Catalytic reduction of 4-nitrophenol by palladium-resin composites

Catalytic reduction of 4-nitrophenol (4-NP) by palladium-resin catalysts is promising for 4-NP detoxification, in situ regeneration of the catalysts, and recycling of the products. In this study, we examined the catalytic reduction of 4-NP by Pd-based catalysts supported on three neutral resins and two anion exchange resins, using hydrogen gas as the electron donor. The reduction kinetics followed the same trend as the adsorption kinetics, i.e., faster adsorption and reduction were observed at acidic pHs for the neutral resins and at basic pHs for the anionic resins (4-NP pK(a) = 7.15). The Langmuir-Hinshelwood model based on surface reaction as the rate determining step fitted the kinetic data well. These findings point to the significant role of adsorption in the overall catalytic reaction. Cl- inhibited the reactivity of the neutral resins less than that of the anion exchangers due to the hydrophobic nature of the former. The longevity of the catalysts was examined in two tests: (1) 1 mM 4-NP was repeatedly spiked into the same catalysts for eight times, where the accumulation of 4-NP and the sole reduction product (4-aminophenol) on the resin surfaces resulted in the gradual loss in the catalytic activity, which was largely restored after regeneration, and (2) the catalysts were subjected to fresh influent in each adsorption and catalytic reduction cycle for seven times, where the catalysts sustained their reactivity through the seven cycles, significantly eliminating the need for frequent regeneration.