Catalytic behaviour of multiphasic oxide catalysts containing lanthanides (La, Ce, Pr, Sm, Tb) in the selective oxidation of isobutene to methacrolein

The catalytic performances of five lanthanide oxides (La2O3, Sm2O3, CeO2, Pr6O11 and Tb4O7) for the selective oxidation of isobutene to methacrolein are evaluated within the framework of the remote control mechanism. Mechanical mixtures of these oxides with typical donor (Sb2O4) or acceptor (MoO3) phases of spill-over oxygen were prepared and tested for their activity in the isobutene-to-methacrolein oxidation at 400 degrees C. Amongst the five lanthanide oxides tested, only CeO2 and Pr6O11 were found to display significant cooperation effects for the investigated reaction, with enhanced yields and selectivity for partial oxidation and concomitant decrease of CO2 production. The fresh and used catalysts were characterized by X-ray diffractometry, and the occurrence of solid state reactions between the partner oxides outside the reaction conditions was investigated in parallel in the temperature range 400-500 degrees C. No new phase was observed in the case of the mixtures with La2O3, Sm(2)O3, CeO2 and Tb4O7. Account taken of the absence of any new phase in the CeO2-MoO3 system, it can be concluded that CeO2 is a potential donor of spill-over oxygen. The situation in the MoO3-Pr6O11 mixtures is more complex, owing to the generation of various praseodymium molybdates, together with the oxocarbonate Pr2CO5. The latter phase was shown to have no intrinsic tendency to produce methacrolein, but it seems that some of the praseodymium molybdates present in the working catalysts may exhibit noticeable catalytic properties.