Suzuki cross-coupling reaction catalyzed by palladium nanoparticles supported on calcium carbonate

Abstract

In this work, we report the first use of an eco-friendly palladium catalyst supported on individual calcium carbonate plates (Pd/ICCP) catalyst which has been made from shell waste to use as heterogeneous catalyst in aqueous Suzuki cross-coupling reaction. The Pd/ICCP is prepared from impregnation-reduction method and completely characterized by SEM, EDX, TEM, and ICP-OES techniques. The results indicate a high dispersion of palladium (0) nanoparticles (5-8 nm) on a highly uniform ICCP support (3 micrometer) providing palladium content of 5.5 %wt. For catalytic activity, the use of 2 mol% of Pd/ICCP is generally sufficient with potassium carbonate as a base for aqueous Suzuki cross-coupling reaction in two solvent systems including EtOH:H₂O (3:2) and CTAB/H₂O. Both reaction conditions successfully catalyze the reaction between various aryl halides (iodide and bromide) and arylboronic acids at 40 °C to give biaryl products in 70-100 % yields without remaining starting materials. In comparison with Pd/CaCO₃, Pd/SP and Pd/C, Pd/ICCP exhibits the highest reusability and lowest metal leaching. It can be reused up to nine times with NMR yield of more than 80% retained. The high catalytic activity and reusability of Pd/ICCP catalyst has been ascribed to the highly uniform ICCP support and strong chelation between Pd(0) and trace protein on ICCP support. Moreover, the microwave process has been successfully applied to Suzuki reaction using Pd/ICCP catalyst which can considerably reduce reaction time to only five minutes without left over starting material. Furthermore, Pd/ICCP catalyst can be adapted to the continuous-flow Suzuki reaction. The use of only 0.5%wt of Pd/ICCP can drive reaction completely within two hours of residence time. Therefore, Pd/ICCP is demonstrated to be a highly effective heterogeneous catalyst for aqueous Suzuki reaction in conventional batch, continuous-flow, and microwave processes.