Oxidation of cyclohexane with hydrogen peroxide over β-zeolites with various Si/Al ratios

Selective oxidation of cyclohexanone to epsilon-caprolactone by H2O2 was investigated on beta-zeolites with various Si/Al ratio. The yield of epsilon-caprolactone was increased with decreasing Si/Al ratio. The beta-zeolites in which Si/Al ratio was lower than 117 showed the almost constant yield of epsilon-caprolactone with slight decrease of selectivity. The hydrolysis product, 6-hydroxycaproic acid, was observed. The conversion of H2O2 was also increased with decreasing Si/Al ratio, whereas the efficiency of H2O2 was kept constant, ca. 80%. At lower Si/Al ratio, the decrease of efficiency was also observed. These results suggest that Bronsted acid sites play an important role in the selective oxidation of cyclohexanone by H2O2. Infrared spectra of cyclohexanone and H2O2 were measured in order to elucidate the role of Bronsted acid sites in the oxidation. The IR spectra suggested that cyclohexanone mainly interacted with silanol groups of beta-zeolite, whereas H2O2 selectively interacted with Bronsted acid sites. The evacuation of beta-zeolite preadsorbed with cyclohexanone and H2O2 at 393 K brought about the formation of the vibration band of epsilon-caprolactone. On Na+ ion exchanged beta-zeolite, no specific peak of epsilon-caprolactone was observed. These results can conclude that the Bronsted acid sites activate H2O2 and the silanol groups interact with ketones over beta-zeolite with various Si/Al ratios. (C) 2012 Elsevier B. V. All rights reserved.