In situ generation of Pd nanoparticles in MCM-41 and catalytic applications in liquid-phase alkyne hydrogenations

Pd–MCM-41 materials were synthetized by using the cationic surfactant tetradecyltrimethylammonium bromide (C14TABr) for both the stabilization of the Pd particles and the construction of the mesoporous structure of MCM-41. Two Pd–MCM-41 samples, for which the Pd particles were generated before and after formation of the MCM-41 framework (Pd-A and Pd-B, respectively), were investigated. Structural characterization of the samples was carried out by ICP-AES, N2 sorption, XRD and TEM measurements. It was established that the highly ordered structure of MCM-41 was not appreciably affected by the formation of the Pd particles. Further, a similar particle size control was achieved for both Pd–MCM samples. However, both the location and the size distribution of the Pd particles were found to depend strongly on the preparation procedure. For Pd-A, the Pd nanoparticles were essentially situated on the external surface of MCM-41, whereas for Pd-B, the particles were found to be encapsulated inside the mesopores. For the liquid-phase hydrogenations of alkynes, the catalytic activity of Pd-A clearly surpassed that of Pd-B, indicating that the external Pd crystallites were more readily accessible for the reactants than those incorporated in the MCM-41 framework. The limited activity observed for Pd-B was attributed to mass transport limitations due to diffusion of the reactants into the mesopores of the MCM-41 host.

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