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Engineering of surface environment of Pd nanoparticle catalysts on carbon support with pyrene-thiol ligands for semi-hydrogenation of alkynes.

A new type of pyrene-thiol-derivative-modified Pd nanoparticle (NP) catalyst on a carbon black support for the efficient semi-hydrogenation of alkynes to alkenes is reported herein. Colloidal Pd NPs surrounded by pyrene-thiol modifiers were prepared using the two-phase Brust method, followed by impregnation of carbon black materials. Based on the structural characterization of the prepared catalyst (PyC12S-Pd/VC) by NMR, UV-vis, FT-IR, TEM, HAADF-STEM, Pd K-edge XAFS, XRD, N2 adsorption and XPS, we show that highly dispersed Pd NPs are immobilized on the catalysts via π-π interaction between pyrene groups bounded to the Pd NPs and carbon black supports. PyC12S-Pd/VC efficiently catalyzes the alkyne semi-hydrogenation reaction while maintaining high alkene selectivity; an alkene selectivity of 94% is attained at 98% conversion after 5 h of reaction, and the selectivity was retained around 80% during 10 h of reaction. This performance is superior to that of a catalyst without pyrene groups and that of a commercial Lindlar catalyst. The steric hindrance of pyrene groups restricts access by the substrates to Pd NP surfaces, suppressing the unfavorable over-hydrogenation of alkenes to alkanes, which is revealed by the solvent and substrate dependency on the catalytic performance and DFT calculation study. Furthermore, the high selectivity and stability of PyC12S-Pd/VC are caused by the strong interaction between pyrene groups and carbon supports, which prevents the separation of pyrene modifiers and the leaching or sintering of Pd NPs during the catalytic reaction. It is demonstrated that the combination of Pd NPs, pyrene-thiol modifiers, and carbon supports offers high activity, alkene selectivity, and stability in the semi-hydrogenation reaction.