Chitosan-based Cryogels Containing Copper Nanoparticles as Efficient Heterogeneous Catalysts for Hydrogenation of 4-Nitrophenol

4-nitrophenol (4NP) is a high-priority pollutant in the chemical and pharmaceutical industries due to its high aqueous solubility, which increases its mobility in the environment. Even at trace concentrations, 4NP can adversely affect vital organs, including the liver, kidneys, and central nervous system. Given these environmental and toxicological concerns, the development of efficient, reusable catalysts for its transformation to 4-aminophenol (4AP) is of significant interest.

In this study, chitosan (CS) and thiourea-functionalized chitosan (CSTU) cryogels containing copper nanoparticles (CuNPs) were prepared and evaluated as heterogeneous catalysts for the hydrogenation of 4NP to 4AP in the presence of NaBH₄ as a hydrogen source. The prepared CS/CuNPs and CSTU/CuNPs composites were comprehensively characterized using FTIR, SEM, and uniaxial compression measurements.

The developed composites exhibited a porous internal structure, facilitating rapid diffusion of pollutants toward the catalytically active CuNPs. Catalytic experiments, performed at a catalyst dosage of 0.66 g/L, demonstrated that the composites achieved over 90% conversion of 4NP to 4AP within only a few minutes, depending on the cryogel’s chemical structure and NaBH₄ concentration. The composites maintained stable catalytic performance up to four consecutive cycles, and retained their effectiveness in complex natural waters. XPS analysis of the spent catalysts showed that their chemical structure was preserved after use. In addition, the ultralightweight properties of the cryogels enable the easy recovery of the composite catalysts from the reaction medium, thus improving their recyclability and operational sustainability.