Catalytic cycle during heteropoly acid catalyzed transfer hydrogenation reaction

12-Molybdophosphoric acid catalyzed transfer hydrogenation of nitrobenzene by hydrazine in a homogeneous phase is studied using UV VIS, ESR, P-31 NMR, FTIR, and XPS, The role of nitrobenzene and solvents on the oxidation stat e of molybdenum and hence the catalytic activity is elucidated. Breakdown o f the Keggin structure leads to a loss in catalytic activity. While hydrazi ne reduces the catalyst to form the active Mo(V) species, nitrobenzene reox idizes the catalyst to the Mo(VI) state, thereby completing the catalytic c ycle, Nitrobenzene also maintains part of the catalyst in the (VI) state, t hus preventing deep reduction and hence structural degradation. (C) 1999 Ac ademic Press.