Short contact time oxidative dehydrogenation of C-2 and C-3 alkanes over noble metal gauze catalysts

The oxidative dehydrogenation of C2H6 and C3H8 have been studied at extreme ly short contact times in the order of 10(-5) s in quartz reactors using Pt or Pt/Rh gauze catalysts. The reactions were studied at 1 bar and gauze te mperatures of 600-1050 degrees C (for C2H6) and 695-935 degrees C (for C3H8 ). The alkane/O-2 ratio of the feed was typically 1.7, Ar was normally used as diluent (30 - 79 vol %). However, some runs have also been performed wi th steam as the diluent giving similar product selectivities. At low and mo derate conversions higher C2H4 selectivities were always obtained in an emp ty reactor than in presence of the gauze catalyst. The product composition approaches the same values as in the presence of a catalyst at more severe conditions giving conversion levels above approximately 55%. The role of th e gauze is to facilitate the ignition of the reaction mixture and to provid e extremely rapid in situ heating of the reactants by combustion. However, the oxidative dehydrogenation seems to proceed mainly in the gas phase. Syn thesis gas formation is important at high conversion, especially in case of C2H6. A H-2/CO ratio close to 1.5 (from C2H6) and close to 1 (from C3H8) w as obtained at high conversions also favoring the formation of C2H4 . C2H2 and benzene are favored by high reaction temperatures. (C) 1999 Elsevier Sc ience B.V. All rights reserved.