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

H2S是一种剧毒、腐蚀性强的环境污染物，其脱除是工业生产中污染控制和工艺要求的必要条件。在我们最近提交的专利（WO 2012175630 A1 20121227）中，我们描述了一种发明，其证明了特别改性的水溶性Fe、Mn、Co和Ni卟啉（特别是高度带正电荷的卟啉）可用于在pH中性介质中通过O2非常有效地催化氧化H2S。不需要单独的催化剂再氧化，因为催化剂在环境条件下通过周围介质中存在的O2自动再生。催化反应将H2S转化为固体（元素硫）或可溶性产物（亚硫酸盐/硫代硫酸盐），其选择可以通过选择适当的pH条件来调节。但是，某些金属卟啉催化活性背后的基元反应步骤，以及中间物种的特征和速率控制步骤是完全未知的。阐明这些点将提供的因素，是负责不同类型的金属卟啉的催化与化学计量反应的理解。作为科学家，我们不仅满足于我们的研究结果的潜在应用，因此我们寻求分子水平上的机理解释，以合理设计催化活性金属配合物，基于对其动力学，热力学，溶液和氧化还原行为的理解。因此，所提出的研究涉及仔细检查和动力学/热力学分析的过程中发生的H2S/HS的混合物在不同的pH值的O2的氧化过程中的每个反应步骤，与选定的一系列的水溶性金属卟啉作为催化剂的应用。此外，预期的研究将与负责氧化事件的反应性中间体的化学性质的鉴定和表征相关联，以及与活性物质的催化能力的检查相关联，所述活性物质的催化能力作为各种因素的函数，所述各种因素源自所应用的卟啉催化剂的化学特性（卟啉环、金属中心、其自旋状态和轴向配体的影响）和所选择的反应条件（温度、反应介质的pH或极性、分子氧和催化剂的浓度等）。特别注意将支付的氧化还原性能的调查高度带电的金属卟啉作为pH值和轴向配体的函数，以解决问题的内部与外部球电子转移机制。在分子水平上，金属卟啉催化氧化H2S的系统机理研究尚未在文献中得到解决。除了在环境和经济上可接受的技术方面的相关性外，拟议的研究还将导致我们对H2S生物效应的理解的进步。

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.