Oxidative dehydrogenation of isobutane on VOx/oxide support catalysts:: Effect of the potassium additive

Effect of the potassium additive on physicochemical and catalytic properties in oxidative dehydrogenation (ODH) of isobutane has been studied for a series of vanadia-based catalysts supported on different oxides (SiO2, Al2O3, TiO2, MgO). The catalysts (denoted as VS where S is a cation of a support), containing 1 or 1.5 monolayer (mnl) of V2O5 were prepared by impregnation method, characterized by BET, XPS, isopropanol decomposition (probe reaction for acido-basic properties) and tested in isobutane ODH in the temperature range 250-450 degrees C. For all the catalysts the K additive decreased the acidity and increased the basicity, decreased the total acitivity, and increased (with the exception of VMg catalyst) the selectivity to isobutene. The increase in the selectivity was attributed to the decrease in acidity and increase in basicity of the catalysts. For VMg, different distribution of carbon oxides and different kinetic scheme were observed (dominance of CO2 formed by a consecutive oath over CO formed by a parallel route) as compared with other catalysts (dominance of CO over CO2). These facts suggest, that the decrease in the selectivity to isobutene by the K additive observed for VMg catalyst is due to modification by K of other than acido-basic properties (most probably interaction of the catalysts with oxygen), which control the selectivity in this case.