On the role of phosphate anion in the MoO3-Al2O3 based catalysts

Abstract The incorporation of phosphate ion into -A1 2 O 3 was studied by chromatographic techniques. Phosphate ion strongly interacts with alumina through a mechanism in-volving surface basic sites; it competes with molybdate in the adsorption onto alumina and, when it is the first to be added, it limits the amount of Mo(V1) which can be adsorbed.The study of the phase transitions of alumina in the range 673-1323 K evidenced a stabilizing effect of phosphate. As phosphorus content increases, the isomerization activity of -A1 2 O 3 decreases in parallel with the adsorption of Mo(VI). In the Mo0 3 -A1 2 O 3 systems, the hydrogen activation properties are strongly promoted by the presence of phosphate. These results are rationalized by hypothesizing the presence on y-alumina of strained Al-O-Al groups which have the character of Lewis conjugated acid-base sites and are responsible for adsorption properties, nucleation of a-alumina and catalytic activity. The role of phosphate is unique in stabilizing and/or deactivating these strained sites.