Selection of new Fe(III)/Fe(II) chelating agents as catalysts for the oxidation of hydrogen sulfide to sulfur by air

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Abstract

The removal and oxidation to sulfur of H 2 S, contained in a broad range of sour gas streams including natural gas, by a Fe(III)/Fe(II) chelate system imposes several requirements on the iron chelate that is employed as the redox catalyst. The solution of the catalyst must be neutral or mildly alkaline to efficiently absorb H 2 S from the gas passed through the solution. The stability of the iron(III) chelate must be high enough that solid Fe(OH) 3 not precipitate at the pH employed. Also the stability of the iron(II) chelate must be high enough to prevent the precipitation of FeS from the reaction mixture. The difference in stabilities of the Fe(III) and Fe(II) chelates must be low enough so that the Fe(III) form can be reduced by HS − and that the Fe(III) chelate can be regenerated by aeration of the solution at alkaline pH. An unexpected result of this study is that chelating agents designed for high stability of iron(III) chelates do not form satisfactory redox catalysts. Ligands that form suitable iron complexes as catalysts have both oxygen and nitrogen donors. All complexes investigated are readily reoxidized from the ferrous to the ferric chelate by air. Key words: iron chelates, stability constants, redox potentials, hydrogen sulfide, redox catalysts.

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