Homogeneous Catalysis Under Ultradilute Conditions: TAML/NaClO Oxidation of Persistent Metaldehyde

TAML activators enable homogeneous oxidation catalysis where the catalyst and substrate (S) are ultradilute (pM–low μM) and the oxidant is very dilute (high nM–low mM). Water contamination by exceptionally persistent micropollutants (MPs), including metaldehyde (Met), provides an ideal space for determining the characteristics and utilitarian limits of this ultradilute catalysis. The low MP concentrations decrease throughout catalysis with S oxidation (k II) and catalyst inactivation (k i) competing for the active catalyst. The percentage of substrate converted (%Cvn) can be increased by discovering methods to increase k II/k i. Here we show that NaClO extends catalyst lifetime to increase the Met turnover number (TON) 3-fold compared with H2O2, highlighting the importance of oxidant choice as a design tool in TAML systems. Met oxidation studies (pH 7, D2O, 0.01 M phosphate, 25 °C) monitored by 1H NMR spectroscopy show benign acetic acid as the only significant product. Analysis of TAML/NaClO treated Met solutions employing successive identical catalyst doses revealed that the processes can be modeled by the recently published relationship between the initial and final [S] (S0 and S∞, respectively), the initial [catalyst] (FeTot) and k II/k i. Consequently, this study establishes that ΔS is proportional to S0 and that the %Cvn is conserved across all catalyst doses in multicatalyst-dose processes because the rate of the k II process depends on [S] while that of the k i process does not. A general tool for determining the FeTot required to effect a desired %Cvn is presented. Examination of the dependence of TON on k II/k i and FeTot at a fixed S0 indicates that for any TAML process employing FeTot < 1 × 10–6 M, small catalyst doses are not more efficient than one large dose.