Effect of Mg-Al hydrotalcite crystallinity on catalytic Baeyer-Villiger oxidation of cyclohexanone with H2O2/acetonitrile

A series of magnesium-aluminum hydrotalcite-like compounds with molar ratio Mg/Al = 3 and different crystal sizes were prepared by variation of the synthesis parameters and characterized with powder XRD, XRF, SEM, N-2 adsorption/desorption (-196 degrees C), FTIR spectroscopy, thermal analysis, selective chemisorption of acrylic acid, and contact angle measurements. The materials were tested as catalysts in the Baeyer-Villiger oxidation of cyclohexanone to e-caprolactone with H2O2/acetonitrile as oxidizing system in mild conditions (70 degrees C, 1 atm). The study revealed that the catalyst primary crystal size constitute an important parameter affecting the catalytic performance. In particular, lowering of the crystal size improved catalyst's selectivity. This effect is discussed in terms of the evolution of catalyst's hydrophilicity upon crystal size diminution and its impact on the interaction with organic and inorganic reagents. The efficiency of H2O2 utilization is significantly lower than expected from the reaction stoichiometry, due to the decomposition of H2O2 and/or occurrence of non-selective reaction pathways.