An in-situ DRIFTS-MS study on elucidating the role of V in the selective oxidation of methacrolein to methacrylic acid over heteropolyacid compounds

Abstract

The role of vanadium in 11-molybdo-1-vanadophosphoricacid (H4PMo11VO40, HPMoV) has been investigated and compared with 12-modybdophosphoricacid (H3PMo12O40, HPMo) using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with mass spectrometry (MS). It has been found for the first time that there are differences in the DRIFT spectra of carbonyl and carboxylate-type species recorded in the region of 1900 – 1200 cm-1 between the two compounds under methacrolein (MAL), MAL+O2, and MAL+O2+H2O reaction conditions at 320 C and the MS results show the faster formation of methacrylic acid over HPMoV than HPMo catalysts. A number of hydrocarbon species adsorbed on the surface have been identified in the DRIFTS including 5-membered ring of lactone-type adsorbed species, 6-membered ring of lactone-type adsorbed species, bidentate methacrylate adsorbed species, monodentate methacrylate adsorbed species, and -adsorbed species with the presence of V in the heteropolyacid Keggin structure favouring the formation of monodentate methacrylate surface species compared with lactone-type surface species under MAL+O2+H2O reaction conditions at 320 C. The monodentate methacrylate surface species is proposed to be the intermediate in the final step for the formation of MAA. It is likely that the same mechanism of selective oxidation of MAL occurs with and without the substitution of Mo with V, with the presence of V resulting in a faster formation of active monodentate methacrylate intermediates.