V/Te/Mo mixed oxides, prepared starting from Anderson-type polyoxometalates precursors, were characterized and used as catalysts for the oxidation of isobutene and of isobutane to methacrolein. Catalysts were active and moderately selective in olefin oxidation. The addition of V increased the activity but decreased the selectivity to methaerolein. The reaction network was complex and included several parallel reactions from isobutene, namely, the oxidation to methacrolein, the formation of heavy byproducts including oxygenated alkylaromatics that originated from the reaction with methacrolein itself and of polymeric materials, and the combustion to carbon oxides. The relative contribution of the different reactions was a function of catalyst composition and reaction conditions. Catalysts were poorly active in the oxidation of isobutane. The Ni/Te/Mo/O catalyst sample was more active than the Te/Mo/O one. Total conversion of isobutene was reached at 350°C. The selectivity to methacrolein initially increased but then declined for isobutene conversions higher than 80%. The yield to methacrolein was however lower than that obtained with the Te/Mo/O catalyst. In the 60-100% isobutene conversion range, the selectivity to methacrolein increased from 10 to 30% while that to carbon oxides only slightly increased. Among the minor products, only the selectivity to methacrylic acid increased but remained low. The reaction network was complex. The Ni/Te/Mo/O and V/Te/Mo/O catalysts were poorly active in the oxidation of isobutane. The main product was carbon dioxide. Isobutene was not produced despite the low conversion such that the catalysts catalyzed the direct transformation of the alkane to carbon oxides. The catalysts tested possessed the functionality necessary to transform isobutene to methacrolein but transformed the olefins to carbon oxides and heavy byproducts. The addition of V or of Ni that hopefully should have increased catalyst activity to isobutane did not yield the expected improvement. The selective oxidation of isobutene to methacrolein is a step in the multi-step transformation of isobutane to methacrylic acid. The discussion covers introduction; experimental; results and discussion (characterization of catalysts; and catalytic performance in isobutene and isobutane oxidation).
|Titolo:||Isobutene and isobutane oxidation over molybdenum oxide-based catalysts, prepared starting from anderson-type polyoxometalates precursors|
|Data di pubblicazione:||2004|
|Appare nelle tipologie:||Relazione (in Rivista)|