Microwave-assisted synthesis of copper-loaded polyamidoxime brushes as an efficient catalytic system for nitroarene reduction

Abstract

This study reports the development of copper-loaded polyamidoxime polymer brushes (Cu-AO-PB) as a heterogeneous catalyst via microwave-assisted graft polymerization for the first time. Microwave-assisted synthesis allows faster and more uniform heating, leading to shorter reaction times, more energy-efficiency and better control over catalyst morphology reducing production costs and environmental impact. In contrast to the traditional methods, catalysts prepared using microwave techniques often demonstrate superior activity and selectivity due to their unique structural characteristics. The influence of different parameters on the grafting of acrylonitrile onto vinyl sepiolite was studied and a maximum of 449% grafting-percentage was obtained with 89% grafting-efficiency. Polyacrylonitrile grafted brushes were chemically modified to polyamidoxime followed by copper complexation. The surface morphology and chemical composition of the synthesized polymer brushes were assessed by SEM, TEM, XRD, TGA and FTIR. The catalytic capability of Cu-AO-PB was assessed systematically for 4-nitrophenol to 4-aminophenol reduction utilizing a UV-visible spectrophotometer. The Langmuir–Hinshelwood model was exploited to study the mechanism of 4-nitrophenol reduction catalyzed by copper-loaded amidoxime-nanoclay polymer brushes. Thermodynamic studies revealed important insights about ΔG, ΔH and ΔS values. Turnover frequency (TOF) was calculated to be 1.65 × 10¹⁰ molecules g⁻¹ s⁻¹. Due to the integrated synergy between Cu and the unique polyamidoxime-nanoclay support, Cu-AO-PB demonstrated elevated catalytic efficacy for nitroarene reduction achieving a significantly higher reaction rate (0.0143 s⁻¹) and lower activation energy (28 kJ mol⁻¹) within 360 s.