Kinetics of long chain n-paraffin dehydrogenation over a commercial Pt-Sn-K-Mg/γ-Al ₂ O ₃ catalyst: Model studies using n-dodecane

A kinetic modeling study on long chain n-paraffin dehydrogenation using a commercial Pt-Sn-K-Mg/γ-Al ₂ O ₃ catalyst was carried out in a continuous flow set-up using n-dodecane as a model component at various temperatures (450-470 °C), pressures (0.17-0.30 MPa), H ₂ /paraffin mole ratios (3:1-6:1) and space times (0.22-1.57 g h mol ⁻¹ ). The commercial catalyst was characterized by XRD, BET, MIP, SEM and CO chemisorption. An empirical exponential equation was found to predict the mono- and di-olefin yields very well. In addition, 6 mechanistic models based on the LHMW mechanism were derived and tested by non-linear least squares fitting of the experimental data. The model which assumes that surface reactions and particularly the dehydrogenation of the metal-alkyl chain to the adsorbed mono-olefin and di-olefin as the rate determining steps was found to give the best fit with the experimental data. In addition, activation energies and adsorption enthalpies for each elementary reaction were obtained. The kinetic testing and modeling have shown that the high mono-olefins selectivity for long chain paraffin dehydrogenation can be obtained by operating at low space time (when P, T and m are same), high pressure (when τ, T and m are same) and high H ₂ /paraffin ratio (when τ, P and T are same), as well as low reaction temperature (when τ, P and m are same) but with little effect.