Abstract
A heterogeneous Janus-type palladium interphase catalyst was obtained by selective surface modification of a hollow mesoporous silica material. The catalyst comprises hydrophobic octyl groups on one side of the silica nanosheets and single-site bis-imidazoline dichlorido palladium(II) complexes on the other. The structure of this composite material has been analyzed by means of elemental analysis, atomic absorption spectroscopy, BET surface analysis, TGA, SEM and solid-state CP-MAS 13C and 29Si NMR spectroscopy. The catalyst showed extraordinary activity for the aqueous-phase oxidation of styrene to acetophenone using 30% hydrogen peroxide as the oxidant. An 88% yield of acetophenone could be achieved after 60 min.
Highlights
The Wacker oxidation is an industrial method for the conversion of ethylene to acetaldehyde
Hollow spheres with octyl groups (R) that are covalently immobilized on the inner surface were prepared by using a modified Stöber method for the preparation of monodisperse silica particles followed by removing of the surfactant to yield the mesoporous hollow structure 1
The loading of palladium in the catalyst was determined based on an atomic absorption spectroscopy (AAS) to be 0.22 mmol·g–1
Summary
The Wacker oxidation is an industrial method for the conversion of ethylene to acetaldehyde It has been performed by employing catalytic amounts of palladium together with stoichiometric amounts of copper salts as the co-catalyst [1]. H2 O2 can be considered as a less hazardous oxidant since water is the sole by-product of the reaction and the use of copper as co-catalyst is not necessary. Heterogeneous catalysts allow a simple recovery and separation of the palladium source. They encounter a number of limitations for the Wacker oxidation: the reaction rate of the heterogeneous catalyst is lower, and longer reaction times are required to achieve desirable conversions. In some cases, up to 10 equiv. of the oxidant had to be added [9], which is not a cost-effective and sustainable protocol for an oxidation reaction
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