Lindlar catalyst

Abstract: Site modification and isolation through selective poisoning comprise an effective strategy to enhance the selectivity of palladium catalysts in the partial hydrogenation of triple bonds in acetylenic compounds. The recent emergence of supported hybrid materials matching the stereo- and chemoselectivity of the classical Lindlar catalyst holds promise to revolutionize palladium-catalyzed hydrogenations, and will benefit from an in-depth understanding of these new materials. In this work, we compare the performance of bare, lead-poisoned, and ligand-modified palladium catalysts in the hydrogenation of diverse alkynes. Catalytic tests, conducted in a continuous-flow three-phase reactor, coupled with theoretical calculations and characterization methods, enable elucidation of the structural origins of the observed selectivity patterns. Distinctions in the catalytic performance are correlated with the relative accessibility of the active site to the organic substrate, and with the adsorption configuration and strength, depending on the ensemble size and surface potentials. This explains the role of the ligand in the colloidally prepared catalysts in promoting superior performance in the hydrogenation of terminal and internal alkynes, and short-chain alkynols. In contrast, the greater accessibility of the active surface of the Pd-Pb alloy and the absence of polar groups are shown to be favorable in the conversion of alkynes containing long aliphatic chains and/or ketone groups. These findings provide detailed insights for the advanced design of supported nanostructured catalysts. Hybrid nanocatalysts: The classical Lindlar and the newly developed NanoSelectTM catalysts are confronted in the semi-hydrogenation of alkynes (see figure). Systematic testing under continuous-flow three-phase conditions, coupled with detailed characterization analyses and molecular simulations, enable the understanding of the structure of the catalysts and the associated activity and selectivity patterns for a wide range of acetylenic compounds.

Abstract: Metallic nanocrystals (NCs) with well-defined sizes and shapes represent a new family of model systems for establishing structure-function relationships in heterogeneous catalysis. Here in this study, we show that catalyst poisoning can be utilized as an efficient strategy for nanocrystals shape and composition control, as well as a way to tune the catalytic activity of catalysts. Lead species, a well-known poison for noble-metal catalysts, was investigated in the growth of Pd NCs. We discovered that Pb atoms can be incorporated into the lattice of Pd NCs and form Pd-Pb alloy NCs with tunable composition and crystal facets. As model catalysts, the alloy NCs with different compositions showed different selectivity in the semihydrogenation of phenylacetylene. Pd-Pb alloy NCs with better selectivity than that of the commercial Lindlar catalyst were discovered. This study exemplified that the poisoning effect in catalysis can be explored as efficient shape-directing reagents in NC growth, and more importantly, as a strategy to tailor the performance of catalysts with high selectivity.