Hydroxyapatite as a bifunctional nanocatalyst for solventless Henry reaction: a demonstration of morphology-dependent catalysis

In the current investigation, HA nanorods and nanoplates with a high surface area have been synthesized using the chemical precipitation method via alcogel formation employing l-arginine as a crystal growth modifier. The prepared HA particles have been characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM), High resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray analysis (EDXA) and nitrogen adsorption-desorption analysis. A representative morphology-dependent catalysis of the synthesized HA particles has been attempted using the Henry reaction under solventless conditions. The HA nanorods have been found to catalyze the Henry reaction with 100% conversion and 84.1% yield while the nanoplates were found to be catalytically inactive. A plausible mechanism supported by DFT studies has been proposed for the observed morphology-dependent catalysis of Henry reaction signifying the importance of surface Ca2+ ions in lowering the LUMO levels of the substrate besides assembling the reactants to proximity.