Butene oxidative dehydrogenation (OXD) on a manganese iron oxide catalyst was studied in batch recirculation and microcatalytic pulse reactors at temperatures between 300C and 400C. Mechanistic features of the reaction were examined using ('14)C-labeled butadiene (Neiman method), deuterium labeled butene (isotopic tracer technique), and ('18)O-labeled carbon dioxide (oxygen isotope exchange) experiments. Temperature programmed desorption (TPD) experiments were used to evaluate desorption kinetics. Solid state changes in the catalyst were examined through x-ray diffraction, magnetization, magnetic susceptibility, thermogravimetric analysis, and electrical conductivity measurements.
Reaction products consist of 1,3-butadiene, carbon dioxide, water, and butene isomers. Chromatographic analyses of the reaction products indicated negligible production of carbon monoxide during reaction. In addition, the TPD experiments demonstrated that carbon monoxide is readily oxidized over the catalyst. Initial rates of formation of butadiene and carbon dioxide are zero order in both oxygen and butene. The reactions are inhibited by product butadiene, where the low conversion data is modeled adequately by a Langmuir-Hinshelwood type rate expression. The activation energy for butadiene formation is 35.2 Kcal/mole, and that for carbon dioxide production is 37.5 Kcal/mole. Microcatalytic pulse experiments carried out in the absence of gas phase oxygen indicated that the reactions may proceed by consuming lattice oxygen. Perdeuterated butene is less reactive than non-deuterated butene. Comparison of the rates of formation and analysis of isotopic composition of the products revealed significant kinetic isotope effects for both OXD and isomerization (Isotope effect for OXD = 2.1, Isotope effect for cis-2-butene to 1-butene isomerization = 1.7, both at 412C). Therefore, carbon-hydrogen bond cleavage is considered rate limiting.
Experimental data are consistent with an oxidation-reduction cycle involving either Fe('3+) or Mn('2+). Butene and oxygen molecules may adsorb into surface anion vacancies associated with either cation. The absence of intermolecular hydrogen-deuterium exchange during isotopic tracer experiments indicates that OXD and isomerization reaction rates are much greater than surface diffusion rates.
A deactivation mechanism consistent with solid measurements is proposed. Major crystallographic transitions occur during reaction and pretreatment operations in agreement with the thermodynamic phase relations for the iron-manganese-oxygen system. X-ray diffraction measurements confirmed that the catalyst aging process parallels the shift from a spinel to a hexagonal crystal structure. This change corresponds to the irreversible oxidation of Mn('2+) in the virgin catalyst to Mn('3+) in the aged catalyst.