Immobilization of Brønsted basic hexaniobate on the Lewis acidic zirconia using an emulsion assisted self-assembly strategy for synergistic boosting of nerve agent simulant decontamination

Abstract

The hydrolytic decontamination of organophosphorus nerve agents can be promoted using Brønsted bases or Lewis acids, but the fabrication of catalytic materials with both Lewis acidic centers and Brønsted base sites is a great challenge due to their essential incompatibility. Herein, a facile and straightforward emulsion assisted self-assembly approach has been developed to immobilize a Brønsted basic hexaniobate cluster on the Lewis acidic zirconia. The obtained [C₁₆H₃₃N(CH₃)₃]₆KHNb₆O₁₉/ZrO₂ (C₁₆N-Nb₆/ZrO₂) exhibits a remarkable catalytic activity for the hydrolysis of a Sarin simulant (DMNP): 100% of DMNP was converted under ambient conditions without using basic additives. Based on the control experiments and spectroscopic analyses, a possible reaction mechanism was proposed, suggesting that such excellent catalytic performance of C₁₆N-Nb₆/ZrO₂ is attributed to the synergistic effect between molecular level accessible Lewis acidic ZrO₂ (activating substrate molecules) and Brønsted basic hexaniobates (activating water molecules). As a broad-spectrum catalyst, C₁₆N-Nb₆/ZrO₂ also shows satisfactory performance for the oxidative decontamination of a sulfur mustard simulant.

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Introduction

Organophosphates have historically been used as chemical warfare agents (nerve agents, such as Sarin, Soman and VX) since World War I, and to date, they are still employed as insecticides or used for terrorist purposes. They continue to pose a safety and security threat to both environment and humans as they can cause rapid death by inhibiting signal transmission from the central nervous system to muscles.^1–4 In this regard, there is a compelling need to develop viable strategies to decontaminate organophosphorus nerve agents in a facile and eco-friendly way. Hydrolysis using Brønsted bases or Lewis acids is a primary way to break the P–X bonds (X = F, CN, etc.) in organophosphorus nerve agents.⁵ Although basic hydrolysis is effective for Sarin and Soman, the use of stoichiometric strong alkalis makes this route less attractive. Recently, the hydrolysis using Lewis acid catalysts, especially Zr-containing catalysts, has attracted increasing attention.^6–11 Since the strongly Lewis acidic Zr sites are available to bind and activate the P=O bond, the hydrolysis kinetics of organophosphonates is significantly improved.^12–14 The work of G. W. Peterson's group reveals that zirconium hydroxide is a promising candidate for hydrolytic decontamination of Soman and VX.¹⁵ In addition, a series of Zr-based metal–organic frameworks (MOFs) exhibit excellent catalytic activities for the hydrolysis of nerve agents, but their performance highly depends on the use of a basic buffer (N-ethylmorpholine, pH = 10).^16–19 Such results imply that the incorporation of Lewis acid catalysts with Brønsted base sites can boost the hydrolytic decontamination of nerve agents.^20,21 Nevertheless, the combination of the Lewis acid catalyst with the Brønsted base site is a great synthetic challenge due to their essential incompatibility.

Polyoxoniobates (PONbs) are a subfamily of polyoxometalates (POMs)^22–24 and both their synthesis chemistry and catalysis chemistry are far behind those of other POM members.^25–31 Recently, the catalytic activities of PONbs in the decontamination of chemical warfare agents and/or their simulants have been explored. Our investigations show that PONbs can promote the oxidative detoxification of sulfur mustard and VX simulants due to their ability to activate hydrogen peroxide.^32–36 In addition, PONbs have basic surface oxygen atoms,^37–39 which can accelerate the hydrolysis of nerve agents.^40,41 Nyman et al. initially demonstrated the hydrolysis of Sarin and Soman using homogeneous Lindqvist [Nb₆O₁₉]⁸⁻.⁴² Then, Hill and Zheng reported that Keggin-type PONb, K₁₂[Ti₂O₂][XNb₁₂O₄₀] (X = Ge or Si), and copper-containing PONb, H₂Li₅Na₅K₅[Cu(en)₂]₇[Nb₄₇O₁₂₈(OH)₆(CO₃)₂]·20H₂O (en = ethylenediamine), as solid bases can catalyze the hydrolysis of nerve agent simulants, but their sluggish hydrolysis kinetics leaves much room for improvement.^43,44 Considering the good catalytic performances of Zr-based catalysts in the hydrolysis reaction, the immobilization of basic PONbs on the Lewis acidic supports would provide an efficient way to realize the rapid decontamination of nerve agents. However, the immobilization of PONbs, which are stable under alkaline conditions with a high negative charge density, is difficult.^33,45,46 To our knowledge, the combination of PONbs with a Lewis acid supporter is rarely explored.

Emulsion assisted self-assembly has been proved to be a facile and straightforward method to prepare functionalized materials, such as polymer particles, graphene oxide and metal oxides.^47–50 In the self-assembly process, an emulsion was formed in the presence of surfactants and the reactions (such as polymerization, hydrolysis, etc.) occur at the interface of the emulsion droplets, leading to the materials with a high specific surface area, tunable size and structure, and narrow molecular weight distribution. Recently, a series of amphiphilic POMs were prepared by the electrostatic assembly of polyanions with surfactants, which not only can stabilize an emulsion but also catalyze organic reactions at the liquid/liquid interface.^51–54 However, the emulsion stabilized by surfactant modified POMs was rarely used to prepare POM-based materials. To our knowledge, the only example was reported in Liu's group where a POM modified polystyrene latex particle was prepared by the emulsion assisted assembly using methyl methacrylate functionalized tri-vacant Keggin-type {PW₉O₃₇} as a surfactant.⁵⁵

In our previous study, amphiphilic PONbs ([C_nH_2n+1N(CH₃)₃]₇HNb₆O₁₉, n = 14, 16 and 18) were synthesized by the electrostatic assembly of hexaniobate with quaternary ammoniums with a long alkyl chain, which can catalyze hydrolytic and oxidative detoxification of nerve agent and sulfur mustard simulants in an emulsion system.³⁵ To advance our research in the design and construction of PONb-based decontamination materials, herein the Brønsted basic hexaniobate was immobilized on the Lewis acidic zirconia by a facile emulsion assisted self-assembly using amphiphilic [C₁₆H₃₃N(CH₃)₃]₆KHNb₆O₁₉ (C₁₆N-Nb₆) as an emulsifier. As a combination of the Brønsted basic Nb₆ with Lewis acidic Zr sites, the synthesized C₁₆N-Nb₆/ZrO₂ exhibits remarkable catalytic performance towards the hydrolytic degradation of dimethyl 4-nitrophenyl phosphate (DMNP), a nerve agent simulant, under basic additive-free conditions. Moreover, the synergistic catalytic mechanism was supported by the control experiments and spectroscopic analyses.

Results and discussion

Synthesis and characterization of the C₁₆N-Nb₆/ZrO₂ composite

The fabrication of the C₁₆N-Nb₆/ZrO₂ composite is illustrated in Scheme 1. First, a stable water-in-oil (W/O) type emulsion (Fig. S1†) was formed by vigorously stirring a mixture of water and toluene containing the amphiphilic hexaniobate (C₁₆N-Nb₆). Then, zirconium n-butoxide (ZB) was dropped slowly into the emulsion and gradually dispersed to the water–oil interface, where ZrO₂ was in situ generated by hydrolysis and simultaneously assembled with C₁₆N-Nb₆ to form the C₁₆N-Nb₆/ZrO₂ composite. During the assembly process, the amphiphilic hexaniobate plays dual roles: as a PONb source and an emulsifier. To our knowledge, this is the first example of using an emulsion to prepare PONb-based materials. Compared with the reported methods for preparing POM-based materials,^56–59 the emulsion assisted self-assembly strategy is facile and straightforward. In addition, the control experiments reveal that the Nb₆ loading amount can be readily regulated by the mass ratio of C₁₆N-Nb₆ to the zirconium n-butoxide precursor, and according to inductively coupled plasma atomic emission spectrometry (ICP-AES), five composites with Nb₆ loading amounts of 4% (4%-C₁₆N-Nb₆/ZrO₂), 6% (6%-C₁₆N-Nb₆/ZrO₂), 9% (9%-C₁₆N-Nb₆/ZrO₂), 11% (11%-C₁₆N-Nb₆/ZrO₂) and 19% (19%-C₁₆N-Nb₆/ZrO₂) were obtained (Table S1†), respectively. C₁₆N-Nb₆/ZrO₂ with an Nb₆ loading amount of 11% was used in the following characterization.

Scheme 1 Illustration of the design and fabrication of the C₁₆N-Nb₆/ZrO₂ composite using an emulsion assisted self-assembly approach.

The Fourier transform infrared (FT-IR) spectra of C₁₆N-Nb₆/ZrO₂, C₁₆N-Nb₆ and ZrO₂ are illustrated in Fig. 1a and S2.† C₁₆N-Nb₆ displays three characteristic peaks in the region of 500–1000 cm⁻¹, attributed to the stretching vibrations of the Nb–O_b–Nb (555 and 645 cm⁻¹, b: bridge oxygen atoms) and Nb=O_t (879 cm⁻¹, t: terminal oxygen atoms) bonds in the hexaniobate cluster. The peaks at 2971 and 2848 cm⁻¹ belong to the asymmetric vibration of CH₂ in the organic ammonium cation.³⁵ For ZrO₂, the weak peak at 838 cm⁻¹ is assigned to the stretching vibration of the Zr–O bond and the broad bands at 1622 and 1331 cm⁻¹ are associated with the bending vibration of water molecules.⁶⁰ The characteristic peaks of Nb₆, cetyltrimethyl ammonium cation and ZrO₂ can be observed in the FT-IR spectrum of C₁₆N-Nb₆/ZrO₂ (red line in Fig. 1a and S2†), suggesting the successful combination of C₁₆N-Nb₆ with ZrO₂. The vibrations of Nb₆ are rather weak due to its overlap with the strong absorption bands of ZrO₂. We speculate that during the assembly process, the organic ammonium cation acts as a protective agent to improve the compatibility of the basic Nb₆ cluster with the acidic ZrO₂. According to the TG curves of C₁₆N-Nb₆ and C₁₆N-Nb₆/ZrO₂ (Fig. S3†), in the composite the weight percentages of C₁₆N-Nb₆ and ZrO₂ are about 49.4 wt% and 50.6 wt%, respectively.

Fig. 1 (a) The FT-IR spectra of C₁₆N-Nb₆/ZrO₂, C₁₆N-Nb₆ and ZrO₂; (b) N₂ adsorption and desorption isotherms of C₁₆N-Nb₆/ZrO₂ and ZrO₂ measured at 77 K; (c) zeta potential distribution of C₁₆N-Nb₆/ZrO₂, C₁₆N-Nb₆ and ZrO₂ in water; (d) high-resolution Nb 3d XPS spectra of C₁₆N-Nb₆/ZrO₂ and C₁₆N-Nb₆.

The N₂ adsorption and desorption isotherms of C₁₆N-Nb₆/ZrO₂ and ZrO₂ are presented in Fig. 1b. C₁₆N-Nb₆/ZrO₂ shows a type IV isotherm pattern with a hysteresis loop, indicating the presence of mesopores in the composite. The Brunauer–Emmett–Teller (BET) surface area of C₁₆N-Nb₆/ZrO₂ is 41.6 m² g⁻¹, which is obviously lower than that of the ZrO₂ (242.8 m² g⁻¹, Table S2†). As shown in Fig. 1c, the ZrO₂ dispersed in an aqueous medium exhibits a positive charge (ξ = 24.04 mV), probably due to the protonation of the surface O atom.^61,62 After combining with C₁₆N-Nb₆ (ξ = −9.76 mV), the zeta potential of C₁₆N-Nb₆/ZrO₂ shifts to a lower value (ξ = 2.09 mV). Based on the above results, it can be concluded that the change in the specific surface area and surface potential distribution reveals the successful immobilization of C₁₆N-Nb₆ on the ZrO₂.

To further confirm the elemental compositions and oxidation states, the X-ray photoelectron spectrum (XPS) of the as-synthesized C₁₆N-Nb₆/ZrO₂ was recorded and compared with those of C₁₆N-Nb₆ and ZrO₂. The survey XPS spectrum in Fig. S3† exhibits the presence of Zr, Nb, C, N and O elements in C₁₆N-Nb₆/ZrO₂. The high-resolution XPS Nb 3d spectrum of C₁₆N-Nb₆/ZrO₂ displays two peaks at 208.8 eV and 206.0 eV (Fig. 1d), which can be assigned to the Nb 3d_5/2 and Nb 3d_3/2 with a +5 oxidation state, respectively.³³ Compared with that of the C₁₆N-Nb₆ precursor (two peaks at 208.5 eV and 205.7 eV correspond to Nb 3d_5/2 and Nb 3d_3/2, respectively), the binding energy of Nb 3d in C₁₆N-Nb₆/ZrO₂ shifts to a higher region by about 0.3 eV, demonstrating the strong interaction between C₁₆N-Nb₆ and ZrO₂ in the C₁₆N-Nb₆/ZrO₂ composite.

The morphology of C₁₆N-Nb₆/ZrO₂ is investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The SEM image of C₁₆N-Nb₆/ZrO₂ (Fig. 2a) shows small particles with a diameter of about 20 nm, which is in line with the morphology of the reported ZrO₂-based materials.^63,64 No lattice fringe was observed in the HRTEM image (Fig. 2b), demonstrating the amorphous structure of the as-prepared composite. In addition, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) elemental mapping were carried out to further determine the elemental distribution. As shown in the HAADF-STEM image (Fig. 2c), the heavy atoms (such as Zr and Nb) are brighter than the light atoms (such as O and N). The EDX mapping analysis reveals that niobium, zirconium and nitrogen are evenly dispersed in the C₁₆N-Nb₆/ZrO₂ composite (Fig. 2d–f). Moreover, broad diffraction PXRD peaks of C₁₆N-Nb₆/ZrO₂ indicate that the as-prepared composite is amorphous (Fig. S4†). There is no characteristic diffraction of C₁₆N-Nb₆, further revealing that the hexaniobate clusters are uniformly dispersed in the C₁₆N-Nb₆/ZrO₂ composite. This result indicates that the emulsion droplets stabilized by amphiphilic PONb provide a special reaction environment, where C₁₆N-Nb₆ assembles with in situ formed ZrO₂ in a controlled way.

Fig. 2 (a) SEM image, (b) TEM image, (c) HAADF-STEM image, and (d–f) EDX elemental mappings of the C₁₆N-Nb₆/ZrO₂ composite.

In addition, the water contact angles of C₁₆N-Nb₆/ZrO₂, ZrO₂, and C₁₆N-Nb₆ were measured. ZrO₂ is hydrophilic with a water contact angle of 26° (Fig. S5†). The combination of ZrO₂ with C₁₆N-Nb₆ makes the C₁₆N-Nb₆/ZrO₂ composite (with a contact angle of 61°) to have similar surface wettability to that of C₁₆N-Nb₆ (with a contact angle of 64°). Importantly, the enhancement of the surface hydrophobicity would promote the contact of organic substrates (such as DMNP) with the hexaniobate and Zr sites on the composite, thereby accelerating the hydrolytic rate of nerve agents.

Hydrolytic decontamination of the nerve agent simulant

Given the coexistence of Zr centers (Lewis acid sites) and hexaniobate clusters (Brønsted base sites) in the as-prepared C₁₆N-Nb₆/ZrO₂ composite, the hydrolysis of DMNP, a nerve agent simulant, was selected to evaluate its catalytic activity. Before the decontamination study, we tested the acid and base sites of the composite. The surface basicity was first investigated using phenolphthalein as an indicator. As shown in the Fig. 3a inset, a colour change from pale to light pink was observed when phenolphthalein was dropped into the slurry containing C₁₆N-Nb₆/ZrO₂, while the colour of the ZrO₂ slurry remains unchanged. This phenomenon reveals that the introduction of C₁₆N-Nb₆ endows the material with Brønsted basicity. To measure the basic site distribution and the basicity capacity of the composite, temperature-programmed desorption of CO₂ (CO₂-TPD) was conducted. A comparison of the CO₂-TPD profiles of C₁₆N-Nb₆/ZrO₂ and ZrO₂ is shown in Fig. 3a, C₁₆N-Nb₆/ZrO₂ has the maximum CO₂ desorption at 126 °C (corresponding to the weak base sites), while ZrO₂ reaches the desorption peak at 106 °C. The total basicity capacity of C₁₆N-Nb₆/ZrO₂ and ZrO₂ was calculated to be 13.5 and 0.92 mmol g⁻¹, respectively. These results verify that the immobilization of hexaniobate on the ZrO₂ renders the composite with Brønsted base sites. In addition, temperature-programmed desorption of ammonia (NH₃-TPD) measurement was performed to detect the Lewis acid sites. As shown in Fig. 3b, the broad desorption peaks at about 148 °C were observed in both the NH₃-TPD profiles of C₁₆N-Nb₆/ZrO₂ and ZrO₂, suggesting the presence of weak acid sites.^65,66 The density of the Lewis acid sites on C₁₆N-Nb₆/ZrO₂ was calculated to be 121 mmol g⁻¹, which is slightly lower than that of ZrO₂ (133 mmol g⁻¹). The coexistence of Lewis acid sites and Brønsted base sites makes the C₁₆N-Nb₆/ZrO₂ composite a promising catalyst for the hydrolytic decontamination of nerve agents.

Fig. 3 (a) CO₂-TPD of C₁₆N-Nb₆/ZrO₂ and ZrO₂, inset: optical photographs of C₁₆N-Nb₆/ZrO₂ and ZrO₂ slurry after adding phenolphthalein. (b) NH₃-TPD of C₁₆N-Nb₆/ZrO₂ and ZrO₂.

In a typical reaction, the C₁₆N-Nb₆/ZrO₂ composite was dispersed in a mixture solution of H₂O and CD₃CN (volume ratio 3 : 2) with an initial pH value of about 9.3 and DMNP was injected to start the hydrolytic reaction. The decontamination process was performed under ambient conditions without using any basic co-catalysts and the reaction progress was monitored by ³¹P NMR spectroscopy at various time intervals. As shown in Fig. 4b, the ³¹P NMR spectral measurements reveal a gradual disappearance of DMNP (δ = −4.8 ppm) and an increase of the peak at δ = 2.6 ppm, attributed to the hydrolysis product, dimethyl phosphate (DMP). Moreover, no toxic byproduct was detected during the hydrolysis (Fig. 4b and S7†). As shown in Fig. 4c, DMNP was completely consumed within 6 h under basic additive-free conditions and the half-life of DMNP is about 30 min. In contrast, a negligible amount of DMNP was converted in the blank experiment without a catalyst (Fig. 5). To our knowledge, the catalytic performance of the C₁₆N-Nb₆/ZrO₂ composite in the present work outperforms those of most of the reported catalytic materials, such as POMs,^{26,43,44,67,68} MOFs⁶⁹ and oxides/hydroxides⁷⁰ (Table S3†).

Fig. 4 (a) Catalytic hydrolysis reaction of the phosphonate-based nerve agent simulant DMNP using C₁₆N-Nb₆/ZrO₂; (b) ³¹P NMR of DMNP hydrolysis catalyzed by C₁₆N-Nb₆/ZrO₂ at various intervals of time; (c) time profile for DMNP degradation. Reaction conditions: DMNP (5 mg), H₂O (300 μL), CD₃CN (200 μL), C₁₆N-Nb₆/ZrO₂ (15 mg), and room temperature for 6 h.

Fig. 5 Catalytic hydrolysis of DMNP with different catalysts without using any basic co-catalysts. Reaction conditions: DMNP (5 mg), H₂O (300 μL), CD₃CN (200 μL), catalyst, and room temperature for 6 h.

To further investigate the influence of Lewis acid sites and Brønsted base sites on the catalytic hydrolysis reaction, the following control experiments were conducted. When ZrO₂ was used, only a trace amount of DMNP was converted under the standard decontamination conditions (Table 1, entry 6 and Fig. 5). The amphiphilic C₁₆N-Nb₆ also gives a negligible conversion (<5%) (Table 1, entry 7 and Fig. 5). The above results indicate that neither the Lewis acidic ZrO₂ nor the Brønsted basic C₁₆N-Nb₆ alone can effectively accelerate the decontamination reaction, implying the interesting synergistic effect between the two components. Although the conversion of DMNP can be increased to 27% using a physical mixture of ZrO₂ and C₁₆N-Nb₆ (Fig. 5), its catalytic activity is still much lower than that of the as-prepared C₁₆N-Nb₆/ZrO₂ composite (100%). We speculate that the molecular level accessible Lewis acidic sites and Brønsted basic sites play a key role in the rapid hydrolysis of DMNP. In addition, the catalytic performance of C₁₆N-Nb₆/ZrO₂ is affected by the content of Nb₆, the conversion of DMNP increased from 83% to 100% with the loading amount of Nb₆ being improved from 4% to 11% (Table 1, entries 1–5), and the C₁₆N-Nb₆/ZrO₂ with 11% Nb₆ loading was used in the following experiments.

Table 1 Hydrolytic degradation of DMNP using various catalysts^a

Entry	Catalyst	Conditions	Conv. (%)
a Reaction conditions: DMNP (5 mg), H2O (300 μL), CD3CN (200 μL), catalyst, and room temperature for 6 h. All the hydrolysis of DMNP was performed in a heterogeneous system. b NaOH was added to the reaction mixture to adjust the pH value.
1	19%-C16-Nb6/ZrO2	15 mg	100
2	11%-C16-Nb6/ZrO2	15 mg	100
3	9%-C16-Nb6/ZrO2	15 mg	97
4	6%-C16-Nb6/ZrO2	15 mg	88
5	4%-C16-Nb6/ZrO2	15 mg	83
6	ZrO2	13.3 mg	Trace
7	C16N-Nb6	1.7 mg	Trace
8^b	ZrO2 + NaOH	13.3 mg, pH = 9.3	29

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To understand the catalytic role of C₁₆N-Nb₆/ZrO₂ in the hydrolysis reaction, the composite was treated with DMNP for 1 h in a non-aqueous system and the XPS measurement was performed on the isolated sample. As shown in Fig. 6a, after treating with DMNP, the high-resolution Zr 3d spectrum of C₁₆N-Nb₆/ZrO₂ exhibits an obvious binding energy shift from 183.3 to 184.2 eV for Zr 3d_3/2 and from 181.0 to 181.9 eV for Zr 3d_5/2, respectively. The obvious increase of the Zr 3d binding energy (0.9 eV) implies the strong interaction of Zr sites with the substrate, which is consistent with previous reports.⁷¹ Furthermore, a new peak at 133.5 eV appeared in the high-resolution P 2p XPS spectrum of DMNP-treated C₁₆N-Nb₆/ZrO₂ (Fig. 6b), which is assigned to P=O of the surface-bound phosphate species.⁷¹ The above results demonstrate that the Lewis acidic Zr sites in the composite can bind and activate the P=O bond of DMNP during the catalytic reaction. The role of solid base Nb₆ was also investigated. As the initial pH value of the reaction solution was 9.3 in the presence of C₁₆N-Nb₆/ZrO₂, we first think that the role of basic C₁₆N-Nb₆ is to generate OH⁻ to attack the P center of the substrate. Then, the control experiment using ZrO₂ with a pH of 9.3 adjusted using NaOH was performed, which only converted 29% of DMNP (Table 1, entry 8 and Fig. S8†). This result implies that the main role of hexaniobate is not to accept the proton of water to release OH⁻, but to activate water molecules by its basic surface oxygen atoms.

Fig. 6 High-resolution XPS spectra of the C₁₆N-Nb₆/ZrO₂ composite treated with DMNP in a non-aqueous system: (a) Zr 3d and (b) P 2p.

Based on the previous hydrolysis studies in the Zr-containing catalytic system and the above experimental data, we proposed a possible mechanism for the catalytic decontamination of DMNP by C₁₆N-Nb₆/ZrO₂ (Fig. 7). First, the Lewis acidic Zr centers coordinate with the phosphoryl oxygen of DMNP, activating the P=O bond. Meanwhile, water molecules interact with the bridging oxygen atoms of the Lindqvist hexaniobate cluster to form intermediate A. Subsequently, the oxygen atom of the activated water molecule attacks the P center of adjacent DMNP giving intermediate B with a pentacoordinated P center and a protonated hexaniobate. Then, the elimination of nitrophenoxide consumes the proton on the hexaniobate cluster, leaving the intermediate C (DMP binds to the Zr sites). Finally, the bonded product dissociates to the reaction solution regenerating the C₁₆N-Nb₆/ZrO₂ catalyst.

Fig. 7 The possible hydrolytic degradation mechanism for DMNP on the C₁₆N-Nb₆/ZrO₂ catalyst.

The heterogeneous nature of the C₁₆N-Nb₆/ZrO₂ catalyst was evaluated using the leaching test. After the hydrolysis was performed for 1 h with a conversion of about 70%, the solid catalyst was filtered off from the reaction mixture and the resulting solution continued to react under the otherwise identical conditions. The negligible conversion was observed for another 5 h (Fig. 8a), suggesting that the hexaniobate is stably immobilized on ZrO₂. Then, we investigated the long-term stability of the C₁₆N-Nb₆/ZrO₂. After the first run, the solid catalyst was filtered from the reaction system and used for the next run after washing. Unfortunately, a decrease in DMNP conversion (about 20%) was observed for the first three cycles (Fig. 8b). Based on the literature research,^20,21 we speculate that the decrease in the catalytic performance is probably related to the adsorption of the acidic product (DMP) on the catalytic active sites. This point was proved by the following control experiments and spectroscopic analysis. First, an obvious decrease in the catalytic activity of C₁₆N-Nb₆/ZrO₂ (the conversion of DMNP decreases from 100% to 69%) was observed upon the addition of extra DMP to the standard catalytic reaction. Moreover, after the third run, the isolated catalyst was washed with acetonitrile thoroughly and measured by FT-IR spectroscopy. As shown in Fig. S9,† the characteristic peaks of hexaniobate and ZrO₂ were maintained, but new peaks at 1164 cm⁻¹ and 1110 cm⁻¹ assigned to the ν_a(O–P–O) and ν_s(O–P–O),⁷¹ respectively, were detected. The isolated sample was further characterized by XPS. After three cycles, the peaks of Zr 3d_3/2 and Zr 3d_5/2 shift to a higher binding energy (Fig. 8c) and a similar trend is observed in the Nb 3d XPS spectra (Fig. 8d), which indicates the interaction of the Zr center and hexaniobate with the acid hydrolysis product (DMP). To gain a better recycle performance, after three cycles the C₁₆N-Nb₆/ZrO₂ composite was activated by soaking in KOH solution (1 M) and the catalytic activity of C₁₆N-Nb₆/ZrO₂ was basically recovered after regeneration with 92% and 90% of DMNP conversion in the fourth and fifth runs, respectively (Fig. 8b).

Fig. 8 (a) Leaching experiment for the hydrolysis of DMNP over C₁₆N-Nb₆/ZrO₂; (b) recycle test for the decontamination reaction using C₁₆N-Nb₆/ZrO₂ as the catalyst. Reaction conditions: DMNP (5 mg), H₂O (300 μL), CD₃CN (200 μL), C₁₆N-Nb₆/ZrO₂ (15 mg), and room temperature for 6 h; (c) high-resolution Zr 3d XPS spectra of C₁₆N-Nb₆/ZrO₂ before and after the catalytic reaction; and (d) high-resolution Nb 3d XPS spectra of C₁₆N-Nb₆/ZrO₂ before and after the catalytic reaction.

Oxidative decontamination of the sulfur mustard simulant

Our previous works indicate that PONbs are active in the oxidative decontamination of the sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES) in the presence of dilute aqueous H₂O₂ (3 wt%).^32–36 To explore the broad-spectrum decontamination activity of the as-prepared C₁₆N-Nb₆/ZrO₂ composite, oxidative degradation of CEES was selected as the second model reaction (Fig. 9a). Under the optimized conditions, 93% of the toxic substrate was consumed after 2 h and 83% of selectivity for the less-toxic product, 2-chloroethyl ethyl sulfoxide (CEESO), was observed using C₁₆N-Nb₆/ZrO₂ as the catalyst (Fig. 9b and S10, S11†). In contrast, the test without the catalyst (blank test) shows a negligible CEES removal (Conv. 7%).

Fig. 9 (a) Catalytic oxidation degradation of the sulfur mustard simulant CEES by C₁₆N-Nb₆/ZrO₂; (b) concentration profiles of CEES, CEESO, and CEESO₂ (2-chloroethyl ethyl sulfone) versus reaction time over C₁₆N-Nb₆/ZrO₂; reaction conditions: CEES (0.4 mmol), 1,3-dichlorobenzene (0.2 mmol), C₁₆N-Nb₆/ZrO₂ (10 mg), and aqueous H₂O₂ (3 wt%, 0.44 mmol) at room temperature for 2 h. (c) Oxidative decontamination of CEES over different catalysts.

To evaluate the contribution of ZrO₂ and the hexaniobate cluster to the oxidative process, control experiments with different catalysts were conducted. As shown in Fig. 9c, C₁₆N-Nb₆ converted 86% of CEES, and a satisfactory selectivity of 96% towards the less toxic CEESO was obtained, while ZrO₂ can also interact with H₂O₂ to form the Zr-peroxo species (Fig. S12†), showing a moderate catalytic performance (Conv. 47%, Sele. 53%). Furthermore, the catalytic activities of the C₁₆N-Nb₆/ZrO₂ composites with various C₁₆N-Nb₆ loadings were tested. A significant improvement in the conversion (from 60% to 94%) was obtained by increasing the loading of Nb₆ from 4% to 9%, but a further increase in Nb₆ loading did not lead to enhanced performance. Based on the above results, we can conclude that the Nb₆ cluster is the main activity center towards the selective oxidation of CEES. In addition, the comparison of FT-IR spectra (Fig. S13†) of the catalyst before and after the oxidative decontamination reaction indicates that the structure of C₁₆N-Nb₆/ZrO₂ remains stable under the turnover conditions.

Conclusions

In this report, a catalytic decontamination material containing both Brønsted basic hexaniobate and the Lewis acidic Zr sites has been successfully prepared using a facile emulsion-assisted self-assembly strategy. The obtained C₁₆N-Nb₆/ZrO₂ composite shows remarkable catalytic performance towards the hydrolytic degradation of an organophosphorus nerve agent simulant, DMNP, while C₁₆N-Nb₆ or ZrO₂ alone shows negligible activity. The in-depth mechanism study indicates that the synergistic effect between C₁₆N-Nb₆ and ZrO₂ is of great importance to boost the hydrolytic kinetics. Moreover, the C₁₆N-Nb₆/ZrO₂ composite also shows catalytic activity in the oxidative decontamination of the sulfur mustard simulant. The reported investigation not only provides a highly effective catalyst for the decontamination of chemical warfare agent simulants but also opens up new prospects for the design and fabrication of PONb-based catalytic materials. The combination of surfactant modified PONbs with other functionalized supports is under investigation in our lab.

Experimental section

Materials and methods

All the starting chemicals and solvents were of reagent grade, purchased from commercial sources, and used without further purification. CAUTION: The simulants of CWAs, DMNP and CEES, are highly toxic and must be handled only by trained personnel using applicable safety procedures in a closed system or a hood under good ventilation.

The XRD patterns were obtained on a Shimadzu XRD-6000 instrument with graphite-monochromatized Cu Kα (λ = 1.5406 Å). The XPS analysis was conducted on a PHI VersaProbe III spectrometer using an Al Kα radiation as the X-ray source. The FT-IR spectra were recorded on a JASCO 6300D instrument in the region of 400–4000 cm⁻¹. The Brunauer–Emmett–Teller (BET) specific surface areas were measured at 77 K with a Micromeritics ASAP 2000 instrument, and the pore size distributions were estimated using the Barrett–Joyner–Halenda (BJH) method. Elemental analysis (C, H and N) was performed on an ElementarVario EL cube Elmer CHN elemental analyzer and ICP-MS was performed on a ThermoiCAP 6000 atomic emission spectrometer. SEM (JOEL JSE-7500F), TEM (JEOL JEM-2010) and field emission transmission electron microscopy (FEI Talos F200X) were used to observe the morphology and microstructure of the samples. The acidic and basic sites of the synthesized material were measured by NH₃-TPD and CO₂-TPD, respectively, with a PCA-1200 instrument. The gas chromatographic (GC) analyses were performed on a Shimadzu GC-2014C equipped with an FID detector and an HP-5 ms capillary column. The GC-MS spectra of the products were obtained using an Agilent 7890A-5975C instrument. The ³¹P NMR spectra were recorded on a Bruker 400 MHz instrument in CD₃CN, and all the chemical shifts were referenced to an aqueous H₃PO₄ solution (85%).

Preparation of C₁₆N-Nb₆/ZrO₂

The C₁₆N-Nb₆/ZrO₂ composite was prepared using an emulsion assisted self-assembly method. First, a certain amount of [C₁₆H₃₃N(CH₃)₃]₄K₃HNb₆O₁₉ (C₁₆N-Nb₆)³² was dispersed in toluene (10 mL) and treated with ultrasound for about 5 min. Then, deionized water (20 mL) was added to the above toluene solution and the mixture was strongly shocked and stirred until a stable emulsion was obtained. A toluene solution (1 mL) containing zirconium n-butoxide was dropped slowly into the above emulsion system (in an ice-bath) and the reaction mixture was stirred for another 6 h. The final white precipitate, C₁₆N-Nb₆/ZrO₂, was filtered from the reaction mixture, washed with ethanol several times, and dried under vacuum. Various C₁₆N-Nb₆/ZrO₂ composites with different Nb₆ loading amounts were obtained by controlling the ratio of zirconium n-butoxide to C₁₆N-Nb₆: 0.3 g of C₁₆N-Nb₆ and 0.5 g of zirconium n-butoxide for C₁₆N-Nb₆/ZrO₂-19%, 0.2 g of C₁₆N-Nb₆ and 0.6 g of zirconium n-butoxide for C₁₆N-Nb₆/ZrO₂-11%, 0.1 g of C₁₆N-Nb₆ and 0.4 g of zirconium n-butoxide for C₁₆N-Nb₆/ZrO₂-9%, 0.1 g of C₁₆N-Nb₆ and 0.6 g of zirconium n-butoxide for C₁₆N-Nb₆/ZrO₂-6%, and 0.1 g of C₁₆N-Nb₆ and 0.8 g of zirconium n-butoxide for C₁₆N-Nb₆/ZrO₂-4%.

Catalytic hydrolysis of DMNP using C₁₆N-Nb₆/ZrO₂

The as-prepared C₁₆N-Nb₆/ZrO₂ catalyst (15 mg) was added to a mixture solution of CD₃CN (0.2 mL) and water (0.3 mL) and stirred for 5 min to disperse homogeneously. Then, DMNP (100 mg mL⁻¹ in CH₃CN, 50 μL) was injected to start the decontamination process. The catalytic hydrolysis reaction was performed with stirring at room temperature and the hydrolysis profiles were recorded by ³¹P NMR measurements at various intervals of time. The conversion of DMNP was calculated depending on the peak area ratio of the detected DMNP to DMNP adding the products.

Catalytic oxidation of CEES using C₁₆N-Nb₆/ZrO₂

The as-prepared C₁₆N-Nb₆/ZrO₂ catalyst (10 mg) was dispersed in CH₃CN (3 mL) and to this solution CEES (0.4 mmol) and 1,3-dichlorobenzene (0.2 mmol, internal standard) were added. After stirring for 5 min, aqueous H₂O₂ (3 wt%, 0.44 mmol) was dropped to initiate the decontamination reaction. The products were qualitatively analyzed by GC-MS and the oxidation profiles were monitored using GC.

Procedure for NH₃-TPD and CO₂-TPD measurements

NH₃-TPD measurements were performed to gain the density distribution and strength of acid sites in C₁₆N-Nb₆/ZrO₂. First, C₁₆N-Nb₆/ZrO₂ (25 mg) was pretreated at 150 °C for 120 min under an Ar blow at a flow rate of 30 mL min⁻¹. Then, the sample was exposed to ammonia at 100 °C until saturation and purged under an Ar flow for 30 min to remove the physically adsorbed ammonia before cooling to room temperature. The sample was heated from 40 to 400 °C at a ramping rate of 10 °C min⁻¹ under an Ar atmosphere, and ammonia was detected using a thermal conductivity detector.

CO₂-TPD measurements were performed to gain the density distribution and strength of base sites in C₁₆N-Nb₆/ZrO₂. The procedure for CO₂-TPD measurements was similar to that of NH₃-TPD, except that He gas was used instead of Ar gas and the adsorption temperature of CO₂ was 80 °C.

Treatment of C₁₆N-Nb₆/ZrO₂ with DMNP

C₁₆N-Nb₆/ZrO₂ was dispersed in the anhydrous CH₃CN solution of DMNP and consecutively stirred for 1 h at room temperature. Then, the solid was separated by filtration, washed and characterized by XPS.

Author contributions

H. F. Liu synthesized and characterized the C₁₆N-Nb₆/ZrO₂ composite catalyst and J. Dong performed the catalytic hydrolysis of DMNP as well as the oxidation of CEES. X. R. Sun and C. P. Liu helped with data analysis and graphic design. Y. N. Chi and C. W. Hu conceived and directed the research. W. Lu, Z. M. Xu, N. Zhen and D. Zhang gave some advice about the preparation of catalysts and the catalytic experiments.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21871026, 21971010 and 22101012).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi02484e

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