Metal-free-photocatalytic reduction of 4-nitrophenol by resin-supported dye under the visible irradiation

Reduction of 4-nitrophenol (4-NP) with NaBH4 in the aqueous medium is a kinetically inert reaction, and generally the reaction occurs in the presence of metal/metal oxide nanoparticles. In the present study, the reduction of 4-nitrophenol (4-NP) has been carried out photocatalytically using the visible irradiation under a metal-free (eco-friendly) condition. By systematic screening, Eosin Y loaded resin is identified to be efficient in the reduction of 4-NP under the visible light. The effectiveness of the catalytic process depends upon the adsorption and the electron transfer. The adsorption favours the reaction molecules to come closer on the resin surface, and then reduction occurs by transfer of electrons. Gradual decrease in the absorbance at 400 nm confirms the decrease in concentration of the 4-NP. Simultaneously, there is an increase in absorbance at 300 nm infers the formation of 4-aminophenol (4-AP) during the photolysis. Results obtained from the irradiation on–off experiment suggests that the visible light irradiation is essential for the above processes, and also it indicates the sensitivity of the photocatalyst. The optimized reaction condition had been arrived from the results obtained after a series of experiments, including variation of catalytic dose, concentrations of 4-NP and the reducing agent (NaBH4). The rate of the reaction is found to be pseudo first order kinetics, and the rate constant (k) value is 6.90 × 10−2 min−1, under the experimental condition. From the control experiments, it is identified that the photocatalyst is primarily serving as an electron carrier in the reaction mechanism.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Photoreduction of 4-Nitrophenol was carried out under the visible irradiation. ► This involves metal-free heterogeneous photocatalyst: resin supported Eosin Y. ► Catalyst is highly light responsive, can be reused for this photoreduction process. ► Interestingly, the borohydride used here does not act as a usual hydride donor. ► Here, borohydride acts as a single electron-hydrogen atom transferring agent.