Homogeneous and heterogeneous oxidation of nitrilotrismethylenephosphonic acid (NTMP) in the presence of manganese(II, III) and molecular oxygen

In the absence of oxygen, NTMP (nitrilotrismethylenephosphonic acid) is oxidized by aqueous suspensions of the Mn-III-containing mineral manganite (MnOOH), yielding iminodimethylenephosphonic acid (IDMP) as the sole phosphonate-containing product, along with orthophosphate and two equivalents of Mull. Hence, both C-P and C-N bond cleavage take place. The reaction proceeds via formation of a Mn-III-NTMP surface complex and oxidation of NTMP by Mn-III via a carbon-centered methylene radical. Mn-II strongly inhibits the reaction, presumably by competition for available surface sites. Homogeneous NTMP oxidation in Mn-II and O-2-containing solutions yields IDMP, formate, phosphate, and hydrogen peroxide. Interception of the methylene radical by O-2 leads to N-formyl-IDMP and a second not-identified product. The dual presence of MnOOH and O-2 yields autocatalytic NTMP oxidation, owing to the generation of dissolved Mn-II. Although the ultimate oxidants in the Mn-II-NTMP-O-2 and MnOOH-NTMP systems are different (O-2 versus Mn-III), both oxidation reactions begin with C-N bond cleavage. This finding supports the hypothesis that Mn-III is generated in the Mn-II-NTMP-O-2 system.