The mechanism of catalytic oxidative dehydrogenation of n-butane to n-butenes and butadiene over Ni-Sn and Mn-Li oxide catalysts has been investigated. The techniques used include kinetic modeling, stable isotopic tracers, temperature programmed desorption, x-ray diffraction, electrical conductivity, and alternate reduction-oxidation tests.
The more selective catalyst for making the dehydrogenated products was Ni-Sn, an amorphous oxide mixture containing 47% Ni, 10% Sn, 6% P, 1% K, 4% S and 32% oxygen by weight. Its BET surface area is 49 m('2)/g. This catalyst gave only the products n-butenes, 1,3-butadiene, water and CO(,2) at temperatures up to 520(DEGREES)C. Over this catalyst n-butane is first converted to 1-butene which isomerizes to 2-butenes. These n-butenes may either be burned to CO(,2) or be oxidatively reduced to 1,3-butadiene which may further react to CO(,2).
Kinetic modeling of this reaction developed a Mars-Van Krevelen type reaction expression for both the oxidative dehydrogenation of 1-butene and deep oxidation of 1-butene and 1,3-butadiene. This model incorporates a Langmuir expression in the rate of dehydrogenation for the competitive adsorption of hydrocarbons. Parity plots indicated that agreement between predicted and measured behavior for the deep oxidation reactions and oxidative dehydrogenation of 1-butene was quite acceptable. However, the five parameter rate expressions did not predict the reaction of n-butane with reasonable accuracy because isomerization to the 2-butenes was not included. Additional information on the rate of isomerization of the n-butenes and the rate of reaction of the 2-butenes would enable this model to be expanded to provide reasonable predictions. The activation energies for deep oxidation of 1,3-butadiene and 1-butene are 23 and 30 kcal/mole. Both the oxidative dehydrogenation of 1-butene and the reoxidation of the catalyst, by gas phase oxygen, exhibit activation energies of 45 kcal/mole.
('18)O tracer studies determined that the oxygen on the Ni-Sn catalyst is easily exchanged by CO(,2) and that the mobile oxygen amounts to 1.9 monolayers at 380(DEGREES)C and 3.5 monolayers at 452(DEGREES)C.
A redox cycle is proposed as the mechanism of oxidative dehydrogenation of n-butane over the Ni-Sn catalyst. This involves using lattice oxygen to abstract the hydrogen atoms from the hydrocarbon to form the olefin or diolefin. The metal cation is reduced during the reaction of the hydrocarbon then is reoxidized by gas phase oxygen. This mechanism accounts for a C(,4)H(,9) species observed in the TPD experiments and is also consistent with the hydrogen-deuterium scrambling noted in the butene and butadiene products. The inhibition of the reaction rate by water is explained in this mechanism by adsorption-desorption equilibrium of water which limits the available oxygen sites.
Since the Mn-Li catalyst (7/1 molar Mn/Li ratio; 2.5 m('2)/g) is far less selective (its major products are CO(,2) and water) than the Ni-Sn catalyst, this material was not studied extensively. The observed product distribution was sensitive to the reactor design with a shallow broad reactor giving a peculiar selectivity hysteresis that could not be explained.