Highly Selective Electrochemical Semi-Hydrogenation Via a Palladium Membrane Reactor.

The selective semihydrogenation of alkynes is a critical industrial process, yet conventional palladium-based methods often suffer from overhydrogenation, leading to poor alkene selectivity. To address this, we report an electrochemical palladium membrane reactor that employs cysteamine, a simple aliphatic amino thiol, as a surface modifier. This system achieves >99% selectivity for the semihydrogenation of phenylacetylene to styrene at high current densities up to 300 mA/cm2, under ambient conditions and continuous electrolysis. Spectroscopic analysis and density functional theory calculations indicate that cysteamine forms a surface S2- adlayer, resulting in a competitive adsorption mechanism where the S2- species prevents styrene readsorption without impeding the initial hydrogenation of phenylacetylene. This work demonstrates a distinct and operationally simple strategy, using a specific sulfur modifier under continuous hydrogen flux, to achieve ultrahigh selectivity in a targeted alkyne semihydrogenation.