Effect of cobalt loading on structure and catalytic behavior of CoOx/SiO2 in CO2-assisted dehydrogenation of ethane

Structural and catalytic properties of flame-made CoOx/silica catalysts for the CO2-assisted dehydrogenation of ethane were observed to vary strongly depending on the Co-loading (0.1-4.5 wt%). X-ray diffraction showed that both fresh and spent catalysts were amorphous. UV-vis, XPS and XAS indicated that at Co-loadings lower than about 1 wt% highly dispersed tetrahedral Co2+ strongly bound to the silica matrix in a cobalt silicate like mixed oxide phase, was the prevalent cobalt species. At higher Co-loadings the presence of CoOx clusters became significant. All cobalt species showed strong resistance towards reduction, as shown by TPR. Comparative XAS analyses of catalysts exposed to ambient atmosphere and thermally pretreated catalysts (400 degrees C, vacuum for 2 h) showed that the presence of water facilitates the transformation of tetrahedrally to octahedrally coordinated Co2+. Best catalytic performance was achieved with a catalyst containing 0.75 wt% Co, affording 46% ethane conversion at 85% selectivity to ethene under the standard reaction conditions applied (700 degrees C, CO2/C2H6 ratio of 2.5 and gas hourly space velocity of 6000 L kg(-1) h(-1)). Some coke deposition in the form of disordered and graphitic carbon was observed with all Co-loaded catalysts. The catalyst with optimal Co-loading (0.75 wt%) showed nearly stable performance during 10 h time-on-stream.