Equilibrating metal-oxide cluster ensembles for oxidation reactions using oxygen in water

Although many enzymes can readily and selectively use oxygen in water-the m ost familiar and attractive of all oxidants and solvents, respectively-the design of synthetic catalysts for selective water-based oxidation processes utilizing molecular oxygen(1-4) remains a daunting task(5,6). Particularly problematic is the fact that oxidation of substrates by O-2 involves radic al chemistry, which is intrinsically non-selective and difficult to control . In addition, metallo-organic catalysts are inherently susceptible to degr adation(5) by oxygen-based radicals, while their transition-metal-ion activ e sites often react with water to give insoluble, and thus inactive, oxides or hydroxides(7). Furthermore, pH control is often required to avoid acid or base degradation of organic substrates or products. Unlike metallo-organ ic catalysts, polyoxometalate anions are oxidatively stable and are reversi ble oxidants(8,9) for use with O-2 (refs 8-10). Here we show how thermodyna mically controlled self-assembly of an equilibrated ensemble of polyoxometa lates, with the heteropolytungstate anion(11,12) [AIV(V)W(11)O(40)](6-) as its main component, imparts both stability in water and internal pH-managem ent. Designed to operate at near-neutral pH, this system facilitates a two- step O-2-based process for the selective delignification of wood (lignocell ulose) fibres. By directly monitoring the central Al atom, we show that equ ilibration reactions typical of polyoxometalate anions(13,14) keep the pH o f the system near 7 during both process steps.