On the Homogeneity/Heterogeneity of Solid Copper Oxide Precatalysts in the Oxidative Homocoupling of Ethynylbenzene


​The heterogeneity of copper oxide supported on γ-Al2O3 (10CuO-Al2O3) and TiO2 (10CuO-TiO2) precatalysts is assessed in the oxidative homocoupling of ethynylbenzene in piperidine, with a focus on the effect of temperature, support, and solvent on copper leaching and catalyst reactivity. It is demonstrated that for this alkyne homocoupling reaction, elevated temperature is needed to solubilize copper and activate it, likely by production of soluble molecular copper-piperidine complexes. Once this activation occurred, the reaction can occur at room temperature, giving good yields of the desired product. Copper oxide is shown to interact differently on the two supports; CuO supported on TiO2 is more dispersed and accessible than CuO supported on γ-Al2O3. Less copper leaches from 10CuO-TiO2 than from 10CuO-Al2O3 at room temperature. Nevertheless, copper species that leach from 10CuO-TiO2 are more active than the species leached from 10CuO-Al2O3, as some of the copper leached from alumina likely exists as fine, inactive CuO nanoparticles. Piperidine has multiple roles in this reaction: it acts as solvent, base, as well as an important ligand. The precatalyst 10CuO-TiO2 is reused three times, with improved performance after each cycle. Nevertheless, it cannot be considered a recyclable catalyst, but rather a reusable solid precatalyst that produced new soluble active copper species in each cycle.