Probing the active sites of aluminated mesoporous molecular sieve MCM-41 by secondary synthesis in the conversion of cyclohexanol

Abstract

The active sites of H-AI-MCM-41 aluminated by secondary synthesis have been probed by the conversion of cyclohexanol and compared with those of HAI-MCM-41 prepared by direct synthesis, purely siliceous MCM-41, AIP04-5 and H-ZSM-5. Conversion of cyclohexanol produces cyclohexene, cyclohexanone and 3-methylpentane in the presence of Bronsted, basic and Lewis acid sites respectively. Whereas cyclohexanol is converted to polyaromatic compounds in the presence of very strong acid sites as those found in H-ZSM-5. The formation of cyclohexene as the only product of conversion over H-AI-MCM-41 prepared by direct and secondary synthesis indicates the presence of Bronsted acid sites in both systems, which is not observed in the purely siliceous MCM-41 and AIP04-5 samples. The larger amount of cyclohexene formed over H-AI-MCM-41 by secondary synthesis suggests that there is a higher degree of Bronsted acidity in this system than that of H-AI-MCM-41 by direct synthesis. However, the strength of acidity in HAI-MCM-41 by secondary synthesis is weaker than that of H-ZSM-5. In addition, the results of this reaction, supported by IR spectroscopy study, do not indicate the presence of Lewis acidity in H-AI-MCM-41 prepared by secondary synthesis.