Nanochannel-based heterometallic {ZnIIHoIII}-organic framework with high catalytic activity for the chemical fixation of CO2

Article abstract of DOI:10.1039/d1ra00590a

The exquisite combination of Zn^IIand Ho^IIIgenerated the highly robust [ZnHo(CO₂)₆(OH₂)]-based heterometallic framework of {[ZnHo(TDP)(H₂O)]·5H₂O·3DMF}_n(NUC-30, H₆TDP = 2,4,6-tri(2′,4′-dicarboxyphenyl)pyridine), which featured outstanding physicochemical properties, including honeycomb nanochannels, high porosity, large specific surface area, the coexistence of highly open Lewis acid-base sites, good thermal and chemical stability, and resistance to most organic solvents. Due to its extremely unsaturated metal tetra-coordinated Zn(ii) ions, hepta-coordinated Ho(iii) and high faveolate void volume (61.3%), the conversion rate of styrene oxide and CO₂into cyclic carbonates in the presence of 2 mol% activatedNUC-30and 5 mol%n-Bu₄NBr reached 99% under the mild conditions of 1.0 MPa and 60 °C. Furthermore, the luminescence sensing experiments proved thatNUC-30could be used as a fast, sensitive and highly efficiency sensor for the detection of Fe³⁺in aqueous solution. Therefore, these results prove that nanoporous MOFs assembled from pyridine-containing polycarboxylate ligands have wide applications, such as catalysis and as luminescent materials.

Full text of DOI:10.1039/d1ra00590a

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PAPER
II
III
Nanochannel-based heterometallic {Zn Ho }–
organic framework with high catalytic activity for
the chemical fixation of CO2†
Cite this: RSC Adv., 2021, 11, 9731
ab
a
a
Qiaoling Li* and Xiutang Zhang *a
a
Tao Zhang, Hongtai Chen, Hongxiao Lv,
II
III
The exquisite combination of Zn and Ho generated the highly robust [ZnHo(CO
2 6 2
) (OH )]-based
0
0
heterometallic framework of {[ZnHo(TDP)(H O)]$5H O$3DMF} (NUC-30,
2
2
n
H
6
TDP ¼ 2,4,6-tri(2 ,4 -
dicarboxyphenyl)pyridine), which featured outstanding physicochemical properties, including
honeycomb nanochannels, high porosity, large specific surface area, the coexistence of highly open
Lewis acid–base sites, good thermal and chemical stability, and resistance to most organic solvents. Due
to its extremely unsaturated metal tetra-coordinated Zn(II) ions, hepta-coordinated Ho(III) and high
faveolate void volume (61.3%), the conversion rate of styrene oxide and CO
2
into cyclic carbonates in the
presence of 2 mol% activated NUC-30 and 5 mol% n-Bu
4
NBr reached 99% under the mild conditions of
Received 22nd January 2021
Accepted 15th February 2021
ꢀ
1
.0 MPa and 60 C. Furthermore, the luminescence sensing experiments proved that NUC-30 could be
3+
used as a fast, sensitive and highly efficiency sensor for the detection of Fe in aqueous solution.
Therefore, these results prove that nanoporous MOFs assembled from pyridine-containing
polycarboxylate ligands have wide applications, such as catalysis and as luminescent materials.
DOI: 10.1039/d1ra00590a
rsc.li/rsc-advances
5
Furthermore, MOFs (e.g., MOF-74/CPO-27 series), as a type of
organic–inorganic hybrid material with marvelous structural
Introduction
Porous metal–organic frameworks (MOFs), as a booming family adaptability, feature the excellent characteristics of high
of organic–inorganic hybrid materials, have aroused much specic surface area and synergistic micropore size, conse-
interest owing to their potential applications as heterogeneous quently leading to a better unique adsorption performance for
catalysts, gas separation and storage materials, biomedical and CO
2
. In particular, the MOF-74 series of modied M
2
(dobpdc)
1
chemical sensors, electrochemical microelectrodes, etc. With (dobpdc ¼ 4,4-diphenyl-3,3-dicarboxylate) has been function-
2
the development of technology and social advancement, the alized with diamine to exhibit excellent CO absorption and
highly optimized synthesis of targeted MOFs has been adopted selectivity under humid conditions, thus eliminating the water
6
and applied stepwise by combining functionalized organic limitation. For instance, in traditional aqueous amine solu-
linkers and special metal clusters to realize excellent functional tions, the diamine-graed M (dobpdc) series fully demon-
2
characteristics including inherent porosity, large surface area, strates the advantage of Lewis acid–base sites, and thus CO
and active sites. Thus far, numerous MOFs have been widely molecules can react with amines to form ammonium carbamate
2
2
applied as a type of effective separation and catalytic material or bicarbonates. Therefore, due to the lower renewable energy
for the separation and chemical conversion of CO , which has cost from solid materials, diamine-appended M
2
(dobpdc)
2
created the serious problem of global warming because of materials with high working ability and energy efficiency are
excessive emissions from modern enterprises and motorized very promising candidates for CO
tools. Particularly, the mild reaction conditions of low promote the selective adsorption capacity of CO
2
capture. Moreover, to
in terms of
3
2
temperature and pressure utilizing MOF catalysts are incom- kinetics and thermodynamics, research interest on MOFs at
parable with other hetero- and homogeneous catalysts, for present is concentrated on their performance optimization by
instance, zeolites, inorganic metal salts or oxides, ionic liquids, designing functional organic ligands to adjust their pore size,
4
amine aqueous solutions, and polymerized organics.
increasing open metal sites to improve their adsorption affinity
for CO , and improving their thermal and chemical stability
e.g., water resistance).
Among the reported MOFs, Zn ions, as one type of d-block
metal ions and potential strong Lewis acid, have been paid
close attention due to their distinctive characteristics
including electronic conguration, binding energy, and
charge distribution, which render various coupling affinities
2
7
(
a
2+
North University of China, Taiyuan 030051, People's Republic of China. E-mail:
qiaolingl@163.com; xiutangzhang@163.com
b
Taiyuan Institute of Technology, Taiyuan 030008, People's Republic of China
†
Electronic supplementary information (ESI) available. CCDC 2002682. For ESI
and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1ra00590a
1
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2021 The Author(s). Published by the Royal Society of Chemistry
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to multifarious donor atoms from ligands. Recently, reported
Zn-MOFs based on tetra-coordinated Zn ions with solvent- heterometallic MOFs with a high porosity distribution were
free channels displayed good selective adsorption perfor- explored by employing the pyridine-containing ligand of 2,4,6-
Paper
8
Considering the above discussion and our recent research,
2
+
0
0
mances for the guest molecules of CO2 and catalyzed the tri(2 ,4 -dicarboxyphenyl)pyridine (H TDP), which possesses
6
chemical transformation of epoxides with CO
2
into the corre- the followed characteristics: (i) ample carboxyl groups to
sponding alkyl carbonates. This is because the exposed active bridge multiple metal cations, which is benecial for the
2
+
Zn ions act as a strong Lewis acid to polarize and activate formation of a three-dimensional architecture and (ii) the
9
CO
Thus, the self-assembly of nanoporous Zn-MOFs with the tending to polarize acidic molecules of CO
purpose of enhancing the amount of exposed active tetra- or a robust, double-walled, nano-channel heterometallic frame-
2
and the ring of ethylene oxide via coordination affinity.
nitrogen atom of pyridine can serve as a Lewis base site,
2
. Herein, we report
2
+
penta-coordinated Zn cations will become a research hot- work of {[ZnHo(TDP)(H O)]$2DMF$4H O} (NUC-30), which is
2
2
n
spot, especially for application in the selective separation or/ based on the exquisite combination of dinuclear
6
ꢁ
2 6 2
and storage of guest molecules, catalysis for some specic [ZnHo(CO ) (H O)] SBUs and TDP ligands. Interestingly, the
1
0
reactions, and optics. Moreover, in the past decade, the desolvated NUC-30 not only possesses dual nanoscopic chan-
research on the structure and chemical bond theory of Ln- nels, high porosity and large specic surface area, but also
2
+
MOFs has constantly increased, leading to a great deal of coexisting Lewis acid–base sites, including 4-coordinated Zn
3
+
new research branches, such as catalysts, monomolecular ions, 7-coordinated Ho ions, uncoordinated carboxyl oxygen
magnets, magnetic refrigeration materials, and uorescence atoms, and Npyridine atoms. Thus, as expected, NUC-30 dis-
1
1
recognition. Thus far, for the reported Ln-MOFs, it has been played high catalytic activity in the cycloaddition of various
3
+
exhibited that the coordination modes of Ln can vary in epoxides into the corresponding alkyl carbonates. In addition,
a wide range with the largest coordination number of 12 due to the luminescence sensing experiments proved that NUC-30
n
2+m
3
the various orbital hybridization of f d sp (n ¼ 0–3, m ¼ 1– can be used as a fast, sensitive and highly efficiency sensor for
3
+
3
) by the electron orbitals of 6s, 6p, 5d, and 4f, which lead to the detection of Fe in aqueous solution.
a series of characteristics corresponding with the intrinsic
1
2
3+
structures. Thus, although the Ln ions included in the re-
ported Ln-MOFs display octa- and nano-coordinated modes,
Experimental
they can still possess strong Lewis acidity and show strong Materials and general methods
affinity to small guest molecules with smaller steric resistance,
such as CO , SO , N , and H S. Recently, the metal–organic
framework of [Eu(BTB)(phen)] reported by Zhao and cow-
0 0
6
,4,6-Tri(2 ,4 -dicarboxyphenyl)pyridine (H TDP) was obtained
from Jinan Henghua Sci. & Tec. Co. Ltd and used as received
2
2
2
X
O
Y
2
without further renement. Infrared (IR) spectra were measured
1
2c
orkers, as the rst example of Ln-MOFs for the xation of
CO , displayed excellent catalytic activity for the chemical
transformation of epoxides and CO into carbonates under
mild conditions, which further conrmed that the stable octa-
using a TENSOR 27 spectrometer in the wavenumber range of
ꢁ1
2
6
00–4000 cm . Thermogravimetric analysis (TGA) was carried
2
out using a TG-209F3 thermal analyzer at a heating rate of
ꢀ
ꢁ1
1
0 C min under an Ar stream. The powder X-ray (PXRD)
3
+
and nano-coordinated state of Ln ions could be viewed as
a strong Lewis acid to polarize and activate approaching guest
molecules.
In addition, some metal cations are also widely found in
nature and living organisms, such as Fe , which is one of the
most crucial metal ions in the Earth's crust and biological
systems, playing an important role in oxygen transportation,
enzyme catalysis, cell metabolism, and DNA and RNA synthesis.
A certain amount of Fe is necessary to promote the formation
of muscle and hemoglobin. Thus, insufficient or excessive Fe
can lead to physical diseases such as anemia, cirrhosis, heart
failure, diabetes, cancer and decreased immunity. Excessive
Fe in the body can lead to neurodegenerative diseases, such as
diffraction analysis of the complexes was performed on
a Rigaku SmartLab (9 kW) diffractometer with Cu-Ka radiation
1
3,14
ꢀ
ꢁ1
at room temperature at the scanning speed of 10 min . The
SEM-EDS images of NUC-30 were observed on a JSM-7200F
scanning electron microscope (SEM, JEOL) at an acceleration
voltage of 10.0 kV. Fluorescence spectra were measured using
a Hitachi F-4600 uorescence spectrophotometer at room
temperature. The cryogenic N2 adsorption and adsorption–
3
+
3
+
2
desorption isotherms of CO at 273 K and 298 K were measured
on an ASAP 2020 Plus instrument.
3
+
3
+
Preparation of NUC-30
3
+
Alzheimer's disease and Parkinson's disease. Moreover, Fe is A homogeneous solution of anhydrous zinc chloride (0.014 g,
a common inorganic pollutant in the aquatic environment, 0.10 mmol), Ho (0.019 g, 0.05 mmol), H TDP (0.034 g, 0.06
which can cause perpetual damage to human health and living mmol), 7 mL DMF, 2 mL H O, and 0.2 mL HNO in a 25 mL
2
O
3
6
2
3
ꢀ
surroundings even at low concentrations. Consequently, autoclave was heated at 110 C for 4 days and then gradually
3
+
ꢀ
ꢁ1
studies on the effective detection of Fe are particularly cooled to room temperature at a rate of 10 C h . Colourless
important. However, traditional analytical methods, such as ion crystals of NUC-30 were collected by ltration and washed with
mobility spectroscopy (IMS), inductively coupled plasma (ICP), DMF/H O (yield: 82% based on H TDP). Anal. calcd for NUC-30
2
6
X-ray dispersion, atomic absorption spectrometry, are generally (C H HoNO Zn): C, 42.91%; H, 1.36%; N, 1.72%. Found: C,
2
9
11
13
15
ꢁ1
expensive and time-consuming.
42.57%; H, 1.42%; N, 1.74%. IR (KBr pellet, cm ): 3422 (vs),
1604 (vs), 1373 (vs), 1105 (w), 1020 (w), 855 (w), 770 (s), 672 (w).
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X-ray crystallography
Result and discussion
The diffraction intensity data for NUC-30 was obtained at 296(2)
K using a Bruker Smart-APEX II CCD area detector (Mo-Ka
radiation, l ¼ 0.071073 nm) with graphite monochromated
radiation. The reection data was corrected for empirical
absorption corrections and Lorentz and polarization effects.
The structure was solved by direct methods and rened by full-
matrix least-squares with the SHELXL package. All non-
hydrogen atoms were rened anisotropically. Hydrogen atoms
except those on water molecules were generated geometrically
with xed isotropic thermal parameters and included in the
structure factor calculations. The block of SQUEEZE in PLATON
was employed to eliminate the highly disordered solvent
molecules. The crystallographic data and renement parame-
ters for NUC-30 are listed in Table S1.† Selected bond lengths
and angles for NUC-30 are presented in Table S2.† Further
details on the crystal structure investigations can be obtained
from the Cambridge Crystallographic Data Centre, with the
depository number CCDC-2002682 for NUC-30.
Description of the crystal structure
The single-crystal XRD analysis exhibited that NUC-30 crystal-
lizes in the trigonal system with the R3m space group and
exhibits a honeycomb anionic structure with dual-channels,
which is built on the exquisite combination of paddlewheel
ꢀ
6
ꢁ
[
ZnHo(CO ) (H O)] SBUs and bifunctional ligands of TDP .
2 6 2
Although both types of alternately arranged nano-channels (I
and II) in NUC-30 shaped by six rows of [ZnHo(CO ) (H O)]
SBUs have an equal amount of exposed active metal sites, they
could be discriminated from their different functionalized
inner surfaces with free carboxyl oxygen atoms (C]O) and
2
6
2
2
coordinated aqueous molecules (H O). It is worth mentioning
that NUC-30 features outstanding physicochemical properties
including honeycomb nanochannels, high porosity, large
specic surface area, coexisting highly open Lewis acid–base
sites, good thermal and chemical stability, and resistance to
most organic solvents. In addition, the activated NUC-30 with
void volume (61.3%) consists of extremely unsaturated metal
ions including tetra-coordinated Zn(II) and hepta-coordinated
Ho(III), which render the host framework a promising hetero-
PXRD and thermal analyses
2
geneous catalyst for the chemical xation of CO .
To detect the phase purity, the samples of NUC-30 and H TDP
6
Specically, the asymmetric unit of NUC-30 contains half
were characterized via X-ray powder diffraction at the scanning
a crystallographically unique Zn(II) ion, half an Ho(III) ion, and
ꢀ
ꢁ1
ꢀ
speed of 10 min in the 2q range of 5–30 . As shown in
Fig. S1,† the powder X-ray diffraction pattern of NUC-30 aer
activation was almost the same as the simulated pattern. The IR
6ꢁ
half a TDP ligand. Firstly, the Ho(1) ion is chelated by two a-
1
1
2
carboxylic groups with the coordination mode of m -h :h
located on the 2-position of the phenyl rings, which are
arranged on the 2- and 6-positions of the pyridine in TDP
Aerwards, the Ho(1) ion is further bridged to the Zn(1) ion to
spectra of NUC-30 and H
6
TDP were measured in the frequency
6ꢁ
.
ꢁ
1
range of 600–4000 cm , as shown in Fig. S2.† The SEM-EDS
images of NUC-30 illustrated that C, O, N, Zn and Ho were
evenly distributed on the crystal surface (Fig. S3†). The thermal
stability of NUC-30 was tested by thermogravimetric analysis
2 6
form one paddle-wheel dinuclear SBU of [Zn(CO ) ] with the
ꢁ
separate Ho(1)/Zn(1) distance of 4.04(9) A by three other g-
carboxyl groups, two of which are provided by two benzene rings
on the 2-position of pyridine and one on the 4-position of
pyridine (Fig. 1a). In addition, the Zn(1) ion is coordinated by
one 2-position carboxyl group from another TDP
(TGA) and the result is shown in Fig. S4.† According to the TGA
curve, the rst weight loss of 8.75% in the temperature range of
ꢀ
2
5–120 C is consistent with the associated water molecules.
6ꢁ
ligand.
O)] SBU is facilitated
ꢀ
The second weight loss in the range of 120–400 C is ascribed to
Thus, the formation of one [ZnHo(CO
2
)
6
(H
2
the solvent of DMF. The framework remained stable up to
6ꢁ
by ve TDP ligands, each of which simultaneously connects
ꢀ
4
00 C, indicating that NUC-30 exhibits high thermal stability.

2 6 2
ve [ZnHo(CO ) (H O)] SBUs to generate one three-dimensional
structure with honeycomb-like nanochannels (Fig. 1b).
Although the Ho(1) ion is linked by seven carboxyl oxygen atoms
and one aqueous molecule to constitute a “twisted double hat
Catalytic experiment operation
The activated sample of NUC-30 was obtained by immersing the tri-prism” geometry (Fig. S5a†), it is worth mentioning that the
newly synthesized crystals into methanol for one week, during associated water molecules can be easily removed under the
which methanol was replaced three times at ambient temper- activation conditions. In addition, the Zn(1) ion is linked by
6
ꢁ
ature. Subsequently, the ltered crystals were dried in a vacuum four carboxyl oxygen atoms from four different TDP ligands to
drying oven at 393 K for one day. The catalytic cycloaddition form a tetrahedron (Fig. S5b†) with the Zn(1)–O bond length in
reactions were performed in a 25 mL stainless clave under the the range of 1.918(2)–1.963(2) ꢁA . Even more exciting is that the
solvent-free conditions of 10 atm CO
heterogeneous catalyst NUC-30. The qualitative detection a natural Lewis acid site.
during the reactive process was monitored by gas chromatog-
It is worth noting that each six rows of [ZnHo(CO ) (H O)]
2
gas and 2 mmol% tetrahedral Zn(1) in a tetrahedral coordination geometry is
2
6
2
1
6ꢁ
raphy and the transformed product was determined by H NMR SBUs are alternately bridged by TDP ligands to form two types
spectroscopy. At the end of the catalytic reaction, the hetero- of edge-sharing nanochannels, namely circular and hexagonal
geneous catalyst NUC-30 was retrieved through simple centri- ones. The circular nanochannels (I) with a pore diameter of 9.18
fugation separation aer each reaction was complete, and then ꢁA are characterized by the associated water molecules on the
3
+
cleaned with strong polar solvent of DMF and volatile solvent of Ho
acetone in sequence.
cations, pointing vertically to the center of the
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aquatic systems was analyzed. As shown in Fig. S6,† the PXRD
spectra demonstrated that no structural collapse occurred aer
NUC-30 was soaked for 10 days at normal temperature, and even
aer soaking in boiling water for 72 h, which reects its strong
water resistance. This may be due to the change in the internal
environment of its nanochannel by heterometallic doping,
resulting in the strong hydrophobicity of the inner surface. The
method of controlling the stability of MOFs by heterometallic
doping also provides a new idea for improving the properties of
porous metal–organic frameworks.
Gas adsorption studies
To explore the permanent porosity of NUC-30, crystal samples
were soaked in methanol and dichloromethane separately three
times a week, the solvent was changed every 24 h, and then
ꢀ
dried under vacuum at 120 C for 12 h. The PXRD pattern of the
activated NUC-30 was consistent with the results of the X-ray
single crystal diffraction data simulation, as shown in
Fig. S7.† The adsorption capacity for N at 77 K displayed that
2
the total pore volume and calculated BET surface area of the
3
ꢁ1
2
ꢁ1
activated NUC-30 were 0.38 cm g and 701 m g , respec-
tively. The adsorption–desorption isotherm of N displayed
2
a typical type-I adsorption conguration, and the hole distri-
bution was in the range of 0.9–1.3 nm, as shown in Fig. S8.†
Due to the inherent structural advantages of the tubular
nanochannels and the exposed high-density active metal sites
of NUC-30, it has the potential to adsorb target molecules with
quadrupole moments, and thus the adsorption of CO was
2
1
6,17
explored at 273 K and 298 K.
As shown in Fig. 2, the
3
maximum absorption of CO at 273 K and 298 K was 93.44 cm
2
ꢁ1
3
ꢁ1
g
and 64.99 cm g , respectively, which are higher than that
in the literature, such as for NUC-5, MOF-5, and Uio-66.
Simultaneously, incomplete adsorption isotherms and CO
6
ꢁ
Fig.
HoZn(CO
H atoms are omitted for clarity). (b) 3D dual-channel framework of
1
(a) Structural building block of the TDP
ligand and
18
[
2
)
6
(H O)] (C, gray; O, red; Zn, cyan; Ho, yellow; N, blue; and
2
2
NUC-30 and its topology. (c) View of the 3D channel structure of desorption lead to a moderate hysteresis loop, which conrms
NUC-30 along the c-axis.
the strong interaction between the host and guest. Further-
more, in quantify the binding force between the CO molecules
2
and the framework of NUC-30, Qst was calculated using the
nanochannel. However, the hexagonal nanochannels (II) with virial method with the resulting value at zero coverage being
ꢁ
1
ꢁ
an aperture of 10.1 A show that the free carboxyl oxygen atoms 25.8 kJ mol , which implies that the adsorbed CO molecules
2
(C]O) reside on the inner walls, as shown in Fig. 1c. Therefore,
channel I and channel II have completely different functional-
ized inner surfaces, except for the C–H organic panels and
similar exposed metal parts on the walls.
Finally, for clarity, in the structure of NUC-30, the host
framework was simplied as an fng-type framework with the
6
4
Schl ¨a i symbol of {4 $6 } as analyzed by TOPOS40. In the
topology, the binuclear [HoZn(CO ) (H O)] SBUs and trigeminal
2
6
2
6
ꢁ
TDP ligand are simplied as ve-connected nodes, as shown
in Fig. 1b.
Water resistance of NUC-30
Due to the obvious advantages of MOFs in gas-phase and liquid-
phase adsorption elds, there is a lot of emphasis on their
stability in the chemical environment. However, the low water
resistance of most MOFs limits their application in the aqueous
15
2
phase. Therefore, the water resistance of NUC-30 in different Fig. 2 CO sorption isotherms of NUC-30 at 273 and 298 K.
9734 | RSC Adv., 2021, 11, 9731–9739
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can be easily released, allowing the host framework of NUC-30 unsatisfactory results obtained by employing single NUC-30 or
to be regenerated, as shown in Fig. S9.†
n-Bu NBr demonstrated that the active open metal sites of NUC-
4
ꢁ
30 and anions of Br played different roles during the cyclo-
addition reaction of CO and epoxides. Therefore, the optimal
reaction conditions used for subsequent experiments were
2
2
Catalytic cycloaddition of CO and epoxides
On account of the abovementioned characteristics including quantied as 1.0 mol% NUC-30 catalyst, 5 mol% n-Bu NBr co-
4
ꢀ
highly exposed metal cations, highly partitioned functionalized catalyst, and 1.0 MPa CO at 60 C for 8 h. Compared with the
2
20
nanochannels, large specic surface area, and good trapping recently reported MOF-based catalysts, NUC-30 showed
performance of CO , the catalytic efficiency of NUC-30 was a superior catalytic performance in terms of reaction conditions
tested for the chemical transformation of epoxides with CO2 (Table S4†), which should be attributed to its intrinsic charac-
under comparatively mild conditions into cyclic carbonates, teristics of active metal sites (Zn and Ho ), uncoordinated
which is viewed as a bifunctional reaction for synergistically pyridine and carboxyl oxygen atoms, and unimpeded void
solving the energy crisis and reducing the amount of green- space. Furthermore, besides the synergistic catalytic effect of
2
II
III
19
house gases. As illustrated in Table 1, 1,2-epoxypropane was NUC-30 and n-Bu NBr, the reaction temperature plays a key role
4
selected as a model to explore the effects of different reaction during the cycloaddition process.
conditions on the conversion of propylene oxide from CO
2
to
Under the established optimal reaction conditions including
propylene carbonate, and the obtained products were deter- 20 mmol epoxide, 1.0 mol% NUC-30, 5 mol% n-Bu NBr, and 10
4
1
ꢀ
mined by H NMR and GC-MS spectroscopy. Throughout the atm CO at 60 C for 8 h, a series of epoxide derivatives with
2
experimental process, the amount of NUC-30 was set at typical substitutes was used to check the catalytic universality of
1
.0 mol%. Entry 1 in Table 1 shows that the yield was 13% only NUC-30. As demonstrated in Table 2, the activated NUC-30
in the presence of only solvation-free NUC-30 at room temper- exhibited signicant catalytic activity for all the chosen
ature, while the co-catalyst Bu NBr under similar reaction substrates with yields higher than 96% under the set reaction
4
conditions had a yield of 6% (entry 2). In contrast, as can be conditions, except for styrene oxide with the relatively high
ꢀ
seen from entry 3, when 1.0 mol% NUC-30 and 2.5 mol% boiling point of 194 C, which may have limited its activity and
ꢀ
Bu
4
NBr were used simultaneously, the yield reached 49%, rendered the nal yield lower at the low temperature of 60 C.
which proved that NUC-30 and Bu NBr possessed completely The comparison of entries of 1–4 exhibited that epoxides with
4
different catalytic efficiency for the second-order reaction of single electron-withdrawing groups are more likely to undergo
cycloaddition from epoxides with CO . In addition, with an
2
increase in temperature, the yield increased correspondingly,
ꢀ
and the nal yield reached 99% aer 24 h at 60 C (entry 6).
However, when the amount of Bu NBr was doubled and NUC-30
4
Table 2 Cycloaddition reaction of CO
NUC-30 as the catalyst
2
and various epoxides with
a
remained unchanged, the yield increased to 98% within 8 h at
ꢀ
6
0 C (entry 10). Furthermore, as shown in entries 1 and 2, the
Table 1 Cycloaddition reaction of CO
various conditions
2
with styrene oxide under
a
a,b
Entry Epoxide
Product
Yield (%) Selectivity (%)
1
2
99
99
>99
>99
NUC-30
(mol%)
4
n-Bu NBr
(mol%)
ꢀ
b
Entry
T ( C)
t (h)
Yield (%)
3
4
98
96
>99
>99
1
2
3
4
5
6
7
8
9
1.0
0
0
25
25
25
40
50
60
60
60
60
60
24
24
24
24
24
24
2
4
6
8
13
6
2.5
2.5
2.5
2.5
2.5
5.0
5.0
5.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
49
72
85
99
41
65
88
98
5
92
>99
a
Reaction conditions: substrate (20 mmol), n-Bu
4
NBr (5 mol%), NUC-30
1
0
ꢀ
catalyst (1.0 mol% based on the Zn metal center), CO
2
(10 atm), 60 C,
a
b
Reaction conditions: 20 mmol propylene oxide, solvent free, CO
2
(10 and 12 h. Determined by GC/MS with n-dodecane as the internal
b
1
atm). Checked by H NMR and GC-MS spectroscopy with n-dodecane standard.
as the internal standard.
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the cycloaddition reaction, which should be attributed to the
fact that the electron absorption effect can effectively reduce the
electron density of ethylene oxide. Furthermore, by comparing
entries 1–4 and 5, it is clear that steric hindrance has a great
inuence on the cycloaddition reaction, which is consistent
with the existing similar reactions catalyzed by porous
21,22
MOFs.
According to our hypothesis, the reason for this is
that the larger substituents on the epoxides not only affect the
convenience of their access to the nanocages, but also reduce
the opportunity of interaction with the active sites. Moreover,
compared to the reported porous MOF-based catalysts of MOF-
23a
23b
23c
2
05(S),
superior catalytic efficiency, which should be ascribed to the
contribution of the [ZnHo(CO (H O)] units on the surface of
MMCF-2,
and Hf-Nu-1000,
NUC-30 exhibits
2
)
6
2
its channels based on the claried reaction mechanism. During
the 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.
Fig. 3 Catalytic mechanism for the cycloaddition of epoxides with
CO .
2
Luminescence characteristic and sensing property
The post-treatment including recovery and cyclic utilization
1
0
for a proposed catalyst is one of the most important factors for MOFs with d transition metal ions as constituents have been
evaluating its practical application besides excellent catalytic reported to be potential uorescent materials in the eld of
performance. Therefore, the ten-fold amplication reaction optics. Consequently, the solid-state luminescence properties of
based on the optical conditions with 1,2-epoxypropane as the the pure H TDP ligand and NUC-30 at room temperature were
6
substrate by employing 500 mg NUC-30 was repeated 5 times, studied, as shown in Fig. S13.† The maximum emission peaks
during which NUC-30 was recycled by ltration aer each cycle of H TDP and NUC-30 appeared at ꢂ400 nm and ꢂ410 nm,
6
and directly reused for the next cycle. When the h cycle was respectively. Compared with the H TDP ligand, the maximum
6
completed, the recovered sample of NUC-30 was rinsed with emission of NUC-30 has a weak red shi. Furthermore, due to
1
0
dichloromethane, and subsequently vacuum dried, leaving their d electronic conguration, it is difficult to oxidize or
a weight of 482 mg with the loss of 3.6%. The ICP analysis of reduce the Zn(II) and Ho(III) ions. Moreover, compared with the
the reaction solution showed that there was only a trace free H TDP, the emission of NUC-30 was observed to be red-
6
amount of Zn(II) (z0.027%) and Ho(III) (z0.018%) ions shied, which is mainly due to the coordination effect
leached from the host framework. Notably, the catalytic effect between the ligand and metal ions. In addition, the excellent
of NUC-30 was almost identical during the ve cycles, which water-resistance of NUC-30 makes it a potential luminescence
was conrmed by the transformation yield, as shown in probe to detect different metal ions.
Fig. S10.† Simultaneously, the PXRD pattern of the recovered
NUC-30 sample aer ve experimental runs was basically
consistent with the original pattern, indicating that the Determination of Fe cation
3
+
structural integrity of NUC-30 was still intact (Fig. S11†). As
The investigation of the sensing ability of NUC-30 for various
shown in Fig. S12,† the catalytic reaction was close to inactive
aer the catalyst was separated from the reaction system for
a period of time, which conrmed the catalytic activity of NUC-
cations was studied by adding 2 mg samples in 2 mL of 0.01 M
3
0.
Given the claried structure of NUC-30 and recently pub-
6,23
lished studies, the speculated catalytic mechanism is shown
in Fig. 3. Primarily, the oxygen atom of the selected epoxide
interacts with the open zinc sites on the inner wall of the
nanotubular channel to decrease the original structural stability
of the epoxide ring. Consequently, the bromide atom of TBAB
poses nucleophilic attack on the less hindered carbon atom of
the activated epoxide to generate one intermediate alkylcar-
bonate anion, which tends to launch an attack to the neigh-
bouring polarized CO molecule caught by the Lewis-acid metal
2
sites in the nanochannel. Hence, it can be inferred that the
whole process of cycloaddition can be boosted by coordination
via the weak Lewis acidic–basic interactions involved in the
porous framework.
Fig. 4 Luminescence intensity of NUC-30 at 397 nm in various
cations.
9736 | RSC Adv., 2021, 11, 9731–9739
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epoxides into the corresponding carbonates under compara-
ꢀ
tively mild conditions of 10 atm CO ow and 60 C. Meanwhile,
2
the water-resistant framework of NUC-30 could selectively and
3
+
sensitively discriminate Fe ions in aqueous solution. In fact,
NUC-30 represents a class of emerging heterometallic nano-
porous materials based on heterometallic binuclear SBUs.
These materials not only feature the characteristics of dual
tubular nanochannels, high porosity and specic surface area,
but also possess tetra-coordinated transition metal ions and
3
+
Fig. 5 (a) Luminescence spectra of the NUC-30@Fe suspensions
3+
with the Fe concentration varying from 0 to 0.75 mM (excited at 397
nm). (b) Plot of relative intensity vs. Fe concentration.
2+
octa-coordinated rare earth ions. Although the ions of Zn and
3+
3
+
Ho were selected to construct the targeted model for the
catalytic study on the chemical xation of CO by our group,
2
other combinations of transition metal ions and octa-
coordinated rare earth ions with the aid of H TDP can also be
6
assembled for other specic functional studies in the future,
such as optics and magnetism.
+
2+
aqueous solutions of twelve cations, M(Cl)
x
(M ¼ Na , Zn ,
2
+
2+
+
2+
2+
2+
2+
3+
3+ 2+ 2+
Co , Hg , K , Ca , Ba , Cu , Mg , Al , Fe , Fe , and Sn )
in water at room temperature. As seen in Fig. 4, the luminescent
n+
intensity of the M @CP suspensions varied with the different
3
+
types of metal cations. The results exhibited that the Fe cation
Conflicts of interest
has the best luminous quenching effects on NUC-30. The rela-
3
+
tionship between the concentration of Fe cations and the
luminescent intensity of the NUC-30 suspension was studied
synchronously. As seen in Fig. 5, the uorescent intensity of
NUC-30 displayed an obvious quenching effect with the addi-
tion of Fe solution. When the concentration of Fe reached
.7 mM, the quenching rate of NUC-30 increased to 97%
The authors have no conicts of interest to declare.
Acknowledgements
3
+
3+
The work was supported by nancial support from the Natural
Science Foundation of China (21101097, 51573124), the
Opening Foundation of Key Laboratory of Laser & Infrared
System of Shandong University (2019-LISKFJJ-005), and the
start-up funds of North University of China.
0
(
3
+
Fig. 5a). The non-linear Stern–Volmer curve for NUC-30@Fe
in aqueous system is consistent with the exponential equation
3
+
of I
0
/I ¼ ꢁ30 ꢃ exp[Fe ]/ꢁ2289 ꢁ 31 for NUC-30 (Fig. 5b). In
addition, as shown in Fig. S14,† NUC-30 had a high KSV value of
4
ꢁ1
3+
2
.413 ꢃ 10 M for Fe ions by tting the experimental data-
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©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 9731–9739 | 9739