Ordered Mesoporous Silica Encased Palladium Catalyst for Enhancing Activity and Selectivity in Semihydrogenation

The semihydrogenation of alkynes to alkenes is the main kind of reaction in the chemical industry. This study reports an ordered mesoporous silica encapsulated Pd catalyst for the efficient semihydrogenation of aromatic alkynes. In the selective hydrogenation of phenylacetylene to styrene, Pd@o-SiO2, wherein subnanometric Pd clusters are encapsulated within the well-ordered channels of silica, exhibits a hydrogenation rate that is approximately 13-fold higher than that of the Pd/C catalyst at 298 K. The Pd@o-SiO2 catalyst with subnanometric Pd clusters shows high styrene selectivity even at nearly full conversion, whereas the 1% Pd@o-SiO2 catalyst with Pd nanoparticles favors the overhydrogenation of styrene to form ethylbenzene. Detailed characterization indicates that the ordered silica encapsulated Pd clusters modulate the adsorption behavior of phenylacetylene and facilitate the desorption of styrene, which are crucial for enhancing styrene selectivity. Further recycling experiments demonstrate the excellent stability of the Pd@o-SiO2 catalyst due to the SiO2 framework, which protects and confines the Pd species against growth and coalescence.