Palladium salts of heteropolyanions as catalysts in heterogeneous Wacker oxidation of 1-butene

Abstract

Palladium salts of heteropolyanions, of the Keggin-series H 3+ n PV n Mo 12− n O 40, supported on silica, have been successfully applied as catalysts in the gas-phase Wacker oxidation of 1-butene. In such catalysts the palladium reaction centre (Pd 2+) and the redox component (HPA) are combined in one complex. At 343 K and atmospheric pressure a high initial yield of more than 0.16 g butanone/g of catalyst/h, in combination with a very high butanone selectivity of more than 98%, can be obtained. In the steady-state, the activity of the catalysts is more than a factor ten lower than initially, due to slow reoxidation of reduced palladium-heteropolyanion complexes. The rate of reoxidation increases with increasing number of vanadium atoms per Keggin unit. Under reaction conditions the PdHPAs are reduced by approximately five electrons per Keggin unit. DRIFT spectra of the used catalyst show that the Keggin unit is not affected by this reduction and that no oxygen is removed from the HPA. The DRIFT spectra also show that the reduction and the reoxidation at reaction temperature in the absence of water is fast in comparison with the standard reaction conditions.