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

The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH 3 [PMo 11 NbO 40 ], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H 3 [PMo 12 O 40 ]) and potassium hexaniobate (K 8 Nb 6 O 19 ) in water, followed by freeze-drying. The all protonic form, H 4 [PMo 11 NbO 40 ], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH 3 [PMo 11 NbO 40 ] and H 4 [PMo 11 NbO 40 ] 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 K 5 [PMo 10 Nb 2 O 40 ] was isolated. H 4 [PMo 11 NbO 40 ] 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 .