Kinetics of the Oxidative Dehydrogenation of Isobutane over Cr₂O₃/La₂(CO₃)₃

Abstract

The oxidative dehydrogenation (ODH) of isobutane over Cr₂O₃/La₂(CO₃)₃ has been investigated in a low-pressure Knudsen cell reactor, under conditions where the kinetics of the primary reaction steps can be accurately determined. By heating the catalyst at a constant rate from 150-300 °C, temperature fluctuations due to non-equilibrium adsorption are minimized. The evolved gas profiles show that ODH to isobutene and water is a primary reaction pathway, while carbon dioxide, which forms from the catalyst during reaction, is the only other product. This CO₂ evolution may enhance the activity of the catalyst. Isobutene formation proceeds with the participation of lattice oxygen from the Cr₂O₃/La₂(CO₃)₃ catalyst. The intrinsic Arrhenuis rate constant for the ODH of isobutane is k(s⁻¹)=10... exp{-((55±5)-ΔH... kJmol⁻¹)/RT} The small pre-exponential factor is expected for a concerted mechanism and for such a catalyst with a small surface area and limited porosity.