Rationalization of the catalytic behavior of lanthanide oxides and praseodymium molybdates in total and selective oxidation of isobutene

The purpose of this work is to examine critically the correlations between ionization potential or absolute hardness and the catalytic behavior of binary and ternary oxides containing lanthanides (Ln) in alkene oxidation. Five lanthanide oxides (La2O3, Sm2O3, Pr6O11, Tb4O7, and CeO2) and three praseodymium molybdates (Pr-2-Mo3O12, Pr6MoO12, and Pr2MoO6, were used as pure phases or in mixtures with MoO3 in the partial and total oxidation of isobutene. The catalytic performances of their mixtures with MoO3 are described in the context of the "remote control mechanism", according to which the lanthanide-based oxides are identified as donors of spillover oxygen. Their donor ability is evaluated through the synergetic effects on methacrolein selectivity and correlated with the physical properties mentioned above. In the case of ternary oxides such as praseodymium molybdates, the concepts of average ionization energy and average absolute hardness are proposed to take into account the fact that the electron density and therefore the reactivity of mobile surface oxygen species are influenced by the presence of both metals in the lattice. The results obtained with the pure Ln-based oxides are in line with the literature concepts of ionization energy and absolute hardness. As far as the mechanical mixtures are concerned, it is shown that the above correlations, when combined with the "remote control" concepts, provide a comprehensive view of the influence of ionization energy or absolute hardness for both single phase and biphasic catalysts exhibiting phase cooperation.