THE HYDROLYSIS BEHAVIOR OF VANADIUM SPECIES IN AQUEOUS-SOLUTIONS AND THEIR ADSORPTION ON ALUMINA, SILICA AND TITANIA SURFACES

Acoustophoresis was applied to study reactions of vanadium species in aqueous solutions both in presence and in absence of counter ions, spe cifically phosphate-ions. Interaction of hydrolyzed vanadium species, with a defined average valence state and either or not in the presence of phosphate-ions, with silica, alumina and titania has also been exa mined with the objective to develop a method for the preparation of di spersed supported vanadium-phosphorus oxide catalysts. With titania an d alumina, phosphate-ions specifically adsorbed on the surface, result ing in a shift of the isoelectric point, for titania from a pH of 7 to 1.8, and alumina from pH >8 to 5.5. Quantification of the amount of p hosphate adsorbed (based on NH4H2PO4) yielded values of 0.035 and 0.2 g/g for titania and alumina, respectively. This corresponds with surfa ce areas of 21.4 and 9.3 Angstrom(2) available per phosphate atom, res pectively. Interaction of dissolved vanadium-phosphorus oxide precurso rs with (the surface of) the support material was demonstrated with al l support oxides. However, the extent of interaction with silica was s mall and resulted in formation of vanadium-phosphorus oxide as unsuppo rted bulk material. The large affinity of alumina towards phosphate re sulted in a supported, phosphate depleted, vanadium-phosphorus oxide p hase. With titania this effect was less pronounced and a well disperse d vanadium-phosphorus oxide phase could be obtained. (C) 1997 Elsevier Science B.V.