Physicochemical and corrosive properties of oxidation catalysts based on solutions of Mo-V-phosphoric heteropoly acids

Abstract

Modified aqueous solutions of Mo-V-phosphoric heteropoly acids (HPAs) are used as high-performance catalysts for the oxidation of substrates from different classes with oxygen. The oxidation of n-butenes to methyl ethyl ketone (MEK) with oxygen in the presence of aqueous solution (6 × 10−3 M Pd + 0.25 M HPA-7′), where HPA-7′ is a modified HPA whose molecular formula is H12P3Mo18V7O85, is an industrially important process. At the first Pd-catalyzed stage of the process, n-C4H8 is oxidized with HP acid. At the second stage, the catalyst is regenerated and the reduced form of HPA-7′ is oxidized with atmospheric oxygen, closing the catalytic cycle. In such two-stage redox processes, HPA solutions act as reversible oxidizers whose physicochemical properties are continuously changing. The modified solutions of HPAs are attractive because of their thermal stability and increased regeneration rates. However, no data have yet been reported in the literature about their physicochemical and corrosive properties. This undoubtedly hinders the practical application of processes involving these solutions. Using the 0.25 M HPA-7′ solution as an example, we show how the physicochemical properties of the catalyst change. During the first stage of the process, the pH, density (ρ), and viscosity (η) of the HPA solution increase to their maxima, but its redox potential (E) decreases to its minimum. At the second stage, E increases, while ρ, η, and pH decrease to their initial values. Thus, in the processes with the alternating reduction and oxidation of the HPA-based catalyst solution, the changes in the physicochemical properties of the catalyst are completely reversible. We first have obtained the data on the corrosive properties of the modified HPA-7′ solution with respect to metals and alloys widely used in industrial processes. The corrosion stability of various engineering materials against 0.25 M solution of HPA-7′ was found to decrease in the series Ti (∼0.009 mm year−1) > 06KhN28MDT > 10Kh17N13M2T > 12Kh18N10T > KhN65MV (∼0.68 mm year−1). The unique ability of the modified Mo-V-P HPA solutions to recover completely their properties after regeneration with oxygen permits one to use them successfully for long periods of time in the many-cycled processes. This opens up good prospects for the development of industrial homogeneous catalytic oxidative processes involving these HPAs.