Simultaneous Determination of Reaction Kinetics and Oxygen Activity in an Oxidic Multicomponent Catalyst during Partial Oxidations

Abstract

The partial oxidation of an aldehyde to a carboxylic acid has been studied over an oxidic multicomponent catalyst, mainly based on Mo, V, and Cu. The experimental setup enabled the simultaneous determination of reaction kinetics and the oxygen activity of the catalyst under working conditions. The kinetic measurements were performed by monitoring the gas phase composition along the length of a fixed bed of catalyst. The reactor was treated as an isothermal plug flow system. The reaction kinetics can be described by a simple triangular network consisting of the main reaction (aldehyde to carboxylic acid), a consecutive reaction (carboxylic acid to by-products), and a parallel reaction (aldehyde to by-products). The simultaneous determination of the oxygen activity in the catalyst has been realized by use of a solid electrolyte potentiometry (SEP) cell, connected to the apparatus. The cell consists of an oxygen-ion-conducting solid electrolyte (ZrO2+8.5 wt% Y2O3) coated with a platinum reference electrode on one side and with the catalytically active electrode on the other side, the latter being made of the same catalyst material used in the tubular reactor. While the measuring electrode was in contact with the gas phase to be analyzed, the reference electrode was always flushed with air. The oxygen activity of the catalyst is derived from the measured potential difference between both electrodes. The results clearly show that the catalyst under working conditions is always in a reduced state rather than in equilibrium with the oxygen in the surrounding gas phase. The axial profile of the oxygen activity along the fixed bed reactor indicates the existence of two different oxidation states of the catalyst. The more reduced state in the first part of the reactor reflects the strong interaction of the aldehyde with the catalyst; the less reduced one in the final part of the reactor indicates the weaker interaction of the catalyst with the intermediate carboxylic acid. Both the kinetic and potentiometric measurements lead to consistent results.