Origin of ligand-driven selectivity in alkyne semihydrogenation over silica-supported copper nanoparticles

Copper nanoparticles supported on silica are an earth-abundant catalyst efficient for alkyne hydrogenation, yielding high selectivity towards (Z)-olefin when organic ligands are added. Here, we investigate the origin of this selectivity by studying kinetics of the hydrogenation of the prototypical 1-phenyl-1-propyne substrate. Hydrogenation occurs stepwise on the unmodified catalyst, with first the formation of the (Z)-alkene followed by overhydrogenation to the alkane. Adsorption isotherms and kinetic modelling evidence that these consecutive processes result from the high adsorption constant of the alkyne onto Cu compared to that of the alkene, as confirmed by DFT calculations. Ligands (tricyclohexylphosphine and an NHC) display adsorption constants similar to that of the alkyne, which allows for its hydrogenation but leads to the displacement of the generated alkene from the catalyst, thereby preventing the overhydrogenation. Our findings thus rationalize the observed selectivity and guide the choice of ligands for selective semihydrogenation to (Z)-olefins with Cu -based catalysts. (C) 2018 Elsevier Inc. All rights reserved.