Evaluation of a highly stable biogenic palladium nanocatalyst for industrially relevant and sustainable nitro reduction.

The catalytic reduction of nitroarenes to aromatic amines is an important transformation for environmental remediation and industrial production of valuable chemicals. In this study, we report the use of a biogenic palladium nanoparticle (AtPdNP) synthesized by the Aspergillus trinidadensis VM ST01' OL587588 fungus for the highly efficient and selective reduction of various nitro compounds. The reduction reactions are successfully carried out in a green aqueous medium at room temperature under mild conditions with sodium borohydride (NaBH4) as the hydride source. The AtPdNP catalyst shows good functional group tolerance over a broad range of aromatic ring systems, including monocyclic, bicyclic, polycyclic and heterocyclic scaffolds. The catalyst exhibits excellent conversions (90-100%) and high isolated yields (80-95%) within short reaction times of 30 to 60 min. Additionally, the AtPdNP shows remarkable chemoselectivity, which enabled the efficient reduction of halogen-substituted nitroarenes without any unwanted side reaction of dehalogenation. Importantly, it shows excellent chemoselectivity between ketone/aldehyde and nitro groups by reducing only the nitro functionality using hydrogen gas. Besides catalytic efficiency, the heterogeneous AtPdNP demonstrates outstanding practical stability with high catalytic activity (>90% conversion and >80% isolated yields) throughout 10 successive reaction cycles. The catalyst also retains its intrinsic activity over a 12 month storage period under ambient conditions. Extreme thermo-mechanical stress studies show no leaching of palladium into the aqueous phase, demonstrating the strong anchoring of Pd nanoparticles within the fungal biomass matrix. Overall, the AtPdNP catalyst is a robust, sustainable and highly reliable heterogeneous platform for selective reduction of nitroaromatics.