oa Chlorine dioxide production on-demand under ambient temperature and pH in water

The oxyanions of chlorine have seen multiple uses from bleaching to oxidizers in rocket fuel. Concerns over their accumulation in the environment, particularly that of perchlorate, have risen over the past two decades. Both chlorate (ClO₃¯) and chlorite (ClO₂¯) are employed in the generation of chlorine dioxide gas (ClO₂), which has been used in water treatment, disinfections, the pulp bleaching industry, and as an oxidizing agent. The lack of stability of ClO₂ and difficulties associated with its storage require its generation on-demand. Current methods employ harsh and corrosive conditions that require sulfuric acid and produce large amounts of salt and unwanted byproducts. Inspired by nature and how microorganisms isolated from contaminated sites degrade the oxyanions of chlorine, our research group developed catalysts based on manganese, a cheap and earth-abundant metal, for the conversion of chlorite to ClO₂ under ambient temperature and pH in aqueous solution. Two families of catalysts have been shown effective for this reaction, water-soluble manganese porphyrin/heme complexes and manganese compounds supported by simple polypyridyl cage ligands. Several spectroscopic techniques such as UV-vis, EPR, X-ray, mass spectrometry, and ion chromatography have been employed in the characterization of intermediates and products. Study of the reaction kinetics in conjunction with isotope labeling experiments has lead to mechanistic insight on the relevant species involved in catalysis. The prevailing pathway appears to involve an unprecedented homolytic Cl-O bond cleavage of chlorite at the metal site to generate high-valent manganese oxo and solvated chlorine oxide radical. The implication of these mechanistic insights on the reaction chemistry and development of future practical catalysts will be discussed.