6s-3d {Ba3Zn4}-Organic Framework as an Effective Heterogeneous Catalyst for Chemical Fixation of CO2and Knoevenagel Condensation Reaction

Article abstract of DOI:10.1021/acs.inorgchem.0c03736

The exquisite combination of Ba2+ and Zn2+ with the aid of 2,4,6-tri(2,4-dicarboxyphenyl)pyridine (H6TDP) under the condition of solvothermal self-assembly generates one highly robust [Ba3Zn4(CO2)12(HCO2)2(OH2)2]-organic framework of {[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n (NUC-27), in which adjacent 2D layers are interlaced via hydrogen-bonding interactions to form a 3D skeleton with peapod-like channels and nano-caged voids. It is worth emphasizing that both Ba2+ and Zn2+ ions in NUC-27 display the extremely low coordination modes: hexa-coordinated [Ba(1)] and tetra-coordinated [Ba(2), Zn(1), and Zn(2)]. Furthermore, to the best our knowledge, NUC-27 is one scarcely reported 2D-based nanomaterial with an unprecedented Z-shaped hepta-nuclear heterometallic cluster of [Ba3Zn4(CO2)12(HCO2)2(OH2)2] as SBUs, which not only has plentiful low-coordinated open metal sites but also has the excellent physicochemical properties including omni-directional opening pores, ultrahigh porosity, larger specific surface area, and the coexistence of Lewis acid-base sites. Just as expected, thanks to its rich active metal sites and pyridine groups as strong Lewis acid-base roles, completely activated NUC-27 displays high catalytic efficiency on the chemical transformation of epoxides with CO2 into cyclic carbonates under mild conditions and effectively accelerates the reaction process of Knoevenagel condensation.

Full text of DOI:10.1021/acs.inorgchem.0c03736

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Article
6s-3d {Ba3Zn4}−Organic Framework as an Effective Heterogeneous
Catalyst for Chemical Fixation of CO2 and Knoevenagel
Condensation Reaction
Hongtai Chen, Liming Fan, Tuoping Hu, and Xiutang Zhang*
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ABSTRACT: The exquisite combination of Ba2+ and Zn2+ with the aid of 2,4,6-
tri(2,4-dicarboxyphenyl)pyridine (H6TDP) under the condition of solvothermal self-
assembly generates one highly robust [Ba3Zn4(CO2)12(HCO2)2(OH2)2]−organic
framework of {[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n (NUC-27), in which
adjacent 2D layers are interlaced via hydrogen-bonding interactions to form a 3D
skeleton with peapod-like channels and nano-caged voids. It is worth emphasizing that
both Ba2+ and Zn2+ ions in NUC-27 display the extremely low coordination modes:
hexa-coordinated [Ba(1)] and tetra-coordinated [Ba(2), Zn(1), and Zn(2)].
Furthermore, to the best our knowledge, NUC-27 is one scarcely reported 2D-
based nanomaterial with an unprecedented Z-shaped hepta-nuclear heterometallic
cluster of [Ba3Zn4(CO2)12(HCO2)2(OH2)2] as SBUs, which not only has plentiful
low-coordinated open metal sites but also has the excellent physicochemical properties
including omni-directional opening pores, ultrahigh porosity, larger specific surface
area, and the coexistence of Lewis acid-base sites. Just as expected, thanks to its rich
active metal sites and pyridine groups as strong Lewis acid-base roles, completely activated NUC-27 displays high catalytic efficiency
on the chemical transformation of epoxides with CO2 into cyclic carbonates under mild conditions and effectively accelerates the
reaction process of Knoevenagel condensation.
for CO2, by which the focused fundamental research on the
chemical cycloaddition of CO2 with epoxides into high value-
added alkyl carbonates is demonstrated to satisfy the green
development stratagem of energy conservation and environ-
mental protection.23−27 MOF-based materials can serve as an
excellent heterogeneous catalyst because of their good
recyclability and easy product separation. Although the
achieved experimental data for the high catalytic efficiency
dedicated by a large amount of MOFs is intriguing, the
realization of application on an industrial scale still poses a
challenge because of extensive requirements including stability,
recyclability, the enterprise’s total cost, etc. Up to this day, only
a few reported MOFs make the chemical transformation of
CO2 simpler, saving energy, minimizing wastage, avoiding the
use of toxic organic solvents, and reducing environmental
pollution in mild conditions, which demonstrate that the in-
depth study is necessary for realizing the industrialization of
MOF catalysts.28−31
INTRODUCTION
■
Metal−organic frameworks (MOFs), as a class of burgeoning
hybrid materials, have been widely investigated in the area of
heterogeneous catalysts. Meanwhile, in order to improve their
catalytic activity, the current hot research topic has been
gradually characterized by developing corresponding two-
dimensional (2D) MOFs catalytic materials.1−6 Two-dimen-
sional nanomaterials, as an more intriguing subclass of MOFs,
have better performance because of their obviously inherent
advantages including well-off open metal sites, omni-direc-
tional opening pores and ultrahigh porosity, larger specific
surface area, and high electrical conductivity, which render
them the excellent physicochemical properties along with the
most promising applications in the fields of energy conversion,
luminescence, photocurrent switching, etc.7−10 Compared to
conventional three-dimensional MOFs, the enhanced catalytic
activity of two-dimensional MOFs is responsible for the
abundant active sites, higher density crystal defects, and
exchangeable coordination sites.11−14
Received: December 25, 2020
Published: February 17, 2021
CO2 could be widely used for human beings in the fields of
electronics industry, medical research and clinical diagnosis,
refrigeration, etc.15−18 However, excessive emission of CO2
from modern enterprises and motorized tools has created a
problem of serious global warming.17−22 Therefore, it becomes
an urgent issue to develop a sustainable enrichment technology
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then the mixture was transferred into a magnetically stirred round-
bottom quartz flask. The catalyst after reaction completion was
retrieved using the same method above. For these two kinds of
catalytic reactions, the conversion rate, the yield of the product, and
the selectivity were monitored by gas chromatography mass
spectrometry (GC-MS), and the transformed products were
In addition, almost all documented studies demonstrate that
the catalytic performance on the chemical fixation of CO2 is
greatly dominated by the constituents of employed MOFs,
especially the geometry of metal ion coordination and the
properties of metal ions.32−35 Our previous researches
exhibited that heavy metal cations possess the excellent
capabilities of wider coordination number and stronger hard
Lewis acidity because of their larger ionic radius and higher
charge density, which entitle them to the favorable congenital
factors for concentrating and catalyzing the cycloaddition of
CO2 molecules by acknowledged polarization behavior.36−39
However, up to now, the MOFs constructed from the alkaline
cation of barium are scarcely reported, which may be due to
that the highly multivariate coordination number and fuzzy
directionality usually result in nonporous highly interpenetrat-
ing frameworks under the self-assembly conditions.40,41 So, in
order to realize the high catalytic efficiency and the application
in multiple fields, the well-proven effective strategy of
introducing heavy metal cations as nodes into MOFs will
undoubtedly receive more and more attention.42−47
In view of the above-mentioned discussions, the favorable
cations of Ba2+ and Zn2+ are employed to assemble
heterometallic SBUs-based frameworks under the solvothermal
conditions in the presence of the bifunctional 2,4,6-tri(2,4-
dicarboxyphenyl)pyridine (H6TDP) ligand. Here, we report
one novel two-dimensional heterometallic−organic framework
of {[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n (NUC-
27). Furthermore, adjacent 2D layers in NUC-27 are
interlaced to each other via rich hydrogen-bonding interaction
around protruded ZnO3(OH2) units to form peapod-like
channels with excellent physicochemical properties including
omni-directional opening pores, ultrahigh porosity, larger
specific surface area, and the coexistence of Lewis acid-base
sites.
1
determined by H NMR spectroscopy.
RESULT AND DISCUSSION
■
Description of the Crystal Structure. Single-crystal XRD
analysis exhibits that NUC-27 crystallizes in the monoclinic
system with P21/n space group, and the asymmetric unit
consists of two crystallographically independent barium ions
(Ba(1) and Ba(2)), two zinc ions (Zn(1) and Zn(2)), one
TDP6− ligand, one formate anion, and one associated water
molecule. In NUC-27, Ba(1) and Zn(1) are exquisitely
spanned together by six carboxyl groups to form one
undocumented heterometallic SBUs of [BaZn2(CO2)6].
While, Ba(2) and Zn(2) are bridged via another three carboxyl
groups to generate one new kind of binuclear SBUs of
{BaZn(CO2)3(OH2)}. Such two kinds of SBUs are further
unified to form unprecedented Z-shaped hepta-nuclear clusters
of [Ba3Zn4(CO2)12(HCO2)2(OH2)2], which are further
propagated by the organic skeletons of TDP6− ligands into
two-dimensional layers. Moreover, adjacent 2D layers are
interlaced to each other via protruded ZnO3(OH2) units along
with rich hydrogen-bonding interactions to form a highly
robust 3D network, in which nano-caged voids with the
diameter of 13.7 Å are shaped by 8 {Ba3Zn4(CO2)12(HCO2)2-
(OH2)2} SBUs and 12 TDP6− ligands. In view of a three-
dimensional perspective, there are three kinds of channels with
the same window size of 8.6 × 6.9 Å. It is worth emphasizing
that both Ba2+ and Zn2+ ions in NUC-27 display the extremely
low coordination modes: hexa-coordinated [Ba(1)] and tetra-
coordinated [Ba(2), Zn(1), and Zn(2)], which render the host
framework a promising heterogeneous catalyst for various
important catalytic process.
EXPERIMENTAL SECTION
■
Preparation of NUC-27. A mixture of anhydrous zinc chloride
(0.014 g, 0.10 mmol), barium chloride (0.021 g, 0.10 mmol), and
H6TDP (0.057 g, 0.10 mmol) was added into a mixed solvent
including 3.0 mL of DMF, 3.0 mL of EtOH, 1.5 mL of H2O, and 0.1
mL of concentrated HNO3 solution. This obtained suspension was
further magnetically stirred for 30 min to form a homogeneous
solution and then sealed in a 25 mL Teflon-lined stainless steel vessel
at 130 °C for 72 h. Colorless crystals were obtained by cooling
gradually to room temperature with a yield of 90% on account of
H6TDP. Anal. Calcd for NUC-27 (C81H85Ba3N9O41Zn4): C, 38.70;
H, 3.14; N, 5.01 (%). Found: C, 38.63; H, 3.21; N, 5.09 (%). IR (KBr
pellet, cm−1): 3448 (vs), 2932 (s), 1665 (s), 1553 (m), 1393 (s),
1255 (v), 1097 (m), 788 (m), 736 (w), 665 (m), 475 (w).
As for its detailed internal links, first, Ba(1) ion is chelated
by six α-carboxyl groups from two TDP6− ligands with the
coordination mode of μ2-η1:η1, by which Zn(1) and Zn(1#)
are further spanned together to form one trinuclear {BaZn2}
unit. In detail, Ba(1) and Zn(1) (or Zn(1#)) are linked by
three μ2-η1:η1 α-carboxyl groups from two separate TDP6−
ligands. Furthermore, Zn(1) and Zn(1#) are separately
coordinated by one formate anion. Thus, with the aid of six
μ2-η1:η1 carboxyl groups from two TDP6− ligands and two
formate anions, Ba(1), Zn(1), and Zn(1#) are exquisitely
spanned together to form one undocumented [BaZn2(CO2)6]
SBU (Figure S1b). The alkaline earth ion of Ba(1) is located in
a heavily distorted octahedron coordination geometry, which is
completed by six carboxyl oxygen atoms with Ba(1)−O bond
distances in the rang of 2.599(8)−2.687(5) Å, much smaller
than the documented normal region of 2.710−2.989 Å. Such
should be ascribed to the combination pattern of
[BaZn2(CO2)6], which is bonded together by six carboxyl
groups of two symmetrical TDP6− ligands. Furthermore, it is
worth mentioning that Ba(1) ion exhibits the unexpected
exposure, which could be verified from its large bond angles of
97.48−107.3°. And meanwhile, in view of common coordina-
tion numbers (7, 8, and 10) in documented Ba-based MOFs,
hexa-coordinated Ba2+ is extremely scarce. While, Ba(2) and
Zn(2) are spanned together to form one binuclear {BaZn-
(CO2)3(OH2)} unit with the help of three μ2-η1:η1 β-carboxyl
Catalytic Experiment Operation. Newly synthesized NUC-27
crystals were first immersed into low-boiling methanol for 3 days
while the methanol was replaced three times in 1 day at ambient
temperature. Then, the samples were collected and the same
operation above was applied using dichloromethane to replace
methanol. Finally, the obtained crystals of NUC-27 were dried in a
vacuum drying oven at 120 °C for 10 h. The catalytic coupling of CO2
and epoxides was performed in a 25 mL stainless clave under the
solvent-free conditions of 1 atm CO2 gas, 2% mmol heterogeneous
catalyst NUC-27 (based on the Zn center), and tetrabutylammonium
bromide (n-Bu4NBr, 4 mol %). After reaction, the heterogeneous
catalyst of NUC-27 was recovered by simple centrifugation
separation, followed by cleaning with strong polar solvent of DMF
and volatile dichloromethane in turn. For the Knoevenagel
condensation reaction, in a model reaction, 20 mmol of
benzaldehydes and 10 mmol of malononitrile were added into 3
mL of ethanol in the presence of activated NUC-27 as catalyst, and
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Figure 1. The coordination environment of TDP6− ligand and the structural details of {Ba3Zn4(CO2)12(HCO2)2(OH2)2} SBUs (a); the 2D sheet
of NUC-27 (b); the hydrogen bonded 3D framework of NUC-27 featured by edging-sharing nanocages (pink polyhedron = BaII; blue polyhedron
= ZnII; H atoms are omitted for clarity) (c).
Scheme 1. Schematic Diagram of the Assembling Procedure of Hydrogen Bonded 3D Network Featured by Podiform
a
Channels
a
Green wave line, hydrogen bond; purple ball, pore area.
groups stationed on 4-position of pyridine from three separate
TDP6− ligands, among which Zn(2) is simultaneously
coordinated by one aqueous molecule (Figure S1b). Ba(2) is
further coordinated by the above-mentioned formate anion
with the coordination mode of μ2-η1:η1, which acts as a
reinforcing plug by linking to Zn(1) and Ba(2) throughout the
whole framework. Thus, the above two kinds of SBUs are
combined into undocumented Z-shaped heptanuclear clusters
of {[BaZn(CO2)3(OH2)](HCO2)[BaZn2(CO2)6](HCO2)-
[BaZn(CO2)3(OH2)]} (abbreviated as {Ba3Zn4(CO2)12-
(HCO2)2(OH2)2}, Figure 1a), which are further expanded
into one two-dimensional structure with the aid of a TDP6−
ligand (Figure 1b). Beyond expectation, Ba(2) ion adopts a
trigonal pyramidal coordination configuration with Ba(2)−O
bond distances in the rang of 2.695(7)−2.761(6) Å (Figure
S1a), which renders Ba(2) ion in a state of extreme openness,
undoubtedly being competent to the task of catalyzing the
cycloaddition of epoxides and CO2 as Lewis acid sites in view
of the recognized reaction mechanism. Moreover, for Zn(1)
and Zn(2), although their coordination modes are slightly
different, their tetra-coordination modes make them strong
Lewis acid sites, serving as another kind of efficient catalytic
sites.
bonded 3D network exhibits that hydrogen-bonding inter-
actions between interlaced adjacent 2D layers generate a
seldom reported podiform semi-closed structure with a slightly
symmetric interlace to comply with intermolecular repulsion.
Such an impressive interlayer precipitation model provides
continuous edge-shared semi-enclosed nanocages with omni-
directional open windows. As shown in Figure S1d, each six
TDP6− ligands and four {Ba3Zn4(CO2)12(HCO2)2(OH2)2}
SBUs in a single 2D layer make up a hemispherical shell, which
is further sutured by hydrogen bonds (O(1w)-H(1w)···N(1),
C(5)-H(5)···O(1w), C(5)-H(5)···O(12), and C(15)-H(15)···
O(12)) among adjacent layers to form an unprecedented
nanocage structure with the diameter of 13.7 Å (Figure S1e).
In the meantime, nanocage-embedded channels with the
window size of 8.6 × 6.9 Å2 exist along three axial directions.
The detailed information on hydrogen bonding is concluded in
Table S3.
In addition, it is worthy of note that the successful
construction of NUC-27 demonstrates that H6TDP is one
productive ligand with very strong coordination ability. Despite
the strong acid reaction conditions, deprotonation behavior for
all carboxyl groups could take place in the process of crystal
formation, and each TDP6− ligand serves as a μ4-bridge to
connect four {Ba3Zn4(CO2)12(HCO2)2(OH2)2} clusters. In
NUC-27, six carboxyl groups of TDP6− display the single
coordination mode of μ2-η1:η1, which is completely different
from previously discovered mixed modes, such as μ1−η1:η1,
bridged μ2−η1:η1, and chelated+bridged μ2−η2:η1. In view of
our speculation, different kinds of metal combinations not only
lead to different host metal−organic frameworks but also lead
In addition, it is worth emphasizing that these two-
dimensional layers in NUC-27 are further stacked into one
3D supramolecular nanoporous architecture through hydro-
gen-bonding association (Figure 1c and Scheme 1). The
potential solvent area volume is calculated by PLATON to be
3285 Å3 (58.8% of unit cell volume). Furthermore, it is
noteworthy that the assembling procedure of the hydrogen
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to different coordination modes of organic ligands along with
their own torsion regulation. In NUC-27, the brand-new
coordination patterns to metal ions of Ba2+ and Zn2+ result in
severe torsion angles between branched phenyl rings and
dominant pyridine with the twist angles of 51.4°, 87.9°, and
53.0°, which confirm that conjugated polycarboxylate ligands
are the best choice to construct porous functional MOFs.
In respect of the topological structure of NUC-27, defining
the trigeminal TDP6− ligand, [BaZn2(CO2)6], and [BaZn-
(CO2)3(OH2)] SBUs as topological nodes, the final host-
framework should be simplified into one novel 3-nodal 4-
Table 1. Catalytic Coupling of CO2 with Propylene Oxide
under Various Conditions
a
NUC-27
(mol %)
n-Bu4NBr
(mol %)
b
entry
T (°C) t (h) yield (%)
1
2
3
4
5
6
7
8
9
2
0
2
2
2
2
2
2
2
2
2
0
rt
rt
rt
12
12
12
12
12
12
12
2
17
49
64
71
78
90
99
42
70
89
99
2.5
2.5
2.5
2.5
2.5
2.5
5.0
5.0
5.0
5.0
̈
30
40
50
60
60
60
60
60
connected two-dimensional sheet with the Schlafli symbol of
{3.64.7}4{32.62.72} shown in Figure S1c.
Water Resistance of NUC-27. Owing to the relative weak
coordination bond of metal−organic framework materials, they
are prone to hydrolyze in an aqueous environment. Therefore,
the low water resistance keeps most of the MOFs from being
widely applied. To test the water resistance of NUC-27,
various aquatic systems were conducted including normal
temperature water and boiling water. As shown in Figure S3,
PXRD patterns after different water treatments are in line with
the as-synthesized one, which excludes the structural collapse
and reflects the strong water tolerance of NUC-27.
Structurally, the internal environment of the nanochannel of
NUC-27 could be adjusted by a heterometallic doping
strategy, leaving a strong hydrophobic inner surface.33
4
6
8
10
11
a
Reaction conditions: propylene oxide (20 mmol), CO2 (1 atm),
b
solvent free. The conversion rate and reaction yield are checked by
GC−MS spectroscopy.
epoxypropane was selected as a model to explore the influence
of different reaction conditions on the conversion of propylene
oxide with CO2 into the targeted propylene carbonate. During
the whole experimental process, the amount of NUC-27 was
set at 2 mol %. Entry 1 in Table 1 exhibits that the yield in the
presence of only solvent-free NUC-27 was less than 17% at
room temperature, and the co-catalyst of n-Bu4NBr with the
similar reaction conditions led to a 49% yield (entry 2).
However, as shown in entry 3, by simultaneously employing 2
mol % NUC-27 and 2.5 mol % Bu4NBr, the yield could reach
64%, which proved that NUC-27 and n-Bu4NBr possessed the
completely different catalytic efficiency for the second-order
reaction of cycloaddition from epoxides with CO2. Further-
more, with the temperature being enhanced every 10 °C, the
yield accordingly increased with the final yield of 99% at 60 °C
after 12 h (entry 7). However, under the conditions that the
quantity of co-catalyst n-Bu4NBr was doubled and NUC-27
was constant, the yield at 60 °C within 8 h could quickly
ascend to 99% (entry 11). Compared to recently documented
MOFs-based catalysts (Table S6), NUC-27 displayed the
better catalytic performance, which should be attributed to the
higher concentration of CO2 around the referential polarized
sites in the void channels of the host framework devoted by
these intrinsic characteristics of active metal sites (ZnII and
BaII), uncoordinated pyridine and carboxyl oxygen atoms, and
unimpeded void space.45,46
Under established optimal reaction conditions including 20
mmol of epoxide, 2 mol % NUC-27, 5 mol % n-Bu4NBr, and 1
atm CO2 at 60 °C within 8 h, a series of epoxide derivatives
with typical substitutes were employed to check the catalytic
universality of NUC-27 and the effect of substituents on the
reaction. Just as demonstrated in Table 2, activated NUC-27
possessed the significant catalytic activity to the chemical
transformation of selected epoxies into corresponding
carbonates with the yield of above 95% under the set reaction
conditions except for styrene oxide with a relatively high
boiling point of 194 °C, which maybe limited its activity agility
and consequently reduced the final yield at the low
temperature of 60 °C. The compassion for entries of 2 and
3 exhibited that an epoxide with one electron-withdrawing
Gas Adsorption Studies. A fully activated sample of
NUC-27 was obtained by solvent exchange and vacuum drying
for 12 h; the activation temperature was determined by the
TGA analysis (Figure S2) to ensure complete removal of water
molecules. The integrity of the solvent-removing sample of
NUC-27 was identified by PXRD analysis, revealing a well
consistence with that of the original NUC-27 sample as
illustrated in Figure S4. To assess the permanent pore
characteristic of guest-free NUC-27, a cryogenic nitrogen
adsorption isotherm was collected at 77 K shown in Figure S5.
A classical type-I sorption isotherm was observed with the BET
surface area of 1078 m2/g and the pore distribution ranging
6.5−14.3 Å. As can be seen in Figure S6, furthermore, the
adsorption behaviors of NUC-27 towards CO2 were
investigated under 273 and 298 K. The steeply increasing
uptakes in the low-pressure region of 0−0.1 bar indicate the
high affinity of the NUC-27 framework to CO2 molecules.
With increasing the pressure, the adsorption capacity inclines
to be saturated and observed at 85.6 and 64.8 cm3/g,
respectively. The isosteric heat of adsorption (Qst) was
employed to quantify the binding force between CO2 and
the NUC-27 framework, As shown in Figure S7, the Qst value
calculated by a virial method slightly increased with the
increase of uptake amount, indicating the energy distribution is
relative homogeneous for the inner surface of NUC-27. The
Qst value at zero loading was observed to be 31.5 kJ/mol,
which is comparable to that of a previously reported MOFs-
based sorbent with a large adsorption capacity of CO2.34−44
Catalytic Coupling of CO2 and Epoxides. On account of
these characteristics including highly exposed metal cations,
highly partitioned pore channels, large specific surface area,
and good trapping performance of CO2, the catalytic efficiency
of NUC-27 was testified for chemical transformation of CO2
with epoxides under comparatively mild conditions into cyclic
carbonates, which have been viewed as a bifunctional reaction
for synergistically solving the energy crisis and reducing the
amount of greenhouse gases.42−44 As illustrated in Table 1, 1,2-
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after each cycle and directly reused for the next cycle. When
the fifth cycle was over, the recovered sample of NUC-27 was
cleansed with dichloromethane and consequently vacuum-
dried. ICP analysis revealed that only trace amounts of Ba(II)
(≈0.037%) and Zn(II) (≈0.019%) ions were detected from
the reaction filtrate, which includes the leaching effect of the
host framework. Furthermore, just as shown in Figure S8, these
principal peaks of the PXRD data for recovered NUC-27 were
in accordance with the ones determined by unused activated
NUC-27, implying the high chemical stability and excellent
reusability. Moreover, it should be noted that the catalytic
effect of NUC-27 was almost the same within these five cycles,
which was confirmed by the transformation yield, shown in
Figure S9. For the further hot filtration experiment, the
catalytic reaction immediately ceased when removing the
catalyst during the catalytic process, while the reaction could
continue in the presence of catalyst (Figure S10), revealing the
heterogeneous nature of NUC-27.
Table 2. Catalytic Coupling of CO2 and Various Epoxides
under Optimal Reaction Conditions
a
The catalytic coupling of CO2 with alkyl epoxides into
carbonates prompted by specific catalysts involves three
consecutive steps including ring-opening of epoxide, nucleo-
philic addition of CO2, and cycle closure. Thus, one inferential
mechanism for the catalytic reaction by NUC-27 is exhibited
in Figure 2. First, guest molecules of epoxide and CO2
a
Reaction conditions: epoxide substrates (20 mmol), NUC-27
catalyst (2 mol %, based on the Zn metal center), n-Bu4NBr (5
b
mol %), CO2 (1 atm), solvent free, 60 °C, 8 h. The conversion rate
c
and reaction yield are checked by GC−MS spectroscopy. TON
(turnover number) = mole of product/mol of catalyst.
group was more likely to proceed the cycloaddition reaction,
which should be ascribed to that the electron absorption effect
could effectively reduce the electron density of propylene
oxide. Furthermore, the comparison between entries of 1−4
and 5 clearly displayed that the steric hindrance has a greatly
adverse influence on the cycloaddition reaction, which was in
accordance with the documented similar reactions catalyzed by
porous MOFs (Table S4).51−53 The reason from our
speculation was that large substituents on epoxides not only
affect the convenience of their access to nanocages but also
affect the opportunity to contact with active sites. Thus, within
the specified time, the corresponding yield for the substrates
with larger substituents was relatively low. Moreover,
compared to reported porous MOF-based catalysts, MOF-
205(S),41 MMCF-2,44 and Hf-Nu-1000,47 NUC-27 exhibited
the faster catalytic efficiency, which should be ascribed to the
contribution of {Ba3Zn4(CO2)12(HCO2)2(OH2)2} units on
the surface of the channel on the basis of the clarified reaction
mechanism.51 During the testified reaction process, the
principle of catalysis is to reduce the reaction energy level
and accelerate the reaction process by polarizing the reactants
of epoxides and CO2. Therefore, the research on NUC-27 set a
benchmark that it is most effective for metal cations contained
in porous MOFs to protrude into the position where the
reaction substrates could be easily hit, by which the high
catalytic activity of an emerged catalyst could be achieved.
The post-treatment including recovery and cyclic utilization
for one emerged catalyst is one of the most important factors
for evaluating its practical application except for the excellent
catalytic performance. Thus, the 10-fold amplification reaction
based on the optimal conditions with 1,2-epoxypropane as
substrate by employing 500 mg of NUC-27 was repeated for
five times, during which NUC-27 was recycled by filtration
Figure 2. Proposed reaction mechanism of coupling epoxides with
CO2 catalyzed by NUC-27.
stationed in the channels of NUC-27 were simultaneously
polarized by exposed Lewis acidic and basic sites on the wall.
Then anions of bromide from n-Bu4NBr initiated one
nucleophilic attack on the α-carbon atoms of propylene
oxide to form the anionic intermediates of bromoalkoxide
weakly coordinated with metal sites, just like bats hanging from
branches. Under such circumstances, bromoalkoxide will easily
fulfill the nucleophilic addition reaction with the polarized
“touch porcelain” molecules of CO2 to form the alkylcarbonate
salts, which were prone to display closed-loop behavior to offer
the five-membered ring carbonates.
Catalytic Knoevenagel Condensation Performance.
The coexistence of bifunctional Lewis acid-base sites uniformly
located in the nanocages makes activated NUC-27 a promising
heterogeneous catalyst for Knoevenagel condensation reac-
tion.48−51 In a typical reaction, the substrates of benzaldehyde
(10 mmol) and malononitrile (20 mmol) were selected as
model reactants to define the optimal reaction conditions. As
can be seen in Table 3, initially, the condensation reaction was
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Table 3. Catalytic Knoevenagel Condensation Reaction
from Substrates of Benzaldehyde and Malononitrile
Table 4. Knoevenagel Condensation Reaction of Aldehyde
Derivatives with Different Groups
a
a
b
entry
NUC-27 (mmol)
time (h)
temperature (°C)
yield (%)
1
2
3
4
5
6
7
8
0
1
1
1
1
1
1
1
1
60
60
60
60
rt
30
40
50
4
63
91
99
34
52
84
92
0.01
0.02
0.03
0.03
0.03
0.03
0.03
a
Reaction conditions: benzaldehyde (10 mmol), malononitrile (20
b
mmol), ethanol (3 mL). The reaction yield is checked by GC−MS
spectroscopy.
nearly stagnant without any catalyst (entry 1); only a trace
amount of 2-benzylidenemalononitrile was detected. In
contrast, the yield of desired product could soar to 63%
within 1 h in the presence of 0.01 mmol NUC-27 at 60 °C
(entry 2), which indicates that the NUC-27 as bifunctional
catalyst plays a crucial role in the Knoevenagel condensation
reaction. Furthermore, when increasing the catalyst amount to
0.03 mmol, the yield of 2-benzylidenemalononitrile product
could reach 99% (entry of 4). In order to further probe into
the interfering factors in the process of catalytic reaction, the
reaction temperature was set from room temperature to 50 °C
in the presence of 0.03 mmol of catalyst. As shown in entries
5−8, the conversion rate increases from 34% to 92% with
increasing the temperature, implying that reaction temperature
is directly related to the yield of product. Hence, the above
three steps concluded the following: the Knoevenagel
condensation reaction of 10 mmol of benzaldehyde and 20
mmol of malononitrile could be efficiently accomplished in the
presence of 0.03 mmol of NUC-27 catalyst at 60 °C.
To further investigate the catalytic generality of NUC-27, a
series of aldehyde derivatives with different substituents and
molecular sizes (Table S5) were explored under optimal
reaction conditions shown in Table 4. The results revealed that
NUC-27 displayed moderate catalytic effect on all six reaction
substrates, while slight differences and rules still could be
found: (i) electron-deficient substituents (F−, Cl−, and
NO2−) with the benzaldehyde are more active than the
electron-rich substituents (MeO− and Me−); (ii) the
conversion rate increased along with electron-withdrawing
ability, enhancing of electron-deficient substituents, that is,
NO2− > F− > Cl−.49,50 Therefore, understandably, the MeO−
with the strongest electron-donating nature among them gave
the lowest conversion rate of 91%. On the other hand, the
nonaromatic hexahydro-benzaldehyde was selected to further
check the catalytic universality as shown in entry 7. The
conversion rate was observed to be 94%, which surpasses most
of the MOFs-based catalysts and verifies that NUC-27 could
efficiently catalyze a broad range of aldehydes including
aromatic and nonaromatic types.
a
Reaction conditions: aldehyde derivatives (10 mmol), malononitrile
(20 mmol), NUC-27 (0.03 mmol, based on the Zn metal center),
ethanol (3 mL), 1 h, 60 °C. The reaction yield is checked by GC−
MS spectroscopy. TON (turnover number) = moles of products/mol
b
c
of catalyst.
further washed with DMF and acetone in turn to make sure
that the attached reactants are completely removed. As shown
in Figure S12, the conversion rate was obtained over 95% for
each cycle, which shows a successive good catalytic effect of
NUC-27. After the recycled catalytic experiment, only a
negligible amount of Zn(II) (≈0.028%) and Ba(II) (≈0.041%)
was determined by ICP analysis. In combination with the
PXRD pattern, it could keep its original shape after the
catalytic process (Figure S11), confirming the strong skeleton
stability of the catalyst during the process of Knoevenagel
condensation reactions. Furthermore, a hot filtration experi-
ment was employed to determine the heterogeneous nature of
NUC-27. As illustrated in Figure S13, the catalytic reaction
could complete within 1 h, whereas, in the parallel experiment,
the reaction was almost stagnant after removing the catalyst,
proving the pure heterogeneous nature of NUC-27.
On account of the coexistence of Lewis acid-base sites of
NUC-27,50−53 the hypothetical catalytic mechanism of the
Knoevenagel condensation reaction should be summarized as
follows: First, the exposed metal ions (Zn(II) and Ba(II)) as
Lewis acidic sites will interact and polarize the carbonyl oxygen
on the benzaldehyde, thus enhancing the electrophilicity of the
carbonyl carbon atom to easily receive the attack from
malononitrile (Figure 3). It should be noted that the Lewis
acid sites could promote the deprotonation of the cyano group
in malononitrile. Then, the carbonyl group from the aldehyde
contacts with the carbanion to form an intermediate (C−C
bond formation), followed by rearrangement and dehydration,
and the desired benzylidenemalononitrile was formed (CC
bond formation).
Recyclability as a measure of the catalyst applicability was
further evaluated. In a typical experiment, by repeatedly using
NUC-27 for the Knoevenagel condensation reaction under
optimal reaction conditions in five cycles, the catalyst can be
easily recovered by centrifugation. The collected catalyst was
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Article
AUTHOR INFORMATION
■
Corresponding Author
Xiutang Zhang − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s
Republic of China; orcid.org/0000-0002-5654-7510;
Email: xiutangzhang@163.com
Authors
Hongtai Chen − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s
Republic of China
Liming Fan − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s
Republic of China; orcid.org/0000-0003-4615-0533
Tuoping Hu − Department of Chemistry, College of Science,
North University of China, Taiyuan 030051, People’s
Republic of China; orcid.org/0000-0001-7437-3246
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03736
Figure 3. Proposed catalytic mechanism of Knoevenagel condensa-
tion reaction catalyzed by NUC-27.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSIONS
■
■
In summary, the hydrothermal self-assembly of Ba2+ and Zn2+
generates one 6s-3d nano-caged heterometallic material of
{[Ba3Zn4(TDP)2(HCO2)2(OH2)2]·7DMF·4H2O}n (NUC-
27), in which two kinds of undocumented heterometallic
SBUs of [BaZn2(CO2)6] and {BaZn(CO2)3(OH2)} are
combined into one intriguing Z-shaped hepta-nuclear cluster
of [Ba3Zn4(CO2)12(HCO2)2(OH2)2] via the connection of
two formate anions. Owing to the unique hydrogen bonded
peapod-like channels and the coexistence of Lewis acid-base
sites, NUC-27 displays high catalytic efficiency on the chemical
transformation of alkyl epoxides with CO2 into cyclic
carbonates under mild conditions. In the meantime, by taking
NUC-27 as a bifunctional heterogeneous catalyst, Knoevenagel
condensation could be effectively accelerated. This work
demonstrates that the schematization and assembly of
functional 2D heterometallic MOFs could be more productive
for enhancing the catalytic efficiency on various chemical
transformations.
The work was supported by financial support from the Natural
Science Foundation of China (21101097), and the Opening
Foundation of Key Laboratory of Laser & Infrared System of
Shandong University (2019-LISKFJJ-005).
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03736.
Crystallographic data and refinement parameters of
NUC-27, selected bond lengths and angles, the TGA
curves of as-synthesized and activated sample of NUC-
27, PXRD patterns of NUC-27 after water treatment, N2
absorption/desorption isotherms of NUC-27 at 77 K
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CCDC 2046852 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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