Deep eutectic solvent mediated synthesis of nickel nanoparticles as effective catalyst for hydrogenation of nitroaromatics

Abstract

Deep eutectic solvents (DESs), grounded in sustainability principles, increasingly supplanted conventional, hazardous, and volatile reagents, capturing considerable attention from the research and industrial communities. This study introduced a DES-mediated methodology for producing morphology and size-controlled Ni nanoparticles (NPs), emphasizing their catalytic potential in the reduction of 4-nitrophenol. Three distinct DES formulations, namely choline chloride and ethylene glycol (ChCl/EG), choline chloride and diethylene glycol (ChCl/DEG), and choline chloride and urea (ChCl/urea), were employed as solvent matrices for the synthesis of Ni-ChCl/EG, Ni-ChCl/DEG, and Ni-ChCl/urea, respectively. Morphology and size-controlled Ni nanomaterials were synthesized through a simple chemical reduction method in the absence of any stabilizers or capping agents. The phase purity, structural composition, morphology, and surface functionality were characterized using X-ray Diffraction (XRD), Energy Dispersive X-ray (EDX), Field Emission Scanning Electron Microscope (FESEM) techniques, and Fourier Transform Infrared Spectroscopy (FTIR). XRD and EDX confirmed the formation of phase-pure Ni with a face-centered cubic (fcc) structure in Ni-ChCl/EG and Ni-ChCl/DEG, while Ni-ChCl/urea exhibited both hexagonal close-packed (hcp) and fcc structures due to the formation of a Ni/Ni(OH)2 nanocomposite. EDX also revealed the presence of a nitrogen-doped carbon matrix in Ni-ChCl/urea. FESEM images confirmed the formation of spike ball, nanoflower, and nanograin-like morphology for Ni-ChCl/EG, Ni-ChCl/DEG, and Ni-ChCl/urea, respectively. Among the synthesized materials, Ni-ChCl/urea exhibited the best catalytic performance in the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The kinetic rate constant of Ni-ChCl/urea (kapp = 0.1559 min-1) was approximately five-fold larger than that of Ni-ChCl/EG (kapp = 0.0369 min-1) and about three-fold larger than Ni-ChCl/DEG (kapp = 0.0593 min-1). The significant difference in catalytic activity was attributed to variations in morphology and size. The superior catalytic performance of Ni-ChCl/urea was due to its amorphous nature, which created surface defects that enriched surface activity, as well as its reduced size, which exposed a higher active surface area. Additionally, the presence of nitrogen-doped carbon in Ni-ChCl/urea improved its electrical properties, facilitating easier electron transport during the reduction of 4-NP.