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

Selective oxidation of cyclohexanone to ɛ-caprolactone by H2O2 was investigated on β-zeolites with various Si/Al ratio. The yield of ɛ-caprolactone was increased with decreasing Si/Al ratio. The β-zeolites in which Si/Al ratio was lower than 117 showed the almost constant yield of ɛ-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 Brønsted 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 Brønsted acid sites in the oxidation. The IR spectra suggested that cyclohexanone mainly interacted with silanol groups of β-zeolite, whereas H2O2 selectively interacted with Brønsted acid sites. The evacuation of β-zeolite preadsorbed with cyclohexanone and H2O2 at 393 K brought about the formation of the vibration band of ɛ-caprolactone. On Na+ ion exchanged β-zeolite, no specific peak of ɛ-caprolactone was observed. These results can conclude that the Brønsted acid sites activate H2O2 and the silanol groups interact with ketones over β-zeolite with various Si/Al ratios.