Preparation of Noble-Metal Colloids in Block Copolymer Micelles and Their Catalytic Properties in Hydrogenation

Abstract

Monometallic and bimetallic colloids were prepared in micelles of the block copolymer polystyrene-b-poly-4-vinylpyridine in toluene and analyzed by electron microscopy and various techniques of X-ray analysis. These metal colloids were studied in hydrogenation of cyclohexene, 1,3-cyclooctadiene, and 1,3-cyclohexadiene. A strong influence of the synthetic pathway to the colloids and the type of reducing agent on the catalytic activity of the colloids was found. The lowest activity was observed for N2H4·H2O reduction which is related to a morphology where only a small number of noble-metal colloids is embedded in the micelle core. The highest activity was obtained for the super-hydride reduction where the data suggest the existence of many metal clusters per micelle. The bimetallic Au/Pd colloids with metal ratios 1/5, 1/4, and 1/3 show the highest activity in hydrogenation of cyclohexene to cyclohexane. The Pd monometallic and Au/Pd bimetallic colloids are also rather selective catalysts (both in the homogeneous and a heterogeneous modification), as shown by the hydrogenation of 1,3-cyclooctadiene and 1,3-cyclohexadiene to the corresponding cycloalkenes.