EVALUATION OF THE ROLE PLAYED BY PRASEODYMIUM MOLYBDATES IN PR6O11-MOO3 CATALYSTS FOR THE SELECTIVE OXIDATION OF ISOBUTENE TO METHACROLEIN

The catalytic role played by praseodymium molybdate phases (Pr6MoO12, Pr2MoO6, Pr2Mo3O12) generated in Pr6O11-MoO3 catalysts during their us e in the selective oxidation of isobutene to methacrolein is evaluated in order to determine whether the observed synergetic effects have to be assigned to cooperation between binary praseodymium and molybdenum oxides, or to the occurrence of praseodymium molybdate phases, that w ould be catalytically active themselves or would act synergetically wi th MoO3 or Pr6O11 X-ray diffraction, infrared spectroscopy, X-ray phot oelectron spectroscopy and time-of-flight secondary ion mass spectrome try (ToF SIMS) are used to determine the composition of the used Pr6O1 1-MoO3 catalysts. Pr6MoO12 appears to be the major ternary phase prese nt in the bulk and Pr2Mo3O12 is predominant at the surface. To underst and the catalytic behaviour of the Pr6O11-MoO3 mixtures, the catalytic performances of the pure praseodymium molybdate phases and those of t heir mixtures with MoO3, Sb2O4 and Pr6O11 were also examined. It is sh own that the cooperative effects previously observed in the MoO3-Pr6O1 1 mixtures most probably cannot be assigned exclusively to the intrins ic catalytic properties of praseodymium molybdates. A cooperation is a lso observed between other couples of oxide phases, i.e. Pr6MoO12 disp lays donor properties with respect to MoO3. It appears also that the e volution of the catalytic properties of Pr6MoO12-MoO3 mixtures with re spect to the Pr/Mo content is completely different from the one observ ed in MoO3-Pr6O11 mixtures. In addition, it does not seem that the obs erved synergetic effects could be assigned to the simultaneous presenc e of ternary PrxMoyOz phases and Pr6O11. The generation of praseodymiu m molybdate phases during the catalytic tests seems therefore to play no significant role in the catalytic behaviour of MoO3-Pr6O11 mixtures . (C) 1998 Elsevier Science B.V. All rights reserved.