ANALYSIS OF ACTIVE-SITES ON COPPER ION-EXCHANGED ZSM-5 FOR CO ADSORPTION THROUGH IR AND ADSORPTION-HEAT MEASUREMENTS

The state of copper ion exchanged in ZSM-5-type zeolite has been inves tigated through the IR and adsorption-heat measurements at 301 K by us ing CO as a probe molecule. As a result, it was proved that there are at least three kinds of adsorption sites for a CO molecule on the copp er ion-exchanged ZSM-5, which are responsible for the IR bands at 2159 , 2149, and 2136 cm(-1), as well as the differential adsorption-heats of 91, 82, and similar to 70 kJ mol(-1). Corresponding to these data, TPD also gives three desorption peaks at 363, 442, and 542 K. An excel lent linear relationship has been established between the stretching v ibrational frequencies due to the adsorbed CO species and the differen tial adsorption-heat values. When CO gas is introduced to the 723 K-tr eated sodium ion-exchanged ZSM-5, this sample provides the adsorption- heat of about 35 kJ mol(-1) in the whole adsorption region studied and the weak IR bands at 2175 and 2112 cm(-1). For the sodium ion-exchang ed ZSM-5 and CO system, the above correlation does not hold in the sam e plot containing Cu-CO species. This fact is interpreted in terms of the difference in the nature of bonding between the electrostatic forc e for the sodium ion-exchanged ZSM-5 and CO system and the covalent na ture for the copper ion-exchanged ZSM-5 and CO system. More detailed d iscussions are also made on the nature of the Cu-CO bond.