Catalytic Reduction of Nitroarenes Enabled by In-situ Formed Silver Nanoparticles within a Hydrogel Matrix

Nitroarenes are toxic environmental contaminants, whereas aryl amines are valuable intermediates in pharmaceuticals, agrochemicals and functional materials. Developing sustainable and efficient nitroarene reduction methods under mild conditions remains a significant challenge. Herein, we report a versatile catalytic system based on in-situ generated silver nanoparticles (Ag NPs) embedded within an alginate hydrogel matrix. The hydrogel is formed via mixing aqueous solution of alginate derivative, guanidine hydrazide, and silver nitrate at room temperature, producing uniformly dispersed silver oxide nanoparticles that are subsequently reduced to Ag NPs using sodium borohydride. This process affords a recyclable, and highly active heterogeneous catalyst that efficiently promotes nitroarene reduction under ambient conditions. A wide range of nitroarenes is rapidly reduced to the corresponding anilines within 15–30 min, affording excellent conversions and yields. Kinetic analyses reveal pseudo-first-order behavior with rate constants ranging from 1.79× 10⁻² to 35.33 × 10⁻² min⁻¹. Electron-withdrawing substituents accelerate the reduction, and Hammett analyses support a Langmuir–Hinshelwood-type surface mechanism involving simultaneous adsorption of the nitroarene and reducing species. Importantly, the reactions proceed in water using minimal amounts of NaBH4, avoiding excess reagents and organic solvents. This hydrogel-supported Ag NPs platform provides a practical, scalable and environmentally benign approach to sustainable nitroarene reduction.