Mechanistic clarification of the elementary reaction steps involved in the metalloporphyrins-catalyzed oxidation of hydrogen sulfide under aerobic aqueous conditions

• Applicant: Professorin Dr. Ivana Ivanovic-Burmazovic
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261818077

H2S is a highly toxic and corrosive environmental pollutant, which removal is necessary for pollution control and processing requirements in industry. In our recently filed patent (WO2012175630 A1 20121227) we describe an invention demonstrating that specially modified water soluble Fe, Mn, Co and Ni porphyrins (in particularly, highly positively charged ones) can be used for very efficient catalytic oxidation of H2S by O2 in pH neutral media. There is no need for separate catalyst re-oxidation, because catalysts are automatically regenerated by O2 present in the surrounding medium at ambient conditions. The catalytic reaction transforms H2S into either solid (elemental sulfur) or soluble products (sulfite/thiosulfate), the choice of which can be tuned by choosing appropriate pH conditions. But, elementary reaction steps behind the catalytic activity of certain metalloporphyrins, as well as the character of the intermediate species and the rate-determining steps are completely unknown. Elucidation of these points will provide the understanding of the factors that are responsible for the catalytic vs. stoichiometric reactivity of different types of metalloporphyrins. As scientists we are not just satisfied with a potential application of our findings and therefore we seek for mechanistic explanations on molecular level to rationally design catalytically active metal complexes based on understanding of their kinetic, thermodynamic, solution and redox behavior. Thus, the proposed studies involve careful examination and kinetic/thermodynamic analysis of each reaction step occurring in the course of the oxidation of the H2S/HS¿ mixtures at different pH by O2, with the application of selected series of water-soluble metalloporphyrins as catalysts. Moreover, the intended investigations will be associated with the identification and characterization of the chemical nature of the reactive intermediates responsible for the oxidation events, as well as with the examination of the catalytic ability of the active species as a function of various factors originating both from the chemical identity of the applied porphyrin catalyst (effects of the porphyrin ring, metal center, its spin-state and axial ligands) and from the selected reaction conditions (temperature, pH or polarity of the reaction medium, concentration of dioxygen and catalyst, etc). Special attention will be paid to the redox properties of the investigated highly charged metalloporphyrins as a function of pH and axial ligands, in order to address the question of inner- vs. outer-sphere electron transfer mechanism. Such systematic mechanistic studies of the catalytic H2S oxidation by metalloporphyrins at the molecular level have not yet been addressed in the literature. Besides the relevance in terms of environmentally and economically acceptable technologies, proposed research will also lead to advances in our understanding of biological effects of H2S.

• DFG Programme: Research Grants