Olefins by Catalytic Oxidation of Alkanes in Fluidized-Bed Reactors

Abstract

The production of ethylene or syngas from ethane and olefins from propane, n-butane, and isobutane in the presence of air or O2 at atmospheric pressure has been examined over 100 μm α-Al2O3 beads coated with noble metals in a static fluidized bed reactor at contact times from 0.05 to 0.2 s. Variations in feed composition, preheating temperature, and flow rate were examined. For ethane on Pt, we observe selectivities to ethylene in excess of 72%, with conversions above 85% at the oxidative dehydrogenation stoichiometry (alkane/O2 = 2.0) for a 62% single-pass yield which is 5% higher than previously observed on Pt monoliths. On Rh, CO, and H2 (syngas) production dominates with CO and H2 selectivities up to 90% at nearly 100% ethane conversion. With propane and n-butane on Pt, we observe 55-60% and 65-70% selectivity to olefins, respectively, with >90% conversion for both alkanes. Ethylene and propylene are the major olefin products for both alkanes. Ethylene production dominates at higher temperatures and longer contact times while propylene production is favored at lower temperatures and shorter contact times. With isobutane on Pt, we observe 60-70% olefins with 80% conversion with isobutylene and propylene as the dominant olefins. For all alkanes, C2H2 formation was ≤ 0.8% and C4H6 and aromatics were less than 0.05%. Although all experiments were carried out in a regime predicted to be severely coke forming, no carbon buildup was observed on any of the catalysts and the catalysts exhibit no evidence of deactivation over several days. A simple reaction mechanism appears to explain the observed product distributions. Reactions are initiated by oxidative dehydrogenation of the alkane by adsorbed oxygen to form a surface alkyl. On Pt, β-hydrogen and β-alkyl elimination reactions of adsorbed alkyl dominate, which lead to olefins rather than cracking to Cs and Hs. On Rh, Ni, and Pd β-hydrogen elimination is not preferred and cracking to Cs and Hs ultimately leads to syngas production (Rh, Ni) or carbon deposition (Pd).