Studies of processes occurring during alkoxide derived V–O–W unsupported catalyst formation

Abstract

The aim of this paper is to investigate the processes occurring during V–O–W unsupported catalyst formation in order to gain the knowledge necessary to design supported catalysts. Sol–gel synthesis of V–W hydroxo-oxide hydrate (W:V=9:1) from tungsten and vanadyl isopropoxides is described. The hydrate structure is shown to be related to WO3·0.33H2O. The role of vanadium in the formation of this structure is discussed. Hydrate dehydration is shown to result in the formation of V–W hydroxo-oxide isostructural with hexagonal WO3. Further heating of hydroxy-oxide causes a loss of hydroxy groups accompanied by the formation of V–W oxide bronze isostructural with tetragonal WO3. Annealing the bronze in an air atmosphere results in the removal of vanadium from the bronze crystallites as a results of its surface segregation. The formation of the surface vanadia-like species on WO3-related crystallites is found to be a result of vanadium segregation. Increasing the annealing temperature causes recrystallisation of the surface species. The synthesis of a V–W/TiO2 oxide catalyst via a mixed oxide formation in the presence of titania sols followed, by surface vanadium segregation in an air containing atmosphere, is proposed. The application of the knowledge gained during our investigation of the formation of the unsupported V–O–W catalyst to the discussion of the morphology of the V–O–W/Ti(Sn)O2 (rutile) catalyst is also demonstrated.