A robust oxidized dextran/sodium alginate composite hydrogel containing small-sized gold nanoparticles for efficient and enhanced recyclable catalysts

Hydrogel-supported catalysts offer significant advantages in terms of enhancing catalytic performance, improving environmental adaptability, and promoting recyclability. In this study, oxidized dextran (Ox-Dex) was prepared, and used as a reducing agent and stabilizer to obtain small, highly uniform, and well-dispersed gold nanoparticles coated with Ox-Dex (AuNPs@Ox-Dex). AuNPs@Ox-Dex were blended with sodium alginate (SA) and cross-linked with Ca²⁺ to produce AuNPs@Ox-Dex/SA composite hydrogel microspheres. In the reduction of 4-nitrophenol, the hydrogel exhibited excellent catalytic activity. The reaction followed pseudo-first-order kinetics and the Langmuir-Hinshelwood mechanism. After five consecutive catalytic cycles, the composite hydrogel retained 97.86% of its relative catalytic activity, with a gold leaching rate of only 1.14%. The hydrogel structure remained essentially intact before and after the catalytic reaction, indicating that this composite hydrogel combines high catalytic efficiency with good stability. Furthermore, the AuNPs@Ox-Dex/SA composite hydrogel demonstrated excellent catalytic reduction capabilities for o- and m-nitrophenols as well as azo dyes (such as methyl orange and Congo red). This work has pioneered a new strategy for designing environmentally friendly heterogeneous catalysts for the treatment of organic pollutants and has expanded the application scope of natural polysaccharides in fields such as green synthesis, materials preparation, and heterogeneous catalysis.