Heteropolyanions as Redox Components in Heterogeneous Wacker Oxidation Catalysts

Heteropolyanions (HPAs) of the Keggin type have been successfully applied as redox components in heterogeneous Wacker catalysts. The catalysts consist of a silica support covered with a layer of HPA of the series H3+nPVnMo12−nO40 on which a submonolayer of palladium sulfate is deposited. The initial butanone yield in the oxidation of 1-butene at 343 K is approximately 0.10 g butanone per gram of catalyst per hour. The steady-state activity is, however, a factor of 10 lower due to the slow reoxidation of reduced HPA. The rate of reoxidation and hence the steady-state activity increases with increasing number (n) of vanadium atoms per Keggin unit and when the protons of the HPA are replaced by Cu2+ or by Ni2+. Even higher activity is obtained with palladium salts of heteropolyacids. In these palladium salts the palladium reaction centre and the redox component are combined in one complex. The butanone selectivity of the catalysts with n > 0 is high, viz. >95%, and increases with increasing values of n to more than 98%. The butanone selectivity also increases when the protons are exchanged for metal cations, such as Cu2+, Ni2+, Pd2+, or Cs+. The stability of the Keggin units under reaction conditions is high. Under Wacker oxidation conditions the HPAs are partly reduced, but DRIFT and ESR spectroscopy and TPR analysis of used catalysts show that the Keggin structure remains intact. Reoxidation of the reduced HPAs is, however, slow under reaction conditions.