Rapid Destruction of Two Types of Chemical Warfare Agent Simulants

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Research Article Cite This: ACS Catal. 2018, 8, 6062−6069

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Rapid Destruction of Two Types of Chemical Warfare Agent Simulants by Hybrid Polyoxomolybdates Modified by Carboxylic Acid Ligands Yujiao Hou, Haiyan An,* Yumeng Zhang, Tao Hu, Wei Yang, and Shenzhen Chang College of Chemistry, Dalian University of Technology, Dalian 116023, People’s Republic of China S Supporting Information *

ABSTRACT: Four chain-like hybrid compounds based on mixed carboxylic acid ligands-modified polyoxomolybdates, K2H[(H2O)4M][AsMo6O21(Ala)(PHBA)2]·nH2O 1−4 (M = Co, Ni, Zn, Mn; Ala = alanine; PHBA = p-hydroxybenzonic acid), were prepared and characterized by elemental analysis, IR spectroscopy, solid diffuse reflective spectroscopy, TG analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. Four isostructural compounds 1−4 not only represent the extended architectures constructed from two different organic ligands-modified polyoxometalates but also can rapidly catalyze the degradation of two chemical warfare agent simulants, 2-chloroethyl ethyl sulfide (CEES) and diethyl cyanophosphonate (DECP), at room temperature. The catalytic results were analyzed and confirmed by GC-FID, GC-MS, and 1HNMR techniques. Within 5 min, CEES was high-selectively oxidized to the corresponding nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) using heterogeneous catalyst 1 with the oxidant H2O2 (conversion % = 98.5%, selectivity % > 99.9%). FTIR, PXRD techniques, and the following cycles also ascertained the stability and structural integrity of 1 in the oxidation reaction. Within 10 min, DECP can be almost entirely hydrolyzed to the nontoxic products catalyzed by 1 (conversion % = 99.0%). To our knowledge, they are in the rank of highly active catalysts for the degradation of CEES and DECP to date, accompanied by the advantages of steady reuse. KEYWORDS: hybrid polyoxomolybdates, carboxylic acids, catalysis, chemical warfare agents, destruction medicine, and material science.7,8 Thanks to the prominent work of Hill, Hu, Nyman, and other groups, POMs as promising catalysts for the degradation of CWAs have been investigated.9,10 In the early days, Fe-containing polyoxotungstates and {PV2Mo10O40} were tested for the degradation of mustard gas by Hill’s group.9 Then polyoxoniobates [Nb6O19]8− and K12[Ti2O2][GeNb12O40]·19H2O were used to catalytically detoxify nerve agents and their simulants by basic hydrolysis.10 Recently, a homogeneous catalyst H13[(CH3)4N]12[PNb12O40(VO)2(V4O12)2]·22H2O and a gel material composed of polyoxovanadate TBA-polyV6 were successfully developed by Hill, Hu, and co-workers to degrade sulfur mustard simulants by oxidation and organophosphate simulants via hydrolysis.11 In this regard, POMs containing niobium or vanadium are mainly studied as catalysts, and only two examples are reported to simultaneously destruct both sulfur mustard by oxidation and nerve agents via hydrolysis to date. Moreover, the low degradation kinetics and low selectivity of these materials offer enormous room for improvement. Therefore, the search for more efficient multifunctional materials based

1. INTRODUCTION Chemical warfare agents (CWAs) have caused mass casualties from World War I to the conflicts in Syria and Malaysia.1 Among the most widely used and toxic chemical weapons, vesicants and nerve agents are two typical common types.2 Sulfur mustard (HD), the most notorious vesicant agent, can cause grievous skin blisters as well as irritation to the respiratory system and eyes or even death at high doses.3,4 The selective oxidation degradation of sulfur mustard to sulfoxide is a more promising route than dehydrohalogenation and hydrolysis that are usually incomplete and slow owing to water immiscibility.5 Nerve agents known as organophosphates can stop nervous function and respiratory and further lead to asphyxiation in minutes. Hydrolysis of the labile P−X bond is used to detoxify organophosphate-based nerve agents.5 Some solid materials including metal−organic frameworks (MOFs) or modified activated carbons display fulfilling characteristics for the removal of CWAs. However, most of the reported materials are mainly centered on destructing one kind of CWA as monofunctional catalyst either by oxidation or by hydrolysis.5,6 Given the possibility of using various CWAs, multifunctional materials that could degrade both sulfur mustard by oxidation and nerve agents via hydrolysis are highly desired. Polyoxometalates (POMs) are a large group of inorganic metal oxide clusters with numerous significant applications in catalysis, © XXXX American Chemical Society

Received: March 12, 2018 Revised: May 10, 2018

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time point was used to confirm the identity of the products. The GC−MS spectra were measured on an Agilent HP6890/ 5973MSD spectrometer. 1H NMR spectroscopy was also used to monitor conversion and ascertain the product. 1H NMR spectra were collected on a 500 MHz Bruker Avance III system in CDCl3. Synthesis of K2H[(H2O)4Co][AsMo6O21(Ala)(PHBA)2]· 6.5H2O (1). In a typical synthesis procedure for 1, Na2MoO4 (0.145 g, 0.6 mmol), As2O3 (0.0197 g, 0.1 mmol), Ala (0.0089 g, 0.1 mmol), and PHBA (0.0274g, 0.2 mmol) were dissolved in 20 mL of water. The pH value of the mixture was adjusted from the original pH value of 6.5 to 3.5 with 4 M HCl solution under stirring. The solution was stirred for 1 h at room temperature. Then CoCl2·6H2O (0.0714 g, 0.3 mmol) and KCl (0.022 g, 0.3 mmol) were added successively under stirring. Finally, the solution was heated and stirred for 1 h at 80 °C. The filtrate was kept undisturbed for 2 weeks under ambient conditions, and then crimson crystals of 1 were isolated in about 62% yield (based on Mo). Anal. Calcd for 1: Mo, 34.33; As, 4.47; Co, 3.51; K, 4.66; C, 12.17; N, 0.84; H, 2.34. Found: Mo, 34.23; As, 4.62; Co, 3.26; K, 4.45; C, 12.33; N, 0.77; H, 2.54. FTIR (cm−1): 3437 (s), 3149 (m), 1599 (s), 1532 (s), 1483 (w), 1402 (s), 1276 (m), 1240(m), 1173(m), 929(m), 888(s), 762 (w), 674 (m), 626(w), 504 (m). Synthesis of K2H[(H2O)4Ni][AsMo6O21(Ala)(PHBA)2]· 9H2O (2). The synthetic procedure for 2 was similar to that used for 1, with NiCl2·6H2O (0.0678g, 0.3 mmol) instead of CoCl2·6H2O (0.0714 g, 0.3 mmol). The filtrate was kept undisturbed for 2 weeks under ambient conditions, and then green crystals of 2 were isolated in about 32% yield (based on Mo). Anal. Calcd for 2: Mo, 33.44; As, 4.35; Ni, 3.41; K, 4.54; C, 11.16; N, 0.81; H, 2.46. Found: Mo, 33.76; As, 4.28; Ni, 3.75; K, 4.23; C, 11.48; N, 0.74; H, 2.29. FTIR (cm−1): 3445 (s), 3145 (m), 1595 (s), 1533 (s), 1475 (w), 1400 (s), 1274 (m), 1204(m), 1172(m), 926(m), 882(s), 761 (w), 667 (m), 628(w), 506 (m). Synthesis of K2H[(H2O)4Zn][AsMo6O21(Ala)(PHBA)2]· 8H2O (3). The synthetic procedure for 3 was similar to that used for 1, with ZnSO4·7H2O (0.0861 g, 0.3 mmol) instead of CoCl2·6H2O (0.0714 g, 0.3 mmol). The filtrate was kept undisturbed for 2 weeks under ambient conditions, and then colorless crystals of 3 were isolated in about 35% yield (based on Mo). Anal. Calcd for 3: Mo, 33.70; As, 4.38; Zn, 3.83; K, 4.58; C, 11.95; N, 0.82; H, 2.36. Found: Mo, 33.45; As, 4.56; Zn, 3.64; K, 4.74; C, 12.17; N, 0.92; H, 2.47. FTIR (cm−1): 3427 (s), 3146 (m), 1589 (s), 1530 (s), 1479 (s), 1401 (s), 1276 (m), 1243(m), 1163(m), 922(m), 878(s), 765 (w), 676 (m), 636(w), 524 (m). Synthesis of K2H[(H2O)4Mn][AsMo6O21(Ala)(PHBA)2]· 7H2O (4). The synthetic procedure for 4 was similar to that used for 1, with MnCl2·2H2O (0.0486 g, 0.3 mmol) instead of CoCl2·6H2O (0.0714 g, 0.3 mmol). The filtrate was kept undisturbed for 2 weeks under ambient conditions, and then light yellow crystals of 4 were isolated in about 44% yield (based on Mo). Anal. Calcd for 4: Mo, 35.58 As, 4.63; Mn, 3.63; K, 4.83; C, 12.62; N, 0.87; H, 2.43. Found: Mo, 35.35; As, 4.22; Mn, 3.55; K, 4.33; C, 12.34; N, 0.88; H, 2.76. FTIR (cm−1): 3423 (s), 3166 (m), 1597 (s), 1526 (s), 1484 (s), 1399 (s), 1266 (m), 1234(m), 1143(m), 924(m), 889(s), 763 (w), 671 (m), 638(w), 520 (m). General Methods for Catalyzing Degradation of CEES and DECP. CEES oxidation: Compound 1 (6.29 mg, 3.75 μmol), H2O2 (0.25 mmol), CEES (0.25 mmol), and ethanol (0.5 mL) were put in a glass bottle. The catalytic reaction was carried out at room temperature for 5 min. Hydrolysis of DECP: Compound 1 (1.7 mg, 1 μmol), DECP (1 mmol), DMF (0.6 mL), and H2O (50 μL) were put in a glass bottle. The catalytic

on POMs that can rapidly and high-selectively destroy two types of CWAs still is a significant and challenging topic. Organic ligand-modified POMs via covalent bonding have evoked much attention during the last decades, not only because they possess remarkable structures and can be valuable synthons to generate for further postfunctionalization but also because synergy between inorganic POMs and organic ligands makes them have significant potential applied in catalysis.12 In this aspect, one hot issue is the modification of POMs with two or more kinds of organic ligands. However, only Cronin, Wei, and Wu’s group reported several mixed organic ligands-modified POMs such as [TBA]3[(MnMo6O18 )((OCH2) 3CC 9H17 )((OCH 2 ) 3 CNHCHC 16 H 9 )], [TBA] 3 {[C 2 H 5 C(CH 2 O) 3 ]CrMo6O18[(OCH2)3CNH2]}, and [TBA]2{CuMo6 O16[(OCH 2) 3 CCH 3]2 (OCH3 )2 },13 respectively, perhaps because the drastic coordination competition of different organic ligands increase the difficulty of synthesis. Carboxylic acids substituted the oxygen atoms of POMs by two carboxyl oxygen atoms can produce a class of carboxylic acidmodified POMs, which far lag behind organoimido or triol ligands-modified POMs.14 Thus far, a few POMs covalently modified by one kind of carboxylate ligands were developed, including [Mo8O26(L) 2]n‑ (L = proline, lysine, alanine, glycine),15 [XMo6O21(O2CRNH3)3]n− (X = Se, Te, As, Sb, Bi),16 and [(YMo6O21)2(O2CRCO2)3]n− (Y = Se, As, P).17 To our knowledge, different carboxylic acids covalently modified POMs are very rare, let alone the extended architectures based on such units. Inspired by the pioneering work of Hill, Hu, Cronin, Wei, Kortz, Yang, and other groups, we intended to prepare the mixed carboxylic acids-modified polyoxomolybdates as molecular synthons to construct extended structures and further investigate their functionality for destroying diverse CWAs. Herein, we successfully isolated four new 1D chains based on mixed carboxylic acid ligands-modified POMs: K2H[(H2O)4M][AsMo6O21(Ala)(PHBA)2]·nH2O (M = Co2+ 1, Ni2+ 2, Zn2+ 3, Mn2+ 4; Ala = alanine, PHBA = p-hydroxybenzonic acid). These compounds represent the first extended architectures based on mixed-ligands-modified POMs and metal cations. Four hybrid compounds can rapidly and high-selectively detoxify sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES) by oxidation and the nerve agent simulant diethyl cyanophosphonate (DECP) via hydrolysis.

2. EXPERIMENTAL SECTION Materials and Methods. We used chemicals that were commercially purchased without further purification. Elemental analyses (H and N) were performed on a PerkinElmer 2400 CHN elemental analyzer; As, Mo, Co, Ni, Mn, Zn, K, and Na were analyzed on a PLASMA-SPEC (I) ICP atomic emission spectrometer. We detected the IR spectra using KBr pellets as the background in the range 400−4000 cm−1 on an Alpha Centaur FT/IR spectrophotometer. TG analyses were performed in flowing N2 at a heating rate of 10 °C min−1 on a PerkinElmer TGA7 instrument. The PXRD patterns of the samples were recorded on a Rigaku Dmax 2000 X-ray diffractometer with graphite-monochromatized Cu Kα radiation (λ = 0.154 nm) and 2θ varying from 5° to 50°. The diffuse reflectivity spectra were performed on finely ground samples with a Cary 500 spectrophotometer equipped with a 110 mm diameter integrating sphere, which were measured from 200 to 800 nm. GC analysis was performed with an Agilent HP6890 spectrometer with a flame ionization detector, which was used to monitor the conversion and selectivity. GC−MS at the final 6063

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asymmetric unit in 1 has one crystallographically independent [AsMo6O21(Ala)(PHBA)2]5− anion, one Co2+ cation, and two K+ cations (see Figure S1). In the polyoxoanion the average bond lengths of Mo−Ob (2.305 Å) (μ2-O of Mo) are obviously longer than the original Mo−Ot (1.703 Å) (terminal oxygen of Mo), which were analogous to the derivatives of other carboxylatesmodified heteropolymolybdates. Unique Co(1) ion has a distorted octahedral geometry, defined by four water molecules [Co−OW 2.073(4)−2.120(4) Å] and two terminal oxygen atoms from two polyoxoanions [Co−O 2.095(4)−2.106(5) Å]. Finally, the organic ligands-modified POMs are joined together by Co2+ cations to produce a 1D linear chain (Figure 1b). To our knowledge, this is the first 1D extended structure based on mixed ligands-modified POMs. Strong hydrogen bonds exist between the hydroxyls of PHBA and oxygen atoms of polyoxoanions (O6−H···O29 = 2.684 Å, O16−H···O28 = 2.795 Å), generating a 2D supramolecular layer (Figure 1c). These 2D layers are then linked together to produce the 3D supramolecular framework via the hydrogen bonds among the polyoxoanions, coordination water molecules, and nitrogen atoms of the Ala ligands (O3W− H···O26 = 2.740 Å; N1−H···O14 = 2.750 Å) (Figure S2). The most important feature is that the four structural components of {AsMo6} unit, Ala, PHBA, and transition metal cation endow these compounds with multiple functionality in catalysis. Bond valence sum (BVS) calculations18 show that the oxidation states of Mo, As, and Co/Ni/Zn/Mn ions in 1−4 are +6, +3, and +2, respectively. IR spectra for compounds 1−4 were recorded in Figure S5. The characteristic vibrational bands of {AsMo6} anions were located in the 1000−400 cm−1 range, and the peaks from 1600 to 1100 cm−1 arose from the carboxylic acid ligands. UV−vis diffuse reflectance spectra of compounds 1−4 exhibit the characteristic O → Mo charge transfer of the {AsMo6} unit (see Figure S8). In the UV region (200−400 nm) there are two absorption bands at 252 and 306 nm for 1, which are assigned to O → Mo charge transfer for {AsMo6} polyoxoanions. In the visible region (400−800 nm), the plots display two absorption bands at 528 and 650 nm for 1, which were, respectively, assigned to the 1A1g → 1T2g and 1A1g → 1 T1g transition of a regular octahedral configuration low-spin Co2+ cations. Thermogravimetric analyses (TGA) displayed in Figure S9 indicated compounds 1−4 successively lost lattice water, coordinated water, and organic ligands from 50 to 850 °C. The phase purities of 1−4 were also proved by the PXRD in Figure S10. The diffraction peaks of these calculated and experimental patterns match well, indicating the phase purities of these compounds. The similar PXRD patterns of the four compounds also indicate the isostructural nature of these compounds. Catalytic Oxidation Studies for CEES. Owning to the danger involved in managing with highly toxic CWAs, simulant molecules with similar chemical behaviors and structures are always used by experiments. Since CEES is considered as an effective simulant molecule for HD, we investigated the oxidative degradation reaction of CEES using compounds 1−4. The catalytic experiment of CEES was monitored by gas chromatography (GC) and performed in ethanol (0.5 mL) with CEES (0.25 mmol), naphthalene (internal standard 0.25 mmol), catalyst (3.75 μmol), and H2O2 (oxidant 0.3 mmol). Figure 2b displays the time profiles for the oxidation of CEES with compounds 1−4 and without catalyst. The results indicate that these compounds efficiently catalyzed the oxidation degradation of CEES within 5 min, and compound 1 shows the best catalytic effect. Compound 1 can convert 98.5% of CEES into the nontoxic oxidation

reaction was carried out at room temperature for 10 min. After the catalytic reaction was finished, GC-FID, GC-MS, and 1H NMR were used to analyze the resulting mixture.

3. RESULTS AND DISCUSSION Structures and Characterization of Catalysts. Compounds 1−4 were assembled under reflux at 80 °C for 1 h by a simple one-pot process in which the raw materials Na2MoO4, As2O3, PHBA, and Ala are mixed together at a ratio of 6:1:2:1. Single-crystal X-ray diffraction analyses reveal that compounds 1−4 are isostructural and crystallize in the space group P-1. The four compounds contain the same polyoxoanion unit {AsMo6}, which is different from the classic Anderson-type POMs. In the {AsMo6} unit, the central As atom with lone pair electrons is coordinated to three oxygen atoms to form a trigonal-pyramidal molecular geometry, and such AsO3 group is surrounded by a ring of six edge-sharing and corner-sharing MoO6 octahedra. Then two different carboxylic acid ligands (two PHBA molecules and one protonated Ala) were covalently bound to the polyoxoanion {AsMo6} by their carboxylate groups on the same side of the cluster to yield the mixed carboxylic acidsmodified POM [AsMo6O21(Ala)(PHBA)2]5− (Figure 1a). The

Figure 1. (a) Direct self-assembly protocol for the synthesis of mixed carboxylic acid ligands-modified POMs. (b) Polyhedral and ball−stick view of the 1D chain of 1. (c) View of the 2D supramolecular layer showing the hydrogen bonds. (color code: As, yellow; Mo, purple; O, red; Co, light blue; N, blue; C, black). 6064

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Figure 2. (a) Catalytic oxidation pathway of CEES to CEESO. (b) Conversion of CEES oxidation for compounds 1−4 and blank. Reaction conditions: [CEES] = 0.25 mmol, 3.75 μmol of catalyst, 0.25 mmol of naphthalene (internal standard), 0.3 mmol of H2O2, and 500 μL of CH3CH2OH at room temperature for 5 min. (c) GC-FID signals indicating the progress of the oxidation of CEES (4.557 min) to CEESO (8.920 min) and internal standard (8.495 min, naphthalene) in the presence of 1.

Table 1. Comparison of CEES Decomposition by Different Materials in Recent Years catalyst

time (min)

oxidant

temperature (°C)

conversion (%)

sulfoxide selectivity (%)

refs

compound 1 compound 2 compound 3 compound 4 PNb12VVVIV4 PW12@NU-1000 TBA-polyV6 fb-PCN-222/MOF-545

5 5 5 5 60 20 30 60

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 O2

25 25 25 25 25 45 25 25

98.5 97.3 95.4 94.3 100 98 99 93

>99.9 >99.9 >99.9 >99.9 67 57 100 100

this work this work this work this work 11a 5b 11b 5c

product 2-chloroethyl ethyl sulfoxide (CEESO) in 5 min. The selectivity for the nontoxic CEESO is 99.9%. Blank reaction exhibited negligible activity without catalyst. Table 1 summarizes the catalytic degradation ability of reported materials in recent years in CEES oxidation. It indicates that compounds 1−4 are notably more active than the reported catalysts based on POMs. Figure 2c shows the progress of the oxidation of CEES to CEESO in the presence of 1 by GC-FID signals, and it displays CEES is almost entirely oxidized to the only product observed CEESO within 5 min under these conditions (the mass spectrum further indicates the only product in Figure S11). Meanwhile, 1H NMR spectroscopy was also utilized to monitor the conversion and ascertain the product to demonstrate the accuracy of the reaction (Figure 3). The reaction solutions at 3 and 5 min reaction time were detected in CDCl3 in the 1H NMR spectrum. For comparison, the 1H NMR spectra of pure CEES and CEESO were also detected. To further investigate whether CEESO might be oxidized into overoxidation product 2-chloroethyl ethyl sulfone (CEESO2), we prolonged the reaction time to 1 h. We were delighted to find that extending the reaction time was not favorable to further oxidation reaction.

Figure 3. Selected regions of the 1H NMR spectra of CEES, CEESO, oxidation reaction (without internal standard) at 3 min, and oxidation reaction (without internal standard) at 5 min with two compounds in CDCl3. Peaks used for calculation are labeled with chemical shift.

In addition, to probe the active components of compound 1 in the selective oxidation of CEES, we studied the catalytic activity of Ala, PHBA, {AsMo 6 (Ala) 3 }, {AsMo 6 (PHBA) 3 }, 6065

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Kinetics Study. The catalytic degradation reaction kinetics of the CEES process was also investigated. Figure 2b shows that the conversion rates of the oxidative degradation reaction change as a function of time. ln(Ct/C0) against the reaction time exhibits a linear correlation, which demonstrates that the oxidation process accords with a first-order reaction of CEES (Ct and C0 represent the CEES concentration at some time and at the begin time). The first-order kinetics constants k1, k2, k3, and k4 for 1−4 are 0.90826, 0.72416, 0.62967, and 0.5945 min−1, respectively (Figure S15). Catalyst Stability and Recyclability. As the heterogeneous catalyst, compound 1 was collected by simple filtration after one cycle of the catalytic reaction. The filtrate showed no catalytic effect in the similar situation. The conversion and selectivity of CEES remain stable for the following 6 cycles using the collected catalyst (Figure 5), which indicate excellent cycling stability of

{Co0.25AsMo6(Ala)3} (compound 5), {Co0.5AsMo6(PHBA)3} (compound 6), and CoCl2 salt. As shown in Figure 4, Ala or

Figure 4. Oxidation of CEES by different catalysts. Reaction conditions: [CEES] = 0.25 mmol, 3.75 μmol of catalyst, 0.25 mmol of naphthalene (internal standard), 0.3 mmol of H2O2, and 500 μL of CH3CH2OH at room temperature for 5 min.

PHBA alone makes very little CEES oxidize. When the ligands coordinated to the {AsMo6} unit construct ligand-modified POMs, {AsMo6(Ala)3} and {AsMo6(PHBA)3} can catalyze CEES oxidation with conversions of 43.4% and 45.5% and selectivity of 92.1% and 93.4% respectively, which are far below that for 1. The structure of {AsMo6(Ala)3} is similar to that reported by Kortz’s group,16 and the formula of {AsMo6(PHBA)3} was inferred by the IR spectrum due to the poor crystal quality (Figure S7). Furthermore, compound 5 constructed from {AsMo 6 (Ala) 3 } and Co2+ cation and compound 6 based on {AsMo6(PHBA)3} and Co2+ cation were prepared and characterized by single-crystal X-ray diffraction (see Figures S3 and S4). Compounds 5 and 6 can convert 58% and 77.3% of the CEES with a selectivity of 93.5% and 95.4%, respectively, which get a little improved compared with {AsMo6(Ala)3} and {AsMo6(PHBA)3} but are lower than that for compound 1 with {AsMo6(Ala)(PHBA)2} and Co2+. When CoCl2 was used, the low conversion is 10.3% with 72% selectivity. From the above results, it can be concluded that the synergistic role between two kinds of carboxylic acids-modified POM and Co2+ cation in 1 promotes the rapid and high-selective oxidation of CEES. Then the factors that influence the activity of compound 1 were continuously tested. The catalytic reaction was limited by the quantity of hydrogen peroxide and catalyst. The optimal ratio of hydrogen peroxide to CEES is 1.2 to 1. The conversion was lowered to 65.6%, and the selectivity remained high when the amount of hydrogen peroxide is 0.8 equiv (Figure S12). When the catalyst loading was lowered from 1.5% to 1% equiv, the conversion of the oxidation reaction was observed to be 67% with a little lower selectivity of 92.1% (Figure S13). The conversion and selectivity almost keep constant if the catalyst loading is above 1.5%. We next investigated the influence of the solvents. The results illustrated that the reaction was susceptible to solvents and in favor of the polar solvents (Table S1). For comparison, O2 was also used and we found that the conversion and selectivity were far lower than H2O2 in 24 h (Table S1, entry 5).

Figure 5. Recycling of the catalytic system for the oxidation of CEES to the corresponding CEESO using compound 1.

compound 1. In addition, the PXRD patterns and the IR spectra of compound 1 before and after the oxidation process remain virtually identical, which further reveal that compound 1 keeps intact after the catalytic reaction (Figures S16 and S17). Possible Catalytic Mechanism Studies. Previous research, especially the POMs being investigated widely at this point, established that the peroxo−metal species are the actual catalysts of the catalytic oxidation reaction in the mixture solution of POMs and H2O2.11,19 In this case, we proposed a possible mechanism of CEES oxidation in Figure 6. To confirm the

Figure 6. Proposed mechanism of the catalytic oxidation of CEES.

existence of the active peroxo intermedium, the UV−vis spectrum of the mixture solution was explored. Indeed, the red shifts of the peaks (248 → 253 nm, 546 → 552 nm) and a new band occurring at 325 nm confirm the interactions between compound 1 and H2O2 (Figure S18). Thus, we think that the catalyst 1 interacts with H 2 O 2 to generate an active peroxomolybdenum and peroxocobalt species, which result in the attack on the S atom of the CEES and produce the nontoxic CEESO. By virtue of the cooperative interactions among molybdenum, cobalt, and carboxylic acid ligands, compound 1 6066

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Figure 7. (a) Hydrolytic decomposition pathway of DECP. (b) Conversion of DECP hydrolysis for compounds 1−4. (c) DECP decomposition by different catalysts. Reaction conditions: [DECP] = 1 mmol, 1 μmmol of catalyst, 0.25 mmol of naphthalene (internal standard), 600 μL of DMF, and 50 μL of H2O at room temperature for 10 min.

hydrolyze, which also are a little more improved than {AsMo6(Ala)3} and {AsMo6(PHBA)3}. Therefore, the control experiments may conclude that both mixed carboxylic acidsmodified POM {AsMo6(Ala)(PHBA)2} and Co2+ cation contribute to the hydrolytic degradation reaction. Kinetics and Recyclability Studies. The kinetics of the catalytic hydrolysis reaction was also studied. Figure S20a displays the rates of the DECP removal change under various amounts of catalysts. The initial rate of the decontamination of DECP shows it is first order in relation to compound 1 (Figure S20b). Similarly, the influence of the amount of substrates was also explored and is exhibited in Figure S20c,d, which demonstrated that the rates of the hydrolytic degradation reaction decreased with more substrates, and it also displayed a first-order reaction with respect to DECP. After DECP was completely converted to the corresponding products, the reusability of 1 was tested by adding the same amount of DECP under a similar situation. The catalytic effect remained stable for the following 7 cycles (Figure 8). To further identify the reusability of compound 1, the catalytic reaction proceeds after 24 h in the same condition. The catalytic capability remained stable when the reaction was reused three times (Figure 8).

exhibits superior catalytic performance in the oxidation degradation of CEES in comparison to the published POMs. Catalytic Hydrolysis Studies for DECP. Given that DECP is an effective simulant for nerve agents by previous studies10,11,20 broad-spectrum detoxification catalyst is highly desired. The hydrolytic degradation reaction of DECP was thus investigated by compounds 1−4, since they have metal cations as Lewis acid sites and pronoated amino and hydroxy groups.21 We were surprised to find out that compounds 1−4 can almost entirely catalyze the hydrolytic reaction within 10 min with a small quantity of catalysts without adding any basic molecule such as N-ethylmorpholine (Figure 7b). The half-life of DECP is about 3.8 min with a turnover frequency (TOF) of 6000 h−1 using 1. Just as Table 2 summarizes, this rate of the catalytic degradation Table 2. Comparison of DECP Decomposition by Different Materials in Recent Years catalyst

time (min)

conversion (%)

t1/2 (min)

refs

compound 1 compound 2 compound 3 compound 4 MOF-808 UiO-66 MgO TiO2 KGeNb PNb12VVVIV4 K8Nb6O19 TBA-polyV6

10 10 10 10 30 30 30 30 30 30 30 30

99 97 95 96 50 32 90 87 100 98 90 100

3.8 4.2 4.7 4.8 24 78 12 12 6 10 12

this work this work this work this work 19b 19b 19a 19a 10b 10b 10b 11b

4. CONCLUSIONS In summary, four extended hybrid compounds 1−4 based on mixed carboxylic acids-modified arsenomolybdates were synthesized and fully characterized. As high-performing multifunctional catalysts, the four compounds can rapidly catalyze selective oxidation degradation of the sulfur mustard simulant CEES and hydrolytic degradation of the nerve agent simulant DECP under mild conditions, indicating the great promise of our compounds for CWA protection. The remarkable catalytic activities of these compounds come from the multicomponent synergy among the {AsMo6} unit, Ala, PHBA, and transition metal cation. This study will shed light on the design of more efficient POM catalysts for destroying diverse CWAs.

reaction is significantly faster than the reported catalysts based on POMs. Figure 7c shows the conversions in different catalysts under similar conditions. The compounds {AsMo6(Ala)3} and {AsMo6(PHBA)3} can catalyze 79.9% and 81.2% of the DECP hydrolysis, while CoCl2 converts 51% of DECP to the nontoxic products, which displayed relatively lower conversion than compounds 1−4 but much higher than the blank reaction. In addition, compounds 5 and 6 can make 85.3% and 86.4% DECP 6067

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Research Article

ACS Catalysis

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Figure 8. Recycle test for hydrolytic degradation of DECP using compound 1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b00972. Synthesis of 5, 6, {AsMo 6 O 21 (Ala) 3 }, and {AsMo6O21(PHBA)3}; ORTEP drawing of 1, 5, and 6; IR spectra, TG plots, PXRD figures, UV−vis diffuse reflective spectra for 1−4; mass spectrum of 2-chloroethyl ethyl sulfoxide (CEESO); time profile for the oxidative decontamination of CEES using 1 under different conditions; kinetic analysis of CEES/DECP; oxidation of CEES under different conditions; crystal data and structure refinement for 1−6; selected bond lengths and angles for 1−6 (PDF) Crysllographic information (CIF) checkCIF/PLATON report (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-411-84657675. E-mail: [email protected]. ORCID

Haiyan An: 0000-0003-3848-5210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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).

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REFERENCES

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DOI: 10.1021/acscatal.8b00972 ACS Catal. 2018, 8, 6062−6069

Research Article

ACS Catalysis

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DOI: 10.1021/acscatal.8b00972 ACS Catal. 2018, 8, 6062−6069