Contributions of heterogeneous and homogeneous chemistry in the catalytic partial oxidation of octane isomers and mixtures on rhodium coated foams

Catalytic partial oxidation experiments with n-octane, 2,2,4-trimethylpentane (i-octane), and an n-octane: i-octane (1:1) mixture were performed on 80 and 45 ppi Rh-coated alpha-alumina foam supports at 2, 4, and 6 SLPM total flow rate in order to explore the effects of chemical structure for single components and binary mixtures on fuel reactivity and product distribution. When reacted as single components, the conversion of i-octane is greater than n-octane at C/O > 1.1 (both fuel conversions are 100% for C/O < 1.1). However, when reacted in an equimolar mixture, the conversion of n-octane is greater than i-octane. All three fuels give high selectivity to syngas (H-2 and CO) on 80 ppi supports for C/O < 1. For C/O > 1, n-octane produces high selectivity to ethylene while i-octane makes i-butylene and almost no ethylene. The fuel mixture produces these species proportional to the mole fractions of n-octane and i-octane within the reacting mixture. Increasing the support pore diameter decreases the selectivity to syngas and increases H2O and olefin selectivity. The reforming of all three fuels is modeled using detailed chemistry by decoupling the heterogeneous and homogeneous chemistry in a two-zone plug flow model. Detailed homogeneous reaction mechanisms with several thousand elementary reactions steps and several hundred species are used to simulate experimentally observed olefin selectivities for all three fuels on 80 and 45 ppi monoliths at 2, 4, and 6 SLPM quite well. These results support the hypothesis that a majority of the observed olefins are made through gas-phase chemistry. (c) 2006 Elsevier Ltd. All rights reserved.