Mo Oxidation State of Cd, Fe, and Ag Catalysts Under Propane Mild Oxidation Reaction Conditions

Activity and selectivity of molybdenum phosphates, Cd1−xFex-MoPO6 (x=0→1) and AgMo3P2O14, were compared for the propane mild oxidation reaction (C3H8/O2/N2=60/20/20, W/F=3.5 g h L−1). Despite different initial molybdenum oxidation states in the CdMoPO6 and AgMo3P2O14 compounds, both catalysts exhibited almost the same performance. At 733 K, propane conversion reached 6.5% and mild oxidation product selectivity (propylene + acrolein) was 75%. On the other hand, Cd1−xFexMoPO6 (x≠0) compounds were completely inactive. X-Ray diffraction and absorption wera performed before and after catalytic tests and even during the reaction in the case of AgMo3P2O14. It was shown that both active catalytic systems were modified under reactive flow; depending on their initial oxidation state, molybdenum species were either partially reduced or partially oxidized. This modification resulted in equilibration of the proportions of MoV and MoVI reaching the Mo5.25 average oxidation state in both equilibrated catalysts. The intrinsic ability of the two frameworks to allow establishment of convenient MoV/MoVI balances of 2/3 and 1/3 was a determinant factor toward activity. EXAFS studies allowed us to provide evidence of the corresponding structural relaxation phenomena. In contrast, in the case of the iron-containing compounds Cd1−xFexMoPO6, molybdenum remained pentavalent and catalytic activity was thus inhibited. The easier competitive oxidation of divalent iron into trivalent iron may be responsible for the inhibition of the MoV/MoVI redox couple.