Importance of site isolation in the oxidation of isobutyric acid to methacrylic acid on iron phosphate catalysts

Isobutyric acid (IBA) oxidative dehydrogenation to methacrylic acid (MAA) r eaction has been studied for a wide variety of iron (hydrogen) phosphates c atalysts and mechanical mixtures of several of these phosphates. It was obs erved that the reaction needs high amounts of water, typically 10-12 mot wa ter per mot IBA, to get stable, active and selective catalysts and follows a Mars and van Krevelen mechanism, as schematised below: Fe-3(3+)(PO3OH)(3)(PO4) + (x/2)IBA --> Fe-3-x(3+) Fe-x(2+)(PO3OH)(3+x)(PO4) (1-x) + (x/2)MAA Fe3-x3+Fex2+ (PO3OH)(3+x)(PO4)(1-x) + (x/4)O-2 --> Fe-3(3+)(PO3OH)(3)(PO4) + (x/2)H2O The best catalysts were found td be composed of trimers of face sharing FeO 6 octahedra, isolated one from the others by cationic vacancies and bounded to the next row by PO4 tetrahedra. Solids having dimers or long chain octa hedra are also active but less selective. Theoretical calculations showed t hat this corresponds to an electron hopping from one Fe2+ to the next Fe3cation during the redox mechanism for a trimer, while no electron hopping o ccurs for dinners and whereas it occurs all over the Fe cations for infinit e octahedra chains. The presence of such vacancies and, to a lesser extent, of excess phosphorus at the surface, as shown by XPS analysis, exemplifies the site isolation concept in partial oxidation reactions. At variance, th e cooperation effect between several phases was not observed.