Metal-catalyzed chemical activation of calcium carbide: New way to hierarchical metal/alloy-on-carbon catalysts

Abstract

A simple and efficient strategy for the synthesis of “metal/alloy–on–carbon” catalysts was developed. A highly ordered extra pure graphite-like carbon material as a catalyst support was obtained after calcium carbide decomposition at 700 °C in a stream of gaseous chlorine. When Pd, Pt, Ag, Au, Co, Ni, Fe, Cu salts were added to calcium carbide prior to decomposition, a metal was reduced from a salt by elemental carbon, despite an oxidizing atmosphere. Metal particles were formed on the surface of the layered carbon material, covered with a thin layer of high–purity carbon and partially immersed in it. A catalytically active remaining metal was available for organic molecules due to the porous structure of carbon. At the same time, a metal was firmly held inside the carbon shells and was not washed out during a reaction and after washing procedures, keeping its catalytic activity unchanged for several cycles. Mixing various salts together before the reaction led to the alloys, and the ratio of the salts simply determined the ratio of the metals in the desired alloy. This approach allowed the synthesis of highly active metals/alloys on carbon catalysts with intrinsic hierarchical organization, which ensures a long-life cycle in the reaction. The obtained catalysts were successfully tested in the Suzuki-Miyaura cross-coupling reaction and showed excellent stability with a yield change less than 1% over several cycles (compared with a 64% yield decrease of commercial catalyst). The obtained catalysts have also shown very good performance in the semihydrogenation of C≡C bonds in phenylacetylene and other alkynes with selectivity up to 96% at 99% conversion.