Kinetic and isotopic study of ethane dehydrogenation over a semicommercial Pt,Sn/Mg(Al)O catalyst

Mechanistic and kinetic information on the ethane dehydrogenation reaction over a semicommercial Pt,Sn/Mg(Al)O catalyst has been elucidated from catalytic testing and isotopic labeling experiments under reaction conditions close to those used in the commercial dehydrogenation process (C2H6/H-2/H2O/inert = 10/1.5/2/32 or C2H6/H-2/CO2/inert = 10/2/5/83, 600-630 degrees C reaction temperature, atmospheric pressure). From kinetic measurements, a negative dependence of the reaction rate in H-2 and C2H4 partial pressures was observed, while the dependence on steam partial pressure was positive. Isotopic labeling experiments showed that the negative effect of H-2 and C2H4 could not be attributed to the reverse reaction, but rather to competitive adsorption at the active sites for dehydrogenation. The observed reaction rate with respect to C2H6 was close to first order. By fitting the experimental data to the rate equation derived from the elementary steps of ethane dehydrogenation, the observed deviation from the first order could be explained by partial occupation of the surface by adsorbed surface species. Methane and ethyne were the main byproducts of the dehydrogenation reaction. Cofeed experiments with (CCH6)-C-13-C-12/(C2H)-C-12, indicated that both methane and ethyne are produced via ethene, and not directly from ethane. D-2/C2H6 cofeeding experiments revealed further that one H at the time is replaced with D in ethene, that addition of CO, does not affect the H-D distribution in ethene and ethyne and that practically no H-D exchange takes place over the support. The activation energy of the dehydrogenation of ethane over the present catalyst has in a previous study been determined to 116 kJ/mol [Virnovskaia, A. Morandi, S.; Rytter, E.; Ghiotti, G.; Olsbye, U. Characterization of Pt,Sn/Mg(Al)O Catalysts for Light Alkane Dehydrogenation by FT-IR Spectroscopy and Catalytic Measurements. J. Phys. Chem. C 2007, 111, 14732]. The apparent activation energy for ethene hydrogenation over the same catalyst could be determined to -40 kJ/mol in the temperature range 497-596 degrees C, indicating that in this temperature range the decrease in surface coverage with temperature overcompensates the increase of the rate constant of the rate determining step of the hydrogenation reaction.