Homogeneous Redox Catalysts Based on Heteropoly Acid Solutions: 5. Developing a Two-Stage Process for Propylene Oxidation to Acetone: A Review

Studies on the development of a homogeneous chloride-free two-stage process (target reaction (first stage) and catalyst regeneration (second stage)) for propylene oxidation to acetone with oxygen in the presence of a \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + Mo–V–P heteropoly acid (HPA-x, where x is the number of V atoms) catalyst have been described. A kinetic equation of the target reaction has been derived; a mechanism of the reaction has been proposed. It has been shown that the most efficient catalysts are those based on high-vanadium modified (non-Keggin) HPA-x_M compositions. It has been found that the kinetics of C₃H₆ oxidation in the presence of Keggin HPA-x and HPA-x_M solutions is identical; however, only \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-x_M catalysts are technologically feasible. They exhibit high thermal stability (up to 180°C) and, therefore, can be rapidly regenerated with oxygen. Owing to this feature, catalysts based on HPA-x_M compare favorably with those based on Keggin HPAs-x, the thermal stability of which is limited to 140°C. The possibility of rapidly regenerating the catalyst has made it possible to close the two-stage catalytic cycle of C₃H₆ oxidation to acetone with oxygen and opened up prospects for the practical implementation of the process in the presence of \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-x_M. The catalyst has effectively passed stability tests.