Silica is preferred over various single and mixed oxides as support for CO2-assisted cobalt-catalyzed oxidative dehydrogenation of ethane

Catalysts containing 4.5 wt% cobalt supported on single oxides of SiO2, Al2O3, TiO2 and ZrO2 and on mixed ones of SiO2-Al2O3, SiO2-TiO2, SiO2-ZrO2 and TiO2-ZrO2 were produced in a single step by flame spray pyrolysis and tested in the CO2-assisted oxidative dehydrogenation of ethane in a continuous fixed-bed microreactor. Structural and chemical properties of the nonporous catalysts were characterized by nitrogen adsorption, XRD, HR-TEM, EDXS, NH3-TPD, H-2-TPR, TGA, DRIFTS, XPS and Raman and UV-vis spectroscopy. Depending on the supporting oxides the reducibility of the cobalt species varied in a broad range, indicating vastly different interaction of the cobalt species with the oxidic support. For catalysts supported on Al2O3 and SiO2-Al2O3 no significant H-2 consumption was observed up to 840 degrees C, while ZrO2-supported CoOx was already reduced at T <500 degrees C. The silica-supported catalysts showed a broad reduction peak at 780 degrees C attributed to well dispersed CoOx embedded in the SiO2 matrix forming cobalt silicate, consistent with HR-TEM, XPS and UV-vis analyses. Among all catalysts, silica-supported cobalt showed the best catalytic performance under the test conditions used (700 degrees C with CO2/C2H6 = 2.5). Admixing Al2O3, TiO2 and ZrO2 to SiO2 did not result in improved catalytic performance. Reducibility of the CoOx species as well as the strength and surface density of acidic sites were identified as crucial catalyst properties for optimal ethene yield. Control of the surface acidity seems particularly important to avoid undesired side reactions (cracking, coking) impairing activity and selectivity. Parametric sensitivity studies of the silica supported cobalt catalyst, including reaction temperature, CO2/C2H6 feed ratio and space velocity, revealed the important role of CO2. In the absence of CO2, only low C2H6 conversion was observed and change of the oxidant to O-2 resulted in poorer catalytic performance. (C) 2016 Elsevier B.V. All rights reserved.