Mechanistic Study on the Selective Oxidation of Acrolein to Acrylic Acid concerning the Role of Water

Abstract

The activity of Mo/V/W mixed oxide catalyst in the gas phase oxidation of acrolein to acrylic acid is tremendously influenced by addition of fed water. This effect of water is investigated and connected to the catalytic mechanism by several stationary kinetic measurements and transient response methods, such as temperature‐programmed reduction, oxidation and desorption experiments, special concentration‐programmed methods as well as H218O‐SSITKA and D2O‐SSITKA experiments. Furthermore, kinetic modeling, based on results of H218O‐SSITKA experiments in a wide temperature and feed water content range gives access to kinetic parameters of single reaction steps in the catalytic mechanism. The developed mechanism is not only able to describe conversion and selectivities, inclusively the effect of water, but also describes the 18O‐degree of labeling of all reaction components quantitatively.

True Catalytically Active Structure in Mo–V-Based Mixed Oxide Catalysts for Selective Oxidation of Acrolein

“…It is well recognized that ACR oxidation over the Mo–V-based mixed oxide catalysts takes place according to the following scheme: (1) adsorption of ACR over the Brønsted acid site (B acid site) on the catalyst surface and its activation to form a surface acetal, (2) abstraction of C-H from the surface acetal, resulting in the formation of a surface acrylate or acyl ester, (3) desorption of the surface acyl ester as AA by the nucleophilic attack of water, leaving an oxygen vacancy, and (4) replenishment of the oxygen vacancy either by gaseous oxygen or by the diffusion of lattice oxygen from the bulk . In this reaction scheme, the rates of (1) and (3) are known to be promoted by the feed of water. − Considering the effects of W introduction on the ACR oxidation, it can be speculated that W promotes the dissociative adsorption of water to create a Brønsted acid site as the adsorption site for ACR. Although this proposal has been theoretically and experimentally verified in W-V mixed oxide systems, , effects of water feed on acid sites were investigated in our experiments by in situ IR experiments after the adsorption of pyridine followed by water introduction at the reaction temperature of ACR oxidation (250 °C).…”