ISOBUTYRIC ACID OXIDATIVE DEHYDROGENATION OVER IRON HYDROXYPHOSPHATES.2. TENTATIVE DESCRIPTION OF THE CATALYTIC SITES BASED ON MOSSBAUER SPECTROSCOPIC STUDY
Jmm. Millet et al., ISOBUTYRIC ACID OXIDATIVE DEHYDROGENATION OVER IRON HYDROXYPHOSPHATES.2. TENTATIVE DESCRIPTION OF THE CATALYTIC SITES BASED ON MOSSBAUER SPECTROSCOPIC STUDY, Applied catalysis. A, General, 124(2), 1995, pp. 205-219
Mossbauer spectroscopy has been used to characterize several hydroxyph
osphate catalysts used in the oxidative dehydrogenation of isobutyric
acid into methacrylic acid. It was found that under the conditions of
catalysis all the tested phases were transformed into phases belonging
to a solid solution of the type Fe4-x3+Fe3x2+(PO4)(3)(OH)(3-3x)O-3x w
ith 0 less than or equal to x less than or equal to 1. Depending upon
their composition, these phases contained clusters of face sharing iro
n octahedra of different sizes of which the largest ones underwent a f
ast electron transport process. This process which gave rise to the ob
servation by Mossbauer spectroscopy of mixed-valent cations, affected
the catalytic properties of the solids by both increasing their select
ivities for acetone and carbon dioxide at the expense of that for meth
acrylic acid and decreasing their total activity. From these results t
ogether with previously published data, it is suggested that the prese
nce of the mixed-valent cations lowers the basicity of the O2- ion inv
olved in the reaction. The change in activity seemed to be related to
the change in selectivity between parallel reactions whose rates depen
d upon the mobility of H+ or O2- within the solid.