Three Keggin-type heteropoly catalysts with different vanadyl species, H4PMo11VO40 (V existing in the primary structure), HVOPMo12O40 (V existing as VO2+ in the secondary structure), and V2O5/H3PMo12O40 (V existing in V2O5 before calcination), were prepared and studied to identify the preferable vanadyl species for oxidation of methacrolein to methacrylic acid. The catalysts were characterized by FT-IR, Raman, XRD, UV-vis, EPR, and XPS before and after calcination at different temperatures. The calcined catalysts were applied in the selective oxidation of methacrolein. The chemical state and position of the vanadyl species varied with the treatment temperature. Compared with the VO2+ species in HVOPMo12O40, the VO2+ species generated in H4PMo11VO40 and V2O5/H3PMo12O40 during calcination processes showed very different promotional effects, and the VO2+ species that interact with the Keggin structure turned to be the main active vanadyl species in all the catalysts. The active vanadyl species changed from an independent ion form to a squashed square pyramidal on Keggin structure or defect Keggin structure during the oxidation reaction. The optimum conversion of methacrolein and selectivity to methacrylic acid were obtained on the V2O5/H3PMo12O40 catalyst calcined at 350 degrees C, so a new means of adding vanadium in the Keggin-type heteropoly catalysts to obtain high selectivity to methacrylic acid is proposed. (C) 2015 Elsevier Inc. All rights reserved.