Fe(III) superoxide radicals in halloysite nanotubes for visible-light-assisted benzyl alcohol oxidation and oxidative C-C coupling of 2-naphthol

Selective oxidation of benzyl alcohols to aldehydes and 2-naphthol to BINOL was achieved by activation of molecular oxygen (O-2) and hydrogen peroxide (H2O2) over an iron-oxide catalyst embedded in halloysite nanotube. Electron spin resonance spectroscopy (ESR), Raman and in situ FTIR spectroscopic analysis provided direct evidence for the involvement of superoxide radical bound Fe-III species in the oxidation reaction. Both the analysis suggested the end-on binding of superoxide radical with Fe-III-centre. The stability of such radical bound Fe-III-species in halloysite nanotube was analyzed through density functional theory (DFT) calculations. Results suggested that end-on (eta(1)) binding was favourable by 13.5 kcal/mol than the side-on (eta(2)) binding mode. The formation of such reactive species was believed to play the crucial role in bringing the high selectivity in the catalytic oxidation of benzyl alcohol and oxidative C-C coupling of 2-naphthol. UV-Vis spectroscopic studies on the oxidation of benzyl alcohol suggested for the initial adsorption of substrate molecule on the catalyst surface followed by its interaction with Fe-III-superoxide/hydroperoxide species generated upon photoirradiation with visible light in presence of O-2. The presence of a suitable band gap similar to 2.14 eV enabled the catalyst to catalyze the reaction under visible light irradiation. Both the reactions (benzyl alcohol and 2-naphthol oxidation) were tested in presence of both O-2 and H2O2 as oxidants at ambient temperature. The influence of different parameters like rate of oxygen flow, amount of peroxide, nature of solvent, and catalyst amount on the conversion and selectivity of the reactions were studied to understand their role in the catalytic reactions. Successful oxidation of 2-naphthol with H2O2 as oxidant was a real success to overcome the limitations associated with this reaction using H2O2 as oxidant.

Automated summary

Selective oxidation of benzyl alcohols to aldehydes and 2-naphthol to BINOL was achieved by activating molecular oxygen and hydrogen peroxide over an iron-oxide catalyst embedded in halloysite nanotube. The involvement of superoxide radical bound Fe-III species in the oxidation reaction was confirmed through ESR, Raman, and in situ FTIR spectroscopic analysis. The stability of such radical bound Fe-III species in halloysite nanotube was analyzed through DFT calculations, suggesting that end-on binding was more favorable than side-on binding.