Modifications of the AgMo3P2O14 Catalyst in the Oxidation of Propane

The catalytic performances in the oxidation of propane of the AgM@2014 catalyst are quite good, leading to 70% of propene selectivity at 7% of propane conversion. We have checked that, before reaction, results of XANES and EXAFS obtained at the MO K-edge (19999 eV) d r m those obtained by X-ray spectroscopy (oxidation state and MO number of oxygen neighbour of 5.66, Mo=O bond length=1.72A). Mer reaction, we have shown that the catalyst is partially reduced (5.66+5.2), the number of oxygen atoms increases to 6 and the mean molybdenyl bond length decreases (1.68A) which is in agreement with the variation of this bond length with the oxidation state of the molybdenum in molybdenum phosphates. XPS measurements also d r m e d a partial reduction of molybednum species and showed a migration of the silver cation to the surface, certainly participating to the surface stabilization. Moreover, a correlation between the catalytic activity and the modification of the catalyst is made showing that an equilibrium between ~o"and M O ~ species in the catalyst leading to the avrerage oxidation state of molybdenum of 5.2 seems to be very important. It has been shown that the catalysts petformances of several systems in the mild oxidation of propane in acrolein are really increased by adding molybdenum atoms in their formulation (for example, in the V205+Mg0 system [l]). Moreover, adding silver in the catalyst synthesis seems to enhance the selectivity in acrolein in the oxidation reaction of propane. The A ~ M O ~ P ~ O ~ ~ [2] compound is effectively a quite efficient catalyst for this reaction and it is also an interesting model system to try to correlate its catalytic performances to its structural properties. A very fine study of the Ag%PZO14 catalyst crystalline structure has been done : the comparison of the catalytic and structural properties of other molybdenum phosphate will allow us to precise the structural parameters which should act positively in the catalytic reaction. X-ray spectroscopy by transmission at the K-edge of molybdenum was performed ex situ on two samples, one corresponding to fie& catalyst and the second one to catalyst after catalytic test. The modification of the catalyst occuring during the reaction could thus be followed. n) RESULTS AND DISCUSSION 1) STRUCTURE DESCRIPTION In [&P2014]m host lattice, the MO atoms are located on three independant crystallographic sites resulting in an M@Ou unit which is composed of two octahedra linked by one edge and one trigonal bipyramid linked to one of the two octahedra by one corner. The exact geometry of this unit is described on figure l in case of AgMo3P2O14 compound. In this case, the average molybdenum oxidation state is 5.66, the mixed valency being a located one with the following repartition ( M O ~ ) . ~ ( M O ~ ) ~ ( M O ~ ) ~ . ~ in the MOjOl5 unit. When strontium (divalent cation) is substituted to silver (monovalent cation), the average molybdenum oxidation state becomes 5.33 with the new repartition ( M O ~ ) ~ ( M O ~ ) ~ ( M O ~ ) ~ ~ .