Oxidative dehydrogenation of isobutane at short contact times

The oxidative dehydrogenation of isobutane over noble metal coated ceramic foam monoliths selectively produces isobutylene with high conversions and yields at short contact times in an autothermal reactor at atmospheric pressure and temperatures of similar to 800-900 degrees C. Maximum selectivity of less than or equal to 71% to total olefins is achieved with O-2 and reactant preheat of 350 degrees C at a fuel/oxygen ratio of 1.7. Operation at a constant fuel/oxygen ratio of 1.2 while varying the space velocity from 8x10(4) to 8x10(5) h(-1) increases selectivities,to C-4-olefins and decreases selectivities to C-2-olefins by similar to 8%, although the fuel conversion drops from 80% to 40%. To investigate the effects of the physical and chemical nature of the catalyst, Pt, Rh, Ir,Pd, and Pt-Sn catalysts were examined, with Pt being optimal for olefin production and catalytic activity decreasing in the order Pt>Pd>Rh>Ir. Pt-Sn deactivated by carbon formation and metal loss while Pd deactivated by rapid carbon formation. Monolithic catalysts with various pore sizes (20, 45, 80 ppi) show that in the fuel lean regime, smaller pores (XO ppi), as compared to catalysts with more open channels (20 ppi), lead to 20% higher fuel conversion but 10-15% lower selectivity to olefins. Results indicate that either a purely catalytic beta-elimination mechanism or a heterogeneously assisted homogeneous reaction mechanism can explain the product distribution. The causes of secondary reactions, oxygen breakthrough, and the effect of increased flow rate, and increased mass transfer are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.