Selective photocatalytic oxidation of aromatic alcohols to aldehydes with air by magnetic WO3ZnO/Fe3O4. In situ photochemical synthesis of 2-substituted benzimidazoles

Abstract

Recently, visible light-driven organic photochemical synthesis has been a pioneering field of interest from academic and industrial associations due to its unique features of green and sustainable chemistry. Herein, WO₃ZnO/Fe₃O₄ was synthesized, characterized, and used as an efficient magnetic photocatalyst in the preparation of a range of 2-substituted benzimidazoles via the condensation of benzyl alcohol and o-phenylenediamine in ethanol at room temperature for the first time. The key feature of this work is focused on the in situ photocatalytic oxidation of benzyl alcohols to benzaldehydes under atmospheric air and in the absence of any further oxidant. This new heterogeneous nanophotocatalyst was characterized via XRD, FT-IR, VSM and SEM. Short reaction time, cost-effectiveness, broad substrate scope, easy work-up by an external magnet, and excellent product yield are the major advantages of the present methodology. A number of effective experimental parameters were also fully investigated to clear broadness and generality of the protocol.

Scheme 1. General route for the preparation of different 2-substituted benzimidazoles.

Fig. 1. FESEM images of WO₃ (a), ZnO (b), and WO₃ZnO/Fe₃O₄ (c) nanoparticles.

Fig. 2. TEM images of WO₃ (a), ZnO (b), and WO₃ZnO/Fe₃O₄ (c) nanocomposites.

Fig. 3. FT-IR spectra of Fe₃O₄, WO₃ZnO, and WO₃ZnO/Fe₃O₄.

Fig. 4. Wide-angle XRD patterns of ZnO (a), WO₃ (b), and WO₃ZnO/Fe₃O₄ (c).

Fig. 5. XPS survey spectrum of WZnO (a), W 4f (b), Zn 2p (c), and O 1s (d) in WO₃ZnO.

Fig. 6. VSM magnetization curves of Fe₃O₄ (a) and WO₃ZnO/Fe₃O₄ (b) nanoparticles.

Fig. 7. Photocatalytic activity of some simple or mixed metal oxides in the synthesis of 2-substituted benzimidazoles. o-Phenylenediamine (1 mmol), benzyl alcohol (1 mmol), and 2 mg of each photocatalyst were mixed in 10 mL ethanol under the irradiation of a high-pressure Hg lamp at 25 °C for 3 h. Yield% refers to the isolated yield.

Fig. 8. Effect of the reaction time on the condensation reaction. The reaction conditions are the same as described for Fig. 7; 20 mg of WO₃ZnO/Fe₃O₄ was used in all cases.

Fig. 9. Effect of the catalyst amount on the photocatalytic efficacy of WO₃ZnO/Fe₃O₄. The reaction conditions are the same as described for Fig. 7. Reactions were performed for 2.5 h. Yield% was increased to 18 and 31% in the absence of photocatalyst after 4 and 24 h, respectively.

Fig. 10. Effect of benzyl alcohol : o-phenylenediamine ratio on the photocatalytic efficacy of WO₃ZnO/Fe₃O₄. Here, 10 mg of WO₃ZnO/Fe₃O₄ was used in all cases. Reactions were performed for 2.5 h.

Fig. 11. Studying recyclability of WO₃ZnO/Fe₃O₄ under the optimized reaction conditions.

Fig. 12. XRD patterns of the fresh (a) and final reused photocatalyst (b).

Scheme 2. Plausible mechanism for the synthesis of different 2-substituted benzimidazoles catalyzed by WO₃ZnO/Fe₃O₄.