Effect of catalyst structure on oxidative dehydrogenation of ethane and propane on alumina-supported vanadia

摘要

The catalytic properties of Al_2O_3-supported vanadia with a wide range of VO_x surface density (1.4-34.2Vm~2) and structure were examined for the oxidative dehydrogenation of ethane and propane.UV-visible and Raman spectra showed that vanadia is dispersed predominately as isolated monovanadate species below approx 2.3Vm~2.As surface densities increase,two-dimensional polyvanadates appear (2.3-7.0Vm~2),along with increasing amounts of V_2O_5 crystallites at surface densities above 7.0Vm~2.The rate constant for oxidative dehydrogenation (k_1) and its ratio with alkane and alkene bombustion (K_2/k_1 and k_3/k_1,respectively) were compared for both alkane reactants as a function of vanadia surface density.Propene formation rates (per V atom) are approximately eight times higher than ethene formation rates at a given reaction temperature,But the apparent ODH activation energies (E_1) are similar for the two reactants and relatively insensitive to vanadia surface density.Ethene and propene formation rates (per V atom) are strongly influenced by vanadia surface density and reach a maximum value at intermediate surface densities (approx 8Vm~2).The ratio of k_2/k_1 depends weakly on reaction temperature,indicating that activation energies for alkane combustion and ODH reactions are similar.The ratio of k_2/k_1 is independent of surface density of ethane but increases silghtly wih vanadia surface density for propane,suggesting that isolated structures prevalent at low surface densities are slightly more selective for alkane dehydrogenation reactions.The ratio of k_3/k_1 decreases markedly with increasing reaction temperature for both ethane and propane ODH.Thus,the apparent activation energy for alkene combustion (E_3) is much lower than that for alkane dyhydrogenation (E_1) and the difference between these two activationenergies decreases withincreasing surface density.The lower alkene selectivities observed athigh vanadia surface densities are attributed to an increase in alkene adsorption enthalpies with increasing vanadia surface density.The highest yield of alkene is obtained for catalysts containing predominantly isolated monovanadate species and operated at high temperatures that avoid homogeneous reactions (< approx 800K).