A novel polyoxovanadate-based Co-MOF: Highly efficient and selective oxidation of a mustard gas simulant by two-site synergetic catalysis

Article abstract of DOI:10.1039/d0ta00537a

Two novel polyoxovanadate-based metal-organic frameworks (MOFs), [Co(bib)]{V2O6} (V-Co-MOF) and [Ni(en)(bib)]{V2O6}·2H2O (V-Ni-MOF) (bib = 1,4-bis(1H-imidazoly-1-yl)benzene, en = ethylenediamine) were facilely synthesized under mild hydrothermal condition

Full text of DOI:10.1039/d0ta00537a

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DOI: 10.1039/D0TA00537A
ARTICLE
A novel polyoxovanadate-based Co–MOF: highly efficient and
selective oxidation of mustard gas simulant by two-site synergetic
catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Hong-Rui Tian, Zhong Zhang, Shu-Mei Liu, Tian-Yi Dang, Xiao-Hui Li, Ying Lu, Shu-Xia Liu*
DOI: 10.1039/x0xx00000x
Two novel polyoxovanadate-based metal–organic frameworks (MOFs), [Co(bib)]{V
Ni(en)(bib)]{V }2H O (V-Ni-MOF) (bib = 1,4-bis(1H-imidazoly-1-yl)benzene, en = ethylenediamine) were facilely
synthesized under mild hydrothermal conditions. Single-crystal X-ray diffraction analysis shows that the V sites in both
compounds adopt {VO } tetrahedral coordination geometries, and the Co center in V-Co-MOF presents four-coordinated
2
O
6
}
(V-Co-MOF) and
[
2
O
6
2
4
distorted tetrahedron configuration (coordinatively unsaturated metal sites, CUMS), while the Ni center in V-Ni-MOF
exhibits six-coordinated octahedral geometry (coordinatively saturated metal sites, CSMS). Given that the CUMS can
generally be used as active site for catalytic reaction, we explored the catalytic activities of these two compounds for the
oxidation of mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES). The experimental results indicate that they can catalyze
oxidation of CEES to give the only nontoxic product, 2-chloroethyl ethyl sulfoxide (CEESO). Significantly, V-Co-MOF exhibits
higher catalytic activity, it converts 100% of the CEES in 10 min, whereas V-Ni-MOF converts only 47.5% of CEES under the
identical conditions. Researching the mechanism of the catalytic reaction found that the excellent catalytic performance of
2 2
V-Co-MOF was attributed to the two-site synergetic effect: (1) the oxidant H O interacts with V site to produce
peroxovanadium with higher oxidation activity; (2) the S atom in CEES coordinates with the four-coordinated Co(II) center
to obtain 2-chloroethyl ethyl sulfonium cation (CEES+), which makes the CEES more easily oxidize to CEESO based on the
oxidation mechanism of peroxovanadium, and shortening the molecular size distance between CEES and the obtained
peroxovanadium, thereby greatly improving the rate of the catalytic reaction. To our knowledge, this is the first dual-active-
site polyoxometalate-based MOFs catalyst for catalysing the oxidative detoxification of CEES.
materials and methods can safely and effectively
decontaminate HD has been extensively concerns.
Introduction
The rapid detoxification of chemical war agents (CWAs) under
mild conditions is a vital issue due to they have been used in
past combats and recent terrorist attacks. Bis-(2-chloroethyl)
sulfide (mustard gas or HD), a highly toxic vesicant agent that
causes serious skin blisters, irritation to respiration system and
eyes, damaging DNA, and even death at high doses, is one of
the most widely used CWAs because of it can be readily and
cheaply produced.3 Generally, the methods of detoxification
Polyoxovanadates (POVs) as
a
special branch of
polyoxometalates (POMs), have attracted great interest
because of their molecular magnetism, redox activity as well as
1
, 2
2
0-22
bioactivity.
In particular, POVs have been used as oxidative
catalyst for catalysing various organic reactions, such as olefin
2
3-25
26-29
epoxidation,
alcohol oxidation
and oxidative
3
0-32
desulfurization,
due to their oxidizing activity. Recently, a
series of POVs have been confirmed to be efficient catalyst for
the selective oxidation of HD and/or its simulants under H as
, 4
2 2
O
5
-8
9, 10
of HD include hydrolysis,
dehydrohalogenation,
and
oxidant based on the oxidation mechanism of
oxidation1 (Scheme 1), among which the partial oxidation of
1-15
1
,
HD to sulfoxide is considered to be the most effective pathway.
1
6
However, it is important that the oxidation should be
selective for the sulfoxide, as overoxidation of the HD to sulfone
1
7-19
that is highly toxicity.
Therefore, the development of new
Key Laboratory of Polyoxometalate Science of the Ministry of Education,
College of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P.
R. China. Email: liusx@nenu.edu.cn.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1 Common pathways of HD detoxification.
peroxovanadium.33-35 Although these POV-based catalysts can
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-
1
eliminate toxicity of HD and/or its simulants, they suffer from 50°, scan rate = 10° min ). The nitrogen adsorption / desorption
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low reaction kinetics, which may be attributed to the fact that measurement was taken by ASAP 2020 V4D.O00I: 1in0.s1t0r3u9m/De0nTtA0a0t57377KA.
these catalysts only have active sites that interact with the X-ray photoelectron spectra (XPS) were collected on Thermo
1
oxidant (H
2 2
O
) to produce higher oxidative peroxovanadium ESCALAB 250Xray photoelectron spectrometer. HNMR spectra
1
and no sites for activating the substrate molecular (HD and/or were recorded on a Bruker Advance 600 ( H: 600 MHz) at
its simulants). In addition, the difficulty in separation and ambient temperature. The Raman spectra were obtained on
reusability of catalyst in certain homogeneous catalytic systems SNFT-SRLab1000 with an excitation wavelength of 785 nm. The
severely restrict their practical application. Therefore, conversion, selectivity and the amounts of capture of CEES were
developing highly efficient, selective, recyclable heterogeneous monitored by a Shimadzu GC-2014 instrument equipped with a
catalyst for catalyzing oxidation of mustard gas still remains flame ionization detector and a SH-Rtx-Wax column. The GC-MS
challenging.
Polyoxometalate-based
2 6
POMOFs) are crystalline hybrid materials constructed by POMs Synthesis of [Co(bib)]{V O
spectra were measured on an Agilent 7890A-5975C instrument
frameworks for confirming the identity of the product.
} (V-Co-MOF)
(24.7 mg, 0.2 mmol), N H O (80%, 4.2μL, 0.068 mmol)
and Co(OAc) 4H O (25 mg, 0.1 mmol) were dissolved in 9.5 mL
metal–organic
(
and metal–organic frameworks (MOFs), which have attracted
great interest in the realm of catalysis because of they can
NaVO
3
H
2 4
2
2
2
3
6-39
integrate multiple catalytically active sites into one.
Moreover, their well-defined structures make it possible to
distilled water. After the mixture was stirred for 0.5 hour, a
solution of bib (21 mg, 0.1 mmol) in DMF (0.5 mL) was added
above mixture. Then the mixture was stirred for 1 hour. The pH
value of the mixture was adjusted with 1M HCl to 4.5 and was
sealed in a 23 mL Telflon reactor and kept under autogenous
pressure at 130 °C for 3 days. Then the mixture was cooled to
room temperature at a rate of 10 °C/hour, and cyan strip
crystals were obtained (yield: 25.6 mg, 54.8% based on V).
4
0-42
explore the structure-function correlations.
successfully synthesized two novel POV-based MOFs,
Co(bib)]{V } (V-Co-MOF) and [Ni(en)(bib)]{V }2H O (V-Ni-
MOF), (bib 1,4-bis(1H-imidazoly-1-yl)benzene), en
ethylenediamine). The two compounds can catalyse oxidation
of mustard gas simulant 2-chloroethyl ethyl sulfide (CEES) to
give the only product, 2-chloroethyl ethyl sulfoxide (CEESO).
Significantly, V-Co-MOF (t1/2 = 2.6 min) shows higher catalytic
activity than V-Ni-MOF (t1/2 = 11 min) under identical reaction
conditions, which can be attributed to two sites cooperative
In this work, we
[
O
2 6
O
2 6
2
=
=
4 6 2
Elemental analysis for C12H10CoN O V : C, 30.86%; H, 2.16%; N,
1
1
1
9
2.00%; Co, 12.62%; V, 21.81%; Found: C, 30.79%; H, 2.24%; N,
-1
1.89%; Co, 12.56%; V, 21.74; FTIR (cm ): 3115(w), 1527(s),
503(w), 1442(w), 1320(m), 1265(m), 1137(w), 1070(s), 955(s),
15(w), 897(w), 827(s), 803(s), 632(s), 620(s), 593(w), 554(w)
work of V-Co-MOF. Specially, (1) V(V) site on {V
2 6
O } reacts with
to produce higher activity peroxovanadium; (2) the S atom
2 2
H O
(
Figure S3a).
Synthesis of [Ni(en)(bib)]{V
NaVO (24.7 mg, 0.2 mmol), en (80%, 60 μL, 0.9 mmol),
Ni(OAc) 4H O (100 mg, 0.4 mmol) and bib (21 mg, 0.1 mmol)
of CEES coordinates with coordinatively unsaturated Co(II) on
the framework to generate a 2-chloroethyl ethyl sulfonium
cation (CEES+). More importantly, the connection mode of V-O-
Co makes the obtained peroxovanadium more accessible to the
CEES+ to achieve the rapid oxidation of CEES to CEESO. To the
best of our knowledge, this is the first dual-active-site POMOF
catalyst for catalysing selective oxidation of CEES and its
catalytic activity is superior to the most documented POVs and
POMOFs-based catalysts.
2 6 2
O }2H O (V-Ni-MOF)
3
2
2
were dissolved in 10 mL distilled water. After the mixture was
stirred for 1 hour, the pH value of the mixture was adjusted with
1
M HCl to 6.5 and was sealed in a 23 mL Telflon reactor and kept
under autogenous pressure at 120 °C for 3 days. Then the
mixture was cooled to room temperature at a rate of 10
°
C/hour, and light green strip crystals were obtained (yield: 40
mg, 71% based on V). Elemental analysis for C14 Ni: C,
9.87%; H, 3.94%; N, 14.93%; Ni, 10.43%; V, 18.10%; Found: C,
22 6 8 2
H N O V
2
2
1
Experimental
Materials and Instrumentation
9.78%; H, 3.98%; N, 14.87%; Ni, 14.85%; V, 17.96%; FTIR (cm-
): 3109(w), 1575(w), 1521(s), 1493(w), 1460(w), 1320(w),
All reagents and solvents used were purchased from 1268(w), 1238(w), 1065(w), 1034(m), 964(w), 952(w), 928(m),
commercial suppliers and without further purification. The 897(w), 797(w), 739(w), 657(s), 605(m), 559(w), 532(w), 496(s)
ligand bib was bought from Jilin Province Extension Technology (Figure S3b).
Co., Ltd. Elemental analyses (C, H and N) were conducted on a Crystallography
Perkin-Elmer 2400 CHN elemental analyzer. V, Co and Ni were
Single-crystal X-ray diffraction (SXRD) data of V-Co-MOF and V-
analyzed on a PLASMA-SPEC (I) ICP atomic emission
Ni-MOF were collected on a Bruker diffractometer using Mo Kα
spectrometer. The Fourier transform infrared (FTIR) spectra
radiation (λ=0.71073 Å) at 298 K. The empirical absorption
were collected on an Agilent Technologies Cary 630FT-IR
correction was based on equivalent reflections. The structure
-1
spectrophotometer in the range 400-4000 cm . Thermal
gravimetric analysis (TGA) was measured on a SDT 2960
Simultaneous DSC-TGA of TA instruments up under nitrogen
atmosphere with a heating rate of 10 °C/min to 600 °C. Powder
X-ray diffraction (PXRD) measurements were performed on a
Smartlab instrument with Cu Kα radiation (λ =1.5418 Å, 2θ = 5°-
was solved by the direct methods and refined by full-matrix
least-squares fitting on F using the SHELXTL crystallographic
software package. Non-hydrogen atoms were refined with
anisotropic displacement parameters during the final cycles. All
hydrogen atoms of the organic molecule were placed by
geometrical considerations and were added to the structure
2
2
| J. Name., 2012, 00, 1-3
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factor calculation. Crystallographic data for V-Co-MOF and V-Ni- Crystal structures and characterizations of V-Co-MOF and V-Ni-
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DOI: 10.1039/D0TA00537A
MOF (1976374 and 1976375) can be obtained free of charge MOF.
from The Cambridge Crystallographic Data Centre. Crystal data
and structure refinement is summarized in Table S1.
General procedure of catalytic oxidation of CEES
Single-crystal X-ray diffraction (SXRD) analysis shows that both
2-
V-Co-MOF and V-Ni-MOF contain 1D {V
abbreviated as {V
2 6
O
} polyanion chains
(
2 6
O }), in which V ions adopt {VO
4
} tetrahedral
CEES (0.25 mmol), biphenyl as internal standard (0.25 mmol), coordination geometry (Fig. 1a). In addition, the four-
catalyst (0.025mmol), 30% H (0.3 mmol) and EtOH (2 mL) coordinated Co(II) ion in V-Co-MOF is ligated by two N atoms
were placed in 10mL sealed vial and stirred at 25 °C for 10 min. from two bib molecules and two O atoms from adjacent {V
The conversion and selectivity of CEES were determined by GC. chains (Fig. 1b, d), while Ni(II) ion in V-Ni-MOF presents six-
2 2
O
2 6
O }
Absorption test of CEES
coordinated structure with two N atoms from two bib
molecules, two N atoms from one en molecule and two O atoms
from adjacent {V O } chains (Fig. 1c, e). It is well known that
2 6
CEES (0.25 mmol), biphenyl as internal standard (0.25 mmol)
and EtOH (2 mL) were placed in 10mL sealed vial and stirred.
After the solution was clarified, 10 μL of solution was taken out
as initial concentration of CEES. The catalyst (0.025mmol) was
added to above reaction vial and the mixture was stirred at 25
coordinatively unsaturated metal sites (CUMS) in the MOFs
created by structural defects can regulate interactions between
MOFs and substrate molecules, to activate substrate molecules
4
3-
and thereby facilitate rapid progress of the desired reaction.
°
C. Then, 10 μL of suspension was taken out and centrifuged to
4
4
Remarkably, both V-Co-MOF (V ions and Co ions) and V-Ni-
remove solid particles at fixed time intervals. The supernatant
was diluted with 100 μL of EtOH, and then 1 μL of the diluted
sample was injected into the GC to monitor absorption amount
of CEES.
MOF (V ions) contain CUMS, which are expected to be of
potential for substrate molecular activation. In addition, two 2D
bimetallic oxide layers, {CoV
Ni-MOF, are constructed from the 1D {V
4 2
through {CoN tetrahedron (Fig. 1b) and {NiN O }
2 6
O } in V-Co-MOF and {NiV
2 6
O } in V-
2 6
O } chains linked
Recycle experiment of V-Co-MOF
2 2
O }
The reusability of catalyst V-Co-MOF was tested using CEES as
the substrate. After the first catalytic reaction was finished, the
catalyst was recovered by centrifugation and then washed
three times with EtOH. The recovered V-Co-MOF was dried
under air overnight and reused for the next run under identical
conditions.
octahedron (Fig. 1c), respectively (Fig. 1d, e). Co(II) ions and
Ni(II) ions in the two 2D oxide networks respectively linked by
ligand bib into two 3D frameworks (Fig. 1f, g).
Bond valence sum (BVS) calculations reveal that the
oxidation states of V and Co / Ni ions in compound V-Co-MOF
2 6
Fig. 1 Schematic of the crystal structures of V-Co-MOF and V-Ni-MOF. (a) The 1D {V O } polyanion chains of V-Co-MOF and V-Ni-MOF. (b, c) Coordination modes of
Co ion in V-Co-MOF and Ni ion in V-Ni-MOF. (d, e) The 2D bimetallic oxide layers of V-Co-MOF and V-Ni-MOF. (f, g) The 3D frameworks of V-Co-MOF and V-Ni-MOF.
Hydrogen atoms, lattice water molecules and carbon atoms in en are omitted for clarity.
and V-Ni-MOF are +5 and +2, respectively (Table S2). The X-ray
photoelectron spectra (XPS) experiments of V-Co-MOF and V-
Ni-MOF were carried out to further confirm the above results
Results and Discussion
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(
Figure S1). In the XPS of V-Co-MOF, the peaks with binding (Fig. 2b). Notably, V-Co-MOF can catalyse complete conversion
View Article Online
DOI: 10.1039/D0TA00537A
energies of 517.01 eV and 781.84 eV match the value of V(V) of CEES in 10 min, while V-Ni-MOF catalyses less than 50%
2
4
5, 46
p3/2 and Co(II) 2p3/2, respectively.
The observation of peaks conversion of CEES under identical conditions (Fig. 2b).
at 516.92 eV and 956.07 eV in XPS of V-Ni-MOF can be assigned Meanwhile, the high catalytic activity and selectivity of the V-
4
5, 46
1
to V(V) 2p3/2 and Ni(II) 2p3/2, respectively.
The experimental Co-MOF are further confirmed by H-NMR spectrum (Figure S7).
powder X-ray diffraction (PXRD) patterns of V-Co-MOF and V- During the whole catalytic reaction process, only CEESO was
Ni-MOF match well with the simulated peaks (Figure S2), obtained and CEES was completely converted within 10 min.
indicating that the phase purities of these compounds. The
Fourier transform infrared (FTIR) spectra of V-Co-MOF and V-Ni- oxidation reaction of CEES with V-Co-MOF as catalyst. As shown
MOF are recorded in Figure S3. The characteristic absorption in Figure S8, both the quantity of catalyst and H can affect
bands in 950-1000 cm and 540-650 cm are corresponding to the oxidation of CEES. The optimal ratio of catalyst (V-Co-MOF)
Subsequently, we research the factors that influence the
2 2
O
-1
-1
4
7
the V=O stretching modes and V-O-V bending, respectively.
to CEES is 0.1 to 1 (Figure S8a). The conversion of CEES was
-1
The peaks of 1100-1600 cm region arise from ligand bib (C=C) lowered to 80.0% when the catalyst amount was reduced to
4
8-49
and en (C-N and C-C).
and V-Ni-MOF are confirmed by thermogravimetric analyses increased to 0.12 eq., the conversion of CEES was almost
TGA) (Figure S4). unchanged. The feasible ratio of H to CEES is 1.2 to 1.0
Figure S8b). When the amount of H was decreased to 1.0 eq.
and 0.8 eq., the conversion of CEES was dented to 86.4% and
5.0%, respectively. In addition, according to the conversions of
The thermal stabilities of V-Co-MOF 0.08 eq. (eq. = equivalent). When the quantity of catalyst was
(
2 2
O
(
2 2
O
Catalytic oxidation for CEES.
Since CEES is a widely used simulant for the study of HD
oxidative degradation, we investigated catalytic activities of
compound V-Co-MOF and V-Ni-MOF using CEES. The catalytic
reaction of CEES (0.25 mmol) was conducted in ethanol (EtOH,
6
the CEES oxidative reaction change as a function of time
investigate the kinetics of CEES oxidation for compound V-Co-
MOF and V-Ni-MOF (Figure S9), which indicates that the
oxidation process of CEES was a pseudo-first order reaction. The
2
.0 mL) with 30% H
2 2
O (0.30 mmol) as oxidant, biphenyl
internal standard, 0.25 mmol), and the catalyst (V-Co-MOF or
(
rate constants k
1
and k
2
for V-Co-MOF and V-Ni-MOF are
V-Ni-MOF, 0.025 mmol, 10% mol) at 25 °C. For a better
dispersion in EtOH media, the V-Co-MOF and V-Ni-MOF crystals
were ground and the Brunauer–Emmett–Teller (BET) surface
area of used catalysts were 171 m /g and 80 m /g (Figure S5),
respectively. The catalytic reaction was monitored by gas
chromatography with flame ionization detection (GC-FID) and
the product was confirmed by gas chromatography-mass
spectrometer (GC-MS). As shown in Fig. 2, 2-chloroethyl ethyl
sulfoxide (CEESO) is the only product and no overoxidized
-1
-1
0
.27044 min and 0.06273 min , respectively, in which their
half-lives (t1/2) are calculated to 2.6 min and 11.0 min, further
manifesting that the catalytic activity of V-Co-MOF is
significantly higher than that of V-Ni-MOF.
To explore the reason of higher catalytic activity of V-Co-
MOF than V-Ni-MOF, we compare the structures and
compositions of these two compounds. As mentioned earlier, in
terms of components, although V-Co-MOF and V-Ni-MOF
2
2
contain the same ligand bib and {V
differences that V-Ni-MOF not only involves an additional ligand
en, but also the adjacent {V } chains are linked by Ni(II) ions,
while the adjacent {V } chains of V-Co-MOF are connected
2 6
O } unit, there are some
2 6
O
2 6
O
with Co(II) ions; In the field of structures, the most significant
difference is that the six-coordinated Ni(II) ions in V-Ni-MOF are
coordinatively saturated, whereas the four-coordinated Co(II)
ions in V-Co-MOF are coordination unsaturated. To evaluate the
influence of the components of V-Co-MOF (bib, Co(II), and V(V)
in {V
the oxidation process of CEES, a series of control experiments
were carried out using bib, en, NiCl , CoCl and NaK{V } as
catalysts, respectively. NaK{V } (Figure S10) was synthesized
according to the literature, which possessed the same {V
2 6 2 6
O }) and V-Ni-MOF (bib, en, Ni(II), and V(V) in {V O }) on
2
2
2 6
O
2 6
O
5
0
2 6
O }
units as V-Co-MOF and V-Ni-MOF. As shown in Table 1, ligand
bib or en alone produces no CEES oxidation, indicating that the
ligand (bib and en) do not contribute to the catalytic activity of
2 2
V-Co-MOF and V-Ni-MOF (Table 1, Entry 2-3). NiCl and CoCl
Fig. 2 (a) Oxidation of CEES to CEESO catalysed by V-Ni-MOF and V-Co-MOF. (b) can catalyse CEES oxidation: conversions are 12.5% and 13.0%,
Time profile for the CEES using V-Ni-MOF and V-Co-MOF as catalyst, and blank.
2 6
respectively, but obviously lower than NaK{V O }, which the
Reaction conditions: CEES (0.25 mmol), 30% H
0.025mmol), biphenyl (internal standard, 0.25 mmol), and EtOH (2.0 mL) at 25 conversion is 55.1% (Table 1, Entry 4-6), thus we can deduce
C for 10 min.
2 2
O (0.30mmol), catalyst
(
°
that V(V) in {V
2 6
O } is the catalytic active site for CEES oxidation.
and CoCl are almost equal,
2
product 2-chloroethyl ethyl sulfone (CEESO ) (Fig. 2a, Figure S6),
In addition, the conversions of NiCl
2
2
indicating that V-Co-MOF and V-Ni-MOF have high selectivity.
Blank experiment showed negligible activity without catalyst
showing that the catalytic activity of V-Co-MOF is higher than
that of V-Ni-MOF is independent of the nature of the metal ions
4
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(
Co(II) and Ni(II)) and may be related to the open sites of the (Fig. 3b and Figure S11). In comparison with the FTIR spectrum
View Article Online
DOI: 10.1039/D0TA00537A
metal ions because of CUMS can interact with substrate to of the free V-Co-MOF, the FTIR spectrum of CEES-absorbing V-
-1
facilitate chemical reaction. The NaK{V
with NiCl and CoCl for CEES oxidation, respectively, and the the character peak of CH
conversions (NaK{V } + NiCl (65.1%); NaK{V } + CoCl obvious red shift from 1215 cm of free CEES indicates that the
65.8%)) are higher than that of V-Ni-MOF (47.5%), but absorption and activation of CEES by V-Co-MOF. There is no
obviously lower than that of V-Co-MOF (100%) (Table 1, Entry significant difference in the FTIR spectra of CEES-absorbing V-
-10). There appears to be some cooperativity between V(V) Ni-MOF before and after, suggesting that V-Ni-MOF cannot
2 6
O } was physically mixed Co-MOF exhibits a new peak at 1208 cm , which correspond to
5
1
2
2
2 2
wagging of the CH -S group. The
-1
2
O
6
2
O
2 6
2
(
7
and coordinatively unsaturated Co(II) in V-Co-MOF. On the basis capture CEES. Meanwhile, we can also know that the
of above experimental results, we speculate that the high coordinatively unsaturated V(V) in V-Co-MOF and V-Ni-MOF
catalytic activity of V-Co-MOF is attributed to synergistic effect cannot absorb CEES. Combing the above experimental results
of the V(V) in {V
on the framework. On the one hand, the V(V) reacts with H
2
O
6
} and the coordinatively unsaturated Co(II) and the structural differences of V-Co-MOF and V-Ni-MOF, we
deduce that V-Co-MOF absorbs and activates CEES is ascribed
2 2
O
to produce higher active peroxovanadium; on the other hand, to its Co(II) with coordinatively unsaturated site interacts with
the coordinatively unsaturated Co(II) coordinates with S atom the lone pairs electrons of S atom in CEES to produce CEES+
in CEES to afford the CEES+, which not only reduces the electron intermediate, and this phenomenon has been confirmed by
5
2
density of the S atom but also shortens the molecular size Bandosz and co-workers. To prove the above assumption, the
distance between CEES and the generated peroxovanadium. XPS measurements were performed on V-Co-MOF-A and V-Ni-
Finally the O atom in peroxovanadium nucleophilic attacks the MOF-A (V-Co-MOF-A and V-Ni-MOF-A refer to V-Co-MOF and V-
sulfonium cation in CEES+ to realize the rapid conversion of Ni-MOF after absorbing CEES). As shown in Figure S12,
CEES to CEESO.
comparing with the Co 2p spectrum of V-Co-MOF, an obvious
Reaction mechanism of V-Co-MOF for catalysing oxidation of CEES downward shift of binding energy of a small amount of Co 2p is
observed for V-Co-MOF-A (Figure S12a). This suggests a higher
To confirm the aforementioned assumptions, the
electron density on the Co ions in V-Co-MOF-A, which indicates
absorption tests of CEES were firstly performed on V-Co-MOF
5
3
the transfer of electrons from CEES to Co ions in V-Co-MOF.
and V-Ni-MOF, respectively, and the amount of absorbed CEES
were determined by GC. As shown in Fig. 3a, V-Co-MOF rapid
captures CEES and the maximum amount of absorption is 14
However, there is no significant difference in the binding energy
of Ni 2p in V-Ni-MOF and V-MOF-A (Figure S12b), thus
reaffirming that V-Co-MOF can absorb CEES due to the
coordinatively unsaturated Co(II) sites. Subsequently, we
explored the active species of V-Co-MOF catalytic oxidation of
Table 1 Oxidation of CEES by different catalystsa.
Cl
catalyst, 30% H2O2
Cl
S
CEES using H
with H
2
O
2
as oxidant. It is well known that POVs interact
S
EtOH, 25 °C
O
54
2
O
2
tend to form peroxovanadium, thus we speculate
CEES
CEESO
that the peroxovanadium is the active species in the CEES
oxidation because the reaction does not occur without H
Table 1, Entry 11-12). In this case, FTIR and Raman spectra
confirm the forming of active peroxides. The FTIR and Raman
2 2
O
Entry
Catalyst
―
en
Conversion (%)e
(
1
2
<2
<2
spectra of H
2
O
2
-treating V-Co-MOF exhibit a new peak at 860
3
bib
<2
-1
-1
cm and 870 cm , respectively (Figure S13), which are assigned
to O-O stretch.
4
5
6
7b
8c
9
NiCl
CoCl
NaK{V
NaK{V
NaK{V
2
12.5
13.0
55.1
65.1
65.8
47.5
100
―
55
2
A possible mechanism was proposed for the catalytic
oxidation reaction of CEES with V-Co-MOF based on above
2 6
O }
}+NiCl
}+CoCl
2
2
O
6
2
5
2, 56, 57
experimental results and reported literatures
Initially, the V(V) and four-coordinated Co(II) in V-Co-MOF (a)
interact with H and CEES, respectively. Specially, the V(V) on
} reacts with H to afford the active peroxovanadium
(Scheme 2).
2
O
6
V-Ni-MOF
V-Co-MOF
V-Ni-MOF
V-Co-MOF
2 2
O
1
0
{V
O
2 6
2 2
O
1
1
1d
2d
and the Co(II) on the framework coordinates with S atom in
CEES to form transitional sulfonium cation (b). Then the O atom
of the resulting peroxovanadium nucleophilic attack on the
―
a
2 2
Reaction conditions: CEES (0.25 mmol), 30% H O (0.30mmol), catalyst
b
(
0.025mmol), and EtOH (2.0 mL) at 25 °C for 10 min. NaK{V
2
O
6
}(0.025 mmol) + sulfonium cation in CEES, and through a five-membered ring
c
d
NiCl
2
(0.025 mmol). NaK{V
2
O
6
}(0.025 mmol) + CoCl
2
(0.025 mmol). No the
transition state (c) to achieve the transfer of the O atom. Finally,
the CEES is oxidized to CEESO and the catalyst returns to the
original (a).
e
oxidant 30% H
2
O
2
in the catalyst reaction system. Yield was determined by GC
using biphenyl as internal standard (0.25 mmol).
Stability and recyclability of V-Co-MOF
mg / g, while V-Ni-MOF exhibited no absorption for CEES. In
addition, the FTIR spectra of V-Co-MOF and V-Ni-MOF after
absorption of CEES were carried out to further confirm that V-
Co-MOF can absorb and activate CEES while V-Ni-MOF cannot
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DOI: 10.1039/D0TA00537A
Fig. 3 (a) Adsorption capacity of V-Co-MOF and V-Ni-MOF for CEES. (b) FTIR spectra
of V-Co-MOF and V-Ni-MOF before and after absorbing CEES molecules, and FTIR
spectrum of CEES molecules. Note: A = After absorbing CEES molecules.
Scheme 2 Proposed catalytic mechanism of V-Co-MOF catalyses CEES oxidation.
The catalyst V-Co-MOF exhibits highly catalytic activity and
selectivity for CEES oxidation through the synergetic effect of
V(V) sites and Co (II) centres, thus its stability and recyclability
are critical for its practical application. A leaching test was
performed to verify the heterogeneity and stability of the
catalyst V-Co-MOF. As is shown in Fig. 4a, the catalyst was
removed by filtration after 2 min, and the solution was
maintained for another 8 min under the same conditions, but
no additional conversion was observed, suggesting that the
catalyst neither dissolved nor loss active species in the reaction
system. In addition, we weighed the recovered catalyst (0.0248
mmol, 11.58 mg) and the results showed that there was almost
no loss of the mass of the recovered catalyst, further proving
the stability of the catalyst V-Co-MOF. More importantly, there
was no significant decrease in conversion and selectivity after
five cycles (Fig. 4b), indicating excellent cycling stability of V-Co-
MOF. The PXRD patterns and the FTIR spectra of the recovered
catalyst matched well with the fresh catalyst (Fig. 4c, d),
revealing its structural integrity.
Fig. 4 (a) Catalyst V-Co-MOF filtered after 2 min. (b) Recycling tests for CEES oxidation
using V-Co-MOF. (c) The PXRD patterns of recovered V-Co-MOF after five catalytic
runs. (d) The FTIR spectra of recovered V-Co-MOF after five catalytic runs.
of CEES is greater than 98% after five cycles). According to the
experimental results and the structural analysis of the catalysts
V-Co-MOF and V-Ni-MOF) confirm that the perfect catalytic
performance of V-Co-MOF is attributed to the two sites synergic
work of V(V) centre on POVs and coordinatively unsaturated
Co(II) site on the framework. This work provides an insight on
designing more efficient POMOFs catalysts for CEES
decontamination.
Comparing with the reported POVs and POMOFs-based
catalysts for selective oxidation of CEES (Table S3), V-Co-MOF
exhibited several advantages. Firstly, as a heterogeneous
catalyst, V-Co-MOF showed higher activity than POVs-based
(
3
3
homogeneous catalyst (Table S3, Entry 3). Secondly, the rapid
detoxification of CEES was realized in V-Co-MOF (conversion of
CEES up to 100% in 10 min) by the cooperative work of two sites,
whereas longer reaction time was necessary in certain catalytic
3
4, 35, 58
systems (Table S3, Entry 3-6).
Thirdly, V-Co-MOF
exhibited higher selectivity for sulfoxide (CEESO) than POMOFs-
based catalyst and the catalytic reaction can proceed at 25°C
without heating (such as 45°C, 70°C) (Table S3, Entry 7-9).
Conflicts of interest
There are no conflicts to declare.
5
9-61
Hence, V-Co-MOF has the potential as a sustainable catalyst for
detoxification of mustard gas in practical applications.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Grant nos. 21872021 and 21671033).
Conclusion
In conclusion, two novel POVs-based MOFs were
synthesized and used as catalysts for the CEES detoxification.
The compound V-Co-MOF not only exhibits extraordinary
efficiency and selectivity in the oxidative detoxification of CEES
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA00537A
Table of Contents
A novel polyoxovanadate-based cobalt organic framework (V-Co-MOF) was
synthesized and it as a heterogeneous catalyst can catalyze the rapid selective
oxidation of mustard gas simulant to non-toxic product by two sites synergistic effect
V
II
of V and Co .