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 y ields at short contact times in an autothermal reactor at atmospheric press ure and temperatures of similar to 800-900 degrees C. Maximum selectivity o f less than or equal to 71% to total olefins is achieved with O-2 and react ant 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 8x1 0(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 catal yst, Pt, Rh, Ir,Pd, and Pt-Sn catalysts were examined, with Pt being optima l for olefin production and catalytic activity decreasing in the order Pt>P d>Rh>Ir. Pt-Sn deactivated by carbon formation and metal loss while Pd deac tivated by rapid carbon formation. Monolithic catalysts with various pore s izes (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 th e product distribution. The causes of secondary reactions, oxygen breakthro ugh, and the effect of increased flow rate, and increased mass transfer are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.