Homogeneous and Heterogeneous Oxidation of Nitrilotrismethylenephosphonic Acid (NTMP) in the Presence of Manganese(II, III) and Molecular Oxygen

Abstract

In the absence of oxygen, NTMP (nitrilotrismethylenephosphonic acid) is oxidized by aqueous suspensions of the MnIII-containing mineral manganite (MnOOH), yielding iminodimethylenephosphonic acid (IDMP) as the sole phosphonate-containing product, along with orthophosphate and two equivalents of MnII. Hence, both C−P and C−N bond cleavage take place. The reaction proceeds via formation of a MnIII−NTMP surface complex and oxidation of NTMP by MnIII via a carbon-centered methylene radical. MnII strongly inhibits the reaction, presumably by competition for available surface sites. Homogeneous NTMP oxidation in MnII− and O2−containing solutions yields IDMP, formate, phosphate, and hydrogen peroxide. Interception of the methylene radical by O2 leads to N-formyl-IDMP and a second not-identified product. The dual presence of MnOOH and O2 yields autocatalytic NTMP oxidation, owing to the generation of dissolved MnII. Although the ultimate oxidants in the MnII−NTMP−O2 and MnOOH−NTMP systems are different (O2 versus MnIII), both oxidation reactions begin with C−N bond cleavage. This finding supports the hypothesis that MnIII is generated in the MnII−NTMP−O2 system.