Role of Surface Basic Sites in Sonogashira Coupling Reaction over Ca5(PO4)3OH Supported Pd Catalyst: Investigation by Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Hydroxyapatite [Ca5(PO4)3(OH); HAP] supported Pd catalyst has been identified as an efficient and reusable heterogeneous catalyst for the Sonogashira coupling reaction under copper and phosphine-free conditions. The 2 wt % Pd/HAP offered excellent yields (96%) compared to 2 wt % Pd/MgO and 2 wt % Pd/Al2O3 catalysts. The formic acid adsorbed diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results revealed that moderate basic sites on 2 wt % Pd/HAP are found to be selective for the Sonogashira coupling reaction. Strong basic sites present on the Pd/MgO catalyst demonstrated homocoupling of the substrate during the Sonogashira reaction. The physicochemical characteristics of the catalysts are rationalized by temperature-programmed desorption of CO2, H2-temperature-programmed reduction, formic acid adsorbed DRIFTS, Brunnauer-Emmet-Teller surface area, X-ray photoelectron spectroscopy, transmission electron microscopy, and CO chemisorption techniques. The 2 wt % Pd/HAP catalyst was recovered and reused for four recycles, showing consistent activity and selectivity. The nature and strength of the surface basic sites influence the coupling reaction through oxidative addition and reductive elimination on surface Pd sites.