Characteristics of silver ions exchanged in ZSM-5-type zeolite, aluminosilicate, and SiO2 samples:: In comparison with the properties of copper ions exchanged in these materials

The IR technique in combination with adsorption microcalorimetry was used to picture the bonding nature of silver ion exchanged or supported on solid materials, such as ZSM-5-type zeolite, aluminosilicate, and SiO2, by utilizing CO as a probe molecule. It has become apparent that there exists an adsorption site on which the CO molecule is adsorbed to give an IR absorption band: IR band at around 2193 cm(-1) for silver-ion exchanged ZSM-5 (AgZSM-5) and aluminosilicate (Ag/SiO2.Al2O3), and at 2177 cm(-1) for silver-ion supported SiO2 (Ag/SiO2). The CO adsorption took place accompanying a large heat evolution of 100-80 kJ mol(-1) on the former two samples and relatively small heat of 70-40 kJ mol(-1) on the latter sample; silver ions exchanged with protons acting as Bronsted acid sites are responsible for the strong adsorption sites for CO adsorption. Taking account of the relationship between the differential heat of adsorption (q(diff)) and the stretching vibrational frequency of adsorbed CO (nu(CO)), it was concluded that the electrostatic interaction is dominantly operative in these systems. The large adsorption heats in the initial stage of CO adsorption on the AgZSM-5 and Ag/SiO2-Al2O3 samples may be successfully explainable by considering a little contribution of a-bonding in addition to the electrostatic interaction. The,quantum chemical calculation was performed to justify the two types of ion-exchange model for silver ion coordinated to two or three lattice oxygen atoms in AgZSM-5, as well as to clarify the bonding nature between the exchanged silver ion and CO molecule. As the results, the two-coordinated silver ions in AgZSM-5 can adsorb CO molecules and give the values of about 100 kJ mol(-1) and 2193 cm(-1), and the three-coordinated silver ions weakly adsorb CO to give the values of about 80 kJ mol(-1) and 2184 cm(-1). These adsorption energies are much smaller and the stretching frequencies due to the adsorbed CO are higher, compared with the case of CO adsorption on copper-ion exchanged ZSM-5 (CuZSM-5). From these results it can be interpreted that the dominant force operating in the AgZSM-5-CO system is electrostatic attraction, as is different from the case of CuZSM-5-CO system in which the alpha-donation is dominant in the bonding. This difference is explained by taking account of the differences in energy gap between 4d-5s for silver ion and 3d-4s for copper ion; in the former case the hybridization of orbital is limited to result in a large alpha-repulsion. The present experiment clearly proved that the evaluation of bonding nature from the relationship between qdiff and nu(CO) is useful in the characterization of exchanged ions. The specific bonding nature between copper ion (Cu+) exchanged in ZSM-5 and CO molecule was also clarified.