Characterization of Fe−MCM-41 Molecular Sieves with Incorporated Carotenoids by Multifrequency Electron Paramagnetic Resonance

Abstract

Multifrequency electron paramagnetic resonance (EPR) spectroscopy was applied to study Fe(III)−MCM-41 mesoporous molecular sieves with incorporated carotenoids. It was demonstrated that high-frequency/high-field EPR is a promising technique to increase spectral resolution for proper assignment of different Fe3+ sites, which cannot be resolved by the X-band experiments. The broad unresolved at 9 GHz EPR line in the g = 2 region is due to overlapping signals from Fe3+ sites with different zero field parameters. The peak with g = 2.45 is tentatively assigned to aggregated Fe3+. The signal with g = 2.07 can be attributed to Fe3+ coordinated to oxygen atoms on the surface of the pore. A narrow line with gx = gy = 2.003, gz = 1.99, and E/D = 0.3 was attributed to a single Fe3+ site. The X-band and 94 GHz EPR measurements indicated that extraframework iron species at the surface of the mesopores are mostly responsible for carotenoid oxidation in molecular sieves. ENDOR measurements revealed the orientation of 7‘-apo-7‘,7‘-dicyano-β-carotene and canthaxanthin within Fe−MCM-41.