Sulfur mustard (HD), bis(2-chloroethyl) sulfide, is a highly persistent and toxic chemical warfare agent. Its decontamination is crucial for environmental and public safety. Selective oxidation of HD to its sulfoxide (HDO) is a key strategy, as HDO is significantly less toxic. Existing methods often involve harsh conditions or the use of environmentally unfriendly solvents. Therefore, the development of a robust, efficient, and environmentally benign decontamination method is of paramount importance. This research aimed to address this challenge by designing a solvent-free, solid catalyst capable of selectively oxidizing HD to HDO using only ambient air as the oxidant at room temperature. The successful development of such a catalyst would have significant implications for the creation of protective materials (e.g., clothing, masks, coatings) capable of effectively decontaminating HD upon contact, thereby offering superior protection against this hazardous agent. The use of a colorimetric indicator in the catalyst allows for visual confirmation of HD presence and successful decontamination. This approach offers significant advantages over current methods by combining high efficiency with environmental friendliness, simplicity, and immediate detection of HD.

Previous research on HD decontamination has focused on various approaches, including chemical oxidation, hydrolysis, and enzymatic degradation. Many of these methods suffer from limitations, such as low efficiency, the need for harsh conditions (high temperatures, pressures, or strong acids/bases), or the generation of harmful byproducts. Existing catalytic systems often rely on homogeneous catalysts, which can be difficult to separate from the reaction mixture, requiring extra steps for efficient removal and regeneration of the catalyst. There has been significant interest in heterogeneous catalysts to improve the ease of separation and the ability for regeneration and reuse, however, the reported results have not been effective in overcoming the challenge of efficient and selective decontamination of HD using ambient air. The authors reviewed the literature on polyoxometalates (POMs), metal-organic frameworks (MOFs), and other materials for catalytic oxidation, providing a context for the design of their proposed catalyst. The literature also highlighted the importance of finding efficient and selective oxidants, preferably environmentally friendly such as molecular oxygen.

The research involved both solution-phase and solid-phase catalytic studies. Solution-phase studies employed acetonitrile as a solvent and investigated the roles of various components (tribromide, nitrate, copper(II), acid) in the catalytic oxidation of 2-chloroethyl ethyl sulfide (CEES), a sulfur mustard simulant. The reaction kinetics were monitored using gas chromatography (GC), NMR spectroscopy, and stopped-flow UV-Vis spectroscopy. The effects of water and zinc(II) were also examined to understand the mechanism and identify the key components for catalytic enhancement. Stopped-flow techniques provided detailed information on the rapid reaction kinetics and allowed the identification of key intermediates. Spectroscopic techniques, such as UV-Vis, NMR, and DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) monitored the reaction process and were used to identify reaction intermediates. To develop the solid-state catalyst (SFC), Nafion (a chemically robust perfluorinated polymer and acid source), tetrabutylammonium tribromide (TBABr3), tetrabutylammonium nitrate (TBANO3), and copper(II) nitrate trihydrate were mechanically mixed. The solid catalyst was then tested using both liquid and vapor phase HD (sulfur mustard) and the simulant CEES. The effectiveness of the SFC was evaluated using GC-MS to determine the extent of conversion of the sulfur mustard to the sulfoxide. Bromine and copper K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies were employed to study the catalyst under reaction conditions, confirming the catalytic nature of the material and providing insights into the regeneration of the active species. DRIFTS was used to monitor the reaction with HD vapor, confirming the formation of the sulfoxide product at the gas-solid interface. Control experiments were also conducted with common battlefield contaminants (octane and CO2) to assess their influence on the catalytic activity.

Solution-phase studies revealed the critical role of Cu(II) in accelerating the reaction rate by increasing the equilibrium concentration of the bromosulfonium ion intermediate. Both water and zinc(II) ions could also shift the equilibrium but did not enhance the overall rate. This is because water and zinc inhibit the oxidation of Br⁻ ions back to Br₂, a key step in the catalytic cycle. Copper, however, does not inhibit this step. The Cu(II) ions also enabled colorimetric detection of HD, with a color change from pale yellow to dark green upon exposure to CEES (the color change was reversible). The development of a solvent-free, solid catalyst (SFC) was successful, achieving complete and selective sulfoxidation of CEES with only ambient air as the oxidant. The SFC was also effective with live agent HD, demonstrating a capability for both liquid and vapor phase decontamination. XANES and EXAFS studies provided evidence for the regeneration of tribromide under catalytic conditions. DRIFTS studies confirmed the absorption of HD vapor and selective conversion to HDO at the gas-solid interface. The SFC showed no significant inhibition of the reaction rate in the presence of octane or CO2, suggesting robustness in relevant battlefield conditions. The catalyst exhibited a color change from light green to dark brown upon exposure to HD, providing a visual indication of HD presence and reaction progress.

The findings demonstrate a significant advance in HD decontamination technology. The solvent-free, solid catalyst is highly effective, selective, and environmentally friendly, overcoming limitations of previous methods. The colorimetric detection feature adds a crucial layer of practical utility. The mechanistic insights into the role of Cu(II) and the reversible formation of the bromosulfonium ion intermediate are valuable for future catalyst design and optimization. The catalyst's performance under simulated battlefield conditions suggests its potential for real-world applications in protective materials. The combination of high efficiency, selectivity, ease of use, and colorimetric detection makes this catalyst a promising candidate for protecting personnel and the environment from HD exposure. The study highlights the power of integrating mechanistic studies with materials design to achieve practical and sustainable solutions for environmental remediation.

This research successfully developed a highly reactive, solvent-free, solid catalyst for the selective, catalytic, air-based oxidation of sulfur mustard (HD) to its less toxic sulfoxide (HDO) at ambient conditions. The catalyst utilizes Cu(II) to significantly increase reaction rates and enable colorimetric detection of HD. Mechanistic studies elucidated the role of Cu(II) in increasing the concentration of a key reactive intermediate. The solid catalyst effectively decontaminates both liquid and vapor HD. Future research could focus on optimizing the catalyst composition, exploring different solid supports, and integrating the catalyst into various protective materials for practical applications.

The study primarily utilized CEES, a simulant of HD, for much of the testing. While the results demonstrate significant promise, further investigation with live HD under various environmental conditions (temperature, humidity, presence of other contaminants) is needed to fully validate the effectiveness of the catalyst. The long-term stability and durability of the SFC under various conditions also warrant further investigation. The scalability of the catalyst synthesis for large-scale production and deployment is another aspect needing further study.