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-vanadophosphoric acid (H₄PMo₁₁VO₄₀, HPMoV) has been investigated and compared with 12-modybdophosphoric acid (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 catalysts under methacrolein (MAL), MAL + O₂, and MAL + O₂ + H₂O reaction conditions at 320 °C and the MS results show faster formation of methacrylic acid over HPMoV than over HPMo catalysts. A number of hydrocarbon species adsorbed on the surface have been identified in the DRIFTS analysis including a 5-membered ring lactone-type species, a 6-membered ring lactone-type species, bidentate methacrylate species, monodentate methacrylate species, and π-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.