Facile Synthesis of Natural Kaolin-Based CuO Catalyst: An Efficient Heterogeneous Catalyst for the Catalytic Reduction of 4-Nitrophenol

Water contamination by nitro compounds from various industrial processes has significantly contributed to environmental pollution and severely threatened aquatic ecosystems. Inexpensive, efficient, and environmentally benign catalysts are required for the catalytic reduction of such nitro compounds. This study reports the fabrication of various nanocomposites (NCs) of copper oxide nanoparticles (CuO NPs) supported on a kaolin sheet using straightforward and simple one-pot synthesis procedures that control the metal precursor to kaolin ratios. The selected as-synthesized CuO/kaolin NC was characterized using a range of advanced spectroscopic and microscopic methods, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), high-angle annular dark-field scanning TEM (HAADF-STEM), and N₂ adsorption/desorption analysis. The characterization results confirmed the successful incorporation of CuO NPs into the kaolin sheets, which had an average size of about 18.7 nm. The fabricated CuO/kaolin NC was used as a heterogeneous catalyst for the efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride (NaBH₄) in an aqueous system at room temperature. The catalyst demonstrated superior catalytic performance with high 4-NP conversion into 4-AP (>99%) in the aqueous phase (50 mL, 20 mg L^-1) within 6 min. In addition, the reaction kinetics of 4-NP reduction was also investigated, and the reaction followed the pseudo-first-order kinetics equation with the apparent rate constant of 1.76 min^-1. Furthermore, the Arrhenius and Eyring parameters for the catalytic hydrogenation reaction of 4-NP were calculated in order to investigate the catalytic reaction process in more detail. Moreover, the catalyst exhibited excellent reusability and stability over seven repeated catalytic test cycles without any noticeable decline in catalytic activity. Therefore, this paper could provide a novel, efficient, and environmentally promising clay-based non-noble metal oxide nanocatalyst to reduce nitro compounds in the aqueous system.