Oxidative dehydrogenation of ethane at millisecond contact times:: Effect of H2 addition

The oxidative dehydrogenation of ethane using Pt/alpha-Al2O3 and various bimetallic catalysts operating at similar to 1000 degrees C and very short contact times is examined with H-2 addition to the feed. When H-2 is added with a Pt catalyst, the ethylene selectivity rises from 65 to 72% but ethane conversion drops from 70 to 52%. However, using a Pt-Sn/alpha-Al2O3 catalyst, the C2H4 selectivity increases from 70 to greater than 85%, while the conversion remains similar to 70%. The process also produces approximately as much H2 as is added to the feed. Effects of other metal promoters, sphere bed and fibermat supports, preheat, pressure, nitrogen dilution, and flow rate are examined in an effort to further elucidate the mechanism. Deactivation of the Pt-Sn catalyst is examined, and a simple method of regenerating the activity on-line is demonstrated. Possible mechanisms to explain high selectivities to ethylene are discussed. Although the process can be regarded as a simple two-step reaction sequence with the exothermic oxidation of hydrogen or ethane driving the endothermic dehydrogenation of ethane to ethylene, the exact contributions of heterogeneous or gas-phase reactions and their spatial variations within the catalyst are yet to be determined. (C) 2000 academic Press.