXRD profiles of the activated MOF-590 and -591 exhibited
no appreciable changes compared with those of the
corresponding as-synthesized samples, confirming that the
structural integrity of MOF-590 and -591 was fully retained
after activation (SI, Figures S9−S10). Otherwise, the PXRD
pattern of activated MOF-592 showed broadened and slightly
shifted peaks, suggesting a possible structural change attributed
to the loss of guest solvent molecules (SI, Figure S11). In
addition, chemical stability tests were also conducted on the
Ln-MOFs, in which all activated Ln-MOF samples were
immersed in water, acidic (pH = 4.0, aqueous acetic acid), and
basic conditions (aqueous sodium hydroxide, pH = 12) at
room temperature for 1 week. In the case of MOF-590, a loss
of crystallinity was appreciated for the basic treated sample,
while only differences in the high-angle area diffraction peaks
were observed for the water and acid treated samples, as
expected (SI, Figure S12). On the contrary, MOF-591 and
-592 were noted to be unstable under the selected basic or acid
conditions.
The thermal stability of MOF-590 to -592 was evaluated by
recording the thermal gravimetric analysis (TGA) on the
activated MOF-590 to -592 under airflow (SI, Section S6).
Accordingly, the TGA curves exhibited no considerable weight
loss up to 400 °C, indicating the high thermal stability of these
Ln-MOFs (SI, Figures S13 and S15). The significant weight
loss observed at ∼400 °C in each TGA curve was associated
with framework decomposition. The analyzed metal oxide
residue weight percent for MOF-590 (27.6%), MOF-591
(15.2%), and MOF-592 (16.5%) was found to be in agreement
with the corresponding calculated value according to elemental
microanalysis (25.3, 14.3, and 15.2%, respectively).
The porosity of MOF-590 to -592 was assessed by
performing the N2 adsorption isotherms at 77 K. Accordingly,
MOF-590 exhibited a nonporous behavior due to the restricted
pore size and narrow pore volume natures. Otherwise, MOF-
591 and -592 revealed Type-I isotherm, typical of microporous
materials (SI, Figure S16). Furthermore, the isotherm of MOF-
592 shows a small additional step with hysteresis around P/P0
= 0.05, indicating framework flexibility or guest molecule
Figure 2. Single-crystal structure of MOF-592. (A) Connection of
BIPA-TC and [Tb2(-COO)8(DMF)4(H2O)2]2− SBUs results in (B)
MOF-592. (C) MOF-592 exhibits the new (3,3,8)-connected
topology. Atom colors: Tb, blue polyhedra; C, black; O, red; N,
green; H of protonated -COOH group, pink spheres; all H atoms,
except those involved in hydrogen bonding, are omitted for clarity.
ular, two carboxylate moieties coordinate in a bridging fashion
to two Tb cations, two carboxylate functionalities connect in
dimonodentate fashion, and four carboxylate groups are in
monodentate type. Solvent molecules, including of DMF and
water, complete the coordination spheres of the Tb atoms, and
DMA+ cations act as counterions occupying the pores.
Interestingly, there are hydrogen bonds between the
protonated carboxylate groups of the linkers, which construct
a zigzag chain (H2BIPA-TC)2− that extends along the [100]
n
direction. Topological analysis reveals that the 3D framework
of MOF-592 belongs to a new trinodal topology with a point
symbol of (4.62)2(42.612.814)(62.8)4 resulting from the linking
Table 2. Summary of the Surface Area, CO2 Uptake Capacity, and CO2/N2 and CO2/CH4 Selectivities for MOF-591 and MOF-
592
ABET
CO2 uptake
N2 uptake
CH4 uptake
Qst
CO2/N
CO2/CH4
d
a
b
b
b
c
2d
MOF
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(kJ mol−1
)
selectivity
selectivity
MOF-591
MOF-592
960
900
36
42
2.3
2.1
7.4
7.0
23
22
21
27
5.6
6.8
a
b
c
d
Calculated by BET method. At 800 Torr and 298 K. Calculated by virial-type expansion equation at near-zero CO2 coverage. Calculated from
the ratios of the initial slope of pure component isotherms based on Henry’s Law.
D
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rearrangement. From these isotherm curves, the Brunauer−
Emmett−Teller (BET)/Langmuir surface areas for MOF-590
to -592 were estimated to be 0, 960/1100, and 900/990 m2
g−1, respectively. These values were in good agreement with
the theoretical accessible surface areas derived from crystal
structure data of MOF-590, -591, and -592 (0, 1010, and 917
m2 g−1, respectively).
CO2 Adsorption and Breakthrough Measurement.
Thermodynamic Uptake Capacity. To evaluate the gas
adsorption properties, single component of CO2, N2, and
CH4 adsorption isotherms were performed at 273, 283, and
298 K for MOF-590 to -592 (SI, Section S6, Figures S17−22).
As depicted in Table 2, MOF-592 displays the highest CO2
capacity among the new materials, with an uptake of 42 cm3
g−1 at 800 Torr and 298 K. On the other hand, the N2 and
CH4 uptake show low capacities (2.1 and 7.0 cm3 g−1,
respectively) at identical temperature and pressure, which
highlights the potential of this MOF for CO2 capture and
separation (Figure 3). Consequently, the coverage-dependent
Figure 4. A binary mixture of CO2/N2 is flown through a fixed bed of
MOF-592. The dashed line indicates the breakthrough time.
separation of CO2 from the N2 stream. From the breakthrough
data, the dynamic CO2 uptake capacity in the CO2/N2
separation was calculated to be 6.2 cm3 g−1 (1.2 wt %). To
ensure the recyclability and facile regeneration of MOF-592,
dynamic breakthrough measurements on activated MOF-592
were conducted over three cycles, without loss of performance
(SI, Figures S24). Furthermore, MOF-592 could be fully
regenerated by simply flowing pure N2 through the MOF-
loaded bed.
One-Pot Oxidative Carboxylation of Styrene and
CO2. MOF-590 is built up from Nd clusters that have a large
number of coordinated H2O molecules (3 and 5 molecules per
Nd1 and Nd2 center, respectively), which in principle can
provide accessible catalytic Lewis and/or Brønsted sites. The
resulting activated MOF-590 decorated with a high density of
active acidic sites, along with presence of π-acidity derived
from naphthalene diimide functionality of the H4BIPA-TC
linker, was proposed to effectively catalyze the oxidative
carboxylation of styrene and CO2. Since the size of styrene (9.8
× 7.1 Å2, optimized by Gaussian 09)43 is larger than the pore
aperture metrics of MOF-590 (8.4 × 2.5 Å2), the catalytic
activity will only be due to the Nd3+ ions that exist on the
external surface of MOF-590. The chemical and thermal
stabilities of MOF-590 were previously demonstrated by
PXRD and TGA (SI, Figures S12 and S13). Motivated by
these highlighted features, MOF-590 was employed as a
reusable catalyst in the one-pot oxidative carboxylation of
styrene and CO2 forming styrene carbonate.
According to crystal structure of MOF-590, the interaction
of the substrate molecules is assumed to originate on the
external surface of MOF-590. Consequently, FT-IR studies
were performed on the samples consisting of the activated
MOF-590 in contact with TBHP (MOF-590@TBHP) and
styrene oxide (MOF-590@SO) in order to confirm the
absorption of the substrates onto this material despite its
nonporous nature (SI, Section S8, Figures S29 and S30).
Accordingly, the emergence of peaks at 844 and 1028 cm−1 in
the IR spectra of MOF-590@TBHP and MOF-590@SO,
respectively, was observed owing to the characteristic peaks of
peroxide O−O stretching and alkoxide C−O−C stretching,
respectively.25,55 The active Nd clusters of MOF-590 can serve
as Lewis acid catalytic sites to react with TBHP and activate
the epoxy ring through the oxygen atoms. These results
validate the ability of MOF-590 to adsorb organic substrates
onto the external surface.
Figure 3. CO2 (red), CH4 (blue), and N2 (green) isotherms at 298 K
of MOF-592. Adsorption and desorption branches are marked by
filled and open symbols, respectively. The connecting curves are
guides for the eye.
isosteric enthalpy of adsorption (Qst) for the studied gases
(CO2, N2, and CH4) were calculated using a virial-type
expansion equation to fit the corresponding isotherms
collected at 273, 283, and 298 K (SI, Figures S23). As
shown in Table 2, the CO2 adsorption enthalpies at near-zero
coverage for MOF-591 and -592 were found to be relatively
high, with values of 23 and 22 kJ mol−1, respectively. To
confirm the ability of selective CO2 adsorption, the CO2/N2
and CO2/CH4 selectivities for MOF-591 and -592 were
estimated from the ratios of the initial slope of pure
component isotherms using Henry’s law (Table 2 and SI,
Section S6). Accordingly, MOF-592 revealed the highest
selectivity toward CO2 over N2 and CH4 (27 and 6.8,
respectively). Subsequently, a dynamic breakthrough experi-
ment was performed on MOF-592 with the binary mixture of
CO2 and N2 to evaluate the reliability of those gas selectivity
calculations and further demonstrate the CO2 separation
property.
Dynamic Adsorption Capacity via Breakthrough Meas-
urement. A bed packed with MOF-592 was exposed to a 20
mL min−1 flow of a 16% dry mixture of CO2 in N2 at room
temperature (SI, Section S7).18 As expected, only CO2 was
captured in the bed while N2 passed through (Figure 4). This
observation demonstrates that MOF-592 proceeds complete
In order to obtain an optimized reaction condition, we first
used MOF-590 as a model platform to investigate the impact
of catalyst amount, with the use of styrene (3.9 mmol), a co-
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catalyst nBu4NBr (8 mol %), anhydrous TBHP (1.9 equiv),
and solvent-free at 80 °C under atmosphere pressure of CO2
for 12 h. Accordingly, the reaction using 0.09 mol % catalyst
proceeded to 88% conversion of styrene and 86% yield of
styrene carbonate.
Table 4. Optimization of Reaction Conditions for One-Pot
Oxidative Carboxylation of Styrene and CO2 Catalyzed by
MOF-590, -591, and -592
a
The conversion, selectivity, and yield of the oxidative
carboxylation of styrene and CO2 increased to 96, 95, and
91%, respectively, upon increasing the amount of catalyst to
0.18 mol %. However, a further increase to 0.27 mol % MOF-
590 was found to be ineffective, resulting in lower conversion
of styrene (90%) and yield of carbonate product (63%) (Table
3, SI, Figures S34−S35). It was noted that the generation of
c
yield (%)
c
c
no.
1
2
3
4
MOF
con. (%)
sel. (%)
SC
SO
MOF-590
MOF-590
MOF-591
MOF-592
93
97
95
98
94
87
85
82
87
84
81
80
3
8
3
5
b
Table 3. . Optimization of MOF-590 Amount for One-Pot
a
Oxidative Carboxylation of Styrene and CO2
a
Reaction conditions: styrene (3.9 mmol), MOF (0.18 mol %, based
on molecular weight), TBHP in decane (7.4 mmol), nBu4NBr (8 mol
%), CO2 (balloon pressure), 80 °C, 10 h. TBHP in H2O. The
catalytic conversion (con.) of styrene, selectivity (sel.) of styrene
carbonate, and yield of products were determined by GC-FID analysis
using biphenyl as the internal standard. SC = styrene carbonate; SO =
styrene oxide.
b
c
b
yield (%)
b
b
no.
MOF-590 (mol %)
con. (%)
sel. (%)
SC
SO
To compare the catalytic efficiency of these MOF catalysts,
we performed the typical oxidative carboxylation reaction using
homogeneous catalysts and Lewis acidic-MOF materials
(Table 5). Accordingly, MOF-590 was shown to be the best
catalyst among the evaluated catalytic platforms. In general, the
homogeneous metal salts, Nd(NO3)3·6H2O, Eu(NO3)3·5H2O,
and Tb(NO3)3·xH2O exhibited poorer performances than
those of MOF-590 (Table 5, entries 1−3). Low selectivities
(55, 30, 52%) and yields (48, 27, 47%) of styrene carbonate
formation were found with the homogeneous Nd, Eu, and Tb
salt catalysts, respectively, even though high conversions (88,
92, 90%, respectively) were initially observed. Additionally, the
reaction using just the H4BIPA-TC linker afforded only 38%
yield of styrene carbonate with a moderate conversion of
styrene (66%, Table 5, entry 4). Interestingly, a reaction
catalyzed by a mixture of Nd(NO3)3·6H2O and H4BIPA-TC
linker proceeded a high conversion of styrene (91%), but only
41% yield of styrene carbonate form under the model reaction
(Table 5, entry 5). These evidences support the high catalytic
activity of Ln sites and the cooperative effect between the Ln
clusters and the naphthalene diimide based linker within
MOF-590, -591, and -592.
1
2
3
4
0.09
0.18
0.27
88
96
90
92
97
95
70
41
86
91
63
38
3
0
0
7
No-MOF
a
Reaction conditions: styrene (3.9 mmol), TBHP in decane (7.4
mmol), nBu4NBr (8 mol %), CO2 (balloon pressure), 80 °C, 12 h.
The catalytic conversion (con.) of styrene, selectivity (sel.) of
styrene carbonate, and yield of products were quantified by GC-FID
with the use of biphenyl as the internal standard. SC = styrene
carbonate; SO = styrene oxide.
b
styrene oxide was observed to be 0 in the presence of 0.18 and
0.27 mol % catalyst. In a control experiment, we found that
only 38% yield of cyclic carbonate product was obtained when
reactions were carried out in the absence of MOF catalyst
(Table 3, entry 4). tert-Butanol (t-BuOH) was permanently
observed as a byproduct of the oxidation transformation. To
emphasize the reproducibility of catalytic results, all catalytic
reactions were conducted at least three times, and the results
obtained in standard deviations of 2, 2, and 3% for the
conversion, selectivity, and yield of the products, respectively.
With the optimized condition, the catalytic activities of all
three compounds MOF-590 to -592 were examined for the
one-pot oxidative carboxylation of CO2 with styrene to obtain
styrene carbonate. MOF-590 demonstrated the best perform-
ance in the Ln-MOF series (Table 4) reaching 93%
conversion, a selectivity of 94%, and 87% yield after 10 h at
80 °C under 1 atm of CO2. A small amount of styrene oxide
still remained (ca. 3%) after 10 h (Table 4, entry 1), but it was
unobserved after 12 h, indicating a completeness of oxidation
of styrene (Table 3, entry 2). As such, MOF-590 afforded the
highest selectivity and yield of styrene carbonate (95 and 91%,
respectively) after 12 h. Interestingly, MOF-590 promoted
reasonable catalysis for the synthesis of styrene carbonate with
the use of aqueous TBHP, yielding high conversion and good
yield of 97 and 84%, respectively (Table 4, entry 2, SI, Figure
S36). Otherwise, MOF-591 and -592 exhibited high
conversion of styrene (95 and 98%, respectively) with
reasonable yields of styrene carbonate (80−81%) (Table 4,
entries 3 and 4).
Similarly, the MOF-based catalysts Al-MIL-53,44 UiO-67-
bpydc,22 and ZIF-845 exhibited low yields of 38, 39, and 22%,
respectively, under identical condition (Table 5, entries 9−11).
HKUST-146 showed no catalytic activity for the carbonate
synthesis with 99% conversion of styrene to benzaldehyde and
2-hydroxy-2-phenylethyl benzoate, and only a trace amount of
both styrene oxide and styrene carbonate were detected (Table
5, entry 12). Similarly, MOF-17747 and Mg-MOF-7422
promoted the one-pot synthesis of styrene carbonate from
styrene and CO2 with moderate yields of 53 and 50%,
respectively (Table 5, entries 13 and 14). By comparing these
results, it is evident that high surface area and high CO2 total
uptake at room temperature are not the most decisive variables
for the catalytic activity in the one-pot oxidative carboxylation.
This is evidenced by the exceptional yield and the even more
exceptional selectivity achieved by nonporous MOF-590 as
opposed to the other representative MOFs with high porosity,
such as MOF-177, Mg-MOF-74, MOF-591, and MOF-592. In
particular, reaction using other nonporous Nd-based MOF,
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To confirm the heterogeneous nature of MOF-590, we
performed a leaching assessment for the model reaction to
prove that the catalytic activity did not promote from any Nd3+
ions leaching from the MOF-590 structure. After the catalytic
reactions were carried out for 2 and 6 h, MOF-590 catalyst was
removed by centrifugation, and the filtrated reactions were
allowed to continue for 12 h. As expected, there was no
significant increase of the yield of styrene carbonate in the 6 h
filtrate reaction (rate of increasing yield of 1%), nor the 2 h
filtrate (6% rate of increasing yield). We notice that this virtual
quenching of the reaction after the solid catalyst removal is
different from the results of the blank experiment conducted
without MOF, where there is a higher increase of the yield
(23% in the initial 6 h, and 7.4% in the final 6 h, Figure S37).
However, it must be considered that after solid filtration, the
reaction medium is expected to be different in each case, due
to inevitable withdrawing of unknown amounts of TBHP,
styrene oxide, and nBu4NBr absorbed to the surface of MOF-
590, and thus the two systems are not directly comparable.
Nonetheless, the hot filtration experiment demonstrates the
heterogeneous nature of the system catalyzed by MOF-590.
Furthermore, the filtrates were also analyzed with inductively
coupled plasma mass spectrometry (ICP-MS), and the
concentration of Nd3+ was found to be <3 ppm, indicative of
no considerable Nd3+ leaching in the reaction solutions. To
assess the reusability of MOF-590 catalyst, recycling studies
were conducted under the optimized condition. Remarkably,
MOF-590 was recovered and reused up to five times without a
significant decrease in catalytic activity, demonstrated by the
reasonable average values of conversion (88%), selectivity
(90%), and yield (79%) of styrene carbonate (SI, Section S9,
Figure S38). The structural crystalline of the recycled MOF-
590 catalyst was proven by PXRD (SI, Section S10, Figure
S39). In addition, the integrity and particle morphology of
MOF-590 were retained after several catalytic reactions, as
indicated by FT-IR analyses (SI, Figure S40) and SEM images
(SI, Figure S41).
Table 5. Comparison of Catalysts for One-Pot Oxidative
Carboxylation of Styrene
a
c
yield (%)
con.
sel.
c
type
c
no.
catalyst
(%)
(%)
SC SO
1
2
3
4
5
Nd(NO3)3·6H2O
Eu(NO3)3·5H2O
Tb(NO3)3·xH2O
H4BIPA-TC
H4BIPA-TC +
Nd(NO3)3·6H2O
88
92
90
66
91
55
30
52
57
45
48
27
47
38
41
15
13
15
10
12
Hom.
6
7
8
9
MOF-590
MOF-591
MOF-592
Al-MIL-53
UiO-67
93
95
98
77
62
67
94
85
82
49
63
33
0
54
61
52
87
81
80
38
39
22
0
53
50
51
3
3
5
15
20
12
0
12
14
0
b
10
MOF
11
12
13
ZIF-8
d
HKUST-1
MOF-177
Mg-MOF-74
Nd-BDC
99
97
82
98
b
14
b
15
a
Reaction conditions: styrene (3.9 mmol), MOF (0.18 mol %, based
on molecular weight), TBHP in decane (7.4 mmol), nBu4NBr (8 mol
b
%), CO2 (balloon pressure), 80 °C, 10 h. Molecular weight
calculated from Zr6O4(OH)4(C12N2O4H6)6, Mg2(C8O6H2)(H2O)2,
and Nd2(C8O4H4)3(DMF)3(H2O) for UiO-67-bpydc, Mg-MOF-74,
and Nd-BDC, respectively. Hom. = homogeneous; SC = styrene
c
carbonate; and SO = styrene oxide. The catalytic conversion (con.)
of styrene, selectivity (sel.) of styrene carbonate, and yield of products
were quantified by GC-FID analysis and the use of biphenyl as the
d
internal standard. Benzaldehyde and 2-hydroxy-2-phenylethyl
benzoate were observed as main products.
The direct synthesis of styrene carbonate from styrene and
CO2 can be described as an oxidative carboxylation process
that combines two consecutive reactions: (i) the epoxidation
of styrene and (ii) the following cycloaddition of CO2 to
styrene oxide. To gain more information on the role of the
catalyst, both reactions, epoxidation of styrene (first step) and
Nd-BDC,48 provided only 51% yield of styrene carbonate
(Table 5, entry 15). This result confirms the outstanding
structure−function of MOF-590 framework, the combinations
of Nd clusters and naphthalene diimide-based linker, for the
catalysis oxidative carboxylation reaction of styrene and CO2.
Table 6. Study of the Influence of Reaction Variables on the Two Reactions Part of the Oxidative Carboxylation of Styrene and
a
CO2
c
yield (%)
c
c
no.
sub.
pro.
MOF-590
nBu4NBr
TBHP
CO2
con. (%)
sel. (%)
SC
SO
1
2
3
4
ST
ST
ST
ST
SO
SO
SO
SO
SO
SO
SO
SO
SC
SC
SC
SC
+
0
+
0
+
0
+
0
0
0
+
+
+
+
+
+
0
0
0
0
0
+
+
+
+
42
16
68
47
94
90
87
32
−
−
−
−
−
−
22
15
45
28
−
−
−
−
+
+
+
+
+
+
−
−
b
5
6
7
8
82
71
99
90
77
64
86
29
b
b
b
0
a
Reaction conditions: styrene (3.9 mmol), MOF (0.18 mol %), TBHP in decane (7.4 mmol), nBu4NBr (8 mol %), CO2 (balloon pressure), 80 °C,
b
10 h. Reaction conditions: styrene oxide (3.9 mmol), MOF (0.18 mol %), TBHP in decane (7.4 mmol), nBu4NBr (8 mol %), CO2 (balloon
pressure), 80 °C, 10 h. The catalytic conversion (con.) of styrene, selectivity (sel.) of styrene carbonate, and yield were calculated by GC-FID
c
analysis using biphenyl as the internal standard. sub. = substrate; pro. = product; ST = styrene; SC = styrene carbonate; SO = styrene oxide; (+) =
presence, (0) = none.
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Scheme 1. Proposed Mechanism for the Oxidative Carboxylation of Styrene and CO2 Catalyzed by MOF-590
cycloaddition of CO2 to styrene oxide (second step), were
independently carried out (Table 6). Consequently, the
epoxidation experiments for the first step were carried out
under the same conditions without the use of CO2 (Table 6,
entries 1−4). Accordingly, in the presence of MOF-590 as
catalyst, the reaction proceeded with a conversion/yield of 42/
22%, while in absence of catalyst, a conversion/yield of 8/15%
was achieved (Table 6, entries 1 and 2). nBu4NBr was also
found to catalyze the reaction, reaching a conversion/yield of
47/28%, and, when combining both MOF-590 and nBu4NBr,
the best performance was observed with a conversion/yield of
68/45% (Table 6, entries 3 and 4).
As expected, minor quantities of benzaldehyde and 2-
hydroxy-2-phenylethyl benzoate were detected as side products
of the one-pot oxidative carboxylation reaction (SI, Section S4,
Figure S42). This observation was consistent with previous
reports, which demonstrated the catalytic activity of nBu4NBr
in the oxidative carboxylation through the bromohydrin
species in situ assumption.28,49,50
With our results, the important role of nBu4NBr in both the
epoxidation of styrene and the cycloaddition of CO2 to styrene
oxide is evidenced, which also proceeds with good conversion
and moderate yield (90 and 64% respectively) in absence of
MOF and presence of TBHP (Table 6, entry 6). However, in
the absence of TBHP and MOF-590, only 32% conversion and
29% yield were observed. MOF-590 exhibited an exceptional
catalytic activity toward the cycloaddition of CO2 to styrene
oxide, with conversion of 87%, selectivity of 99%, and yield of
86% in the formation of styrene carbonate in absence of TBHP
and nBu4NBr (Table 6, entry 7). The selectivity to the
formation of styrene carbonate was slightly diminished through
the addition of TBHP (82%, Table 6, entry 5).
The combination of both MOF-590 and nBu4NBr results in
a highly efficient catalytic system for the addition of CO2 to
styrene oxide, reaching 94% conversion and 77% yield of
styrene carbonate in the presence of TBHP (Table 6, entry 5)
and 87% conversion and 86% yield of styrene carbonate in
absence of TBHP (Table 6, entry 7). It is suggested that the
Lewis and/or Brønsted acid sites derived from Nd clusters of
MOF-590 work together with the base nBu4NBr co-catalyst to
complete the activation of the epoxy ring for the cycloaddition
of CO2 and styrene oxide. On the other hand, with the use of
aqueous TBHP (Table 4, entry 3), the conversion of styrene
and the amounts of styrene oxide, benzaldehyde, and 2-
hydroxy-2-phenylethyl benzoate were also found to slightly
increase as compared to those found with the use of anhydrous
TBHP, indicating that the impact of water on the one-pot
oxidative carboxylation catalyzed by MOF-590 is minimal.
An epoxidation catalyzed by metal-based catalyst has been
reported to proceed through a radical pathway.51−54 In order
to demonstrate if radicals were involved in such reactions, a
radical scavenger, 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO, 1 equiv to mole of TBHP), was added separately
into the model experiments of the epoxidation reaction and the
oxidative carboxylation reaction after 2 h (SI, Section S11,
Figure S43). Consequently, the reactions were continued
under the same condition and monitored for 10 h. It was
observed that the yield of styrene oxide (14%) obtained from
epoxidation reaction was lower than that of typical epoxidation
reactions (45%, Table 6, entry 3). Notably, upon addition of
the radical scavenger, the oxidative carboxylation reaction
ended immediately with 17% yield of styrene carbonate after
10 h (SI, Section S11, Figure S43). Therefore, we suggest that
the overall mechanism of oxidative carboxylation indeed
proceeds mainly via radical process.
Plausible Reaction Mechanism. Based on our observations
and previous literature, the mechanism of the oxidative
carboxylation of styrene and CO2 should consist of the
mechanism of the epoxidation catalyzed by MOF-590/
nBu4NBr-TBHP and the mechanism of the subsequent
cycloaddition catalyzed by MOF-590/nBu4NBr. Accordingly,
in light of the catalytic contribution of MOF-590 catalyst, the
radical pathway depicted in Scheme 1 takes place between
Nd3+ clusters of MOF-590 and TBHP to form Nd4+-peroxy
species.53,54,56−58 The Nd4+-peroxy species then releases a t-
butoxy radical and regenerates MOF-590. Consequently, a
reaction between t-butoxy radical and styrene produces a t-
butoxperoxy intermediate, which further undergoes migration
of oxygen to yield the styrene oxide product, along with t-
BuOH as byproduct. An involvement of oxygen with t-
butoxperoxy intermediate could result in benzaldehyde as the
side product.
Subsequently, in the cycloaddition of CO2 to styrene oxide
reaction, the O atom of styrene oxide is first activated through
an involved coordination of the Nd unit. The epoxide
coordination molecule is accordingly attacked by Br− acting
as nucleophile derived from the co-catalyst nBu4NBr, leading
to form a metal-coordinated bromoalkoxide. The CO2
molecules then insert into the Nd−O bonds, forming metal−
carbonate intermediates. Finally, a ring-closing step of metal−
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Inorganic Chemistry
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Notes
carbonate intermediate takes place, in which a styrene
carbonate is formed, and, simultaneously, the MOF-590
catalyst and nBu4NBr co-catalyst are regenerated (Scheme 1).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
SUMMARY
■
We are grateful to Prof. O. M. Yaghi (UC Berkeley), Mr. Kyle
E. Cordova (UC Berkeley), and the Berkeley Global Science
Institute for continuous support of research activities. We wish
to thank Dr. H. Furukawa (UC Berkeley), Prof. Jaheon Kim
(Soongsil University), and Dr. H. T. C. Ho for scientific
inspirations and valuable discussions. We are indebted to Prof.
M. O’Keeffe (Arizona State University) for his useful
discussion on topological analysis.
In conclusion, the synthesis and full characterization of a novel
series of lanthanide-based MOFs constructed from a
benzoimidephenanthroline tetracarboxylic acid linker were
demonstrated. MOF-591 and MOF-592 showed moderate
CO2 uptake capacities at 800 Torr and 298 K (36 and 42 cm3
g−1, respectively) with corresponding zero coverage isosteric
heats of adsorption being 23 and 22 kJ mol−1, respectively. To
ensure the selective CO2 adsorption property, breakthrough
measurements were employed on MOF-592 with a binary gas
mixture containing CO2 and N2. As expected, MOF-592 was
proven as a promising adsorbent for the CO2 separation over
N2 (1.2 wt % CO2 capacity). The effective CO2 separation by
MOF-592 was successfully demonstrated in three consecutive
cycles of dynamic measurement with simple N2 flow for
regeneration. Otherwise, all of the new Ln-MOFs revealed
promising catalytic activity for the heterogeneous one-pot
oxidative carboxylation of styrene and CO2 affording styrene
carbonate. These materials exhibited excellent conversion,
selectivity, and yield of styrene carbonate under soft reaction
conditions, in absence of solvent, 1 atm pressure of CO2, and
80 °C for 10 h, without the need of preliminary isolation of
styrene oxide. Remarkably, MOF-590 was found to be an
excellent recyclable catalyst with the use of anhydrous and
aqueous TBHP oxidant in the oxidative carboxylation reaction.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02293.
■
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Accession Codes
CCDC 1831029−1831031 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: gandara@icmm.csic.es.
*E-mail: ntkphuong@inomar.edu.vn.
ORCID
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́
(10) D’Vries, R. F.; Iglesias, M.; Snejko, N.; Gutierrez-Puebla, E.;
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Huong T. D. Nguyen: 0000-0002-7965-9742
́
Felipe Gandara: 0000-0002-1671-6260
Phuong T. K. Nguyen: 0000-0002-9149-2279
Funding
This work was financially supported by Vietnam National
University Ho Chi Minh City (VNU-HCM) under grant
C2018-50-03. F.G. acknowledges Spanish Ministry of Econo-
my and Competitiveness for funding through the “Ramon y
Cajal” program and grant CTQ2017−87262-R.
́
I
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