Preparation and isolation of mono-Nb substituted Keggin-type phosphomolybdic acid and its application as an oxidation catalyst for isobutylaldehyde and Wacker-type oxidation

Abstract

The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH₃[PMo₁₁NbO₄₀], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H₃[PMo₁₂O₄₀]) and potassium hexaniobate (K₈Nb₆O₁₉) in water, followed by freeze-drying. The all protonic form, H₄[PMo₁₁NbO₄₀], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH₃[PMo₁₁NbO₄₀] and H₄[PMo₁₁NbO₄₀] in pure forms is the evaporation of water using the freeze-drying method. Using a similar procedure, the potassium salt of the di-Nb substituted compound K₅[PMo₁₀Nb₂O₄₀] was isolated. H₄[PMo₁₁NbO₄₀] exhibited high catalytic activity for oxidizing isobutylaldehyde to methacrolein and moderate catalytic activity for the Wacker-type oxidation of allyl phenyl ether when combined with Pd(OAc)₂.