Partial oxidation of ethanol to acetaldehyde over LaMnO₃-based perovskites: A kinetic study

A set of experiments was carried out in a continuous fixed-bed reactor to investigate the relative catalytic activities of LaMnO₃ and LaMn _0.95Pd_0.05O₃ for the partial oxidation of ethanol to acetaldehyde. Both catalysts were prepared by the sol-gel method. The resultant data have indicated that LaMn_0.95Pd_0.05O ₃ is more active than LaMnO₃ and that acetaldehyde selectivities of both are close at around 90%. To gain an in-depth understanding of perovskite's chemistry involved, kinetic analysis of the data has been conducted with the differential method. Accordingly, eight elementary reactions have been proposed by resorting to the Mars-van Krevelen redox cycle. Subsequently, these elementary reactions have been lumped into five steps comprising ethanol adsorption, oxygen adsorption, surface reaction, acetaldehyde desorption, and water desorption. This has rendered it possible to derive a set of rate equations based on the Langmuir-Hinshelwood-Hougen-Watson formalism. The exploration of these rate equations has revealed that surface reaction, evolving chemisorbed ethanol and oxygen, is rate-limiting. The estimated activation energies of 11.72 kcal/mol for LaMnO₃ and 12.44 kcal/mol for LaMn_0.95Pd_0.05O₃ are nearly identical; however, the pre-exponential factor of the latter is about twice the value of the former. This can be attributed to the better reducibility of Pd-promoted LaMnO₃, thereby leading to greater reactivity in ethanol partial oxidation.