Selective Oxidation of Methacrolein towards Methacrylic Acid on Mixed Oxide (Mo, V, W) Catalysts. Part 2. Variation of Catalyst Composition and Comparison with Acrolein Oxidation

While the oxidation of acrolein on mixed oxide (Mo, V, W) catalysts shows an exceptionally high selectivity with respect to acrylic acid, the selectivity of the analogous oxidation of methacrolein is only moderate. This is due to a considerably lower oxidation rate of methacrolein compared to that of acrolein, while the undesired subsequent oxidation of methacrylic acid is of the same order of magnitude as the oxidation of acrylic acid. Comprehensive kinetic studies in particular by transient experiments have led to a sound hypothesis to explain this difference and to show ways of catalyst improvement for the conversion of methacrolein to methacrylic acid. A moderate increase of methacrylic acid selectivity could be reached by variation of the Mo/V ratio of the mixed oxide. Surprisingly the addition of phosphoric acid and Cs-acetate effected a reduction of the subsequent oxidation of methacrylic acid without affecting the methacrolein oxidation, leading to a marked increase of the selectivity toward methacrylic acid. Furthermore, the comparison of methacrolein and acrolein oxidation results in a confirmation of the hypothesis that aldehyde oxidation and subsequent oxidation of acid occur on different domains of the mixed oxide catalyst

Selective Oxidation of Methacrolein towards Methacrylic Acid on Mixed Oxide (Mo, V, W) Catalysts. Part 1. Studies on Kinetics

Comprehensive kinetic studies of the oxidation of methacrolein on mixed oxide (Mo, V, W) catalysts have been undertaken in order to find ways of catalyst improvement to achieve a high selectivity toward methacrylic acid. Steady-state kinetic experiments were carried out in a differential recycle reactor that behaves like an ideal continuous stirred tank reactor (CSTR). A kinetic model based on the scheme of Mars−van Krevelen was developed for a fair representation of the reaction rates of methacrolein conversion, methacrylic acid formation, byproduct formation, and selectivity toward methacrylic acid. For a better understanding of the kinetics, transient experiments were carried out in an apparatus to render possible sorption studies as well as transient kinetic experiments, which are powerful tools to study independently both the oxidation of the aldehydes on the catalyst in the absence of oxygen and the reoxidation of the catalyst. Steady-state kinetic data and transient experiments agree well. It could be clearly shown that the conversion of methacrolein is mainly determined by the reoxidation of the catalyst and that the selective oxidation of the aldehydes and the consecutive oxidation of the acids occur on different domains of the catalyst

Steady state and transient kinetic experiments for rational catalyst design: Differences of acrolein and methacrolein oxidation on mixed oxide catalysts

Acrolein (ACR) is oxidised with exceptionally high selectivity and activity on Mo-Voxides to give acrylic acid (AA). On the other hand heteropoly acids are the best catalysts for the partial oxidation of methacrolein (MAC). Our experiments have shown, that the oxidation of both starting materials is independent of the further oxidation of the corresponding acids and vice versa [1, 2]. This implies that the selective oxidation and the further oxidation proceed on different domains of the catalyst. Crosswise exchange of catalysts and reaction leads to considerably lower selectivities of both reactions. The variation of the Mo/V-ratio of heteropoly acid catalysts (HPA) has shown that catalysts of high selectivity generally exhibit low activity [3]. Due to this, variation of redox properties of the catalyst was restricted to mixed oxides. Alteration of the Mo/V-ratio of the mixed oxide has revealed that the different catalyst domains can be separately influenced, thus leading to an optimized selectivity