Fe3O4/Pd NPs immobilized on triazine-based polyurethane microspheres as a magnetically recoverable catalyst for the reduction of nitroarenes

Abstract

This study presents the design of a sustainable, triazine-containing, polyurethane-based magnetic catalyst (Fe₃O₄@PUN-Pd) that exhibits better catalytic activity and desirable reusability for the efficient reduction of nitroarenes. Uniform polyurethane (PUN) microspheres were prepared by surfactant-free precipitation polymerization of toluene diisocyanate (TDI), the triazine-based polyol 3-THA and subsequently decorated with magnetic nanoparticles (MNPs) and palladium (Pd) to form magnetically separable Fe₃O₄@PUN-Pd catalysts. Comprehensive characterisation using FT-IR, XRD, HRSEM, HRTEM, EDX, TGA, DLS, UV-vis, ICP-OES and XPS confirmed the molecular interactions, structural integrity, thermal stability, and uniform dispersion of Pd NPs. HRSEM confirmed PUN's spherical shape, while XPS determined the surface-bound Pd's oxidation state. The resulting catalyst offers a sustainable platform with substantial recovery and reuse potential. The synthesized catalysts were evaluated for their hydrogenation activity towards 4-nitrophenol (4-NP) in an aqueous medium. The catalytic efficiency varied depending on the method used to prepare the MNPs and the Pd NP loading. Among the prepared catalysts with different synthesis methods, Fe₃O₄@PUN-Pd(2) demonstrated the best performance in the aqueous-phase hydrogenation of 4-nitrophenol (4-NP), delivering a high rate constant of 0.430 min⁻¹ and maintaining ∼95% conversion over 10 consecutive cycles, possessing high stability and magnetic recoverability. The catalyst showed good activity and stability in water compared to organic solvents viz., methanol, ethanol, and acetonitrile, underscoring its green applicability. Additionally, the reduction of various hazardous nitroarenes, such as 4-nitroaniline (4-NA), 2-nitroaniline (2-NA), nitrobenzene (NB) and 2-nitrotoluene (2-NT), was also investigated in aqueous medium. The catalyst showed potent stability and good catalytic performance in water, a green solvent, compared to methanol, ethanol, and acetonitrile.