Catalytic investigation of Pd particles supported on MCM-41 for the selective hydrogenations of terminal and internal alkynes

Pd-MCM-41 samples with different Pd contents were synthetized by using PdCl2, tetraethyl orthosilicate (TEOS) and the cationic surfactant cetyltrimethylammonium bromide (CTAB). The template-free materials were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), N2 sorption, H2 chemisorption and TEM measurements. The representative samples Pd-MCM(1.39) and Pd-MCM(5.85) had Pd contents 1.39 and 5.85%, respectively. Although the formation of the Pd particles was found to decrease the crystalline character of the host material, the structure of the MCM-41 framework was retained for both samples. The evidence that the Pd crystallite sizes exceeded 5 nm suggested that the Pd particles were mainly situated on the surface of MCM-41, although some of the Pd content of Pd-MCM(5.85) may also be embedded in the mesopores. Accordingly, the Pd-MCMs can essentially be regarded as MCM-41-supported Pd materials. Both samples proved to be active and selective catalysts for the liquid-phase semihydrogenations of phenylacetylene, 3-butyn-1-ol, 4-octyne and 1-phenyl-1-butyne. The initial activity of Pd-MCM(1.39) surpassed that of Pd-MCM(5.85) for each reaction, indicating that the catalytic activity was dependent on the Pd crystallite size. In contrast, the selectivity of alkene formation was irrespective of the particle diameter for most reactants. For the semihydrogenation of 4-octyne, Pd-MCM(1.39) proved to be an extremely efficient catalyst. The pronounced (Z)-alkene stereoselectivities obtained for the hydrogenations of internal alkynes over both Pd-MCMs may be attributed to the participation of high-coordination terrace atoms as active sites. The catalytic activities of the Pd-MCMs were considerably higher than those of silica-supported Pd catalysts.