Vanadium oxides supported on TiO2-Sepiolite and Sepiolite:Preparation,structural and acid characterization and catalytic behaviour in selective oxidation of toluene

摘要

The synthesis of vanadium oxide systems(5-20 wt% of V2O5)supported on TiO2-Sepiolite(with titania loading around the theoretical monolayer,12 wt%)and on Sepiolite was carried out by the wet impregnation method.Materials were characterized by EDX,XRD,DR.UV-vis,Raman,29Si and 1H MAS NMR,TEM and nitrogen adsorption.Surface acid properties were evaluated through pyridine chemisorption.Their catalytic behaviour in the selective oxidation of toluene was studied.All solids exhibited the type IV isotherm,which corresponds to mesoporous solids.The decomposition of the Sepiolite into magnesium silicate and silica due to the effects of the vanadium incorporation process and calcination was not observed.The nature of the vanadium species depended on the support type and vanadium loading.Thus,in general,at the lowest vanadia loadings,isolated tetrahedral and polymetavanadate type species predominated.At higher loadings(15 wt% or above),vanadium oxides were also presented as crystalline V2O5 nanoparticles and,especially in %V/Sepc systems,as pyro- and meta-magnesium vanadates.On the other hand,in relation to the supports,the vanadium systems exhibited new acid sites whose density and strength both depended on the vanadia species structures.The catalytic performance of the vanadium oxide systems in the selective oxidation of toluene was also influenced by the structure and nature of vanadia species.Thus,the species formed on Ti02-coated Sepiolite support exhibited more activity in the formation of benzaldehyde(and benzoic acid)than the ones on Sepiolite.This formation is favoured by the polymeric vanadia species.