Copper-free sonogashira coupling reactions using palladium supported on shell particles as reusable heterogeneous catalyst

A novel palladium catalyst supported on individual calcium carbonate plates (Pd/ICCP) has been prepared from Asian green mussel (Perna viridis) shell for the copper-free Sonogashira coupling reaction. Treatment of virgin shell under sonication with hydrogen peroxide results in the formation of ICCP. Impregnation of ICCP with Pd(0) generates the a Pd nanoparticle embedded on ICCP. Morphology of the prepared Pd/ICCP catalyst has been fully characterized by SEM, TEM, EDX, XRD, ICP-OES, TGA and IR showing that the ICCP supported catalyst are uniform with 3 micron size and Pd nanoparticle is uniformly dispersed onto the surface of ICCP at 6.1% (wt./wt.). For catalytic performance, the optimized study reveals that the use of 1 mol% of Pd/ICCP in the present of potassium carbonate in ethanol can catalyze the copper-free Sonogashira coupling reaction between a variety of aryl iodides and terminal alkynes at 77 0C without using external ligand to provide the corresponding diarylethynes in 57-98 %yields. The catalytic activity and recyclability of the prepared Pd/ICCP along with the shell particle and calcium carbonate supported palladium (Pd/SP and Pd/CaCO3) catalysts have been evaluated. Pd/ICCP demonstrates comparable catalytic activity with Pd/CaCO3 but higher than Pd/SP. Importantly, Pd/ICCP exhibits better reusability than both Pd/SP and Pd/CaCO3. The remarkable Pd/SP activity and stability has been attributed to the well uniform and the chelation of Pd (0) with the surface protein of the supported ICCP.