Simulation of the Kinetics from Isotopic Exchange Experiments on Mo/V/W‐Mixed Oxide Catalysts for the Oxidation of Acrolein

Abstract

Mittels der Isotopenaustauschmethode SSITKA (steady‐state isotopic‐transient kinetic analysis) ergaben sich Rückschlüsse zum Mechanismus der Partialoxidation von Acrolein zu Acrylsäure an Mo/V/W‐Mischoxidkatalysatoren. Der Isotopenaustausch erfolgte mit den beiden Sauerstoffisotopen 16O2 und 18O2. Dabei findet am Katalysator ein Austausch des Carbonylsauerstoffs im Acrolein mit Festkörpersauerstoff statt. Zusätzlich wird die Kinetik der unterschiedlichen Isotopologenspezies durch eine Austauschreaktion von Sauerstoff zwischen belegten und unbelegten Zentren an der Oberfläche und in der Volumenphase des Katalysators signifikant beeinflusst. Gestützt durch einen erweiterten Modellvorschlag zum Mechanismus konnten über Simulationen die experimentell gewonnenen Isotopologenkinetiken verifiziert werden.

Activity Hysteresis during Cyclic Temperature‐Programmed Reactions in the Partial Oxidation of Acrolein to Acrylic Acid

“…The role of bulk oxygen in the catalytic mechanism has already been discussed in previous works , , , , , , . This participation is considered by a reversible reaction between bulk and surface and, thus, is directly related to the concentration of reduced bulk and surface sites or to the bulk and surface degree of reduction, respectively.…”

“…From previous works, the individual reaction steps (reversible adsorption of ACR by the formation of a surface acetal, conversion of the surface acetal into a surface acrylate (redox step), and desorption of AA) are known , , but are of less importance in a first approach for modeling of the activity hysteresis. Nevertheless, such details are required for the explanation of the results from the steady‐state isotopic transient kinetic analysis experiments or the activity‐enhancing effects of water , , , , . In addition to selective oxidation of ACR (Eqs.…”

“…D ax is the axial dispersion coefficient, u is the flow velocity, both in direction of the spatial coordinate x , and ν i,j are the stoichiometric coefficients of the component i in the reaction j. D ax was estimated from residence time experiments to a constant value of 2 × 10 −5 m 2 s −1 , , . Thus, an independency of D ax from temperature and gas composition is approximated.…”

“…From X‐ray absorption (XAS) measurements it is known that Mo +VI and V +V can be reduced under reaction‐relevant conditions to Mo +IV and V +IV , respectively. Tungsten as W +VI does not change its oxidation state , , , . Hence, 0.27 mmol of oxygen atoms could be removed from 50 mg mixed oxide.…”

“…However, at elementary level, the mechanism is still not fully understood. For this purpose, additional aspects, such as the effect of water , , , or the participation of oxygen from the bulk phase of the mixed oxide have to be considered .…”