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

* Chemistry and Pulping Research Work Unit, USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin 53705, USA † Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA ‡ Department of Chemical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA ....................................................................................... Although many enzymes can readily and selectively use oxygen in water-the most familiar and attractive of all oxidants and solvents, respectively–the design of synthetic catalysts for selec­ tivewater-basedoxidation processes utilizing molecular oxygen1–4 remains a daunting task5,6. Particularly problematic is the fact that oxidation ofsubstrates by O2 involves radical chemistry, which is intrinsically non-selective and difficult to control. In addition, metallo-organic catalysts are inherently susceptible to degradation5 by oxygen-based radicals, while their transition-metalion active sites often react with water to give insoluble, and thus inactive, oxides or hydroxides7. Furthermore, pH control is often requiredto avoidacidorbase degradation oforganic substrates or products. Unlike metallo-organic catalysts, polyoxometalate anions are oxidatively stable and are reversible oxidant8,9 for use with O2 (refs 8-10). Here we show how thermodynam­ ically controlled self-assembly of an equilibrated ensemble of polyoxometalates, with the heteropolytungstate anion11,12 [AIVVW11O40]6– as its main component, imparts both stability in water and internal pH-management. Designed to operate at near-neutral pH, this system facilitates a two-step O2-based process for the selective delignification of wood (lignocellulose)