Synthesis of multi-amine functionalized hydrogel for preparation of noble metal nanoparticles: utilization as highly active and recyclable catalysts in reduction of nitroaromatics

Abstract

A new and one-pot protocol for mass-producable synthesis of the poly(ethylene oxide propylphosphonamidate) (PEOPPA) hydrogel bearing multi-amine and multi-amino groups with excellent swellability and flexibility has been described. The structural characterization of the formed gel was well analyzed by infrared, solid state ³¹P-NMR and X-ray photoelectron spectroscopies, and scanning electron microscopy. Rheological properties and swellability are carefully measured and discussed as well. The PEOPPA hydrogels have been used as a template and reducing agent for the synthesis of a series of noble metal nanoparticles (MNPs) (Au, Ag, Pd, Pt and Ru NPs) in the absence of any other reducing agents and stabilizers. In situ formed MNPs were uniformly impregnated inside the hydrogel without secession, efficiently avoiding aggregation-induced loss of catalytic activity. The MNP-impregnated hydrogels were employed for the reduction of various nitroaromatics in the presence of sodium borohydride in aqueous media, showing very high catalytic activity. These systems exhibit excellent recyclability in an easy way. The catalytic activity in enhancing the conversion rate increases in accordance with a series: PEOPPA hydrogel supported (HS) Ru NP < HS Ag NP < HS Au NP < HS Pt NP < HS Pd NP. The reaction rate constants at 10, 30 and 50 °C and the activation energies with other activation parameters were estimated. The activation energies for the reduction of 2,6-dinitrophenol, 2,4-dinitrophenol, 4-nitrophenol and 2,4,6-trinitrophenol catalyzed by the HS Au NP are 35.13, 29.08, 22.03 and 32.01 kJ mol⁻¹, respectively.