Physicochemical properties of catalysts based on aqueous solutions of Mo–V–phosphoric heteropoly acids

Various organic substrates can be selectively oxidized with O2 in two stages using aqueous solutions of Mo–V–phosphoric heteropoly acids (HPA) as catalysts. In stage (1), a substrate is oxidized with a HPA solution to the desired product that is separated from the reduced form of HPA. The latter is oxidized with O2 in stage (2) closing a catalytic cycle. All physicochemical properties of the homogeneous catalysts based on HPA solutions continuously alter during these redox processes. Using as an example a solution of the modified high-vanadium HPA having gross composition H16P3Mo17V7O84 (HPA-7′), it was shown that viscosity and pH of this solution reach their maxima after reaction (1) and attain their minima after reaction (2). The reverse situation is observed for redox potential of the HPA-7′ solution. Many-cycled testing of 1-butene oxidation in stages (1) + (2) in the presence of the Pd + HPA-7′ catalyst confirms that the alterations of physicochemical properties of the catalyst are completely reversible.

Graphical abstractIn processes of catalytic oxidation of organic substrate, Su, with O2 via reactions (1) + (2) going on in the presence of Mo–V–phosphoric heteropoly acid (HPA) solution, all physicochemical properties of this solution, i.e. viscosity, density, redox potential, and pH continuously alter during these reactions.equation(1)m2Su+m2H2O+HPA→m2SuO+HmHPAequation(2)HmHPA+m4O2→HPA+m2H2OBy the example of one high-vanadium HPA solution, it is shown that the alterations of the properties in redox processes (1) + (2) are completely reversible. Such alterations are to be taken into account in the course of elaboration of industrial processes (1) + (2).Figure optionsDownload full-size imageDownload as PowerPoint slide