REDOX BEHAVIOR AND MOBILITY OF COPPER IONS IN NAZSM-5 ZEOLITE DURING OXIDATION

The redox behavior and mobility of the Cu ions in Cu ion-exchanged NaZSM-5 zeolites (Cu-NaZ) during oxidation of benzyl alcohol were examined by CO adsorption, IR absorption measurement of CO adsorbed on the Cu-NaZ, and diffuse reflectance spectra measurement. The properties and oxidation state of the Cu ions in Cu ion-exchanged SiO2 (CuSiO2) were also investigated by similar methods, focusing on the comparison with those in the Cu-NaZ zeolites. The Cu-NaZ zeolites with higher degrees of Cu ion exchange were found to have at least two different kinds of Cu ions, in contrast to the CuSiO2 which showed the presence of only one kind of Cu ion. The redox process of benzyl alcohol oxidation catalyzed by Cu-NaZ was followed by the IR measurement using CO as a probe molecule. Benzyl alcohol, ethanol, and isopropyl alcohol were found to cause the Cu ions, which located in recessed sites, to relocate to a more open space such as a channel intersection. The Cu ions were found to be reversibly displaced depending on the presence or absence of the alcohol molecule. In the oxidation process, the adsorbed benzyl alcohol was suggested to have the function of withdrawing Cu ions from recessed sites to sites accessible by the reactant molecule, at which benzyl alcohol was oxidized to benzaldehyde. The catalytic oxidation activity for benzyl alcohol was found to correlate with the amount of Cu ions which can be geometrically displaced in the Cu-NaZ zeolite.