Versatile catalysts constructed from hybrid polyoxomolybdates for simultaneously detoxifying sulfur mustard and organophosphate simulants

Article abstract of DOI:10.1039/c9cy00094a

Chemical warfare agents (CWAs) containing sulfur mustard and organophosphates are among the most toxic materials known to mankind and have caused lots of casualties during wars and terrorist attacks. Development of catalysts with selective oxidation and hydrolysis abilities for detoxification of these two types of CWAs is highly desired. Here, we report a series of carboxylic acid modified polyoxomolybdate derivatives with high catalytic performance in the degradation two typical chemical warfare agent simulants: 2-chloroethyl ethyl sulfide (CEES) and diethyl cyanophosphonate (DECP), at room temperature. Our results demonstrate that CEES can be highly-selectively degraded to the corresponding far less toxic 2-chloroethyl ethyl sulfoxide (CEESO) with the oxidant H₂O₂ within 12 minutes, and DECP is almost completely hydrolysed to nontoxic products within 10 minutes. The catalytic activities of these compounds were found to be influenced by the metal cations, carboxylic acid ligands and heteroatoms of polyoxoanions. The corresponding order is respectively Co²⁺ > Ni²⁺, Zn²⁺, Mn²⁺; PABA > Ala, Ac; AsMo > TeMo.

Full text of DOI:10.1039/c9cy00094a

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DOI: 10.1039/C9CY00094A
Journal Name
ARTICLE
Versatile
catalysts
constructed
from
hybrid
polyoxomolybdates for simultaneously detoxifying sulfur
mustard and organophosphate simulants
Received 00th January 20xx,
Accepted 00th January 20xx
Yujiao Hou, Haiyan An,* Shenzhen Chang, Jie Zhang
DOI: 10.1039/x0xx00000x
www.rsc.org/
ABSTRACT: Chemical warfare agents (CWAs) containing sulfur mustard and organophosphates are among
the most toxic materials known to the mankind and have caused lots of casualties during the wars and terrorist
attacks. Development of catalysts with selective oxidation and hydrolysis for detoxification of these two types
of CWAs is highly desired. Here, we report a series of carboxylic acids modified polyoxomolybdate derivatives
with high catalytic performance in the degradation two typical chemical warfare agent simulants: 2-chloroethyl
ethyl sulfide (CEES) and diethyl cyanophosphonate (DECP), at room temperature. Our results demonstrate that
CEES can high-selectively degrade to the corresponding far less toxic 2-chloroethyl ethyl sulfoxide (CEESO)
with the oxidant H2O2 within 12 minutes, and DECP was almost completely hydrolysed to nontoxic products
within 10 minutes. The catalytic activities of these compounds were found to be influenced by the metal cations,
carboxylic acid ligands and the heteroatoms of polyoxoanions. The corresponding order is respectively Co2+
Ni2+, Zn2+, Mn2+; PABA > Ala, Ac; AsMo > TeMo.
>
active site can obviously influence the catalytic activity.4,6,7 So far,
most reported materials only can catalyze the degradation of one
kind of CWAs, either sulfur mustard or nerve agents. Thus, for the
practical application concerns, the exploitation of high-efficient
broad-spectrum catalysts with multiple detoxification pathways
remains a significant challenge.
1 Introduction
The destruction of chemical warfare agents (CWAs), including
vesicants such as sulfur mustard and nerve agents known as
organophosphates, has been always an active area of research
owing to their high toxicity and dangerousness to human beings
and the environment.1,2 Sulfur mustard can be effectively
destroyed by means of the selective oxidation to the far less toxic
sulfoxide.3,4 Organophosphates can be detoxified by the
hydrolysis of the labile P-X bond.4,5 In view of the hazard in
dealing with highly toxic CWAs, effective simulant molecules for
CWAs with similar chemical behaviours and architectures are
always handled by experiments. Various materials that catalyze
selective oxidation of sulfur mustard (or simulants) or hydrolysis
of organophosphates (or simulants) into nontoxic corresponding
products, have been developed, such as, metal organic frameworks
(MOFs), inorganic oxides, zeolites and modified activated
carbons.4-10 Hupp, Farha and coworkers have pioneeringly
documented that some Zr-based MOFs (UiO-66, UiO-67, MOF-
88 and Nu-1000) can efficiently catalyze the degradation of
CWAs, and found the chemical environment around a catalytic
Polyoxometalates (POMs), as a large group of inorganic metal
oxide clusters, bear important applications in catalysis, magnetism,
optics, medicine, etc.11-26 Currently, POMs have been explored as
catalytic materials for destroying CWAs by Hill, Nyman, Hu, Liu
and other groups, because of their designable structures, versatile
redox potentials and tunable acidity/basicity.27-35 Hill’s group
firstly reported H5PV2Mo10O40 can degrade the sulfur mustard gas
simulant 2-chloroethyl ethyl sulfide (CEES) in 1994.27 Since then,
some
polyoxotungstates ([(Fe(OH2)2)3(PW9O34)2]9-, [{PW11O39Zr(μ-
OH)(H2O)}2]8- ([Nd6O19]8-,
and polyoxoniobates
inorganic
POMs
such
as
metal-substituted
)
K12[Ti2O2][GeNb12O40]) were successively investigated in the
catalytic degradation of one kind of CWAs, either by oxidation or
by hydrolysis.28 Recently, a polyoxoniobate–polyoxovanadate
complex, H13[(CH3)4N]12[PNb12O40(VO)2(V4O12)2]·22H2O, as
bifunctional catalyst, was successfully developed to
homogeneously degrade sulfur mustard simulants by oxidation
and organophosphate simulants via hydrolysis.31 In the search for
hererogeneous POM materials for this application, two
College of Chemistry, Dalian University of Technology, Dalian 116023, P. R. China
Email:anhy@dlut.edu.cn; Fax:+86-411-84657675; Tel:+86-411-84657675
Electronic Supplementary Information (ESI) Synthesis of 13, 14 and 15; ORTEP drawing
of 1, 5, 9, 10 and 12, 1D supramolecular chain of 1, 5, 9 and 12; 2D supramolecular sheet
of 5 and 12; IR spectra, UV−Vis diffuse reflective spectra, TG plots, PXRD figures for
1−12; time profile for the oxidative reaction of CEES using 1 under different conditions;
kinetic analysis of CEES and DECP; crystal data and structure refinement for 1−12; the
formulas of compounds; selected bond lengths and angles for 1−12 is provided.
POM@MOF
hybrid
materials,
PW12@NU-1000
and
PV2Mo10@MIL-101, were prepared and tested for the degradation
of sulfur mustard and its simulant with high conversion but
relatively low selectivity.34,35 Catalytic degradation of CWAs
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using POM-based compounds has turned into a burgeoning 0.3 mmol)/ MnCl2·2H2O (0.0486 g, 0.3 mmol) andVKiewCAlrt(ic0l.e0O2n2linge,
DOI: 10.1039/C9CY00094A
research field in recent years. However, on the one hand, the 0.3 mmol) were added successively under stirring. Finally, the
reported POM-based catalysts still suffered from a lot of problems, solution was heated and stirred for one hour at 80 °C. The filtrate
such as slow degradation rate, low selectivity and poor was kept undisturbed for two weeks under ambient conditions, and
recyclability; on the other hand, broad-spectrum POM materials then crystals of 1−4 were isolated in about 52% yield (based on
simultaneously catalyzing the degradation of sulfur mustard and Mo). Elemental analyses: calcd for 1 Mo, 33.77; As, 4.39; Co,
nerve agents are still lacking.
1.73; K, 5.73; C, 14.79; N, 2.46; H, 2.39 (%). Found: Mo, 34.03;
As, 4.62; Co, 1.66; K, 5.45; C, 14.53; N, 2.27; H, 2.54 (%). FTIR
data (cm-1): 3360 (s), 1607 (s), 1548 (s), 1413 (w), 1271 (m),
1178(m), 923(m), 894(s), 777 (w), 678 (m), 620(w), 512 (m).
Calcd for 2: Mo, 33.94; As, 4.37; Ni, 1.71; K, 5.70; C, 14.71; N,
2.45; H, 2.44 (%). Found: Mo, 33.76; As, 4.28; Ni, 1.55; K, 5.43;
C, 14.48; N, 2.34; H, 2.29 (%). FTIR data (cm-1): 3361 (s), 1605
(s), 1546 (s), 1411 (w), 1271 (m), 1178(m), 924(m), 894(s), 776
(w), 678 (m), 619(w), 512 (m). Calcd for 3: Mo, 33.71; As, 4.39;
Zn, 1.91; K, 5.72; C, 14.75; N, 2.46; H, 2.18 (%). Found: Mo,
33.95; As, 4.56; Zn, 1.64; K, 5.94; C, 14.67; N, 2.32; H, 2.07 (%).
FTIR data (cm-1): 3360 (s), 1606 (s), 1548 (s), 1414 (w), 1272 (m),
1179(m), 922(m), 894(s), 778 (w), 679 (m), 620(w), 513 (m).
Calcd for 4: Mo, 33.46 As, 4.35; Mn, 1.60; K, 5.68; C, 14.64; N,
2.44; H, 2.23 (%). Found: Mo, 35.35; As, 4.22; Mn, 1.69; K, 5.33;
C, 14.75; N, 2.18; H, 2.30 (%). FTIR data (cm-1): 3358 (s), 1609
(s), 1548 (s), 1411 (w), 1272 (m), 1178(m), 926(m), 895(s), 779
(w), 678 (m), 622(w), 512 (m).
One promising method to enhance the catalytic activity and
stability of POMs as broad-spectrum catalysts is to graft organic
ligands onto POM clusters via covalent bonds to get novel organo-
modified materials, which would strengthen the interactions
between catalysts and substrates. In 2017, Hill’s group prepared a
class of hybrid polymers composed of polyoxovanadate clusters
covalently bound by 1, 3, 5-benzenetricarboxamide linkers, which
catalyze the degradation of CEES and diethyl cyanophosphonate
(DECP) within 30 minutes.36 Very recently, our group found that
mixed carboxylic acids modified POMs linked by metal cations
can rapidly and high-selectively destroy CEES and DECP, which
indicate the great potential of carboxylic acid ligands modified
POMs in destroying CWAs.37 In light of these discoveries, it is of
interest to study the new structures and catalytic properties of
POMs modified by different carboxylic acid ligands that can
provide similar connectivity to the polyoxoanions but different
pendent groups. Based on this strategy, we chose carboxylic acid
ligands containing pendent amino groups as the organic Syntheses of Cs4X[M(H2O)m][TeIVMo6O21(PABA)3]2·nH2O
components, and successfully synthesized a series of new hybrid (5−8)
materials with transition metal cations as linkers:
In a typical synthesis procedure for 5−8, Na2MoO4 (0.145 g, 0.6
K5H[M(H2O)m][AsIIIMo6O21(PABA)3]2·nH2O
1−4,
mmol), Na2TeO3 (0.022 g, 0.1 mmol), and PABA (0.0411g,
0.3mmol) were dissolved in 20 mL of water. The pH value of the
solution was adjusted from the original pH value of 6.5 to 4.0 with
HCl solution under stirring. The solution was stirred for one hour
at room temperature. Then, CoCl2·6H2O (0.0714 g, 0.3
mmol)/NiCl2·6H2O (0.0678g, 0.3mmol)/ZnSO4·7H2O (0.0861 g,
0.3 mmol)/MnCl2·2H2O (0.0486 g, 0.3 mmol) and CsCl (0.034g,
0.2mmol) and KCl (0.007g, 0.1mmol) were added successively
under stirring. Finally, the solution was heated and stirred for one
hour at 80 °C. The filtrate was kept undisturbed for two weeks
under ambient conditions, and then crystals of 5−8 were isolated
in about 45% yield (based on Mo). Elemental analyses: calcd for
5: Mo, 29.03; Te, 6.43; Co, 1.49; Cs, 13.40; Na, 1.16; C, 12.72; N,
2.12; H, 2.19 (%). Found: Mo, 29.43; Te, 6.62; Co, 1.66; Cs,
13.15; Na, 1.29; C, 12.33; N, 2.27; H, 2.34 (%). FTIR data (cm-1):
3385 (s), 1604 (s), 1533 (s), 1406 (s), 1281 (w), 1178(m), 903(s),
768 (w), 728 (w), 637(s), 513 (w). Calcd for 6: Mo, 28.88; Te,
6.62; Ni, 1.52; Cs, 13.79; K, 2.03; C, 13.09; N, 2.18; H, 1.83 (%).
Found: Mo, 28.53; Te, 6.49; Ni, 1.77; Cs, 13.51; K, 2.12; C, 12.83;
N, 2.07; H, 1.92 (%). FTIR data (cm-1): 3384 (s), 1604 (s), 1535
(s), 1406 (s), 1282 (w), 1178(m), 905(s), 768 (w), 728 (w), 637(s),
516(w). Calcd for 7: Mo, 29.84; Te, 6.61; Zn, 1.69; Cs, 13.78; K,
1.01; C, 13.07; N, 1.45; H, 1.96 (%). Found: Mo, 29.44; Te, 6.44;
Zn, 1.77; Cs, 13.31; K, 1.21; C, 13.33; N, 1.32; H, 2.24 (%). FTIR
data (cm-1): 3382 (s), 1606 (s), 1533 (s), 1406 (s), 1286 (w),
1178(m), 905(s), 768 (w), 728 (w), 635(s), 513 (w). Calcd for 8:
Mo, 29.37; Te, 6.51; Mn, 1.40; Cs, 13.56; K, 1.00; C, 12.87; N,
2.14; H, 2.13 (%). Found: Mo, 29.46; Te, 6.29; Mn, 1.58; Cs,
13.42; K, 1.21; C, 12.54; N, 2.31; H, 2.23 (%). FTIR data (cm-1):
Cs4X[M(H2O)m][TeIVMo6O21(PABA)3]2·nH2O 5−8 (M = Co2+,
Ni2+, Zn2+, Mn2+; X = Na2 5, K2 6, KH 7, 8; PABA = p-
aminobenzoic
acid),
Rb2HT[M(H2O)6][M(H2O)m][AsIIIMo6O21(Ala)2(Ac)]2·nH2O
9−11 and K2NaH[Mn(H2O)3]2[AsIIIMo6O21(Ala)2(Ac)]2·nH2O 12
(M = Co2+, Ni2+, Zn2+; T = H 9, 11, Na(H2O)6 10; Ala = β-Alanine;
Ac = Acetic Acid). Compounds 1−8 are constituted of PABA ligands
modified
polyoxomolybdates
[AsMo6O21(PABA)3]4-
/[TeMo6O21(PABA)3]4- as building units, and Co2+/Ni2+/Zn2+/Mn2+
cations as linkers to generate the novel dimeric architectures.
Compounds 9−12 also have a dimeric structure composed of Ala and
Ac ligands modified polyoxomolybdate [AsMo6O21(Ala)2(Ac)]4-
bridged by metal cations. All these compounds can degrade sulfur
mustard simulant CEES with high selectivity within 12 minutes
and nerve agent simulant DECP within 10 minutes, representing
the rare examples that can simultaneously destroy two kinds of
CWAs.
2 Experimental section
Syntheses of K5H[M(H2O)m][AsIIIMo6O21(PABA)3]2·nH2O
(1−4)
In a typical synthesis procedure for 1−4, Na2MoO4 (0.145 g, 0.6
mmol), As2O3 (0.0197 g, 0.1 mmol), and PABA (0.0411g,
0.3mmol) were dissolved in 20 mL of water. The pH value of the
solution was adjusted from the original pH value of 6.8 to 4.0 with
HCl solution under stirring. The solution was stirred for one hour
at room temperature. Then, CoCl2·6H2O (0.0714 g, 0.3
mmol)/NiCl2·6H2O (0.0678g, 0.3mmol)/ZnSO4·7H2O (0.0861 g,
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3387 (s), 1601 (s), 1536 (s), 1406 (s), 1283 (w), 1178(m), 907(s), finished, GC-FID, GC-MS and 1HNMR were used to analyze the
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DOI: 10.1039/C9CY00094A
768 (w), 728 (w), 638(s), 513 (w).
resulting mixture.
Syntheses
of 3 Results and discussion
Rb2HT[M(H2O)6][M(H2O)m][AsIIIMo6O21(Ala)2(Ac)]2·nH2O
Synthesis
(9−11)
In this paper, we got twelve hybrid compounds based on PABA
In a typical synthesis procedure for 9−11, Na2MoO4 (0.145 g,
0.6 mmol), As2O3 (0.0197 g, 0.1 mmol), Ala (0.0267g, 0.3mmol)
were dissolved in 20 mL of water. The pH value of the solution
was adjusted from the original pH value of 6.8 to 4.0 with 4 M
HCl solution under stirring. The solution was stirred for one hour
at room temperature. Then, Co(OAc)2·4H2O (0.0747 g, 0.3
ligands
modified
POMs
[AsMo6O21(PABA)3]4-/
[TeMo6O21(PABA)3]4-, and Ala and Ac ligands modified POMs
[AsMo6O21(Ala)2(Ac)]4-. In 1−12, these carboxylic acids modified
POMs were joint together to form interesting dimeric structures
through the linkage of transition metal cations (Co2+, Ni2+, Zn2+,
Mn2+). Other metal cations were also attempted to be introduced
to this reaction system. For example, when Cu2+ cation was used,
only coordination complex containing Cu2+ and organic ligands
was produced. Rare earth metal cations, such as La3+, Ce3+, and
Nd3+, were also attempted to be the linkages to synthesize the
similar compounds, however only precipitation was obtained. If
no transition metal cations were added to the reaction solutions of
mmol)/Ni(OAc)2·4H2O(0.0747g,
0.3mmol)/Zn(OAc)2·2H2O
(0.0659 g, 0.3 mmol), and RbCl (0.024g, 0.2mmol) were added
successively under stirring. Finally, the solution was heated and
stirred for one hour at 80 °C. The filtrate was kept undisturbed for
two weeks under ambient conditions, and then crystals of 9−11
were isolated in about 39% yield (based on Mo). Elemental
analyses: calcd for 9: Mo, 36.95; As, 4.81; Co, 3.78; Rb, 5.48; C,
6.16; N, 1.80; H, 2.52 (%). Found: Mo, 36.51; As, 4.67; Co, 3.47;
Rb, 5.13; C, 5.99; N, 1.95; H, 2.87 (%). FTIR data (cm-1): 3439
(s), 3150 (m), 1595 (s), 1432 (s), 1401 (w), 1276 (m), 1240(m),
1173(m), 929(m), 889(s), 762 (w), 674 (m), 627(w), 505 (m).
Calcd for 10: Mo, 34.51; As, 4.49; Ni, 3.52; Na, 0.69; Rb, 5.12; C,
5.76; N, 1.68; H, 2.99 (%). Found: Mo, 34.82; As, 4.55; Ni, 3.82;
Na, 0.62; Rb, 5.32; C, 6.10; N, 1.78; H, 2.72 (%). FTIR data (cm-
1): 3441 (s), 3151 (m), 1596 (s), 1434 (s), 1402 (w), 1275 (m),
1241(m), 1172(m), 929(m), 889(s), 763 (w), 675 (m), 628(w), 504
(m). Calcd for 11: Mo, 36.37; As, 4.73; Zn, 4.13; Rb, 5.40; C, 6.07;
N, 1.77; H, 2.61 (%). Found: Mo, 36.21; As, 4.52; Zn, 4.40; Rb,
5.69; C, 6.28; N, 1.70; H, 2.76 (%). FTIR data (cm-1): 3439 (s),
3149 (m), 1592 (s), 1433 (s), 1402 (w), 1277 (m), 1241(m),
1173(m), 929(m), 889(s), 764 (w), 675 (m), 627(w), 506 (m).
1−4,
the
poor-quality
crystal
compound
K3[AsMo6O21(PABA)3]·nH2O (compound 13) with K+ as cations
was synthesized. Above parallel experiments demonstrate that
metal cations play an important role for the formation of these
hybrid compounds. In the synthetic process of 9−12, Ala and Ac
ligands were simultaneously added to the reaction solution. If Ac
ligand was not added, hybrid compounds based on only Ala
modified POM K6Na2H2[(H2O)6Co][AsMo6O21(Ala)3]4·41H2O
(14) were isolated, which show a mono-supporting structure. Thus,
the metal cations Co2+/Ni2+/Zn2+/Mn2+ and the organic ligands play
crucial roles in constructing the dimers with carboxylic acid
ligands modified POM subunits.
The alkali metal counter cations also are important to the
preparation of these compounds. In the synthesis of 1−4, the K+
cations need to be introduced to the reaction system because the
K+ cation participates in the formation of 1−4. In the absence of
K+, no crystal could be formed. During the preparation of 5−8, the
Cs+ and K+ cations were all necessary to get the similar dimeric
structures with 1−4. If no Cs+ cation was added to the reaction
solution, only precipitate was obtained, and only compound 5 can
be obtained without K+ in the existence of Cs+. When Cs+ cation
was added to the reaction system of 1−4 in the existence of K+
cations, no influences on the final architectures were found.
Besides, in the synthesis of 9−11, the Rb+ cation was the
indispensable component, whereas the K+ was the necessary
element in the preparation of 12. If no Rb+ was introduced to the
reaction system of 9−11, the poor-quality crystals would be
obtained.
Syntheses
of
K2NaH[Mn(H2O)3]2[AsIIIMo6O21(Ala)2(Ac)]2·12H2O (12)
The synthetic procedure for 12 was similar to that used for 9,
with Mn(OAc)2·2H2O(0.0804g, 0.3 mmol) instead of
Co(OAc)2·4H2O (0.0747g, 0.3 mmol) and RbCl was replaced by
KCl (0.016g 0.2mmol). The filtrate was kept undisturbed for two
weeks under ambient conditions, and then light yellow crystals of
12 were isolated in about 38% yield (based on Mo). Calcd for 12:
Mo, 38.59; As, 5.02; Mn, 3.68; K, 2.62; Na, 0.77; C, 6.44; N, 1.88;
H, 2.40 (%). Found: Mo, 38.75; As, 4.84; Mn, 3.42; K, 2.48; Na,
0.73; C, 6.33; N, 1.75; H, 2.66 (%). FTIR data (cm-1): 3442 (s),
3147 (m), 1592 (s), 1431 (s), 1402 (w), 1276 (m), 1241(m),
1173(m), 929(m), 891(s), 762 (w), 672 (m), 627(w), 505 (m).
General methods for catalyzing degradation of CEES and
DECP
In addition, the parallel experiments were proceeded and
indicated that the reaction stoichiometry, temperature and pH
value can also influence the quality of the crystals. In the synthetic
process, compounds 1−12 can be well obtained in one pot reaction
of Na2MoO4, As2O3/Na2TeO3, PABA (Ala and Ac) and TM with
the ratio of Mo/As(Te)/PABA/TM = 6:1:3:3. The quality of
crystals and the yields will vary considerably over the ratio of
these raw materials. The optimal reaction temperature is 80 °C. At
higher temperature (100 °C), compounds were synthesized with
poor quality crystals, at room temperature (25 °C), no reaction
CEES oxidation: POM catalyst (2.5 μmol), H2O2 (0.3 mmol),
CEES (0.25 mmol), and acetonitrile (0.5 mL) were put in a glass
bottle. The catalytic reaction was carried out at room temperature
for 12 minutes. Hydrolysis DECP: POM catalyst (0.6 μmol),
DECP (1 mmol), DMF (0.6 mL) and H2O (50 μL) were put in a
glass bottle. The catalytic reaction was carried out at room
temperature for 10 minutes. After the catalytic reaction was
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occurred. The optimal pH value is from 3.4 to 4.5, whether for a
low pH (pH = 2.5) or high pH (pH = 5.5), only precipitations can
be obtained.
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DOI: 10.1039/C9CY00094A
Crystal structures of 1−12
Single crystal X-ray analyses reveal that compounds 1−8
crystallize in the space group P-1. The eight compounds include
the similar polyoxoanion unit {ZMo6O21} (Z = As, Te), which are
composed of a ring of six almost coplanar edge-sharing and
corner-sharing MoO6 octahedra. In this unit, the central As/Te
atom is situated slightly above the MoO6 unit and connects to three
oxygen atoms to generate a trigonal-pyramidal molecular
geometry. This coordinated mode and coordination geometry of
{ZMo6O21} are obviously different from the classic Anderson-
type POMs. Next, the polyoxoanions {ZMo6O21} are covalently
coordinated to three carboxylic acid ligands (PABA) via the
carboxyl units to form the single-side organically modified POMs
{ZMo6O21(PABA)3} in 1−8 (Fig. 1a). Given compounds 1−8 have
similar structures, only compounds 1 and 5 were taken as
examples to be discussed here. In the PABA modified POMs, the
average bond length of Mo-Ob (2.308Å) (μ2-O of Mo) in 1 and
Mo-Ob (2.273Å) (μ2-O of Mo) in 5 are akin to other carboxylates
modified POM derivatives, and distinctly longer than the initial
Mo-Ot (1.703 Å) (terminal oxygen of Mo). In addition, three
classes of O atoms exist in the cluster in view of the different
coordinated manners and environment. Hence, the Mo-O bond
lengths are divided into three different groups: Mo–Ot 1.704(4)–
1.731(5) Å, Mo–Ob 1.905(5)–2.337(6) Å, Mo–Oc 2.164(3)–
2.232(4) Å in 1; Mo–Ot 1.709(5)–1.724(6) Å, Mo–Ob 1.911(4)–
2.286(4) Å, Mo–Oc 2.226(3)–2.277(4) Å in 5. The angles of O-
As-O change from 95.78(18) to 96.67(18) in 1; the angles of O-
Te-O change from 93.70(18) to 94.04(18) in 5. Corresponding to
the reported study, the bond lengths and angles of this report are
within the reasonable ranges.
Fig. 1 a) The direct self-assembly protocol for the synthesis of carboxylic acid
ligands modified POM [AsMo6O21(PABA)3]4-; b) Ball-stick and polyhedral
representation of dimeric polyanion for 1; c) View of the 2D supramolecular
framework showing the hydrogen bonds. (color code: As yellow, Mo purple,
Co light blue, O red, N blue, C black).
In the asymmetric unit of 1, it contains one crystallographically
independent [AsMo6O21(PABA)3]3- polyoxoanion, half one Co2+
cation, and two and half K+ cations (Fig. S1). The asymmetric unit
of 5 was similar with 1, except Cs+ and Na+ were cations instead
of the K+ (Fig. S2). Unique Co(1) ion in 1 (5) is coordinated to
three water molecular [Co–OW 1.997(6)–2.107(4) Å in 1;
2.056(5)–2.165(5) Å in 5] and two terminal oxygen atoms
originated from two polyoxoanions [Co–O 2.000(6)–2.046(5) Å
in 1; 2.110(5)–2.133(5) Å in 5] to generate a distorted square
pyramidal geometry. Then, two PABA modified POMs were
linked together to produce a dimer by Co–O–Mo bonds, related by
an inversion center (Fig. 1b). Such dimers based on carboxylic
acids modified POMs and metal cations have never been known
up to now. Interestingly, strong hydrogen bonds exist between the
N atoms of PABA and O atoms of polyoxoanions (N3−H···O26 =
2.920 Å in 1; N1−H···O12 = 2.886 Å in 5), generating a 1D
supramolecular chain (Fig. S3, S4). These 1D chains are joint
together to produce a 2D layer (Fig. 1c, S5) also through the
hydrogen bonds between the N atoms of PABA and O atoms of
polyoxoanions (N2−H···O7 = 2.879 Å, O1W−H···O10 = 2.940 Å
in 1; N1−H···O9 = 2.737 Å in 5).
When Ala and Ac ligands were used as the organic components,
compounds 9−12 were isolated, which crystallize in the space
group P21/c and have the same polyoxoanion unit {AsMo6O21}
with compounds 1−4. Then two different carboxylate ligands (two
protonate Ala and one Ac) were coordinated together with the
polyoxoanion {AsMo6O21} on the one side to form the mix
carboxylic acids modified POMs [AsMo6O21(Ala)2(Ac)]4- (Fig.
2a). Similar to compounds 1−4, the Mo-O bond lengths are
classified into three kinds: Mo–Ot 1.700(3)–1.725(6) Å, Mo–Ob
1.893(3)–2.328(6) Å and Mo–Oc 2.184(6)–2.225(4) Å in 9. The
angles of O-As-O change from 96.1(3) to 96.6(3) in 9. The
asymmetric unit in 9−11 contain one crystallographically
independent [AsMo6O21(Ala)2(Ac)]4- polyoxoanion, one Co2+/
Ni2+/Zn2+ cation and one Rb+ (compound 10 also has half one Na+)
(Fig. S6). The asymmetric unit of 12 was similar with 9 except the
Rb+ was replaced by K+ (Fig. S9). A dimeric structurewas formed
through two modified POMs jointed together by M–O–Mo bonds
in 9−12 (Fig. 2b). However, Co2+ cations in 9 and 1 (5) displayed
different coordination behaviors, thus producing different dimeric
structures. The unique Co2+ cation in 9 is defined by three O atoms
originated from the polyoxoanions [Co–O 2.103(5)–2.201(6) Å]
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and three water molecules [Co–OW 2.193(5)–2.277(5) Å] to form
the distorted octahedron, which is different from the coordination The calculated and experimental patterns of the diffraction peaks
mode for 1 (three water molecules and two terminal oxygen atoms matched well, which certified the phase purities of these
to produce the distorted square pyramidal). Strong hydrogen compounds. In addition, the similar diffraction peaks among 1−8
bonds between N atoms of Ala and O atoms of polyoxoanions indicate the isostructural nature of these compounds, which are
produce a 1D supramolecular chain in 9 (Fig. S8) (N2−H···O6 = different with those of 9−12. Moreover, these conclusions agree
2.778 Å). These 1D chains were further linked together to form a with the results obtained from the single crystal X-ray analysis.
ARTICLE
Fig. S21~S23 displayed the PXRD patterns of compounds 1−12.
View Article Online
DOI: 10.1039/C9CY00094A
2D layer (Fig. 2c) through the hydrogen bonds (O2W−H···O18 =
2.854 Å).
Catalytic oxidation study
Given CEES is regarded as an effective simulant molecule for
Bond valence sum (BVS) calculations indicate that the
sulfur mustard, we attempted to investigate the catalytic oxidation
oxidation states of Mo, As and Te are respectively +6, +3, +4, and
degradation effect of these compounds for CEES. To perform the
Co/Ni/Zn/Mn are +2.
oxidative degradation reaction, CEES (0.25 mmol), catalyst (2.5
μmol), H2O2 (oxidant 0.3 mmol), and naphthalene (internal
standard 0.25 mmol) were dispersed in acetonitrile (0.5 ml) at
room temperature, the reaction was detected by gas
chromatography (GC). We were delighted to find out that CEES
can almost entirely be rapidly degraded to the far less toxic
product 2-chloroethyl ethyl sulfoxide (CEESO) and hardly any 2-
chloroethyl ethyl sulfone (CEESO2) within 12 minutes. Fig. 3b, 3c,
3d illustrate the conversion of the oxidation of CEES with
compounds 1−12 and blank, which indicate that these compounds
can efficiently degrade the CWAs simulant molecule CEES within
12 minutes. Compound 1 shows the best catalytic activity, and can
oxidize 98.9% of CEES into the far less toxic product CEESO
(sele.% = 98.0%); compounds 5 has better catalytic activity than
other isostructural species 6−8, making 97.4% of CEES degrade
to CEESO (sele.% = 96.5%); compound 9 also displays better
activity than 10−12, making 93.8% of CEES degrade to CEESO
(sele.% = 89.0%) at 12 minutes. There appeared to be negligible
activity without catalyst. GC-FID signals were recorded in Fig. 4a,
showing the process of the oxidative degradation for CEES in the
presence of 1, which indicated CEES is almost entirely converted
to the corresponding products within 12 minutes under mild
conditions. In addition, Fig. 4b also displays the progress of the
1
degradation reaction of CEES in the presence of 1 by H NMR
spectroscopy. And it can be employed to monitor the reaction
process, confirm the products and further ascertain the accuracy
Fig. 2 a) The direct self-assembly protocol for the synthesis of carboxylic acid
ligands modified POM [AsMo6O21(Ala)2(Ac)]4-; b) Ball-stick and polyhedral
of the reaction. The reaction processes at 6 and 12 minutes were
representation of dimeric polyanion for 9; c) View of the 2D supramolecular performed in CDCl3 in the 1H NMR spectrum. To further
1
framework showing the hydrogen bonds. (color code: As yellow, Mo purple,
illuminate the reaction, the H NMR spectra of the pure CEES,
Co light blue, O red, N blue, C black).
CEESO and CEESO2 were also detected. The spectrum contrast of
actual reaction and pure compounds further demonstrates that
CEES can be almost oxidized to the far less toxic CEESO within
12 minutes. Subsequently, to explore the active components of
compound 1 in the selective oxidation degradation of CEES, we
investigated the catalytic effect of PABA, compound 13
(K3[AsMo6O21(PABA)3]·nH2O), and CoCl2 salt. The formula of
13 was deductive by IR spectrum and EDS data because of the
poor crystal quality (Fig. S14). As shown in Fig. 5, the ligand
PABA alone oxidizes very little CEES, and compound 13 can
convert 72.0% CEES with 82.0% selectivity, which are all lower
than that for 1. Furthermore, the transition metal salt CoCl2 also
oxidized only 13.0% CEES with the selectivity 74.0%. Above all,
we can speculate that the combination of Co site and PABA
modified POM can promote the catalytic oxidation reaction with
rapid reaction process and wonderful selectivity in the degradation
TGA and PXRD Characterization
The TGA of these compounds are shown in Fig. S18~S20,
which demonstrate these compounds display multistep weight loss
processes. The total weight loss of 34.9%, 35.2%, 35.0%, 34.7%,
30.1%, 35.1%, 35.4%, 37.4%, 27.9%, 31.0%, 28.3%, and 26.8%
meet the calculated weight loss 34.4%, 34.8%, 34.4%, 35.2%,
30.7%, 34.8, 35.9%, 37.2%, 27.4%, 31.3%, 28.1%, and 26.5%
from 50 to 800 °C. Take compound 1 as an example, the first
weight loss from 50 to 150 °C is 9.5%, corresponding to all the
lattice water and partial coordinated water. The second weight loss
of 7.5% from 150 °C to 380 °C was mainly attributed to the
remaining coordinated water and partial organic ligands. The last
weight loss of 21.5% from 380 °C to 750 °C arose from the loss
of organic ligands and partial oxygen molecules.
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of CEES.
that of 9 based on {AsMo6(Ala)2(Ac)} and Co2+ for the
View Article Online
degradation of CEES (Table 1). The catalDyOticI: 1a0c.1ti0v3i9ty/C(9cCoYn0v0.0%94=A
98.9%, sele.% = 98.0%) of compound 1 is higher than the activity
(97.4%, 96.5%) of compound 5, indicates that the central hetero
atom produces certain role on the CEES oxidation. In contrast, the
catalytic activity of compound 1 is obviously higher than the
activity (93.8%, 89.0%) of compound 9, which might be due to
the carboxylic acid ligands with different structures. PABA is the
coordinated carboxylic acid in 1, and Ala and Ac are the organic
ligands in 9, which reveals that PABA ligand modified POM is
more in favour of the catalytic reaction.
Fig. 4 a) GC-FID signals indicating the progress of the oxidation of CEES
(4.56 min) to CEESO (8.920 min) and CEESO2 (9.23), and internal standard
1
(8.495 min, naphthalene) in the presence of 1; b) Selected regions of the H
NMR spectra of CEES, CEESO, CEESO2 oxidation reaction (without internal
standard) at 6 min and oxidation reaction (without internal standard) at 12 min
with two compounds in CDCl3. The peaks used for calculation are labeled with
chemical shift.
For understanding how the carboxylic acid ligands impact the
catalytic
activity,
two
compounds,
Fig. 3 a) Catalytic oxidation pathway of CEES to CEESO and CEESO2; b, c,
d) The conversion of CEES oxidation for compounds 1−12 and blank.
Reaction conditions: 0.25 mmol CEES, 2.5 μmol catalyst, 0.25 mmol
naphthalene (internal standard), 0.3 mmol H2O2 and 500μL CH3CN at room
temperature for 12 min.
K6Na2H2[(H2O)6Co][AsMo6O21(Ala)3]4·41H2O (14) with alanine
ligand (Ala) and K10[(H2O)6Co][AsMo6O21(PHBA)3]2·23H2O (15)
with p-hydroxybenzonic acid (PHBA) were prepared and detected for
the catalytic oxidation degradation of CEES. Compounds 14 and 15
can respectively degrade 73.0% and 92.1% of CEES with 94.2% and
93.4% selectivity under similar conditions (Fig. 5). The catalytic
activities of compounds 14 and 15 are observably lower than that of
Furthermore, the catalytic oxidation activity of 1 was compared
with that of 5 constructed from {TeMo6(PABA)3} and Co2+, and
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compound 1. The three compounds contain the same {AsMo6O21} peroxomolybdenum and peroxocobalt species, which lead to the
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anion, the Co2+ cation, but different carboxylic acid ligands. By reaction with S atom of CEES, generating the corresponding
DOI: 10.1039/C9CY00094A
comparing the structure of PABA ligand in 1 with PHBA ligand in products.
15, amino group (NH2) is more advantageous than hydroxyl group
(OH) for the catalytic oxidation of CEES; in the contrast of PABA in
1 and Ala in 14, the carboxylic acid ligand with aromatic group is
more beneficial. From the catalytic results, it can be found that PABA
ligand with aromatic amine is better than Ala, PHBA and Ac ligands
for the oxidation of CEES, when these ligands are used to modify
POMs. These results manifested carboxylic acid ligands that modified
POMs play a key role in the oxidation of CEES.
Table 1. Comparison of CEES decomposition by different
materials.
Catalyst
Conversion (%)
Sulfoxide
selectivity (%)
98.0
Compound 1
Compound 5
Compound 9
Compound 13
Compound 14
Compound 15
98.9
97.4
93.8
72.0
92.1
73.0
96.5
89.0
82.0
93.4
94.2
Reaction conditions: CEES (0.25 mmol, 1 equiv), catalyst (1 mol%), H2O2 (1.2
equiv), naphthalene (0.25 mmol) and acetonitrile (0.5 mL), for 12 min, at room
temperature.
Then, other elements that influence the activity of these
compounds were also investigated. When we compared
compound 9 with compound 14, which constructed from the same
polyoxoanion {AsMo6O21} and organic ligand Ala, we found that
compound 9 with more Co2+ cation displayed higher catalytic
activity. This result exhibits Co2+ cation is the catalytic active site,
and the higher quantity has the higher activity. However, when
focusing on the catalytic effect of compounds 1 and 9, the catalyst
9 with more Co2+ shows lower conversion and selectivity of CEES,
which might be attributed to the differences of carboxylic acid
ligands playing the leading part. So, the cooperation between Co2+
and PABA modified {AsMo6O21} in 1 promotes the catalytic
performance in the catalytic degradation process for CEES.
Fig. 5 a) The oxidation of CEES by different catalysts; b) The conversion and
selectivity of CEES oxidation for different catalysts. Reaction conditions: 0.25
mmol CEES, 2.5 μmol catalyst, 0.25 mmol naphthalene (internal standard), 0.3
mmol H2O2 and 500 μL CH3CN at room temperature for 12 min.
Then, we continued to study the factors that influence the
degradation reaction with compound 1 as catalyst. Obviously,
both the quantity of catalyst and H2O2 restrict the catalytic reaction.
The optimized catalyst loading for compound 1 is 1% equiv (Fig
S26). The conversion of CEES was lowered to 73.0% when the
catalyst loading was decreased from 1% to 0.8% equiv in 12 min.
The conversion of CEES was almost unchanged when the catalyst
loading was increased from 1% to 1.2% equiv. This phenomenon
is often observed in the heterogeneous catalytic system.31,32,39 And
the feasible ratio of oxidant to CEES is 1.2 to 1.1. When the
oxidant was lowered to 0.8% equiv, the conversion of CEES was
observed to be 70.5 % and keep almost constant with high
selectivity (Fig. S27). When the oxidant H2O2 was replaced to
TBHP, the conversion we found were far below than that for H2O2.
The catalytic oxidation reaction kinetics of CEES was next
investigated. Fig. 3 displays the time profiles for the oxidation
reaction process of CEES. Ln(Ct/C0) against the reaction time
shows a linear correlation, which indicates that the oxidation of
In light of the abovementioned consequences and the previous
researches, especially investigated widely based on POMs, the
peroxo-metal species play an important role in the catalytic
oxidation reaction in the presence of POMs and H2O2.31,32,38 To
understand the reaction mechanism, radical scavengers, such as
2,6-Di-tert-butyl-p-cresol, diphenylamine and p-benzoquinone
were introduced to the catalytic systems, respectively. The
conversion of CEES remained unchanged after adding the
scavengers, indicating that no radical species are responsible for
the oxidation reaction of CEES. In this regard, we summarized a
possible nonradical reaction mechanism of CEES oxidation
degradation in Fig. 6a. Indeed, the interaction between 1 and H2O2
was confirmed by the changes of the UV-vis spectrum of the
mixture solution of before and after H2O2 was added to the
solution of 1 (248 → 253nm, 546 → 552nm) (Fig. 6b, c).
Therefore, the catalyst interacts with H2O2 to form an active
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CEES accords with a first-order reaction (Ct and C0 represent the
CEES concentration at some time and at the begin time). The first-
order kinetics constants k1~k12 for 1−12 are 0.38211 min−1, 0.3479
min−1, 0.30719 min−1, 0.2804 min−1, 0.33069 min−1, 0.27797
min−1, 0.31077 min−1, 0.22987 min−1, 0.21809 min−1, 0.21486
min−1, 0.20998 min−1, and 0.19482 min−1, respectively (Fig.
S28~S30).
and after the catalytic process (Fig. 7b, c)V.iewTAhrtuicsle,Ontlhinee
abovementioned results manifest that comDpOouI:n1d0.1103is9/aCn9CeYx0c0e0ll9e4nAt
heterogeneous oxidation catalyst of CEES.
Fig. 7 a) Recycling of the catalytic system for the oxidation of CEES using
compound 1; b) IR spectrum for (a) as-synthesized compound 1 and (b)
recovered catalyst after catalysis reaction; c) Powder X-ray diffraction (PXRD)
patterns of 1: calculated pattern from crystal data (black line); experimental
pattern before catalysis (red line); recovered catalyst 1 after 10 catalytic runs
of the oxidation of CEES (blue line).
Fig. 6 a) Proposed mechanism of the catalytic oxidation of CEES; b) UV-vis
spectra of compound 1 in dimethyl sulfoxide with one-drop portions of H2O2
on the UV region (200−400 nm); c) UV-vis spectra of compound 1 in dimethyl
sulfoxide with one-drop portions of H2O2 on the vision region (400−800 nm).
The red shift of the absorption spectrum after adding H2O2 show the
interactions between compound 1 and H2O2.
Catalytic hydrolysis study
The recyclability and stability of the heterogeneous catalysts
were further researched in the catalytic process. The catalyst was
easily collected by simple filtration. In the following 10 cycles, the
conversion and selectivity of CEES keep almost constant using the
recovered catalyst, which demonstrate wonderful cycling stability
of 1 (Fig. 7a). And the filtrate showed no catalytic activity on the
oxidation of CEES in the similar situation. Moreover, the IR
spectra and the PXRD patterns of 1 keep virtually identical before
As we all know, the broad-spectrum catalyst against CWAs
possessing multiple detoxification pathways, such as oxidation
and hydrolysis, is highly desired. Then, the hydrolytic degradation
of organophosphate-based nerve agent simulant DECP was thus
examined since compounds 1−12 have metal cations as Lewis acid
sites and amino groups have the great effect on the hydrolysis
according to the literatures.7 Fig. 8b, 8c, 8d show the conversion
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of the hydrolysis of DECP with compounds 1−12. These amount of catalysts. Compound 1 exhibits the best catalytic
View Article Online
DOI: 10.1039/C9CY00094A
activity, and can hydrolyze 100% of DECP into the nontoxic
product. The turnover frequency (TOF) of the catalytic
degradation reaction using 1 is about 10000 h-1 with the half-life
of 4.2 min. To our knowledge, this catalytic activity for the
degradation of DECP using 1 is significantly higher than most
reported catalysts built on POMs. Table 2 displays the comparison
of the catalytic hydrolysis activities for DECP among 1, 5 and 9.
The initial rates were ascertained from the slope of the amount of
DECP versus time. The catalytic activities of compounds 1 (100%)
and 5 (99.5%) are almost approximate, but the difference of these
initial rates (7.8 and 6.9 mM/s) is distinct, which indicates the
central hetero atom produces certain effect on the DECP
hydrolysis. Under the similar conditions, the catalytic activity of
compound 9 (98.2%, 6.3 mM/s) is obviously lower than that of
compound 1 (100%, 7.8 mM/s), which might be originated from
the carboxylic acid ligands with different structures and reveals
that PABA ligand modified POM is more beneficial for the
catalytic hydrolysis reaction.
To reveal the active sites of compound 1 for hydrolyzing DECP,
the
catalytic
activities
of
compound
13
(K3[AsMo6O21(PABA)3]·nH2O) and CoCl2 salt were determined
(Fig. S31). Compound 13 can hydrolyze 69.9% of DECP, while
CoCl2 converts 51% of DECP to the nontoxic products, which
show higher conversion than the blank but much lower activity
than compound 1 (100%). The experiment results indicate that
both PABA modified POM {AsMo6(PABA)3} and metal cation as
the Lewis acid together enhance the hydrolytic degradation
activity. Besides, the catalytic activities of compounds 14 and 15
respectively based on {AsMo6(Ala)3} and {AsMo6(PHBA)3} unit
for hydrolysing DECP, were detected (Table 2). Compounds 14
and 15 can respectively convert 80.3% and 82.5% of DECP with
the initial rates of 4.2 and 4.5 mM/s, which displayed much lower
catalytic activity than 1 (100%, 7.8mM/s). The observed
difference in the catalytic hydrolysis activity is mainly owing to
the covalently bonded carboxylic acid ligands. The PABA ligand
with aromatic amine in 1 is more advantageous for the hydrolysis
reaction of DECP. Similar phenomenon has also been investigated
in metal-organic frameworks by other groups.7
Table 2. Comparison of DECP decomposition by different
materials.
Catalyst
Conversion
initial rate
(%)
100
(mM/s)
7.8
Compound 1
Compound 5
Compound 9
Compound 13
Compound 14
Compound 15
99.5
98.2
69.9
80.3
82.5
6.9
6.3
3.6
4.2
4.5
Fig. 8 a) The hydrolytic decomposition pathway of DECP; b, c, d) The
conversion of DECP hydrolysis for compounds 1−12. Reaction conditions: 1
mmol DECP, 0.6 μmol catalyst, 0.25 mmol naphthalene (internal standard),
600 μL DMF and 50 μL H2O at room temperature for 10 min.
compounds can catalyze the hydrolytic degradation of DECP to
the nontoxic diethyl phosphate within 10 minutes under a small
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work will help the development of more efficienVtiePwOArMticl-ebOansleinde
materials for destruction of diverse CWAsD. OI: 10.1039/C9CY00094A
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (21371027, 20901013), Natural
Science Foundation of Liaoning Province (2015020232) and
Fundamental Research Funds for the Central Universities
(DUT15LN18, DUT15LK02).
References
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to the catalytic systems for DECP. No obvious change was found
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hydrolysis of DECP were performed in the same reaction vessel.
The catalytic results show that 96.7% CEES can be converted to
the corresponding CEESO with the selectivity of 97.1% and
DECP was completely decomposed, indicating both CWA
simulants can be simultaneously decomposed to their
corresponding products with high efficiency.
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4. Conclusions
In summary, twelve new hybrid dimers 1−12 based on different
carboxylic acid ligands modified POMs and metal cations were
designedly synthesized, and used as catalysts for rapid destruction
of two CWA simulants (CEES and DECP) at room temperature.
The excellent catalytic performances of these compounds
originate from the synergetic effects among carboxylic acid
ligands with pendent amino group, {AsMo6}/ {TeMo6} unit, and
transition metal cations. In addition, the comparison study of
catalytic activities of these compounds indicates that i) as linker to
the polyoxoanion, Co2+ cation shows the better catalytic ability
than Zn2+, Ni2+ and Mn2+ cation, and the quantity of metal cation
will influence the catalytic activities; ii) as organic ligand
modified the POM, the PABA ligand with aromatic amine is more
advantageous than Ala, Ac and PHBA ligand for the oxidation of
CEES and the hydrolysis of DECP; iii) the central heteroatoms
(As, Te) of polyoxoanions can also impact their catalytic activities,
and with As as heteroatom catalysts have the better activity. This
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Page 12 of 12
View Article Online
DOI: 10.1039/C9CY00094A
Versatile catalysts constructed from hybrid polyoxomolybdates
for simultaneously detoxifying sulfur mustard and
organophosphate simulants
Yujiao Hou, Haiyan An,* Shenzhen Chang, Jie Zhang
College of Chemistry Dalian University of Technology, Dalian 116023, P. R. China
Twelve new hybrid dimers based on carboxylic acid ligands modified
polyoxomolybdates were prepared, which can rapidly oxidize the mustard gas
simulant, CEES, and hydrolyze the nerve agent simulant, DECP, at room temperature.