Oxidative Dehydrogenation Of Propane Over Mo-P-Modified Carbon Nanotube Catalysts

Conversion of light alkanes to the corresponding high value-added olefins is one of the most challenging tasks in catalysis. Among them, oxidative dehydrogenation of propane to propylene (ODH) is an important reaction. In recent years, due to its unique chemical and physical properties, carbon nanotubes (CNTs) have been used as catalysts and/or catalyst supports in heterogeneous catalysis. Su used the metal-free surface modified CNTs in the oxidative dehydrogenation of alkanes. The catalysts show a good catalytic performance. This study attempts to use transition metal loaded surface modified CNTs as a catalyst for oxidative dehydrogenation of propane (ODH).First, cheap and commercial multi-wall carbon nanotubes were treated by mixed acid oxidation process (oCNTs), and the structure was characterized by TEM, XRD, FTIR and UV-Raman techniques. Then molybdenum and phosphorus oxide were loaded on oCNTs by impregnation method, and tested by the propane oxidation dehydrogenation reaction.Firstly, the reaction conditions (temperature, space velocity, O2/propane ratios) of oCNTs catalysts for ODH were optimized. The catalytic properties of carbon nanotubes before and after acid treatment were compared. Compared with that of the untreated CNTs catalyst, propylene selectivity of the oCNTs increases by 15% for the same propane conversion. The surface oxygen groups on the oCNTs, especially the ketonic C=O groups, are proposed as the active sites.Effects of the loadings of MoOx/ oCNTs and POX/ oCNTs catalysts on the catalytic performance were investigated. The results show that the addition of low loading of molybdenum can significantly improve the activity, but reduce the propane selectivity at high temperature. As the loading further increases, the conversion slightly increases, but the selectivity drops significantly. Deactivation is due to the formation of crystalline MoO3. In contrast, adding phosphorus decreases the reactivity and increases the selectivity. Low phosphorous loading improves the selectivity by suppressing the combustion of propane and/or propylene. However, high phosphorous loading leads to coverage of active sites, hence resulting in a sharp decline in catalytic properties.Adding phosphorus and molybdenum elements can maintain the activity, improve the selectivity, and improve the stability of the catalyst at high temperature.20% of C3H8 conversion and 57% of C3H6 selectivity are obtained at 500℃. This is because that adding phosphorus can promote the dispersion of molybdenum on the surface of carbon nanotubes, hence improve the catalytic performance.