Oxidative Dehydrogenation of Ethane with CO2 over Flame-Made Ga-Loaded TiO2

The influence of the Ga content (0-17 wt %) on the structural properties and the catalytic behavior of flame-synthesized TiO2-supported gallium oxide in the oxidative dehydrogenation of ethane (ODHE) has been investigated in a continuous tubular microreactor using CO, as a mild oxidant. The gallium oxide-titania powders consisted of nonporous spherical particles of about 10 nm average diameter, as indicated by HRTEM and X-ray line broadening, and had a specific surface area of about 120 m(2)/g. XRD showed no reflections corresponding to Ga2O3 in the as-prepared samples, indicating high dispersion of the Ga constituent. At higher Ga loading (>14 wt %) stronger acidic sites became prominent, as indicated by NH3-TPD and DRIFTS. The ethene yield increased with Ga loading up to about 10 wt %. The molar CO2/C2H6 ratio in the feed, reaction temperature, and space velocity were decisive parameters for achieving maximum ethene yield. The ethene yield achieved was 22% using a CO2/C2H6 ratio of 2.5, 700 degrees C, and a space velocity of 6000 L/((kg of cat.) h), corresponding to 57% ethane conversion at 38% selectivity to ethene. After reaction (1 h on stream) all catalysts showed significant coking and a loss of surface area depending on the Ga content. Catalysts with higher Ga content were more resistant toward sintering but showed more severe coking due to the presence of stronger acidic sites. Raman spectroscopy revealed that all spent catalysts were covered with both D- and G-type carbon. Both carbon deposition and reduction of catalyst surface area led to a significant decrease in activity with time on stream, while the selectivity to ethene increased up to above 70%. The catalysts could efficiently be regenerated by exposing them for 10 min to air at a reaction temperature of 700 degrees C, providing a base for the development of a dynamic process consisting of alternative ODHE/regeneration cycles.