Oxidative dehydrogenation of ethane to ethylene over alumina-supported vanadium oxide catalysts: Relationship between molecular structures and chemical reactivity

摘要

The influence of vanadium oxide loading in the supported VO_x/Al_2O_3 catalyst system upon the dehydrated surface vanadia molecular structure,surface acidic properties,reduction characteristics and the catalytic oxidative dehydrogenation (ODH) of ethane to ethylene was investigated.Characterization of the supported VO_x/Al_2O_3 catalysts by XPS surface analysis and Raman spectroscopy revealed that vanadia was highly dispersed on the Al_2O_3 support as a two-dimensional surface VO_x overlayer with monolayer surface coverage corresponding to approx 9 Vm~2.Furthermore,Raman revealed that the extent of polymerization of surface VO_x species increases with surface vanadia coverage in the sub-monolayer region.Pyridine chemisorption-IR studies revealed that the number of surface Br0nsted acid sites increases with increasing surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region.The reducibility of the surface VO_x species was monitored by both H_2-TPR and in situ Raman spectroscopy and also revealed that the reducibility of the surface VO_x species increases with surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region.The fraction of monomeric and polymeric surface VO_x species has been quantitatively calculated by a novel UV-Vis DRS method.The overall ethane ODH TOF value,however,is constant with surface vanadia coverage in the sub-monolayer region.The constant ethane TOF reveals that both isolated and polymeric surface VO_x species possess essentially the same TOF value for ethane activation.The reducibility and Br0nsted acidity of the surface VO_x species,however,do affect the ethylene selectivity.The highest selectivity to ethylene was obtained at a surface vanadia density of approx 2.2 Vm~2,which corresponds to a little more than approx 0.25 monolayer coverage.Below 2.2 Vm ,exposed Al support cations are responsible for converting ethylene to CO.Above 2.2 Vm~2,the enhanced reducibility and surface Br0nsted acidity appear to decrease the ethylene selectivity,which may also be due to higher conversion levels.Above monolayer coverage,crystalline V_2O_5 nanoparticles are also present and do not contribute to ethane activation,but are responsible for unselective conversion of ethylene to CO.The crystalline V_2O_5 nanoparticles also react with the Al_2O_3 support at elevated temperatures via a solid-state reaction to form crystalline AlVO_4,which suppresses ethylene combustion of the crystalline V_2O_5 nanoparticles.The molecular structure-chemical characteristics of the surface VO_x species demonstrate that neither the terminal V=O nor bridging V-O-V bonds influence the chemical properties of the supported VO_x/Al_2O_3 catalysts,and that the bridging V-O-Al bond represents the catalytic active site for ethane activation.