Tailoring Oxidation State of Manganese Enables the Direct Formation of Todorokite.

Abstract

Todorokite, an octahedral molecular sieve (OMS-1), is characterized by its large 3 × 3 tunneled structure (∼1 nm) formed by a metal oxide framework. Todorokite-type manganese oxides have garnered interest in materials science and environmental chemistry due to their nanoporous ionic conductive channels and natural ubiquity. However, conventional synthesis of todorokite involves energy- and time-consuming processes: (1) synthesis of a layered Mn oxide, (2) Mg-intercalation into the layer, and (3) hydrothermal transformation of the layered Mn oxide to todorokite. Here, we report a rapid electrochemical synthesizing method and its mechanisms of todorokite, achieved within 30 min using Mn²⁺(aq) in 1 M MgCl₂ at pH 8.5 and room temperature. We demonstrate that structured Mn(III), which exhibits Jahn-Teller distortion, forms through comproportionation at pH 8.5 and subsequently rearranges to create the todorokite framework. In addition, we found that aqueous Mg²⁺ species, specifically Mg(OH)⁺, stabilize the structured Mn(III) and contribute to the formation of the todorokite framework during electrodeposition. Our facile and direct synthesis method of todorokite promises to enhance its utility in engineering applications, offers an approach for synthesizing and controlling the crystalline structure of Mn oxides through the principles of sustainable chemistry, and advances the fundamental understanding of the natural occurrence of todorokite in environmental chemistry.