A solvent-free solid catalyst for the selective and color-indicating ambient-air removal of sulfur mustard

Sulfur mustard (HD) is a highly persistent chemical warfare agent; effective decontamination requires selective oxidation of HD to the far less toxic sulfoxide. Existing oxidation systems often demand added oxidants or solvents and may lack selectivity or practicality for protective materials. This study investigates an ambient-air, solvent-free catalytic approach that selectively converts HD (and simulants) to the sulfoxide using a solid formulation incorporating tribromide, nitrate, Cu(II), and a solid acid. The research asks whether Cu(II) can enhance the key bromosulfonium-mediated aerobic sulfoxidation pathway and enable a practical, color-indicating solid catalyst capable of decontaminating both liquid and vapor HD under ambient conditions.

Prior work has established aerobic sulfide oxidations using heterogeneous catalysts and halogen-based systems, including tribromide-mediated processes and nitrate/nitrite redox cycles for regenerating active bromine species. Reports include tribromide-based oxidations on supports, NO2/Br O2-based sulfoxidations, and alternative approaches such as singlet oxygen generation from MOFs. Metal nitrates and bromides have been implicated as redox mediators for selective sulfoxidation, and copper-catalyzed aerobic oxidations are broadly known. Building on mechanistic models of nitrate/tribromide systems and halogen-mediated sulfoxidations, this work integrates Cu(II), tribromide, nitrate, and a solid acid into a solvent-free, solid formulation designed for selective HD decontamination with ambient O2.

Mechanistic and catalytic studies were conducted in solution and in the solid state, followed by live-agent tests and in situ spectroscopies. - Solution studies: Stock solutions (typically 50 mM in acetonitrile) of tetrabutylammonium tribromide (TBABr3), tetrabutylammonium nitrate (TBANO3), p-toluenesulfonic acid (p-TsOH), and Cu(NO3)2·3H2O (or Cu(ClO4)2·6H2O in some cases) were prepared. Reactions (total 2 mL) in 20 mL vials contained internal standard (1,3-dichlorobenzene), optional H2O, and CEES (approx. 103 mM) to initiate oxidation under O2 headspace maintained by oxygen balloons. Conversions were tracked by GC-FID; selectivity verified by 13C NMR. - Spectroscopic kinetics: Stopped-flow UV-Vis and conventional UV-Vis monitored Br2/Br3 dynamics and CuBrx complex formation. CuBr3 formation was followed via LMCT band at 635 nm as an indirect probe of bromosulfonium intermediate formation. Effects of added H2O and Zn(BF4)2 were examined, including Br− oxidation by nitrous acid (HNO2) using stopped-flow. - Solid-formulation catalyst (SFC) preparation: A solvent-free solid formulation was prepared by mechanically mixing Nafion (150 mg; 136 mmol H+ equivalents), TBABr3 (145 mg; 300 mmol), TBANO3 (61 mg; 200 mmol), and Cu(NO3)2·3H2O (24 mg; 100 mmol). The target composition corresponds to a 5.0:3.3:1.7:2.3 molar ratio of TBABr3:TBANO3:Cu(NO3)2·3H2O:Nafion (as H+ equivalents). The green solid was stored airtight. - SFC testing with simulant (CEES): For liquid-phase, SFC (about 25 mg) received CEES (50 µL, 430 µmol) directly on the solid; O2 headspace maintained by balloons. CEES and product were extracted into toluene containing 1,3-DCB internal standard and quantified by GC; selectivity confirmed by 13C NMR. Interference studies included octane (gasoline surrogate) and CO2 exposure. - Live-agent HD studies: Liquid HD tests placed 5 mg SFC in sealed vials with O2 headspace; 5 µL HD applied to the solid. Exposures were sampled at 1, 2, 4, 8, 24, and 96 h; products analyzed by GC-MS after chloroform extraction/filtration. Vapor HD tests used a DRIFTS setup: SFC in a diffuse reflectance cell under 2% RH Zero Air, then exposed to HD vapor generated by a micro-fritted saturator at 20 °C; spectra collected for 4 h. - X-ray absorption spectroscopy (XAFS): Br K-edge and Cu K-edge XANES/EXAFS were collected at NSLS-II 7-BM (QAS). For solid-liquid CEES exposure, 30 mg SFC was contacted with 50 µL CEES under air and sampled at 0, 3, and 76 h; aliquots loaded into Kapton capillaries. For in situ vapor exposure, 4 mg SFC in a capillary was mounted in a sealed cell containing CEES vapor. Data were processed with Athena/Artemis; EXAFS fitted using models for Br-Br (approx. 2.30 Å) and Cu-O, Cu-Br, Cu-S paths. - DRIFTS: Nicolet 6700 with DiffusIR accessory; background under 2% RH Zero Air; difference spectra collected over 4 h during HD vapor exposure to monitor adsorption and S=O formation. - Additional analytical details: GC on HP 6890 with HP-5 column and FID; NMR on Bruker 600 MHz and Varian 400 MHz systems; UV-Vis on Agilent 8453; kinetic mixing via HI-TECH KinetAsyst SF-61sX2.

- A solvent-free, solid catalyst (SFC) composed of TBABr3, TBANO3, Cu(NO3)2·3H2O, and Nafion (molar ratio 5.0:3.3:1.7:2.3; Nafion as H+ equivalents) selectively oxidizes sulfur mustard (HD) and its simulant (CEES) to the corresponding sulfoxides using ambient O2 at room temperature. - Cu(II) shifts the equilibrium toward the bromosulfonium intermediate, increasing the overall sulfoxidation rate approximately four-fold and enabling colorimetric detection via formation of CuBr3 (green LMCT band at 635 nm); the solid changes from green (no sulfide) to brown upon exposure to HD/CEES and returns to yellow as Br3− is regenerated after completion. - In contrast to Cu(II), while H2O and Zn(II) also shift the initial Br2/Br3 plus sulfide equilibrium toward bromosulfonium, they slow the overall catalytic rate by inhibiting Br− re-oxidation to Br2 by nitrous acid; with 5.0 mM Zn(II), Br2/Br3 is consumed in under 10 s, yet overall sulfoxidation is slower. With 1.0 M H2O, the overall rate is similar to or slower than control; adding H2O to Cu(II)-containing reactions slows them. - XANES/EXAFS confirm catalytic turnover of the bromine species: Br K-edge pre-edge (1s→4p) at ~13473 eV indicates Br3− consumption upon CEES exposure and regeneration upon completion. EXAFS shows average Br–Br distances of 2.55 ± 0.01 Å (before) and 2.54 ± 0.01 Å (after), demonstrating tribromide regeneration. - Cu K-edge EXAFS suggests formation of Cu–Br and Cu–S coordination under reaction conditions, consistent with CuBrx complexation during turnover. - DRIFTS under HD vapor shows initial HD adsorption (e.g., CH2 wag at ~1300 cm−1) and growth of S=O bands at 1083 and 1041 cm−1, confirming surface formation of sulfoxide (HDO). GC-MS corroborates HDO formation after DRIFTS experiments. - The SFC operates without added solvent and remains effective in the presence of typical battlefield interferents: exposure to octane (gasoline surrogate) and CO2 produced no measurable inhibition of the reaction rate. - Live-agent performance: Applying 5 µL HD to 5 mg SFC (approx. 10 turnovers based on Br3−) under O2 headspace showed progressive conversion over 1–96 h by GC-MS; SFC also decontaminates HD vapor under 2% RH air as shown by DRIFTS. - Mechanistic studies support reversible formation of the bromosulfonium intermediate and identify nitrous acid-mediated re-oxidation of Br− to Br2 (via NOBr) as part of the catalytic cycle; Cu(II) maintains effective Br− re-oxidation rates while sequestering Br− to favor bromosulfonium formation.

The results demonstrate that a tribromide/nitrate-based catalytic system, when combined with a solid acid (Nafion) and Cu(II), enables selective aerobic oxidation of HD to its sulfoxide under ambient conditions with no added solvent. The mechanistic data indicate that the bromosulfonium ion is a key reversible intermediate; Cu(II) accelerates catalysis by complexing Br−, shifting the equilibrium toward bromosulfonium without hindering the nitrous acid-driven re-oxidation of Br− to Br2. In contrast, water and Zn(II) disrupt this re-oxidation step and slow turnover, explaining why only Cu(II) affords a net rate enhancement. Spectroscopic evidence (Br K-edge XANES/EXAFS and Cu K-edge EXAFS) confirms tribromide regeneration and Cu–Br/S interactions under turnover, supporting a closed catalytic cycle. DRIFTS and GC-MS show that the solid catalyst is effective against both liquid and vapor HD at the gas–solid interface, while its color change provides real-time indication of exposure and completion. These findings address the need for practical, selective HD decontamination materials by providing a robust, solvent-free, ambient-air catalyst with built-in colorimetric reporting.

A solvent-free, solid catalytic formulation comprising TBABr3, TBANO3, Cu(NO3)2·3H2O, and Nafion selectively oxidizes HD to its sulfoxide using ambient oxygen at room temperature. Guided by mechanistic insights from solution studies, Cu(II) was identified as crucial for increasing bromosulfonium concentration and enhancing rate, while also enabling colorimetric detection. XAFS and DRIFTS provide in situ evidence of tribromide regeneration and selective sulfoxide formation at the solid interface. The catalyst effectively decontaminates both liquid and vapor HD and resists interference from common co-contaminants (octane, CO2). Future work could optimize formulation durability and kinetics under a broader range of environmental conditions, explore scalability in protective textiles and coatings, and extend the approach to other persistent sulfide-based threats.

- Water and Zn(II) slow the overall catalytic turnover by inhibiting the nitrous acid-mediated re-oxidation of Br− to Br2, indicating sensitivity of the cycle to certain components present in the environment. - While Cu(II) accelerates the reaction, complete decontamination of liquid HD under the studied conditions can require hours (up to 96 h in some tests), suggesting kinetics may vary with loading, geometry, and environmental factors. - Performance was validated against selected interferents (octane, CO2); broader environmental matrices and long-term stability under varied humidity and temperature were not detailed in the provided text.