Ligand-effect regulated selective catalytic oxidation of 2-chloroethyl ethyl sulfide in aqueous phase by Keplerate-type molybdenum-oxygen cluster {Mo₁₃₂}

Abstract

Keplerate-type nano-polyoxometalates {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} were respectively used as catalysts, and H₂O₂ as the oxidant, for the catalytic oxidation of 2-chloroethyl ethyl sulfide (CEES), a simulant of mustard gas, in an aqueous solution. The decontamination conditions were optimized via the single-factor method. Under the optimized conditions, the CEES conversion rates catalyzed by {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} were 98.59% and 99.69%, respectively. After 5 cycles of reuse, both catalysts maintained a CEES conversion rate of over 90% and a selectivity of over 99%. The catalytic decontamination processes of CEES by {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} both followed first-order reaction kinetics, with kinetic constants of 0.39 min⁻¹ and 0.60 min⁻¹, respectively. Compared with {Mo₁₃₂-Ac⁻}, {Mo₁₃₂-SO₄²⁻} with more negative charges exhibited higher catalytic activity, showing a significant ligand effect. By adjusting the amount of the catalyst and H₂O₂, CEES can be selectively catalytically oxidized to low-toxicity 2-chloroethyl ethyl sulfoxide (CEESO).

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1 Introduction

Mustard gas (HD) is an infamous chemical warfare agent. The decontamination of HD and its simulant 2-chloroethyl ethyl sulfide (CEES) has long been a research focus in chemical protection and environmental remediation.^1,2 The decontamination mechanisms of HD and CEES mainly include physical adsorption,³ dehydrochlorination,⁴ oxidation,^5,6 and hydrolysis.⁷ Catalytic decontamination materials developed based on these mechanisms include hypochlorites,⁸ zeolites,⁷ metal oxides,⁶ metal–organic frameworks (MOFs),^9–11 and polyoxometalates(POMs).^12–16 Among these, physical adsorption is prone to cause secondary pollution from the adsorbent;¹⁰ dehydrochlorination generally shows low kinetic efficiency for HD/CEES decontamination;¹⁷ oxidation carries the risk of over-oxidation (possibly generating more toxic sulfone products),¹⁸ and some systems rely on harsh conditions such as light irradiation. In contrast to the above three pathways, hydrolysis yields less toxic products but is constrained by the poor water solubility of HD and its simulants; additionally, the formation of intermediates impedes the further progress of the reaction, thus limiting its practical application.²

Compared with traditional catalytic decontamination materials (hypochlorites, zeolites, metal oxides), POMs possess dual-functional characteristics (acidic sites and redox-active sites) and core advantages of tunable structure (performance optimization via transition metal doping or organic functional group modification), making them widely applicable in all four decontamination mechanisms. In recent years, an increasing number of novel and efficient POM catalysts have been used for HD/CEES decontamination.^12–16

Currently, in POM-based HD/CEES decontamination involving solvents, the extensive use of organic solvents is a prominent feature of mainstream schemes.⁴ The core consideration is to improve substrate solubility, which is premised on the low solubility of POMs in organic solvents. Especially at low POM amount, the insufficient exposure of active sites cannot be alleviated by the good solubility of the substrate; moreover, this scheme fails to consider the potential secondary pollution risk of organic solvents, which constitutes a major limitation for technical application.

To overcome the inherent defects of the above organic solvent systems, this study selected water as the green solvent and H₂O₂ as the green oxidant, and specifically adopted Keplerate-type POMs as the catalytic core. Due to their unique cage-like topology, Keplerate-type POMs have a large specific surface area, high active site density, and sufficient site exposure. Their abundant coordination sites and tunable charge properties not only adapt to the aqueous reaction environment to alleviate the poor solubility of traditional POMs but also exhibit excellent catalytic selectivity and stability in thioether oxidation reactions.^12,19,20

Based on this, two Keplerate-type nano-polyoxometalates were selected as research objects: (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca.300H₂O·ca.10CH₃COONH₄ (denoted as {Mo₁₃₂-Ac⁻})²¹ and (NH₄)_72−n[(H₂O)_81−n⁺(NH₄)_n ⊂ {(Mo^VI)Mo^VI₅O₂₁(H₂O)₆}₁₂{Mo^V₂O₄(SO₄)₃₀}]·ca.200H₂O (denoted as {Mo₁₃₂-SO₄²⁻}).²² The difference between the two lies in the ligands on the anionic {Mo₂} units: the anions of {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} carry 42 and 72 negative charges, respectively (Fig. 1). Using water as the solvent and H₂O₂ as the oxidant, the decontamination conditions were optimized via the single-factor method. The catalytic decontamination performance, reaction kinetics, and decontamination mechanism of the two catalysts for CEES were investigated.

Fig. 1 The capsular structure of nano-polyoxometalate cluster {Mo₁₃₂} (color code: Mo^VI, blue and Mo^V, cyan).

2 Experimental

2.1 Reagents and instruments

CH₃COOH (analytical grade), N₂H₄·H₂SO₄ (analytical grade), (NH₄)₆Mo₇O₂₄·4H₂O (analytical grade), CH₃COONH₄ (analytical grade), CEES (analytical grade), dichloromethane (analytical grade), thymolphthalein (analytical grade), 1,3-dichlorobenzene (chromatographic grade), ethanol (analytical grade), NaOH (analytical grade). All chemicals were purchased directly and used without further purification.

Fourier transform infrared (FT-IR) spectrometer (ICAN9 model): tested in the range of 4000–400 cm⁻¹ by KBr pellet method (for catalyst characterization). UV-visible (UV-vis) spectrophotometer (for aqueous phase absorbance determination). GC-FID (GC-5890): equipped with a Kb-5 column (30 m × 320 µm × 0.25 µm); column temperature: 120 °C, inlet temperature: 220 °C, detector temperature: 250 °C (for qualitative and quantitative analysis of reaction materials). X-ray photoelectron spectrometer (XPS, Axis Supra). Magnetic stirrer (TH70-85-2). Reactor: glass screw-cap sample vial (3 mL, 16 × 35 mm).

2.2 Catalyst preparation

{Mo₁₃₂-Ac⁻}²¹ and{Mo₁₃₂-SO₄²⁻}²² were synthesized according to the literature and characterized by FT-IR and XPS.

2.3 Decontamination of CEES

In 500 µL deionized water, 5 µL CEES (0.042 mmol) and a preset amount of catalyst {Mo₁₃₂-L} (L = Ac⁻, SO₄²⁻) were added, followed by H₂O₂ (30%) to initiate the reaction. After reaction at room temperature for a designated time, 500 µL of dichloromethane and 2.4 µL of 1,3-dichlorobenzene (21 µmol) as the internal standard were added. The mixture was stirred for 10 min to facilitate extraction, followed by standing for phase separation. The aqueous layer was carefully withdrawn using a pipette, and quantitative analysis of the product was performed by gas chromatography (GC) employing the internal standard method.

2.4 Catalyst reusability

0.77 µmol of catalyst {Mo₁₃₂-L} (L = Ac⁻, SO₄²⁻) was used for the reusability test. After each reaction cycle, the catalyst was recovered by concentration, dried at 60 °C, and reused for the next catalytic run.

3 Results and discussion

3.1 Characterization

{Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} were synthesized strictly following the protocols reported in the literature,^21,22 and characterized by FT-IR, PXRD and XPS. Notably, their FT-IR spectra (including characteristic peak positions and intensities) and XPS results (e.g., Mo valence state distribution) were in full accord with those documented in the aforementioned references, confirming the successful reproduction of the target catalysts as reported.^21,22

The PXRD patterns of {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} (Fig. S1) exhibit characteristic low-angle peaks (2θ ≈ 8°, 11°) consistent with {Mo₁₃₂}-type Keplerate clusters. No impurity peaks are observed, confirming the high phase purity and good crystallinity of the samples, which is in line with literature reports and verifies the successful synthesis.^21,22

3.2 CEES decontamination performance of catalysts

3.2.1 Effect of the amount of catalyst. 5 µL CEES (0.042 mmol), 6 µL H₂O₂ (30%, 0.059 mmol), and different amount of {Mo₁₃₂-Ac⁻} (0, 5, 10, 20, 30 mg) were added to 500 µL H₂O, and the reaction was conducted at room temperature for 10 min. Results showed that CEES was barely degraded without the catalyst, indicating that H₂O₂ alone cannot effectively decontaminate CEES. When the catalyst amount increased from 5 mg to 10 mg, the conversion rate increased from 95.46% to 98.84% (Fig. 2(b)), with a small amount of highly toxic CEESO₂ detected alongside the target product CEESO (Fig. 2(a)). Decontamination products were qualitatively identified by comparison with standard substances. The GC-FID chromatograms of CEES, internal standard (1,3-dichlorobenzene), CEESO, and CEESO₂ standards are shown in Fig. S2. GC-FID quantification was performed by the internal standard method with 1,3-dichlorobenzene. Response factors of CEES, CEESO, and CEESO₂. The internal standard were calibrated for quantitative accuracy. No significant interference peaks were observed, eliminating matrix effects on results.

Fig. 2 (a) GC-FID signals show the effects of the amount of { Mo₁₃₂-Ac⁻} on CEES decontamination. (b) CEES conversion rate as a function of {Mo₁₃₂-Ac⁻} amount. Data are the average of three parallel experiments with an experimental error of ±2%.

At a catalyst amount of 20 mg, the conversion rate was 98.59%, slightly lower than that at 10 mg, but CEESO₂ formation was minimized. To avoid CEESO₂ and ensure high conversion, the catalyst amount was set to 0.77 µmol.

3.2.2 Effect of the amount of 30% H₂O₂ on CEES oxidation. 5 µL CEES (0.042 mmol), 20 mg {Mo₁₃₂-Ac⁻} (0.77 µmol), and different amount of H₂O₂ (30%, 0, 4, 6, 8, 10 µL) were added to 500 µL H₂O, and the reaction was conducted at room temperature for 10 min. Results showed that almost no decontamination occurred without H₂O₂. With the increase of H₂O₂ amount, the conversion rate increased to 91.12% (4 µL), 98.59% (6 µL), 98.84% (8 µL), and 99.59% (10 µL) (Fig. 3(b)). When H₂O₂ amount exceeded 6 µL, CEESO₂ was detected alongside CEESO (Fig. 3(a)). Thus, the amount of H₂O₂ (30%) was set to 6 µL.

Fig. 3 (a)GC-FID chromatograms illustrating the oxidation progress of CEES with varying amounts of H₂O₂ (30%). (b) Effect of H₂O₂ (30%) volume (µL) on CEES conversion rare. Data are the average of three parallel experiments with an experimental error of ±2%.

3.2.3 Effect of reaction time. Based on the optimized amount of catalyst and oxidant, experiments were conducted to investigate the effect of reaction time (0, 1, 3, 5, 7, 10 min) on the decontamination rate. For {Mo₁₃₂-Ac⁻}, the conversion rate increased significantly in the first 5 min: 46.01% (1 min), 61.48% (3 min), 77.13% (5 min). After 7 min, the conversion rate plateaued, reaching 98.59% at 10 min. For {Mo₁₃₂-SO₄²⁻}, the conversion trend was consistent but higher: 54.80% (1 min), 79.17% (3 min), 94.13% (5 min), 99.29% (7 min), 99.69% (10 min), with nearly complete CEES decontamination at 7 min (Fig. 4). Notably, under optimized conditions, CEESO was the dominant product at 10 min for both catalysts, indicating that product control relies on appropriate amount of catalyst and H₂O₂.

Fig. 4 GC-FID chromatograms show the time profiles for the decontamination of CEES: (a) {Mo₁₃₂-Ac⁻}, (b) {Mo₁₃₂-SO₄²⁻}. Effect of reaction time on conversion of CEES (c). Data are the average of three parallel experiments with an experimental error of ±2%.

Plots of ln(C_t/C₀) (C_t: CEES concentration at time t; C₀: initial CEES concentration) versus time showed approximately linear relationships for both catalysts, confirming first-order reaction kinetics. The first-order kinetic constants of {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} were 0.39 ± 0.041 min⁻¹ and 0.60 ± 0.046 min⁻¹, respectively (Fig. 5).

Fig. 5 Kinetic analysis of CEES oxidation catalyzed by {Mo₁₃₂-L} (L = Ac⁻, SO₄²⁻): (a) {Mo₁₃₂-Ac⁻}, (b) {Mo₁₃₂-SO₄²⁻}.

Compared with recent aqueous-phase POM-based CEES decontamination systems, {Mo₁₃₂-SO₄²⁻} exhibits superior comprehensive performance. POM@CTF catalyst showed a kinetic constant of 0.061 min⁻¹ and 99% selectivity,¹³ while {PMo₁₁MoCu₈} was used for 4 cycles.¹⁶ In contrast, {Mo₁₃₂-SO₄²⁻} achieves a higher 0.60 min⁻¹ kinetic constant, >99% selectivity, and >90% conversion after 5 cycles without organic solvents, highlighting the ligand regulation strategy and green aqueous system's application potential.

3.3 Decontamination mechanism

Notably, in the decontamination system, CEES was barely degraded when only the catalyst {Mo₁₃₂-L} or H₂O₂ was added. In contrast, the CEES conversion rate increased significantly when both were present simultaneously. Thus, CEES decontamination relies on the synergistic effect of the catalyst and oxidant. Upon reaction with H₂O₂, the terminal Mo₂=O sites are converted into peroxo-molybdenum species (Mo(O₂)) via peroxo coordination, as verified by XPS. These peroxo species act as the actual active intermediates for the selective oxidation of CEES to CEESO.

In both catalysts, Mo exists in two valence states: +5 and +6. The chemical formulas with valence state labels for {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} are as follows: (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]·ca. 300H₂O·ca. 10CH₃COONH₄ (note: slight discrepancy from the formula in the introduction) and (NH₄)_72−n[(H₂O)_81−n⁺(NH₄)_n ⊂ {(Mo^VI)Mo^VI₅O₂₁(H₂O)₆}₁₂{Mo^V₂O₄(SO₄)₃₀}]·ca. 200H₂O. Their XPS characterization results are shown in Fig. 6 and S3.

Fig. 6 XPS of Mo 3d of {Mo₁₃₂-Ac⁻} (a), Mo 3d of {Mo₁₃₂-Ac⁻} after adding H₂O₂ (b), Mo 3d of {Mo₁₃₂-Ac⁻} after adding H₂O₂ and substrate CEES (c).

As presented in Fig. 6(a), before the addition of H₂O₂, the high-resolution Mo 3d spectrum of {Mo₁₃₂-Ac⁻} exhibited four peaks at 231.5, 232.7, 234.7, and 235.8 eV.¹² The peaks at 231.5 eV and 234.7 eV were assigned to Mo^V 3d_5/2 and Mo^V 3d_3/2, respectively, while those at 232.7 eV and 235.8 eV corresponded to Mo^VI 3d_5/2 and Mo^VI 3d_3/2, according to the reported XPS reference values for molybdenum-based compounds.²³ After adding H₂O₂ (Fig. 6(b)), the Mo 3d spectrum of {Mo₁₃₂-Ac⁻} showed only two peaks at 232.7 eV and 235.8 eV, which were attributed to Mo^VI 3d_5/2 and Mo^VI 3d_3/2, indicating that Mo^V was completely oxidized to Mo^VI by H₂O₂.

As shown in Fig. 6(c), after the simultaneous addition of H₂O₂ and the substrate CEES to the catalyst, the high-resolution Mo 3d spectrum was restored to that before H₂O₂ addition (four peaks). The peaks at 231.5 eV and 234.9 eV were assigned to Mo^V 3d_5/2 and Mo^V 3d_3/2, respectively, and those at 232.8 eV and 235.9 eV corresponded to Mo^VI 3d_5/2 and Mo^VI 3d_3/2, in line with the reported XPS reference values for molybdenum-containing compounds.²³ These results confirm that the {Mo₁₃₂-Ac⁻} catalyst was regenerated after the addition of H₂O₂ and CEES.

The {Mo₁₃₂-SO₄²⁻} catalyst exhibited an identical trend. Prior to H₂O₂ addition, the high-resolution Mo 3d XPS spectrum of {Mo₁₃₂-SO₄²⁻} (Fig. S3a) displayed four characteristic peaks at 231.5, 232.9, 234.9, and 235.9 eV. The peaks at 231.5 eV (Mo^V 3d_5/2) and 234.9 eV (Mo^V 3d_3/2) are indicative of Mo^V species, while those at 232.9 eV (Mo^VI 3d_5/2) and 235.9 eV (Mo^VI 3d_3/2) correspond to Mo^VI. Upon H₂O₂ introduction (Fig. S3b), only two peaks at 232.6 eV and 235.8 eV were observed in the Mo 3d spectrum of {Mo₁₃₂-SO₄²⁻}, which can be ascribed to Mo^VI 3d_5/2 and Mo^VI 3d_3/2, confirming the complete oxidation of Mo^V to Mo^VI by H₂O₂.

Following the co-addition of H₂O₂ and substrate CEES to the catalyst (Fig. S3c), the high-resolution Mo 3d spectrum of {Mo₁₃₂-SO₄²⁻} was recovered to its state prior to H₂O₂ addition, exhibiting four peaks at 231.5, 232.9, 234.8, and 236.0 eV. These peaks are assigned to Mo^V (3d_5/2: 231.5 eV; 3d_3/2: 234.8 eV) and Mo^VI (3d_5/2: 232.9 eV; 3d_3/2: 236.0 eV), consistent with the initial spectral profile. These findings verify the regeneration of the {Mo₁₃₂-SO₄²⁻} catalyst after the combined treatment with H₂O₂ and CEES.¹²

The negative charges of {Mo₁₃₂-Ac⁻} (42⁻) and {Mo₁₃₂-SO₄²⁻} (72⁻) have been confirmed by elemental analysis and charge balance calculations.^21,22 For the {Mo₁₃₂-Ac⁻}, the pH of the reaction solution was kept at 4.6–4.8 before and after the addition of H₂O₂. Similarly, for the {Mo₁₃₂-SO₄²⁻}, the reaction pH was also maintained at 3.5–3.7 throughout the process. Notably, the Keplerate {Mo₁₃₂} clusters possess a well-defined porous spherical structure with pore openings of ca. 9 Å, which allow the entry of CEES molecules into the clusters. However, only the terminal Mo=O groups exposed on the outer spherical surface are accessible to H₂O₂ and responsible for the formation of peroxo-molybdenum species (Mo(O₂)). Consequently, the catalytic oxidation of CEES to CEESO proceeds exclusively on the outer surface of {Mo₁₃₂}. In the acidic aqueous system, the sulfur atom in CEES is prone to protonation to form the R–S⁺(H)–R′ intermediate. Compared with {Mo₁₃₂-Ac⁻}, {Mo₁₃₂-SO₄²⁻} with more negative charges exhibits stronger electrostatic interactions with the positively charged R–S⁺(H)–R′, bringing the substrate closer to the catalyst surface. Peroxo-molybdenum species (Mo(O₂)) are formed from the reaction of the 132 terminal Mo=O groups on {Mo₁₃₂} with H₂O₂. All these Mo=O oxygen atoms lie on the outer spherical surface, while the trans-positioned H₂O ligands of the 72 Mo^VI centers point toward the center of the sphere. These peroxo intermediates then efficiently oxidize CEES to CEESO. This inference is fully consistent with the catalytic performance data of the two catalysts: {Mo₁₃₂-SO₄²⁻} shows a higher kinetic constant (0.60 ± 0.046 min⁻¹ vs. 0.39 ± 0.041 min⁻¹ for {Mo₁₃₂-Ac⁻}) and a faster reaction rate (nearly complete conversion within 7 min), highlighting the catalytic advantage brought by ligand-regulated charge properties,²⁴ as shown in Fig. 7.

Fig. 7 Comparison of the difficulty of selective oxidation of CEES with {Mo₁₃₂-L} (L = Ac⁻, SO₄²⁻) (color code: Mo^VI, blue and Mo^V, cyan).

3.4 Catalyst reusability

Reusability tests showed that after 5 cycles, {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} maintained a CEES conversion rate >90% and selectivity >99% (Fig. 8). FT-IR spectra of the recovered catalysts showed no significant changes compared with fresh catalysts (Fig. 9), indicating good stability of {Mo₁₃₂-Ac⁻} (or {Mo₁₃₂-SO₄²⁻}) under reaction conditions. Meanwhile, XPS spectra confirmed that the catalysts were regenerated after CEES catalytic decontamination (Fig. 6 and S3).

Fig. 8 Influence of catalyst cycle times on conversion and selectivity: (a) {Mo₁₃₂-Ac⁻}, (b) {Mo₁₃₂-SO₄²⁻}. Data are the average of three parallel experiments with an experimental error of ±2%.

Fig. 9 IR spectra of the fresh and recovered catalysts after catalysis reaction: (a) {Mo₁₃₂-Ac⁻}, (b) {Mo₁₃₂-SO₄²⁻}.

4 Conclusions

Keplerate-type {Mo₁₃₂} polyoxometalates with different ligands (Ac⁻ and SO₄²⁻) were successfully applied for CEES decontamination in green aqueous phase using H₂O₂ as oxidant. The ligand effect was systematically investigated: {Mo₁₃₂-SO₄²⁻} with more negative charges exhibits a higher first-order kinetic constant (0.60 min⁻¹) and CEES conversion (99.69%) than {Mo₁₃₂-Ac⁻}, attributed to the enhanced electrostatic interaction between catalyst and CEES. By optimizing the catalyst and H₂O₂ dosage, CEES is selectively oxidized to low-toxic CEESO with selectivity >99%. Both catalysts maintain excellent activity (>90% conversion) and selectivity (>99%) after 5 cycles, supported by FT-IR and XPS, which confirms structural stability. This work provides a new strategy for regulating catalytic performance via ligand engineering of Keplerate-type POMs and offers a practical, green solution for rapid detoxification of mustard gas simulants.

Notably, this work focuses on the decontamination of pure CEES in aqueous media; future studies could explore the catalytic performance in complex matrices (e.g., soil extracts or wastewater) to further validate practical applicability. Additionally, the ligand regulation strategy reported herein provides a general approach for designing high-performance POM-based catalysts for thioether oxidation reactions.

Author contributions

Jianbo Yin: investigation, writing – original draft, review & editing, methodology, formal analysis, data curation. Liming Sun: formal analysis, data curation. Yunshan Zhou: conceptualization, methodology, resources, writing – review & editing, supervision, project administration, funding acquisition. Lijuan Zhang: writing – review & editing, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting the findings of this study are available within the article and its supplementary information (SI). The SI contains detailed procedures for the synthesis of {Mo₁₃₂-Ac⁻} and {Mo₁₃₂-SO₄²⁻} catalysts, GC-FID spectra of CEES, CEESO, CEESO₂ and the internal standard (1,3-dichlorobenzene) for quantitative analysis (Fig. S2), and full XPS Mo 3d characterization data of {Mo₁₃₂-SO₄²⁻} (Fig. S3). All other relevant data are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra00608f.

Acknowledgements

The financial support from the National Natural Science Foundation of China (No. 21976013) is acknowledged.

Notes and references

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