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

硫化氢是一种剧毒、腐蚀性强的环境污染物，工业生产中的污染控制和处理要求对其进行脱除是必不可少的。在我们最近提交的专利(WO2012175630A1 20121227)中，我们描述了一项发明，证明了在pH中性介质中，特殊修饰的水溶性铁、锰、钴和镍卟啉(特别是高正电荷的)可以非常有效地催化O2氧化H2S。不需要单独的催化剂再氧化，因为在环境条件下，催化剂会通过周围介质中存在的O2自动再生。催化反应将H_2S转化为固体(单质硫)或可溶产物(亚硫酸盐/硫代硫酸盐)，可通过选择合适的pH条件来选择。但是，某些金属卟啉催化活性背后的基本反应步骤，以及中间物种的特征和速度决定步骤是完全未知的。阐明这些观点将有助于理解导致不同类型金属卟啉的催化与化学计量反应性的因素。作为科学家，我们不仅对我们的发现的潜在应用感到满意，因此我们在分子水平上寻求机理解释，以基于对其动力学、热力学、溶液和氧化还原行为的了解，合理地设计具有催化活性的金属配合物。因此，拟议的研究涉及使用选定的系列水溶性金属卟啉作为催化剂，对不同pH下的H_2S/HS混合物被O2氧化过程中发生的每个反应步骤进行仔细的检查和动力学/热力学分析。此外，预期的研究将与鉴定和表征导致氧化事件的活性中间体的化学性质有关，以及考察活性物种的催化能力作为各种因素的函数，这些因素源于所应用的卟啉催化剂的化学同一性(卟啉环、金属中心、其自旋态和轴向配体的影响)和选定的反应条件(反应介质的温度、pH或极性、氧气和催化剂的浓度等)。将特别关注所研究的高电荷态金属卟啉的氧化还原性质随pH和轴向配体的变化，以解决内球与外球电子转移机制的问题。在分子水平上对金属卟啉催化氧化H_2S的机理进行系统的研究尚未见文献报道。除了在环境和经济上可接受的技术方面的相关性外，拟议的研究还将促进我们对硫化氢生物影响的理解。

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.