Preparation of Pd particles coated with mesoporous silica layers

Pre-formed polycrystalline Palladium aggregates (ca. 200 nm in diameter) were coated by thin mesoporous silica layers to form metal-mesoporous silica composite materials. To synthesize the composites, Pd aggregates were dispersed in an aqueous solution of C16H33N(CH3)3Br to which tetraethylorthosilicate (TEOS) was then added to give composite precipitates. The surfactant template in the precipitate was subsequently removed by extraction with an acetic acid/ethanol solvent. TEM images of the composite (containing as much as 65 wt% Pd) revealed that the Pd aggregates were fully surrounded by mesoporous silica layers (ca. 50 nm in thickness) having ordered pores that penetrate to large extent from the exterior to the surface of Pd aggregates. When the materials were used as catalysts for cinnamaldehyde hydrogenation in toluene, the reactant molecules were concentrated in the meso channels by a factor of ca. 3. Accordingly, the composites showed comparable or higher activity than that observed for neat Pd aggregates. This study highlights the preparation of unique metal-mesoporous silica composite materials by simple synthetic procedures and which are usable as active Pd catalysts.