Effects of base concentration and cation on hydrothermal processes of cetyltrimethylammonium permanganate in various aqueous media

Abstract

Hydrothermal reactions of cetyltrimethylammonium permanganate (CTAMnO₄) in various aqueous media were systematically studied. The final products were strongly dependent on base concentrations and alkali metal cations in the medium. The transformation of CTAMnO₄ into CTAOH was faster in 1.5 M LiOH solution than in 0.3 M LiOH solution, and the Hofmann elimination of CTAOH resulted in the formation of 1-hexadecene. The reaction of the alkene with lithium permanganate resulted in the co-precipitated phase of lithium carbonate and manganese dioxide, in which Li₄Mn₅O₁₂ was formed. Further reaction between Li₄Mn₅O₁₂ and lithium carbonate produced a stable compound Li₂MnO₃ in the environment that contained organic compounds. No precipitated phase of lithium carbonate was observed in 0.3 M LiOH solution, and manganese dioxide was formed which could further react with organic compounds in dilute base environments, leading to the formation of Li-birnessite. Li-birnessite was transformed into the spinel with an extended reaction time. In KOH or NaOH solution, the molar ratio of Mn³⁺/Mn⁴⁺ in final products decreased with increasing base concentration. Mn³⁺-based compounds (Mn₂O₃ and γ-MnOOH) were observed in 0.1 M KOH solution, whereas K-birnessite was a stable product in 1.5 M KOH solution. The mesophase was observed in KOH solution, and its diffraction intensity increased with increasing base concentration. The effects of base concentration and alkali metal cation on the products were discussed as well.