Jc. Vedrine et al., MOLECULAR DESCRIPTION OF ACTIVE-SITES IN OXIDATION REACTIONS - ACID-BASE AND REDOX PROPERTIES, AND ROLE OF WATER, Catalysis today, 32(1-4), 1996, pp. 115-123
Oxidation reactions in heterogeneous catalysis usually involve a Mars
and van Krevelen mechanism which includes activation of the substrate
on a metallic cation, insertion of oxygen from lattice oxygen ions, a
redox mechanism on the catalyst surface, and the transfer of several e
lectrons. It follows that such a reaction necessitates both acid-base
and redox properties of a catalyst the acid site being of Lewis type (
cations) and the basic sites being the surface O2- or OH- species whic
h could exhibit electrophilic or nucleophilic properties. The active s
ite should be able to fulfil the following requirements: H abstraction
from the substrate, oxygen insertion, and electron transfer. It has b
een shown to correspond to an ensemble of atoms of limited size in an
inorganic molecular complex, It could correspond to local structural d
efects including steps, kinks, coordinatively unsaturated cations or t
o clusters of atoms on the surface. Some examples are described namely
: (i) n-butane oxidation to maleic anhydride on (VO)(2)P2O7 catalyst w
here four dimers of vanadyl cations on the (100) face were suggested t
o form the active site; (ii) isobutyric acid oxidative dehydrogenation
to methacrylic acid on iron hydroxy phosphates where trimers of iron
oxide octahedra were shown to constitute the most efficient and select
ive catalytic site while water was observed to be absolutely necessary
to facilitate the reaction which corresponds to hydroxylated surface
sites ensuring the redox mechanism; (iii) propane oxidative dehydrogen
ation to propene on VMgO samples which was shown to depend both on VOx
arrangements with respect to MgO and on the basicity of the material
induced by MgO while vanadium cations induced acidic features.